Muscle tone

Muscle tone is the resistance a muscle offers to passive movement. It reflects both neural activity and the mechanical properties of muscle and surrounding tissues.

Stretch reflex: A reflex contraction triggered when a muscle is stretched, mediated by sensory feedback and motor neurons. Stretch-sensitive feedback contributes to resting resistance, especially when a muscle is lengthened.
Neurological examination: A clinical assessment of sensory, motor, reflex, coordination, and cognitive functions of the nervous system. Passive limb movement is one component of the motor examination.
Skeletal muscle: Striated muscle tissue attached to the skeleton and controlled largely by somatic motor neurons. Its fibers and connective tissues provide the structures whose passive resistance is felt.
Upper motor neuron lesion: Damage to motor pathways descending from the brain or brainstem to spinal and brainstem motor circuits. Such lesions can disrupt descending control and produce abnormal tone.
Muscle spindle: A sensory receptor within skeletal muscle that detects changes in muscle length and the speed of stretching. Its afferent signals help regulate the reflex response to passive stretch.
Spasticity: A velocity-dependent increase in stretch reflexes and muscle tone associated with upper motor neuron lesions. It is a particular pathological pattern, not a synonym for all increased tone.
Motor unit: A motor neuron and all the skeletal muscle fibers it innervates. Motor-unit activity contributes active force that can affect measured tone.
Lower motor neuron lesion: Damage to motor neurons or their axons that directly connect the central nervous system to skeletal muscle. It commonly reduces reflex activity and tone, contrasting with many upper motor neuron syndromes.
Gamma motor neuron: A motor neuron that adjusts the tension of intrafusal muscle fibers inside muscle spindles. Gamma activity tunes spindle sensitivity and thereby shapes stretch-related tone.
Rigidity: Increased resistance to passive movement that is relatively independent of movement speed, often associated with parkinsonism. Its speed-independent resistance contrasts with the velocity-sensitive pattern of spasticity.

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