Motor Systems: Cortex, Ganglia, Cerebellum

• These study notes cover Motor Systems, Basal Ganglia, and the Cerebellum.

Motor Cortex

• The primary motor cortex is in the frontal lobe pre-central gyrus and is responsible for precise voluntary movement control.
• Neurons in the primary motor cortex are referred to as Upper Motor Neurons.
• Upper Motor Neurons are topographically organized, with neurons innervating the lower body located near the midline.
• Neurons that innervate the head muscles are located near the lateral sulcus.
• The motor homunculus is the upside-down body map of the motor cortex.
• Head and face muscles (eyes, jaw, tongue, facial expressions) are regulated by the Upper Motor Neuron sending projection fibers to all brainstem levels via the corticonuclear tract system.
• Limb and trunk muscles are regulated by Upper Motor Neurons, which send projection fibers to the spinal cord via the corticospinal tract system.
• UMN = Upper Motor Neurons

Corticospinal Tracts Overview

• Axons from upper motor neurons descend through white matter as corticospinal tracts.
• They synapse on lower motor neurons in the spinal cord gray matter.
• Corticospinal tracts change names in different regions:
• Subcortical white matter = internal capsule (contains ascending sensory fibers)
• Midbrain white matter = cerebral peduncles
• Medulla white matter = pyramids

Corticospinal Tracts Details

• 90% of corticospinal fibers cross to the contralateral side at the caudal medulla (pyramidal decussation) and move to a lateral position as the lateral corticospinal tract.
• These fibers descend in the lateral funiculus of the spinal cord.
• About 10% of corticospinal fibers remain uncrossed and descend as the ventral (anterior) corticospinal tract in the anterior funiculus.
• Lateral corticospinal fibers terminate on lower motor neurons (LMN) in the lateral portion of the spinal cord ventral horn (distal limb muscles).
• Ventral corticospinal fibers terminate bilaterally on lower motor neurons in the medial portion of the spinal cord ventral horn (trunk muscles).
• Ventral corticospinal fibers cross in the anterior white commissure.
• The cell bodies of origin for lateral and ventral corticospinal fibers are located in the pre-central gyrus, which is made up of Upper Motor Neurons.

Lateral vs Medial Corticospinal Tracts

• Lower motor neurons of trunk muscles are in the medial nucleus of the spinal cord ventral horn
• Lower motor neurons of distal limb muscles are in the lateral nucleus of the spinal cord ventral horn.

Upper and Lower Motor Neuron Lesions

• A lesion to the descending lateral corticospinal motor tract results in contralateral or ipsilateral upper motor neuron signs/deficits:
• Spastic paralysis (spasticity) = excessive muscle contraction
• Hyper-reflexia
• Muscle atrophy
• Lesions above the pyramids lead to contralateral spasticity, while lesions below them lead to ipsilateral spasticity.
• A lesion in the brainstem or the ventral horn of the spinal cord results in ipsilateral lower motor neuron signs/deficits:
• Flaccid paralysis = muscles become limp and can't contract
• Hypo-reflexia
• Muscle atrophy
• Spinal cord ventral horn lesions lead to ipsilateral flaccid paralysis.

Rubrospinal Tract

• Fibers arise from the red nucleus in the rostral midbrain and immediately cross to the contralateral side in the midbrain tegmentum.
• Descending fibers travel through the brainstem and then in the lateral funiculus of the spinal cord.
• Facilitates flexor muscles and inhibits extensor muscles of the limbs/trunk and neck.
• Lesions at or below the level of the red nucleus result in damage to the rubrospinal tract, causing a decerebrate posture.
• This is characterized by arms and legs being straight out, toes pointed downward, and the head and neck arched backward.

Vestibulospinal Tract

• The lateral vestibulospinal tract arises from the lateral vestibular nucleus located at the pontomedullary junction in the brainstem and does not cross.
• It descends in the anterior funiculus to all levels of the spinal cord.
• Facilitates extensor muscles associated with the axial region and lower limbs (anti-gravity muscles).
• The medial vestibulospinal tract descends from the medial vestibular nuclei and becomes part of the descending portion of the medial longitudinal fasciculus (MLF) in the anterior funiculus.
• Terminates in cervical and upper thoracic levels of the spinal cord without crossing.
• Facilitates neck musculature and controls the positioning of the head.

Tectospinal Tract

• Arises from the superior colliculus of the midbrain and immediately crosses to the contralateral side of the midbrain.
• Travels in the anterior funiculus.
• Fibers terminate in the first four spinal cord segments on interneurons in ventral gray matter.
• Mediates reflex movements in response to visual input to move the head and neck to visualize an object.

Basal Ganglia

• Basal Ganglia nuclei includes the:
• Striatum
• Globus pallidus
• Subthalamic nucleus
• Substantia Nigra
• It Receives crude motor signals via projection fibers from the motor cortex and sends refined motor signals back to motor areas via the thalamus (ventral anterior nucleus).
• Plays a major role in motor planning (voluntary movement initiation) and muscle tone control.
• The basal ganglia system is mainly supplied by deep branches from the anterior, middle, and posterior cerebral arteries.
• Anterior cerebral arteries predominantly supply the striatum; the middle cerebral arteries supply the striatum and globus pallidus; posterior cerebral arteries supply the substantia nigra and subthalamic nucleus.
• Lesions to the basal ganglia system affects thalamocortical activity so movements are defective.
• Damage to the basal ganglia on one side affects the ipsilateral motor cortex -> damage to the basal ganglia on one side causes contralateral movement disorder
• Hypokinetic: Parkinson's disease
• Hyperkinetic: Huntington's disease

Parkinson's Disease

• Neuropathology involves the degeneration of dopamine-producing neurons in the substantia nigra, which project to the striatum.
• Bradykinesia (shuffling gait), rigidity (cogwheel or lead-pipe), resting tremor (70% of patients), and stooped posture and/or unstable balance are motor symptoms seen in Parkinson's Disease.

Huntington's Disease

• Neuropathology involves the degeneration of GABA-producing neurons in the striatum, which project to the globus pallidus.
• Enlarged lateral ventricles are due to striatal cell loss and striatal atrophy.
• Chorea (rapid, involuntary movement of the extremities and trunk), lack of coordination and balance, walking difficulty, slurred speech, and chewing & swallowing difficulty are motor symptoms seen in Huntington's Disease.

Cerebellum Anatomy

• Two transverse fissures divide the cerebellum into three lobes.
• The Primary fissure subdivides the cerebellum into the anterior and posterior lobes
• The Posterolateral fissure subdivides the cerebellum into the flocculonodular lobe.
• The medial region of the cerebellum is the vermis, and the lateral regions (on each side of the vermis) are the cerebellar hemispheres.

Deep Cerebellar Nuclei

• 3 pairs of deep nuclei; bilaterally located in cerebellar white matter.
• Each associated with specific anatomical subdivision along the medial-tolateral axis:
• vermis
• paravermis
• lateral hemisphere
• Deep-nuclei neurons integrate sensory and motor signals to regulate lateral and medial descending motor systems

Cerebellum Function

• Neurons integrate motor signals to regulate lateral and medial descending motor systems for movement timing & direction
• Lateral hemisphere = Dentate nucleus = over/under shooting
• Paravermis = Interposed nucleus = voluntary movements of the extremities
• Vermis = Fastigial nucleus = voluntary movements of the trunk

Lateral, Interposed, and Fastigial Nuclei

• Dentate nucleus neurons integrate motor signals to regulate lateral and medial descending motor systems for movement timing & direction.
• Lateral hemisphere
• over/under shooting
• Interposed nucleus neurons integrate sensory signals to regulate lateral descending motor systems for adjustments of ongoing voluntary movements of the extremities.
• Paravermis
• Fastigial nucleus neurons integrate sensory signals to regulate medial descending motor systems for adjustments of ongoing voluntary movements of the trunk.
• Vermis

Cerebellum Dysfunction

• Motor incoordination, balance, and gait abnormalities due to altered activity in the cerebellum

Cerebellum Damage

• Lateral cerebellar lesions involving the dentate nucleus cause unilateral deficits of the ipsilateral distal muscles.
• Symptoms - Finger-to-nose test inaccuracy (dysmetria), overshoot (hypermetria), undershoot (hypometria), and corrective movements that result in a terminal tremor
• Difficulty in rapid alternating movements (dysdiadochokinesia) is caused by irregular pattern
• Midline cerebellar lesions (in the vermis and paravermis) typically cause bilateral deficits d/t the medial motor systems influence on the trunk muscles.
• Incoordination of lower trunk/leg muscles, ataxic gait/postural instability, and a wide-based compensatory standing position are the clinical signs/symptoms
• Patients flex hip to place to knee and run heel down the tibial crest (shin) to ankle on a heel-to-shin test, abnormal heel oscillates above the knee and their foot slips off-shin.