Does The Brain Control Muscles Or Movements?

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Does The Brain Control Muscles Or Movements?

http://medicalnewscenter.com/out/out.cgi?

http://www.sciencedaily.com/releases/2008/05/080507133321.htm

One of the major scientific questions about the brain is how it can

translate the simple intent to perform an action--say, reach for a

glass--into the dynamic, coordinated symphony of muscle movements

required for that action. The neural instructions for such actions

originate in the brain's primary motor cortex, and the puzzle has

been whether the neurons in this region encode the details of

individual muscle activities or the high-level commands that govern

kinetics--the direction and velocity of desired movements.

Now, Ajemian and his colleagues, analyzing muscle function in

monkeys, have created a mathematical model that captures the control

characteristics of the motor cortex. It enabled the researchers to

better sort out the " muscles-or-movement " question.

The researchers described their model in an article in the May 8,

2008, issue of the journal Neuron, published by Cell Press.

Researchers have been thwarted in their efforts to measure and model

the neural control of complex motions because muscle forces and

positions constantly change during such motions. Also, the position

sensors, called proprioceptors, in joints and muscles feed back

constantly changing signals to the neurons of the motor cortex.

Ajemian and colleagues overcame these complexities by simplifying the

experimental design. Rather than asking monkeys to carry out complex

movements, they trained the animals to push on a joystick in

different, specified ways to move a cursor on a screen to a desired

target. This use of isometric force greatly simplified the

measurements the researchers needed to make to define muscle and

joint action.

As the monkeys carried out the isometric tasks, the researchers

analyzed the patterns of muscle activations that corresponded with

the isometric forces in different directions and at different

postures. They then developed a model that enabled them to test

hypotheses about the relationship between neuronal activity that they

measured in the animals' motor cortex and the resulting actions.

They said that their " joint torque model can be tested at the

resolution of single cells, a level of resolution that, to our

knowledge, has not been attained previously. "

They concluded that their model " suggests that neurons in the motor

cortex do encode the kinetics of motor behavior. "

" This model represents a significant advance, because it is

strikingly successful in accounting for the way that the responses of

individual [primary motor cortex] neurons vary with posture and force

direction, " commented Bijan Pesaran and Movshon in a preview

of the article in the same issue of Neuron.

" The results of Ajemian et al's analysis provide strong evidence that

it is useful to think of the output of [primary motor cortex] neurons

in terms of their influence on muscles. Their model, in effect,

defines a 'projection field' for each [primary motor cortex] neuron

that maps its output into a particular pattern of muscle actions. "

Pesaran and Movshon commented that " perhaps we should set aside the

somewhat artificial dichotomy between muscles and movements, between

the purpose and its functional basis, and recognize that the

activation pattern of motor cortex neurons does two things--it

specifies for the peripheral motor system both what to do and how to

do it. "

The researchers include Ajemian, McGovern Institute for Brain

Research, Massachusetts Institute of Technology, Cambridge, MA;

Green, Universite de Montreal, Canada; Bullock,

Department of Cognitive and Neural Systems, Boston University,

Boston, MA; Center of Excellence for Learning in Education, Science,

and Technology, Boston, MA; , York University, Toronto,

Canada; Kalaska, Universite de Montreal, Canada; and

Grossberg, Department of Cognitive and Neural Systems, Boston

University, Boston, MA, Center of Excellence for Learning in

Education, Science, and Technology, Boston, MA.

Journal reference: Ajemian, Green, Bullock,

, Kalaska, and Grossberg. Assessing the

Function of Motor Cortex: Single-Neuron Models of How Neural Response

Is Modulated by Limb Biomechanics. Neuron, Vol 58, 414-428, 08 May

2008.