Page 347 - Atlas of Histology with Functional Correlations

nerve impulse in a postsynaptic cell depends on the summation of excitatory

or inhibitory effects of many synapses on the target cell allowing a more
precise regulation of responses from postsynaptic neurons, muscles, or
glands. Thus, the synapses regulate neuronal activity in the nervous system
by inducing either excitatory or inhibitory effects on the target cells after
which the neurotransmitters are rapidly removed from the synaptic cleft by

enzymes, diffusion, or endocytosis.

FIGURE 9.8 | Motor Neurons: Anterior Horn of

Spinal Cord

The large, multipolar motor neurons (7) of the CNS have a large central
nucleus (11), a prominent nucleolus (12), and several radiating cell processes—

the dendrites (10, 16). A single, thin axon (5, 14) arises from a cone-shaped,
clear area of the neuron; this is the axon hillock (13). The axons (5, 14) that
leave the motor neurons (7) are thinner and much longer than the thicker and
shorter dendrites (10, 16).

The cytoplasm, or perikaryon, of the neuron is characterized by coarse

granules (basophilic masses). These are the Nissl bodies (4, 8), and they
represent the granular endoplasmic reticulum of the neuron. When the plane of
section misses the nucleus (4), only the dark-staining Nissl bodies (4) are seen in
the perikaryon of the neuron. The Nissl bodies (4, 8) extend into the dendrites

(10, 16) but not into the axon hillock (6, 13) or into the axon (5, 14). This feature
distinguishes the axons (5, 14) from the dendrites (10, 16). The nucleus of the
neuron stains light because of the uniform dispersion of the chromatin, whereas
the nucleolus (12) is prominent, dense, and stains dark. The nuclei (2, 9) of the

surrounding neuroglia (2, 9) are stained prominently, whereas their small
cytoplasm remains unstained. The neuroglia (2, 9) are nonneural cells of the
CNS; they provide the structural and metabolic support for the neurons (7).

Surrounding the neurons (7) and the neuroglia (2, 9) are numerous blood

vessels (1, 3, 15) of various sizes.

346