Chapter 3 The Different Parts of the Nervous System 77 Finally, a large proportion of the fibers serve the signal traffic, and hence cooperation, between the segments of the cord. Since the first two groups of axons become successively more numerous in the rostral direction, the proportion of white to gray matter increases from caudal to rostral. The white matter is divided into funiculi, or columns, by drawing lines in the transverse plane from the sulci on the surface of the cord to the center (Fig. 3.4). Thus, in each half of the cord, the white matter is divided into a ventral or anterior funiculus, a lateral funiculus, and a dorsal or posterior funiculus. For the latter, the term dorsal column is used most frequently. The Spinal Gray Matter Contains Three Main Types of Neurons Among neurons in the gray matter of the spinal cord, there are both morphological and functional differences. Three main types may be identified according to where they send their axons: (1) neurons sending their axons out of the central nervous system; (2) neurons sending their axons to higher levels of the central nervous system (such as the brain stern); and (3) neurons sending their axons to other parts of the spinal cord. We will now consider in some detail each of these three groups. Efferent Fibers from the Cord Control Muscles and Glands The cell bodies of the first kind of neuron listed above are located in the ventral horn and at the transition between the dorsal and ventral horns. The motor neurons or motoneurons have large, multipolar perikarya and are in the ventral horn proper (Figs. 1.2,3.7,3.8, 3.10, and 11.2). The dendrites extend for a considerable distance in the gray matter. The axons leave the cord through the ventral root, follow the spinal nerves, and end in skeletal muscles (muscles that are controlled voluntarily). The motoneurons are further discussed in Chapter 11. There is also another group of neurons that sends its axons out of the cord through the ventral root. These supply smooth muscles and glands with motor signals, and belong to the autonomic nervous system. The autonomic system controls the vascular smooth muscles and visceral organs throughout the body. The cell bodies lie in the lateral horn (Fig. 3.8). Most of them form a long, slender column, the intermediolateral cell column. This column is present only in the thoracic and upper two lumbar segments of the cord and belongs to the sympathethic part of the autonomic nervous system. A corresponding, smaller group of neurons is present in the sacral cord (S3-S4) and belongs to the parasympathetic part of the autonomic nervous system. The motor neurons and those in the intermediolateral cell column (and those in the sacral cord) are under synaptic influence from higher levels of the central nervous system. => page 213    start page