Microscopic Anatomy

As indicated earlier, the brain contains extraordinary numbers of neurons and glial cells. The remarkable capacities of the brain result in part from the wealth of neurons and their connections, and the glia provide important support for the optimal function of the neuronal population. In this section, we consider microscopic aspects of the major cell types in the brain.

The neuron is the fundamental functional unit of the nervous system. Neurons are anatomically specialized to transmit information, which may take the form of a sensory stimulus received from the periphery or a motor signal destined to produce movement in a muscle. In the brain, neurons have many additional functions that can be generally termed ''information processing'', and presumably this category includes all of the cognitive and emotional operations traditionally associated with mental activity and behavior. Most brain neurons, in fact, are classified as interneurons because they are interposed between sensory input and motor output and mediate these other, ''higher'' functions. To accomplish all of these various tasks, neurons have a typical arrangement that includes a cell body, a variable number of dendrites, and an axon (Fig. 2). The cell body, also known as the soma or perikaryon, contains the cell nucleus and other organelles that maintain the metabolic status of the neuron and synthesize macromolecules essential for its function. Dendrites are relatively short processes extending from the cell body that receive input from adjacent neurons via synaptic contacts (see later discussion).

Dendrites

Figure 2

Drawing of a typical neuron, with its characteristic cell body, dendrites, and axon. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds. (2000). Principles of Neural Science, 4th ed., p. 24. McGraw-Hill, New York.

The axon is a long, cylindrical process that provides for the output of the neuron, again via synaptic contact with adjacent neurons.

The transfer of information in the nervous system is both electrical and chemical in nature (Fig. 3). Within a single neuron, the signal is electrical and takes the form of an action potential. This is an electrical impulse (often referred to as a spike) propagated along the axon by virtue of a sudden influx of sodium ions that transiently reverses the polarity of the axonal membrane. The resting membrane potential is quickly restored after the action potential passes, and after a short refractory period another spike can be propagated. The action potential is an all-or-none phenomenon that depends on the balance between excitatory and inhibitory input received by the neuronal den-drites.

Whereas the neuron itself conducts information electrically, the junction between neurons, known as the synapse, operates by means of chemical transmission (Fig. 3). A synapse is a specialized region at which a chemical messenger known as a neurotransmitter from one neuron (presynaptic) diffuses across a narrow synaptic cleft to activate another neuron (postsynaptic). When the neurotransmitter binds with the receptor on the postsynaptic membrane, it acts to produce either a depolarization (excitatory stimulus) or hyperpolarization (inhibitory stimulus), and the summation of these many postsynaptic potentials determines whether an action potential is generated in the postsynaptic neuron. Neurotransmitters are generally small amines, amino acids, and neuropep-tides, and their pharmacology promises many avenues for the successful manipulation of abnormal physical and mental states.

Far more abundant even than neurons in the brain are glial cells. Glia of the brain and spinal cord are classified into four types: astrocytes, oligodendrocytes, microglia, and ependymal cells. Astrocytes are star-shaped cells found in both gray and white matter that have a role in the mechanical support of neurons, contribute to metabolic regulation of the microenvironment of the brain, and participate in its response to injury. Oligodendrocytes are confined mainly to white matter, where they are responsible for the myelination of brain axons, just as Schwann cells perform this function in the PNS. Microglia are small cells found in gray and white matter that serve as the phagocytes of the brain, migrating as necessary to damaged areas where they consume pathogens and neuronal debris.

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Figure 3 General depiction of information transfer between neurons. Chemical signaling by neurotransmitter release onto a neuron (1) causes the generation of an action potential (2), which is an electrical signal propagated down the axon (3). The action potential then causes further neurotransmitter release onto subsequent neurons (4). Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 145. Mosby, St. Louis.

Figure 3 General depiction of information transfer between neurons. Chemical signaling by neurotransmitter release onto a neuron (1) causes the generation of an action potential (2), which is an electrical signal propagated down the axon (3). The action potential then causes further neurotransmitter release onto subsequent neurons (4). Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 145. Mosby, St. Louis.

Ependymal cells line the ventricles of the brain, and at a specialized structure called the choroid plexus, one of which is found in each ventricle, ependymal cells form a secretory epithelium that produces the CSF that fills the ventricles and bathes the entire CNS.

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