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Lectures are not necessarily in chronological order
Lecture 1 - Introduction
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Lecture 2 - Organ Systems
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Lecture 3 - Organs of Information and Control
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Lecture 4 - Nervous System and Brain
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Lecture 5 - The Skeleto-Muscular System
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Lecture 6 - Hormones
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Lecture 7 - Circulation and Gas Exchange
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Lecture 8 - Nutrition, Digestion and Excretion
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Overheads - Excretion
Lecture 9 - Reproduction
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Practice Exam
Practice Exam - Answers
The METAZOA have a long evolutionary history. Their origins are uncertain but have been placed between 800 million and about 600 million years before the present (mypb).
FOSSIL EVIDENCE suggest that already 540 mybp in the Lower Cambrian there existed animals that had body plans recognizably like Arthropods, scale worms, jawless fish like lampreys and hagfish, mollusks, echinoderms, sponges, and sipunculids. The arthropod fossils of the Xianjiang formation (S.E China) and Burgess Shale Formation (in Canada) suggest great evolutionary divergence had occurred before 520 mybp.
FOSSIL EVIDENCE IS, HOWEVER, INCOMPLETE. Fossil containing rocks 550 millions years and older are not common. For example there are only a few localities known where pre-Cambrian fossils can be found. These are from the VENDIAN ERA and mainly come under the grouping of Ediacarian Fossil, after the locality in Australia where they were first found. These fossils are quite unlike the Cambrian biota, however, and may represent a group of organism that went extinct about 600 million years ago.
EXTINCTION EVENTS have caused mass die-off of animals and plants. One happened at the end of the Vendian (when there were no land plants yet. Land was colonized by plants only 430 million years ago, in the LOWER SILURIAN period).
The most catastrophic EXTINCTION EVENT happened about 250 mybp. This is the PERMIAN-TRIASSIC EXTINCTION EVENT (the P-T event). This event may have been protracted, lasting some hundreds of thousands of years. Huge tectonic changes occurred with massive and widespread volcanic activity. Sea levels dropped, the atmosphere was high in CO2 and the earth's surface temperature rose,possibly by as much as 80C (Interestingly, this are the worst case scenario for predictions of global warming). As a consequence possibly more than 90% of all animal species, and much of the plants, went extinct. Only one species of sea urchin is thought to have survived this cataclysm, but during the following recovery period, sea urchins underwent divergent evolution and habitat radiation, which is why there are many species ofthem today.
Another consequence of the P-T event was the demise of the trilobites, a major group of arthropods, which we know only from fossils.The Crinoids also went extinct at this time. Even insect suffered severe species loss.
The accompanying figure is a highly simplified history showing some of the major evolutionary events that advanced one or another group of animals. TAGMOSIS. Primitive arthropods had bodies that were made up of many almost identical segments (like millipedes and centipedes are today) One major evolutionary advance was the coalescing of certain groups of segments into special body regions, like the thorax or abdomen, accompanied by local segment-specific specializations of limb organization: This resulted in the differentiation of walking limbs, limbs that were used for feeding (mouthparts), limbs that were used for swimming, breathing. Similarly, certain internal organs were no longer repeated in each segment, but became localized to a single segment.
(Click to enlarge)
GENE DUPLICATION. It is known that in modern representatives of primitive chordates, like ascidians, or lancelets (Amphioxus), that only a single Hox gene is expressed in the front most part of the dorsal nerve cord. In jawless fish (which are thought to have descended from ascidian-like or lancelet-like ancestors) these genes are doubled, and in bony fish doubled again. One result is major changes of the front part of the nervous system to provide more complex brains. Another consequence is the evolution of additional skeletal and muscle elements to provide gill arches and jaws.
EMERGENCE ONTO LAND. The first animals to colonize land in the Late Ordovician period were probably scorpion and centipede-like species. Fossil marine scorpions look very like modern terrestrial scorpions (there are today no marine scorpions!). The first fossil insects are found in rocks from the Mid-Silurian but these were fragile animals and the fossil record is very incomplete. The first vertebrates colonization of the land was in the about 370 mybp in the Late Devonian.
FLIGHT The first fossilized winged insects come from 390 mybp rocks of the Mid-Devonian. Thus, it may have taken about 75 million years to accomplish this major change and another 100 million before insects appeared that could fold their wings over their backs.
EVOLUTION CAN THUS BE RAPID OR "EXPLOSIVE" AS IN THE LOWER CAMBRIAN, AND BE FOLLOWED BY PERIODS OF NEGLIGIBLE OR VERY SLOW DIVERSIFICATION. BUT AFTER A MASS EXTINCTION, RECOVERY CAN BE RAPID. This occurred after the P-T event, when there was a period of great diversification of marine fauna (fish, coral, and mollusks), of insects, and of reptiles.
Today there are representatives of ancient types of reptiles (the crocodiles). But there are no dinosaurs of the kind that went extinct at the end of the Cretaceous Period. However, it is now known that Dinosaurs were ancestors of the birds. Thus, in a sense, dinosaurs are amongst us.
The extinction event that ended the Cretaceous period resulted in the demise of the dinosaurs, but did not end mammalian evolution.
The fossil record suggests that the first mammals (as defined by the possession of hair and mammary glands) appeared in the second half of the Triassic Period, as did the first birds. Primates, with us as one of this evolutionary line, appeared towards the end of the Cretaceous. Bipedal hominids appeared about 23 million years ago, and Homo neanderthalis about 2 mybp. Homo sapiens (which is us, despite the misnomer "sapiens") exists for about the last 100,000 years.
The MODERN EXTINCTION EVENT is largely man made and is progressing at terrible speed. Predictions are that its impact will be as great as the END-CRETACEOUS event that saw off the dinosaurs. However, despite man's "intelligence", collectively and politically we are in state of denial and continue to destroy our planet. It is up to you to reverse this course.
1. Differences between plants, animals, and fungi.
2. How do protostomes differ from deuterostomes?
Which have spiral cleavage of their early embryos? Which has a blastopore that forms the mouth? In Protostomes, mesoderm forms from the lip of the blastopore, and then splits to form the coelom. Is this the same in Deuterostomes? What is meant by an indeterminate early embryo? How many cell divisions can occur in which each progeny cell could become a new embryo? What is the difference between radial and spiral cleavage?
3. If all deuterostomes are related by virtue of descent from a common ancestor, then are vertebrates closer to a cockroach or to a starfish? Please refer to Fig. 30.1.
4. In Fig. 29.3 an acoelomate, pseudocoleomate, and coleomate are compared. What are the major differences between these three body plans? What germ layers do they have in common (i.e. do all have an ectoderm, mesoderm, endoderm and what structures derive from these?).
WHAT KINDS OF ORGAN SYSTEMS ARE WE TALKING ABOUT?
Examples? Skin, cuticle.
What animals possess a cuticle?
The two organ systems are:
What are the crucial differences between them?
A. Organs for the generation of haploid cells that will become sperm and ova.
B. Organs for the transmission or dispersal of sperm and ova.
WHAT ARE ORGANS MADE OF?
FOUR BASIC TISSUE TYPES
ORGANS ARE COMPOSED OF TISSUES TISSUES ARE COMPOSED OF COHORTS OF CELL TYPES THAT FORM:
How many kinds of muscle are there? What are they called? What cells comprise nervous tissue? What are they called? Are Schwann cells neurons or a special form of glial cell?
WHAT IS EPITHELIAL TISSUE? Answer: Sheets of tightly connected cells.
WHERE DOES ONE FIND EPITHELIAL TISSUE? e.g. Skin and lining of hollow organs. Think of four organs as examples.
WHAT ARE SOME OF THE FUNCTIONS OF EPITHELIAL TISSUE?
Epithelial tissue can be secretory. What does this mean? What is actually secreted from the skin? How complex an organ is the skin? Skin is protective; but why?
Epithelial tissue, as in the lining of the gut, can have motile functions. What would do the moving? Muscles or cilia? And, is epithelium in the gut absorptive? Is the skin?
And when we consider skin, how complex is it? What kinds of glands does it have? What inert structures are produced by the skin, and what is the special system of cells that gives rise to this structure? Are there nerves in the skin? And why does hair "stand on end?"
WHERE DOES ONE FIND CONNECTIVE TISSUE?
How about in ligaments, dermis, bone, fatty tissue? Where else?
AND, AGAIN, WHAT KIND OF MUSCLES ARE THERE?
Name the type of muscle that we use for walking, lifting, and other voluntary actions. How does the structure of this type of muscle differ from the muscle that keeps your heart beating? What kind of muscle provides involuntary actions, and where is that type of muscle found? Think of at least two examples.
SKELETAL MUSCLE is innervated by motor neurons. These originate where? What provides information the brain? What are the non-neuronal cells of the central nervous system?
Organ systems. We will discuss more detailed features of these in the remaining lectures. However, you should have an overview of these systems and should know where the organs are located relative to each other in the human body.
A. Organs of reproduction. Please refer to Fig. 37.5 in your book and also Fig. 39.9 which describes the reproductive tract of the human male, and Fig. 39.12, which describes the reproductive tract of the human female.
B. Fig. 37.6 Organs of nutrition. The esophagus, liver, stomach, and intestine comprise the digestive system.
C. Fig. 37.6 Organs for gas exchange. Trachea, lungs, and the diaphragm. The movement of the diaphragm allows inhalation and exhalation.
D. Fig. 37.8. Organs of excretion and water and salt balance. Kidneys are supplied by blood vessels that wrap around the structures that filter, absorb, and process urine and conserve water. These are the "nephrons" (see Fig. 48.7), which we shall consider in more detail in the last lecture.
Urine is sent from the kidney to the bladder via the ureter, and expulsion of urine via the urethra is under voluntary muscle control. Distention of the bladder stimulates stretch receptors that inform the central nervous system that the bladder should be emptied.
E. Organs of protection, support and movement. i.e. skin, skeleton, and striate muscle system (Fig. 37.4).
F. Organs of transport. There are two major systems: the circulatory system and the lymphatic system (Fig. 37.7). Note what the blood transports. It is not merely red blood cells transporting oxygen, but also nutrients, wastes, hormones, heat. Where are white blood cells situated? This is a question that confuses many in exams. The answer is that white blood cells are present in the circulatory system and lymphatic system. They provide immune defense and the lymphoid organs produce large numbers of such cells. Where are red blood cells produced?
Q. Are the thymus and spleen part of:
a) The digestive system?
b) The hormonal system?
c) The lymphatic system?
d) none of the above?
What are lymph nodes?
G. Organs of control and information. Fig. 37.3
Both systems possess a kind of "master controller" What are these?
The endocrine system consists of glands that secrete into extracellular space. Hormones are carried to their targets via which organ of transport? What is the hypothalamus and how does it relate to the pituitary gland?
The nervous system comprises the central (voluntary) and autonomic nervous systems. What are the main sensory structures that detect external information?
How do primitive nervous systems in diploblastic or triploblastic organisms differ from the more advanced ones?
Answers: Nerve nets are primitive, whereas nervous systems that have clearly organized nerve bundles and ganglia are thought to be more advanced.
What does cephalization mean? What is a common feature amongst the central nervous systems of earthworms, insects, octopus, flatworms, and mice? See Fig. 41.1
Differences between nervous systems and endocrine systems
THE ENDOCRINE SYSTEM provides signals that effect change over distance. Signals are mediated by chemicals called hormones. The signal travels relatively slowly.
THE NERVOUS SYSTEM provides signals that effect change over distance by electrical signals carried by contiguous cells called neurons. The signal is transmitted within the millisecond range; that is, extremely rapidly.
We discussed homeostasis and the regulation of internal environments in complex organisms.
What type of organism doesn't need to maintain independence of internal environment from the external one?
The answer is that single cells, or diploblastic multicellular organisms such as sponges, jellyfish, and coral polyps are at equilibrium with the external environment. However, this equilibrium has its own inherent drawbacks, doesn't it? If the external environment is perturbed then these organisms are unable to adapt. It was discovered last year that much of the coral had died world-wide due to a global rise in the sea temperature. Corel us unable to maintain independence from such events and the polyps are incredibly sensitive to even the smallest changes of temperature. This is an example of mass extinction. What are its wider consequences (I won't ask this in the exam, but it's worth thinking about)?
What types of organisms have self regulating internal environments?
What is the advantage of self regulating internal environments?
CONTROL OF INTERNAL ENVIRONMENT: HOMEOSTASIS
Internal environment must remain constant within an optimal range despite changes of:
Maintenance is by homeostasis - that is, the control and regulation of internal environments. Information pathways for homeostasis are......?
Control is the mechanism for setting and altering parameters.
Regulation is the mechanism for maintaining a variable parameter.
Questions: what is a set point? What is its purpose? What is an error signal? What is feedback? What is feedforward? See Fig. 37.9
Negative feedback reduces or reverses a process. Thus, if a process is going too fast negative feedback will slow it. If a process is progressing too slowly negative feedback will reverse this and the process will.....what?
Positive feedback increases a process. What is an example of positive feedback?
MANY PROCESSES ARE TEMPERATURE DEPENDENT
What is the Q10 of a process? the Q10 is a quotient which is calculated how....? Hint, if a process is not temperature sensitive then its Q10 = 1.
Think of animals that can maintain their body temperatures.
Can a bird? A mouse? A shark? A bluefin tuna? a frog?
Some animals can generate heat from muscle; can a moth? Can a flatworm?
Endotherms are animals that can maintain their temperature independent of the environment Another term for them are homeotherms.
Poikilotherms are exothermic: they have body temperatures that reflect the external environment. A lizard is a poikilotherm. To regulate its body temperature it must move from places that are cool to places that are warmer. It uses temperature sensitive receptors to drive this motor behavior. The behavior is mediated by the central nervous system.
A heterotherm is an animal that can go into a "cool" state, lowering its body temperature and thus conserve energy. Think of examples of animals that do this.
Metabolic rates change as the environmental temperature changes. The metabolic rate/temperature relationships of ecto- and endotherms differ remarkably. How? (see Fig. 37.13 in your text book). What is an upper and lower critical temperature (see Fig. 37.19 and read the relevant text).
How does a "hot" fish conserve heat carried by its circulatory system if its blood from the gills is as cold as the water passing over the gills? Hint: there is a principle here, called counter current exchange. You will find this diagrammed in Fig. 37.19. Warmth from venous blood passes to the inflowing (colder) oxygenated arterial blood because the venous and arterial blood vessels are arranged side by side, with of movement of the blood in opposite direction (counter current) thus allowing temperature exchange from the warmer to the colder vessels.
Where and how is temperature regulated?
Where? In the brain.
What is the structure?. The hypothalamus
What happens if the hypothalamus is cooled in cold blooded animals?
Answer: thermotactic behavior: the animal seeks a warmer environment.
What happens if the hypothalamus is cooled in cold blooded animals?
Answer: several phenomena, all logical, contributing to the conservation of warmth and increase of heat production. Name two such phenomena. This should be easy if you consider what happens in a homeotherm if the hypothalamus is warmed? Blood vessels dilate to allow heat dissipation, the organisms may sweat, pant. In very hot conditions, the organism consumes less food because ..........?
The hypothalamus creates a set point for the optimal temperature of the organism. In order to do this, the hypothalamus needs what...............?
.............INFORMATION!
What is this information, and from where does it come?
When does the hypothalamus change its set point, and what is the adaptive advantage of this? When does a rise in body temperature contribute to the survival of the organism? What role do pyrogens play in this and from what do pyrogens derive? Pyrogens don't just affect the hypothalamus but they stimulate the production and activity of macrophages. These release interleukins, which stimulate other parts of the immune system.
HORMONES: THE ENDOCRINE SYSTEM
There are 4 major types of hormone
How have hormones evolved?
Because the same chemical substance may serve as a hormone mediating different functions in different organisms it is thought that chemicals serving as hormones evolved early, are highly conserved, and are used by different organisms for many different signaling functions.
Even protists such as slime molds can communicate chemically, releasing 3'5' cyclic adenosine monophosphate (cAMP) which stimulate slime mold aggregates to form fruiting bodies. In metazoans, cells that are the target of hormones possess receptor molecules that are "tuned" to that particular hormone. The water soluble hormone binds to the receptor. This activates a submembrane catalytic component of the receptor which in turn drives the activation of an adjacent G protein that then activates adenylate cyclase (a submembrane receptor molecule) which catalyzes the conversion of ATP to cAMP. cAMP functions as a "second messenger" to trigger other responses by the cell. Please refer to Fig. 38.13 and to Fig. 38.14.
This concludes notes to lecture 3. Please do not neglect to look at your own notes, particularly your diagrams. And please read the relevant chapters in your text book.
This part of the notes will cover the lectures on the nervous system; the nerve impulse, the synapse, the central and autonomic nervous system and sensory systems.
A neuron is a specialized type of cell. Usually, neurons have two types of processes - dendrites and axons. Which of these processes receive information? Which provide information. And, to what does a neuron provide information? Muscles? Other neurons? Glial cells? Blood vessels? One of the proceeding is wrong. One is a special case as in the hypothalamus (remember what a neurohemal release site is? Where a neurosecretory neurons releases substances into the blood). So, neurons provide information to other neurons. Certain neurons (motorneurons) provide information to muscle.
Question: What is the main temporal difference between the hormonal system and nervous system with respect to the transmission of information? Answer: the hormonal system may respond to a signal over minutes or even hours whereas the nervous system responds in the millisecond range.
A neurons conducts information along its axon by action potentials.
First consider the organization of ions inside and outside the nerve cell axon. There are more potassium (K+) cations within the axon than outside and more sodium (Na+) cations outside the axon than inside it. The axon also contains negatively charged protein complexes. Measured across the axon membrane there is a higher negative charge within the axon and a higher positive charge outside it. This is true when the axon is "at rest." The resting potential of the axon, measured between the two electrodes, one outside and one that penetrates the inside of the axon, is about -60 mV (millivolts).
The axon membrane contains many channels that are permeable to ions. Other are opened when the voltage across the membrane reaches a certain value. These are voltage-gated (gated meaning openable, like a gate) ion channels.
When the axon is at rest, sodium channels are closed, and potassium ions can move into and out from the axon through open potassium channels. Potassium is maintained at a higher concentration within the axons by channels that actively pump out sodium from inside the axons and let in potassium. The potential across the membrane begins to shift towards a more positive value when gated sodium channels open allowing more sodium into the axons. The change in potential across the membrane results in the opening of other voltage-gated sodium channels thereby suddenly increasing the inward flow of sodium ions into the axons. The negative charge within the axon has now suddenly decreased and the there is a short-lived change of voltage across the membrane (the action potential) during which the potential across the membrane reaches a value of about +50 mV. This depolarization now results in the opening of voltage-gated potassium channels and an efflux (outward movement) of potassium ions and the closing (inactivation) of voltage-gated sodium channels. The potential across the membrane falls to below - 60Mv and is said to "hyperpolarize" (the opposite of depolarize). After this potassium reenters the axon, sodium is actively pumped out and the resting potential is re-established.
Please refer to Fig. 41.9 to follow these events.
How does an action potential at one location in an axon trigger an action potential in an immediately adjacent part of the axons? Depolarization spreads: depolarizing current renews the action potential by causing voltage gated ------ channels to open and allow influx (inward movement into the axons) of ------ ions. Fill in the blanks.
What is a Schwann cell? How does the arrangement of Schwann cells facilitate an increase in the conductance velocity down the axon? Please refer to Fig. 41.12. What is a myelin sheath. What is a node of Ranvier? How does myelin insulate lengths of the axon? What is prevented from leaving or entering the axon along these portions between nodes of Ranvier?
Eventually, after a few milliseconds, the action potential reaches the end of the axon. It spreads into the terminal branches of the axon and finally arrives at presynaptic sites. What happens at the synapse?
Now, not only are voltage-gated sodium channels opened, but a divalent cation also enters the cell. This cation is.......calcium. Calcium facilitates the fusion of synaptic vesicles within the synapse with the synapse's membrane thereby releasing transmitter substance into the space between the synapse and the postsynaptic membrane of the next (postsynaptic) neuron's dendrites. See Figs. 41.15.
There are several types of transmitter substance. Acetylcholine is recognized by acetylcholine receptors in the dendritic membrane of the postsynaptic neuron. These channels are "ligand-gated." That is, they open when acetylcholine binds to them. When this happens there is an influx into the postsynaptic neuron of........... what cation?
This cation flows into the postsynaptic neuron's dendrites at postsynaptic sites. What type of potential is generated there? The answer is the excitatory postsynaptic potential or EPSP. When many synapses provide a lot of EPSPs in the postsynaptic neuron, then these small changes of voltage can summate spatially to provide an action potential. A new action potential can also be caused by temporal summation of EPSPs. See Fig 41.16.
Why do neurons communicate with other neurons via chemical synapses when it would be simply faster to have neurons coupled electrically? What is the advantage to the nervous system in having chemical synapses? Do all synapses excite the next neuron? No: GABA is a known inhibitory transmitter substance. Thus, what kind of receptors would there have to be in the postsynaptic neuron's dendrites such that the postsynaptic neuron would hyperpolarize. What kind of ions might be involved? Would inhibition be a depolarization or a hyperpolarization in the postsynaptic neuron? Are all synapses fast (the time between the arrival of an action potential at a fast chemical synapse and the generation of an EPSP in the next neuron is called the synaptic delay and may be 1-2 milliseconds long). The answer is no. There are slow synapses that act via second messengers, as shown in Fig. 41.17.
Neurons comprise the central nervous system. But how is information acquired by it?
The answer is via receptors - specialized cells that usually occur in specialized receptor organs. Receptor cells can originate from the periphery, as in the nasal epithelium, but are nevertheless often called receptor neurons.
What is a receptor? A receptor is a cell that detects a physical or chemical change and transduces this as an electrical signal. Sensory signals come in all guises: pressure (e.g.,. touch, sound); heat (thermosensors); chemicals (taste, smell - chemosensors); photons (photosensors); gravity (linear acceleration; mechanoreceptors). In each case proteins in the cell membrane of the sensory cell respond to a specific type of physical or chemical stimulus. Please refer to Fig. 42.1.
What is the sequence of events when a stimulus impinges on a receptor cell?
Stimulus -----> exchange in a receptor protein ----> ion channel opens ----> a potential change occurs across the membrane of the receptor cell (receptor potential) ----> this may be amplified (generator potential) to provide an action potential ------> neurotransmitter is released at the synapse to give rise to an EPSP in the postsynaptic neuron. See Fig. 42.2.
How is information encoded by a receptor cell? Think of what happens in a muscle spindle that provides information about the amount of contraction or extension occurring in your biceps. How would this data be encoded? A useful model of this is provided by the stretch receptor in a crayfish, as shown in Fig. 42.3. What happens when a weak stimulus is given? How do the spike trains differentiate between a weak and strong stimulus? Where is the generator potential initiated? Where is the receptor potential initiated? What is the difference between a receptor potential, a generator potential, and a sequence of action potentials? All the answers are found in Fig. 42.3.
1. Odor receptors. Please study Fig. 42.7, showing the sequence of events at an odor receptor membrane. Why is this so interesting with respect to other types of chemical receptors that we have discussed? Think about receptors that are tuned to specific hormone molecules. Consider the ubiquitous role of "second messengers."
Odor receptors in humans are situated in the nasal epithelium. Their axons terminate in the olfactory bulb. The olfactory bulb is relatively small in humans, but is proportionately much larger in dogs and in dogfish (related to sharks). What does this suggest with respect to the role that odors play in signaling a dog's world versus that of a human?
2. Acceleration receptors. These are found in the inner ear of mammals. The receptors are hair cells, each equipped with apical stereocilia that are bent when otoliths are displaced during acceleration of the inner ear. What displaces the otoliths? What kind of event occurs at the membrane of such a receptor? Refer to Figs. 42.1 and 42.12.
3. Equilibrium receptors. Again, hair cells are involved. But here (no pun intended) several hair cells have their stereocilia embedded in a dome-like structure called a cupola. Cupolas occur within special areas (ampulla) at the base or vestibule of each semicircular canal. Fluid is displaced in the semicircular canal when the head is tilted in one of the three directions (pitch -forward; yaw - to the side; roll - around the body axis). This results in the cupola bending, thereby causing a distortion of the membrane of the stereocilia. Axons of the hair cells project to the brain via the vestibular nerve.
4. Acoustic receptors. Again, these are hair cells. They are supported by support cells that lie on the basilar membrane of the cochlea. The tips of the hair cells extend upwards where they are attached to an overhanging and rigid structure called the tectorial membrane. When pressure distorts the tympanic membrane of the outer ear, this distortion is transduced as mechanical displacement of the oval window via three interconnected bones, the maleus, incus, and stapes (evolutionarily derived from the jaws of fish). The oval window is then distorted and this distortion sets up a pressure wave that travels along the cochlea. Depending on the pitch of sound, pressure waves travel to a specific position along the vestibular canal. At this position the pressure wave causes a distortion of the basilar membrane, and thus a shearing distortion of the hair cells. Low pitch (400 Hz) distorts the membrane near the end of the cochlea. High pitch (22,000 Hz) distorts the basilar membrane near the beginning of the canal. This is shown in Fig. 42.14. Increase in intensity (volume) increases the shearing action between hair cells and the tectorial membrane because oscillations of the basilar membrane at that position relate to intensity. The organization of the transducing elements between the tympanum and oval window (at the top of the cochlea) is shown in Fig. 42.13. The arrangement of hair cells and the basilar membrane are shown in Fig. 42.13. Please familiarize yourselves with these.
5. Pressure receptors and other skin receptors. The positions and structure of different types of receptor in the skin can give you an idea of what they are for. For example, the Pacinian corpuscle is a receptor embedded in many concentric layers of myelin and lies deep in the skin. It is sensitive to strong pressure. In contrast Meissner's corpuscles are situated near the surface of the skin, just beneath the epidermis. They are strategically placed so as to respond to light touch. What are Ruffini's corpuscles for? What are hair follicle receptor endings sensitive too? Please study Fig. 47.9.
6. Photoreceptors. We will discuss these receptors later when dealing with the eye and how the retina computes the first simple reconstructions of the visual image. Right now, remember that the photoreceptors of humans come in two "flavors": the rods and cones. Which of these are used for diurnal (daylight) vision? Which for nocturnal (night time) vision?
Now we can return to the central nervous system. Please remember how the brain derives from the neural tube. Is the neural tube endoderm, mesoderm, or ectoderm? When does it first appear in the embryo?
What are the fore, mid-, and hind-brain? What is the largest part of our brain? What is unique about the brains of humans? Think hard about this: and remember that areas of the cortex can be described as sensory and motor. Thus, whatever it is that is unique to humans, it is not simply that different parts of the body are mapped onto the cortex (this also occurs in mice, for example) or that specific areas of the cortex relate to the control of muscle action in various parts of the body (also true of mice).
What does the brain do? List all the phenomena that the brain mediates: to start off with: dreams, emotions,...........
The brain is an anterior specialization of a segmental central nervous system. More posteriorly, the central nervous system comprises the spinal cord, itself composed of 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral segments (in humans). Like the brain, the spinal cord processes information. This is nicely summarized in Fig. 43.4. A stimulus excites a receptor cell, which carries information to the gray matter (where neurons contact each other) of the spinal cord's dorsal horn. There, the sensory neuron synapses onto an interneuron (all neurons that are neither sensory nor motor are generally referred to as interneurons) which, in turn, synapses onto motor neurons in both left and right ventral horns of gray matter. Motor neurons terminate on muscles. On the side of the stimulus, muscle contraction results in raising the leg from the noxious stimulus. On the other side, other muscles co-contract to stiffen the opposite leg, providing increased support. Thus the actions of a single stimulus are computed by the central nervous system to give appropriate motor actions, which may differ on each side of the body. Information about the stimulus is also relayed to interneurons that send their axons forward to the brain. Axons extend through the spinal cord in the white matter. Why is white matter called "white matter"?
Information about sensations is mapped into specific regions of the cortex. For example, information about touch is mapped into the somatosensory cortex. What parts of the body are more represented than others in the somatosensory cortex? Are the surfaces of the hand and fingers more represented than the surface of the chest? Are the lips and face more represented in the somatosensory cortex than the hand? Are the genitalia more or less represented than the legs or neck?
Likewise, in motor cortex, more cerebral volume is dedicated to the control of the hand and fingers than to the legs. A huge amount of cortex is dedicated to the control of the face, and to the tongue and throat. Why is this? What is its evolutionary significance? Do cats or dogs have facial expressions? Do other primates, like Bonobos or Baboons?
Other parts of the cortex are known to be specialized for processing information about sounds, tastes, language, reading, awareness of one's own body, visual processing, and for associating different types of sensations (different "modalities") such as vision and hearing, or vision, hearing, and taste (see Fig. 43.7).
Many subcortical regions play vital roles in controlling overall neural activity. For example, the reticular system (Fig. 43.5) is crucial in controlling alertness and also sleep and wakefulness. The limbic system (Fig. 43.6) is a deep and evolutionarily primitive part of the forebrain that is closely linked to the olfactory bulb. This may explain why odors are so suggestive of other sensory events and emotions since the limbic system controls motivation, emotions, and contains centers for encoding pleasure and pain. Associated within the limbic system is an area called the hippocampus, which is crucial in forming memories about place.
This is divided into two components: the sympathetic and parasympathetic nervous systems. Together, these control increase or decrease of visceral activities. For example, activity in the sympathetic nervous system increases heart rate, blood pressure, cardiac output, decreases digestion, stimulates glucose release, inhibits gut mobility, relaxes the urinary tract, relaxes airways - all functions that are advantageous if you are challenged by a threat and need to move very fast so as to run away (please try not to allow certain of these reactions when you read the exam - relax, read all the questions first before answering any of them. Leave your running shoes at home).
All the final outputs from the sympathetic nervous system (called postganglionic neurons) carry the transmitter substance norepinephrin. Preganglionic neurons of the sympathetic nervous system originate from the thoracic and lumbar segments of the spinal cord. They carry acetylcholine. Preganglionic neurons terminate on postganglionic neurons in the sympathetic ganglia and in two other ganglia, the celiac ganglion (supplying norepinephrin containing postganglionic neurons to the stomach pancreas, liver, and gut) and the inferior mesenteric ganglion (supplying norepinephrin containing postganglionic neurons to the urinary bladder and to the genitalia).
The parasympathetic nervous system slows the heart rate, stimulates digestion and gut mobility, for example. It stimulates genital blood flow and stimulates salivation, and constricts the pupils. Its preganglionic neurons originate from the brain stem and from the 2-4 sacral segments of the spinal cord. Its postganglionic neurons originate from ganglia close to or actually on the target organs themselves. Both the pre- and postganglionic neurons of the parasympathetic nervous system carry acetylcholine.
The action of the sympathetic and parasympathetic postganglionic neurons are opposite: thus norepinephrine from postganglionic neurons of the sympathetic nervous system accelerate the heart beat. Acetylcholine released by postganglionic neurons of the parasympathetic nervous system slows the heartbeat.
Please study carefully Fig. 43. 11.
During which we discussed PRINCIPLES OF HORMONAL SYSTEMS AND ACTION and introduced the embryonic origin of the NERVOUS SYSTEM.
CHEMICAL COMMUNICATION - earliest form exists in protists, like slime molds. For example, 3',5' cyclic adenosine monophosphate (cAMP) is used by slime molds during aggregation into fruiting bodies. cAMP is also ubiquitous as a "second messenger." Please refer to Fig. 38.13 and text to familiarize yourselves with the principles of second messenger action. Also refer to the lecture notes 4 on senses and look at passages that discuss the sense of odor perception, in which second messengers play a crucial role in sensory transduction. A second messenger system is not limited to one simple step. As shown in Fig. 38.14, the reception of epinephrine at an epinephrine receptor of a liver cells results in a cascade of six successive reactions before glycogen is converted to glucose.
Hormones are CONSERVED across different species but may have different functions. The hormone thyroxin controls metamorphosis in newts and other amphibia but elevates metabolism in mammals.
The action of hormones has been well studied in an insect "model system." (A model system means a biological system that is more accessible for analysis in a relatively simple experimental animal, often evolutionary very distant from ourselves, which provides a similar set of structures, cell organizations, or chemical events as in our own bodies).
MODEL SYSTEMS SHOWING HORMONAL ACTION
In the 1950s, the English biologist Vincent Wigglesworth (who was later knighted - which doesn't happen to many biologists) showed that metamorphosis in the blood-sucking bug Rhodnius prolixus is controlled by a hormone, the release of which is triggered by injection of a blood meal. About a week after the 3rd instar (third stage of juvenile growth) insect ingests blood it molts into the adult form. Sir Wigglesworth showed that this was due to a substance secreted by the brain. If the 3rd instar insect was decapitated soon after its blood meal, it would never molt. But if it was decapitated a week after its blood meal it would molt. However, if the insect was decapitated soon after its blood meal, and was then fused to another insect that had been decapitated a week after its meal, both insects would molt to headless adults. This is because the insect that was decapitated a week after its meal already had hormone freely diffusing through its system and could pass this to the freshly decapitated insect that had been joined to it. This experiment is summarized nicely in Fig. 38.3 (except that the caption mistakenly says the insects are beetles or Coleoptera. They are not; they are Hemiptera or true bugs).
Further experiments by a number of researchers in the USA have shown that growth and maturation of insects like moths and butterflies is controlled by the relative amounts of the hormone ecdysone and juvenile hormone, both of which are controlled initially by the brain. The brain releases an initial brain hormone which then triggers the release of the hormone ecdysone from the corpora allata/cardiaca complex. As the animal matures, the brain releases less and less juvenile hormone. Ecdysone release induces molting. Juvenile hormone decrease results in the growth of the insect to ever more mature forms after each molt. Finally, the amount of juvenile hormone diminishes completely, and ecdysone triggers the final larval modification to the pupal form of the insect, which then undergoes complete organ reorganization, the growth of limbs, antennae, and reproductive organs, all of which comprise the final sexually mature adult moth. The combined action of ecdysone and juvenile hormone are summarized in Fig. 38.4.
AN EXAMPLE OF HUMAN HORMONAL PATHWAYS:
1. THE HYPOTHALAMUS-PITUITARY COMPLEX
The hypothalamus is part of the brain. Like parts of the moth brain it contains highly specialized nerve cells (hypothalamic neurons) that secrete hormones. There are two types of such neurons: those that send their axons directly to the posterior pituitary gland (also embryonically derived from the mid-brain), and those that send shorter axons to blood vessels of the portal organ situated in the dorsal part of the anterior pituitary. Blood vessels from the portal organ carry these neurohormones into the endocrine tissue of the anterior pituitary. Neurohormones stimulate or inhibit the release of hormones by the endocrine tissue. Together the hypothalamus and anterior pituitary make up a "master gland" complex.
What is an exocrine gland, by the way?
Thus there is a cascade of events. Hypothalamus -----> release of neurohormones (neurohormones that stimulate release of tropic hormones from the anterior pituitary. Neurohormones include thyrotropin-releasing hormone; gonadotropin-releasing hormone; adrenocorticotropin-releasing hormone; or hormones that inhibit the release of hormones from the anterior pituitary, such as prolactin release-inhibiting hormone) -----> neurohormones act on anterior pituitary----> to release tropic hormones (e.g. thyrotropin for the synthesis of thyroxin; or adrenocorticotropin for release of hormones from the adrenal cortex) which are carried by the circulatory system to other endocrine glands (thyroid gland, adrenal cortex).
Consult table 38.1 in your text book to see whole range of hormones and their actions and study Fig. 38.2 carefully to familiarize yourselves with where the various glands are situated.
QUESTIONS: What other tropic hormones have you listed in your lecture notes? What does luteinizing hormone do? Where does it act?
What is the target tissue of prolactin? What does prolactin stimulate?
2. THE THYROID AND CALCIUM REGULATION
What events are associated with the hormonal regulation of calcium and which endocrine organ plays the central role in the formation of new bone? What happens when there is an imbalance of calcium, such as when calcium is too low in the blood? Please study Fig. 38.9.
Now we come to THE EMBRYONIC ORIGIN OF THE NERVOUS SYSTEM
Most of the ground-breaking studies of embryogenesis were done on amphibian eggs, like those of frogs.
After many cell divisions of the fertilized egg, a series of crucial events occurs, which transform the simple blastula stage of the embryo into a gastrula. This process is called gastrulation.
EVENT 1. Formation of the blastopore.
EVENT 2. Expansion of sheets of outer cells (expansion of course involves many cell divisions) in the upper hemisphere of the blastula (this cell layer lines the upper part of the blastocoel) into the lower hemisphere of the embryo. This sheet of cells will form the ectoderm. Expansion of the ectoderm is accompanied by inward movement of the larger cells in the lower hemisphere. These cells will form the endoderm. Inward migration of cells situated just above the dorsal lip of the blastopore under the outer cell layer will form the endoderm.
EVENT 3. Now the embryo (which is still a round ball) consists of three layers. In the upper hemisphere of the gastrula ectoderm lies over mesoderm, which is itself lined by a thin layer of endoderm. In the lower hemisphere, ectoderm lies over a thick layer of endoderm. Please refer to the first three images in Fig. 40.11.
EVENT 4. Now the embryo begins to elongate. Part of the ectoderm in what will be the front of the embryo differentiates into the neural plate. This extends down the dorsal length of the embryo which is stiffened by specialized mesodermal cells that form the notocord. Meanwhile mesodermal cells migrate and proliferate ventrally to interpose between the endoderm and ectoderm. Refer to the fourth and fifth images of Fig. 40.11.
What happens if some of the gastrula's ectoderm that will later form the neural plate is transplanted into ventral ectoderm? The answer depends on the age of the embryo when this operation is performed. If the transplantation is done to an early gastrula before ectodermal cells have begun the program of differentiation no effect is seen. But what happens later? Consult Fig. 40.13.
So, then, what embryonic tissues make what adult tissues?
The answer is full of surprises: Did you know that ectoderm gives rise to:
And the mesoderm differentiates into:
But the mesoderm does not give rise to the liver!
This is formed from differentiated endodermal cells, as are the:
Now let's get back to the formation of the brain and spinal cord. Please refer to Fig. 40.16, which shows that during the transformation of the embryo from a gastrula to a neurula (these are terms that denote major stages of embryonic growth and tissue formation) the neural plate starts to get a thick ridge around its edge and that the plate is broader in the front of the embryo than in the back. This ridge, which is called the neural fold, migrates over the neural plate, moving inwards from the edges to the midline, so as to form a neural tube. This will become the brain and spinal cord.
The neural tube starts to swell and fold anteriorly. This is a very distinctive set of events which, for humans, is shown in Fig. 43.3.
There are three main divisions of the brain: the forebrain, the midbrain, and the hindbrain, and then the spinal cord. In humans as in other mammals the forebrain will provide two major subdivisions, the telencephalon (which will become the cerebral cortex) and the diencephalon. Note that the developing eye is an outgrowth of the diencephalon. The diencephalon will form the evolutionary more ancient subcortical regions, like the limbic system and hippocampus. It will also provide the thalamus and hypothalamus, and the pituitary. The hindbrain provides the cerebellum and the pons - the part of the brain linking it to the spinal cord.
Question: What happens if a small part of the motor cortex is injured? What happens if the pons sustains a lesion?
Question: does a sharks brain look more like the brain of a 25 day old human embryo or like that of a 50 day old embryo. Does a shark have a cerebral cortex? Does a frog?
Finally, consider how the overall organization of the whole nervous system.
What is the difference between the peripheral nervous system (PNS) and the central nervous system (CNS)?
What is the voluntary NS and what does it control?
What is the autonomic nervous system and what does it control?
What carries signals to the CNS.
These and other basic questions are answered in Fig. 43.2 of your text book. Please familiarize yourself well with this figure.
Effectors and the skeleton (motility, muscle, and contractile tissue, bone, levers, joints).
EFFECTORS:
The simplest cases are actions in unicellular organisms. TWO IMPORTANT STRUCTURES ARE INVOLVED: Microtubules and Microfilaments. In unicellular organisms, cilia and flagella are driven by microtubule activity.
Microtubules:
Are hollow tubes of a protein called tubulin. In flagella there is a cylindrical arrangement of paired (double) tubes that are fused together. One of the pair of tubes has side arms of a protein called dynein. The head of each protein molecule is attached to a neighboring cylinder of tubulin; namely, to the member of the pair that doesn’t have dynein. It is the side arms of dynein that generate force. There is a conformational change in the structure of dynein such that the head bends downwards pushing the neighboring pair of tubes downwards and the tube to which the dynein molecular is attached upwards (along with its fused partner). In a flagellum, there are 9 pairs of tubulin tubes, with one of each pair attached by dynein to the other tube of the neighboring pair. Because all the tubes are anchored at the base, they can’t really be pushed up or down, but instead have to bend in a rotatory fashion thus driving the flagellum in a sort of propellerlike motion.
What single haploid cell of a multicellular organisms has a flagellum? That question is really easy. Here’s a harder one. One types of receptor neuron in the vertebrate has the rudiments of an axoneme in the cell itself. The answer to that is the rods and cones of the retina!
For a detailed explanation of the structure of a flagellum (the configuration of the circle of 9 pairs of microtubules surrounding a central pair = an axoneme) see Fig. 44.4.
Reminder: Microtubules are
Microfilaments are
Microfilaments comprise strands (filaments) of a protein called actin. These strands can change their length when they undergo a conformational change of their molecular structure (this is a polymerization or the reverse, a depolymerization).
COLLABORATIVE ACTION OF ACTIN AND MYOSIN TO PROVIDE CONTRACTION
Together actin and another protein called Myosin provide for muscle contraction.
In muscle the actin filament consists of a double helical arrangement of actin monomers which are enwrapped by a double helix of another protein called tropomyosin.
Each actin monomer has a binding site to which the head of a myosin molecule can attach.
BUT.........attachment occurs only when the binding site is exposed. How does this occur? The answer is that the tropomyosin can be pulled away from the actin double helix and when this happens binding site for myosin attachment are exposed.
HOW DOES THIS PULLING AWAY OCCUR? Molecules of troponin are spaced along the tropomyosin helix. When calcium binds to troponin there is a conformation change such that tropomyosin relaxes pulling away from the underlying actin monomers.
Now the heads of the myosin filaments can attach to the actin binding sites.
GO TO FIG. 44.10 in your book and study the cycle of events just described. Question: Is energy, in the form of ATP used to bind the head of a myosin molecule to the binding site on actin or to detach the head of a myosin molecule from the binding site? Question: What happens to calcium after the head is detached from actin. Answer : it is released from troponin. Question: Where is calcium sequestered (stored) in the muscle fiber? Answer : In the sarcoplasmic reticulum.
But what triggers release of calcium ions and their biding to troponin? It is the action potential that spreads through the muscle fibre that causes current gated release of Ca2+. And, what triggers the action potential in the muscle? That is easy. If you don’t know, look at Fig. 44.9
In Fig. 44.9 and 44.7 you will see terms like "sarcomere," "H zone," "A band," "M band," "I band," and "Z line." I sympathize with you: these are awful things to learn, but they are important because if you grasp their arrangements then you have understood how the actin filaments are arranged with respect to the myosin filaments. For example, if the filaments of actin and myosin have to slide against each other for the muscle to contract, then there has to be some room for them to do so. This is provided by the I band and H zone. There has to be a place where one set of these fibres is anchored, otherwise myosin filaments couldn’t exert force on the actin filaments. The anchor point is the M band.
So now work backwards from 44.10, then remember how the molecules are structured in Fig. 44.8. Now go to the bottom of Fig. 44.7 and look at how lots of these actin and myosin filaments are arranged, stacked on top of each other and beside each other in an incredibly orderly fashion (it is such a pseudocrystalline structure that the arrangements shown were discovered by X-ray diffraction). Now look how the single sarcomeres are contiguous, end o end, to form a myofibril. Lots of myofibrils are arranged together ensheathed by the sarcolemma. This set, surrounded by the sarcolemma (which has many nuclei) comprises a single muscle fibre. Many fibres form a bundle of fibres, each bundle surrounded by connective tissue, many fibres form a muscle.
There you have it!......but we are not quite finished. The muscle fibre has other structures, mentioned above. These are the T-tubules and sarcoplasmic reticulum. Action potential spread from the postsynaptic sites at the neuromuscular junction, through the T-tubules, and release calcium cations from the sarcoplasmic reticulum. That’s shown in Fig. 44.9.
AGAIN FOLKS: HERE ARE THE SALIENT EVENTS WHEN AN ACTION POTENTIAL IN A MOTOR NEURON AXON REACHED THE SYNAPSE AT THE NEUROMUSCULAR JUNCTION.
It follows then, that the more myosin and actin bind the greater the contraction. It follows that the more calcium cations are released from the sarcoplasmic reticulum the more binding sites on actin will be exposed to binding of myosin and the more the muscle will contract and it follows the greater number of nerve impulses depolarizing muscle, the greater the amount of calcium cations released etc. Etc. Thus, muscle contraction is proportional to the action in a muscle motor neuron.
What examples of organs have muscle cells (not just striate, but any kind of muscle cells - and by the way, how many kinds of muscle are there: striate, and ......., and ..........). Where in our eyes do we have muscle?
THE SKELETON
We have internal skeletons. Insects don’t, nor do crustaceans. They have exoskeletons. In insects exoskeletons are made of cuticle.
Study Fig. 44.15, just to get the feel of it all, so to speak. How are muscles attached to bone. What is a tendon, what is a ligament?
How are muscles arranged between bones? Answer is: for power or for speed.
For power: the le is further towards the end of the load arm than it is for speed. For power, the load arm-power arm ratio is LOW. For speed the muscle is much nearer the joint and the load arm-power arm ratio is high. The axiom here is: less speed more force, low ratio but high speed, less force, high ratio.
Most bones are articulated with other bones (not all are, but most). These points of articulation are called the joints. How many types of joints could one buy at a comprehensive hardware store? How many types of joints have been invented by engineers through the ages, but have always existed within us. The answer is: all of them! Look at Fig. 44.21.
BONE GROWS, BONE RESHAPES, BONE REPAIRS.
Bone is a dynamic tissue. Look at the stages of development in Fig. 44.17 and then look at the section across the head of femur in Fig. 44.18. You will see the lines of mechanical stress revealed as struts of bone within the shell of the femur. This provides enormous strength where it is needed, and also contributes to the lightness of bone. In the adult, osteoclasts tunnel new pathways for blood vessels, and osteoblasts make new bone. This occurs in repair and also when bone is subject to new stress forces. Look at these structures in Fig. 44.16.
What does bone marrow produce?
One tip: if a question asks if all of the above are incorrect (or correct), then if you decide one is incorrect, they all have to be incorrect. Such a question would not mix up correct and incorrect statements.
The same strategy as for lecture 1-5: Notes interspersed with questions that will direct your reading of relevant material.
The Circulatory System.
The heart.
The heat contracts due to the coordinated contraction of cardiac muscle. Cardiac muscle has many unique properties. Please think about these and list them. For example, cardiac muscle is unlike striate muscle in that the fibres are arranged not as parallel longitudinal units but as ................. (?). Also, certain cardiac muscle fibres are arranged as specialized aggregates called nodes (e.g. the sino-atrial node), which serve as pace makers.
Blood pressure changes initiate feedback pathways
What happens when the pressure an a major artery falls?
1. Stretch receptors relax, this information results in decrease neural activity in their axons which lead to the hypothalamus.
2. Vasopressin is released from the hypothalamus
3. Blood vessels constrict
4. Arterial pressure rises.
AT THE SAME TIME WHAT ELSE OCCURS?
Pressure in a major artery falls
1. Decrease blood flow to tissues
2. Accumulation of wastes
3. Widening of vessels
4. Pressure in artery falls
Blood Pressure is Regulated
1. Stretch sensors in arteries detect change of pressure.
2. They signal information to medullary cardiovascular control center (MCCC)
3. This center also receives information from higher brain centers (which is why emotion, anticipation, and stress can all affect the heart rate)
4. The medullary cardiovascular control center supplies information to the two parts of the autonomic nervous system. What are they called?
5. The sympathetic nervous system supplies connections from the MCCC to the adrenal gland (which, don’t forget, is situated on each kidney). This secretes epinephrine, which increases the heart rate.
6. Chemosensory receptors on the aorta and carotid arteries also supply information about O2 to the MCCC which communicates information to the parasympathetic nervous system, which in turn decreases the heart rate due to the release of...........?
Blood
"Mud, mud, wonderful mud
There’s nothing quite like it for cooling the blood
So let us all Follow
Down to the Hollow
And there let us Wallow
In GLORIOUS mud."
(The famous Hippopotamus’s song)
But what does blood actually consist of?
It’s 60% plasma and 40% cellular
The Plasma contains water, which is the solvent, and salts, such as Na+, K+, Ca2+, Mg2+, Cl, HCO3-, all of which are involved in maintaining osmotic balance and/or pH buffering, and/or regulation of membrane potentials. Plasma also contains proteins, such as albumin, fibrinogen and immunoglobulins. These are also involved in osmotic balance, but more crucially in clotting and in the immune response. Which is involved in clotting?
The cellular fraction contains cells or bits of cells (like the platelets, which are involved in clotting).
| components: | Erythrocytes | Leukocytes (including lymphocytes) | platelets |
| amount/1ml: | 5-10,000 | 5-6 million | 250-400,000 |
| functions: | transport of O2, CO2 | destroy foreign cells, clean up particles
of foreign matter, produce antibodies, roles in allergic reactions |
clotting |
New blood Erythrocytes derive from stem cells in the bone marrow (especially marrow of the ribs, breastbone, pelvis, and vertebrae) Production of erythrocytes is controlled by a hormone called erythropoietin. This is released from the kidneys is too little O2 in blood. White cells also produced by bone marrow.
Two types of precursor cells:
| Totipotent | ones produce only lymphocytes (found also in tonsils, spleen, lymph nodes |
| Pluripotent | ones produce other types |
The circulatory system
Arteries that supply oxygenated blood to your muscles, and other organs are initially of large diameter and under great pressure.
Blood must somehow be dissipated in order that the erythrocytes are transported as close as possible to the target tissue so that oxygen can diffuse to other cells.
Think of a river, carrying a massive volume of water across a flat delta. What happens to the course of the river? With that in mind you will gain some insight into how arteries anastomise into ever smaller vessels, finally branching as arterioles.
Oxygen is mainly provided to tissue by a means of a transport molecule although some oxygen is supplied dissolved in blood plasma.
O.K. so now you’ve remembered it: HEMOGLOBIN! 1 molecule of hemoglobin contains 4 subunits of heme, which bind 4 molecules of O2. What happens when the partial pressure of oxygen is low in tissue? Erythrocytes release about 33% of their oxygen.
There is another vital pigment molecule in your (and my) tissue that carries oxygen. This is myoglobin, which is concentrated within striate muscle. The more myoglobin in the muscle the darker the muscle. This is why the poor Thanksgiving turkey has dark muscle in its legs.
At high altitudes, the percentage of blood-saturated oxygen is higher than at sea level.
The answer to the latter is the acidity of tissue. The more acidic the tissue the greater the release of O2 into it. For example, tissue pH is lowered by lactic acid, carbon dioxide, and fatty acids.
Carbon dioxide is produced by tissue. Essentially it a waste product of metabolism and must be removed. Does CO2 simply replace O2 in hemoglobin? No, relatively little CO2 is carried from tissue by red blood corpuscles. Instead CO2 combines with water to form ....what? And which enzyme is involved in this reaction? What is the role of this enzyme - carbonic anhydrase? Does it facilitate the production of H2CO3? Does it have a second role, splitting this molecule back to CO2 and H2O? The answer is that indeed the second reaction is also driven by carbonic anhydrase where the capillary invests the alveolus of the lung.
Alveoli are little bags of tissue that walls of which are one cell thick, separating the lumen of the alveolus and the smallest capillaries of the body. These are the venous capillaries, which anastomize (branch) from the pulmonary venule. They surround alveoli where they give up their CO2 across the alveolus wall and absorb O2 from the alveolus lumen. The oxygenated blood returns to the pulmonary arteriole via converging arterial capillaries. Arterioles converge to the left (or right, depending on which lung we are looking at) branch of the pulmonary artery. The alveoli are arranged in clusters, arising from a thicker hollow branch called the bronchiole, which is itself a tributary of a system of bronchi that originally branch off the base of the left or right branch of the trachea. The lung is suspended in the plural cavity and muscle action of the thorax and abdomen provide the upward and downward movement of the diaphragm that results in inspiration and exhalation.
Detection of CO2 and O2 by the nervous system
Again we return to the nervous system. The base of the carotid arteries have specialized patches of tissue, the carotid bodies, that contain sensory receptors.
They are chemoreceptors that signal to the respiratory center of the brain’s medulla oblongata information about O2 levels. There is also another area of chemoreceptors. Where are these? What do they detect? The answer is, also O2 at the aortic body. So, where does the detection of CO2 occur? This is at a sensor region on the underside of the medulla oblongata that relays information to the respiratory center via interneurons. Outputs from the respiratory centers innervate which muscles?
This brings us to the end of lecture notes 6. Please be aware of the questions asked here, but don’t assume that only some of these could be included in the examination.
A tip for the next exam. When you receive your exam papers, read all the questions first. Answer those that you know you can answer confidently because certain questions may actually provide important hints about answers to other questions that may be more difficult at first sight.
Organisms as diverse as earthworms, insects, and humans all have tubular structures that actively (by transport across membranes or by pressure across membranes) accrue dissolved salts and nitrogenous waste. Tubules (or their analogues) of the excretory system concentrate these wastes while returning to the circulatory system ions and water that are needed for ionic and osmotic balance.
In earthworms "metanephridia" are the organs that perform these functions. Dissolved waste material is transported into the bulbous head of the metanephridium, called the nephrostome. Blood capillaries invest the collecting tubule, which is continuous with the nephrostome, and water is reabsorbed from the tubule. Dissolved waste material increases in concentration and is stored in the bladder to be eventually expelled from the nephridiopore.
The earthworm consists of many identical segments and is bilaterally symmetrical.
All these questions are easy to answer by studying figure 48.6, and thinking about what direction water flows across a semipermeable membrane if the outside has a higher concentrate of solutes than the inside.
So far, we have seen that excretion is a two step process: filtration and reabsorbtion. First, the solvent, which is water, carries a variety of solutes (ions, uric acid) into the tubule. These are filtered to exclude larger molecules. Then there is selective reabsorbtion from the tubule of those solutes (e.g. sodium, potassium) that the organism cannot do without. It follows that water is reabsorbed by differences of osmotic pressure between the inside of the tubule and outside (if sodium and potassium are actively transported from the inside to the outside of the tubule, then the increased concentration of ions outside will drive movement of water from the inside to the outside).
The kidney is constructed into discrete functional subunits called the cortex and medulla, the latter divided into renal pyramids. This is where the nephrons are situated.
What constitutes a nephron? Please study Fig. 48.10.
In the cortex, arterial blood is carried through densely branching clusters of capillaries, called a glomerulus, that are semi-encapsulated by the head of the nephron's tubule, called the Bowman's capsule. The glomerulus is the site where water and dissolved wastes are forced from it into the capsule, across its cell wall, by arterial pressure. A summary of the nephron is shown in Fig. 48.7. The renal tubule is not straight like in the diagram. Instead, it is thrown into three loops, as schematized in Fig. 48.11. Note the arrangement of blood vessels with the Bowman's capsule and renal tubule.
Each of our kidneys contains a million nephrons.
The kidneys are hugely supplied by blood vessels.
They are receiving 20% of your arterial blood, even as you read this. (Hopefully much more, percentagewise, is reaching our brains!)
Even so, the kidneys get a lot of blood. And they need to, being exquisitely designed filtration machines.
->-> The input to the kidneys (it follows from the % above) is 1 liter per minute or 1440 liters each day. 12% of this is across into the nephrons. But we don’t urinate 172 liters a day. In fact, we urinate 2-3 liters a day.
How is this achieved?
Answer: by a TWO-STEP system.
1. FILTRATION from glomeruli into Bowman's capsule.
2. REABSORBTION from the tubules. This involves active transport of salts (NaCl) from the tubules into the extracellular fluid. NaCl in the extracellular fluid contributes to the differences of osmolarity between the inside of the tubule and extracellular fluids. It is important that you carefully study the principle of counter concentration reabsorbtion, which is nicely illustrated in 48.11.
Note that osmolarity (milliosmoles/liter) is low in the proximal tubule. Osmolarity increases down the descending limb of the tubule due to water being driven out of the tubule because of the differential between osmolarity inside the tubule and outside the tubule in the extracellular fluids (given to the left of Fig. 48.11).
In the loop of Henle, the osmolarity in the tubule is very high indeed due to the increased concentration of uric acid. However this concentration is eventually increased even further by the following mechanism:
2. This contributes to the graded increase of osmolarity in the extracellular fluid - low osmolarity in the outer medulla increasing to high osmolarity in the inner medulla.
3. It is this gradient, once again, that provides the necessary driving force for further extraction of water from the collecting duct, leading to an even further increase in the concentration of uric acid.
4. Thus, at the inner part of the duct, osmolarity is again very high, mainly due to the concentration of uric acid that has been built up due to the selective reabsorbtion of water.
Finally, let us consider the concomittant pathways regulating blood pressure and blood osmolarity. This is diagrammed in Fig. 48.14 of the text book. But let’s go through the two pathways step by step.
| START | START |
| 1. Rise in blood pressure..... | 1. Rise in blood osmolarity..... |
| 2. Detected by pressure receptors..... | 2. detected by osmosensors..... |
| 3. .....inhibit activity of neurons in the
neurons to the pituitary |
3. .....which stimulate hypothalamus that
lead to the posterior hypothalamic posterior pituitary which..... .....releases antidiuretic hormone(ADH) |
| 4. ADH constricts blood vessels | 4. ADH increases the permeability of nephron tubules |
| 5. | Thus:
A) decreased osmolarity inhibits osmosensors, and antidiuretic hormone release from post pituitary stops. B) Reabsorbtion of water maintains blood volume |
| 6. Due to B, construction of peripheral
blood vessels provides elevation of blood pressure, detected by sensors, and further inhibition of hypothalamic neurons, thus decrease of ADH release by post pituitary. |
And now think of obvious questions that an examiner might ask.
One of the statements below is incorrect. Which is it?
B) Elevation of blood pressure triggers release of ADH from the posterior pituitary
C) Decrease of blood osmolarity indirectly inhibits release of ADH
D) ADH increase permeability of nephron loops.
You got the right answer: it's B.