Structure And Function
Although there are variations related to function, a typical neuron consists of dendrites (also termed dendritic processes), a cell body, an axon, and an axon terminus.
Dendrites are the (filamentous) terminal portions of neuron that bind neurotransmitter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by microfilaments and microtubules.
The nerve cell body contains a nucleus, a nucleolus, and cytoplasm containing the cell (such as mitochondria, endoplasmic reticulum, and so on). Unique to the nerve cell body are Nissl bodies, which are rough surfaced vesicles in the endoplasmic reticulum (cytoplasm located near the nucleus), and are involved with protein synthesis. Another characteristic structure of nerve cells are the neurofibrils, which are delicate threadlike structures that help to maintain the shape of the cell, and which transport substances between the cell body and the axon terminals. The plasma membrane around the cell separates the cytoplasm on the inside of the cell from the extracellular
fluid on the outside. Cell membranes of neurons contain electrically gated channels, which when properly stimulated allow electrically charged particles (such as sodium and potassium ions) to pass across the barrier. This ionic exchange is the basis for the flow, or action potential, of the nerve impulse.
The axon is a cytoplasmic continuation of the cell body specialized for the electrical conduction of neural signals. The axon may be long—up to a yard in length in humans—or short, depending upon the neuron's position and function. The cell membranes of the neural axon transmit neural signals via changes in action potentials that sweep down the membrane.
At the junction of the cell body and axon is a region termed the axon hillock. At the axon hillock, chemical signals received by the dendrites may reach a threshold level to cause a wave of electrical depolarization and hyperpolarization of the axon cell membrane. The net movements of ions across the cell membrane are responsible for these changes that move down the axon to the axon terminus as an action potential.
At the axon terminus, neurotransmitters are released into the synaptic gap. Through synaptic gaps, a typical neuron may interconnect with thousands and tens of thousands of other neurons. Axon terminals have knob-like swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a synapse. Under very high magnification, a very tiny space, the synaptic cleft or gap (about one millionth of an inch, or mm), can be detected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers, anesthetics, nicotine, and caffeine target the synapse and can cause an alteration of impulse transmission.