Movement 1 and 2 NA

Lower motor neuron (LMN)

A motor nerve cell that directly carries action potentials from the brainstem or spinal cord to skeletal muscle fibers.

Rationale for calling the lower motor neuron the "final common pathway"

All descending commands and peripheral sensory inputs converge on it, making its activation the definitive command for muscle contraction.

Motor unit

A single alpha motor neuron and all the individual skeletal muscle fibers it innervates.

Two mechanisms used to grade muscle contraction force

Recruiting a variable number of motor units (spatial recruitment) and modulating the firing rate of action potentials (temporal frequency coding).

Four fundamental classes of motor behavior

Simple patterned reflexes, postural adjustments, rhythmical movements, and voluntary goal-directed movements.

Simple patterned reflex

A rapid, involuntary, and stereotyped motor response executed via spinal or brainstem circuits without necessitating cerebral cortex participation.

Postural adjustment

An automatic motor correction mediated largely by the brainstem and spinal cord that preserves balance and center of gravity.

Rhythmical movement

A repetitive, patterned motor sequence (such as walking) that can be volitionally initiated and stopped, but runs automatically once engaged.

Voluntary movement

A purposeful, flexible motor act guided by high-level cortical systems acting through brainstem and spinal networks.

Function of ongoing sensory feedback in motor performance

Provides real-time proprioceptive and environmental information to update, calibrate, and correct motor commands.

Cortical lobe containing the motor areas

The frontal lobe.

Prefrontal cortex (PFC) role in movement

Directs executive motor planning, cognitive decision-making, formulation of behavioral goals, and selecting appropriate actions.

Supplementary motor area (SMA) functions

Programs and executes internally generated movement sequences, coordinates bimanual movements, and supports mental motor rehearsal.

Premotor cortex (PMC) primary functional role

Prepares and guides movements cued by external sensory stimuli (such as visual or auditory targets).

Primary motor cortex (M1) function

Directly issues motor commands to downstream tracts for the execution of voluntary muscle contractions.

Motor homunculus

A topographic cortical map in which body parts requiring fine motor control are allocated disproportionately large surface areas.

Pyramidal system

The direct corticospinal and corticonuclear pathways originating from the cerebral cortex dedicated to voluntary movement control.

Target musculature of the corticospinal tract

Muscles of the trunk and limbs located caudal to the head and neck.

Corticonuclear (corticobulbar) tract function

Conveys voluntary motor commands from the cortex to cranial nerve motor nuclei in the brainstem to control head and neck muscles.

Rostral-to-caudal course of the corticospinal pathway

Cerebral cortex → corona radiata → internal capsule (posterior limb) → brainstem → spinal cord → lower motor neurons.

Anatomical site where most corticospinal fibers decussate

The lower medullary pyramids (pyramidal decussation), conferring contralateral motor control.

Functional divergence between lateral and anterior corticospinal tracts

The lateral tract controls fine, fractionated movement of the distal limbs, whereas the anterior tract predominantly modulates axial and proximal trunk posture.

Medial longitudinal fasciculus (MLF)

A heavily myelinated brainstem tract that interconnects abducens, trochlear, and oculomotor nuclei to coordinate conjugate eye movements with vestibular signals.

Three longitudinal zones of the brainstem

The tectum (dorsal roof), tegmentum (intermediate core), and basis (ventral base).

Brainstem division: pyramidal vs. extrapyramidal pathways

Pyramidal tracts travel in the ventral basis, whereas extrapyramidal pathways descend within the tegmentum.

Extrapyramidal system

Subcortical and brainstem descending tracts (reticulospinal, vestibulospinal, tectospinal, rubrospinal) regulating automatic muscle tone, posture, and coordination.

Reticulospinal tract

A descending brainstem pathway that modulates postural tone, activates antigravity musculature, and coordinates broad locomotor synergies.

Lateral vestibulospinal tract

Projects ipsilaterally to facilitate extensor muscle tone in the trunk and limbs for maintaining upright equilibrium.

Medial vestibulospinal tract

Projects bilaterally to the cervical spinal cord to stabilize head and neck orientation in response to angular acceleration.

Vestibulo-ocular reflex (VOR)

Compensatory ocular movement driven by vestibular activation in the direction opposite to head rotation to stabilize images on the fovea.

Tectospinal tract

Originates in the superior colliculus to reflexively turn the head and eyes toward novel visual or auditory cues.

Rubrospinal tract

Originates in the midbrain red nucleus and projects to the spinal cord to facilitate flexor muscle activation, mostly in the upper limbs.

Supraspinal reflex

An automatic postural restoration mechanism coordinated by brainstem nuclei rather than solely local spinal circuits.

Circuitry difference between a spinal stretch reflex and a supraspinal reflex

A stretch reflex is confined to segmental spinal circuitry, whereas a supraspinal reflex loops through brainstem integration centers.

Dorsolateral vs. ventromedial descending motor systems

Dorsolateral pathways govern fractionated, independent distal limb movements; ventromedial pathways govern axial posture and bilateral balance.

Clinical presentation of a lower motor neuron (LMN) lesion

Flaccid weakness or paralysis, hypotonia, hyporeflexia, muscle fasciculations, and rapid neurogenic atrophy.

Clinical presentation of an upper motor neuron (UMN) lesion

Spastic paresis, hypertonia (clasp-knife rigidity), hyperreflexia, clonus, and an extensor plantar response (positive Babinski sign).

Clinical presentation of extrapyramidal dysfunction

Involuntary movements, resting tremor, lead-pipe or cogwheel rigidity, and akinesia/bradykinesia, with relative preservation of muscle strength.