Spinal Cord: Descending Motor Pathways

Vocabulary flashcards covering the anatomy, pathways, bulbospinal systems, reflex circuits, and clinical lesion characteristics of the descending motor pathways of the spinal cord.

General Motor System Organization

The multi-tiered motor network wherein the cerebral cortex projects to the basal ganglia, cerebellum (via the pons), thalamus, and brainstem, which subsequently direct descending lateral and medial motor systems to the spinal cord.

Anterior Horn Somatotopic Organization

The spatial arrangement within the ventral horn where medial motor nuclei innervate proximal and axial muscles, while lateral motor nuclei innervate distal limb muscles.

Lateral Corticospinal Tract

A descending motor pathway originating in areas 4 and 6 whose fibers ($\sim 85\text{--}90\%$) cross at the pyramidal decussation, descend in the lateral funiculus, and synapse in the anterior horn across all spinal cord levels to control voluntary movements of distal limbs.

Anterior Corticospinal Tract

A descending motor pathway comprising $10\text{--}15\%$ of corticospinal fibers that remain uncrossed at the medulla, descend in the anterior funiculus, and cross largely through the anterior white commissure to terminate only in the cervical and upper thoracic anterior horns to control neck, shoulder, and upper back muscles.

Pyramidal Decussation

The crossing point of approximately $85\text{--}90\%$ of corticospinal tract axons located in the caudal medulla near the spinomedullary junction.

Upper Motor Neuron (UMN)

A neuron whose cell soma originates in motor cortices (such as areas 4 and 6) or brainstem nuclei, coursing through the central nervous system to terminate primarily on spinal interneurons and alpha motor neurons.

Lower Motor Neuron (LMN)

A motor neuron with its cell body located in the anterior horn of the spinal cord (or cranial nerve motor nuclei) whose axon exits the central nervous system via ventral roots to directly innervate skeletal muscle.

Alpha Motor Neuron ($\alpha$-motor neuron)

A large lower motor neuron located in the anterior horn whose axon innervates extrafusal skeletal muscle fibers to produce voluntary movement.

Gamma Motor Neuron ($\gamma$-motor neuron)

A smaller lower motor neuron located in the anterior horn whose axon innervates intrafusal fibers of the muscle spindle to regulate receptor sensitivity during contraction.

Primary Motor Cortex (Brodmann Area 4)

The neocortical region situated on the precentral gyrus of the frontal lobe organized by movements rather than individual muscles; low-level electrical stimulation elicits localized movements.

Motor Homunculus

A topographical cortical representation depicting the disproportionate amount of motor cortex dedicated to specific body regions relative to physical size (e.g., large representation for hands, medial for lower extremities, lateral for the face).

Premotor Cortex (Brodmann Area 6)

A cortical region containing at least six subregions that requires higher currents to elicit motor responses and is responsible for selecting, planning, organizing, and executing movement sequences based on internal or external cues.

Dyspraxia

An impaired ability to plan and perform purposeful motor tasks (e.g., pouring coffee, plugging in an appliance) despite maintaining normal baseline muscle strength, often resulting from a premotor cortex lesion.

Mirror Neurons

A specialized population of neurons within the premotor cortex that fire both when performing a motor action and when passively observing another individual perform that same action.

Posterior Limb of the Internal Capsule

The deep subcortical white matter pathway through which descending pyramidal tract axons originating in Brodmann areas 4 and 6 pass before entering the cerebral peduncles of the midbrain.

Effects of Upper vs. Lower Motor Neuron Damage

A clinical comparison showing that LMN lesions cause decreased muscle tone, decreased stretch reflexes, severe atrophy, and fasciculations/fibrillations, whereas chronic UMN lesions cause increased tone, hyperreflexia, mild disuse atrophy, clonus, and positive Babinski signs.

Spinal Shock

An initial transient period of hypotonia (flaccidity) and areflexia immediately following acute upper motor neuron injury, resulting from the sudden loss of descending supraspinal drive before spinal cord reflexes regain function.

Fasciculations

Spontaneous, involuntary contractions of entire groups of muscle fibers that are visible through the skin as small twitches, typically characteristic of lower motor neuron damage.

Fibrillations

Spontaneous contractions of single individual muscle fibers that are not grossly visible through the skin but can be detected on electrical recordings (EMG), typical of lower motor neuron injury.

Clonus

A rapid series of rhythmic, alternating involuntary muscle contractions in response to sudden, sustained stretch (such as ankle dorsiflexion), caused by impaired descending supraspinal inhibition over spinal reflex circuits.

Babinski Sign (Positive Extensor Plantar Response)

An abnormal reflex characterized by extension (dorsiflexion) of the great toe and fanning (abduction) of the lateral four toes upon firm plantar stimulation; normal in healthy infants but indicative of upper motor neuron pathology in adults.

Myotatic Reflex (Stretch Reflex Arc)

A monosynaptic reflex wherein stretch of a muscle spindle activates Ia sensory afferents that directly excite alpha motor neurons supplying the agonist muscle while exciting an inhibitory interneuron to relax the antagonist muscle.

Reciprocal Inhibition

The neural mechanism during a myotatic reflex whereby Ia sensory afferent collaterals synapse on inhibitory interneurons in the spinal cord to suppress alpha motor neuron discharge to antagonist muscles.

Muscle Spindle

A stretch-sensitive proprioceptive mechanoreceptor located within skeletal muscle composed of intrafusal muscle fibers innervated by gamma motor neurons that detects muscle length and rate of length change.

Lateral Vestibulospinal Tract

An uncrossed descending bulbospinal tract originating from the lateral vestibular nucleus that projects ipsilaterally to all spinal cord levels to facilitate antigravity extensor muscles and maintain upright posture.

Medial Vestibulospinal Tract

A descending bulbospinal pathway originating from the medial vestibular nucleus that projects bilaterally to cervical and upper thoracic spinal cord levels to coordinate head and neck movements in response to vestibular stimuli.

Pontine (Medial) Reticulospinal Tract

A descending brainstem pathway from the pontine reticular formation that acts primarily via gamma motor neurons to facilitate axial and postural extensor muscles, maintaining upright posture and anticipatory postural control.

Medullary (Lateral) Reticulospinal Tract

A descending brainstem pathway from the medullary reticular formation that exerts predominantly modulatory actions, suppressing spinal reflex activity, inhibiting extensors, and facilitating flexors.

Tectospinal Tract

A descending pathway originating in the superior colliculus of the midbrain that decussates and terminates in the contralateral cervical anterior horn to mediate orienting head and neck movements toward visual, auditory, or tactile stimuli.

Rubrospinal Tract

A descending bulbospinal tract originating in the magnocellular division of the red nucleus that decussates in the ventral tegmental decussation and terminates in the cervical and upper thoracic cord to facilitate upper limb flexor motor activity.

Decerebrate Posturing

An abnormal posture resulting from a brainstem lesion at the rostral pons (below the red nucleus but above vestibular and reticular nuclei), presenting clinically with trunk extension, lower extremity extension with ankle plantarflexion, and upper extremity extension at the elbow.

Decorticate Posturing

An abnormal posture resulting from a lesion superior to the midbrain/red nucleus (e.g., in the thalamus, internal capsule, or white matter), characterized by trunk extension, lower extremity extension, and upper extremity flexion at the elbow due to intact rubrospinal activity.