The nervous system

Nervous systems range from the simple nerve nets of jellyfish and sea anemones, which have no brain and relatively few interconnections, to the nervous system of humans, with brains of staggering complexity. The human brain contains many millions of cells, each of which may communicate with thousands of other nerve cells. Their interconnections form circuits which enable us to control our muscles, think, remember, and even study our own brains.

All the various animal nervous systems are fast-acting communication systems containing nerve cells, neurones,which convey information in the form of nerve impulses (electrochemical changes). Neurones take various forms but each has a cell body,containing a nucleus, and nerve fibres,long extensions that transmit nerve impulses rapidly from one part of the body to another. Fibres carrying impulses away from the cell body are called axons;those carrying impulses towards the cell body are called dendrons.Apart from the main nerve fibre, there may be small dendrons (dendrites)extending from the cell body.

In mammals, sensory neuronescarry messages from peripheral sense organs to a central nervous system(CNS) consisting of the brain and spinal cord. The CNS acts as an integration centre and processes information from many sources. Motor neuronesconvey instructions from the CNS to effector organs (mainly muscles and glands).

A mammalian motor neurone can convey information rapidly over considerable distances; for example, a single nerve impulse may be transmitted from the spinal cord to the feet in a few milliseconds. These fast-conducting neurones are enclosed along most of their length by a thick insulating material called the myelin sheath. The myelin sheath is produced by special supporting cells called Schwann cells. The sheath is essentially a series of cell membranes, each produced by a Schwann cell and wrapped many times around the axon. Gaps between the membranes of each Schwann cell, called the nodesof Ranvier, are the key to the fast transmission of nerve impulses.

Fast transmission enables mammals to respond almost instantaneously to stimuli. Nerve impulses can be directed along the nerve fibres to specific points in the body so that responses can be very localised.