Motor Programs

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When the brain wishes to execute a movement a multitude of factors must be considered.

First the visual system must analyse the 3D path needed to execute the movement.

Current body position must be identified and a plan of what joints and muscles will activate when.

How fast will the movement be, how fine and precise?

Multiple trajectories can be used for any one movement, the brain must analyse and pick the best one of the lot.

Major problem in robotics, huge amount of information to be processed and analysed.

Motor Programs involve the storage of plans for movements that can be quickly accessed and adapted for changing situations.

For example, Walking is in essence the same set of movements every time. These movements need minor adjustments dependent on variances in Terrain, Speed etc.

The plan for walking can be stored and quickly retreived and adapted instead of doing all the normal calculations required to get a walking movement.

There are 5 key pieces of evidence supporting the idea of Motor Programs:

1) Persistence of Triphasic EMG in absence of movement (observed mainly in fast accurate movements).

When throwing a dart, three muscles need to activate to preform the movement to completion.

Triceps must initiate Elbow Extension.

Biceps ends the movement with elbow flexion

Triceps stabilises end of movement

These three muscle activations form a Triphasic EMG. This is still observed even if the arm is blocked from performing the last two movements.

2) Reaction Time increases if the task is more complex. If more movements are needed to perform a certain task, then more planning is needed.

3) If information about an upcoming task is known, RT will decrease. Less programming time is needed because the prescribed movements are already known

4) A fourth piece of evidence for the existence of motor programs is that Learnt movements show similar form even at different scales. An example of this is your signature. If you were to write your signature with your toes as opposed to your hands, the end results would be a rough approximation. Though an entirely different set of joints and muscles are trying to write your signature, the signature still looks like your charasteristic one eg. Curvy G.

This is suggests that your brain has a stored blueprint ready to utilise.

Cell assembly Theory:

Subset of cortical neurons activated. Cycle around to refine selection. Movement begins when output is sufficient to activate an Alpha motor Neuron.

If these connections are utilised continuously its theorized that these connections can strengthen resulting in a retrievable blueprint.

Cells have a preferred direction of movement and their activity under stimulation correlates with this preferred direction.

We use vectors to represent a direction coded by activity of a group of cells.

A vector for a cell is always drawn in its preffered direction regardless of what way it is actually moving. So a cell that prefers to move at 90 degrees will have all of its vectors drawn at 90 degrees but the length of the vector will identify whether this is its preferred direction.

A net population vector is drawn as the sum of all vectors in the population.

Muscles work in synchronous producing a variety of vectors in a variety of directions that finally is summed producing the intended movement.