Muscle tone - anatomy, hysiology, examination of muscle tone, syndromes of disturbed muscle tone
Definition: Muscle tone is the continuous, involuntary, slight contraction of muscles present at rest. It maintains posture, stabilizes joints, and prepares muscles for movement.
Anatomy and Physiology
- Muscle tone depends on the integrity of:
- Muscle spindles (stretch receptors)
- Afferent sensory fibers (Ia fibers)
- Spinal reflex arc
- Anterior horn cells (α-motor neurons)
- Descending pathways (pyramidal and extrapyramidal systems)
- Cerebellum
Stretch Reflex Mechanism
Muscle stretch → Muscle spindle activation → Ia sensory fibers → Spinal cord → α-motor neuron activation → Muscle contraction
This reflex is the main mechanism maintaining normal muscle tone.
Regulation of Muscle Tone
Pyramidal system: modulates voluntary movement
Extrapyramidal system: regulates posture and tone
Cerebellum: coordinates and adjusts tone
Examination of Muscle Tone
The patient should be relaxed.
Passive Movements: The examiner moves the patient's limbs through their range of motion and assesses resistance.
Upper Limb
- Flexion and extension of elbow
- Pronation and supination of forearm
- Wrist movements
Lower Limb
- Flexion and extension of knee
- Rotation of hip
- Dorsiflexion and plantar flexion of foot
Assessment
Tone may be: Normal. Increased (hypertonia). Decreased (hypotonia)
Syndromes of Disturbed Muscle Tone
1. Hypotonia (Decreased Tone)
- Features: Reduced resistance to passive movement. Flaccid limbs. Hypermobile joints. "Rag-doll" appearance
- Causes: Lower motor neuron lesions. Peripheral neuropathy. Acute cerebellar lesions. Acute spinal shock. Muscle diseases
- Associated Findings: Muscle weakness. Hyporeflexia or areflexia. Muscle atrophy (LMN lesions)
2. Hypertonia (Increased Tone)
A. Spasticity (Pyramidal Hypertonia): Due to upper motor neuron lesions.
Features
- Increased tone predominantly in antigravity muscles
- Resistance increases with rapid movement
- Clasp-knife phenomenon
- Associated with hyperreflexia and Babinski sign
Causes: Stroke. Multiple sclerosis. Spinal cord lesions
B. Rigidity (Extrapyramidal Hypertonia): Due to basal ganglia disorders.
Features
- Uniform resistance throughout movement
- Independent of movement speed
Types:
Lead-pipe rigidity – constant resistance
Cogwheel rigidity – ratchet-like resistance
Causes: Parkinson disease. Parkinsonian syndromes.