Archive for February, 2010

Comparative Animal Respiration

Introduction

Respiration is the process of obtaining sufficient oxygen from an organism’s environment to support its cellular metabolic requirements. Respiration is not just the mechanical process of breathing – the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction. It is also the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and ATP (energy).

In a typical scenario, organisms find greater difficulty obtaining oxygen (O2) than dealing with the waste gas product of respiration – carbon dioxide (CO2). This is because CO2 is more readily dissolved in plasma and so can be excreted faster than oxygen is obtained.

For oxygen to enter body fluid/plasma it must first dissolve in a fluid layer. There must be a thin layer between the fluid layer and body fluid/plasma.

This comparative article will look at the way different species all tackle the problem of obtaining oxygen for aerobic respiration, starting with insects, then amphibians, fish, mammals and finally birds.

After you’ve read this article, you might want to participate in this poll below:

Insect Respiration

Insects are able to obtain all the oxygen they need for cellular metabolism without lungs. Instead insects have a hard exoskeleton which contains valve like openings called spiracles. Typically there is one pair of spiracles per body segment. Air flow is regulated by small muscles that operate flap-like valves within each spiracle which contract to close the spiracle, or relax to open it.

Oxygen travels along the spiracles which branch off into a network of tracheal tubes, further branching into tracheoles that terminate as sacs. These sacs provide the thin, moist interface necessary for gaseous exchange.

Oxygen transported this way is delivered directly to the tissues that require it, meaning it is not transported via the circulatory system. Because of this the circulatory system of insects is greatly reduced (as compared to a mammal for example).

Oxygen diffuses into cell cytoplasm along a concentration gradient (having first dissolved in the moist tracheole surface); in a similar manner CO2 diffuses out of the cytoplasm to leave the insect via the spiracles.

Because of the inefficiency of this method of respiration, insects must retain small body sizes. As body size or activity of an insect increases, it may be able to increase oxygen intake by manipulating the spiracle and abdominal muscles to increase ventilation. If activity decreases, due to the toxic nature of elevated oxygen levels, the insect can close spiracles to prevent increased oxygen intake.

Amphibian Respiration

Amphibians have lungs which they use to respire, but they are also able to obtain oxygen through their skin. As expected with a gaseous exchange surface, the skin is thin, moist and well vascularised. Oxygen is therefore able to dissolve in the moist layer of the skin and diffuse directly into the blood. When submerged beneath the water surface, amphibians obtain all their oxygen through their skin.

This does however pose a problem, because the skin is a very thin barrier (due to it having evolved as a ‘respiratory organ’) it provides very little in the way of protection when compared to mammalian skin for example (which has evolved as a protective barrier). This restricts amphibians to generally aquatic/damp habitats where less stress is posed on the skin.

As mentioned above, amphibians also have a pair of saclike lungs, although they have a limited surface area and are not nearly as efficient as mammalian lungs. Without ribs or a diaphragm (and the lack of involvement of the chest muscles in ventilation) amphibians must use a force pump mechanism to ventilate the lungs.

The force pump mechanism of breathing is very energy expensive, the mechanism is as follows. With the mouth and nostrils closed, the amphibian will lower the floor of the oral cavity (resulting in the distinctive expansion of the throat) resulting in a reduction of pressure. The nostrils are then opened allowing air to rush into the mouth (as pressure equalises). The air in the mouth is then forced into the lungs by further contraction of the oral cavity floor.

Fish Respiration

Fish live in predominantly aquatic environments however there are exceptions, such as the lung fish which are able to utilise lungs to obtain oxygen. Aquatic fish however use a set of respiratory organs known as gills.

Gills are highly vascularised with a large surface area, short diffusion distance and an always moist surface. Their structure consists of:

  • Four gill arches (either side of the head) or two gill arches in cartilaginous fish
  • Each gill arch then has two rows of gill filaments. There are many gill filaments per column.
  • On the upper and lower sides of each gill filament there are many lamellae. The blood flow within the lamellae is counter-current meaning the deoxygenated blood flow direction is opposite to the oxygenated water flow direction. This mechanism maintains a concentration gradient thus increasing the efficiency of the respiration process. Cartilaginous fish do not have a counter current flow system as they lack bones which are needed to have the opened out gill that bony fish have.

In order to pass the oxygenated water over the gills fish have two main methods, either ram ventilate (swim through the water with an open mouth forcing water to pass over the gills, stopping would cause the fish to drown) or use a similar method to amphibians – a double force pump. Both of these methods are unfortunately energy expensive.

The mechanism of the double force pump method is as follows. The fish utilises the expansion and contraction of two cavities, the buccal cavity (mouth) and the opercular cavity (gills):

  • Water is sucked into the buccal cavity by expanding both the buccal and opercular cavities. The operculum (the bony flap on the outside of the gills which is used to protect them) remains closed to prevent water escaping.
  • By contracting the buccal cavity water is now forced over the gills into the opercular cavity. The mouth and operculum remain closed to keep the cavities sealed.
  • With the mouth still closed, the operculum opens and the opercular cavity now contracts forcing the deoxygenated water out.

The exposure of the gills to water poses certain problems concerning salt movement. In freshwater salt is able to easily diffuse out of the fish into the hypotonic water. To maintain a healthy salt concentration within their body, they:

  • Do not drink freshwater (which would dilute the body salt levels)
  • Excrete a mucus over their body surface to minimise water and salt movement
  • Excrete large volumes of diluted urine
  • Obtain more salt from their diet and absorb as much as possible through the gills

Marine fish on the other hand, have the completely opposite problem. Water is continuously lost from the fish and salt is continually being taken up. To maintain healthy levels of water and salt marine fish:

  • Excrete a small volume of isotonic urine (The kidneys lack loops of Henle meaning they are unable to concentrate their urine)
  • Chloride is actively pumped out of the fish by specialised chloride cells in the epithelium of the gill, whilst sodium diffuses out between epithelial cells along the electrical gradient
  • Marine, cartilaginous fish have a specialised salt excreting gland in their rectum
  • They are also able to increase the osmolarity of their tissue fluids enough so that they actually extract water from seawater by osmosis
  • To increase their water levels, they must drink the seawater and excrete the salt via the specialised rectum glands – this is energy expensive however.

Mammalian Respiration

Compared to other species, mammalian respiration is highly efficient; there is a very large surface area within the lungs which is maximised by the bubble like structure of the alveoli. The lungs also benefit from very thin membranes between the moist layer within the alveoli and the blood. The blood supply to the lungs is very great.

Mammals have a sealed thoracic cavity, which is sealed by the diaphragm. In conjunction with the ribs, the two sets of muscles are able to control breathing.  Normally, the diaphragm’s relaxed position recoils (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). This process works in conjunction with the intercostal muscles connected to the rib cage. Contraction of these muscles lifts the rib cage, thus aiding in increasing the thoracic volume. Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. The intercostal muscles simultaneously relax, further decreasing the volume of the lungs. This increased pressure forces air out of the lungs. Conversely, contraction of the diaphragm increases the volume of the (partially empty) lungs, decreasing the pressure inside, which creates a partial vacuum. Environmental air then follows its pressure gradient down to fill the lungs. The ventilation system of mammals is basically a suction pump.

Avian Respiration

Just like mammals, birds have ribs, although they lack a diaphragm to seal the thoracic cavity. The thoracic and abdominal cavities are thus not separated and this single large body cavity is known as the coelom.

The lungs of the bird are connected to the wall of the coelom by connective tissue and are unable to enlarge themselves like mammalian lungs. Instead air is moved in and out of the bird by expanding the coelom; this enlarges air sacs connected to the coelom causing air to pass through the lungs and into the air sacs.

A complete avian respiratory cycle involves two inspirations and two expirations (unlike mammals which involve only one). Whereas mammals have lungs in which air is inspired and then exhaled, you can think of birds as having 3 sets of lungs. They have anterior (cranial) air sacs, posterior (caudal) air sacs and the lungs themselves. Air is shifted around these 3 sets of respiratory organs during inhalation and exhalation, the exact mechanism is below:

  1. During the first inspiration the air stream is divided into two as the trachea divides. The air does not go directly to the lung, but instead travels to the caudal (posterior) air sacs. A small amount of air will pass through the caudal air sacs to the lung.
  2. During the first expiration, the air is moved from the posterior air sacs into the lungs. Blood capillaries flow through the lungs and this is where the oxygen and carbon dioxide are exchanged.
  3. When the bird inspires the second time, the air moves to the cranial air sacs. (Whilst more air is inhaled into the caudal air sacs)
  4. On the second expiration, the air moves out of the cranial air sacs, into the trachea and out of the nostrils. (Whilst the air inspired during the second inspiration moves through the lungs).

Notice how efficient this is, before the initial breathe has even been exhaled, more air has been brought into the respiratory system.

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Introduction to Kin Selection

Introduction

Some organisms tend to exhibit strategies that favour the reproductive success of their relatives, even at a cost to their own survival and/or reproduction. The classic example is a eusocial (highly social) insect colony, with sterile females acting as workers to assist their mother in the production of additional offspring. Many evolutionary biologists explain this by the theory of kin selection. Natural selection should eliminate such behaviours; however, there are many cases, such as alarm calling in squirrels, helpers at the nest in scrub jays, and sterile worker castes in honey bees, in which these animals cooperate despite an obvious disadvantage to the donor.

This sacrifice of individual success for the aid of other individuals is known as altruism.

There are thought to be four possible ‘routes’ to altruism – why it might arise, these are:

  • Kin selection – Keeping altruism in the family, possibly shared in the genes. Altruism within a family helps it to proliferate well.
  • Reciprocal altruism – ‘One good turn, deserves another.’ Altruism expressed by an individual is at some point returned. E.g. social grooming in primates, the individual doing the grooming is eventually groomed back.
  • Selfish mutualism – ‘What’s in it for me?’ Altruism which is expressed only because an individual also gains from it. E.g. feeding in house sparrows, they will call for help to break up large pieces of food which they are unable to carry alone thus losing some of the resource but gained more than they would have alone.
  • Group selection – ‘For the good of the group.’ Groups within a population – not necessarily family – which benefit by co-operation.

Kin Selection

John Maynard Smith described Kin Selection in 1964 as “…The evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which do not require any discontinuities in the population breeding structure.”

It goes on the idea that because similar genes are more prevalent within a family (either by kind [species] or by descent [ancestral]), any altruistic genes expressed within the family are more likely to become more prevalent within the entire species.

Kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals. Under natural selection, a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population; and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behaviour (altruistic genes), may nonetheless increase in frequency, because relatives often carry the same gene; this is the fundamental principle behind the theory of kin selection. According to the theory, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour.

Hamilton’s Rule

Whether or not altruism is favoured within a family or species depends on whether or not Hamilton’s rule is met:

Hamilton’s Rule: rB-C>0 or rearranged rB>C

Altruism is favoured when rB>C

  • C – The cost of displaying altruism, any disadvantages to the individuals.
  • B – Benefit to the individual(s) who receive aid.
  • r – The coefficient of relatedness. The probability that 2 individuals contain a gene identical by descent at the same locus. It has a value of 0-1.

Possible r values:

Relationship Coefficient of Relatedness
Identical Twins 1.0
Parent to an offspring 0.5
Siblings 0.5
Half Siblings 0.25
Unrelated individuals 0.0

Hamilton’s rule therefore predicts that we expect closer related individuals to express greater amounts of altruism. For example:

Would a mother warn her child of a predator, thus exposing herself as a target? If doing so has an arbitrary value of 2, but the benefit of saving the child of 5 then using Hamilton’s Rule the following must be true:

r(0.5) x B(5) – C(2) > 0

0.5 x 5 – 2 = 0.5

Thus as the value is greater than zero, altruism in this situation is favoured, would the same be true between half siblings? (r=0.25)

r(0.25) x B(5) – C(2) > 0

0.25 x 5 – 2 = -0.75

Because the result of Hamilton’s rule is less than 0, altruism in the same situation but a half-sibling attempting to warn a half-sibling, is not favoured.

Introduction to the Evolution of Animal Fighting Behaviour

Introduction

Animal fighting behaviour can be introduced using the simple models discussed here; one of these is the ‘Hawk/Dove’ model by Maynard Smith. From this model, we can construct payoff matrixes which can then be used to determine evolutionarily stable strategies (defined below).

Evolutionarily stable strategy – An evolutionary stable strategy or ESS is a strategy which, if adopted by most members of the population, cannot be invaded by a mutant strategy which is initially rare.

Maynard Smith’s Model

The evolution of ritualised behaviour has evolved for the benefit of individuals. Simply, we can imagine that a ritualised behaviour has evolved to allow an animal to avoid participating in conflict, when it is aware that the opposition is more capable. By avoiding conflict, the animal does not have to pay a ‘cost’ of injury. This is looked at in the Maynard Smith Hawk/Dove model.

There are many examples, such as the domestic dog will roll on to its back making itself vulnerable to the opposition, signifying that it does not want to participate in the conflict and the opposition may take the resource for which the conflict arose

Another example is the roaring behaviour of the Red Deer. Typically a roaring contest (another type of ritualised behaviour) will mark the start of a conflict; this allows each Red Deer to gauge the prowess of one another, from which they can decide whether or not to elevate the aggression.

The payoff of conflict is frequency dependent i.e. if there are very few dominant aggressive males, they will obtain a large majority of the resources as the other males are highly likely to show submissive behaviour and flee.

The payoff of conflict can be broken down into two simple values:

  • V – The value of the resource (This could be food, females etc.)
  • C – The cost of the conflict (Injury), however if the animal retreats, no cost is therefore paid

From this we can build the Hawk/Dove Model.

Building the Hawk/Dove Model

The hawk/dove model is a simplistic model concerning the possibilities and outcomes of conflict. By simplifying the animal’s behaviour, we can break down their responses to conflict into 3 choices, either:

  • The animal displays ritualised behaviour (E.g. the roaring contest of the Red Deer)
  • The animal retreats
  • Or the animal elevates the conflict and engages in a fight

Using these three possible outcomes, 2 strategies are built, the Hawk strategy and the Dove strategy. Animals can be related to either the dove strategy or the hawk strategy.

The Hawk strategy consists of the following behaviour:

  • The hawk will engage in conflict immediately and only retreat if it becomes injured

The Dove strategy consists of the following behaviour:

  • The dove will display immediately and only retreat should its opponent escalate the conflict

By winning in a conflict, the animal will gain the resource V, by losing the animal will have to pay the cost C. Some possible scenarios are listed below (Hawk – H, Dove – D)

  • H vs. H – Both will escalate the conflict meaning one gets injured and retreats. This means there is a 50% chance of winning resource V and a 50% chance of paying cost C. (This is assuming both individuals are equally matched and of the same fitness – see assumptions section below). In mathematical terms, E (the energy gain from the conflict, or payoff) of the conflict between two hawks is E(H,H). We can equate this to the value and cost, so:
  • E(H,H) = (V-C)/2
  • H vs. D – The hawk immediately escalates the fight and so the dove retreats. This means the hawk always gets the resource V and the dove always gets nothing – but does not pay a cost as it retreats. So:
  • E(H,D) = V, E(D,H) = 0
  • D vs. D – Both immediately display but as both are of equal fitness (see assumptions below) they must either share the resource or one randomly wins the resource, either way they receive the equivalent of half the resource. So:
  • E(D,D) = V/2

Assumptions

The Hawk/Dove model retains its simplicity due to some assumptions, these are:

  • All individuals are of the same Darwinian fitness, making them evenly matched.
  • The V gained and C paid are the same for all individuals e.g. the cost of an injury costs the same amount of energy in all ‘dove’ individuals.
  • All interactions are completely at random.

Payoff Matrixes

Payoff matrixes are grids which determine whether an ESS is in place, by inputting the values of V and C we can see whether or not hawks, doves or a mixture of both give an ESS.

From previous knowledge (above) we know the following information:

Vs. > H D
H (V-C)/2 V
D 0 V/2

To determine which strategy is an ESS depends on whether or not V<C or V>C, which would depend on the situation. Each has been equated below:

V>C

If V>C e.g. V=4, C=2, we would get the following information (by substituting the values into the table above):

Vs. > H D
H 1 4
D 0 2

What this shows us is that, because in column 1, H vs. H = 1 and D vs. H = 0, Doves are unable to invade a hawk population – This means that the Hawk Strategy when V>C is an ESS. We back this up by looking at column 2 and seeing D vs. D = 2 and H vs. D = 4. This means Hawks are able to invade doves, so doves therefore cannot be an ESS.

V<C

If V<C e.g. V=2, C=4, is there a difference when compared to V>C? Again by substituting the values into the table above, the following information is obtained:

Vs. > H D
H -1 2
D 0 1

Column 1 – D vs. H = 0, H vs. H = -1. This means that Doves are able to invade hawks, does this mean hawk is not an ESS?

Column 2 – D vs. D = 1, H vs. D = 2. This means that Hawks can invade doves.

As both strategies are able to invade one another, when V<C, a mixed ESS arises.

Determining Proportions in a Mixes ESS

In a mixed ESS, we are able to determine the proportion of each strategy by using a simple equation, p=V/C. The equation is derived initially from a more complex equation however:

W=Fitness, W(H) = Fitness of hawks, W(D) = Fitness of doves, W0 = basic fitness p = proportion

We assume that W(H) = W(D)

  • W(H) = W0 + p[(V-C)/2] + pV
  • Where p[(V-C)/2] is the proportion of occasions that we see H vs. H
  • Where pV is the proportion of occasions we see H vs. D
  • W(D) = W0 + p0 + p[V/2]
  • Where p0 is the proportion of occasions we see D vs. H
  • Where p[V/2] is the proportion of occasions we see D vs. D

Because we assume that W(H)=W(D) we can equate these equations to one another, therefore:

  • W0 + p[(V-C)/2] + pV =  W0 + p0 + p[V/2]
  • p[(V-C)/2] + pV = p[V/2]
  • p=V/C

So when simplified we get p = V/C which means the proportion of a strategy in the mixed ESS depends entirely on the value of the resource and cost of injury. Using the values we saw in the V<C example above (V=2, C=4) we get p(H)=2/4. This equates to 0.5 or 50%, therefore the proportion of hawks in this mixed ESS is 50%.

What we can conclude from this is that behavioural variation in a population is suited to evolve that way, especially when V<C. Also that it is frequency dependent. Also as the cost of injury increases, more contests for resources within the species will be settled by ritualised displays.

Introduction to optimal foraging theory

Introduction

Darwinian fitness – The rate of increase of a gene in the population, this is difficult to measure. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual’s genes in all the genes of the next generation. If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.

Survival value – Survival value is how a form of behaviour contributes to survival.  For example the removal of an eggshell to prevent predation, how will this affect the survival of the organism within the egg shell?  Early removal of the egg shell removes the stimulus of the egg shell which is a sign to predators that there is food available (the egg) for them to eat, however the early removal of the egg shell means that the young will be underdeveloped and are less likely to survive. Also in certain species of gulls this leaves the newly hatched gulls prone to cannibalism of each other, in this species of gull the parents remove the shell after 30 minutes the chicks hatch. This is also an example of a trade-off.

Trade-off – A trade-off is a situation that involves losing one quality or aspect of something in return for gaining another quality or aspect. It implies a decision to be made with full comprehension of both the upside and downside of a particular choice.  In terms of animal behaviour, these aspects are different behaviours.  See the above example.

Optimal foraging theory

Optimal foraging theory or the optimal patch model is the exploitation of resource patchiness.  Food tends to be clumped which gives rise patches of food/resources.  If we think of Tt as the travel time between patches and Tp as the time spent in the patch we can begin to develop optimal patch model.  More time spent per patch means more energy used for foraging, as prey numbers in the patch decrease, less time spent in the patch means relatively large amounts of time will be spent on travelling.  This is the basis of the optimal patch model.

This graph shows the period of travelling Tt, in relation to the period in the patch Tp. The grey line shows the prey consumed whilst the red line (gradient) is the energy gain:

The gradient (energy gain) is equal to:

E/Tp+Tt   (E = energy gain, Tp = Time in patch, Tt = Travel Time)

The optimal patch time (the optimal amount of time spent in the patch) is when the gradient (energy gain) is at a tangent. At this point any further time spent in the patch will not produce sufficient energy gain from prey as the prey resource has decreased. It is now more efficient to move onto the next patch.

When the distance between patches increases, the time spent in the patch for efficient energy gain also increases.  We can see this by using the graph, as the travel time increases the gradient becomes less meaning it reaches a tangent at a later point.  This later point is the optimal patch time.  This is a benefit to the organism because of the increased travel time between patches which require more energy; due to the increase in energy requirements it is more beneficial to remain in the patch for a longer period of time to extract more energy from the resource i.e.  Consume more prey.

This model does make some assumptions however:

  • That the patches are equidistance apart
  • That the patches are equally stocked with prey

Meninges, CSF & Venous Drainage

Meninges

The meninges are a group of three membrane layers which wrap around the brain and central nervous system. The meninges consist of three layers (meninx), these include:

  • The Dura Mater
  • The arachnoid membrane
  • The Pia Mater

The primary function of these layers is to protect the central nervous system.

Fig. 1 The meninges surrounding the brain. Meninges of the spinal cord would have another layer between the periosteum and Dura mater – the epidural space and lack the bone layer (skull)

The Dura Mater

The outermost layer of the meninges and therefore closet to the skull is the Dura mater; it is a thick and tough layer composed of dense fibrous tissue which wraps almost entirely around the whole brain and spinal cord. There is little space between the Dura mater layer and the layer below/the bone above it, the exception to this is when (not in the meninges of the spinal cord) the Dura mater splits into two layers forming a gap between them, which is known as the venous sinus. The Dura contains larger blood vessels which split into the capillaries in the pia mater.

In the cranium, the Dura mater also acts as another layer of periosteum, however in the spine there is an epidural space (filled with fat) which separates the Dura mater and periosteum.

The Arachnoid Membrane

The arachnoid layer is much thinner than the Dura mater and contains a lot of underlying space which is filled with CSF (cerebrospinal fluid). This space is known as the subarachnoid space. The arachnoid space is so called because of its spider web-like appearance. It provides a cushioning effect for the central nervous system.

The Pia Mater

This is a very thin and delicate layer which intimately follows the surface of the brain and spinal cord. It is highly vascularised tissue and its capillaries are responsible for nourishing the brain.

Clinical Aspects of the Meninges

The meninges are prone to infection/inflammation – two well-known afflictions associated with the meninges are:

  • Meningitis – This is the inflammation of the meninges surrounding the brain
  • Meningioma – The formation of tumours within the meninges, typically of the Dura mater

Cerebrospinal Fluid

Cerebrospinal fluid (CSF) is a clear, saline bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain. It is produced continuously at a steady rate and is essential for the normal functioning of the CNS. There is very little protein and virtually no cells present in normal CSF, only around 35mg per 100ml compared to 7,000mg per 100ml in typical serum. It is therefore a greater proportion of water (99%) compared to 93% in serum.

Production

CSF is produced in the brain by modified ependymal cells in the vascular choroid plexus (approx. 50-70%), and the remainder is formed around blood vessels and along ventricular walls. Both filtration and secretion occurs by epithelial cells.

Flow

Generally CSF flows from the lateral ventricles, through the foramina of Monroe to the 3rd ventricle, then through the cerebral aqueduct (of Sylvius) to the 4th ventricle. It then mostly flows out of the lateral foramen of Luschka and into the cisterna magna (a dilation of the subarachnoid space), or caudally into the central canal of the spinal cord.

Exit/Drainage

Generally CSF will exit via the arachnoid villi. There are other possible routes of drainage however; such as absorption by venules in the pia mater, through spinal veins and lymphatics (around the roots of the spinal nerves) and by direct venous drainage from the subarachnoid space into the venous sinuses.

Sampling

CSF can be sampled from an organism by cisternal puncture. The CSF should flow into the container without the need to draw it in, due to pressure. No more than a maximum of 1ml per 5kg of body tissue should be extracted.

Things to look out for in the CSF sample include:

  • Colour – Red/Yellow staining of the normally clear fluid could indicate a haemorrhage. (Poor technique may also cause some blood to be extracted with the CSF however)
  • Protein content – If an increase in protein content is observed, this can act as a nonspecific indicator of CNS disease
  • Chemical content – A decrease in CSF glucose may indicate bacterial or fungal meningitis
  • Cell count – CSF cell count is usually low, a cloudy appearance may indicate higher cell counts which could be a sign of CNS disease

Venous Drainage

Cranial Venous Drainage

The brain and spinal cord use a series of dural sinuses, in addition to veins to remove CSF and venous blood. Dural sinuses have no valves, (the venous system of the cranium generally has few valves).

Spinal Venous drainage

A similar system exists to drain venous blood from the spinal cord. There is a direct continuity with the cranial sinuses. CSF and venous blood drain via intervertebral veins into either the vertebral, azygos veins or the vena cava.

Arterial Blood Supply to the Brain

Introduction

Compared to other tissues, the brain is extremely dependent on a stable and efficient blood supply. Despite making up only 2% of total body mass, the brain requires 15-20% of total cardiac output; this makes the brain extremely sensitive to hypoxia. Any hypoxic damage caused to the brain becomes irreversible after only a few minutes.

In normal tissue, there are three typical forms of metabolites utilisable for energy, these include; glucose, fatty acids and ketone bodies. However, in the brain only glucose can be utilised, except under extreme conditions, such as starvation. During these harsh conditions for the brain, ketone bodies may be used for energy. Because of the brains dependence on glucose, hypoglycaemia will result in dizziness and confusion as the brain is starved of energy.

An overview of Trophic Support:

  • The heart pumps blood into the arterial system
  • This moves blood to the skull
  • The blood passes through a series of membranes (meninges)
  • From here it moves into:
    • Capillaries
    • Neuronal extracellular fluid
    • Cerebrospinal fluid (CSF)
    • Used blood then flows into venous sinuses (blood filled cavities between the skull & brain)
    • Draining back into venous blood
    • Moving back to the heart

The Circle of Willis

The Circle of Willis is a circular network of arteries that supply blood to the brain. It acts as a redistribution centre for blood which is supplied to the Circle of Willis; blood is brought together here and then moved to the brain. The Circle of Willis sits directly beneath the brain. The arrangement of the brain’s arteries into the circle of Willis creates redundancies in the cerebral circulation. If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia (restriction of blood supply).

There are 4 routes which blood can take to reach the Circle of Willis, these are:

  1. Common carotid arteries -> internal carotid arteries (This is the most direct route)
  2. External carotid arteries -> maxillary arteries -> anatomising ramus ->internal carotid
  3. Vertebral arteries -> rete mirabile -> internal carotid
  4. Vertebral/Vertebral spinal arteries -> basilar artery

Fig. 1 – The arterial supply to the Circle of Willis, the different routes all begin by flowing in through the aorta.

The routes are colour coded to correspond with the above four routes:

  1. Black
  2. Blue
  3. Yellow
  4. Green

In effect there are only two final routes into the Circle of Willis, either:

  • Through either internal carotid artery
  • Through the Basilar artery

At the Circle of Willis, the arteries anastamose (join together) to form a ring.

Fig. 2 – The Circle of Willis, upon reaching the Circle of Willis blood is then distributed further heading towards the brain. The blood supply from the Circle of Willis is shown in Fig. 3.


Fig. 3 (below) – Blood Supply from the Circle of Willis. The lines in red show which arteries the blood leaves the Circle of Willis from, whilst the lines in pink show the arteries which blood use to enter the Circle of Willis

Blood Supply From the Circle of Willis

There are essentially 5 pais of arteries which supply the brain with blood. 4 of these are derived from the cerebral arterial circle, this is the red circle in Fig. 3, only the caudal cerebellar arteries are not derived from this ‘circle’. The five pairs of arteries which therefore supply the brain are:

  • Rostral cerebral arteries
  • Middle cerebral arteries
  • Caudal cerebral arteries
  • Rostal cerebellar arteries
  • Caudal cerebellar arteries

The terms rostral (front) and caudal (back) are interchangable with the terms anterior and posterior respectively (these terms are used in human medicine).

The blood from different arteries emerging from the Circle of Willis tend to supply different parts of the brain:

  • The medial surface of the brain is mainly supplied by:
    • Rostral cerebral arteries
    • Caudal cerebral arteries
    • Whilst the lateral surface of the brain is mainly supplied by:
      • Middle cerebral arteries

Species Differences

Retia Mirabilia

A rete mirabile is present in sheep, goats, swine, ox and dogs and is located in the venous cavernous sinus. The cat has a rete mirabile present also, however it is located extracranially. Retia Mirabilia are absent in the rat and rabbit.

Arterial Blood Supply to the Circle of Willis – Cats

During embryonic development in the cat, the internal carotid artery degenerates. In other species the internal carotid artery is typically a major artery in terms of direction of blood flow (see fig. 1). The maxillary artery compensates and becomes the major supply of blood to the Circle of Willis. Additional supply is also aided by a greater developed pharyngeal artery. This degeneration of the internal carotid also occurs in adult sheep, cows and pigs.

Variation of the Circle of Willis

The Circle of Willis’ structure can differ between species; this has even been observed between different breeds of dogs. The typical example is that some dogs completely lack a rostral communicating cerebral artery altogether.

Supply of Arterial Blood to the Cortex

The route of supply of blood to the brain may also differ between species:

  • Human & Dog – The Circle of Willis receives blood by both the internal carotid and the basilar arteries. (Fig. 1) This means blood supply to the forebrain mainly originates from the internal carotids. Caudal areas of the brain are typically supplied with vertebral arterial blood.
  • Cats, Sheep & Pigs – The internal carotid is much less important; the maxillary artery supplies the Circle of Willis via the anatomising ramus. In this situation the basilar artery carries blood away from the Circle of Willis (unlike in man). This means most blood supply to the forebrain and midbrain is derived from the maxillary artery.
  • The Ox – Blood again flows away from the Circle of Willis via the basilar artery. Blood enters the Circle of Willis via maxillary and vertebral artery pathways, which are well mixed -meaning all areas of the brain, are supplied with blood from mixed maxillary and vertebral arterial origins.

Arterial Supply to the Spinal Cord

Like the brain, the spinal cord requires an equally rich supply of arterial blood, the general arrangement:

  • Ventral spinal artery – a large, broad artery which runs the length of the ventral surface of the spinal cord
  • Dorsal spinal arteries – a pair of smaller arteries which run parallel to the ventral spinal artery along the dorsolateral surface of the spine
  • These arteries are joined by various anatomising arteries which form an arterial ring between the ventral spinal and dorsal spinal arteries.

Regulation of Arterial Blood Supply to the CNS

Should levels of CO2 or O2 alter in the blood, certain regulatory methods are in place to aid the return to normal levels. Hypoxic conditions occur when oxygen levels are low, whilst anoxic conditions occur when there is no oxygen. There are three main regulatory methods:

  • Chemical autoregulation – In all systems, blood flow is locally regulated. When O2 tension falls or pCO2 increases local vasodilation occurs. This results in increased blood flow (Increased oxygen delivery and CO2 removal). The situation is similar for the brain within the restrictions of the inelastic skull.
  • Sympathetic control – A sympathetic drive constricts blood vessels (vasoconstriction) which causes hypoxia. Vasodilation occurs as a result which increases the volume of the brain within the skull causing pressure and pain.
  • Myogenic autoregulation – As pressure increases within cerebral arteries, muscle responds by constricting to prevent increased blood flow.

The Blood Brain Barrier

To protect the brain tissue, a blood-brain barrier is in place. This prevents unwanted substances crossing into brain tissue freely; certain transport systems are in place to selectively transport required substances.

Glucose and ketone bodies do not freely pass through the blood-brain barrier they are instead specifically transported across the barrier. It is believed than even molecules such as water require selective transport across the blood-brain barrier, water is moved across by aquaporin transport proteins.

Structure

In simple terms the brain capillaries are surrounded by ‘astrocyte feet’. These astrocytic feet serve to regulate the blood brain barrier in some way.

Differences between a regular capillary tissue boundary and the blood-brain barrier:

  • The capillaries are not fenestrated, but bound together with tight junctions. This prevents simple diffusion of substances between the gaps as would happen outside the CNS.
  • Pericytes site alongside the capillaries, these may be involved with capillary proliferation.
  • Astrocytic feet surround the capillaries and their role appears to be in the formation and maintenance of the blood-brain barrier complex.

An Introduction to Animal Behaviour and Sociobiology

Tinbergen’s four Questions of Ethology

Explanations to Tinbergen’s questions can be split into two groups; evolutionary (ultimate) and proximate. Ultimate explanations pertain to the evolution of a species and include:

Function (adaptation) – This type of explanation for animal behaviour usually concerns a trait that is functional to the reproductive success of the organism which is a result of natural selection. Why an organism is the way it is

Evolution (phylogeny) – This type of explanation for animal behaviour encapsulates all evolutionary explanations other than function/adaptation, it include the history of the species reconstructed from as far back as possible. Sequential changes in a species through time

Proximate explanations pertain to individuals within the species and include:

Mechanism (causation) – This type of explanation describes an organism’s structure and how biological mechanisms of the organism are able to work. What organism’s structures are like and how they work

Development (ontogeny) – This type of explanation describes how an organism has developed, from changes in its DNA code to the different forms of life stages. Sequential changes in individuals across their lifespan

Examples of Explaining Tinbergen’s four Questions of Ethology

Example 1: Why do birds sing in spring?

• Mechanism – Alterations in day length affect hormone secretions within birds

• Function – Singing helps to defend the bird’s territory as well as attract females

• Evolution – Here you would need to compare the songs of a species of bird throughout the species’ evolution

• Development – This concerns the learning of an individual bird’s song, which would be easier to observe than the evolution aspect above

Example 2: Why do we see?

• Mechanism – The lens of the eye focuses light on the retina visual system

• Function – We see so we can find food easier and avoid danger

• Evolution – The vertebrate eye initially developed with a blind spot, the lack of adaptive intermediate forms prevented the loss of this blind spot

• Development – Neurons connect the eye to the brain and require photo-stimulation to transmit information

Example 3: Why do we not find siblings attractive? (Westermarck effect)

• Mechanism – Little is known about this neuromechanism

• Function – To discourage inbreeding which would otherwise decrease the number of viable offspring

• Evolution – This mechanism is found in a number of mammalian species which suggests it may have evolved tens of millions of years ago

• Development – Results from familiarity with another individual early in life, especially in the first 30 months for humans. The effect is also manifested in nonrelatives raised together

The Comparative Method

In ethology, comparative is used in specific sense, meaning to indicate systematic comparisons between different species (including humans). The comparative method has been used by Lorenz to make inferences about the evolution of behaviour and by Gittleman to study evolutionary function.

One way of using the comparative method to make comparisons about species is to gather information on two variables which are believed to be related. Collect data from each species and plot this on a graph. If the variables are related, there will be a positive linear relationship and the differences between species can be compared. For example, body weight against testes weight gives a positive linear relationship.

The outcome of the method becomes useful when groups are made within the data, using the testes example, by forming groups of species which are either monogamous, have one dominant male or have multiple males we can compare relative testes weights. We can see that on average multi-male species have larger testes, whilst monogamous species have smaller.