Neural Control and Coordination: Class 11 Biology · Chapter 18

The bus is already pulling away from the stop, so you sprint. Within seconds your leg muscles are burning fuel many times faster than at rest, and they need extra oxygen to do it. You breathe faster and more deeply, your heart rate climbs from about 72 to perhaps 150 beats a minute, and the arteries to your legs widen to carry more blood. Once you are seated on the bus, all of this winds down again over the next few minutes. No organ made these changes on its own. Muscles, lungs, heart, blood vessels and even the kidneys shifted together, by the right amount and at the right moment.

Biologists call such teamwork coordination (समन्वय): organs interacting so that the work of each supports the work of the rest. It is how the body holds its internal conditions steady, a state called homeostasis, while the demands on it keep changing.

Two systems share the job (Fig. 18.1). The neural system works like a telephone line: an electrical message runs along a chain of cells to one particular muscle or gland and arrives within milliseconds. The endocrine system works more like a radio broadcast: a gland releases a hormone into the blood, the hormone reaches almost every cell, and only cells with a matching receptor respond. Hormonal effects start more slowly but can last much longer. Together the two systems integrate the organs so that they work in step. This chapter follows the neural route: how a nerve impulse arises, how it travels and how it crosses from one cell to the next. Chapter 19 takes up the hormones.

18.1 Neural System

Every animal's neural system, from a jellyfish to a human, is built from neurons (तंत्रिका कोशिका). These cells are specialised for three tasks: picking up stimuli, receiving signals from other cells and passing signals on. What changes from one animal group to the next is how the neurons are arranged (Fig. 18.2).

In Hydra, a tiny freshwater cnidarian, neurons form a loose network spread through the body wall. There is no brain and no command centre: a touch on one tentacle sends signals spreading out in all directions through the net, and the whole animal contracts. Insects show a big step up. Their neurons gather into clusters called ganglia; a brain sits in the head, and a chain of ganglia joined by nerve cords and other neural tissue runs along the body. Vertebrates have the most developed arrangement: a large brain inside a protective skull, a spinal cord inside the backbone, and nerves reaching every part of the body.

18.2 Human Neural System

The human neural system has two parts. The brain and the spinal cord together make up the central neural system (CNS), the part that takes in information, makes sense of it and decides what the body should do. Everything else, the nerves that branch out from the brain and spinal cord to reach muscles, glands, skin and organs, forms the peripheral neural system (PNS). Picture a city's traffic-control room and the cables running to every signal and camera: the room decides, the cables carry (the comparison below).

Nerve fibres of the PNS carry traffic in two directions. Afferent fibres (अभिवाही) bring reports in to the CNS: the pressure of a pen on your fingertip, or the stretch of a full stomach. Efferent fibres (अपवाही) carry commands out from the CNS to the muscles, glands and organs that must respond. A handy memory aid: afferent arrive, efferent exit (Fig. 18.3).

The PNS is also divided by where its outgoing impulses go. Messages bound for the skeletal muscles, the ones you move at will when you write or kick a ball, travel through the somatic neural system. The autonomic neural system serves the parts you cannot move at will: the involuntary organs and the smooth muscles of the heart, gut, bladder and blood vessels. It has two divisions, the sympathetic and the parasympathetic neural systems, which often act on the same organ in opposite directions: the sympathetic side speeds up the heart, for example, and the parasympathetic side slows it (Table 18.2).

Some PNS terms describe the organs served rather than the direction of travel. The nerves, ganglia and plexuses (meshworks of interlacing nerves) that link the CNS with the viscera, the organs of the chest and abdomen, are together called the visceral nervous system. Its traffic runs both ways: commands out to organs such as the stomach and the bladder, and reports back from them.11PNS nerves leave the CNS at fixed points: in humans, 12 pairs of cranial nerves arise from the brain and 31 pairs of spinal nerves from the spinal cord.

18.3 Neuron as Structural and Functional Unit of Neural System

A neuron is far too small to see without a microscope, yet it is built to a clear plan with three main parts: a cell body, several dendrites and one axon (Fig. 18.4).

Start with the cell body, the neuron's control centre. Besides the nucleus it has all the ordinary organelles of a cell, and when stained it shows dark clumps called Nissl's granules: clusters of ribosomes and rough endoplasmic reticulum, the neuron's protein factories.22They are named after the German neuroscientist Franz Nissl, who devised the stain that shows them. Sprouting from the cell body are the dendrites, a bushy tangle of short, much-branched fibres. Think of them as the neuron's antennae: they collect signals and pass them inwards to the cell body, and like the cell body they contain Nissl's granules.

Signals leave along the axon, the neuron's output cable: a single long fibre that, in a nerve to the foot, can stretch for about a metre. Only at its far (distal) end does it fan out into branches. Each branch swells at its tip into a synaptic knob, stocked with synaptic vesicles, tiny membrane sacs of chemical messengers called neurotransmitters. Here the signal is handed on, to another neuron across a synapse or to a muscle fibre at a neuromuscular junction. Unlike the cell body and the dendrites, the axon has no Nissl's granules.

Count the fibres leaving the cell body and you can sort any neuron into one of three types (Fig. 18.5). Several fibres, one of them an axon and the rest dendrites, make a multipolar neuron, the common type in the cerebral cortex. Exactly two, one dendrite and one axon, make a bipolar neuron, found for example in the retina of the eye. A single fibre, the axon alone, makes a unipolar neuron; such neurons occur mainly at the embryonic stage.

Axons in the nerves of the body are looked after by Schwann cells, in one of two ways (Fig. 18.6). In a non-myelinated fibre, a Schwann cell simply cradles the axon in a fold of its membrane, with no wrapping. In a myelinated fibre, the Schwann cell rolls itself round and round the axon, like a mat rolled round a pole, and the many stacked layers of its membrane make an insulating myelin sheath. One Schwann cell covers only a short stretch, so a long axon carries a row of them with tiny bare gaps in between, the nodes of Ranvier. As for where they occur: myelinated fibres make up the spinal and cranial nerves, while non-myelinated fibres are common in the autonomic and somatic neural systems.

18.3.1 Generation and Conduction of Nerve Impulse

Why can a neuron fire at all? Because, even at rest, its membrane is polarised: positive charge lines its outer face and negative charge its inner face, a little like the two terminals of a battery. That stored charge difference is what makes neurons excitable cells. It exists because the ions on the two sides are unevenly spread, and the unevenness is kept up by the membrane's ion channels, protein pores that are selective about which ions they let through.

Measure the fluids on the two sides of a resting axon (Table 18.3) and they are strikingly different. The axoplasm holds about 28 times as much $\mathrm{K}{\phantom{X}}^{+}$ as the fluid outside, while the outside holds almost ten times as much $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$; large protein molecules carrying negative charge are trapped inside. The channels explain why the difference persists. At rest many $\mathrm{K}{\phantom{X}}^{+}$ channels stay open, so $\mathrm{K}{\phantom{X}}^{+}$ leaks across fairly easily; the $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ channels are shut, so almost no $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ gets through; and no channel admits the proteins at all. Each ion therefore sits on a concentration gradient, $\mathrm{K}{\phantom{X}}^{+}$ tending to leak out and $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ tending to push in (Fig. 18.7).

Leaks would slowly run these gradients down, so the cell spends energy restoring them. The sodium–potassium pump, a membrane protein powered by ATP, moves ions against their gradients: each cycle throws three $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ out of the cell and brings two $\mathrm{K}{\phantom{X}}^{+}$ in. Every cycle therefore exports one more positive charge than it imports. Add the $\mathrm{K}{\phantom{X}}^{+}$ that keeps leaking out while the negative proteins stay behind, and the result of leak and pump together is a steady charge separation: positive along the outside of the membrane, negative along the inside. The voltage across this resting membrane is the resting potential (विराम विभव), about $-70\mathrm{m}\mathrm{V}$ in a typical neuron. The minus sign says the inside is the negative side, by the convention of (18.1).

Now give the axon a small electric jolt, or let a sense organ excite it, at one spot, P (Fig. 18.8). The stimulus throws open the sodium channels there, and for a moment $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$ can pour through the membrane at P. Sodium has two reasons to rush in: there is far more of it outside, and the negative interior attracts it. In well under a millisecond so many positive ions have entered that the charges at P flip, positive inside and negative outside. This flip is depolarisation, and the reversed voltage across the membrane at P, the action potential, is the nerve impulse itself.

What carries the impulse forward? Compare P with its resting neighbour Q. Inside the axon P is now positive and Q negative; outside, Q is positive and P negative. Positive charge drifts from + towards −, so a little loop of current forms: forward along the inside, from P to Q, and back along the outside, from Q to P. That local current is enough to tip Q over: its sodium channels open, sodium floods in and Q fires. Q's current then fires R, R fires the next spot, and the impulse runs along the fibre like a line of falling dominoes, each knocked over by the one before. Because every site makes its own fresh action potential, the impulse reaches the end of even a metre-long axon undiminished.

Each site stays depolarised for only an instant. The sodium channels close again almost as soon as they open, and potassium channels open instead. $\mathrm{K}{\phantom{X}}^{+}$, plentiful inside, streams out, taking positive charge with it, and the inside swings back to negative: repolarisation. Within a millisecond or two the site is back at its resting potential and ready to fire again. That short recovery also explains one-way travel: the site the impulse has just left is still recovering, so the wave cannot double back.33This brief unresponsive spell is called the refractory period.

Myelin changes the picture. The sheath is a good insulator, so along a myelinated fibre ions can cross the membrane only at the nodes of Ranvier. The local current from one active node spreads rapidly under the sheath and depolarises the next node, and the impulse seems to leap from node to node. This saltatory conduction (Latin saltare, to leap) is much faster than the continuous, site-by-site conduction along a non-myelinated fibre. The fastest myelinated fibres in your body conduct at around $100\mathrm{m}\mathrm{/}\mathrm{s}$, while thin non-myelinated fibres manage about $1\mathrm{m}\mathrm{/}\mathrm{s}$. It also saves energy, because the pumps have fewer ions to put back. The time a signal needs follows (18.2).

18.3.2 Transmission of Impulses

Neurons do not join end to end like a wire; each is a separate cell. So when an impulse reaches the tip of an axon, it must be handed across a junction to the next cell. That junction, a synapse (अंतर्ग्रथन), has two sides: the pre-synaptic neuron, whose terminal brings the impulse, and the post-synaptic neuron, which receives it. Whether the two membranes practically touch or are held apart by a narrow gap, the synaptic cleft, decides how the hand-over works and gives us two kinds of synapse (Electrical and chemical synapses).

In an electrical synapse the gap almost vanishes. The two membranes are pressed so close that the local current of an arriving impulse simply flows on into the next neuron,44The current crosses through gap junctions, narrow protein channels that link the cytoplasm of the two cells. just as it would flow along the next stretch of a single axon. Nothing has to be released or recognised, so this is the fastest possible hand-over, quicker than any chemical synapse. Your neural system, however, uses it only rarely.

Most of your synapses are chemical synapses. Here the cleft is a real, fluid-filled gap, too wide for the local current to bridge, so a chemical messenger carries the signal across instead (Fig. 18.10):

1. The action potential reaches the axon terminal.
2. Synaptic vesicles are drawn to the inner face of the pre-synaptic membrane, merge with it and spill their neurotransmitter into the cleft.
3. The neurotransmitter molecules drift across the narrow cleft and lock onto receptors made for them on the post-synaptic membrane.
4. Each occupied receptor opens an ion channel, and ions pour into the post-synaptic neuron.
5. The ion flow changes the voltage of the receiving membrane. If the change pushes that neuron towards firing, it is excitatory; if it holds the neuron back, it is inhibitory.

Acetylcholine is one well-known neurotransmitter; there are many others. Two consequences of this design are worth remembering. Only the pre-synaptic side stores neurotransmitter and only the post-synaptic side carries the receptors, so a chemical synapse passes signals in one direction only. And release, diffusion and binding take time, so each chemical synapse adds a synaptic delay of roughly half a millisecond.

18.4 Central Neural System

Every decision you take, and thousands you never notice, passes through the brain, the body's central information processor and its command and control system. A quick inventory of its work:

• Conscious work: seeing, hearing, speaking, remembering, thinking and feeling emotions, and the intelligence that ties them together
• Movement: every voluntary action, and keeping the body balanced while it moves
• Housekeeping you never notice: running vital involuntary organs such as the heart, lungs and kidneys, holding body temperature steady, and signalling hunger and thirst
• Timing: the body's 24-hour (circadian) rhythms, such as the sleep–wake cycle
• Management: directing several endocrine glands, and shaping behaviour

Bone and membrane guard this soft organ. The skull is the hard outer case. Beneath it, three membranes called the cranial meninges (मस्तिष्कावरण) wrap the brain like the layers of a parcel (Fig. 18.11). Working inwards, the dura mater ('tough mother') is thick and strong; the arachnoid, named for its cobweb-like texture, is extremely thin; and the pia mater ('tender mother') is a delicate film that hugs the brain tissue, following every fold. From the brain outwards the initials spell PAD: pia, arachnoid, dura.

Looked at as a whole, the brain has three major regions, named for their order from front to back in the early embryo: the forebrain, the midbrain and the hindbrain (Fig. 18.12).

18.4.1 Forebrain

Three structures make up the forebrain: the cerebrum, the thalamus and the hypothalamus. The cerebrum dwarfs everything else in the human brain. Seen from above it has two great halves, the left and right cerebral hemispheres, separated by a deep groove that runs from the forehead to the back of the head. At the floor of that groove a broad bridge of nerve fibres, the corpus callosum, links the halves so that they can work as one (Fig. 18.13).

Each hemisphere has a skin of neurons a few millimetres thick, the cerebral cortex, wrinkled into ridges and grooves. The wrinkling packs a huge sheet into the limited room of the skull, the way a dupatta crumples to fit into a small bag. Crowded with neuron cell bodies, the cortex has a greyish tint and is called grey matter. Under it lies the white matter: bundles of nerve fibres whose fatty myelin sheaths make it look pale.

Map the cortex and you find three kinds of territory. Sensory areas receive what the sense organs report, and motor areas issue the commands that move muscles. The rest, a large share of the cortex, does neither job directly. These association areas do the higher work of combining input from several senses, storing memories and handling communication. When a familiar voice on the phone instantly brings your cousin's face to mind, association areas are linking what you hear with what you remember.

Buried under the hemispheres sits the thalamus, a relay hub where sensory and motor signals are coordinated on their way to and from the cortex. Just beneath it is a structure about the size of an almond with a huge range of duties, the hypothalamus. It is your thermostat, keeping body temperature steady; it makes you feel hungry or thirsty; and some of its neurons are neurosecretory cells that release the hypothalamic hormones (Chapter 19).55The hypothalamus also houses the body's master clock, a small cluster of neurons called the suprachiasmatic nucleus, which keeps the circadian rhythms in time with day and night.

Emotions have their own circuitry. The limbic system (or limbic lobe) is built from the innermost parts of the hemispheres together with deep structures linked to them, among them the amygdala and the hippocampus. In partnership with the hypothalamus it governs motivation and sexual behaviour and shapes how we express emotions: excitement before a cricket final, pleasure, rage, fear. It also takes part in the sense of smell and in autonomic responses.

18.4.2 Midbrain

Below the thalamus and hypothalamus, and above the pons, lies a short segment of brain, the midbrain. Down its core runs a narrow channel of cerebrospinal fluid, the cerebral aqueduct. Its back (dorsal) surface carries four round bumps, the corpora quadrigemina (literally 'quadruplet bodies'). Signals from the eyes, the ears and the skin meet and are combined here.66The upper pair of these lobes is linked mainly with vision and the lower pair with hearing.

18.4.3 Hindbrain

The hindbrain has three parts: the pons, the cerebellum and the medulla oblongata (usually just the medulla). The pons, whose name is Latin for 'bridge', is built mostly of bundles of nerve fibres that link one part of the brain with another. One of its centres, the pneumotaxic centre, also moderates the breathing rhythm set by the medulla, as you saw in Chapter 14.

The cerebellum sits behind the pons, under the back of the cerebrum. Its surface is folded even more finely than the cortex, which gives room for an enormous number of neurons: it makes up only about a tenth of the brain's mass yet holds roughly four out of every five of its neurons. The cerebellum combines information from the semicircular canals of the inner ear and from the auditory system, and keeps you balanced and your movements smooth, whether you are cycling through a crowded market or standing in a swaying local train.

Lowest of all, the medulla tapers into the spinal cord. Small as it is, life depends on it: its centres keep you breathing, adjust heart rate and blood pressure through cardiovascular reflexes, and control gastric secretion, which is why an injury here can be fatal. Taken together, the midbrain, the pons and the medulla form the brain stem, the stalk on which the rest of the brain sits and through which it is wired to the spinal cord.