Neuron

The term used to describe a nerve cell. A typical neuron consists of a cell body, several branching projections called dendrites, and a filamentous projection called an axon (also known as a nerve fibre). An axon branches at its end to form terminals through which electrical signals are transmitted to target cells. Most axons are coated with a layered insulating myelin sheath, which speeds the transmission of the signals. The myelin sheath is punctuated along its length by gaps called nodes of Ranvier, which help this process. Because the myelin sheath is nonconductive, ion exchange (depolarization) only occurs at a node, and signals leap from node to node along the length of the axon. The nervous system contains billions of neurons, of which there are 3 main types: sensory neurons, which carry signals from sense receptors into the central nervous system (CNS); motor neurons, which carry signals from the CNS to muscles or glands; and interneurons, which form all the complex electrical circuitry within the CNS itself. When a neuron transmits (“fires”) an electrical impulse, a chemical called a neurotransmitter is released from the axon terminals at synapses (junctions with other neurons). This neurotransmitter may make a muscle cell contract, cause an endocrine gland to release a hormone, or affect an adjacent neuron. Different stimuli excite different types of neurons to fire. Sensory neurons, for example, may be excited by physical stimuli, such as cold or pressure. The activity of most neurons is controlled by the effects of neurotransmitters released from adjacent neurons. Certain neurotransmitters generate a sudden change in the balance of electrical potential inside and outside the cell (an “action potential”), which occurs at one point on the cell’s membrane and flows at high speed along it. Others stabilize neuronal membranes, preventing an action potential. Thus, the firing pattern of a neuron depends on the balance of excitatory and inhibitory influences acting on it. If the cell body of a neuron is damaged or degenerates, the cell dies and is never replaced. A baby starts life with the maximum number of neurons, which decreases continuously thereafter.