The Sensorimotor System PDF

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Summary of the sensorimotor system, touch and other sense perceptions...

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The Sensorimotor System Three Principles of Sensorimotor Function The Sensorimotor system is hierarchically organised The main advantage of a hierarchal organisation is that the higher levels of the hierarchy are left free to perform more complex functions. The sensorimotor system is a parallel hierarchical system, in which signals flow between levels over multiple paths. It is also characterised by functional segregation; each level tends to be composed of different units, each of which performs a different function. In the sensorimotor system, information mainly flows down. Motor input is guided by sensory input The eyes, organs of balance, receptors in skin, muscles and joints all monitors the body’s responses and they feed their information back into sensorimotor circuits. Sensory feedback plays an important role in directing the continuation of the responses that produced it. Ballistic movements are not normally influenced by sensory feedback; theses are high speed movements e.g. swatting a wasp. Learning changes the nature and locus of sensorimotor control During the initial stages of motor learning, each individual response is performed under conscious control; then, after much practice, individual responses become organised into continuous integrated sequences of action that flow smoothly and are adjusted by sensory feedback without conscious regulation. General model of sensorimotor function

Sensorimotor Association Cortex Association cortex is at the top of the sensorimotor hierarchy. There are two major areas of sensorimotor association cortex: the posterior parietal cortex and the dorsolateral prefrontal association cortex. These are composed of several different areas, each with different functions. Posterior Parietal Association cortex The posterior parietal association cortex plays an important role in directing behaviour by providing spatial information and in directing attention. The posterior parietal cortex is classified as association cortex because it receives input from more than one sensory system. It receives information from the three sensory systems that play roles in the

localisation if the body and external objects in space: the visual system, the auditory system, and the somatosensory system. In fMRI studies and transcranial magnetic stimulation (TMS) in humans indicate that the posterior parietal cortex contains a mosaic of small areas, each specialised for guiding particular movements of eyes, head, arms, or hands (Man et al., 2015; Wang et al., 2015). Damage to the posterior parietal cortex can produce a variety of deficits, including deficits in the perception and memory of spatial relationships, in accurate reaching and grasping, in the control of eye movement, and in attention. However, apraxia and contralateral neglect are the two most striking consequences of posterior parietal cortex damage. Apraxia is a disorder of voluntary movement that is not attributable to a simple motor deficit or to any deficit in comprehension or motivation. Although its symptoms are usually bilateral, apraxia is often caused by unilateral damage to the left posterior parietal cortex or its connections. Contralateral neglect is a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite (contralateral) to the side of a brain lesion in the absence of simple sensory or motor deficits. Most patients with contralateral neglect have difficulty responding to things to the left. The deficits in responding occur for stimuli to the left of their own bodies, referred to as egocentric left. Ecocentric left is partially defined but gravitational coordinates: when patients tilt their heads, their field of neglect is not normally tilted with it. Many patients tend not to respond to the left sides if objects, regardless of where the objects are in their visual fields. Some of these patients who are said to have suffered from object-based contralateral neglect, fail to respond to the left side of objects (e.g. the left hand of a statue) even when the objects are presented horizontally or upside down. Patients with contralateral neglect can however perceive unconsciously. Dorsolateral prefrontal association cortex Receives projections from the posterior parietal cortex, and it sends projections to areas of secondary motor cortex, to primary motor cortex, and the frontal eye field. The response properties of dorsolateral prefrontal neurons suggest that decisions to initiate voluntary movements may be made in this area of cortex, but these decisions depend on critical interactions with posterior parietal cortex and other areas of frontal cortex. Secondary motor cortex Areas of secondary motor cortex are those that receive much or their input from association cortex and send much of their output to primary motor cortex. The supplementary motor area wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure, and the premotor cortex runs in a strip from the supplementary motor area to the lateral fissure. Identifying the areas of secondary motor cortex With neuroanatomical and neurophysiological research with monkeys, at least 8 areas of secondary motor cortex in each hemisphere have been suggested. Three different supplementary motor areas, two premotor areas and three small areas- the cingulate motor areas- in the cortex of the cingulate gyrus. To qualify as a secondary motor area, an area must be appropriately connected with association and secondary motor areas. Neurons in an area of secondary motor cortex often become more active just prior to the initiation of a voluntary movement and continue to be active throughout the movement.

Areas of secondary motor cortex are thought to be involved in the programming of specific patterns of movements after taking general instructions from dorsolateral prefrontal cortex. Mirror Neurons Mirror neurons are neurons that fire when an individual performs a particular goal-directed hand movement or when the observe the same goal-directed movement performed by another. Neurons that seemed to encode particular goal subjects; these neurons fired when a monkey reached for one object (e.g. a toy) but not when the monkey reached for another object. Mirror neurons provide a possible mechanism for social cognition. Mapping the actions of other onto one’s own actions repertoire would facilitate social understanding, cooperation and imitation. Support for this idea comes from demonstrations that these neurons respond to the understanding of the purpose of an action, not to some superficial characteristic of the action itself. E.g. mirror neurons that reacted to the sight of an action that made a sound were found to respond just as robustly to the sound alone- i.e. they responded fully to the particular action and its goal regardless of how it was detected. Many ventral premotor mirror neurons fire even when a monkey does not perceive the key action but just creates a mental representation of it. There is no direct evidence that mirror neurons are responsible for human findings (areas of human motor cortex that are active when a person performs watches or imagines a particular action. It is possible that different neurons in the same cortical areas contribute to the functional MRI activity in these different conditions. However, the mirror mechanisms identified by functional MRI in humans tend to be in the same areas of cortex as those identified by single cell recording in macaques. Primary Motor Cortex Located in the precentral gyrus of the frontal lobe. It is the major point of convergence of cortical sensorimotor signals, and it is the major, but not the only, point of departure of sensorimotor signals from the cerebral cortex. Conventional view of primary cortex motor cortex function The somatotopic layout of the human primary motor cortex is commonly referred to as the motor homunculus. Each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints that the site influences. Until recently, each neuron was thought to encode the direction of movement. The main evidence for this was the finding that each neuron in the arm area of the primary motor cortex fires maximally when the arm reaches in a particular direction, and that each neuron has a different preferred direction. Current view of primary motor cortex function Recent studies have revealed a crude somatotopic organisation, that is, stimulation in the face area tended to elicit face movements. However, the elicited responses were complex species-typical movements that often involved several parts of the

body, rather than individual muscle contractions. Also, sites that moved a particular body part overlapped greatly with sites that moved other body parts. If a monkey reached toward a particular location, primary motor cortex neurons sensitive to that target location tended to become active regardless of the direction of the movement that was needed to get to the target. The neurons of the primary motor cortex play a major role in initiating body movements. The effects of primary motor cortex lesions Extensive damage to the human primary motor cortex has less effect than you might expect, given that this is the major point of departure of motor fibres from the cerebral cortex. Large lesions may disrupt a patient’s ability to move one body part (e.g. one finger) independently of others, may produce astereognosia, and may reduce the speed, accuracy, and force of a patient’s movements. But these lesions do not eliminate voluntary movements, presumably because there are parallel pathways that descend directly from secondary and association motor areas to subcortical motor circuits without passing through primary motor cortex. Cerebellum and Basal Ganglia Both the cerebellum and basal ganglia interact with different levels of the sensorimotor hierarchy and, in doing so, coordinate and modulate its activities. Cerebellum The functional complexity of the cerebellum is suggested by its structure and complex connectivity with other brain structures. The cerebellum contains more than half of the brain’s neurons. The cerebellum receives information from primary and secondary motor cortex, information about descending motor signals from brain-stem nuclei, and feedback from motor responses via the somatosensory and vestibular systems. Though to compare these three sources of input and correct ongoing movements that deviate from their intended course. It is believed to play a major role in motor learning, particularly in the learning of sequences of movements in which timing is a critical factor. The effects of diffuse cerebellar damage on motor function are devastating. The patient loses the ability to control precisely the direction, force, velocity, and amplitude of movements and the ability to adapt patterns of motor output to changing conditions. The functions of the cerebellum were once thought to be entirely sensorimotor. Patients with cerebellar damage often display diverse sensory, cognitive, and emotional deficits. There are currently no accepted theories of cerebellar function that account for this diversity. Basal Ganglia The basal ganglia does not contain as many neurons as the cerebellum, but in one sense they are more complex. The basal ganglia are a complex heterogenous collection of interconnected nuclei. The anatomy of the basal ganglia suggests that, like the cerebellum, they perform a modulatory function. They are part of neural loops that receive cortical input from various cortical areas and transmit it back to the cortex via the thalamus. Many of these loops carry signals to and from the motor area of the cortex. The traditional view of the basal ganglia was that they play a role in the modulation of motor output. Now, the basal ganglia are thought to be involved in a variety of cognitive functions. This view is consistent with the fact that they project to cortical area known to have cognitive functions. Descending Motor Pathways Neural signals are conducted from the primary motor cortex to the motor neurons of the spinal cord over four different pathways. Two pathways descend in the dorsolateral region of the spinal cord – collectively

known as the dorsolateral motor pathways, and two descend in the ventromedial region of the spinal cord – collectively known as the ventromedial motor pathways. Dorsolateral Corticospinal tract and dorsolateral Corticorubrospinal tract One group of axons that descends from the primary motor cortex does so through the medullary pyramids (two bulges on the ventral surface of the medulla) then decussates and continues to descend in the contralateral dorsolateral spinal white matter. The group of axons constitutes the dorsolateral corticospinal tract. Most notably are the Betz cells – extremely large pyramidal neurons of the primary motor cortex. Most axons of the dorsolateral corticospinal tract synapse on small interneurons of the spinal grey matter, which synapse on the motor neurons of distal muscles of the wrist, hands, fingers, and toes. Primates and a few other mammals that are capable of moving their digits independently have dorsolateral corticospinal tract neurons that synapse directly on digit neurons. A second group of axons that descends from the primary motor cortex synapses in the red nucleus of the midbrain. The axons of neurons in the red nucleus decussate and descend through the medulla, where some of them terminate in the nuclei of the cranial nerves that control the muscles of the face. The rest continue to descent in the dorsolateral portion of the spinal cord. This pathway is called the dorsolateral Corticorubrospinal tract (rubro refers to the red nucleus). These axons synapse on interneurons that in turn synapse on motor neurons that project to the distal muscles of the arms and legs. Ventromedial Corticospinal Tract and Ventromedial Cortico-brainstem-spinal Tract There are two major divisions of the ventromedial motor pathway, one direct and one indirect. The direct ventromedial pathway is the ventromedial corticospinal tract, and the indirect one is the ventromedial cortico-brainstem-spinal tract. The long axons of the ventromedial corticospinal tract descend ipsilaterally from the primary motor cortex directly into the ventromedial areas of the spinal white matter. As each axon of the ventromedial corticospinal tract descends, it branches diffusely and innervates the interneuron circuits in several different spinal segments on both dies of the spinal grey matter. The ventromedial cortico-brainstem-spinal tract comprises motor cortex axons that feed into a complex network of brain stem structures. The axons of some of the neurons in this complex brain stem motor network then descend bilaterally in the ventromedial portion of the spinal cord. Each side carries signals from both hemispheres, and each neuron synapses on the interneurons of several different spinal cord segments that control the proximal muscles of the trunk and limbs. There are four major brain stem structures that interact with the ventromedial cortico-brainstem-spinal tract: (1) the tectum, which receives auditory and visual information

about spatial location, (2) the vestibular nucleus, which receives information about balance from receptors in the semi-circular canals of the inner ear, (3) the reticular formation, which (among other things) contains motor programs that regulate complex species-typical movements such as walking, swimming and jumping and (4) the motor nuclei of the cranial nerves that control the muscles of the face.

Comparison of the two dorsolateral motor pathways and the two ventromedial motor pathways Both are composed of two major tracts; one whose axons descend directly to the spinal cord and another whose axons synapse in the brain stem on neurons that in turn descend to the spinal cord. However, these two differ in two major respects:  The ventromedial tracts are much for diffuse. Many of their axons innervate interneurons on both sides of the spinal grey matter and in several different segments. Those axons of the two dorsolateral tracts terminate in the contralateral half of one spinal cord segment, sometimes directly on a motor neuron.  The motor neurons activated by the two ventromedial tracts project to proximal muscles of the trunk limbs (e.g. shoulder muscles). Whereas the motor neurons activated by the two dorsolateral tracts project to distal muscles (e.g. finger muscles). Because the four descending motor tracts originate in the cerebral cortex, all are presumed to mediate voluntary movement. However, major differences in their routes and destinations suggest they have different functions. Lawrence and Kuypers (1968) used monkeys in two experiments. In the first experiment (1968a) they transected the left and right dorsolateral corticospinal tracts of their subjects in the medullary pyramids, just above the decussation of the tracts. The monkeys after surgery could stand, walk and climb quite normally; however, their ability to use their limbs for other activities was impaired. E.g. their reaching movements were weak and poorly directed. After a few weeks the monkeys never regained the ability to move their fingers independently of one another; when they picked up their pieces of food, they did so by using their fingers as a unit. Also, they never regained the ability to release objects from their grasp, once they picked up a piece of food, they often had to root for it in their hand (like a pig). However, it is remarkable that they had no difficulty releasing their grasp on the bars of their cage when they were climbing. This shows that the same response performed in different contexts can be controlled by different parts of the CNS. In their second experiment (1968b), additional transections were made in the monkeys whose dorsolateral corticospinal tracts had already been transected in the first experiment. The dorsolateral Corticorubrospinal tract was transected in one group of these monkeys. They could still walk, stand and climb but when they sat down their limbs hung limply by their sides. When they did try to reach it was very poor. The other group in the 2nd experiment had both their ventromedial tracts transected. These subjects had severe postural abnormalities; they had great difficulty walking or sitting. If they did manage to sit or stand without clinging onto the bars, the slightest disturbance would make them fall. The additional transection of the ventromedial tracts eliminated their ability to control their shoulders. When they fed, they did so with elbow and whole-hand movements while their upper arms hung limply by their sides. These experiments suggest that the two ventromedial tracts are involved in the control of posture and whole-body movements (e.g. walking and climbing) and that they can exert control over the limb movements involved in such activities. Both of the dorsolateral tracts control the movements of limbs. Only the corticospinal division is capable of mediating independent movements of the digits.

Sensorimotor Spinal Circuits Muscles Motor units are the smallest units of motor activity. Each motor unit is comprised of a single motor neuron and all of the individual skeletal muscle fibres that it innervates. When the motor neuron fires all the fibres if its unit contract together. The units with the fewest fibres, like the fingers and face, permit the highest degree of selective motor control. A skeletal muscle are hundreds of thousands of fibres bound together in a tough membrane and attached to a bone by a tendon. Acetylcholine which is released by motor neurons at neuromuscular junctions which activates the motor end-plate on each muscle fibre and causes the fibre to contract. Contraction is the only method muscles have for generating force, thus any muscle can generate force in only one direction. All of the motor neurons that innervate the fibres of a single muscle are called its motor pool. Skeletal muscles are often considered to be two basic types: fast muscles fibres and slow muscle fibres. Fast muscles fibres contract and relax quickly and are capable of generating great force. But these fatigue quickly because they are poorly vascularised (few blood vessels). Slow muscl...