The nerve cell

Nerve cells, also known as neurons, have a number of distinctive properties. They may be very long and thin (spinal cord to toe length, for instance). They are very long-lived (in many cases the lifetime of an individual) but cannot divide by mitosis; hence, if a nerve cell is destroyed, it cannot be replaced by cell division. They have a very high rate of metabolism, requiring glucose and oxygen to support this, and if deprived of oxygen for more than a few minutes will die.

All nerve cells have a cell body, an enlarged part in which are found the nucleus and all the biosynthetic apparatus of the cell (including rough endoplasmic reticulum), from which extend various projections. The dendrites are multiply branched extensions from the cell body, involved in receiving information from the environment and other nerve cells. The axon is a long, slender, usually unbranched projection, extending from the cell body to the point where the nerve cell will exert its effects. At the distal end (far end) the axon may branch, extending several 'feet' to the target tissue or organ (Fig. 7.1).

At the end of each 'foot' of the axon, contact is made with another neuron or with another type of cell through the structure known as a synapse. The synapse is formed by a swelling on the end of the axon facing, across a small space known as the synaptic cleft, a specialised receptor area on the cell which will receive the signal. There are two sorts of synapses. Electrical synapses occur between two neurons; ion channels effectively connect the cytoplasm of the two cells, and the electrical signal being transmitted along

one continues almost without interruption along the next. However, more relevant from the point of view of regulation of metabolism are the chemical synapses. At a chemical synapse, vesicles containing a neurotransmitter substance are stored within the swelling at the end of the axon. There are a great many neurotransmitters used by different neurons, but of particular relevance to us will be acetylcholine and noradrenaline (Fig. 7.2). When an electrical impulse arrives, the neurotransmitter substance is released into the synaptic cleft, and acts on receptors on the target cell. The nature of a nerve impulse, and the events occurring at a synapse, are discussed in Boxes 7.1 and 7.2.

A synapse may be with another neuron. Alternatively, it may be with a muscle cell, in which case it is called a neuromuscular junction, or with an endocrine cell, in which case it is sometimes known as a neuroglandular junction. The neuromuscular junction is a specialised structure, activation of which leads to muscle contraction. It will be considered in more detail later (Section 7.2.2.3).

When we speak of a nerve in the anatomical sense, that refers to a specialised structure containing a number of axons and associated supporting cells, together with fine blood vessels.

ch3 o

CH3 - N -ch2-ch2-O-C-ch3 Acetylcholine

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