Chapter 5 — Coordination – The Linking System

Living things react to their surroundings. A dog pricks up its ears at a sound, a sunflower turns towards the sun, our hand jerks away from a hot pan, and our mouth waters at the smell of food. Any change in the environment, inside or outside the body, that produces a reaction in an organism is a stimulus (plural stimuli), and the reaction is the response. Light, sound, heat, touch, pressure, chemicals, gravity and water are all stimuli. The ability to detect a stimulus and respond to it is called irritability or sensitivity, and it is one of the defining properties of life.

For a response to happen, three things are needed. A receptor detects the stimulus; receptors are specialised cells, often collected into sense organs such as the eye, ear, nose, tongue and skin, and there are also receptors inside the body that sense blood pressure, carbon dioxide, temperature and the stretch of muscles. An effector carries out the response; the effectors are the muscles, which contract, and the glands, which secrete. Between them there must be a link that carries the information from receptor to effector and decides what to do. In animals the link is provided by two systems: the nervous system, which sends fast electrical signals along nerves, and the endocrine system, which sends slower chemical signals, hormones, through the blood. The working together of the receptors, the linking systems and the effectors, so that the body acts as a whole, is called coordination, and in the human body it is also called control, since the systems not only link but regulate.

Why is coordination necessary? Consider a runner at the start of a race. The ear hears the gun; the brain sends impulses to dozens of leg and arm muscles in a precise order; the heart is made to beat faster and the breathing to deepen; blood is diverted from the intestine to the muscles; the liver releases glucose; sweat glands are switched on. Every one of these responses is timed and measured, and none would be useful alone. Or consider the simpler matter of standing upright: hundreds of muscles are being adjusted many times a second in response to signals from the ears, eyes and the stretch receptors of the legs. Without coordination the body would be a collection of organs each working on its own.

Plants respond too, though usually slowly and by growth: roots grow towards water, shoots towards light, tendrils coil round a support, and a touch-me-not folds its leaves in a second. Plants have no nerves and coordinate entirely by chemical messengers, the plant hormones, which are treated at the end of the chapter.

The response to a stimulus may be simple and automatic, as in a reflex, or complex and learned, as in reading this page. Both use the same cells, the neurons, which are studied next.

The nervous system is built of special cells called neurons or nerve cells, together with supporting cells. A neuron is the longest cell in the body, some reaching over a metre from the spinal cord to the toe, and it is specialised for one job: to receive a signal at one end and carry it as an electrical impulse to the other end. There are about 100 billion neurons in the human brain alone.

A neuron has three parts. The cell body (cyton) contains the nucleus, cytoplasm and the usual organelles, together with granules called Nissl bodies that make the proteins of the cell. From the cell body arise short, much-branched processes, the dendrites, which receive signals from other neurons or from receptors and carry them towards the cell body. One process is much longer than the others and carries impulses away from the cell body; this is the axon or nerve fibre. The axon may be very long; it is usually covered by a fatty white sheath, the myelin sheath, laid down by Schwann cells, with gaps between the Schwann cells called nodes of Ranvier. The sheath insulates the axon and speeds up the impulse, which jumps from node to node. Myelinated fibres look white and make up the white matter of the brain and spinal cord; cell bodies and unmyelinated fibres look grey and form the grey matter. At its end the axon branches into fine terminals, each ending in a small swelling, the synaptic knob, which lies close to the dendrite of the next neuron or to a muscle fibre.

Neurons are of three kinds according to their function. Sensory neurons (afferent) carry impulses from the receptors and sense organs towards the central nervous system; their cell bodies lie in ganglia beside the spinal cord. Motor neurons (efferent) carry impulses from the central nervous system to the effectors, the muscles and glands; their cell bodies lie in the grey matter of the spinal cord and brain. Relay neurons (interneurons, association neurons) lie entirely within the brain and spinal cord and connect sensory neurons to motor neurons and to one another; they are by far the most numerous and are the basis of all thinking, memory and decision.

A nerve is not a single neuron but a bundle of hundreds or thousands of axons bound together by connective tissue, like wires in a cable, with its own blood supply. Nerves carrying only sensory fibres are sensory nerves, such as the optic nerve from the eye; those with only motor fibres are motor nerves; most spinal nerves carry both and are called mixed nerves. A collection of cell bodies outside the central nervous system is a ganglion.

Neurons do not divide after birth, and a neuron destroyed by injury or disease is usually not replaced, which is why damage to the spinal cord or brain is so often permanent. A cut peripheral nerve can regrow slowly along its old sheath if the cell body survives.

The message that travels along a neuron is a nerve impulse, a wave of electrical change that sweeps along the membrane of the fibre. It is not an electric current flowing like electricity in a wire; it is a rapid, self-propagating change in the distribution of ions across the membrane.

Resting state. A resting neuron pumps sodium ions out and potassium ions in, and its membrane is more permeable to potassium, so the inside of the fibre is negatively charged relative to the outside, by about 70 millivolts. The membrane is said to be polarised.

The impulse. When a dendrite or the cell body is stimulated strongly enough, the membrane at that point suddenly becomes permeable to sodium; sodium ions rush in and the inside becomes momentarily positive. This reversal, the action potential, disturbs the next part of the membrane, which does the same, and so a wave of depolarisation runs along the axon. Behind the wave, potassium flows out, the membrane returns to its resting state, and the pumps restore the ions. The impulse is all-or-none: a stimulus below a certain strength (the threshold) produces nothing, and one above it produces a full-sized impulse; a stronger stimulus produces more impulses per second, not bigger ones. In a myelinated fibre the impulse jumps from one node of Ranvier to the next, reaching speeds of over 100 metres per second; in unmyelinated fibres it crawls at about 1 metre per second. An impulse travels in one direction only, from dendrite to axon terminal, because of the arrangement at the synapse.

The synapse. Neurons do not touch. Between the synaptic knob at the end of one axon and the dendrite or cell body of the next neuron is a microscopic gap of about 20 nanometres, the synaptic cleft, and the whole junction is the synapse. When an impulse reaches the knob, it causes tiny sacs (vesicles) in the knob to release a chemical, a neurotransmitter such as acetylcholine, into the cleft. The chemical diffuses across, fits on to receptor molecules in the membrane of the next neuron and starts a new impulse in it. The transmitter is then destroyed by an enzyme so that the synapse is ready for the next signal. Since only the knob has vesicles and only the far side has receptors, the impulse can cross in one direction only. Crossing a synapse takes about half a millisecond, so a pathway with many synapses is slower; the delay is the price of flexibility, because at a synapse signals from many neurons can be added together, or one signal can inhibit another, and this is where the nervous system does its computing. The junction between a motor neuron and a muscle fibre is a similar chemical synapse called the neuromuscular junction.

Many drugs and poisons act at synapses. Anaesthetics block the transmission of pain; nerve gases and some insecticides stop the enzyme that destroys acetylcholine, so muscles go into spasm; curare, the arrow poison, blocks the receptors, paralysing the muscles; caffeine and nicotine stimulate, alcohol and sleeping pills depress, synaptic transmission. The chemical nature of the synapse is thus of great practical importance.

If you touch a hot iron, your hand is pulled away before you feel the pain and long before you have decided anything. This sudden, automatic, involuntary response to a stimulus, carried out without the conscious involvement of the brain, is a reflex action. Reflexes are the simplest form of nervous coordination, they are the same in every person, and they are protective: they act in a fraction of a second, faster than any decision could. Withdrawing the hand from heat or a pin, blinking when something approaches the eye, sneezing when the nose is irritated, coughing, the narrowing of the pupil in bright light, the knee jerk when the tendon below the kneecap is tapped, salivation at the taste of food, and the swallowing of a bolus are all reflexes. Most of them are handled by the spinal cord; some, such as blinking and pupil reflexes, by the brain stem.

The pathway along which the impulses travel in a reflex is the reflex arc, and it has five parts. (1) The receptor, for example the pain receptors in the skin of the finger, detects the stimulus and generates impulses. (2) The sensory neuron carries the impulses along its axon in a spinal nerve into the spinal cord through the dorsal root; its cell body lies in the dorsal root ganglion. (3) In the grey matter of the spinal cord the impulse crosses a synapse to a relay neuron. (4) The relay neuron passes it, across another synapse, to a motor neuron, whose cell body lies in the ventral horn of the grey matter and whose axon leaves the cord through the ventral root and runs in the spinal nerve to the arm. (5) The effector, the biceps muscle of the arm, contracts and the hand is jerked away. The whole circuit is completed in the spinal cord, and that is why the response is so fast. In the knee jerk the sensory neuron synapses directly with the motor neuron, with no relay neuron, the simplest reflex arc of all.

At the same time as the relay neuron passes the message to the motor neuron, other neurons carry it up the spinal cord to the brain. That is how we become aware of the pain and know what happened, a moment after the hand has already moved. The brain can also modify or override a reflex to some extent: a person can hold a hot cup for a moment to avoid dropping it, and a child learns to control the emptying of the bladder, which in infancy is a pure reflex.

The reflexes just described are inborn or unconditioned: they are present from birth and need no learning. Conditioned reflexes are acquired by experience. The Russian physiologist Pavlov rang a bell each time he fed a dog; after some days the dog's mouth watered at the bell alone, though a bell has nothing to do with food. The reflex of salivation had been linked to a new stimulus by association. Mouth-watering at the sight of a favourite dish, braking a cycle at the sight of a pothole, and typing without looking at the keys are conditioned reflexes; they are learned, they can be lost, and they involve the cerebrum.

Reflexes are used by doctors to test the nervous system: the knee jerk, the pupil's response to light and the plantar reflex reveal whether the spinal cord, brain stem and nerves are intact.

The human nervous system has two main divisions. The central nervous system (CNS) consists of the brain and the spinal cord; it receives information from every part of the body, processes it and issues commands. The peripheral nervous system (PNS) consists of the nerves that connect the CNS with the rest of the body: 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord. The peripheral system is further divided by function into the voluntary (somatic) system, which carries sensory impulses from the skin and sense organs and motor impulses to the skeletal muscles under our conscious control, and the autonomic system, which controls the internal organs without our awareness.

Protection of the CNS. The brain, a soft jelly-like organ weighing about 1.4 kg, is enclosed in the bony box of the skull (cranium), and the spinal cord runs down the canal formed by the vertebrae. Inside the bone, both are wrapped in three membranes, the meninges: the tough outer dura mater, the web-like middle arachnoid, and the delicate inner pia mater that clings to the surface. Between the arachnoid and pia mater, and in the ventricles (cavities) inside the brain and the central canal of the spinal cord, is the cerebrospinal fluid (CSF), a clear liquid that cushions the brain against jolts, keeps the pressure even, and supplies nutrients and removes wastes. Infection of the meninges is meningitis, a serious disease diagnosed by drawing a sample of CSF from the lower back.

General plan of the brain. The brain develops from three swellings at the front of the embryonic nerve tube, and these remain its three main regions. The forebrain consists of the cerebrum, which is the largest part, together with the thalamus and hypothalamus beneath it. The midbrain is a short stalk connecting the forebrain to the hindbrain, carrying fibre tracts and containing centres for visual and auditory reflexes. The hindbrain consists of the cerebellum, the pons and the medulla oblongata, which is continuous with the spinal cord. The midbrain, pons and medulla together form the brain stem.

The brain is made of grey matter, the cell bodies of neurons, and white matter, the myelinated fibres. In the cerebrum and cerebellum the grey matter lies on the outside as a cortex and the white matter inside; in the spinal cord the arrangement is reversed, with grey matter forming an H-shape in the centre and white matter around it. The brain uses about 20 percent of the body's oxygen and glucose although it is only 2 percent of its weight, and its cells die within minutes if the blood supply stops, which is why a stroke or a blocked airway is so urgent.

The brain receives its information through the cranial nerves and the spinal cord, and it sends its commands out the same way. Each region has its own functions, described in the next topic.

Cerebrum. The cerebrum forms about four-fifths of the brain. It is divided by a deep longitudinal groove into two cerebral hemispheres, joined underneath by a thick band of fibres, the corpus callosum. Its surface, the cerebral cortex, is grey matter thrown into folds (gyri) and grooves (sulci), which triple the surface area and allow room for some 10 billion cell bodies. Each hemisphere is divided into four lobes named after the bones above them: frontal, parietal, temporal and occipital. The cerebrum is the seat of intelligence, memory, reasoning, will, emotion, speech and conscious sensation. Its sensory areas receive impulses from the sense organs and interpret them: the visual area in the occipital lobe, the hearing area in the temporal lobe, and the area for touch, pain and temperature in the parietal lobe behind the central sulcus. Its motor areas in the frontal lobe, in front of the central sulcus, send commands to the voluntary muscles; the left hemisphere controls the right side of the body and the right hemisphere the left, because the fibres cross in the medulla. The association areas between them link sensation with memory and decide on action; the frontal association area is concerned with planning, judgement and personality, and the speech area, usually in the left hemisphere, with language. Damage to a particular area produces a particular loss: a stroke in the left motor area paralyses the right arm and leg.

Thalamus and hypothalamus. Beneath the cerebrum, the thalamus is a relay station through which nearly all sensory impulses pass on their way to the cortex and where crude sensations of pain and temperature are first felt. Below it, the small hypothalamus is the centre for the internal environment: it controls body temperature, hunger, thirst, sleep, the water balance through ADH, and the emotions of fear and anger, and it controls the pituitary gland, thereby linking the nervous and endocrine systems.

Midbrain. The midbrain connects the forebrain and hindbrain and contains reflex centres for the movements of the eyes and head in response to sights and sounds, and for the pupil reflex.

Cerebellum. The cerebellum, the little brain, lies behind and below the cerebrum and is also folded and two-lobed. It does not initiate movement but coordinates it: it receives information from the muscles, joints, eyes and the balance organs of the ear, and adjusts the strength and timing of muscle contractions so that movements are smooth, accurate and balanced. Walking, writing, cycling, threading a needle and keeping the body upright all depend on it. A person whose cerebellum is damaged, or numbed by alcohol, walks unsteadily, reaches clumsily and has slurred speech.

Pons. The pons is a bridge of fibres connecting the two halves of the cerebellum and the cerebrum with the medulla; it also helps regulate breathing.

Medulla oblongata. The medulla is the lowest part of the brain, continuous with the spinal cord. It controls the involuntary vital functions: the rate of the heartbeat, the diameter of the blood vessels and hence blood pressure, the rate and depth of breathing, and the reflexes of swallowing, vomiting, coughing, sneezing, hiccupping and salivation. All nerve fibres between the brain and the spinal cord pass through it, and most cross from one side to the other here. A blow to the back of the head or pressure on the medulla can stop breathing and the heart, which is why injury to this region is so often fatal.

Spinal cord. The spinal cord is a cylindrical cord of nervous tissue about 45 cm long and as thick as a little finger, running from the medulla down through the vertebral canal to the level of the second lumbar vertebra, below which only a bundle of nerve roots continues. It is protected by the vertebrae, the meninges and cerebrospinal fluid. In cross section it shows a central canal containing CSF, an H-shaped or butterfly-shaped core of grey matter (cell bodies of motor and relay neurons), and an outer layer of white matter made of myelinated fibres running up and down. The two arms of the H pointing backwards are the dorsal horns, which receive sensory fibres; the two pointing forwards are the ventral horns, which contain motor cell bodies. The spinal cord has two functions. It is the centre for spinal reflexes, such as the withdrawal reflex and the knee jerk, which it carries out without the brain. And it is the pathway between the brain and the body: ascending tracts in the white matter carry sensory impulses up to the brain and descending tracts carry motor commands down. If the cord is cut by an accident, all sensation and voluntary movement below the cut are lost, though the reflexes below the cut, including bladder emptying, still occur.

Spinal nerves. Thirty-one pairs of spinal nerves leave the cord through gaps between the vertebrae. Each arises by two roots: a dorsal root, purely sensory, with a swelling, the dorsal root ganglion, containing the cell bodies of the sensory neurons; and a ventral root, purely motor, carrying axons of the motor neurons in the ventral horn. The two roots unite into a mixed nerve, which then branches to supply a region of skin and a group of muscles. The cranial nerves, twelve pairs from the brain, supply the head and neck: the olfactory nerve (smell), the optic nerve (sight), the nerves of the eye muscles, the facial nerve, the auditory nerve (hearing and balance), and the vagus, which wanders down to the heart, lungs and abdominal organs.

Autonomic nervous system. The heart, the smooth muscle of the gut, the blood vessels, the bladder, the iris, the sweat glands and the endocrine glands are controlled automatically, without our awareness or will, by the autonomic nervous system, a part of the peripheral system whose motor fibres run through chains of ganglia beside the spinal cord to the organs. It has two divisions with opposite effects, and every organ receives fibres from both, so that its activity can be turned up or down. The sympathetic division, arising from the thoracic and lumbar cord, prepares the body for emergency, the fight-or-flight response: it speeds and strengthens the heartbeat, raises blood pressure, dilates the pupils and the air passages, diverts blood from the skin and gut to the muscles, releases glucose from the liver, makes the hair stand up and the skin sweat, and slows digestion. Its effects are reinforced by the hormone adrenaline. The parasympathetic division, arising from the brain stem (chiefly the vagus nerve) and the sacral cord, restores calm, the rest-and-digest state: it slows the heart, lowers blood pressure, constricts the pupils, stimulates the secretion of saliva and digestive juices and the movements of the gut, and empties the bladder. Both divisions are ultimately regulated by the hypothalamus and the medulla. The balance between them at any moment sets the state of the body: on a peaceful afternoon after a meal the parasympathetic dominates; when a snake is seen the sympathetic takes over within a second.

The sense organs are the receptors of the body, each specialised for one kind of stimulus: the eye for light, the ear for sound and balance, the nose for smell, the tongue for taste and the skin for touch, pressure, pain, heat and cold. The eye is the most important of them and the one examinations ask about most.

Structure. The eyeball is a nearly spherical organ about 2.5 cm across, lodged in a bony socket of the skull, the orbit, moved by six muscles and protected by the eyelids, eyelashes, eyebrows and the tears secreted by the lacrimal gland, which wash the surface and contain the germ-killing enzyme lysozyme. Its wall has three layers. The outer sclera is a tough white coat that keeps the shape of the eyeball; in front it becomes the transparent, curved cornea, through which light enters and which does most of the bending (refraction) of the light. The middle choroid is a dark layer rich in blood vessels that nourishes the eye and absorbs stray light; in front it becomes the ciliary body, whose muscles change the shape of the lens, and the iris, the coloured disc with a central hole, the pupil, whose size the iris muscles adjust to control the amount of light entering. The inner retina is the light-sensitive layer containing the receptor cells: about 120 million rods, sensitive to dim light but not to colour, and 6 million cones, working in bright light and giving colour vision, most concentrated at the yellow spot (fovea) directly behind the lens, where vision is sharpest. Where the optic nerve leaves the retina there are no receptors, the blind spot. Behind the pupil hangs the transparent, elastic, biconvex lens, held by suspensory ligaments to the ciliary body. The space between the cornea and the lens is filled with a watery fluid, the aqueous humour, and the large space behind the lens with a jelly, the vitreous humour, which keep the eyeball firm.

Working. Light from an object passes through the cornea, aqueous humour, pupil, lens and vitreous humour and is focused by the cornea and lens as a small, real, inverted image on the retina. The rods and cones convert light into nerve impulses, which travel along the optic nerve to the visual area of the cerebrum in the occipital lobe, where the image is interpreted and turned the right way up. The two eyes give slightly different images, and the brain combines them to judge distance (binocular vision).

Accommodation is the adjustment of the lens so that both near and distant objects can be focused. For a distant object the ciliary muscles relax, the suspensory ligaments become taut and the lens is pulled thin; for a near object the ciliary muscles contract, the ligaments slacken and the elastic lens bulges, becoming more convex and bending the light more. The nearest point that can be focused, the near point, is about 25 cm in a young adult and recedes with age as the lens stiffens (presbyopia), which is why people over forty need reading glasses. The pupil reflex protects the retina: in bright light the circular muscles of the iris contract and the pupil shrinks; in dim light the radial muscles contract and it widens.

Defects. In myopia (short sight) the eyeball is too long or the lens too strong and distant objects focus in front of the retina; a concave lens corrects it. In hypermetropia (long sight) the eyeball is too short and near objects focus behind the retina; a convex lens corrects it. A cataract is a clouding of the lens with age, cured by replacing it with an artificial lens. Deficiency of vitamin A causes night blindness because the rods cannot make their pigment, rhodopsin. Colour blindness, usually the inability to tell red from green, is an inherited defect of the cones, commoner in men.

The ear has two functions, hearing and balance, and three regions. The outer ear consists of the pinna, the fleshy flap that collects sound, and the auditory canal, about 2.5 cm long, lined with hairs and wax-secreting glands that trap dust and insects, ending at the eardrum (tympanic membrane), a thin, tightly stretched membrane. The middle ear is a small air-filled cavity in the temporal bone containing three tiny bones, the ear ossicles: the hammer (malleus), attached to the eardrum, the anvil (incus) and the stirrup (stapes), whose footplate fits into the oval window, an opening into the inner ear. The Eustachian tube connects the middle ear to the throat and opens when we swallow or yawn, equalising the air pressure on the two sides of the eardrum; that is why the ears pop in an aircraft or on a hill road and why a cold can spread to the middle ear. The inner ear is a set of fluid-filled chambers and tubes embedded in bone, the labyrinth. The cochlea, coiled like a snail shell, contains the hearing receptors, the hair cells of the organ of Corti. The three semicircular canals, set at right angles to each other, and two small sacs, the utricle and saccule, contain the receptors of balance.

Hearing. Sound waves collected by the pinna travel down the canal and make the eardrum vibrate. The three ossicles carry the vibrations across the middle ear, acting as levers that amplify them about twenty times, and the stirrup presses on the oval window, setting up pressure waves in the fluid of the cochlea. The waves bend the hairs of the hair cells, which generate nerve impulses in the auditory nerve. The impulses reach the hearing area in the temporal lobe of the cerebrum, where the sound is perceived and recognised. Different parts of the cochlea respond to different pitches. The human ear hears frequencies from about 20 to 20,000 hertz, the upper limit falling with age.

Balance. When the head moves, the fluid in the semicircular canals lags behind and bends the hairs of receptor cells at the base of each canal; since the three canals lie in three planes, any rotation is detected. The utricle and saccule contain hair cells with tiny chalk granules resting on them, which are pulled by gravity and signal the position of the head and straight-line movements. The information goes to the cerebellum, which adjusts the muscles to keep the body balanced. Spinning round makes the fluid go on moving after we stop, and we feel giddy; the rocking of a boat overloads the system and causes seasickness.

Nose. The roof of the nasal cavity is lined with olfactory epithelium containing millions of receptor cells with fine hairs that respond to chemicals dissolved in the mucus. Their impulses travel along the olfactory nerve to the smell area of the cerebrum. Smell is very sensitive, detects thousands of different substances, tires quickly for a continued odour, and is closely linked to taste and memory; with a blocked nose food tastes flat.

Tongue. The surface of the tongue bears small projections, papillae, with taste buds, groups of receptor cells opening by a pore, that respond to substances dissolved in saliva. Four basic tastes are distinguished, sweet, salt, sour and bitter, with a fifth, umami (savoury), now added; all regions of the tongue respond to all tastes, though sensitivity differs. The flavour of food is the combination of taste, smell, texture and temperature.

Skin. The skin is the largest sense organ. In the dermis lie several kinds of receptors: free nerve endings for pain, Meissner's corpuscles for light touch (most numerous in the fingertips and lips), Pacinian corpuscles deeper down for pressure, and separate endings for heat and cold. The distribution is uneven: a fingertip can distinguish two pin points 2 mm apart, the back only when they are 5 cm apart. Pain receptors also lie in the internal organs, and the pain of an internal organ is often felt at the surface, as heart pain is felt in the left arm.

Besides the nervous system, the body has a second means of coordination, the endocrine system, which works by chemical messengers. An endocrine gland (endo = within, crine = to secrete) has no duct; it pours its secretion directly into the blood flowing through it, and the blood carries the secretion to every part of the body. Glands with ducts, such as the salivary and sweat glands, are exocrine. The pancreas is both: its digestive juice leaves by a duct, its insulin by the blood. The secretions of endocrine glands are hormones (Greek: to excite), organic substances, mostly proteins or steroids, produced in tiny amounts, which act on particular target organs or tissues far from the gland, regulate their activity, and are then destroyed by the liver or excreted, so they must be secreted continuously. Their effects are usually slower and longer-lasting than those of nerves, and they control processes that go on over hours, months or years: growth, metabolism, the response to stress, the balance of sugar, salt and water, sexual development and reproduction. The endocrine glands are themselves coordinated, chiefly by the pituitary and the hypothalamus, and most hormones are regulated by negative feedback: when the level of a hormone or of the thing it controls rises, the secretion of the hormone falls.

Pituitary gland. A pea-sized gland hanging from the hypothalamus at the base of the brain, called the master gland because several of its hormones control other glands. Its front lobe secretes growth hormone, which stimulates the growth of bones and tissues (too little in childhood causes dwarfism, too much gigantism, and too much in an adult acromegaly, with enlarged hands, feet and jaw); thyroid-stimulating hormone; adrenocorticotrophic hormone (ACTH) which stimulates the adrenal cortex; the gonadotrophins FSH and LH, which control the ovaries and testes; and prolactin, which stimulates milk secretion. Its back lobe releases ADH, which controls water reabsorption by the kidney, and oxytocin, which contracts the uterus at childbirth and ejects milk. The pituitary is itself controlled by releasing hormones from the hypothalamus, so the nervous system stands at the head of the endocrine system.

Thyroid gland. A butterfly-shaped gland in front of the windpipe in the neck, secreting thyroxine, which contains iodine and controls the rate of metabolism, the speed at which every cell uses food and oxygen, and is essential for growth and mental development. Too little in a child causes cretinism, stunted growth and mental retardation; in an adult it causes myxoedema, with sluggishness, weight gain and dry puffy skin. Too much causes exophthalmic goitre, with a racing heart, weight loss, nervousness and protruding eyes. When the diet lacks iodine the gland cannot make thyroxine, the pituitary drives it harder and it swells into a simple goitre, once common in the Himalayan foothills and prevented today by iodised salt. The thyroid also secretes calcitonin, which lowers blood calcium, and the four small parathyroid glands embedded behind it secrete parathormone, which raises it; together they keep the calcium of the blood constant for bones, nerves and muscles.

Pancreas (islets of Langerhans). Scattered among the digestive cells of the pancreas are clusters of endocrine cells, the islets, which secrete insulin and glucagon. Insulin lowers the blood glucose after a meal by making cells take up glucose and the liver store it as glycogen; glucagon raises it between meals by releasing glucose from glycogen. Lack of insulin causes diabetes mellitus: glucose accumulates in the blood, spills into the urine, the patient passes large volumes of urine, is thirsty, hungry and tired, loses weight and, over years, suffers damage to the eyes, kidneys, nerves and blood vessels. It is treated with insulin injections, or in the milder adult form with diet, exercise and tablets. India has among the largest numbers of diabetics in the world.

Adrenal glands. A pair of small glands, one capping each kidney, each with an outer cortex and an inner medulla that are really two glands in one. The adrenal medulla secretes adrenaline (epinephrine), the hormone of emergency, in response to fear, anger, pain, cold or exertion, on the orders of the sympathetic nervous system. Adrenaline prepares the body to fight or flee: the heart beats faster and harder, blood pressure rises, the air passages widen and breathing quickens, the pupils dilate, the liver releases glucose into the blood, blood is diverted from the skin and gut to the muscles and brain, the hairs stand on end and the skin sweats, and digestion is suspended. Its effects are the same as those of the sympathetic nerves, but they last longer because the hormone circulates for some minutes. The pounding heart, dry mouth and trembling of stage fright are adrenaline at work. The adrenal cortex secretes steroid hormones: cortisol, which raises blood glucose, helps the body withstand prolonged stress and suppresses inflammation (its synthetic relatives are the cortisone drugs), and aldosterone, which makes the kidney retain sodium and so regulates salt and water balance and blood pressure. Destruction of the cortex causes Addison's disease, with weakness, low blood pressure and darkening of the skin.

Gonads. The testes of the male secrete testosterone, which at puberty produces the secondary sexual characters, the deepening of the voice, the growth of the beard and body hair, the broadening of the shoulders and the growth of muscle, and maintains sperm production. The ovaries of the female secrete oestrogen, which at puberty produces the growth of the breasts, the widening of the hips and the female distribution of fat, and controls the menstrual cycle, and progesterone, which prepares the uterus for pregnancy and maintains it. The gonads are controlled by the gonadotrophins FSH and LH of the pituitary, whose secretion begins at puberty. The thymus in the upper chest, large in childhood and shrinking after puberty, secretes thymosin, which matures the lymphocytes of the immune system. The pineal gland in the brain secretes melatonin, which regulates the daily rhythm of sleep.

Nervous and hormonal coordination compared. The two systems work together and each has its strengths. The nervous system sends its message as an electrical impulse along a fixed path, a nerve fibre, to one particular effector, at speeds up to 100 metres a second; the response begins in milliseconds, is short-lived and precisely localised, and ends when the impulses stop. It is suited to quick, exact actions: withdrawing a hand, catching a ball, speaking. The endocrine system sends its message as a chemical carried by the blood to every part of the body, acting on whichever cells have the right receptors; the response takes seconds to days to appear, may last for hours or years, and may be widespread. It is suited to slow, sustained processes: growth, metabolism, the sugar level, sexual development. Nerves are like a telephone call to one person; hormones are like a broadcast that only the tuned-in receive. The two are linked at the hypothalamus, which is nervous tissue but controls the pituitary, and at the adrenal medulla, which is a modified sympathetic ganglion. Some chemicals, such as adrenaline and noradrenaline, serve both as hormones and as neurotransmitters. In a frightened animal the sympathetic nerves act first, in a second, and adrenaline follows to keep the state going.

Plants have no nervous system, no sense organs and no muscles, yet they respond to stimuli. Most of their responses are movements of growth: one part of an organ grows faster than another, so the organ bends. Because growth is slow, so are these movements, taking hours or days; but they are directed and purposeful, and they allow a rooted plant to place its roots in the wettest soil and its leaves in the brightest light. A growth movement in which the direction of bending is determined by the direction of the stimulus is a tropism or tropic movement. If the organ grows towards the stimulus the tropism is positive; if away from it, negative.

Phototropism is the response to light. Stems and shoots are positively phototropic: a plant on a windowsill bends its shoots towards the window, and a seedling grown in a box with a hole on one side grows out through the hole. Roots are negatively phototropic or indifferent. The bending occurs because the hormone auxin, produced at the tip of the shoot, moves to the shaded side and makes the cells there elongate faster, so the shoot curves towards the light. This places the leaves where they can photosynthesise best.

Geotropism (gravitropism) is the response to gravity. Roots are positively geotropic: a seed planted in any position sends its root downwards, and if a seedling is laid on its side the root bends down within a day. Shoots are negatively geotropic and grow upward, which is why a fallen plant raises its tip. In the root, auxin accumulating on the lower side inhibits growth there, so the upper side grows faster and the root bends down; in the shoot the same accumulation stimulates the lower side and the shoot bends up. The response can be shown with a klinostat, a slowly rotating wheel on which a seedling is fixed horizontally; because gravity acts equally on all sides in turn, the root and shoot grow straight.

Hydrotropism is the response to water. Roots are positively hydrotropic and grow towards moisture, which can be stronger than geotropism: a root that meets a porous pot of water buried nearby will bend towards it and even grow upwards to reach it. This is how tree roots find and invade drains and wells.

Chemotropism is the response to chemicals. The classic example is the growth of the pollen tube down the style towards the ovule, guided by sugars secreted by the ovule; without it fertilisation could not take place. Fungal hyphae grow towards food, and roots away from harmful salts.

Thigmotropism is the response to touch or contact. The tendrils of pea, gourd, cucumber and passion flower, and the twining stems of bean and money plant, coil round any support they touch: the cells on the side away from the contact grow faster and the tendril curls round the object within an hour or two, hoisting the plant into the light without the cost of building a thick stem.

Tropisms show that a plant can perceive the direction of a stimulus, transmit the information, and respond by unequal growth, the whole thing being managed by chemicals. The chemical messengers, the plant hormones, are treated next.

Nastic movements. Some plant movements are responses to a stimulus but their direction is not determined by the direction of the stimulus; the plant part moves the same way whichever side the stimulus comes from. These are nastic movements. Most are not growth movements at all but changes of turgor, the water pressure in special cells, and they can be fast. The most striking is the touch-me-not (Mimosa pudica, chhui-mui): when a leaflet is touched, shaken or heated, the leaflets fold together and the whole leaf droops within a second or two, and the disturbance passes to neighbouring leaves. The movement is caused by the sudden loss of water from cells in a swelling, the pulvinus, at the base of each leaflet and leaf stalk; the cells become flaccid, the leaf collapses, and it recovers in ten to twenty minutes as the cells refill. The signal travels through the plant as an electrical change and a chemical, at a few centimetres a second, faster than any growth response but far slower than a nerve impulse. This is thigmonasty, and it probably protects the plant from browsing animals and insects. Other nastic movements are the daily opening and closing of flowers such as the lotus and dandelion (photonasty, in response to light), the opening of the tulip and crocus when warmed (thermonasty), the folding of the leaves of bean, tamarind and gulmohar at night, the so-called sleep movements (nyctinasty), and the snapping shut of the leaf trap of the Venus fly-trap on an insect. Nastic movements are compared with tropisms thus: both are responses to stimuli, but in a tropism the direction of the response depends on the direction of the stimulus and the movement is by growth and slow, whereas in a nastic movement the direction is fixed by the structure of the organ, the movement is usually by turgor change, and it can be rapid and reversible.

Plant hormones (phytohormones). All the responses of plants are coordinated by chemicals made in one part and acting in another, in tiny amounts, on growth and development. Five main groups are recognised.

Auxins are produced at the growing tips of shoots and roots and move away from the tip. They cause cell elongation and are responsible for phototropism and geotropism, as described above. They also promote the growth of the main shoot while suppressing the side buds (apical dominance), which is why a gardener pinches off the tip to make a plant bushy; they stimulate the formation of roots on cuttings, and synthetic auxins are sold as rooting powder; they prevent the premature fall of fruit; and in high concentration they kill broad-leaved weeds, the basis of the weedkiller 2,4-D.

Gibberellins cause the elongation of stems and the growth of the whole plant; sprayed on a dwarf pea it grows tall, and on grapes it produces larger, seedless fruit. They break the dormancy of seeds and buds and are used in the malting of barley. Cytokinins promote cell division and are found where cells are dividing rapidly, in fruits and seeds and the tips of roots; they delay the ageing of leaves and are used to keep cut flowers and vegetables fresh. Abscisic acid is the hormone of stress and dormancy: it inhibits growth, closes the stomata during drought, promotes the dormancy of buds and seeds, and causes the shedding (abscission) of leaves and fruit; it is a growth inhibitor. Ethylene is a gas produced by ripening fruit; it hastens ripening, which is why a ripe banana placed among green ones ripens them, and why fruits are ripened commercially in ethylene chambers; it also promotes leaf fall and flowering in pineapple.

Plant hormones are thus the plant's entire coordinating system, doing by chemistry alone what animals do with nerves and hormones together, and their use in agriculture, in rooting cuttings, ripening fruit, killing weeds and producing seedless grapes, is one of the practical rewards of this chapter.