Organisation of the nervous system

Two divisions of the nervous system

The nervous system contains of the order of 86 billion neurones. It is classified by a series of divisions, and each division is based on a single criterion.

The first division is anatomical. The central nervous system, or CNS, is the brain and the spinal cord: the coordinating region, containing the relay neurones. The peripheral nervous system, or PNS, is everything else: the sensory neurones carrying impulses from receptors into the CNS, and the motor neurones carrying them out to effectors. A nerve is not a neurone: it is a bundle of the axons and dendrons of many neurones wrapped in connective tissue.

Describe the drawing

A tree diagram in three levels. The nervous system at the top splits into the central nervous system, the brain and spinal cord, and the peripheral nervous system. Only the peripheral side branches again, into somatic, serving skeletal muscle voluntarily, and autonomic, serving glands, smooth muscle and cardiac muscle involuntarily. The autonomic box splits once more into sympathetic and parasympathetic, and three lines beneath show them acting oppositely on heart rate, pupils and the gut.

The second division separates the motor side of the PNS by whether its responses are under conscious control. The somatic nervous system carries impulses to skeletal muscle, and its responses are voluntary or reflex: walking, writing, pulling your hand off a pan. The autonomic nervous system carries impulses to glands, to the smooth muscle of the gut and blood vessels and to cardiac muscle, and its responses are not under conscious control. The secretion of gastric juice, for example, is regulated without any voluntary input.

The autonomic system then splits once more, into two systems that innervate the same organs and pull in opposite directions. The sympathetic system readies the body for action, and the parasympathetic system runs it at rest, which the old tags 'fight or flight' and 'rest and digest' summarise fairly. Most of the time both are active at once, and the state of an organ reflects the balance between them, which is the same antagonistic logic as insulin and glucagon.

Central nervous system

The brain and the spinal cord: the coordinating part of the nervous system, containing the relay neurones.

Peripheral nervous system

The neurones outside the brain and spinal cord, carrying impulses into and out of the CNS.

Somatic nervous system

The part of the peripheral system carrying impulses to skeletal muscle, under voluntary control.

Autonomic nervous system

The part of the peripheral system carrying impulses to glands, smooth muscle and cardiac muscle, not under conscious control.

The pupil: two muscles in the iris

The pupil is the opening in the middle of the iris, and its width sets how much light reaches the retina. The muscles that change it are effectors under autonomic control.

The iris contains two sets of smooth muscle set at right angles to each other. The circular muscles form rings round the edge of the pupil, so when they contract the ring tightens and the pupil narrows. The radial muscles run outwards from the edge of the pupil like the spokes of a wheel, so when they contract they pull that edge outwards and the pupil widens. A muscle can only pull, so each set undoes the work of the other: they are antagonistic, and when one set contracts the other relaxes.

Describe the drawing

Two front views of the iris side by side. On the left, in bright light, the ring of circular muscle around the pupil is drawn thick because it has contracted, the spokes of radial muscle are thin and relaxed, and the pupil is small; the labels say parasympathetic nerves and pupil constricts. On the right, in dim light, the radial muscle spokes are thick and contracted, the circular muscle is thin and relaxed, and the pupil is wide; the labels say sympathetic nerves and pupil dilates.

In bright light the pupil constricts. Light on the rods and cones produces impulses in the optic nerve, which pass to a coordinating centre in the midbrain. Impulses return along parasympathetic motor neurones to the iris, the circular muscles contract, the radial muscles relax, and the pupil narrows. Less light enters, which protects the rods and cones from damage by intense light. In the five-stage sequence, the stimulus is the increase in light intensity, the receptors are the rods and cones, the coordinator is the brain, the effectors are the iris muscles and the response is the narrowing of the pupil.

In dim light the pupil dilates. Parasympathetic impulses to the circular muscles fall away, sympathetic impulses make the radial muscles contract, and the circular muscles relax and are stretched. The wider pupil lets more light reach the retina. The same sympathetic pathway dilates the pupils in fear or excitement, so the pupils sit in the sympathetic row of the table above.

This is a reflex: it is automatic and cannot be overridden by choice. The coordinating centre sends impulses to both irises, so shining a light into one eye makes both pupils constrict.

Circular muscles

Rings of smooth muscle round the pupil in the iris; their contraction constricts the pupil.

Radial muscles

Smooth muscle fibres running outwards from the pupil in the iris; their contraction dilates the pupil.

Pupil reflex

The automatic change in pupil diameter with light intensity: constriction in bright light, dilation in dim light.

The brain, region by region

Gross structure means what is visible on a cut brain without a microscope. Five regions are named at A level, each with a distinct function.

Describe the drawing

A side view of the brain drawn as five colour-coded regions with a matching legend. A large cyan circle is the cerebrum; an amber circle below and behind it is the cerebellum; a coral column running down from the underside of the cerebrum, between the pituitary and the cerebellum, is the medulla oblongata, continuing downwards as the spinal cord; a small white disc at the base of the cerebrum is the hypothalamus, with the grey pituitary gland hanging beneath it on a stalk, in front of the medulla. The legend gives each region one job: thought and movement, balance and coordination, heart rate and breathing, monitoring the blood, and the hormone link.

The cerebrum is the great folded pair of hemispheres that makes up most of the brain's mass. Its outer few millimetres, the cerebral cortex, handle conscious thought, language, memory, learning and the initiation of voluntary movement: everything the somatic system does on purpose starts here.

The cerebellum sits below and behind it, and coordinates rather than commands. It takes in a stream of information from the organs of balance in the inner ear and from stretch receptors in muscles and tendons, and adjusts the timing and strength of contractions so that movement is smooth. None of this is conscious: the cerebrum initiates a movement and the cerebellum coordinates its execution.

The medulla oblongata is the swollen top of the spinal cord, and it holds the regulatory centres of the autonomic system: the cardiovascular centre that sets heart rate and blood vessel diameter, and the ventilation centre that sets breathing rhythm. Damage to the medulla is rapidly fatal because these centres are not duplicated elsewhere.

The hypothalamus monitors the composition of the blood. It holds the osmoreceptors used in osmoregulation and the thermoregulatory centre, and it manufactures ADH. Osmoregulation and thermoregulation are taught in the hormones unit. The pituitary gland hangs beneath it on a stalk and is its link to the endocrine system. The posterior lobe stores and releases the hormones the hypothalamus makes. The anterior lobe secretes its own under hypothalamic control. Together these two structures link nervous and endocrine coordination.

Cerebrum

The paired, folded hemispheres responsible for conscious thought, learning, memory and the initiation of voluntary movement.

Cerebellum

The region below and behind the cerebrum that coordinates balance and the timing of movement, without conscious involvement.

Medulla oblongata

The base of the brainstem, containing the autonomic centres controlling heart rate, vessel diameter and breathing.

Hypothalamus

The region beneath the thalamus that monitors the blood, controls thermoregulation and osmoregulation, and directs the pituitary.

Pituitary gland

The endocrine gland hanging beneath the hypothalamus, whose two lobes release hormones under its control.

Three neurones, told apart by structure

Sensory, relay and motor neurones are usually named by their function in a reflex arc. They also differ in structure, and OCR A requires each type to be identified from a diagram. Two terms carry the distinction. A dendron is a fibre carrying impulses towards a cell body, and an axon carries them away from it; dendrites are the fine branches that feed a dendron or the cell body itself.

Describe the drawing

Three neurones drawn one above another with the impulse running left to right in each. The sensory neurone at the top has a long dendron from the receptor, a cell body raised on a short stalk in a ganglion two thirds of the way along, and a short axon into the CNS. The relay neurone in the middle is short, with its cell body on the fibre and processes radiating from it. The motor neurone at the bottom starts at a cell body ringed with dendrites in the CNS and runs one long axon out to the effector.

A motor neurone has its cell body in the CNS, bearing many short dendrites that collect input from other neurones. One long axon then runs all the way out to the effector, a metre of it if the effector is in the foot. Cell body first and long axon after is the identifying arrangement.

A sensory neurone is arranged differently. Its cell body sits neither at the start nor at the end but on a short side branch part-way along, housed with thousands of its neighbours in a ganglion just outside the spinal cord. The long fibre from the receptor is therefore a dendron, because the impulse in it is travelling towards the cell body, and only the short fibre from the cell body into the CNS is an axon. A neurone drawn with its cell body on a side branch of the fibre is therefore sensory.

A relay neurone, also called an intermediate neurone, lies entirely within the CNS, and it is small: a cell body with many short processes and no long fibre, because nothing it connects to is far away. It links sensory pathways to motor pathways within the CNS, which is its position in a reflex arc.

Dendron

A fibre of a neurone that carries impulses towards the cell body.

Axon

A fibre of a neurone that carries impulses away from the cell body.

Ganglion

A swelling containing the cell bodies of many neurones, outside the central nervous system.

The spinal cord in cross-section

The spinal cord runs from the medulla oblongata down a canal through the vertebrae, which protect it, and a cross-section cut at any level shows the same arrangement.

Describe the drawing

A cross-section of the spinal cord with the dorsal side at the top. An oval of white matter surrounds a butterfly-shaped core of grey matter, with the small central canal at its middle. On the right a dorsal root leaves the top of the grey matter and swells into the dorsal root ganglion outside the cord, and a ventral root leaves the lower part; the two join as the spinal nerve. A reflex arc is drawn over it: a cyan sensory neurone runs in along the nerve and dorsal root, with its cell body in the ganglion, an amber relay neurone crosses the grey matter, and a coral motor neurone with its cell body in the ventral horn runs out through the ventral root.

The centre is a region of grey matter shaped roughly like a butterfly or a letter H. It is made of neurone cell bodies, dendrites, relay neurones and the synapses between them, with few myelinated fibres. Its two upper projections, on the back of the body, are the dorsal horns; its two broader lower projections are the ventral horns. Running down the middle of it is a narrow central canal containing cerebrospinal fluid.

Round the grey matter is white matter, made of myelinated axons running up and down the cord, carrying impulses to and from the brain. The myelin is rich in lipid, which gives this region its pale colour. Grey matter is where neurones synapse and inputs are combined; white matter is where impulses are conducted over long distances, so myelin is concentrated there.

At each level a spinal nerve leaves the cord on each side, 31 pairs in a human, and each nerve joins the cord by two roots. The dorsal root, at the back, carries sensory neurones in, and it bears a swelling, the dorsal root ganglion, which holds their cell bodies. The ventral root, at the front, carries motor neurones out; their cell bodies are in the ventral horn of the grey matter. The two roots join just outside the cord, so every spinal nerve carries both sensory and motor fibres.

A withdrawal reflex follows this anatomy. Impulses arrive along the sensory neurone, pass its cell body in the dorsal root ganglion, and enter the grey matter by the dorsal root. There the sensory neurone synapses with a relay neurone, which synapses with a motor neurone in the ventral horn, and impulses leave by the ventral root to the effector. Other branches carry impulses up the white matter to the brain, which is how the stimulus is perceived a moment after the response has begun.

Grey matter

The central region of the spinal cord, containing cell bodies, dendrites, relay neurones and synapses.

White matter

The outer region of the spinal cord, made of myelinated axons carrying impulses up and down the cord.

Central canal

The narrow channel through the centre of the grey matter of the spinal cord, containing cerebrospinal fluid.

Dorsal root

The root by which sensory neurones enter the spinal cord, at the back, bearing the dorsal root ganglion.

Ventral root

The root by which motor neurones leave the spinal cord, at the front.

Three kinds of muscle and the division that controls each

An effector is a muscle or a gland, and the muscle category covers three distinct tissues. Skeletal muscle is covered in more detail in the muscles and movement lesson.

Skeletal muscle is the tissue attached to bone: long multinucleate fibres with the regular striations the sliding filament lesson explains. It contracts quickly and powerfully, fatigues with use, and takes orders from the somatic system. Involuntary muscle, also called smooth muscle, lines the gut, the blood vessels, the airways and the iris. Its small spindle-shaped cells have one nucleus and no striations, because the filaments are not in register. It contracts slowly and steadily without tiring, under autonomic control. This tissue carries out peristalsis, the wave of contraction that moves food along the gut.

Cardiac muscle differs from both. Its cells are striated like skeletal muscle but short, branched and joined end to end at intercalated discs, so the tissue contracts as a connected sheet. It is myogenic: the beat starts in the muscle itself, at the sinoatrial node, and the autonomic system only adjusts the rate, sympathetic impulses raising it and parasympathetic impulses lowering it, under the control of the medulla. Cardiac muscle does not fatigue, which allows contraction to continue throughout life.