Cortical and Brain Stem Control of Motor Functions PDF

Cortical and brain stem control of motor functions Mona A Hussain Assistant Prof. in Physiology department Tracking and striking a tennis ball require a sophisticated system of motor control. Vander’s human physiology  Co...

Cortical and brain stem control of motor functions Mona A Hussain Assistant Prof. in Physiology department Tracking and striking a tennis ball require a sophisticated system of motor control. Vander’s human physiology  Control of voluntary movement Commands for voluntary movement originate in cortical association areas. The cortex, basal ganglia, and cerebellum work cooperatively to plan movements. Movement executed by the cortex is relayed via the corticospinal tracts and corticobulbar tracts to motor neurons. The cerebellum provides feedback to adjust and smoothen movement.  The motor cortex The motor cortex is anterior to the central cortical sulcus, occupying approximately the posterior one third of the frontal lobes. SOME SPECIALIZED AREAS OF MOTOR CONTROL FOUND IN THE HUMAN MOTOR CORTEX  Motor cortex subareas The motor cortex is divided into three subareas: (1) the primary motor cortex, (M1 or area 4) (2) the premotor area, and (3) the supplementary motor area  Primary motor cortex (area 4) The motor presentation: 1- The feet at the top of the gyrus and the face at the bottom (inverted). 2- The facial area is represented bilaterally, but the rest of the representation is generally unilateral. 3- The cortical motor area controlling the musculature on the opposite side of the body (Crossed). 4- The cortical representation of each body part is proportional in size to the skill with which the part is used in fine, voluntary movement. 5- The areas involved in speech and hand movements are especially large in the cortex; use of the pharynx, lips, and tongue to form words and of the fingers and opposable thumbs to manipulate the environment are activities in which humans are especially skilled. 6- excitation of a single motor cortex neuron usually excites a specific movement rather than one specific muscle. To do this, it excites a “pattern” of separate muscles, each of which contributes its own direction and strength of muscle movement. Primary motor cortex  PLASTICITY 1- When a small focal ischemic lesion is produced in the hand area of the motor cortex of monkeys, the hand area may reappear, with return of motor function, in an adjacent undamaged part of the cortex. 2- The finger areas of the contralateral motor cortex enlarge as a pattern of rapid finger movement is learned with the fingers of one hand; this change is detectable at 1 week and maximal at 4 weeks. Thus, the maps of the motor cortex are not immutable, and they change with experience. (Plasticity)  PREMOTOR AREA The premotor area, lies 1 to 3 centimeters anterior to the primary motor cortex. It extends inferiorly into the sylvian fissure and superiorly into the longitudinal fissure, where it abuts the supplementary motor area, which has functions similar to those of the premotor area. The topographical organization of the premotor cortex is roughly the same as that of the primary motor cortex, with the mouth and face areas located most laterally; as one moves upward, the hand, arm, trunk, and leg areas are located.  PREMOTOR AREA Nerve signals generated in the premotor area cause much more complex “patterns” of movement than the discrete patterns generated in the primary motor cortex. For instance, the pattern may be to position the shoulders and arms so that the hands are properly oriented to perform specific tasks. To achieve these results, the most anterior part of the premotor area first develops a “motor image” of the total muscle movement that is to be performed Then, in the posterior premotor cortex, this image excites each successive pattern of muscle activity required to achieve the image. This posterior part of the premotor cortex sends its signals either directly to the primary motor cortex to excite specific muscles or, often, by way of the basal ganglia and thalamus back to the primary motor cortex.  Supplementary motor area Contractions elicited by stimulating this area are often bilateral rather than unilateral. For instance, stimulation frequently leads to bilateral grasping movements of both hands simultaneously; these movements are perhaps rudiments of the hand functions required for climbing. This area functions in concert with the premotor area to provide body-wide attitudinal movements, fixation movements of the different segments of the body, positional movements of the head and eyes, and so forth, as background for the finer motor control of the arms and hands by the premotor area and primary motor cortex. supplementary motor area may be involved primarily in organizing or planning motor sequences, while M1 executes the movements.  POSTERIOR PARIETAL CORTEX The somatic sensory area and related portions of the posterior parietal lobe project to the premotor cortex. Lesions of the somatic sensory area cause defects in motor performance that are characterized by inability to execute learned sequences of movements such as eating with a knife and fork. Some of the neurons are concerned with aiming the hands toward an object and manipulating it, whereas other neurons are concerned with hand–eye coordination. Neurons in this posterior parietal cortex contribute to the descending pathways involved in motor control.  TRANSMISSION OF SIGNALS FROM THE MOTOR CORTEX TO THE MUSCLES 1- Direct (Motor cortex - corticospinal tract - spinal cord) 2-Indirect (Motor cortex- multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem.) ***In general, the direct pathways are concerned more with discrete and detailed movements, especially of the distal segments of the limbs, particularly the hands and fingers.  Corticospinal (Pyramidal) Tract The most important output pathway from the motor cortex is the corticospinal tract, also called the pyramidal tract. The corticospinal tract originates: 1- about 30 percent from the primary motor cortex, 2- 30 percent from the premotor and supplementary motor areas, 3- and 40 percent from the somatosensory areas. Corticospinal tracts  Termination of corticospinal tract Corticospinal tract is terminating principally on the interneurons in the intermediate regions of the cord gray matter; a few terminate on sensory relay neurons in the dorsal horn, and a very few terminate directly on the anterior motor neurons that cause muscle contraction.  Corticospinal tract Lesion in the lateral corticospinal tract results in loss of control of the distal musculature of the limbs, which is concerned with fine-skilled movements. On the other hand, lesions of the ventral corticospinal tract produce axial muscle deficits that cause difficulty with balance, walking, and climbing.  Other Fiber Pathways From the Motor Cortex The motor cortex gives rise to large numbers of additional, mainly small, fibers that go to deep regions in the cerebrum and brain stem, including the following: 1. short collaterals back to the cortex. These collaterals are believed to inhibit adjacent regions of the cortex, thereby “sharpening” the boundaries of the excitatory signal. 2. A large number of fibers pass from the motor cortex into the caudate nucleus and putamen. From there, additional pathways extend into the brain stem and spinal cord mainly to control body postural muscle contractions. 3. A moderate number of motor fibers pass to red nuclei of the midbrain. From these nuclei, additional fibers pass down the cord through the rubrospinal tract. Corticorubrospinal tract  Function of the Corticorubrospinal System The magnocellular portion of the red nucleus has a somatographic representation of all the muscles of the body, as does the motor cortex. Stimulation of a single point in this portion of the red nucleus causes contraction of either a single muscle or a small group of muscles. However, the fineness of representation of the different muscles is far less developed than in the motor cortex, especially in human beings, who have relatively small red nuclei.  Function of the Corticorubrospinal System The corticorubrospinal pathway serves as an accessory route for transmission of relatively discrete signals from the motor cortex to the spinal cord. When the corticospinal fibers are destroyed but the corticorubrospinal pathway is intact, discrete movements can still occur, except that the movements for fine control of the fingers and hands are considerably impaired. Wrist movements are still functional. the corticospinal and rubrospinal tracts together are called the lateral motor system of the cord.  Other Fiber Pathways From the Motor Cortex 4. A moderate number of motor fibers deviate into the reticular substance and vestibular nuclei of the brain stem; from there, signals go to the cord by way of reticulospinal and vestibulospinal tracts, and others go to the cerebellum by way of reticulocerebellar and vestibulocerebellar tracts. 5. A tremendous number of motor fibers synapse in the pontine nuclei, which give rise to the pontocerebellar fibers, carrying signals into the cerebellar hemispheres. 6. Collaterals also terminate in the inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum. Thus, the basal ganglia, brain stem, and cerebellum all receive strong motor signals from the corticospinal system every time a signal is transmitted down the spinal cord to cause a motor activity.  “Extrapyramidal” System The term extrapyramidal motor system has been used in clinical circles to denote all the portions of the brain and brain stem that contribute to motor control but are not part of the direct corticospinal- pyramidal system. These portions include pathways through the basal ganglia, the reticular formation of the brain stem, the vestibular nuclei, and often the red nuclei. CONTROL OF MOTOR FUNCTIONS BY THE BRAIN STEM  CONTROL OF MOTOR FUNCTIONS BY THE BRAIN STEM Pontine Reticular System The pontine reticular nuclei transmit excitatory signals downward into the cord through the pontine reticulospinal tract in the anterior column of the cord. The fibers of this pathway terminate on the medial anterior motor neurons that excite the axial muscles of the body, which support the body against gravity—that is, the muscles of the vertebral column and the extensor muscles of the limbs. The pontine reticular nuclei have a high degree of natural excitability. In addition, they receive strong excitatory signals from the vestibular nuclei.  Medullary Reticular System The medullary reticular nuclei transmit inhibitory signals to the same antigravity anterior motor neurons by way of a different tract, the medullary reticulospinal tract, located in the lateral column of the cord. The medullary reticular nuclei receive strong input collaterals from (1) the corticospinal tract, (2) the rubrospinal tract, and (3) other motor pathways. These tracts and pathways normally activate the medullary reticular inhibitory system to counterbalance the excitatory signals from the pontine reticular system, so under normal conditions the body muscles are not abnormally tense.  Vestibulo-spinal tracts in association with the pontine reticular nuclei to control the antigravity muscles. The vestibular nuclei transmit strong excitatory signals to the antigravity muscles by way of the lateral and medial vestibulospinal tracts in the anterior columns of the spinal cord. Without this support of the vestibular nuclei, the pontine reticular system would lose much of its excitation of the axial antigravity muscles. The specific role of the vestibular nuclei, however, is to selectively control the excitatory signals to the different antigravity muscles to maintain equilibrium in response to signals from the vestibular apparatus. UMNL vs LMNL  Clasp-knife response Passive flexion of elbow meets immediate resistance due to stretch reflex in the triceps muscle. Further stretch activates inverse stretch reflex. The resistance to flexion suddenly collapses, and the elbow flexes. Clasp-knife response refers to a Golgi tendon reflex with a rapid decrease in resistance when attempting to flex a joint passively. It is one of the characteristic responses of an UMNL. It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied. Planter response or Babinski sign