Neuroscience Online (iii,3,3)

Section II: Sensory Systems
3. Motor Cortex
Part 3 of 5

James Knierim, Ph.D.
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Encoding of Movement by Motor Cortex

Primary Motor Cortex

As discussed above, the primary motor cortex does not generally control individual muscles directly, but rather appears to control individual movements or sequences of movements that require the activity of multiple muscle groups. 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.

What is encoded by the neurons in primary motor cortex? Clues have come from recording the activity of these neurons as experimental animals perform different motor tasks. In general, primary motor cortex encodes the parameters that define individual movements or simple movement sequences.

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.
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 (Figure 3.7). 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.

Figure 3.7 A

Motor cortex encodes the force necessary to make a movement. (Evarts 1968)

(A) When there is little load, a motor neuron in primary motor cortex that controls an extension of the wrist fires when the wrist extends. A motor neuron that controls wrist flexion does not change its low rate of activity. Note that the extension motor neuron begins to fire spikes before the onset of the movement.

(B) When a 5 lb. load is placed on the left pulley, more force must be used to initially hold the weight steady and then lift it. The extension motor neuron in primary motor cortex fires more strongly to produce the greater force.

(C) When a 5 lb. load is placed on the right pulley, the load is on the flexor. Thus, primary motor cortex neurons for flexion are activated to keep the weight stable. When the wrist extends, the neurons are quieter, as the force of the movement is actually produced by the weight itself. (Note that motor cortex encodes the force of a movement, such as wrist extension or more complicated, multi-joint movements. The force of individual muscles is encoded by alpha motor neurons in the spinal cord and brain stem.)

Figure 3.7 B

Figure 3.7 C

Primary motor cortex encodes the direction of movement. Many neurons in the primary motor cortex are selective for a particular direction of movement. For example, one cell may fire strongly when the hand is moved to the left, whereas it will be inhibited when the hand is moved to the right (Figure 3.8).

Figure 3.8

Directional tuning of motor cortex neurons. The cell fires maximally when the hand is moved in the 135º or 180º directions, moderately when the hand moves in the 90º and 225º directions, and is silent when the hand moves in the opposite directions (0º, 45º, 270º, and 315º) (Georgopoulos et al., 1982).

Figure 3.9

Velocity profile of targeted movements.

Primary motor cortex encodes the extent of movement. The firing of some neurons is correlated with the distance of a movement. A monkey was trained to move its arm to different target locations that varied in direction and distance from the center. The firing of many neurons was correlated with the direction of movement (as in Point 3), whereas the firing of other neurons was correlated with the distance of the movement. Interestingly, some neurons were correlated with the interaction of a particular distance and direction; that is, they were correlated with a particular target position.

Primary motor cortex neurons encode the speed of movement. Almost all targeted movements follow a typical bell-shaped curve of velocity as a function of distance (Figure 3.9). For example, when the hand moves an object such as a coffee cup from one location to another (the target), the hand accelerates during the first half of the movement, reaches a peak velocity approximately halfway to the target, and then decelerates until it reaches the target. The firing rate of some primary motor cortex neurons in monkeys correlates with this bell-shaped speed profile, demonstrating that information about movement speed is contained in the spike trains of these neurons.

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