Motor Cortex Areas: Premotor, Primary Motor, and Supplementary Motor

Fig. 56.1 - Motor functional areas of the cerebral cortex (Guyton & Hall)

1. Primary Motor Cortex (Area 4)

• Face and mouth - most lateral (near sylvian fissure)
• Hand and arm - midportion
• Trunk - near apex
• Leg and foot - dip into the longitudinal fissure (paracentral lobule on medial surface)

Penfield & Rasmussen motor homunculus - note the disproportionately large area for hands and speech

Motor homunculus (Adams & Victor) - A = motor cortex, B = sensory cortex

• Direct execution of voluntary movement - sends signals via the corticospinal and corticobulbar tracts to spinal motor neurons
• Stimulation at low threshold evokes contralateral discrete muscle contractions
• Controls fine, skilled movements, especially of the distal limbs (hands, fingers, face, speech muscles)
• More than half the primary motor cortex is devoted to the hands and speech muscles, reflecting the importance of manual dexterity and communication
• Each neuron typically excites a pattern of muscles producing a specific movement, not a single isolated muscle (concept of movement representation, not muscle representation - as articulated by Hughlings Jackson and Sherrington)
• Parts of the body capable of the most delicate movements have the largest cortical representation

2. Premotor Area (Area 6 - lateral portion)

• Generates more complex patterns of movement than area 4 - for example, positioning the shoulder and arm so the hand can perform a specific task
• The anterior premotor cortex develops a "motor image" of the total movement to be performed
• The posterior premotor cortex then translates this image into sequential muscle activation patterns, sending signals either directly to area 4 or via the basal ganglia-thalamus loop back to area 4
• Stimulation of the caudal premotor area produces responses similar to area 4 stimulation (via direct propagation to area 4); stimulation of the rostral premotor area elicits more general movement patterns of proximal limb musculature via pathways independent of area 4
• Contains mirror neurons - these fire both when performing an action and when observing someone else perform the same action. They are thought to be important for understanding others' actions and for learning by imitation
• Together with basal ganglia, thalamus, and area 4, it forms the circuit for complex coordinated muscle activity

3. Supplementary Motor Area (SMA, area 6β / medial area 6)

• Controls body-wide attitudinal and postural movements - provides background postural support for fine movements performed by the arms/hands
• Stimulation often produces bilateral responses (unlike area 4 which is predominantly contralateral) - e.g., bilateral grasping movements of both hands simultaneously (thought to be remnants of climbing behavior)
• Can also induce:
  • Gross ipsilateral or contralateral limb movements
  • Bilateral tonic contractions of the limbs
  • Contraversive movements of head and eyes with contralateral arm tonic contraction
  • Inhibition of voluntary motor activity and vocal arrest
• Provides fixation movements of body segments and positional movements of the head and eyes as background for finer movements
• Functions in concert with the premotor area to coordinate proximal/axial movements

Summary Comparison