Coordination

1. What This Chapter Covers

Sharpening a pencil, grasping a door knob, walking, running, driving — every one of these involves well coordinated movements made with well balanced postures. Even standing upright is a challenge of balancing on two feet with a narrow base, and yet we do it while walking and running as well.

No function is carried out by one system alone. In movement we never use just the skeletal or just the muscular system, and even within the muscular system several muscles work in a systematic manner.

All our body parts, and the tissues and cells in them, work by picking up signals of change — stimuli — from their surroundings and then responding. That is why we move to the side of the road when we hear or see a car approaching.

The index allots this chapter 10 periods in September and runs it from page 100 to page 122.

2. Responding to Stimuli (Textbook 5.1)

All living organisms respond to stimuli. A cat runs because it saw a mouse; plants grow towards sunlight; we sweat when the surroundings are hot and humid. The ability to react to a particular stimulus in a particular situation is of great importance in ensuring survival.

The book breaks a response into four stages in sequence: detecting a change in the outer or inner environment, transmitting the information, processing it, and executing the appropriate action.

Activity 1 — holding a falling stick

Take a scale or stick at least half a metre long. Have a friend hold it near the top with the lower end hanging between your open thumb and forefinger, leaving a gap of about a centimetre. Mark the scale in pencil at the level of your fingers.

When your friend lets go, try to catch it, then mark where you actually caught it. How much higher that second mark is measures the delay of your whole detect-transmit-process-act chain.

Responses are brought about by rapid changes in muscles, and the speed of a response tells you there is an efficient communication system linking the parts that perceive the stimulus to the parts that act.

3. How Nervous Coordination Was Worked Out (Textbook 5.2)

The Greeks believed all body functions were controlled by the brain, because damage to it produced remarkable changes in behaviour, but they had little idea how.

Galen, a Greek physiologist of A.D. 129-200, made one notable observation. A patient who had suffered a blow to the neck falling from his chariot complained of loss of feeling in the arm while still retaining normal muscular control of it. Galen concluded that nerves were of two kinds, one related to sensation and one to action, and that the blow had damaged the sensory nerves only.

How nerves work as integrating systems was little known until the late 18th century, when physiologists connected recent work on electricity with existing proposals about the nervous system and found how signals were transmitted. The book is honest that even now we know the pathways better than the working mechanism of the nerve cell itself.

4. Structure of a Nerve Cell (Textbook 5.3)

Activity 2 asks you to observe a permanent slide of a neuron under the microscope and draw and label its parts.

Each nerve cell has a cell body, the cyton, with a prominent nucleus. Two kinds of fine projections extend from it: the short dendrites, and one long axon that extends to different parts of the body.

The axon is surrounded by a specialized insulating sheath, the myelin sheath, which is interrupted at regular intervals at the nodes of Ranvier. The sheath is made of Schwann cells and consists chiefly of fatty material, and it separates adjacent axons.

Axons with a sheath are myelinated; those without are non-myelinated. Most are non-myelinated.

The nerve cell body lies in the brain, the spinal cord, or the dorsal or ventral root ganglion of the spinal cord. Inside the brain or spinal cord it is hard to tell a dendrite from an axon by length alone; the presence of the sheath is what distinguishes them.

The nerve cell is the structural and functional unit of the nervous system, and ours contains more than 100 billion of them.

Synapse

A synapse is the functional region of contact between two neurons, where information from one is relayed to another. Dendrites of one nerve cell connect to the axons of the next through it.

These are regions of minute gaps, and neurons have essentially no protoplasmic connection between them, yet information passes across in the form of chemical or electrical signals or both.

Synapses are mainly found in the brain, the spinal cord and around the spinal cord. Beyond those areas, the axon carries signals from brain and spinal cord to the relevant part of the body.

5. From Stimulus to Response (Textbook 5.4)

On the basis of the pathways they follow, nerves fall into three types.

In Activity 1 the eye was the detector and the finger muscles the effector, and the coordination between them ran along this chain.

Reflexes

Some situations demand a response we cannot control, and those are reflexes.

Activity 3 — knee jerk. Sit with the legs crossed so the upper leg hangs freely. Strike the area below the knee cap sharply with a rubber hammer while firmly grasping the front of the thigh with the other hand, and watch the shape of the thigh muscles change.

Although we are fully conscious, we cannot prevent the thigh muscle from contracting. That makes it an involuntary reflex — and yet the same thigh muscle operates voluntarily when we kick a football.

The knee jerk was first noted in 1875, and at first it was doubted whether a nervous reflex was involved at all. It was settled by showing that in an anaesthetized monkey whose spinal nerves to the limb had been cut, the knee jerk would not occur. Clearly a nerve pathway was involved.

Involuntary actions that must happen in very short intervals take a short pathway that does not go up to the brain; voluntary pathways are longer and pass through the brain.

6. The Reflex Arc (Textbook 5.5)

The reflex was not understood in terms of pathways until the end of the nineteenth century. A single pathway running from detectors up to the spinal cord and back out to effectors is a reflex arc.

If you accidentally step on a sharp surface, several such arcs operate together to make the leg muscles withdraw the foot. The book's figure labels the chain: detector, sensory nerve, spinal cord with its interneuron, motor nerve, effector muscle.

The same effectors can also be driven by the conscious mind. In a football game the leg muscles operate both by reflex and voluntarily, and the book notes the everyday consequence — if you start thinking about where your feet are going while running upstairs, you often stumble.

Nerve transmission from stimulus to response can occur at a maximum speed of about 100 metres per second.

The chapter is built on one comparison. Anything needed in a fraction of a second goes by nerve; anything that has to be sustained goes by hormone; and the autonomous system and adrenalin show the two working on the same organs at once.

7. The Central Nervous System (Textbook 5.6, 5.7)

The central nervous system includes the brain and spinal cord, and it coordinates all neural functions.

The brain

The human brain is quite large compared with body weight. It sits in a hard bony box, the cranium, and is covered by three layers, the meninges, which cover the spinal cord as well. The space between the layers holds cerebro-spinal fluid, which acts as a shock-absorbing medium.

Nerve cell bodies together with capillaries form a mass called grey matter, while myelinated axons form white matter. In the brain the grey matter is usually on the periphery and the white matter towards the centre.

An adult brain weighs approximately 1400 g. Though it is a little more than 2 per cent of body weight, it uses 20 per cent of the whole body's energy. An average adult male brain weighs about 1375 g and a female brain about 1275 g.

The three divisions and their functions

Forebrain — olfactory lobes. Club-shaped, widely spread bodies visible only from the ventral surface, concerned with the sense of smell.

Forebrain — cerebrum. Two lobes, the cerebral hemispheres. Its surface has many folds appearing as elevations, gyri, and depressions, sulci; the prominent sulci divide each hemisphere into four lobes, and the corpus callosum connects the two hemispheres. It is the seat of mental abilities, controlling thinking, memory, reasoning, perception, emotions and speech, and it interprets sensations and responds to cold, heat, pain and pressure.

Forebrain — diencephalon. A rhomboid lobe visible from the inferior surface, lying between cerebrum and midbrain, divided into thalamus and hypothalamus. It is the relay centre for sensory impulses such as pain, anger and happiness; a reflex centre for muscular activities; a centre for certain emotions; and the centre for water balance, blood pressure, body temperature, sleep and hunger. The hypothalamus is the master control centre of the endocrine system and controls the pituitary gland.

Midbrain — optic lobes. A small, thick, stalked portion connecting the forebrain with the cerebellum and pons of the hindbrain. It relays motor impulses from the cerebral cortex to the spinal cord and sensory impulses from the spinal cord to the thalamus, and shows reflexes for sight and hearing.

Hindbrain — cerebellum. Below the cerebrum and above the medulla oblongata, with two large hemispheres. It maintains posture, equilibrium and muscle tone, and coordinates the voluntary movements initiated by the cerebrum.

Hindbrain — medulla oblongata. Almost triangular, extending from the pons to the spinal cord. It contains centres for cardiac, respiratory, blood pressure and vasomotor activities — vasomotor meaning actions on a blood vessel that alter its diameter — and coordinates reflexes like swallowing, coughing, sneezing and vomiting.

8. The Spinal Cord (Textbook 5.8)

The spinal cord extends from the back of the medulla oblongata to the lumbar region, running through the neural canal of the vertebral column, and is almost cylindrical.

Unlike the brain, in the spinal cord the white matter is towards the periphery and the grey matter towards the centre. Myelinated axons leave the cord from both sides of the vertebral column.

The Greek idea of complete control by the brain turned out to be wrong. Leonardo da Vinci (1452-1519) and Stephen Hales (1677-1771) both recorded that frogs whose brains had been removed survived and still produced muscular movements when the skin was pinched or pricked — but that the animal died as soon as the spinal cord was damaged by pushing a needle down it.

That suggested the spinal cord is not only a pathway for instructions from the brain but a control centre in its own right.

9. The Peripheral Nervous System (Textbook 5.9)

Nerves attached to the spinal cord have two types of root: one to the back, the dorsal side, and one to the front, the ventral side.

Charles Bell in Scotland and Francois Magendie in France showed in the early nineteenth century that these roots have different functions. Cutting the dorsal roots of an experimental animal produced no obvious reaction; touching the ventral roots even lightly made the muscles they served twitch violently.

In 1822 they proposed that the dorsal root carries messages of sensation inwards while the ventral pathway carries instructions for muscular contraction outwards.

The peripheral nervous system is the vast system of dorsal and ventral root nerve cell heads together with the network of spinal and cranial nerves, linked to the brain and spinal cord at one end and the muscles at the other.

There are 12 pairs of cranial nerves originating from the brain and 31 pairs of spinal nerves from the spinal cord.

10. The Autonomous Nervous System (Textbook 5.10)

The PNS involuntarily controls several functions of the internal organs, blood vessels, and smooth and cardiac muscles, and that part of it is the autonomous nervous system. It also has voluntary control of some skin muscles and the skeletal muscle.

The everyday example is the pupil. Entering a dark room, we cannot see immediately; then the pupil widens, letting in more light. Coming out into daylight, the diameter decreases so less light falls on the retina. Both happen under autonomous control.

Ganglia near the vertebral column are connected to the spinal cord by nerves. The sympathetic system is formed by the chain of ganglia on either side of the vertebral column and their associated nerves. The parasympathetic system is formed by nerves arising from the ganglia of the brain and the posterior part of the spinal cord. Together they make the autonomous nervous system, considered a part of the PNS.

Besides the CNS and PNS there is a system of neurons in the digestive tract that can function independently of either. It is called the enteric nervous system, or second brain.

11. Coordination Without Nerves (Textbook 5.11)

The story of insulin

In 1868, Paul Langerhans, Professor of Pathology at the University of Freiburg, working on the structure of the pancreas, noticed patches of cells quite different in appearance from the normal tissue and richly supplied with blood vessels. They were named the Islets of Langerhans — islets meaning islands — but their function stayed unknown.

Others found that removing the pancreas from an experimental animal produced a disease like the human ailment sugar diabetes, in which free sugar in the blood and urine is high. The cause in man was unknown, but the evidence pointed to the pancreas.

The decisive step came when it was found that tying up the pancreatic duct where it emerges from the duodenum caused the pancreas to degenerate while the Islets of Langerhans remained normal — and an animal treated that way did not develop diabetes. That was strong evidence linking blood sugar to the islet cells.

By 1912 workers were convinced the islets produced a secretion liberated directly into the blood. From the Latin insula, island, the name insulin was coined, even though no method of isolating it had been found.

Ten years later in Toronto, Banting, Best and Macleod succeeded in extracting insulin from degenerated animal pancreases whose ducts had been tied. Given by intravenous injection to a dog without a pancreas, it kept the dog alive and healthy with a low blood sugar level.

Hormones (5.11.2)

In 1905 the English physiologist Starling coined the term hormone, from the Greek hormo, to impel, for substances secreted into the blood that control events elsewhere in the body.

The glands that secrete them were called ductless glands, because they have no tube to carry their products away — these pass straight into the blood. That is what distinguishes them from glands like the liver and pancreas, whose secretions pass down ducts to other organs. The whole system of ductless, or endocrine, glands is the endocrine system.

Glands do not produce hormones at a steady rate; the adrenal gland, for instance, normally has a low output.

The book's illustration is fear. Faced with a dog, the heart rate increases, breathing becomes faster, blood pressure rises, the hair becomes erect and we get goose bumps; less obviously the pupils dilate, the skin becomes more sensitive, and rarely the bladder and rectum may be emptied. All of this is carried out under the influence of adrenalin, from the adrenal gland.

The general rule the book draws from this: the various actions of the body are controlled by hormones and coordinated by the nervous system.

The endocrine glands

12. Feedback Mechanism (Textbook 5.12)

A feedback mechanism is a loop in which a product controls its own production, and the production of several hormones is controlled this way.

The pituitary hormone prolactin stimulates the mammary glands to produce milk; as the baby sucks, more prolactin is produced, enhancing milk production.

When blood glucose rises above normal, certain pancreatic cells respond by producing more insulin, which lowers blood glucose; once the level returns to normal, insulin secretion is automatically reduced.

Hormones must therefore be secreted in precise quantities, and the timing and amount are controlled by this inbuilt feedback. The chapter's closing caution is that none of the systems, nervous or endocrine, is totally exclusive of the other.

13. Control Mechanisms in Plants (Textbook 5.13)

Activity 4. Touch the leaves of Mimosa pudica, the touch-me-not, and watch them fold.

Mimosa pudica leaves have pad-like swellings at the base called pulvini, whose cells hold a lot of water and have large intercellular spaces. Water pressure in the pulvinus holds the leaf erect. Touch generates an electrical impulse that acts on a plant hormone, and water in the pulvini cells nearer the leaf vein migrates to the other side. The pulvinus loses its firmness and the leaf folds. After 20 to 30 minutes the water returns, firmness is restored and the leaf becomes erect. This response to touch is thigmonasty.

Both plants and animals respond to stimuli, but not in the same way. Higher animals have a nervous system and an endocrine system; plants have no well-defined nervous or endocrine system, and control instead by chemicals.

Plants sense light, heat, water, touch, pressure, chemicals and gravity. The hormones that handle these are phytohormones, and because they coordinate the plant's activities usually by controlling some aspect of growth, they are also called growth regulating substances.

Activity 5 — watching a seedling turn

Fill a glass jar with soil and sow a bean seed near the wall so you can watch root and shoot. After four or five days it germinates; keep the jar in the sun and observe. Then tilt the jar and lay the plant horizontally, and watch the direction of root and shoot growth for over a week.

The explanation the book gives is direct: more auxin collects on the shaded side of the stem, so cells on that side grow faster, while cells on the lit side grow slowly, which bends the stem. Cutting transverse sections of a bent and a straight stem and comparing the shape of the epidermal cells under a microscope shows this.

Darwin and Went

Charles Darwin and his son Francis covered the tip of a coleoptile with a cylinder of metal foil and lit the plant from the side. The characteristic bending did not occur. When light was allowed to penetrate the cylinder, bending occurred normally. They concluded that when seedlings are exposed to lateral light, some "influence" is transmitted from the upper to the lower part, causing the bend.

In 1926 the Dutch plant physiologist F.W. Went separated that influence from the plant. He cut coleoptile tips from oat seedlings, placed them on a slice of agar for about an hour, then cut the agar into small blocks and placed a block on one side of a decapitated seedling's stump, keeping everything in the dark throughout.

Within an hour he saw distinct bending away from the side on which the block was placed. An agar block that had never touched a coleoptile tip produced no bending, or only slight bending towards that side.

Went read this as proof that the tip acts by a chemical stimulus rather than a physical one such as an electrical impulse. The chemical became known as auxin, from the Greek for "to increase", and it was the first plant hormone discovered.

14. Tropic and Nastic Movements (Textbook 5.14)

Movement of an individual plant part when it is subjected to an external stimulus is tropism, or tropic movement. Sometimes the direction of the stimulus determines the direction of movement; where it does not, the response is a nastic movement.

Phototropism. A creeper near a window bends its shoots towards sunlight.

Geotropism. Roots always grow downwards, responding positively to gravitational force.

Hydrotropism. Roots of a plant near a rock or wall all grow in one direction, away from the rock, towards where water is available in the soil.

Thigmotropism. In creepers like cucumber and bitter gourd the stem is weak and thin, so the plant cannot grow erect. Tendrils — thin thread-like growths on leaves or stems — grow towards a support and wind around it. That response to contact is thigmotropism.

Chemotropism. A ripe stigma secretes a sugary substance, which stimulates the pollen grain that lands on it. The pollen responds by germinating and producing a pollen tube that reaches the ovule for fertilization.

One general rule underlies several of these: unequal distribution of auxins affects root and stem growth, and a high concentration of auxin stimulates stem growth while inhibiting root growth.

Key words from the chapter

Response, stimuli, neuron, Schwann cell, axon, synapse, sensory nerves (afferent), motor nerves (efferent), association nerves, central nervous system, brain, spinal cord, cerebrospinal fluid, peripheral nervous system, insulin, endocrine glands, hormones, feedback mechanism, plant hormones, tropic movements, nastic movements.

15. Summary

The nervous system and the endocrine system are the two systems that control and coordinate the body's functions, and the responses of the nervous system can be classified as reflex, voluntary and involuntary actions.

The human nervous system is studied under two divisions, the central nervous system and the peripheral nervous system. The central system is the brain and the spinal cord; the peripheral system is further divided into the somatic and the autonomous nervous systems.

The autonomous system has two parts, sympathetic and parasympathetic, which cause physical reactions opposite to each other.

The nerve cell is the structural and functional unit of the nervous system, and the synapse is the gap across which signals pass from one neuron to the next.

Hormones produced in one part of the body move to another part to achieve the desired effect, and a feedback mechanism regulates their action so that the amount released matches what the body needs.

Directional movements in plants in response to specific stimuli such as light or chemicals are tropic movements.

Plant hormones are usually growth promoters or inhibitors. The promoters are auxins, cytokinins and gibberellins; the inhibitors are abscisic acid and ethylene.