Motor Cortex and Brain Stem Control

Motor Cortex

• Divided into 3 sub-areas: primary motor cortex, premotor area, and supplemental motor area
• Primary motor cortex:
  • Unequal topographic representation
  • Elicits fine motor movement with stimulation
• Premotor area:
  • Topographical organization similar to primary motor cortex
  • Stimulation results in movement of muscle groups to perform specific tasks
  • Works in concert with other motor areas
• Supplemental motor area:
  • Topographically organized
  • Stimulation often elicits bilateral movements
  • Functions in concert with premotor area to provide attitudinal, fixation, or positional movement for the body

Specialized Areas of the Motor Cortex

• Broca's area:
  • Damage causes decreased speech capability
  • Closely associated area controls appropriate respiratory function for speech
• Eye fixation and head rotation area:
  • Coordinates head and eye movements
• Hand skills area:
  • Damage causes motor apraxia (inability to perform fine hand movements)

Transmission of Cortical Motor Signals

• Direct pathway:
  • Corticospinal tract
  • For discrete detailed movement
• Indirect pathway:
  • Signals to basal ganglia, cerebellum, and brain stem nuclei

The Corticospinal Tract

• Originates in primary motor cortex (30%), supplementary motor areas (30%), and somatic sensory areas (40%)
• Majority of fibers cross to opposite side in medulla and descend in lateral corticospinal tracts
• Corticospinal fibers synapse with interneurons, anterior motor neurons, and a few sensory relay neurons in the cord gray matter
• Giant pyramidal cells (Betz cells) give rise to large fibers with fast transmission rates (70m/sec)

Corticospinal Fibers

• 34,000 Betz cell fibers make up only about 3% of the total number of fibers
• 97% of the 1 million fibers are small-diameter fibers that conduct background tonic signals and feedback signals from the cortex to control intensity of sensory signals to the brain

Other Pathways from the Motor Cortex

• Betz collaterals back to cortex sharpen the boundaries of the excitatory signal
• Fibers to caudate nucleus and putamen
• Fibers to the red nucleus, which then sends axons to the cord in the rubrospinal tract
• Fibers to reticular substance, vestibular nuclei, and pons, then to the cerebellum
• Therefore, the basal ganglia, brain stem, and cerebellum receive a large number of signals from the cortex

Incoming Sensory Pathways to Motor Cortex

• Subcortical fibers from adjacent areas of the cortex, especially from somatic sensory areas of parietal cortex and visual and auditory cortex
• Subcortical fibers from opposite hemisphere, which pass through corpus callosum
• Somatic sensory fibers from ventrobasal complex of the thalamus (i.e., cutaneous and proprioceptive fibers)

Red Nucleus and the Rubrospinal Tract

• Substantial input from primary motor cortex
• Primary motor cortex fibers synapse in the lower portion of the nucleus (magnocellular portion), which contains large neurons similar to Betz cells
• Magnocellular portion gives rise to rubrospinal tract
• Magnocellular portion has somatotopic organization similar to primary motor cortex

Sensory Feedback is Important for Motor Control

• Feedback from muscle spindle, tactile receptors, and proprioceptors fine-tunes muscle movement
• Length mismatch in spindle causes auto-correction
• Compression of skin provides sensory feedback to motor cortex on degree of effectiveness of intended action

Excitation of Spinal Motor Neurons

• Motor neurons in cortex reside in layer V
• Excitation of 50-100 giant pyramidal cells is needed to cause muscle contraction
• Most corticospinal fibers synapse with interneurons
• Some corticospinal and rubrospinal neurons synapse directly with alpha motor neurons in the spinal cord, especially in the cervical enlargement
• These motor neurons innervate muscles of the fingers and hand

Control of Motor Function by the Brainstem

• Brainstem as an extension of the spinal cord
• Performs motor and sensory functions for the face and head (i.e., cranial nerves)
• Similar to spinal cord for functions from the head down
• Contains centers for stereotypic movement and equilibrium

Support of the Body Against Gravity

• Muscles of the spinal column and the extensor muscles of the legs support the body against gravity
• These muscles are under the influence of brainstem nuclei
• Pontine reticular nuclei excite the antigravity muscles
• Medullary reticular nuclei inhibit the antigravity muscles