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Voluntary Movement
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Voluntary movements

1.Voluntary movements are organized around the performance of purposeful task.

2.The effectiveness of Voluntary movements improves with experience and learning.

3.Unlike reflexes Voluntary movements are not mere responses to environmental stimuli but can be generated internally. The higher level of our motor system can therefore dissociate two aspects of stimulus-its informational content and its capacity to trigger movement. In the cortex the information content of a stimulus signals where to move or what to do, but the occurrence of the stimulus may or may not actually initiate the appropriate movement. In reflexes these aspects of stimulus are linked.

Reflex actions are relatively stereotyped and repertory of movement is limited.

                The motor areas of the cerebral cortex are subdivided into a

Primary motor area and several Pre-motor areas.

(Each area contains populations of neurons that project from cortex to the brain and spinal cord.)

There are four main pre motor areas

-Two on the lateral convexity 1. Lateral ventral pre motor area 2.Lateral dorsal pre motor area

Two on the medial wall of the hemisphere are

1.supplementary motor area 2.cingulate motor area, located in the banks of the cingulate sulcus.

-Motor maps of the face and extremities can be delineated in each pre motor area. However, unlike primary motor cortex, where stimulation typically evokes simple movements of single joints, stimulation of the pre motor area evokes more complex movements involving multiple joints & resembling natural coordinated hand shaping or reaching movements.

-Stimulation of the supplementary motor area, can give rise to bilateral movements, suggesting that this area has a role in coordinating movements on the two sides of the body.

-All pre-motor areas project both the primary motor cortex and spinal cord.

-In the spinal cord the areas of termination of pre-motor and primary motor projections overlap.

-The existence of these direct monosynaptic connections suggests that the pre-motor neurons can control hand movements independently of the motor cortex.

 

 

Voluntary movement is organized in the cortex

The primary motor cortex controls simple features of movement

Contra lateral pre-central gyrus (Brodmans area 4), the region now called primary motor cortex, proved to be the area in which lowest intensity stimulation elicited movements. The motor maps show an orderly arrangement along the gyrus of control areas for the face, digits, hand, arm, trunk, leg and foot. However, the fingers, hands, and face-which are used in tasks requiring the greatest precision and finest control-have disproportionately large representations in the motor area cortex, much as the inputs from regions of the body that have important roles in perception predominate in sensory area of the cortex.

Consistent with the overall somatotopic organization, lesions in arm representation lead to degeneration of myelinated fibers in the cervical cord, while lesions in leg representation produced degeneration extending all the way to lumbar spinal cord.

                The early mapping experiments stimulating the cortical surface electrically led to the simplistic idea that primary motor cortex act as massive switchboard with a switch controlling individual muscles or small group of adjacent muscles.

-Weakest stimuli may evoke the contraction of individual muscles; the same muscles are invariably activated from several sites as well, indicating the neurons in several cortical sites project axons to same target.

-Most stimuli activate several muscles rarely being activated individually.

-Sites influencing distal muscles are contained at center of a wider area containing sites that influence more proximal muscles, while sites in peripheral ring around this central area influence proximal muscles alone.

Implication-Inputs to motor cortex from other cortical areas can combine proximal and distal muscles in different ways in different tasks.

 

Each cortical area receives unique cortical and sub cortical inputs

The primary motor cortex receives somatotopically-organized inputs directly from two sources.

1.It receives inputs from primary somatosensory cortex (areas 1,2,3).

2.Posterior parietal area 5.

-Posterior parietal areas 5 and 7 are involved in integrating multiple sensory modalities for motor planning.

The pre-motor areas receive major inputs from areas 5 and 7 as well from area 46 in the prefrontal cortex. Each pre motor area has its own pattern of inputs from distinct locations from areas 5 and 7.Area 46 projects mainly to the ventral pre motor area and is important in working memory; it is thought to store information about location of objects in space only long enough to guide a movement. There are also dense connections between the premotor areas themselves. These connections are thought to allow working memory to influence specific aspects of motor planning that are mediated by the different pre motor sub regions.

                The pre motor areas and primary motor cortex also receive input from the basal ganglia and cerebellum via different sets of nuclei in the ventrolateral thalamus. The BG and cerebellum do not project directly to the spinal cord.

-The connections between cortical areas and subcotical structures are the reciprocal nature. Each cortical area appears to have a unique pattern of cortical and sub cortical input. Thus there are many cortico-subcortical loops, each one making a different contribution to a motor behavior.

 

The somatotopic organization of the motor cortex is plastic

The somatotopic organization of the motor cortex is not fixed but can be altered during motor learning and following injury. Characteristic feature of voluntary movement is that they improve with practice. This may be associated with cortical reorganization. Functional MRI scans reveal that the area of cortex activated during performance of the trained sequence was larger than that activated during a novel untrained sequence. Experience dependent change in the primary motor cortex is likely to be important for the acquisition and retention of other motor skills.

 

Corticospinal axons influence spinal motor neurons through direct and indirect connections

Corticospinal neurons make powerful and direct excitatory connections with alpha motor neurons in spinal cord. A unique feature of the corticospinal synapse is that successive cortical stimuli produce progressively larger excitatory postsynaptic potentials in spinal motor neurons.

-Corticospinal fibers also terminate on interneurons in the spinal cord, which in turn project to alpha motor neurons. These indirect connections with motor neurons regulate a larger number of muscles than do the direct connections and so may contribute to the organization of the multijointed movements such as reaching and walking.

-Corticospinal projections also have inhibitory effects on spinal motor neurons. The corticospinal inhibition is mediated by Ia inhibitory interneuron, the same interneuron responsible for the reciprocal inhibition of stretch reflex.

-Because these spinal interneurons receive peripheral inputs and are able to respond directly to ongoing changes in somatic sensory input, the higher centers of the brain are freed from the need to mange all the details of movements and instead can use the spinal circuits as components of more complex behaviors

 

The Primary motor cortex executes movements and adapts them to new conditions

Activity in individual neurons of the primary motor cortex is related to muscle force

Individual neurons are maximally activated during movement of a particular joint and particular direction of movement. The changes in neuronal activity begin some 100ms or more before the onset of movement.

-Firing of primary motor cortex neurons varied with the amount of force, not with the amplitude of the displacement.

-The activity of these cortical neurons therefore appears to signal the direction and amplitude of muscle force required to produce a movement rather than the actual displacement of the joint.

Preparatory set-intent to perform the movement alters the firing pattern of neurons in the primary motor cortex hundreds of milliseconds before the movement takes place. Movement or set related neurons might be concerned with early changes in postural muscle activity or some other process, rather than with the voluntary movement.

-Neurons in the primary motor cortex that project directly to motor neurons called corticmotorneuronal (CM) cells. They found that individual CM cells project monosynaptically to more than one motor nucleus and sometimes to muscles controlling different joints. Thus muscles are not mapped one-to-one in cortical output neurons. Most of the neurons have phasic-tonic pattern of activity, firing most briskly in dynamic phase of movement and settling down to lower tonic rate when a steady force is reached. For almost all neurons there is a range over which force is related linearly to firing rate.

Direction of movement is encoded by populations of cortical neurons

Most movements involve multiple joints and require sequential and temporally precise activation of multiple muscles. Movement in a particular direction is determined not by action of single neurons but by the net action of a large population of neurons. The contribution of each neuron to movement in a particular direction be represented as a vector whose length indicates the level of activity during movement in that direction. The contribution of individual cells could be added vectorially to produce a population vector. In fact, the directions of such population vectors match the directions of movement. Directionally tuned neurons are modulated strongly by the presence of external loads during reaching movements in given direction, and this modulation depends on the force required to displace the limb. A cells firing rate increases if a load opposes movement of the arm in the cells preferred direction; it decreases if the load pulls the arm in the cells proffered direction. This dependence of firing rate on load suggests that the activity of neurons in the primary motor cortex varies with direction of forces as well as with movement direction during reaching with the whole arm

-Motor cortex activity signals not only lower level movement parameters, such as muscle force, but also higher level parameters related to the trajectory of the hand during reaching. This feature of motor cortex neurons distinguishes them from alpha motor neurons.

 

Neurons in the primary motor cortex are activated directly by peripheral stimulation under particular conditions

Individual movement of digits is controlled by patterns of activity in a population of cortical neurons

Primary motor cortex plays a special role in producing individuated movement of digits.

-Although individual neurons fire maximally when a particular finger is moved, these neurons are dispersed throughout the hand control area of primary motor cortex. (The digits are biomechanicallly coupled by common tendons and thus are not anatomically independent of each other. Moving a single digit alone requires activating and inhibiting muscles acting on all the digits.)

-The cells activated will depend on the task in which the muscle is used.

-Cells that are active during the precision grip remain silent during the power grip, even though the contraction of the target muscle is stronger for the power grip than the precision grip.

-Certain motor cortical cells fire less and less often as muscle force increases i.e. their activity is correlated negatively with force. They discharge only during tasks that require precise control of force and smooth changes in force. Thus their function may be to provide more precise derecruiting of motor units than would be afforded simply by inhibiting the so-called positive cortical neurons.

Each pre motor area contributes to different aspects of motor planning

Damage to pre motor areas causes more complex impairments than does damage to primary motor cortex. Pre-motor areas are involved in planning movement.

Basic features of neural organization of motor preparation

1.movements that are initiated internally by the subject-such as sequencing of finger movements when manipulating an object-involve primarily the supplementary motor area.

2.Movements triggered by external sensory events involve primarily the lateral pre-motor areas.

-The lateral dorsal pre-motor area is concerned with delayed action (delayed later on cue)

-Lateral ventral pre motor area is concerned with conforming the hand to the shape of objects.

3.Mental rehearsal of a movement i.e. the visual imagery to plan a movement invokes same patterns of activity in pre-motor and posterior parietal cortical areas as those that occur during performance of the movement.

4.The pre motor areas activated during a particular task are not the same over time but change progressively as performance becomes automatic.

 

The supplementary and pre-supplementary motor areas play an important role in

Learning sequences of discrete movements.

Motor actions are often self-initiated without an environmental cue. Nearly a full second before a self initiated voluntary movement begins, a characteristic negative shift in cortical potentials in medial pre motor regions, where supplementary motor area is situated, signals the planning that occurs before movement is executed.

Complex movement sequences require more planning than do simple repetitive movements. Imagining complex movements might require the same amount of planning as real movements. A complex sequence of finger movements was simply imagined, regional blood cerebral flow increases within the supplementary area.

When complex sequence of finger movements was simply imagined, regional blood flow increased in an area anterior to the supplementary motor area on both sides. The supplementary motor area seems to be involved in preparing movement sequences from memory in the absence of visual cues.

Pre supplementary motor area provides main cortical input to the supplementary area. Whereas the supplementary motor area is involved in setting the motor programs for learned sequences, the pre supplementary motor area is thought to be involved in learning these sequences. When proficiency and skill are gained, the neural control of task performance can also shift from supplementary motor area to the primary motor cortex.

-Much as extended practice influences the extent of motor representation in primary motor cortex, a shift in representation occurs in the supplementary motor cortex as a task goes from being novel to automatic.

 

The lateral pre-motor areas contribute to the selection of action and to sensorimotor transformations

The ability to learn new, adaptive responses to particular environmental stimuli is crucial to effective and accurate movement. Set related activity when movement is executed

In primary motor cortex specific parameters of particular movement

In supplementary motor area-specific order of responses

In lateral pre motor area-how visual or other sensory stimuli are to be used to direct the movement.

Set related activity in the premotor area persists during the entire interval between an anticipatory cue and signal to move.

Set related activity lateral dorsal premotor area is related to sensory stimuli that do not convey spatial cues to direct movement. Lateral dorsal pre motor area is involved in learning to associate a particular sensory event to a specific movement (associative learning)

 Reaching and grasping are mediated by separate parieto-pre motor channels

Goal-directed movements require transformation of sensory representations of the environment into muscle-control signals, a process termed sensorimotor transformation.

Visuo-motor transformation

1.System is responsible for transforming information about the location of objects in extra personal space into direction of reaching movement

2.System is responsible for transforming visual information about properties of objects, such as shape and size, into commands for effective grasping

Reference:

John Krakquer, Claude Ghez (2000), voluntary movement, in Principles of Neural Science 4th Ed., Editors Eric Kandel et al., (pp.756-781) McGraw Hill