Excitation of the Spinal Cord Motor Control Areas by the Primary Motor Cortex and Red Nucleus
Vertical Columnar Arrangement of the Neurons in the Motor Cortex. We pointed out that thecells 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 some-times stimulating just a single muscle. Also, each column has six distinct layers of cells, as is true throughout nearly all the cerebral cortex. The pyram-idal cells that give rise to the corticospinal fibers all lie in the fifth layer of cells from the cortical surface. Con-versely, 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 cere-bral cortex itself.
Function of Each Column of Neurons. The neurons ofeach 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 simulta-neously. 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 defin-itive muscle contraction.
Dynamic and Static Signals Transmitted by the Py- ramidal Neurons. If a strong signal is sent to a muscleto 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 developmentof force. Then the static neurons fire at a much slowerrate, but they continue firing at this slow rate to main-tain the force of contraction as long as the contractionis 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 cere-bellum, and the cerebellum plays an important role in rapid initiation of muscle contraction.
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 pyram-idal cells further excite the muscle, helping its con-traction 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
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 thereciprocal 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.
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.
Removalof 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 ofdiscrete movements of the distal segments of the limbs, especially of the hands and fingers. This does not meanthat the hand and finger muscles themselves cannot con-tract; rather, the ability to control the fine movements isgone. From these observations, one can conclude thatthe 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 AreasAdjacent to the Motor Cortex. The primary motor cortexnormally exerts a continual tonic stimulatory effect on the motor neurons of the spinal cord; when this stimu-latory 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 adja-cent 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 acces-sory 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. This is the spasticity that normally accompanies a “stroke” in a human being.
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