There are a great many neurons in the body, performing a wide variety of functions, although the nervous system as a whole is highly integrated, and also interacts with many other bodily systems. However, the nervous system can be subdivided in certain ways according to the general function of different groups of neurons.
The term central nervous system (CNS) refers to the brain and spinal cord. The peripheral nervous system refers to other parts of the nervous system, mainly nerves running to and from the spinal cord (spinal nerves) and to and from the brain (cranial nerves). This is a structural definition. There is also a functional classification, listed below:
(1) The autonomic nervous system, a system of nerves carrying impulses from the CNS to other organs and tissues. This part of the nervous system cannot be controlled voluntarily - it seems to be autonomous, and hence its name. (It is sometimes referred to as the involuntary nervous system.) It controls such functions as heart rate, some aspects of digestive function, and some aspects of hormone secretion and of metabolism. The autonomic nervous system can be subdivided into:
• the sympathetic nervous system, which acts as though 'sympathetic' to the body's needs; it speeds up the heart when we are excited or exercising, for instance. This appears to be counteracted by:
• the parasympathetic nervous system - which appears in many ways to counter the sympathetic system; for instance, it slows the heart.
(2) The somatic nervous system is the system of nerves which runs from the CNS to the skeletal muscles, causing them to contract. It is sometimes called the voluntary nervous system since we can activate specific parts of it voluntarily (e.g. lift a hand to scratch our nose).
(3) The afferent nervous system refers to those nerves which bring signals
A synapse is where an axon makes contact with another cell. When an action potential (see Box 7.1) reaches the synaptic terminal (or axonal terminal), it leads to opening of voltage-gated calcium channels, which allow extracellular Ca2+ ions to enter the cell; these lead (as in other secretory cells) to exocytosis of the secretory vesicles containing the neurotransmitter. Exocytosis involves the granules - each of which is surrounded by a phospholipid membrane - fusing with the synaptic membrane and discharging their contents into the space outside, the synaptic cleft.
The neurotransmitter molecules can then diffuse across the narrow gap of the synaptic cleft, and bind to specific receptors on the target cell membrane. Events then depend upon the nature of the target cell. It may be another neuron, in which case binding of the neurotransmitter will open ion channels and begin the passage of an action potential along the new neuron. If it is a skeletal muscle cell, the result will be increased permeability to Na+ ions, depolarisation spreading across the membrane and the opening of Ca2+ channels which allow Ca2+ ions to enter the intracellular space; it is these calcium ions which trigger muscle contraction. On the other hand, the target cell may not be another excitable cell. If the neurotransmitter is noradrenaline, the target cell may have P-adrenergic receptors; binding of noradrenaline to these will activate (through the G-protein system) adenylyl cyclase and raise the cellular level of cAMP.
from tissues and organs back to the CNS. This includes, for instance, pain receptors, chemoreceptors monitoring the pH of the blood, and receptors monitoring the presence of digestion products in the intestinal tract; we have met some examples of these under the consideration of digestion (Chapter 3).
(4) A fourth component of the nervous system is recognised, the enteric nervous system, regulating gastrointestinal function. This is closely connected with both the sympathetic and the parasympathetic nervous systems, but also functions to some extent autonomously, with local 'circuits' so that one part of the intestinal tract can regulate the function of another without the involvement of the CNS. It is highly complex, and will not be considered further here.
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