Lecture 15: Upper Motor Flashcards
Overview of descending motor control - lower motor neurons and axons from the motor cortex
The most medial part of the ventral horn contains lower motor neuron pools that innervate axial muscles or proximal muscles of the limbs, whereas the more lateral parts contain lower motor neurons that innervate the distal muscles of the limbs.
Somatotopic organization of the ventral horn in the cervical enlargement. The locations of descending pro- jections from the motor cortex in the lateral white matter and from the brainstem in the anterior-medial white matter are shown.
Distribution of local circuit neurons
The local circuit neurons, which lie primarily in the intermediate zone of the spinal cord and supply much of the direct input to the lower motor neurons, are also topographically arranged. Thus, the medial region of the intermediate zone of the spinal cord gray matter contains the local circuit neurons that synapse primarily with lower motor neurons in the medial part of the ventral horn, whereas the lateral regions of the intermediate zone contain local neurons that synapse primarily with lower motor neurons in the lateral ventral horn.
Schematic illustration of the major pathways for descending motor control.
What does the medial ventral horn contain?
The medial ventral horn contains lower motor neurons that govern posture, balance, locomotion, and orienting movements of the head and neck during shifts of visual gaze. These medial motor neurons receive descending input from pathways that originate mainly in the brainstem, course through the anterior-medial white matter of the spinal cord, and then terminate bilaterally.
What does the lateral ventral horn contain?
The lateral ventral horn contains lower motor neurons that mediate the expression of skilled voluntary movements of the distal extremities. These lateral motor neurons receive a major descending projection from the contralateral motor cortex via the main (lateral) division of the corticospinal tract, which runs in the lateral white matter of the spinal cord.
Difference in terminal fields of upper motor neuron axons
The axons of the upper motor neurons that project to the medial part of the ventral horn and the medial region of the intermediate zone terminate over many SC segments among medial cells groups on both sides fo the SC.
The axons that project from the motor cortex to the SCcourse through the lateral white matter of the SC and terminate in the lateral parts of the ventral horn, with terminal fields that are restricted to only a few SC segments.
What have microsimulation of the motor cortex and recordings of muscle electrical activity revealed to us about the spatial map of the motor cortex?
Even the smallest currents capable of eliciting a response initiated the excitation of several muscles (and the simultaneous suppression of others), suggesting that organized movements rather than individual muscles are represented in the map.
Within major subdivisions of the map (e.g., forearm or face regions), a particular movement could be elicited by stimulation of widely separated sites, supporting the argument that neurons in nearby regions are linked by local circuits in the cortex and spinal cord to organize specific movements.
The regions responsible for initiating different movements overlap substantially.
Conclusion: movements—or action goals—rather than the contractions of individual muscles are encoded in the cortex
Where are the primary motor cortex and premotor areas located in the human cerebral cortex?
The primary motor cortex is located in the precentral gyrus. The mosaic of premotor areas is more rostral.
These cortical areas all receive regulatory input from the basal ganglia and cerebellum via relays in the ventrolateral thalamus. as well as inputs from sensory regions of the parietal lobe.
The low threshold for eliciting movements is an indicator of a rel- atively large and direct pathway from the primary area to the lower motor neurons of the brainstem and spinal cord.
what kinds of neurons are there in the primary motor cortex - upper motor neurons of the primary motor cortex
The pyramidal cells of cortical layer 5 are the upper motor neurons of the primary motor cortex. This include Betz cells which are the largest neurons (by soma size) in the human CNS.
Betz cells - too few of them to account for the number of axons that project from the motor cortex to the brainstem and spinal cord; play an important role in the activation of lower motor neurons that control muscle activities in the distal ex- tremities.
Smaller, non-Betz pyramidal neurons of layer 5 - found in the primary motor cortex and in each division of the premotor cortex. The axons of these upper motor neurons descend in the corticobulbar and cortico- spinal tracts.
The cortical spinal and corticobulbar tracts
Neurons in the motor cortex give rise to axons that travel through the internal capsule and coalesce on the ventral surface of the midbrain, within the cerebral peduncle. These axons continue through the pons and come to lie on the ventral surface of the medulla, giving rise to the medullary pyramids. As they course through the brainstem, corticobulbar axons (gold) give rise to bilateral collaterals that innervate brainstem nuclei.
Most of the corticospinal fibers (dark red) cross in the caudal part of the medulla to form the lateral corticospinal tract in the spinal cord. Those axons that do not cross (light red) form the ventral corticospinal tract, which terminates bilaterally.
More on corticobulbar axons
Most corticobulbar axons that govern the cranial nerve motor nuclei (see the Appendix) terminate bilater- ally on local circuit neurons embedded in the brainstem reticular formation. These local circuit neurons, in turn, coordinate the output of different groups of lower motor neurons in the cranial nerve motor nuclei. So, damage to the corti- cobulbar fibers on only one side does not result in dramatic deficits in function.
Exceptions - there is significant bias in favor of inputs from the contralateral
motor cortex. Related to lower facial movements. Lower facial movements that may be performed unilaterally—such as pushing the tongue against one cheek, biting on one side of the mouth, or raising or low- ering one corner of the mouth—are governed primarily by the contralateral motor cortex. Most other motor functions governed by cranial nerve nuclei in which the movements of the two sides are largely in synchrony (e.g., vocalization, salivation, tearing, swallowing) are subject to symmetrical, bilateral upper motor neuronal control.
Lateral corticospinal tract vs ventral corticospinal tract; fibres and decussation
Near the caudal end of the medulla, nearly all of the fibers in the medullary pyramids are corticospinal axons. Just before entering the spinal cord, about 90% of these axons cross the midline—decussate—to enter the lateral columns of the spinal cord on the opposite side, where they form the lateral corticospinal tract. The remaining 10% of the pyramidal tract fibers enter the spinal cord without crossing; these axons, which constitute the ventral (ante- rior) corticospinal tract, terminate bilaterally. Collateral branches of these axons cross the midline via the ventral white commissure of the spinal cord to reach the opposite ventral horn.
Lateral corticospinal tracts vs ventral corticospinal tract - termination
The lateral corticospinal tract forms a direct pathway from the cortex to the spinal cord and terminates primarily in the lateral portions of the ventral horn and intermediate gray matter. Some of these axons (including those derived from Betz cells) synapse directly on α motor neurons that govern the distal extremities. This is restricted to a subset of α motor neurons that supply the muscles of the forearm and hand; most axons of the lateral corticospinal tract, in contrast, terminate among pools of local circuit neurons that coordinate the activities of the lower motor neurons in the lateral cell columns of the ventral horn that innervate different muscles. This difference in terminal distribution implies a special role for the lateral corticospinal tract in the control of the hands.
Topographic map of movement in the primary motor cortex
Focal electrical stimulation (Sherrington) applied to the surface of the cortex in great apes confirmed Jackson’s early evidence for motor maps in the cortex (cf epilepsy). Penfield found that this motor map shows the same general disproportions observed in the somatosensory maps in the postcentral gyrus (see Chapter 9). Thus, the musculature used in tasks requiring fine motor control (such as movements of the face and hands) is represented by a greater area of motor cortex than is the musculature requiring less precise motor control (such as that of the trunk).
What happens with particular movements of specific parts of the body?
The discharge of individual pyramidal tract neurons varies.
The discharge of a motor cortex neuron with an axon that projects down the pyramidal tract is recorded while monkey makes a sequence of flexion and extension movements of the wrist. The three parts of the figure show three consecutive flexion-extension cycles, proceeding from top to bottom. In the trace showing wrist position the direction of flexion is down and extension up. The pyramidal tract neuron discharges before and during extensions and is reciprocally silent during flexion movements. It does not discharge during movements of the body parts. Other motor cortex neurons show the opposite pattern of activity, discharging before and during flexion movements.
The activity of some primary motor cortex neurons cane correlated with what? And implications on the motor cortex map?
Particular patterns of muscle activity. The bursts of activity in a single corticomotorneuron during a reach and grasp movement to retrieve food pellets from a small well are correlated with bursts of contractile activity in several of its target muscles at different times during the movements.
the map in the motor cortex is far more complex than a columnar representation of particular muscles. Individual pyramidal tract axons are now known to terminate on sets of spinal motor neurons that innervate different muscles. Not one corticomotorneuron = one muscle. And stimulating it does not cause contraction in one muscle at one specific time. More complex.
The motor cortex contains a topographic map of motor output to different parts of the body
Studies by Clinton Woolsey and colleagues confirmed that the representation of different body parts int he monkey follows an orderly plan: Motor output to the foot and leg is medial, whereas the arm, face, and mouth areas are more lateral. The areas of cortex controlling the foot, hand and mouth are much larger than the regions controlling other parts of the body.
Penfield and colleagues showed that the human motor cortex motor map has the same general mediolateral organisation as the monkey. The areas controlling the hand and mouth are even larger in humans, whereas the area controlling the foot is much smaller. Penfield’s map was meant to show the relative size of the representation of each body part, not that each body part was controlled by a single separate part of the motor map.
Complexity of motor map. What does the motor map actually represent?
cortical microstimulation experiments have shown that contrac- tion of a single muscle can be evoked by stimulation over a wide region of the mo- tor cortex (about 2 to 3 mm in macaque monkeys) in a complex, mosaic fashion. It seems likely that horizontal connections within the motor cortex and local circuits in the spinal cord create ensembles of neurons that coordinate the pattern of firing in the population of ventral horn cells that ultimately generate a given movement.
the fine structure of the map is far more abstract.
Maps of movement.
Muscle field and implications for understanding of motor cortex motor map
the activity of multiple different muscles is directly facilitated by the discharges of a given upper motor neuron. This peripheral muscle group is referred to as the muscle field of the upper motor neuron.
These observations, which confirmed that single upper motor neurons contact several lower motor neuron pools, are consistent with the general conclusion that the activity of the upper motor neurons in the cortex controls movements, rather than individual muscles.
Purposeful movements of the contralateral arm and hand in macaque monkey - experiment and revelations.
Prolonged microstimulation of primary motor cortex sites near the middle of
the precentral gyrus elicits coordinated movements of the hand and mouth (A) or movements of the arm that bring the hand toward central space, as if to visually inspect and manipulate a held object . Graziano et al.
When such stimuli are applied to the precentral gyrus, the resulting movements are sequentially distributed across multiple joints and are strikingly purposeful. Examples of motor patterns frequently elicited with prolonged microstimulation of the precentral gyrus are movements of the hand to the mouth as if to feed, movements that bring the hand to central space as if to inspect an object of interest, and defensive postures as if to protect the body from an impending collision.
==> purposeful movements are organized by the circuitry of the primary motor cortex and that their somatotopic organization is best understood in the context of ethologically relevant behaviors
Directional tuning of an upper motor neuron in the primary motor cortex
The commands to perform precise movement patterns are encoded in the activity of a large population of upper motor neurons integrated by intracortical circuitry.
Recording from cortical neurons during visually guided reaching movements of the arm and hand.
Using this paradigm, the direction of arm movements in monkeys could be predicted by calculating a “neuronal population vector” derived simultaneously from the discharges of a population of upper motor neurons that are “broadly tuned” in the sense that each neuron dis- charges prior to movements in many directions. These observations showed that the discharges of individual upper motor neurons cannot specify the direction of an arm movement, simply because they are tuned too broadly. Each arm movement must be encoded by the concurrent discharges of a large population of such functionally linked neurons.
What are the implications of the same site in the 1º motor cortex encoding diff trajectories of motion depending on the starting position of the limb?
Multiple parameters of movement may be selected by the relevant ensemble of upper motor neurons to achieve a behaviorally useful action. Populations of upper motor neurons whose output encodes not simply the trajectory of arm motion, but also the final position of the hand in the context of an action goal.
What happens after the pyramidal tract is lesioned?
Abolishment of fine grasping movements. The monkey can remove the food only by grabbing clumsily with the whole hand; cannot make individuated movements of wrist, fingers, thumb. Change results mainly from the loss of direct inputs from corticomotorneurons onto spinal motor neurons.
Is the motor cortex the only area devoted to motor control?
No, multiple areas of the cerebral cortex are devoted to motor control and many are somatotopically organised.
