Neuroscience: Motor Systems, Vestibular Function, Cerebellum, Basal Ganglia, and Spinal Cord Injury

Comprehensive vocabulary flashcards reviewing vestibular anatomy and pathology, spinal cord injury stages and syndromes, cerebellar and basal ganglia circuits, and upper versus lower motor neuron pathways.

Vestibular Apparatus

An inner ear structure consisting of three semicircular canals, one utricle, and one saccule that contains hair cell sensory receptors responding to gravity and head movements.

Kinetic Labyrinth

The three semicircular canals containing hair cells that respond to the motion of endolymph during rotational head movements and angular acceleration, serving to stabilize vision.

Static Labyrinth

The otolithic organs (utricle and saccule) located inside the vestibule that detect head position relative to gravity and linear acceleration or deceleration (such as elevator motion or car acceleration) via macular crystals bending hair cells.

Medial Longitudinal Fasciculus (MLF)

A brainstem pathway that ascends bilaterally to motor cranial nerves III, IV, and VI to influence conjugate eye and head movements for gaze stabilization.

Vestibulospinal Pathways

Descending motor pathways that project to spinal cord lower motor neurons to coordinate postural adjustments and maintain the head in an upright position.

Vestibulo-ocular Reflex (VOR)

A reflex producing conjugate eye movements in the direction opposite to head movement in order to maintain visual fixation on the environment.

Vertigo

The illusion of motion or false perception that oneself or the environment is spinning or moving, representing the most common symptom of vestibular dysfunction.

Benign Paroxysmal Positional Vertigo (BPPV)

An inner ear disorder caused by canalithiasis (otolithic crystals displacing into semicircular canals) leading to acute vertigo and nystagmus with head position changes lasting less than $2\text{ minutes}$.

Meniere's Disease

A vestibular disorder characterized by abnormal fluid pressure in the inner ear causing spontaneous episodes of fullness, tinnitus, severe vertigo, nausea, vomiting, and hearing loss.

Vestibular Neuritis

Inflammation of the vestibular nerve, typically viral in origin, producing acute unsteadiness, nausea, and vertigo that peaks over $3\text{ days}$ and subsides over $2\text{ weeks}$, with hearing unaffected.

Complete Spinal Cord Injury (ASIA A)

A spinal cord injury classification defined by the complete absence of motor and sensory function in the sacral segments, trunk, and extremities below the neurological level of injury.

Primary Damage (Spinal Cord Injury)

Immediate structural trauma occurring at the time of insult to the spinal cord, categorized into concussion, contusion, or laceration.

Secondary Damage (Spinal Cord Injury)

Progressive tissue destruction developing days to $4\text{ weeks}$ after the initial trauma, driven by ischemia, ion derangement, demyelination, inflammation, and apoptosis.

Excitotoxicity

Neuronal and glial cell death triggered by excessive release of glutamate following spinal cord injury, which overstimulates glutamate receptors and creates an intracellular ion imbalance.

Spinal Shock

A transient physiological phenomenon immediately following spinal cord injury characterized by flaccidity, absent tendon reflexes, impaired autonomic control, and complete loss of sensation below the lesion.

Cerebellum

The brain region that compares planned motor commands with actual sensory output to make unconscious, real-time corrections to posture and voluntary coordination.

Ataxia

Impaired movement coordination presenting as jerky, inaccurate voluntary movements and an unsteady, staggering, veering gait that is not due to weakness, spasticity, or contractures.

Dysdiadochokinesia

The inability to perform rapid, alternating movements smoothly, tested clinically through rapid pronation/supination or toe tapping.

Dysmetria

The inability to judge the distance or range of a movement, presenting as past-pointing or overshooting an intended target.

Action Tremor

Involuntary shaking of an extremity during voluntary movement that intensifies in amplitude as the limb approaches its intended target.

Substantia Nigra

A midbrain dopaminergic nucleus within the basal ganglia control circuit that undergoes neurodegeneration in Parkinson's disease.

Huntington's Disease (HD)

An autosomal dominant neurodegenerative disorder marked by striatal pathology and an expanded trinucleotide ($\text{CAG}$) repeat on the Huntingtin ($\text{HTT}$) gene, resulting in chorea, behavioral changes, and dementia.

Cervical Dystonia

The most common focal dystonia, characterized by involuntary, sustained contractions of neck muscles leading to abnormal head turning or tilting (including torticollis, laterocollis, retrocollis, and antecollis).

Feedforward

The anticipatory use of sensory information to prepare motor systems before a movement is initiated.

Feedback

The use of sensory inputs during or following a movement to monitor performance and make reactive corrections.

Upper Motor Neurons (UMN)

Neurons with cell bodies located in the cerebral cortex or brainstem whose descending axons modulate and synapse onto lower motor neurons in the brainstem and spinal cord.

Lower Motor Neurons (LMN)

Motor neurons located in the cranial nerve motor nuclei of the brainstem and the ventral horn of the spinal cord that directly project to and innervate extrafusal and intrafusal skeletal muscle fibers.

Lateral Corticospinal Tract

The principal descending pathway controlling voluntary fractionation and fine motor control of limbs; approximately $88\%$ of its fibers cross at the medullary pyramidal decussation to descend contralaterally.

Corticobrainstem Tract

A motor pathway originating in the primary motor cortex that projects to cranial nerve motor nuclei controlling voluntary movements of the face, tongue, pharynx, larynx, trapezius, and sternocleidomastoid.

Central Facial Palsy

A unilateral upper motor neuron lesion of the corticobrainstem tract that causes contralateral weakness of the lower facial muscles while sparing the forehead and upper face due to bilateral cortical innervation.

Alpha Motor Neuron (AMN)

Large lower motor neurons residing in the ventral horn that directly innervate extrafusal skeletal muscle fibers to generate contractile force.

Gamma Motor Neuron (GMN)

Lower motor neurons that innervate intrafusal fibers within the muscle spindle to regulate its sensitivity to stretch across varying muscle lengths.

Motor Unit

A functional motor entity composed of a single alpha motor neuron and all the skeletal muscle fibers it innervates.

Henneman's Size Principle

The physiological concept dictating that smaller motor units controlling fine precision are recruited before larger motor units that generate high contractile force.

Spasticity

A velocity-dependent form of hypertonia characterized by increased resistance to passive muscle stretch that increases with higher stretch velocities, commonly seen in upper motor neuron lesions.

Rigidity

A velocity-independent form of hypertonia characterized by sustained, constant resistance to passive stretch throughout the entire range of motion.

Decorticate vs. Decerebrate Rigidity

Abnormal posturing patterns; decorticate rigidity exhibits upper extremity flexion with lower extremity extension (upper midbrain/cortex lesions), whereas decerebrate rigidity exhibits rigid extension of both upper and lower limbs with internal rotation and plantarflexion (lesions between midbrain and pons).

Brown-Séquard Syndrome

A spinal cord hemisection resulting in ipsilateral loss of voluntary motor function and proprioception/vibration alongside contralateral loss of pain and temperature sensation below the lesion level.

Anterior Cord Syndrome

A spinal cord syndrome caused by anterior spinal artery compromise or flexion injury, resulting in bilateral loss of voluntary motor function and pain/temperature sensation with preserved proprioception and vibration.

Central Cord Syndrome

An incomplete spinal cord injury occurring typically in the cervical region secondary to hyperextension, producing greater motor and sensory deficits in the upper extremities than the lower extremities.

Posterior Cord Syndrome

A rare spinal cord condition impacting the dorsal columns, resulting in bilateral loss of proprioception, vibration, and fine touch, leading to sensory ataxia.

Post-polio Syndrome

A lower motor neuron condition occurring $30\text{--}40\text{ years}$ after acute poliovirus infection, caused by metabolic failure and degeneration of enlarged collateral motor unit terminal sprouts.