Tone (Muscle Tone) in the CNS

Definition

"The resistance of a muscle to stretch is often referred to as its tone or tonus."

• Ganong's Review of Medical Physiology, 26th Ed.

Neural Basis of Tone - The Key Components

Mechanism of Normal Tone

1. γ motor neurons fire tonically at rest, keeping the intrafusal fibers of muscle spindles under continuous mild tension.
2. This tonic stretch keeps the spindle Ia afferents active, delivering a low-level excitatory drive to α motor neurons.
3. α motor neurons respond with a low level of efferent firing, maintaining background muscle contraction - this is tone.
4. Descending supraspinal pathways (corticospinal, reticulospinal, vestibulospinal, rubrospinal) continuously modulate this loop from above.

"Some gamma motor neurons are tonically active at rest, keeping the intrafusal muscle fibers taut and more sensitive to active and passive changes in muscle length."

• Adams & Victor's Principles of Neurology, 12th Ed.

CNS Control - Supraspinal Influences on Tone

"Bulbospinal upper motor neurons influence strength and tone but are not part of the pyramidal system. The descending ventromedial bulbospinal pathways originate in the tectum, vestibular nuclei, and reticular formation and are involved in the maintenance of posture."

• Harrison's Principles of Internal Medicine, 22nd Ed.

Abnormalities of Tone

1. Hypotonia (Decreased Tone)

• Definition: Reduced resistance to passive movement; muscles feel "floppy."
• Causes:
  • Lower motor neuron (LMN) lesion: peripheral nerve or anterior horn cell damage
  • Posterior column / proprioceptive pathway damage
  • Acute upper motor neuron (UMN) lesions - spinal shock phase
  • Cerebellar lesions (ipsilateral hypotonia)
  • Severe hypotonia allows passive hyperextension of joints.

2. Spasticity (UMN type Hypertonia)

• Definition: Velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks.
• Mechanism: Loss of descending inhibitory (mainly reticulospinal) influence on α motor neurons → hyperactive stretch reflex.
• Clasp-knife phenomenon: On rapid passive stretch, there is initial resistance, then sudden release - caused by activation of the inverse stretch reflex (GTOs) and Ib inhibitory interneurons.
• Features: Increased tone with free interval, velocity-dependent, brisk tendon reflexes, Babinski sign.
• Causes: Stroke, spinal cord injury, cerebral palsy, multiple sclerosis.

"Rapid passive displacement of the extremity results in increased resistance followed by relaxation (clasp-knife phenomenon). Resistance varies with the speed and direction of passive motion."

• Bradley & Daroff's Neurology in Clinical Practice

3. Rigidity (Extrapyramidal type Hypertonia)

• Definition: Increased resistance to passive movement that is uniform throughout the range of motion and not velocity-dependent, affecting both flexors and extensors equally.
• Lead-pipe rigidity: Smooth, uniform resistance like bending a lead pipe.
• Cogwheel rigidity: Ratchet-like, rhythmically interrupted resistance - represents tremor superimposed on rigidity (seen in Parkinson's disease).
• Mechanism: Overactivity of the stretch reflex due to loss of basal ganglia inhibition on the thalamo-cortical loop; γ motor neuron hyperactivity.
• Features: No free interval, not velocity-dependent, tendon reflexes NOT enhanced, limb does not resume original position after manipulation.
• Causes: Parkinson's disease, Wilson's disease, neuroleptic drugs, MSA, PSP.

"The increased resistance on passive movement that characterizes rigidity is not preceded by an initial free interval and has an even or uniform quality throughout the range of movement, like bending a lead pipe."

• Adams & Victor's Principles of Neurology, 12th Ed.

4. Paratonia (Gegenhalten)

• The patient appears to actively resist passive movement in any direction.
• Seen in frontal lobe disease, dementia, confusional states.
• Reflects inability to relax, not a true basal ganglia disorder.

Comparison: Spasticity vs. Rigidity

Clinical Examination of Tone

1. Inspection: Spontaneous posture - spasticity causes fixed flexed posture of upper limbs.
2. Palpation: Hypertonic muscles feel firm; hypotonic muscles feel "doughy."
3. Passive movement: Ask the patient to relax completely. Move the limb passively through its full range at varying speeds.
   • Normal: uniform, low-level resistance throughout, same at all velocities.
   • Spastic: resistance increases with faster movement; clasp-knife release.
   • Rigid: even, "lead-pipe" resistance at all velocities.
   • Hypotonic: abnormally easy passive movement; joints may hyperextend.
4. Heel-dragging test (lower limb): In a supine patient, grasp behind the knee and lift briskly. Normally the heel stays in contact with the table; in spasticity the heel rises off the table.

Summary