Central and Peripheral Motor Neuron Syndromes

A collection of vocabulary terms and definitions covering upper and lower motor neuron syndromes, specific clinical signs, tract anatomy, and Amyotrophic lateral sclerosis.

Primary motor cortex

Part of the motor cortex that encodes the force, direction, extent, and speed of a movement.

Premotor cortex

Part of the motor cortex that signals movement preparation, integrates sensory aspects of a motor act, and signals correct vs. incorrect movements in a behavioral context.

Supplementary motor area (SMA)

Area responding to movement sequences and mental rehearsal that transforms spatial distances and angles into dynamic input for muscle contraction.

Hemiparesis

A partial motor deficit involving the face, arm, and leg on one side of the body.

Hemiplegia

A total motor deficit involving the face, arm, and leg on one side of the body.

Paraparesis

A partial motor deficit of both legs, often localized to the bilateral medial frontal lobe, bilateral basis pontis, or spinal cord below T1.

Paraplegia

A total motor deficit of both legs.

Quadriplegia / Tetraplegia

A motor deficit affecting all four limbs.

Betz cells

Giant pyramidal neurons located in the 55th layer of cells in the motor frontal cortex (Brodmann area 4) where the corticospinal tract originates.

Lateral corticospinal tract

The pathway formed by the 90% of axons that cross over at the pyramidal decussation in the caudal medulla.

Anterior corticospinal tract

The pathway formed by the 10% of axons that do not cross at the medulla, which later cross at the spinal segment via the anterior white commissure.

Weber syndrome

A midbrain lesion characterized by IIIrd nerve (oculomotor) palsy on the lesion side and a pyramidal deficit on the opposite side.

Milard-Gubler syndrome

A pons lesion involving facial (VII) and abducens (VI) nerves on the lesion side and a pyramidal deficit on the opposite side.

Dejerine syndrome

A medulla lesion involving the XIIth nerve on the lesion side, with pyramidal and proprioceptive deficits on the opposite side.

Hypertonia / Spasticity

An increase in muscle tone described as elastic; a velocity-dependent resistance to passive movement seen in upper motor neuron disorders.

Spastic catch

A point during quick passive movement where the muscle sharply resists the movement.

Clasp-knife reflex

A sudden drop in resistance during passive movement occurring when the autogenic inhibition reflex is initiated.

Elastic spring sign

A clinical sign where forced passive pronation of the fist is followed by a rapid return to the initial position after release.

Clonus

An abnormal oscillation at $5-7\,Hz$ when a muscle is rapidly stretched and held at a constant length, felt during forced dorsal flexion of the sole.

Babinski sign

A pathological reflex where forceful stroking of the lateral plantar surface of the foot causes slow, tonic dorsiflexion of the great toe and fanning of the other toes.

Oppenheim sign

The Babinski phenomenon evoked by an intense stroke along the edge of the tibia from the knee downward.

Gordon sign

The Babinski phenomenon evoked by pressing or forcefully squeezing the calf muscles.

Rossolimo sign

A toe flexor reflex consisting of flexion of the second through fifth toes in response to a tap on their distal phalanges from the plantar side.

Hoffmann sign

A pathological reflex where pinching the nail of the median finger produces flexion of the distal phalanx of the thumb.

Synkinesis movement

Involuntary movements in a paretic territory induced by movements in other territories, yawning, or coughing.

Raimiste's Phenomenon

A global synkinesis where resistance to hip abduction or adduction of the noninvolved extremity causes the same motion in the paretic leg.

Homolateral synkinesis

A flexor movement elicited in the involved arm when the patient attempts to flex the leg or leg flexion is resisted.

Fasciculation

A visible local twitch of a muscle caused by spontaneous action potentials from a damaged alpha motor neuron.

Fibrillation

Spontaneous twitching of individual muscle fibers, invisible to the eye, detectable only via electrophysiological recordings (electromyogram).

Amyotrophic lateral sclerosis (ALS)

A fatal motor neuron disorder characterized by progressive loss of both upper and lower motor neurons at spinal or bulbar levels.

Riluzole

A medication used for ALS that has shown very modest results in treatment.