Neurons
The basic cellular unit of the nervous system is the neuron. The unique morphological and intercellular structure of the neuron is dedicated to the efficient and rapid transmission of neural signals. Within the neuron, the neural signal travels electrically. At the synapse, the gap between neurons, neural signals are conveyed chemically by a limited number of chemicals termed neurotransmitters. Specialized parts of the neuron facilitate the production, release, binding, and uptake of these neurotransmitters.
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 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.
Multipolar neurons have many processes and serve principally as motor neurons. Motor neurons, efferent because they conduct impulses away from the central nervous system (the brain and spinal cord) regulate the function of muscles and glands. Afferent neural pathways that send signals to the (CNS) are generally composed of unipolar neurons. Unipolar neurons also serve as sensory neurons--their filamentous dendritic processes exposed and elaborated into or connected to sensory receptor cells. Interneurons are neurons that connect neurons along a neural pathway.
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