Cortical Control of Motor Functions Notes

General Principles of Cortical Motor Control

• According to Guyton and Hall (13th Ed.), most voluntary movements initiated by the cerebral cortex are not executed by the cortex alone. Instead, the cortex activates stored functional patterns in lower brain areas.
• These lower brain areas include the spinal cord, brain stem, basal ganglia, and cerebellum.
• These lower centers are responsible for sending the specific, final control signals directly to the muscles for execution.

Subareas of the Motor Cortex

• The motor cortex is anatomically and functionally divided into three distinct subareas:
  • Primary motor cortex.
  • Premotor area.
  • Supplemental motor area.
• These areas are located anterior to the central sulcus, occupying approximately the posterior one-third of the frontal lobes.
• The somatic sensory area 1 (Somatic area 1) and somatic association area are located posterior to the central sulcus in the parietal lobe, providing essential sensory information for motor control.

The Primary Motor Cortex (Brodmann's Area 4)

• Located in the pre-central gyrus of the frontal lobes.
• Topographic Representation:
  • There is an unequal topographic representation of the body (motor homunculus).
  • Large cortical areas are devoted specifically to the hands and the muscles of speech, reflecting the complexity and precision required for these movements.
  • The topographical map follows a vertical arrangement: legs and feet are represented in the longitudinal fissure, followed by the trunk, arm, hand, face, and mouth as one moves laterally and downward toward the Sylvian fissure.
• Functional Stimulation:
  • Stimulation of the primary motor cortex elicits fine motor movements.
  • It produces specific muscle movements rather than complex tasks (e.g., flexion of a single finger).
• Neural Composition:
  • Contains large pyramidal cells known as Betz cells, located in Layer V of the cortex.
  • While Betz cells account for only $3\%$ of the total outgoing fibers of the corticospinal tract, they are the largest and fastest conducting fibers.
  • These cells are involved in direct connections with lower motor neurons in the spinal cord.

The Premotor Area

• Located immediately anterior to the primary motor cortex.
• Topographical Organization: Similar to the primary motor cortex, with the face and mouth represented inferiorly and the trunk and leg areas represented more superiorly.
• Movement Complexity:
  • Generates more complex patterns of movement than the primary motor cortex.
  • Stimulation typically results in the movement of muscle groups reorganized to perform a specific task (e.g., positioning the shoulders and arms so the hands can perform a specific movement).
• Mirror Neurons:
  • These neurons become active when an individual learns new skills by imitation.
  • They fire both when the individual performs a task and when they observe someone else performing the same task.
• Integration: Works in concert with other motor areas to plan and execute tasks.

The Supplemental Motor Area

• Located in the longitudinal fissure but extending onto the superior frontal cortex.
• Topographically organized in a manner consistent with other motor regions.
• Functional Stimulation: Stimulation often elicits bilateral, coordinated movements (e.g., the simultaneous grasping of both hands).
• Supportive Function:
  • Functions in concert with the premotor area to provide attitudinal, fixation, or positional movements for the body.
  • It provides the essential background for fine motor control performed by the arms and hands, which are then refined by the premotor and primary motor cortex.

Specialized Areas of Motor Control

• Broca Area:
  • Known as the motor speech area.
  • Damage to this area causes a significant decrease in speech capability.
  • Responsible for coordinating the muscles of the tongue, mouth, respiration, and larynx specifically for the production of speech.
• Eye Fixation and Head Rotation Area:
  • Located above the Broca area.
  • Responsible for coordinated head and eye movements.
  • Stimulation elicits head rotation.
  • Also controls blinking and eyelid movements.
• Hand Skills Area:
  • Located in the premotor area anterior to the primary motor cortex area for the hands.
  • Damage to this area results in motor apraxia, which is the inability to perform fine, purposeful hand movements (movements become uncoordinated and non-purposeful).

Transmission of Cortical Motor Signals

• Direct Pathway (Corticospinal Tract):
  • Used for discrete, detailed, and fine movements, particularly of the distal limbs.
• Indirect Pathway:
  • Sends signals through the basal ganglia, cerebellum, and various brainstem nuclei.
  • Primarily used for postural control or generalized, gross body movements.

The Corticospinal (Pyramidal) Tract

• Fiber Origin Distribution:
  • $30\%$ originates in the primary motor cortex.
  • $30\%$ originates in the supplementary motor areas.
  • $40\%$ originates in the somatic sensory areas posterior to the central sulcus.
• Anatomical Pathway:
  • After leaving the cortex, fibers pass through the posterior limb of the internal capsule.
  • They descend through the brainstem, forming the pyramids of the medulla.
• Decussation (Crossing Over):
  • The majority of fibers cross to the opposite side in the lower medulla.
  • These decussated fibers descend in the lateral corticospinal tracts to synapse with interneurons and anterior motor neurons (Lower Motor Neurons) in the spinal cord.
• Non-decussating Fibers:
  • Some fibers do not cross over in the medulla and descend as the ventral corticospinal tract.
  • These are typically involved in bilateral postural control and movements of the axial muscles.
• Transmission Speed:
  • Giant pyramidal cells (Betz cells), found only in the primary motor cortex, give rise to large fibers with very fast transmission rates of approximately $70\,m/sec$.

Accessory Pathways from the Motor Cortex

• The motor cortex sends large numbers of auxiliary signals to other brain regions:
  • Basal Ganglia: Fibers pass to the caudate nucleus and putamen; this is critical for the gating and initiation of movement.
  • Red Nucleus: Fibers pass through the corticorubral tract to the red nucleus of the midbrain, then via the rubrospinal tract to the cord to regulate muscle tone and motor learning.
  • Cerebellum: Fibers pass to the reticular substance and vestibular nuclei before reaching the cerebellum, facilitating motor learning, coordination, and equilibrium.

Sensory Feedback in Motor Control

• The motor cortex operates in association with the basal ganglia and cerebellum to excite appropriate motor actions once sensory information is received.
• Important Incoming Fibers:
  • Subcortical fibers from adjacent cortical areas (somatosensory parietal cortex, visual cortex, and auditory cortex).
  • Subcortical fibers from the opposite hemisphere passing through the corpus callosum.
  • Somatosensory fibers from the thalamus (cutaneous and proprioceptive fibers).
• Mechanisms of Feedback fine-tuning:
  • Muscle Spindles: Any length mismatch in the spindle causes an auto-correction in the motor signal.
  • Tactile Receptors: Compression of the skin (e.g., when holding an object) provides sensory feedback to the motor cortex regarding the degree of effectiveness of the intended action and how much force (strength) should be applied.

Clinical Implications and Lesions

• Primary Motor Cortex Lesions:
  • Result in the loss of voluntary control of discrete movements in distal limb segments, particularly the hands and fingers.
  • The ability to perform fine, intricate movements is permanently lost.
• Premotor and Supplementary Motor Cortex Lesions:
  • Result in reduced spontaneous movement and impairment in the sequencing of movements.
  • Causes difficulty with bilateral coordination and grasping.
  • Does not typically result in complete paralysis.

Upper vs. Lower Motor Neuron Lesions

• Upper Motor Neuron (UMN) Lesions:
  • Common Causes: Brain injury or spinal cord injury.
  • Symptoms: Muscle weakness and spasticity (increased stiffness/muscle tone).
  • Reflexes: May cause hyperreflexia (increased reflexes) and possibly clonus (involuntary rhythmic muscle contractions).
• Lower Motor Neuron (LMN) Lesions:
  • Common Causes: Spinal muscular atrophy or peripheral neuropathy.
  • Symptoms: Muscle weakness and hypotonia (reduced muscle tone; "floppy muscles").
  • Reflexes: May cause hyporeflexia (decreased reflexes) or areflexia (total absence of reflexes).