The neuron is the structural and functional unit of the nervous system
The human nervous system contains over 10 billion neurons. Although neurons show the greatest variation in size and shape of any group of cells in the body, they fall into three general categories:
• Sensoiy neurons convey impulses from receptors to the CNS. Processes of these neurons are included in somatic afferent and visceral afferent nerve fibers. Somatic afferent fibers convey sensations of pain, temperature, touch, and pressure from the body surface. In addition, these fibers convey pain and proprioception (noncon-scious sensation) from organs within the body (e.g., muscles, tendons, and joints) to provide the brain with information related to the orientation of the body and limbs. Visceral afferent fibers transmit impulses of pain and other sensations from mucous membranes, glands, and blood vessels.
• Motor neurons convey impulses from the CNS or ganglia to effector cells. Processes of these neurons are included in somatic efferent and visceral efferent nerve fibers. Somatic efferent neurons send voluntary impulses to skeletal muscles. Visceral efferent neurons transmit involuntary impulses to smooth muscle, cardiac conducting cells (Purkinje fibers), and glands (Fig. 11.1).
• Interneurons, also called intercalated neurons, form a communicating and integrating network between the sensory and motor neurons. It is estimated that more than 99.9% of all neurons belong to this integrating network.
The functional components of a neuron include the cell body, axon, dendrites, and synaptic junctions
The cell body of a neuron contains the nucleus and those organelles that maintain the cell. The processes extending from the cell body constitute the single common structural characteristic of all neurons. Most neurons have only one axon, usually the longest process extending from the cell, which transmits impulses away from the cell body to a spe
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