Neurons: Functional Units of Nervous Systems
Neurons: Functional
Units of Nervous
Systems
A neuron, or nerve cell, may assume many shapes, depending on its function and location; a typical kind is shown diagrammatically in Figure 35-1. From the nucleated cell body extend cytoplasmic processes of two types: one or more dendrites, in all but the simplest neuron, and a single axon. As the name dendrite suggests (Gr. dendron, tree), these neurons are often profusely branched. They, and the entire cell body surface, are the nerve cell’s receptive apparatus, often receiving information from several different sources at once. Some of these inputs are excitatory, others inhibitory.
The single axon (Gr. axon, axle), often a long fiber (meters in length in the largest mammals), is relatively uniform in diameter, and typically carries impulses away from the cell body. In vertebrates and some complex invertebrates, the axon is often covered with an insulating sheath of myelin.
Neurons are commonly classified as afferent, or sensory; efferent, or motor; and interneurons, which are neither sensory nor motor but connect neurons with other neurons. Afferent and efferent neurons lie mostly outside the central nervous system (brain and nerve cord) while interneurons, which in humans make up 99% of all neurons in the body, lie entirely within the central nervous system. Afferent neurons are connected to receptors. Receptors function to convert environmental stimuli into nerve impulses, which are carried by the afferent neurons into the central nervous system. Here impulses may be perceived as conscious sensation. Impulses also move to efferent neurons, which carry them via the peripheral system to effectors, such as muscles or glands.
In vertebrates, nerve processes (usually axons) are often bundled together in a well-formed wrapping of connective tissue to form a nerve (Figure 35-2). Cell bodies of these nerve processes are located either in the central nervous system or in ganglia, which are discrete bundles of nerve cell bodies located outside the central nervous system.
Surrounding neurons are nonnervous neuroglial cells (often simply called “glial” cells) that have a special relationship to neurons. Glial cells are extremely numerous in the vertebrate brain, where they outnumber neurons 10 to 1 and may form almost half the volume of the brain. Some glial cells form intimate insulating sheaths of lipid-containing myelin around nerve fibers. Vertebrate nerves are often enclosed by myelin, an insulating sheath laid down in concentric rings by special glial cells called Schwann cells (Figure 35-3) in the peripheral nervous system, and oligodendrocytes in the central nervous system. Certain glial cells, called astrocytes, because of their radiating, starlike shape, serve as nutrient and ion reservoirs for neurons, as well as a scaffold during brain development, enabling migrating neurons to find their destinations from points of origin. Astrocytes, and smaller microglial cells, are essential for the regenerative process that follows brain injury. Unfortunately, astrocytes also participate in several diseases of the nervous system, including Parkinsonism and multiple sclerosis. Other functional roles of glial cells are still being determined.
A neuron, or nerve cell, may assume many shapes, depending on its function and location; a typical kind is shown diagrammatically in Figure 35-1. From the nucleated cell body extend cytoplasmic processes of two types: one or more dendrites, in all but the simplest neuron, and a single axon. As the name dendrite suggests (Gr. dendron, tree), these neurons are often profusely branched. They, and the entire cell body surface, are the nerve cell’s receptive apparatus, often receiving information from several different sources at once. Some of these inputs are excitatory, others inhibitory.
The single axon (Gr. axon, axle), often a long fiber (meters in length in the largest mammals), is relatively uniform in diameter, and typically carries impulses away from the cell body. In vertebrates and some complex invertebrates, the axon is often covered with an insulating sheath of myelin.
Neurons are commonly classified as afferent, or sensory; efferent, or motor; and interneurons, which are neither sensory nor motor but connect neurons with other neurons. Afferent and efferent neurons lie mostly outside the central nervous system (brain and nerve cord) while interneurons, which in humans make up 99% of all neurons in the body, lie entirely within the central nervous system. Afferent neurons are connected to receptors. Receptors function to convert environmental stimuli into nerve impulses, which are carried by the afferent neurons into the central nervous system. Here impulses may be perceived as conscious sensation. Impulses also move to efferent neurons, which carry them via the peripheral system to effectors, such as muscles or glands.
In vertebrates, nerve processes (usually axons) are often bundled together in a well-formed wrapping of connective tissue to form a nerve (Figure 35-2). Cell bodies of these nerve processes are located either in the central nervous system or in ganglia, which are discrete bundles of nerve cell bodies located outside the central nervous system.
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Surrounding neurons are nonnervous neuroglial cells (often simply called “glial” cells) that have a special relationship to neurons. Glial cells are extremely numerous in the vertebrate brain, where they outnumber neurons 10 to 1 and may form almost half the volume of the brain. Some glial cells form intimate insulating sheaths of lipid-containing myelin around nerve fibers. Vertebrate nerves are often enclosed by myelin, an insulating sheath laid down in concentric rings by special glial cells called Schwann cells (Figure 35-3) in the peripheral nervous system, and oligodendrocytes in the central nervous system. Certain glial cells, called astrocytes, because of their radiating, starlike shape, serve as nutrient and ion reservoirs for neurons, as well as a scaffold during brain development, enabling migrating neurons to find their destinations from points of origin. Astrocytes, and smaller microglial cells, are essential for the regenerative process that follows brain injury. Unfortunately, astrocytes also participate in several diseases of the nervous system, including Parkinsonism and multiple sclerosis. Other functional roles of glial cells are still being determined.
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| Figure 35-2 Structure of a nerve showing nerve fibers surrounded by various layers of connective tissue. A nerve may contain thousands of both efferent and afferent fibers. |
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