Question

Does the motor cortex send an individual axon for each M1 neuron down the corticospinal tract down to lower motor neurons in the spinal cord? And then would lower motor neurons receive all these axons and calculate the population code on their own? Or does the motor cortex calculate the population code of all of its neurons (or all of the neurons that represent the corresponding muscle) and send this information to the muscles? Also at what level are the signals muscle-specific?

Answer 1

This concept is a longtime battle among the community of motor control scientists.

A number of prominent theories are present.

One interesting theory concerning  whole limb movement was proposed by Georgopaolos in 80s. According to it M1(primary motor cortex) neurones follow a population vector model which determines the direction of actual whole limb movement. The postulation was tested in lab monkeys where they were given a particular reaching task. The results showed M1 is more regulated by the RESULTANT SPIKES OF GROUP OF NEURONES rather than INDIVIDUAL TRAIN OF ACTION POTENTIALS.

In later years there have been several studies where both the dynamic and static components of movement have been studied.

As per the present concept M1 definitely follows a particular kinesmatic pattern in case of more curved movement. However small joints and individual lower motor neurones provide more dominant input and fine tuning in case of STRAIGHTwhole limb movements.

The research is still on and so many questions are still unanswered.