Motor Cortex PDF 2021-2022

Motor Cortex Dr. Ali Alkaleel Cyprus International University Faculty of Medicine 2021-2022  2 3 1 4 7 6 5  Central sulcus Motor Corte...

Motor Cortex Dr. Ali Alkaleel Cyprus International University Faculty of Medicine 2021-2022  2 3 1 4 7 6 5  Central sulcus Motor Cortex longitudinal fissure Anterior to the central cortical sulcus, occupying approximately the posterior one third of the frontal lobes, is the motor cortex. Posterior to the central sulcus is the somatosensory cortex, which feeds the motor cortex many of the signals that initiate motor activities. Sylvian fissure  Central sulcus longitudinal fissure Divided into 3 sub areas 1. Primary motor cortex (area 4) lies in the first convolution of the frontal lobes anterior to the central sulcus. It begins laterally in the sylvian fissure, spreads superiorly to the uppermost portion of the brain, and then dips deep into the longitudinal fissure. (This area is the same as area 4 in Brodmann’s classification of the brain cortical areas). – Unequal topographic representation. – Fine motor movement elicited by stimulation. Sylvian fissure This mapping was done by electrically stimulating the different areas of the motor cortex in human beings who were undergoing neurosurgical operations Note that more than one half of the entire primary motor cortex is concerned with controlling the muscles of the hands and the muscles of speech. The cortical representation of each body part is proportionate to the skill of that part: fine, voluntary, precise movements. Point stimulation in these hand and speech motor areas on rare occasion causes contraction of a single muscle, but most often, stimulation contracts a group of muscles. To express this in another way, excitation of a single motor cortex neuron usually excites a specific movement rather than one specific muscle. 2. 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. Topographical organization similar to primary motor cortex. Nerve signals generated in the premotor area cause much more complex “patterns” of movement than the discrete patterns generated in the primary motor cortex Stimulation results in movement of muscle groups to perform a specific task. The most anterior part develops a “motor image” of the movement. The posterior premotor cortex excites each successive pattern of muscle activity required to achieve the image. This posterior part 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. A special class of neurons called mirror neurons; he/she observes (heard or seen) the same task performed by others. (learning new skills by imitation) works in concert with other motor areas to control of complex pattern of muscle activity. 3. Supplemental motor area Topographically organized. Simulation often elicits bilateral movements. functions in concert with premotor area to provide attitudinal, fixation or positional movement for the body. It provides the background for fine motor control of the arms and hands by premotor and primary motor cortex.  Specialized Areas of the Motor Cortex Broca’s area Word formation / Directs muscles of tongue, lips, and throat that are used in speech production. damage causes decreased speech capability. closely associated area controls appropriate respiratory function for speech. Eye fixation and head rotation area for coordinated head, eye and eyelid movements Damage to this area prevents a person from voluntarily moving the eyes toward different objects. Hand skills area damage causes motor apraxia the inability to perform fine hand movements. Incoming Sensory Pathways to Motor Cortex subcortical fibers from adjacent areas of the cortex especially from somatic sensory areas of parietal cortex, visual and auditory cortex. subcortical fibers from opposite hemisphere which pass through corpus callosum. Thalamus somatic sensory fibers from ventral posterior complex (i.e. cutaneous and proprioceptive fibers). ventrolateral and ventroanterior nuclei of thalamus for coordination of function between motor cortex, basal ganglia, and cerebellum. fibers from the intralaminar nuclei of thalamus (control level of excitability of the motor cortex), some of these may be pain fibers. Transmission of Cortical Motor Signals Direct pathway (for discrete detailed movement) – corticospinal tract. Indirect pathway – signals to basal ganglia, cerebellum, and brainstem nuclei.  1. The Corticospinal Tract (pyramidal tract) The most important output pathway from the motor cortex is the corticospinal tract, also called the pyramidal tract originates in the primary motor cortex (30%), and supplementary motor areas (30%), and somatic sensory areas (40%). majority of fibers (80%) cross to opposite side in medulla and descend in the lateral corticospinal tracts.(mostly concerned with the movement of hand and legs), rest = ventral tract. corticospinal fibers synapse with interneurons anterior motor neurons and a few sensory relay neurons in the cord gray matter. giant pyramidal cells (Betz cells) give rise to large fibers with fast transmission rates (70m/sec)  34,000 Betz cell fibers, make up only about 3% of the total number of fibers. 97 % of the 1 million fibers are small diameter fibers. conduct background tonic signals feedback signals from the cortex to control intensity of the various sensory signals to the brain A few of the fibers do not cross to the opposite side in the medulla but pass ipsilaterally down the cord in the ventral corticospinal tracts. Many, if not most, of these fibers eventually cross to the opposite side of the cord either in the neck or in the upper thoracic region, (concerned with control of bilateral postural movements by the supplementary motor cortex) 2. Extrapyramidal system (indirect pathways) I. Betz collaterals back to cortex sharpen the boundaries of the excitatory signal. Inhibit adjacent regions of the cortex, thereby “sharpening” the boundaries of the excitatory signal II. A large number of Fibers to caudate nucleus and putamen. III. Fibers to the red nucleus (corticorubral), which then sends axons to the cord in the rubrospinal tract. IV. To reticular substance, and vestibular nuclei; 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. V. To pontile nuclei and inferior olivary nuclei,, which give rise to the pontocerebellar fibers and olivocerebellar fibers Therefore the basal ganglia, brain stem and cerebellum receive a large number of signals from the motor cortex  From Brainstem to Spinal Cord All portions of the brainstem that contribute to the motor control but are not a part of the direct corticospinal-pyramidal system Red nuclei (Rubrospinal tract) Superior colliculus (Tectospinal tract) Reticular formation of the brain stem (Reticulospinal tract) Vestibular nuclei (Vestibulospinal tract)  Red Nucleus and the Rubrospinal Tract Substantial input from primary motor cortex (corticorubral tract + branching fibers from the corticospinal tract) Primary motor cortex fibers synapse in the lower portion of the nucleus called the magnocellular portion contains large neurons similar to Betz cells gives rise to rubrospinal tract has somatotopic organization similar to primary motor cortex stimulation of red nucleus causes relatively fine motor movement but not as discrete as primary motor cortex Rubrospinal tract: crosses in the lower brain stem and courses in the lateral columns of the spinal cord  Superior colliculus (Tectospinal tract) The tectospinal tract projects from the midbrain to the spinal cord and is important for postural movements that are driven by the superior colliculus. Terminate in the cervical region. Head movement in response to visual stimuli. Reticular formation of the brain stem (Reticulospinal tract) This tract influences trunk and proximal limb muscles related to posture and locomotion. Pontine Reticular Nuclei receive strong excitatory signals from vestibular nuclei and deep nuclei of the cerebellum transmit excitatory signals through pontinereticulospinal tract. pontine reticular nuclei have a high degree of natural excitability. when unopposed they cause powerful excitation of the antigravity muscles. Medullary Reticular Nuclei these nuclei receive collateral input from the corticospinal tract, rubrospinal tract, and other motor pathways. transmit inhibitory signals to the antigravity muscles through the medullaryreticulospinal tract. these systems can activate the inhibitory action of the medullary reticular nuclei and counterbalance the signals from the pons. The Decerebrate Animal Develops Spastic Rigidity: When the brain stem of an animal is sectioned below the midlevel of the mesencephalon, the pontine and medullary reticular systems as well as the vestibular system are left intact, and the animal develops a condition called decerebrate rigidity. This rigidity does not occur in all muscles of the body but does occur in the antigravity muscles: the muscles of the neck and trunk and the extensors of the legs. The cause of decerebrate rigidity is blockage of normally strong input to the medullary reticular nuclei from the cerebral cortex, the red nuclei, and the basal ganglia. Lacking this input, the medullary reticular inhibitory system becomes nonfunctional; full over activity of the pontine excitatory system occurs, and rigidity develops.  Vestibular nuclei (Vestibulospinal tract) The vestibulospinal tract connects the brain stem nuclei of the vestibular system with the spinal cord. This allows posture, movement, and balance to be modulated on the basis of equilibrium information provided by the vestibular system. Maintaining the balance Influence axial muscles The specific role of the vestibular nuclei is to selectively control the excitatory signals to the different antigravity muscles to maintain equilibrium in response to signals from the vestibular apparatus.  Lateral Motor System: -Lateral corticospinal tract -Rubrospinal tract Innervate the distal flexors: fine, skilled movements Medial Motor system: -Ventral corticospinal tract -Vestibulospinal tracts -Reticulospinal tracts Innervate the proximal, axial muscles (extensors) Posture, equilibrium, antigravity Thank you Reflexes The reflex Arc 1. Receptor 2. Sensory neuron 3. Local Spinal Circuits 4. Motor neuron 5. Effector Classification of reflexes Sensory Receptor The sensory signal activates divergent polysynaptic reflex pathways. One excites motor neurons that innervate flexor muscles of the stimulated limb, whereas another inhibits motor neurons that innervate the limb’s extensor muscles (reciprocal innervation). Muscle Spindles Muscle spindles are small encapsulated sensory receptors that have a spindle-like or fusiform shape and are located within the fleshy part of a muscle. Their main function is to signal changes in the length of the muscle within which they reside. Changes in length of muscles are closely associated with changes in the angles of the joints that the muscles cross. Thus muscle spindles are used by the central nervous system to sense relative positions of the body segments. Each spindle has three main components: (1) intrafusal muscle fibers with central (noncontractile) (2) sensory fibers that terminate in the noncontractile central regions (3) motor axons that terminate in the polar contractile regions of the intrafusal fibers. Stretching of sensory ending increase their firing rate.  (The fusimotor system) Gamma Motor Neurons Adjust the Sensitivity of Muscle Spindles The contraction of the intrafusal fibers by the gamma motor neurons keeps the spindle under tension, thus maintaining the firing rate of the Ia fibers within an optimal range for signaling changes in length, whatever the actual length of the muscle. This alpha-gamma co-activation thus stabilizes the sensitivity of the muscle spindles and is used in many voluntary movements.  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.  Stretch Reflexes Reinforce Central Commands for voluntary Movements attempt to lift a heavy suitcase that we believe to be empty  crossed-extension reflex: The reflex can produce an opposite effect in the contralateral limb (in Flexion-withdrawal reflexes), that is, excitation of extensor motor neurons and inhibition of flexor motor neurons. This crossed- extension reflex serves to enhance postural support during withdrawal of a foot from a painful stimulus. Co-contraction Contract the prime mover and the antagonist at the same time. has the effect of stiffening the joint and is most useful when precision and joint stabilization are critical. An example of this phenomenon is the co- contraction of flexor and extensor muscles of the elbow immediately before catching a ball. Central Neurons Regulate the Strength of Spinal Reflexes at Three Sites in the Reflex Pathway 1. Alpha motor neurons.. 2. Interneurons 3. The presynaptic terminals of the afferent fibers. Signals from higher-level neurons regulate the strength of reflexes by changing the background (tonic) level of activity at any of the three sites in the spinal reflex pathway. So by working on; An increase in tonic excitatory input to the alpha motor neurons moves the membrane potential of these cells closer to threshold so that even the slightest reflex input will more easily activate the motor neurons. Modulating the strength of reflexes is to change the physiological properties of motor neurons and perhaps interneurons. Stretch Reflexes Jaw jerk reflex cn V Biceps reflex C5-6 Brachioradialis reflex C5\C6 Triceps reflex C7-8 Patellar reflex (knee jerk) L2-3-4 Ankle jerk reflex S1-S2  Golgi Tendon Reflex The opposite of the stretch reflex. Contracting the muscle activates the Golgi tendon organs. Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated. As a result, the contracting muscle relaxes and the antagonist muscle contracts. Thank you