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Chapter 2: Homeostasis in animals


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If you are not familiar with the general principles of homeostasis in biology, you should read the previous Chapter of these notes, "Homeostasis; some general principles" before you continue.




Homeostasis in animals is achieved by either nerves or hormones or frequently a combination of both nerves and hormones.

THE NERVOUS SYSTEM

Nerves are generally associated with the specialised receptor cells which detect a stimulus from the external environment. They are also often involved in the relay of information from receptors to effectors, especially when a rapid response is necessary.

The ability of organisms to detect changes in their external environment, and to react appropriately, is sometimes called sensitivity. Sensitivity is necessary if an animal is to
Many of these activities require a rapid response to external stimuli, and where speed of response is required, nerves are the internal communication method of choice.

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Neurones (or neurons in America)

Nerve cells are called neurones, and they have certain structural features in common, but also specialised features for specialised functions. In the body, neurones generally occur in bundles, with each individual neurone in the bundle having its own contact points (synapses) with other cells at each end.


Diagram of a motor neurone.

This type of neurone typically carries a message from its dendrite, through the cell body, and along the axon to synapse at a muscle. Motor neurone axons are wrapped in a layer of myelin which enables nerve impulses to travel more quickly. A motor neurone initiates a response in its target muscle; the muscle fibres contract (shorten) and the muscle causes some part of the body to move.

Neurones, like almost all cells, have a nucleus and various organelles. The nucleus is often not actually in the centre of the neurone, but is in an attached projection, the cell body. Other organelles are found where their functions are needed, for example, mitochondria are located in synaptic knobs if ATP is required there.

Neurones typically have projections of cytoplasm reaching out from each side of the cell body. Through these projections, nerve impulses (action potentials) travel. Nerve impulses moving towards a cell body travel along a dendrite. Nerve impulses travelling away from a cell body travel along an axon. Nerve impulses always go one way, they can not travel 'backwards' along a neurone. Nerve impulses travel along neurones as tiny waves of electricity, generated by the movement of ions across the dendrite or axon membrane. Like waves in the ocean, the cell returns to its resting electrical state before the next wave comes along.

The lengths and special adaptations at the synapses of dendrites and axons vary, according to the function of the particular neurone.

Students should check their text book for illustrations of sensory neurones, interneurones, motor neurones and others. Look for similarities and differences in the different types of neurones and think about how these are related to each neurone's function.


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Synapses

At synapses, the message needs to pass from one cell to another. Chemicals called neurotransmittors do this. The neurotransmitter is released into the synaptic space and receptors on the cell or tissue on the other side of the space combine with the neurotransmittor molecules. This triggers a response in the target tissue. Neurones can communicate in this way with a variety of cells and tissues, depending on the specialisation of each neurone. It is important for survival that this rapid means of communication is possible when a rapid response is needed.


Diagram of a synapse.

This is the detail of a synapse of a motor neurone at a muscle. (Remember that not all nerves are motor nerves and that not all nerves synapse with muscle tissue, but the general principles are the same.) At the target tissue, here muscle, the association of the neurotransmitter with the appropriate receptor across the synapse will trigger a sequence of biochemical events in the muscle cells. These events will lead to the specific response, contraction, of that muscle.

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Reflex arcs

Reflex arcs are particularly rapid response systems. They do not involve neurones of the brain, but merely recieve an impulse, transmit the message to the spinal cord, and initiate a response -usually muscle movement- from there. The familiar knee-jerk reaction is an example of this, although we are not usually aware that it is occurring; a stretch to the tendon under the knee bone results in a small adjustment of our posture with the thigh muscles. This is one of the mechanisms by which humans manage to balance on two legs rather than four. And we are not even consciously aware that it occurs (until someone hits our knee with a little hammer)!

Diagram of a reflex arc.

Follow the path of the message from when the stimulus (pin prick) is detected, until the response (finger moves away from pin) occurs. Can you explain what happens at each section of the path?




Parts of the nervous system

In mammals, the nervous system is often divided into different parts, depending on the role each part plays. The simplest division is as follows:

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THE ENDOCRINE SYSTEM

The endocrine system includes the organs that secrete hormones into the blood or tissue fluids to influence the activity of metabolism in target tissues. Recently, it has been discovered that single cells, or small groups of cells may also secrete hormones with very localised effects. These are also part of the endocrine system.


A hormone is a substance produced by one cell, or group of cells, that has an effect on another, different cell or group of cells. Sometimes hormones are called chemical messengers.

Hormones are released in response to a stimulus detected by a receptor. The hormone then becomes the relay system to initiate a response in the appropriate target tissue.

Speed of hormone responses

Because hormones usually need to be synthesised in the gland which produces them, there can be a considerable delay before the hormone concentration in the blood builds to a level high enough to initiate a response. However, in some circumstances, stores of the hormone are available in the endocrine gland, and the stimulus releases the stored hormone so that a quicker response is possible.

Once the hormone reaches its target tissue, the response there can also be fast or slow. If a simple response such as increased transport across the cell membrane occurs, this can be quite rapid once the hormone binds to receptor proteins on the cell membrane. On the other hand, some hormones enter the cells and initiate responses needing enzyme synthesis, this takes considerably longer, but is likely to be sustained for longer as well.

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Types of hormones and their actions



Hormones fall into two main groups, related to their structure and their method of action at the target tissue.

Protein hormones are, as the name suggests, made of chains of amino acids. These polypeptide chains vary in size from twenty or thirty amino acids to hundreds of amino acids. Regardless of the size of the protein hormone, it is inevitably too big to pass through the membranes of the target tissue's cells.

Instead of entering cells, the protein hormones bind to the cell membrane, using specific receptor sites. This in turn triggers a sequence of reactions in the cell, usually started with the release of the 'second messenger', cyclic AMP (cAMP) which is formed from ATP. The cAMP diffuses through the cell's cytoplasm, binds with enzymes there and triggers some sort of metabolic response. The nature of the response depends on the type of cell.

Steroid hormones are different chemicals. Thier structure is based on a molecule similar to cholesterol. Because they are small, lipid soluble molecules, steroid hormones can pass through the lipid portions of cell membranes and enter the cell's cytoplasm. The hormones bind with receptor molecules in the cytoplasm then diffuse to, and enter, the nucleus. Once in the nucleus, the hormone-receptor complex can affect gene expression on the chromosomes. So the activated genes initiate protein (probably an enzyme) synthesis, and a biochemical pathway within the cell begins to run.

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Students need not 'rote learn' all the human hormones and their actions, a knowledge of insulin and glucagon in the control of blood [glucose] is all that is specified in the Study Design for VCE Unit 3. An understanding of the control of body temperature is also expected, but this is not a regulatory mechanism involving hormones.

The Table which follows is a summary of the important human hormones to develop familiarity, as it would be reasonable for an ewxam question to give some background on one of these hormones and expect that students could use their understanding of hormones and feedback to apply that information.

Table of Endocrine glands, their hormones and the hormone action.
ENDOCRINE GLAND HORMONE ACTION
THYROID Throxine, other thyroxine-like hormones Stimulation and regulation of rate of cellular metabolism
THYROID Calcitonin Inhibits release of calcium from bone.
PARATHYROID Parathyroid hormone (PTH) Stimuates release of calcium from bone.
ADRENAL CORTEX Aldosterone Affects water and salt balance by reabsorption of sodium and excretion of potassium.
ADRENAL CORTEX Cortisol, other corticosteriods Affect carbohydrate, fat and protein metabolism.
ADRENAL MEDULLA Adrenaline and noradrenaline Increase blood [glucose], alter diameter of blood vessels (some constrict, others dilate), increase rate and strength of heart beat. (The 'fight or flight' response to stress.)
OVARY Oestrogens Develop and maintain female sex characteristics. Initiate accumulation of blood on uterine wall during menstrual cycle.
OVARY Progesterone Promotes continuation of growth of uterine lining during menstrual cycle.
TESTIS Androgens Promote sperm production. Develop and maintain male sex characteristics.
PANCREAS Insulin Stimulates uptake of gluocse into tissues in response to high blood [glucose].
PANCREAS Glucagon Stimulates release of glucose from tissue glycogen stores in response to low blood [glucose].
POSTERIOR PITUITARY Anti diuretic hormone (ADH) Controls water excretion by kidneys.
POSTERIOR PITUITARY Oxytocin Stimulates uterine contraction during childbirth. Stimulates milk secretion.
ANTERIOR PITUITARY Follicle-stimulating hormone (FSH) Stimulates follicle formation in female and sperm formation in male.
ANTERIOR PITUITARY Lutenising hormone (LH) Stimulates ovulation in females and androgen secretion in males.
ANTERIOR PITUITARY Thyroid stimulating hormone (TSH) Stimulates and maintains metabolism by regulating the thyroid gland.
ANTERIOR PITUITARY Adrenocorticotropic hormone (ACTH) Stimulates and regulates the adrenal cortex.
ANTERIOR PITUITARY Growth hormone (GH) or somatotropin Stimulates bone growth, inhibits glucose breakdown, stimulates fat breakdown.
ANTERIOR PITUITARY Prolactin Stimulates milk production
ANTERIOR PITUITARY Melanocyte stimulating hormone (MSH) Increases synthesis of melanin (skin pigment).
HYPOTHALAMUS Releasing hormones (many) (RH) These hormones travel from hypothalamus to the ant. pituitary, triggering the release of the appropriate ant. pituitary hormone.

This Table hints at the complexity of some hormone feedback systems. Often two hormones have opposite actions and the body maintains a balance between the two. For example, think about the roles of calcitonin and parathyroid hormone in calcium balance. In other situations, a hormone must be released form the hypothalamus to initiate the release of another hormone from the anterior pituitary which in turn may regulate hormone release from another gland. The sequence of {Stimulus ....TSHRH ...... TSH ...... Thyroxin ...... Increase metabolic rate (Response)} is an example of this.

Students should follow through the sequences of many feedback loops involvin hormones to become familiar with these mechanisms. Such sequences will be found in any comprehensive senior secondary Biology textbook.

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Other types of secretory cells and their targets

Research in biochemistry and physiology has shown that there are many other secretory cells and tissues apart from those of the traditional endocrine system. Some of these are clearly not secreting hormones, others could be considered to produce hormones but their mechanisms of action are not 'classical'. Some of these are listed in the Table which follows, not because they are part of the course, but to indicate that there are always 'exceptions to the rules' in biology.

Some other secretions and their actions.
Secretion or secretory tissue Comment on action
Exocrine glands These glands do not produce hormones, their secretions are enzymes or other molecules which leave the gland via ducts which empty them into the location where they are used. Examples, digestive enzymes from pancreas, tears.
Paracrine cells Cells which release hormones not into the circulatory system, but directly into adjacent cells or the surrounding extracellular fluid, where they have a very localised effect. These hormones are fatty acids, and include the prostaglandins which have a role in the sensation of pain. (Interestingly, asprin's action has been shown to inhibit the synthesis of prostaglandin - and so stop the sensation of pain.)
Neurocrine cells Hormone secreting cells whose secretory activity is stimulated directly by nerve endings. Example: Nerves in the hypothalamus stimulate certain regions of the posterior pituitary gland which, in turn, release hormones.
Neurotransmittors These chemicals qualify as hormones according to the definition of a hormone, but their special action in the nervous system sets them apart.
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COMPARISON OF NERVOUS AND ENDOCRINE COMMUNICATION


Nerves and hormones: A comparison
Nervous Communication Endocrine Communication
Response to a stimulus, internal or external. Response to a stimulus, internal or external.
Rapid response (Seconds or fractions of a second). Relatively slow response (Seconds or minutes, perhaps longer).
Impulse travels electrochemically along axons and chemically across synapses as neurotransmitter. Message generally travels chemically as hormones in the circulation.
Response is specific in target neurone, muscle or gland. Response is often widespread in several target tissues in the body
Response is generally short-lived. Response is generally long-lasting in regulating metabolism
Initiates short acting chemical changes in target tissue. Influences specific chemical changes and regulates metabolism, growth or reproduction.

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MAINTENANCE OF BODY TEMPERATURE

The ability to control body temperature is extremely important if animals are to survive. Recall from Area of Study 1 that enzymes have optimal temperatures for their activity.

If the enzyme, or the cell it is in, is too cold, collisions between enzyme and substrate occur infrequently, and metabolic processes slow to rates which may no longer support life.

If the enzyme, or the cell it is in, is too hot, the sensitive proteins denature and their function is lost, usually permanently. Without functioning enzymes, life is not possible.

For these reasons, animals have evolved with a variety of ways to maintain their body core temperature within functioninal tolerance limits.

Ectotherms, also called poikilotherms (but NOT called 'cold blooded'!) are animals which rely on heat from the external environment to maintain their body temperatures. They need to gain heat from the environment when they become cool, and they need to lose heat to the environment when they become too warm.

Ectotherms rely on specific behaviours to regulate their heat gain and heat loss.

An ectotherm (for example a snake or frog) will seek out a sunny spot on a cold morning and bask to warm up. But in the heat of the day, when temperatures in the sun may be high, the same snake would be found sheltering under plants or rocks, and the frog would be in cool water. In general, these behavioural mechanisms for temperature regulation are not always available to an ectotherm, so it is fortunate that these animals have a wider range of temperature tolerance limits than most endotherms.

Endotherms or homeotherms (but NOT called 'warm blooded'!) are animals which are able to generate enough internal metabolic heat to maintain their body temperatures.

It is still necessary to fine tune the endotherm's body temperature in response to fluctuations caused be either external influences (very hot or very cold) or internal events (raised or lowered metabolic rate, and its accompanying heat generation), This is achieved, after detection of the stimulus by the hypothalamus, by a combination of functional and behavioural responses. The Table below summarises these responses.

Endotherm's responses to temperature variation.
Stimulus The problem Behavioural response Functional response
Internal temp. too high (hyperthermia) Heat loss to external environment needs to be faster. Reduce physical activity. Seek shade. Stretch out in shade to increase surface area. Take off clothes or reduce fur/feather cover. Dilate surface blood vessels to increase heat loss by radiation. Sweat or pant to increase heat loss by evaporation. Lie on cool surface, or swim in cool water to increase heat loss by conduction.
External temp. too high Heat gain from external environment too high. Reduce physical activity. Seek shade. Stretch out in shade to increase surface area. Take off clothes or reduce fur/feather cover. (If behavioural mechanisms are not sufficient to prevent hyperthermia, then the physiological cooling mechanisms above would begin.)
Internal temp. too low (hypothermia) Internal heat production lower than heat loss. Increase physical activity voluntarily (hand rubbing), or involuntarily (shivering). Huddle up to reduce surface area. Find others to huddle up with. Constrict surface blood vessels to reduce heat loss by radiation. Raise hairs (goose bumps), fur or feathers to trap a layer of warm air next to body. Put on extra clothes.
External temp too low. Heat loss from internal environment to external environment too great. Increase physical activity voluntarily (hand rubbing), or involuntarily (shivering). Huddle up to reduce surface area. Find others to huddle up with. (If behavioural mechanisms are not sufficient to prevent hypothermia, then the physiological warming mechanisms above would begin.)

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Last update :14 March 97