Co-Ordination, The Nervous System, The Endocrine System And Homeostasis



THE various physiological processes in living animals have been described so far as if they were quite separate functions of the body, the total result of which constitutes a living organism.

Although these processes can be usefully considered individually, they are in fact all very closely linked and dependent on each other. The digestion of food, for example, would be of little value without a blood stream to absorb and distribute the products. The working together of these systems is no haphazard process. The timing and location of one set of activities is closely related to the others. For example, in walking, the legs are moved alternately, not both at the same time, without the walker's having to think consciously about it. During exercise, when the body needs more food and oxygen, the breathing rate is automatically increased and the heart beats faster, so sending a greater volume of oxygenated blood to the muscles. When one is eating a meal the position of the food is recorded by the eyes, and as a result of this information the arms are moved to the right place to take it up, not by trial and error, but with precision and accuracy. As the food is raised to the mouth the latter opens to receive it at just the right moment, chewing movements begin, and saliva is secreted. At the moment of Swallowing, many things happen simultaneously as described on. In the stomach, the gastric glands begin to secrete enzymes which will digest the food when it arrives.

In the sequences described above, many bodily functions come into action at just the right moment, with the result that no unnecessary movements are made and no enzymes are wasted by their being secreted when no food is present.

The linking together in time and space of these and other activities is called co-ordination. Without co-ordination the bodily activities would be thrown into chaos and disorder: food might pass undigested through the alimentary canal for lack of enzyme secretion, even assuming it negotiated thee hazard of the windpipe; both legs might be bent simultaneously in an attempt at walking; a runner would collapse after a few metres from lack of an increased oxygen supply, and so on.

So-ordination is effected by the nervous system and the endocrine system. The nervous system is a series of conducting tissues running to all parts of the body, while the endocrine system is a number of glands in the body which produce chemicals that circulate in the blood stream and stimulate certain organs.

The Nervous System

Nerve cells (neurones). The units which make up the nervous system are nerve cells. These are small masses of cytoplasm with a central nucleus. One or more branching. cytoplasmic filaments called dendrites conduct impulses to- wards the cell body, while a single, long fibre called an axon conducts impulses away. In sensory neurones the single, elongated dendrite is called a dendron. Dendrons and axons, collectively called nerve fibres in this account, consist of fluid filled, cytoplasmic tubes, in certain cases surrounded by an insulating sheath of fatty material. In mammals, the cell body is usually in the brain or spinal cord, while the axon or dendron extends the whole distance to the organ concerned. often for considerable lengths, for example, from the base of the spine down to the big toe.

The nerve fibre has the special property of being able to transmit electrical impulses very rapidly down its entire length and pass them on to the next nerve cell in line. This transmission is not the same as electrical conduction in a metal where the current flow depends on the voltage applied. The axon builds up within itself an electrical charge which is released when the nerve 1s stimulated and has to be built in again before the next impulse can pas. Nerve cells usually transmit impulses in one direction only. If the impulse travels in a dendron from a sensory organ or receptor to the nerve centres, it is called a sensory fibre. If the impulse passes from a nerve centre to a muscle or gland, the axon is called a motor fibre.

The synapse. A nervous impulse is passed from one neurone to another by means of a synapse. Branching fibres from one neurone are applied to the dendrites or cell body of another. There is no cytoplasmic connexion between the two and it is thought that the impulse is transmitted by the secretion of a chemical into a microscopic space which exists between the termination of the fibre and the membrane of the cell body. A single impulse does not necessarily get across the synapse. It may take two or three impulses arriving in rapid Succession or perhaps arriving simultaneously from two or more fibres, to start an impulse in the next neurone.

An individual cell body may have synapses with many in- coming fibres and it is via the synapses that the different parts of the body and brain are kept in communication. Because of the enormous possibilities of inter-connexion and since the simultaneous arrival of impulses at a cell body may stimulate or inhibit the relaying of a subsequent impulse, the synapse is probably the basic computer unit of the central nervous System, making possible effective co-ordination and learning.

The grey matter of the brain and spinal cord consists of cell bodies and their synapses. The white matter consists of large tracts of fibres.

Nervous systems. In relatively simple animals like the sea anemone and Hydra, the nerve cells spread fairly evenly under the skin in all directions, so that an impulse started at one point spreads out slowly in all directions encountering many synapses. In higher animals the nerve cells are bundled together into nerves which run in distinct paths from the nerve centres to important organs. Nerves are bundles of fibres, the cell bodies of sensory fibres sometimes forming a bulge, or ganglion, in their length. Most nerves contain both motor and sensory fibres, though one or other type may predominate.

Central nervous system. In vertebrate animals the central nervous system consists of the brain and spinal cord. Most of the cell bodies lie in the central nervous system. Since all impulses from or to the body pass through it, it is possible to have a vast number of cross-connexions and linkages that could not arise if the nerves simply ran from one organ to another. The following analogy of the telephone exchange illustrates the advantages of a centralized nervous system- though in their actual mechanism the nervous system and the telephones are completely different.

If you were to connect your telephone directly to all the people you were likely to want to call up, hundreds of wires would be needed, and if everybody did this the numbers and confusion of wires would be overwhelming. Even so, you would be limited to only those few hundred possible calls. A telephone exchange makes it possible for you to be placed in communica- tion with anyone in the country, rapidly and efficiently. This gives some idea of the increased efficiency of co-ordination as a result of having a central nervous system.

Reflex action is a rapid, automatic response to a stimulus, by an organ or system of organs, which does not involve the brain for its initiation. For example, the iris of the eye can contract or dilate the pupil in response to changing light intensity without our being aware that it is happening. More commonly we are aware of a reflex Occurring but are unable to control it. Blinking when a oreign particle touches the cornea is a reflex action which protects the eyes. We know it is happening but can do nothing to prevent it or modity it. Sneezing is a reflex response to a stimulus in the nose. The knee-jerk is another example. If the right leg is crossed over the left and struck sharply just above or below the knee cap, the lower leg jerks outwards by reflex action.

Reflex arc. It is possible to trace, in a simplified form, the path taken by the impulses involved in a spinal reflex action see. If one unexpectedly touches a hot object, the hand is rapidly removed from the source of heat. Heat or pain receptors in the skin are stimulated and fire off impulses which travel along the sensory fibres in a nerve of the arm. 1The sensory fibres enter the spinal cord via the dorsal root, their cell bodies producing the swelling known as the dorsal root ganglion. In the grey matter of the spinal cord, the impulses pass from the sensory fibre to a relay or association neurone across a synapse. The relay neurone, in turn, makes a synapse With one or more motor fibres. The impulses are thus transmitted to the motor fibres which leave the spinal cord through the ventral root and pass in a nerve, probably the same one in which the sensory impulse travelled, to a muscle, in the example given in the biceps. The impulse causes the muscle to contract, so removing the hand from the painful stimulus and preventing damage to the tissues.

A similar reflex arc produces the knee jerk, but in this case the sensory fibres make a synapse directly with the motor fibre and there is no relay fibre. Striking the tendon below the knee cap stimulates stretch receptors in the leg extensor muscle. The impulse travels round the reflex arc and causes the same muscle to contract. Reflex actions are not usually so simple as de- scribed here. Several receptors of different kinds may be stimulated at once and many sets of muscles or glands may be brought into action, involving many more than the three nerve cells mentioned in the reflex arc described above. The relay nerve cell usually allows connexions to many other motor fibres. In addition, since we are usually aware that a reflex is taking place, narve fibres must conduct the impulses passing in the reflex arc up the spinal cord to the brain.

The term spinal reflex refers to reflex actions in regions below the head, and in an animal such as the frog spinal reflex actions will occur even if the brain is destroyed. Reflex actions concerning organs of the head take place in the brain (cranial reflex).

Conditioned reflexes. In most simple reflexes, the stimulus and response are related. For example, the chemical stimulus of food in the mouth produces the reflex of salivation. After a period of learning or training, however, it is possible for a different and often irrelevant stimulus to produce the same response. In such a case, a "conditioned reflex" has been established, and the animal is said to be conditioned to this stimulus. Pavlov, a Russian biologist, carried out with dogs a great many experiments on conditioned reflexes, one of which is now something of a classic.

The smell and taste of food is a stimulus that activates a dog's salivary glands, making its mouth water. For several days, Pavlov rang a bell at the time the food was given to the dogs.

Later, the sound of the bell alone was a sufficient stimulus to cause a dog's mouth to water, without sight or smell of the food. 

The original chemical stimulus of the food had been replaced by an unrelated stimulus through the ears. The training of animals is done largely by conditioning them to respond to new stimuli. Many of our own actions, such as walking and riding a bicycle, are complicated sets of conditioned reflexes which we acquired in the first place by concentration and practice.

The spinal cord consists of a great number of nerve cells, both fibres and cell bodies, grouped into a cylindrical mass, running from the brain to the tail and protected by the bone of the spinal column. From between the vertebrae, spinal nerves emerge and run to all parts of the body. The fibres of these nerves may be concerned with spinal reflexes or may be carrying Sensory impulses to the brain or motor impulses from the brain to the muscles and other organs of the body.

The nerve cell bodies are grouped in the centre of the cord, making a roughly H-shaped region of grey matter. Outside this 1s the white matter consisting of nerve fibres running up and down the cord or passing out to the spinal nerves. The spinal cord is concerned with spinal reflex actions and the conduction of nervous impulses to and from the brain.

The brain. During evolution, the increasing specialization of the organs of the head, particularly the eyes, ears and nose, has led to more and more sensory fibres entering the front part of the spinal cord. Consequently, this region has grown and developed to form the brain of the vertebrate animal. Like the spinal cord, it consists of nerve cells with a great concentration of cell bodies.

The brain is thus an enlarged, specialized front region of the spinal cord. In the simpler vertebrates, and in the course of development of the more advanced ones, three regions are distinguishable in the brain: the fore-, mid- and hind-brain. The fore-brain receives impulses from the nasal organs. The mid-brain receives impulses from the eyes. Impulses from the ears, and semicircular canals enter the hind-brain, as do also the sensory impulses from the skin From the roof of the hind-brain a thickening develops which forms the cerebellum. This region controls and co-ordinates the balancing organs and the muscles, thus making precise and accurate movements possible.

The floor of the hind-brain thickens to form the medulla oblongata, and situated here are the involuntary centres which control the heart-beat, blood vessels and breathing movements.

The size of these principal regions of the brain usually bears a relation to the most important senses of the animal. In the dogfish, which hunts its prey by smell, the front lobes of the fore-brain are very large and well developed. In the salmon, which depends more on its sight for capturing food, the optic lobes of the mid-brain are much larger than the fore-brain.

MOTOR AREAS. As well as receiving impulses from the sense organs, the brain can send off from certain motor areas impulses which initiate activity in the body. Sometimes these are simple reflexes in response to external stimuli or responses to internal stimuli such as sensations of hunger from the stomach.

ASSOCIATION CENTRES. Certain areas of the brain are supplied with fibres from the principal sense centres of the fore-, mid- and hind-brain, so that several impulses from different sense organs may be correlated. For example, the smell of meat may stimulate the nose of a dog and cause sensory impulses to be sent to the fore-brain. These would normally be relayed to the motor areas and produce co-ordinated movement towards the food, but the sight of another dog in possession and the sound of its ferocious growling will stimulate other sense organs and other brain centres. All these impulses will be relayed to the association centres and, according to the strength of the stimulation and the past experience "stored" in the brain, the motor areas will receive impulses from the association centre resulting in fight or flight.

Without association centres, conditioned reflexes and learning would not be possible. In Pavlov's experiment, sensations of hearing a bell would be most unlikely to produce salivation if only taste and smell centres were connected to the salivary glands.

CEREBRAL HEMISPHERES (cerebrum). In mammals, large outgrowths from the fore-brain spread backwards over the rest of the brain and form two lobes called the cerebral hemi- spheres. These are important association centres where linkages between thousands of nerve cells allow intelligent behaviour, memory and, in ourselves at least consciousness of our own activities. In animals without cerebral hemispheres the behaviour is a matter of simple and conditioned reflexes and inborn behaviour patterns called instinct.

Certain regions of the cerebral hemispheres have been shown to affect particular regions of the body or to be concerned with impulses from a particular sense organ.

Functions of the brain. To sum up: 

1. The brain receives impulses from all the sensory organs of the body.

2. As a result of these sensory impulses, it sends off motor impulses to the glands and muscles, causing them to function accordingly.

3. In its association centres it correlates the various stimuli from the different sense organs.

4. The association centres and motor areas co-ordinate bodily activities so that the mechanisms and chemical reactions of the body work efficiently together.

5. It "stores" information so that behaviour can be modified according to past experience.

The endocrine system

Co-ordination is also effected by chemicals called hormones, secreted from the endocrine glands. These glands have no ducts or openings. The chemicals they produce enter the blood stream as it passes through the glands and they are circulated all over the body. When the hormones reach particular parts of the body they cause certain changes to take place. Their effects are much slower and more general than nerve action and they control rather long-term changes such as rate of growth, rate of activity and sexual maturity. When they pass through the liver, the hormones are converted to relatively inactive compounds which are excreted, in due course, by the kidneys (hence the tests on úrine for the hormonal products of pregnancy). The liver in this way limits the duration of a hormonal response which might otherwise persist indefinitely.

Thyroid. The thyroid gland is in the neck, in front of the wind-pipe. It produces a hormone, thyroxine, which in young animals controls the rate of growth and development. In tadpoles, for example, thyroxine brings about metamorphosis. Feeding tadpoles on thyroxine induces early metamorphosis.

In adult humans thyroxine controls the rate of chemical activity, particularly respiration: too little tends to lead to over weight and sluggish activity; too much can cause thinness and over-activity. Deficiency of the thyroid in infancy causes a certain type of mental deficiency called cretinism, which can be cured in the early stages by administering thyroxine.

Adrenal. The adrenal glands are situated just above the kidneys. The outer layer of the adrenal body, the cortex, produces several hormones, including cortisone, one of whose functions is to accelerate the conversion of proteins to glucose. Secretion by the adrenal cortex is stimulated by certain pituitary hormones.

The inner zone, medulla, of the adrenal gland is stimulated by the nervous system and produces adrenaline. When the sense organs of the animal transmit to the brain, impulses which are associated with danger or other situations needing vigorous action, motor impulses are relayed to the adrenal medulla

which releases adrenaline into the blood. When this reaches the heart it quickens the heart-beat. In other regions it diverts blood from the alimentary canal and the skin to the muscles it makes the pupils dilate and speeds up the rate of breathing and oxidation of carbohydrates. All these changes would increase the animal's efficiency in a situation that might demand Vigorous activity in running away or putting up a fight, In ourselves, they do the same but, together with the nervous system, they produce also the sensation of fear: thumping heart, hollow feeling in the stomach, pale face, ete, In humans, adrenaline may be secreted in many situations which promote anxiety or excitement, and not only in the face of danger.

Pancreas. As well as containing cells which secrete digestive Juices, the pancreas contains endocrine cells which control the use of sugar in the body. The hormone is called insulin; it determines how much sugar is converted to glycogen and how much is Oxidized for energy.

Insulin (a) accelerates the rate at which blood sugar is converted to glycogen in the liver, (b) promotes the uptake of glucose from the blood by the body cells and increases protein synthesis in some cells. The failure of the pancreas to produce sufficient insulin leads to diabetes. The diabetic cannot effectively regulate the blood sugar level. It may rise to above 160 mg/100 cm3 and so be excreted in the urine, or fall to below 40 mg/100 cm3 leading eventually to convulsions and coma. The diabetic condition can be corrected by regular injections of insulin.

Reproductive organs. The ovary produces several hormones called oestrogens of which oestradiol and oestrone are the most potent. These oestrogens (i) control the development of the secondary sexual characters at puberty, cause the lining of the uterus to thicken just before an ovum is released, and (ii), in some mammals at least, oestradiol brings the animal "on heat", i.e. prepares it to accept the male.

Progesterone, the hormone produced from the corpus luteum after ovulation, promotes the further thickening and vascularization of the uterus. Progesterone also prevents the uterus from contracting until the baby is due to be born.

Testosterone is the male sex hormone, produced by the testis. It promotes the development of the masculine secondary sexual characters.

Duodenum. The presence of food stimulates the lining of the duodenum to produce a hormone, secretin, which on reaching the pancreas in the blood stream, initiates the production of pancreatic enzymes. In this way, the enzymes are secreted only when food is present.

Pituitary. The pituitary gland is an outgrowth from the base of the fore-brain. It releases into the blood several different hormones. Some of them appear to have a direct effect on the organ systems of the body. For example ant-diuretic hormone (ADH) controls the amount of water reabsorbed into the blood by the kidneys Growth hormone influences the growth of bone and other tissues Injection of growth hormone in experimental animals causes them to grow larger and to continue growing for longer than usual. Growth, however, is affected by other endocrine glands as well, in particular the thyroid and pancreas, and the growth hormone may exert its influence through these glands rather than directly on the tissues.

In fact, the majority of the pituitary hormmones do act upon and regulate the activity of the other endocrine glands to such an extent that the pituitary is sometimes called the master gland t is a pituitary hormone which, acting on the ovary causes the Graham follicle to develop and secrete its hormone, oestrogen. Another pituitary hormone stimulates the thyroid gland to grow and to produce thyroxine and a third acts on the cortex of the adrenal gland and promotes the pro duction of cortisone.


The foregoing account shows how the hormones (e.g adrenaline) co-ordinate the organs of the body to meet various contingencies, to produce rhythmic patterns of activity (e.g the sex hormones), and to maintain control over long-term processes such as the rate of growth (thyroid and pituitary). It can also be seen that they fulfil a homeostatic function in regulating the composition of the internal environment. If the blood sugar level rises, the pancreas is stimulated to secrete insulin which increases the amount of glucose removed from the blood and stored as glycogen in the muscles and liver. A fall in the blood sugar level suppresses the pro- duction of insulin from the pancreas. A fall in the osmotic potential of the blood results in the release of ADH from the pituitary gland and the consequent reabsorption of water from the kidney tubules.

Interaction and feed-back"

For effective control, two opposing systems are needed. The car needs an accelerator and brakes, a muscle must have its antagonistic partner. The hormones too have antagonistic effects. Adrenaline promotes the release of sugar into the blood while insulin has the opposite effect. Oestrogen stimulates the growth of the follicle while progesterone suppresses it.

A fine adjustment of the balance of these antagonistic hormones helps to maintain the controlled growth, development and activity of the organisms in constantly changing conditions.

Such a balance is maintained partly by the "feed-back" effect of hormones, i.e. a system whereby "information is "fed back" to a source "telling it" about events in the body and so enabling it to adjust its output accordingly. A pituitary hormone stimulates the thyroid to produce thyroxine but thyroxine production is kept in check by the fact that when thyroxine reaches the pituitary via the circulation, production of thyroid- stimulating hormone is suppressed. The "feed-back" of thyroxine to the pituitary regulates the output of the latter. The ovarian follicles are stimulated to produce oestrogen by a pituitary hormone but when the oestrogen in the blood reaches a certain level it suppresses the secretion of follicle-stimulating hormone by the pituitary. A delay in the feed-back effect leads to rhythmic changes. For example, it may take two weeks for the level of oestrogen in the blood to affect the pituitary, by which time the uterus lining has thickened and the ovum has been released from the follicle. The output of follicle-stimulat ing hormone is diminished as a result of increasing oestrogen and this in turn reduces the output of oestrogen from the ovary which, in the absence of fertilization and the development of the corpus luteum, leads to the breakdown of the uterine lining, characteristic of menstruation.


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