Sensory These carry signals to the central nervous system (CNS) – the BRAIN and SPINAL CORD – from sensory receptors. These receptors respond to di?erent stimuli such as touch, pain, temperature, smells, sounds and light.
Motor These carry signals from the CNS to activate muscles or glands.
Interneurons These provide the interconnecting ‘electrical network’ within the CNS.
Structure Each neurone comprises a cell body, several branches called dendrites, and a single ?lamentous ?bre called an AXON. Axons may be anything from a few millimetres to a metre long; at their end are several branches acting as terminals through which electrochemical signals are sent to target cells, such as those of muscles, glands or the dendrites of another axon.
Axons of several neurones are grouped
together to form nerve tracts within the brain or spinal cord or nerve-?bres outside the CNS. Each nerve is surrounded by a sheath and contains bundles of ?bres. Some ?bres are medullated, having a sheath of MYELIN which acts as insulation, preventing nerve impulses from spreading beyond the ?bre conveying them.
The cellular part of the neurones makes up the grey matter of the brain and spinal cord – the former containing 600 million neurones. The dendrites meet with similar outgrowths from other neurones to form synapses. White matter is the term used for that part of the system composed of nerve ?bres.
Functions of nerves The greater part of the bodily activity originates in the nerve cells (see NERVE). Impulses are sent down the nerves which act simply as transmitters. The impulse causes sudden chemical changes in the muscles as the latter contract (see MUSCLE). The impulses from a sensory ending in the skin pass along a nerve-?bre to affect nerve cells in the spinal cord and brain, where they are perceived as a sensation. An impulse travels at a rate of about 30 metres (100 feet) per second. (See NERVOUS IMPULSE.)
The anterior roots of spinal nerves consist of motor ?bres leading to muscles, the posterior roots of sensory ?bres coming from the skin. The terms, EFFERENT and AFFERENT, are applied to these roots, because, in addition to motor ?bres, ?bres controlling blood vessels and secretory glands leave the cord in the anterior roots. The posterior roots contain, in addition to sensory ?bres, the nerve-?bres that transmit impulses from muscles, joints and other organs, which among other neurological functions provide the individual with his or her
proprioceptive faculties – the ability to know how various parts of the body are positioned.
The connection between the sensory and motor systems of nerves is important. The simplest form of nerve action is that known as automatic action. In this, a part of the nervous system, controlling, for example, the lungs, makes rhythmic discharges to maintain the regular action of the respiratory muscles. This controlling mechanism may be modi?ed by occasional sensory impressions and chemical changes from various sources.
Re?ex action This is an automatic or involuntary activity, prompted by fairly simple neurological circuits, without the subject’s consciousness necessarily being involved. Thus a painful pinprick will result in a re?ex withdrawal of the affected ?nger before the brain has time to send a ‘voluntary’ instruction to the muscles involved.
Voluntary Actions are more complicated than re?ex ones. The same mechanism is involved, but the brain initially exerts an inhibitory or blocking e?ect which prevents immediate re?ex action. Then the impulse, passing up to the cerebral hemispheres, stimulates cellular activity, the complexity of these processes depending upon the intellectual processes involved. Finally, the inhibition is removed and an impulse passes down to motor cells in the spinal cord, and a muscle or set of muscles is activated by the motor nerves. (Recent advances in magnetic resonance imaging (MRI) techniques have provided very clear images of nerve tracts in the brain which should lead to greater understanding of how the brain functions.) (See BRAIN; NERVOUS SYSTEM; SPINAL CORD.)
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