Motor Cortex

Motor Cortex

The areas of the cerebral cortex that are responsible for producing conscious, planned movement were introduced in Unit 11, as we were subdividing cortical real estate based on the Brodmann classification. Areas responsible for movement, and their Brodmann numbers, are all in the frontal lobe. They include:

• Area 4: (precentral gyrus): the primary motor cortex, sending axons to the α motor neurons of spinal cord (executing movement).
• Area 6: premotor cortex and supplementary motor area (planning or imagining movement). Among the interesting neurons found here are mirror neurons, which are only active when watching someone else perform an action that you are trying to plan.
• Area 8: Frontal eye fields. This is the area of premotor cortex that plans eye movements.
• Areas 44 and 45: Broca’s area. In most people, Broca’s area on the left side is responsible for the production of symbolic language. This can include speech, sign language, or writing. A lesion in this area produces aphasia, as we saw in Unit 11 Objective 11.

For the time being, we will focus on the primary motor cortex (Brodmann 4, or precentral gyrus).

As for other motor and sensory areas of the brain, neurons which control movement form an orderly map of the body surface within the brain. Recall from Unit 11 Objective 7 that this orderly map is called a homunculus (“little man”).

The motor homunculus, pictured here, is very similar to the sensory homunculus. The face areas are found in the most lateral part of the precentral gyrus. As we move towards the midline, we encounter, in order: hand, arm, shoulder, trunk, hip; in the medial longitudinal fissure, the map continues with leg and toes.

The lateral surface of the brain is supplied by a different set of arteries and arterial branches than the medial surface of the brain (Unit 11 Objective 8). Recall that the right side of the brain controls the left side of the body and vice versa. Therefore, a stroke (infarction, loss of blood supply) in the right lateral part of the precentral gyrus would produce paralysis of the left side of the face while a stroke in the right medial part of the precentral gyrus would produce paralysis of the left leg.

Information about conscious, willed movement is planned in the supplemental and premotor cortices. These areas then transfer their plan to the motor cortex, which convenes a neural “committee” to “vote” on which axons are going to be activated, and how many nerve impulses will be sent through each axon. The pyramid-shaped cells in layer V of area 4 (precentral gyrus) are called Betz cells. Betz cells are the largest neurons in the human body, and at 0.1 mm across, the cell bodies are visible to the naked eye in stained tissue. Accordingly, these cells have axons that are among the largest (and fastest) in the human body.

Apostolos Georgopoulos and colleagues have demonstrated how this vote occurs in an elegant series of experiments that beautifully illustrate the process. Rhesus monkeys were trained to push a red button when it lights up. The buttons were set on a panel in a row, an equal distance apart. To push them requires a movement at a known angle. Hundreds of cells in motor cortex fire at the same time, and because of the way they are wired to the output pyramidal neuron, the output pyramidal neuron generates a train of action potentials which activates the appropriate pool of motor neurons, each by the appropriate amount, in the anterior (ventral) horn of the cervical spinal cord gray matter. The activity of the output pyramidal neuron, and the direction of movement it causes, is shown by the green vector in the photograph above.

The activation of the chosen pyramidal cell in motor cortex causes a contraction of the muscles of the arm and forearm that will approximate movement toward the desired target, in this case a lighted button. The movement that results is shown by a yellow vector. As described elsewhere, the slight mismatch between the preferred direction and force of contraction, and the actual direction and force of contraction, is corrected by the cerebellocortical circuitry.

This image shows the motor axons, along with all other axons going to and from the cortex, in a structure that is collectively called the corona radiata (“radiating crown”). The technique shown here, diffusion tensor imaging, is useful in seeing when the Betz cell axons or any other axon bundles are damaged, as in concussion.