Excitation of the Spinal Cord Motor Control Areas by the Primary Motor Cortex and Red Nucleus

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Vertical Columnar Arrangement of the Neurons in the Motor Cortex. In Chapters 47 and 51, we pointed out that the cells in the somatosensory cortex and visual cortex are organized in vertical columns of cells. In like manner, the cells of the motor cortex are organized in vertical columns a fraction of a millimeter in diameter, with thousands of neurons in each column.

Each column of cells functions as a unit, usually stimulating a group of synergistic muscles, but sometimes stimulating just a single muscle. Also, each column has six distinct layers of cells, as is true throughout nearly all the cerebral cortex. The pyramidal cells that give rise to the corticospinal fibers all lie in the fifth layer of cells from the cortical surface. Conversely, the input signals all enter by way of layers 2 through 4. And the sixth layer gives rise mainly to fibers that communicate with other regions of the cerebral cortex itself.

Function of Each Column of Neurons. The neurons of each column operate as an integrative processing system, using information from multiple input sources to determine the output response from the column. In addition, each column can function as an amplifying system to stimulate large numbers of pyramidal fibers to the same muscle or to synergistic muscles simultaneously. This is important, because stimulation of a single pyramidal cell can seldom excite a muscle. Usually, 50 to 100 pyramidal cells need to be excited simultaneously or in rapid succession to achieve definitive muscle contraction.

Dynamic and Static Signals Transmitted by the Pyramidal Neurons. If a strong signal is sent to a muscle to cause initial rapid contraction, then a much weaker continuing signal can maintain the contraction for long periods thereafter. This is the usual manner in which excitation is provided to cause muscle contractions. To do this, each column of cells excites two populations of pyramidal cell neurons, one called dynamic neurons and the other static neurons. The dynamic neurons are excessively excited for a short period at the beginning of a contraction, causing the initial rapid development of force. Then the static neurons fire at a much slower rate, but they continue firing at this slow rate to maintain the force of contraction as long as the contraction is required.

The neurons of the red nucleus have similar dynamic and static characteristics, except that a greater percentage of dynamic neurons is in the red nucleus and a greater percentage of static neurons is in the primary motor cortex. This may be related to the fact that the red nucleus is closely allied with the cerebellum, and the cerebellum plays an important role in rapid initiation of muscle contraction, as explained in the next chapter.

Somatosensory Feedback to the Motor Cortex Helps Control the Precision of Muscle Contraction

When nerve signals from the motor cortex cause a muscle to contract, somatosensory signals return all the way from the activated region of the body to the neurons in the motor cortex that are initiating the action. Most of these somatosensory signals arise in (1) the muscle spindles, (2) the tendon organs of the muscle tendons, or (3) the tactile receptors of the skin overlying the muscles. These somatic signals often cause positive feedback enhancement of the muscle contraction in the following ways: In the case of the muscle spindles, if the fusimotor muscle fibers in the spindles contract more than the large skeletal muscle fibers contract, the central portions of the spindles become stretched and, therefore, excited. Signals from these spindles then return rapidly to the pyramidal cells in the motor cortex to signal them that the large muscle fibers have not contracted enough. The pyramidal cells further excite the muscle, helping its contraction to catch up with the contraction of the muscle spindles. In the case of the tactile receptors, if the muscle contraction causes compression of the skin against an object, such as compression of the fingers around an object being grasped, the signals from the skin receptors can, if necessary, cause further

Figure 55-6

Convergence of different motor control pathways on the anterior motor neurons.

Figure 55-6

Convergence of different motor control pathways on the anterior motor neurons.

excitation of the muscles and, therefore, increase the tightness of the hand grasp.

Stimulation of the Spinal Motor Neurons

Figure 55-6 shows a cross section of a spinal cord segment demonstrating (1) multiple motor and senso-rimotor control tracts entering the cord segment and (2) a representative anterior motor neuron in the middle of the anterior horn gray matter. The corti-cospinal tract and the rubrospinal tract lie in the dorsal portions of the lateral white columns. Their fibers terminate mainly on interneurons in the intermediate area of the cord gray matter.

In the cervical enlargement of the cord where the hands and fingers are represented, large numbers of both corticospinal and rubrospinal fibers also terminate directly on the anterior motor neurons, thus allowing a direct route from the brain to activate muscle contraction.This is in keeping with the fact that the primary motor cortex has an extremely high degree of representation for fine control of hand, finger, and thumb actions.

Patterns of Movement Elicited by Spinal Cord Centers. From Chapter 54, recall that the spinal cord can provide certain specific reflex patterns of movement in response to sensory nerve stimulation. Many of these same patterns are also important when the cord's anterior motor neurons are excited by signals from the brain. For example, the stretch reflex is functional at all times, helping to damp any oscillations of the motor movements initiated from the brain, and probably also providing at least part of the motive power required to cause muscle contractions when the intrafusal fibers of the muscle spindles contract more than the large skeletal muscle fibers do, thus eliciting reflex "servo-assist" stimulation of the muscle, in addition to the direct stimulation by the corticospinal fibers.

Also, when a brain signal excites a muscle, it usually is not necessary to transmit an inverse signal to relax the antagonist muscle at the same time; this is achieved by the reciprocal innervation circuit that is always present in the cord for coordinating the function of antagonistic pairs of muscles.

Finally, other cord reflex mechanisms, such as withdrawal, stepping and walking, scratching, and postural mechanisms, can each be activated by "command" signals from the brain. Thus, simple command signals from the brain can initiate many normal motor activities, particularly for such functions as walking and attaining different postural attitudes of the body.

Effect of Lesions in the Motor Cortex or in the Corticospinal Pathway—The "Stroke"

The motor control system can be damaged by the common abnormality called a "stroke." This is caused either by a ruptured blood vessel that hemorrhages into the brain or by thrombosis of one of the major arteries supplying the brain. In either case, the result is loss of blood supply to the cortex or to the corticospinal tract where it passes through the internal capsule between the caudate nucleus and the putamen. Also, experiments have been performed in animals to selectively remove different parts of the motor cortex.

Removal of the Primary Motor Cortex (Area Pyramidalis).

Removal of a portion of the primary motor cortex—the area that contains the giant Betz pyramidal cells— causes varying degrees of paralysis of the represented muscles. If the sublying caudate nucleus and adjacent premotor and supplementary motor areas are not damaged, gross postural and limb "fixation" movements can still occur, but there is loss of voluntary control of discrete movements of the distal segments of the limbs, especially of the hands and fingers. This does not mean that the hand and finger muscles themselves cannot contract; rather, the ability to control the fine movements is gone. From these observations, one can conclude that the area pyramidalis is essential for voluntary initiation of finely controlled movements, especially of the hands and fingers.

Muscle Spasticity Caused by Lesions That Damage Large Areas Adjacent to the Motor Cortex. The primary motor cortex normally exerts a continual tonic stimulatory effect on the motor neurons of the spinal cord; when this stimulatory effect is removed, hypotonia results. Most lesions of the motor cortex, especially those caused by a stroke, involve not only the primary motor cortex but also adjacent parts of the brain such as the basal ganglia. In these instances, muscle spasm almost invariably occurs in the afflicted muscle areas on the opposite side of the body (because the motor pathways cross to the opposite side). This spasm results mainly from damage to accessory pathways from the nonpyramidal portions of the motor cortex. These pathways normally inhibit the vestibular and reticular brain stem motor nuclei. When these nuclei cease their state of inhibition (i.e., are "dis-inhibited"), they become spontaneously active and cause excessive spastic tone in the involved muscles, as we discuss more fully later in the chapter. This is the spasticity that normally accompanies a "stroke" in a human being.

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