Neuroscience/Objectives/Lecture 40

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Motor system and motor cortex

Introduce the various components of the motor system.

The motor system comprises the motor cortex, brainstem, spinal cord, cerebellum, and basal ganglia. Its functions permit voluntary movements, reflexes, and rhythmic motor patterns.

Differentiate pyramidal from extrapyramidal signs.

Pyramidal signs are associated with lesions of the corticospinal tract, while extrapyramidal signs are associated with lesions of other corticofugal pathways.

Describe the Babinski reflex and what it indicates.

The Babinski reflex is an abnormal fanning of the toes in adults when a pen is traced along the bottom of the foot (a positive sign is normal as an infant). In normal adults, the Babinski reflex is suppressed by descending inhibitory control exerted over alpha motor neurons by the corticospinal tract. Thus a positive Babinski sign suggests a lesion to the corticospinal tract. This property makes the Babinski test a reasonable way to distinguish between pyramidal and extrapyramidal lesions.

Be familiar with the anatomical subdivisions of the motor cortex, including all Brodmann areas.

• Classical motor areas:
  • Primary motor cortex: BA 4
  • Premotor areas (including premotor cortex and supplementary motor area): BA 6
• Classical sensory areas needed for motor control:
  • Somatosensory cortex: BA 3,1,2 (provide proprioceptive information needed to organize movements of body and limbs)
  • Posterior parietal cortex: BA 5,7 (needed to process stimuli leading to purposeful movements)

Describe the functions of the primary motor cortex and the premotor cortex.

The primary motor cortex is responsible for low-level motor control (eg, controlling simple movements). It participates in the initiation of movement and codes for force and speed of movement. Neurons of the primary motor cortex regulate spinal reflex activity and directly control alpha motor neurons, especially those for the distal limb musculature.

The premotor area (BA 6) plans and coordinate complex movements. Their neurons have slightly higher thresholds than those of the PMC. The supplementary motor area (rostral, medial BA 6) is important in coordinating posture and voluntary movements. Damage to any of the premotor areas results in apraxia, the inability to develop an appropriate strategy to move. Apraxic patients show neither weakness nor sensory loss, and their ability to produce simple movements is preserved. However, coordination may be severely impaired in apraxia.

Be familiar with the somatotopical organization of the primary motor cortex (homunculus). Be able to trace this organization through the descending motor pathways.

(H:7-10) The primary motor cortex is somatotopically arranged with the face and upper extremity represented laterally, the trunk medially, and the lower extremity represented most medially (inside the longitudinal fissure). Corticofugal fibers descending through the posterior limb of the internal capsule maintain a somatotopic organization, with the upper extremity represented anteriorly and lower extremity posteriorly. Before it divides, the corticospinal tract contains upper extremity fibers most medially and lower extremity fibers laterally. The lateral corticospinal tract maintains this organization, but the anterior corticospinal tract has an anterior-posterior somatotopy: posterior fibers carry descending information to the upper extremity, while anterior fibers carry information to the lower extremity.

Define the corticofugal (corticospinal and corticobulbar) pathways.

Corticofugal pathways connect the cortex to the brainstem and spinal cord. Fibers of the corticospinal tract originate in primary motor (BA 4), premotor (BA 6), and somatosensory (BA 3,1,2) cortices and descend through the posterior limb of the internal capsule, crus cerebri of the mesencephalon, longitudinal fibers of the basilar pons, and into the pyramids of the medulla. At the medullospinal junction, 90% of the corticospinal fibers decussate, forming the lateral corticospinal tract, which controls the fine muscles of the hands and fingers. The remaining 10% of fibers form the uncrossed anterior corticospinal tract, which synapses on neurons in lamina VII that innervate axial muscles. Most lateral corticospinal neurons innervate interneurons in the intermediate zone of the spinal gray, although some directly innervate lower motor neurons.

Corticobulbar fibers arise from the same areas as the corticospinal tract, descend through the genu of the internal capsule, and terminate on neurons in the brainstem reticular formation, sensory relay nuclei (nucleus gracilis, NTS, etc.), and cranial nerve nuclei. The motor trigeminal, motor facial, and hypoglossal nuclei receive bilateral corticobulbar input, while other nuclei are innervated by polysynaptic input from the reticular formation. An exception to the bilateral corticobulbar innervation rule is the ventral portion of the motor facial nucleus, which receives predominantly contralateral input.

Describe the symptoms of central seven and of Bell's palsy.

Central seven is a upper motor neuron lesion to the corticobulbar tract innervating the motor facial nucleus. Because the neurons innervating muscles of the upper face receive bilateral corticobulbar input, a unilateral lesion of the corticobulbar tract does not impair the control of muscles of the upper face. However, because neurons innervating muscles of the lower face receive mainly contralateral corticobulbar input, a corticobulbar lesion results in contralateral paresis to the lower face.

Bell's palsy is a lesion to the lower motor neurons of the motor facial nucleus. It presents with paresis of the ipsilateral muscles of facial expression.

Review corticocortical interactions.