It has been estimated that there are several billion neurons that comprise the mammalian brain which, together with their surrounding glial cells, form a unique network of connections which are ultimately responsible for all thoughts and actions. While the glial cells may play a critical role in brain development, their main function in the mature brain is to maintain the structure and metabolic homeostasis of the neurons which they surround.
A typical neuron consists of a cell body and an axonal projection through which information in the form of an action potential passes from the cell body to the axonal terminal. Information is received by the cell body via a complex array of dendrites which make contact with adjacent neurons. The structural complexity and the number of dendritic processes vary according to the type of nerve cell and its physiological function. For example, the granule cells in the dentate gyrus of the hippocampus (a region of the brain which plays a role in short-term memory) receives and integrates information from up to 10 000 other cells in the vicinity.
The majority of the inputs to the granule cells are excitatory, each of which provides a small depolarizing current to the membrane of the cell body. The point of contact between the axonal projection from the neuron and an adjacent cell is termed the synapse which under the electron microscope appears as a swelling at the end of the axon. Most synapses are excitatory and are usually located along the dendritic branches of the neuron. The contributions of the individual excitatory synapses are additive and, as a result, when an excitatory stimulus occurs a wave of depolarizing current travels down the axon to stimulate the adjacent cell body. However, some synapses are inhibitory, usually fewer in number and strategically located near the cell body. These synapses, when activated, inhibit the effects of any excitatory currents which may travel down the dendritic processes and thereby block their actions on the neuron (Figure 2.1).
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