Summary

This chapter has traced how the human nervous system detects, processes and responds to information. The neuron, in its various forms (multipolar, bipolar and unipolar), is the basic signalling unit; it generates a nerve impulse by briefly reversing the resting potential of its membrane (depolarization followed by repolarization) and conducts that impulse along its axon, far more rapidly in myelinated fibres through saltatory conduction than in unmyelinated ones. Impulses cross from one neuron to the next, or to an effector, at a synapse — usually a chemical synapse, in which a neurotransmitter is released from the presynaptic terminal and acts on receptors on the postsynaptic membrane, giving transmission its characteristic one-way direction.

At the organisational level, the central nervous system (brain and spinal cord) integrates information relayed to it by the peripheral nervous system (twelve pairs of cranial nerves and thirty-one pairs of spinal nerves). Within the brain, the forebrain (cerebrum with its cortex, thalamus, hypothalamus and limbic system) is responsible for voluntary movement, sensation, higher mental functions, homeostatic regulation and emotion; the midbrain relays visual and auditory reflexes; and the hindbrain (pons, medulla and cerebellum) regulates vital automatic functions such as breathing and cardiovascular reflexes and coordinates movement and balance. The spinal cord both conducts signals to and from the brain and independently organises simple reflexes.

The autonomic nervous system, through its antagonistic sympathetic ('fight or flight') and parasympathetic ('rest and digest') divisions, keeps the body's internal organs working in balance without conscious effort. Reflex action — a fast, automatic response carried by the fixed pathway of a reflex arc (receptor, sensory neuron, CNS relay, motor neuron, effector) — allows the body to react to certain stimuli before the brain has consciously processed them, as illustrated by the monosynaptic knee-jerk reflex, and reflexes may be innate (simple) or learned (conditioned). …