Organisation of the nervous system

Two divisions of the nervous system

The nervous system contains of the order of 86 billion neurones and is classified by two divisions, each made 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.

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 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.

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. Next topic for both control systems: osmoregulation and thermoregulation 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, and the anterior lobe secretes its own under hypothalamic control. Together these two structures link nervous coordination to 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.

A motor neurone has its cell body in the CNS, wearing 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 at all, 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.

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. Next topic for the first of them in detail: muscles and movement.

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

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.