Significance of Neurons in Grey Matter of Spinal Cord
The ventral horn cells of the spinal cord may be functionally divided into three major categories as follows.
a. The most prominent neurons with large cell bodies and prominent Nissl substance are designated alpha neurons. These are somatic efferent neurons (Fig. 3.1). Their axons (alpha efferents) leave the spinal cord through the ventral nerve roots of spinal nerves and innervate skeletal muscle. They occupy lamina IX of the ventral grey column.
b. Some smaller neurons designated as gamma neurons are also located in lamina IX. They supply intrafusal fibres of muscle spindles (Fig. 4.2). Sensory impulses arising in the spindle travel to the spinal cord and reach the alpha neurons. The gamma neurons thus influence the activity of alpha neurons indirectly through muscle spindles.
c. A considerable number of smaller neurons in the ventral grey column are internuncial neurons. They are most abundant in Lamina VII. Some ramifications of the central processes of cells in the dorsal nerve root ganglia (bringing afferent impulses from the periphery), and axons descending from higher centres, terminate in relation to these internuncial neurons. The axons of internuncial neurons convey these impulses to alpha and gamma neurons.
d. Another variety of neuron that is believed (on physiological grounds) to exist in the ventral grey column is the so calledRenshaw cell. These cells receive the terminations of collaterals arising from the axons of alpha neurons. The axons of Renshaw cells carry the impulses back to the cell bodies of the same alpha neurons, and thus help to regulate their activity.
The neurons of the dorsal grey column may be subdivided as follows.
a. Some of these are internuncial neurons similar to those in the ventral grey column.
b. Many dorsal column neurons receive afferent impulses through the central processes of neurons in dorsal nerve root ganglia. These dorsal column neurons give off axons that enter the white matter of the spinal cord either on the same or opposite side.
These axons may behave in one of the following ways.
1. They may ascend or descend for some segments before terminating in relation to neurons at other levels of the spinal cord. Such axons constitute intersegmental tracts.
2. A considerable number of axons arising from dorsal column neurons run upwards in the spinal cord and constitute ascending tracts which terminate in various masses of grey matter in the brain. These tracts form a considerable part of the white matter of the spinal cord.
The neurons of the intermediolateral group (lateral grey column) are visceral efferent neurons. They are present at two levels of the spinal cord.
a. One group is present in the thoracic and upper two or three lumbar segments. These are preganglionic neurons of the sympathetic nervous system. Their axons terminate in relation to postganglionic neurons in sympathetic ganglia (and occasionally in some other situations). Axons of these postganglionic neurons are distributed to various organs, and to blood vessels (Figs. 19.1, 19.2).
b. The second group of visceral efferent neurons is found in the second, third and fourth sacral segments of the spinal cord. These are preganglionic neurons of the parasympathetic nervous system. Their axons leave the spinal cord through the ventral nerve roots to reach spinal nerves. They leave the spinal nerves as the pelvic splanchnic nerves which are distributed to some viscera in the pelvis and abdomen (Fig. 19.6). They end by synapsing with ganglion cells located in intimate relationship to the viscera concerned. The postganglionic fibres arising in these ganglia are short and supply smooth muscle and glands in these viscera.
The location of the various types of neurons described above in relation to laminae of spinal grey matter is of interest.
Afferent fibres carrying sensations from the skin end predominantly in laminae I to IV. Proprioceptive impulses reach laminae V and VI. These laminae also receive numerous fibres from the cerebral cortex.
Lamina VII gives off fibres that reach the midbrain and cerebellum (through spino-cerebellar, spino-tectal and spino-reticular tracts). It receives fibres from these regions through tectospinal, reticulo-spinal and rubrospinal tracts.
Renshaw cells are located in a forward extension of lamina VII (into the interval between laminae VIII and IX).
Lamina VIII is made up mainly of interneurons that receive fibres from various sources. They give efferents to gamma neurons and thus influence muscle spindles.
Lamina IX contains the alpha and gamma neurons that give off efferent fibres to skeletal muscle. Motor neurons in different parts of the ventral grey column show remarkable differences in the orientation and extent of their dendritic fields. Many of the dendrites of neurons in the ventromedial, central and ventrolateral columns run longitudinally in the form of bundles. These neurons, therefore, come into intimate contact with each other. This arrangement is seen in relation to neurons that supply postural muscles. In contrast, the dendrites of neurons in the dorsolateral column have very little contact with those of neighbouring neurons. This column supplies muscles in distal parts of the limbs and the discrete nature of the neurons may be associated with fine control necessary for movements produced by these muscles.