Question

When motor cortex instructs the spinal cord to move a limb in a specific direction, a large number of neurons in the motor cortex fire action potentials. Despite the activity of many neurons, movement is performed in a single, well-defined direction. How do the firing rates of the population of neurons in the motor cortex specify well-defined movements (e.g., a specific direction)?

Answer 1

Most motor cortical neurons were found to have specific preferred directions in which their mean firing rate during reaching movement was maximal . Some studies showed that the activity of cortical neurons correlated with torques at joints and muscle tensions , or with other kinematic variables ] It has also been suggested that the activity of a particular cortical neuron may correlate with more than one movement parameter . Although many previous studies addressed these phenomena (see for review , the origin of the directional tuning properties continues to be subject to debate. Neuronal activity in the motor cortex may reflect the complexity of movement dynamics as well as higher-order features such as cognitive information related to a motor task or function . This complexity has led to different viewpoints and interpretations .

The motor cortex not only plays a role in triggering movements. It also gets feedback from the movements it helps trigger. Neurons of the motor cortex receive sensory feedback from the muscle fibers they innervate (Asanuma, 1981). This allows for tight coupling between efferent and afferent functions.

Alpha motor neurons in the spinal cord, in turn, encode the force of contraction of groups of muscle fibers using the rate code and the size principle. Thus, in accordance with the concept of hierarchical organization of the motor system, the information represented by motor cortex is a higher level of abstraction than the information represented by spinal motor neurons.

1. Primary motor cortex neurons fire 5-100 msec before the onset of a movement. Thus, rather than firing as the result of muscle activity, these neurons are involved in relaying motor commands to the alpha motor neurons that eventually cause the appropriate muscles to contract.
2. Primary motor cortex encodes the force of a movement. The amount of force required to raise the arm from one location to another is much greater if one is holding a bowling ball than if one is holding a balloon. Many neurons in primary motor cortex encode the amount of force that is necessary to make such a movement . Note the distinction between movement force and muscle force. Whereas a minority of primary motor cortex neurons encodes individual muscle force, a larger number encodes the amount of force necessary for a particular movement, regardless of which individual muscles are used. Alpha motor neurons, in turn, translate the commands of the motor cortex neurons and control the amount of force generated by individual muscles to accomplish that movement, under the principles of the rate code and the size principle.
3. Primary motor cortex encodes the direction of movement.
4. Primary motor cortex encodes the extent of movement
5. Primary motor cortex neurons encode the speed of movement.