The nerve cell, or neuron, which serves to store and process information, is the functional unit of the nervous system. The other cellular constituent include1:
- The neuroglial cell, or glia, which functions to provide structural and metabolic support for the neurons.2 Glial cells outnumber the neurons 10:1.3
- Microglia: phagocytize pathogens and cellular debris within the CNS.
- Schwann cells: surround axons of all peripheral nerve fibers, forming a neurolemmal sheath; wrap around many peripheral fibers to form myelin sheaths.
- Satellite cells (ganglionic gliocytes): support ganglia within the PNS.
- Oligodendrocytes: form myelin sheaths around axons, producing white matter of the CNS.
- Astrocytes: vascular processes that cover capillaries within the brain and contribute to the blood-brain barrier.
- Ependymal cells: form the epithelial lining of brain cavities (ventricles) and the central canal of the spinal cord.
Although neurons come in a variety of sizes and shapes, there are four functional parts to each nerve (Table 9–1, Figure 9–1).
Table 9–1. Functional Parts of a Nerve
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Table 9–1. Functional Parts of a Nerve
Serves a receptive function, receiving information from other nerve cells or the environment.
Conducts information to other nerve cells.
Contains the nucleus of the cell and has important integrative functions.
The transmission site for action potentials, the messengers of the nerve cell.
The motor neuron. (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology: Text and Atlas,10th ed. New York, NY: McGraw-Hill. 2003:162.)
The communication of information from one nerve cell to another occurs at junctions called synapses, where a chemical is released in the form of a neurotransmitter (Figure 9–2). A synapse is the functional connection between a neuron and a second cell.1 In the CNS this other cell is also a neuron. In the PNS, the other cell may be either a neuron or effector cell within a muscle or gland. Neuron-neuron synapses usually involve a connection between the axon of one neuron and the dendrites, cell body, or axon of a second neuron. In almost all synapses, transmission is in one direction only—from the axon of the first (or presynaptic) neuron to the second (or postsynaptic) neuron. Synaptic transmission can be either electrical or chemical.
- Electrical synapses: in order for two cells to be electrically coupled, they must be joined by areas of contact with low electrical resistance. Adjacent cells that are electrically coupled are joined together by gap junctions. Gap junctions are present in cardiac muscle, some smooth muscles, and in various regions of the brain, although the functional significance of the latter is presently unknown.
- Chemical synapses: transmission across the majority of synapses in the nervous system ...
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