Peripheral Motor System: Muscles, Movements, and Motor Units

Vocabulary flashcards covering the components of movement, muscle types and actions, spinal cord somatotopy, and motor unit physiology from Functional Neuroanatomy.

Reflexes

Rapid, stereotyped, involuntary movements that largely exist to protect the body from harm (e.g., stretch reflex and withdrawal reflex).

Rhythmic motor patterns

Movements featuring a mixture of reflexive and voluntary components (e.g., breathing, walking, running, and chewing) that can be initiated voluntarily but are sustained automatically by central pattern generators and reflexes.

Central pattern generators (CPGs)

Groups of interconnected neurons whose activity, once initiated, causes sustained rhythmic movements—often alternating limbs—until a decision is made to stop them.

Voluntary movements

Purposeful, goal-directed movements that can be learned and improve with practice (e.g., typing, driving a car, and playing a piano).

Agonist muscle

The primary muscle in an operating muscle pair responsible for generating a particular movement by pulling on a bone.

Antagonist muscle

A muscle that decelerates movement produced by the agonist muscle and pulls a bone in the opposing direction.

Flexor

A muscle whose contraction serves to decrease a joint angle, such as the biceps pulling the forearm closer to the upper arm.

Extensor

A muscle whose contraction serves to increase a joint angle, such as the triceps pulling the forearm away from the upper arm.

Axial muscles

Muscles that move the trunk and maintain posture.

Proximal muscles

Muscles that move joints close to the trunk (including the shoulder, elbow, pelvis, and knee) and are critical for posture and locomotion.

Distal muscles

Muscles that move joints far from the trunk (hands, feet, fingers, toes, eyes, face) and are utilized for fine manipulation, binocular vision, and facial expressions.

Medial part of ventral horn

The somatotopic region in the spinal cord's ventral horn containing motor neurons that control proximal and axial/trunk muscles to maintain posture.

Lateral part of ventral horn

The somatotopic region in the spinal cord's ventral horn containing motor neurons that control distal muscles involved in fine manipulation and motor control.

Dorsal part of ventral horn

The somatotopic division of the ventral horn that houses motor neurons controlling flexor muscles.

Ventral part of ventral horn

The somatotopic division of the ventral horn that houses motor neurons controlling extensor muscles.

Motor unit

A functional motor entity composed of one motor neuron and all the muscle fibers it innervates.

Paresis

Partial paralysis resulting from damage to a single spinal nerve, occurring because individual muscle fibers across a muscle receive input from multiple motor neurons and spinal nerves.

Motor unit size precision relationship

The principle that motor unit size is inversely correlated with movement precision; high-precision muscles have small motor units (few fibers per neuron), whereas lower-precision muscles have large motor units (more fibers per neuron).