Tone Management

Spasticity and Rigidity

Chloe Kerstein PT, DPT

What is Muscle Tone

“Tone” refers to a relaxed muscle’s resistance to passive stretch/tension

The muscle resists the stretch through both active and passive mechanisms

Passive component

The passive mechanisms refer to viscoelastic structures that are non-contractile.

  • Actin & Myosin
  • Connective tissue
  • etc

Note

Forces created through passive mechanisms are invisible to EMGs

Active component

The active mechanisms refer to contractile motor units

These are controlled by spinal and supraspinal mechanisms

Note

Forces created through active mechanisms can be measured by EMGs

Anatomy

  • Muscle spindle: Intrafusal fibers
  • Muscle belly: Extrafusal fibers
  • Tendon: Connective tissue connecting muscle to bone

Intrafusal Fibers

  • “Muscle spindle”
  • Sensory organs
  • Detects changes in Length AND Velocit

Extrafusal Fibers

  • “Muscle belly”
  • Contraction creates force and movement

Tendon

  • Connects muscle to bone

Intrafusal fibers

Two types: Nuclear bag fibers and Nuclear chain fibers

Nuclear bag fibers:

  • Length + velocity
  • Ia Afferents → SC

Nuclear chain fibers:

  • length
  • Ia and II Afferents →SC

Both Synapse onto Gamma motor neurons in Spinal Cord

Extrafusal fibers

  • Contracts muscle itself
  • Activated by Alpha Motor Neurons from SC

Stretch Reflexes

Muscle spindles generate tone via the stretch reflex (dynamic or static):

2 types

Dynamic: sudden rapid stretch of a muscle → Ia → SC→ alpha motor neuron causes sudden contraction of muscle

Static: Sustained stretch→ type II → spinal cord → alpha motor efferents → cause asynchronous contraction of muscle fibers (motor units not all discharging together) = mild sustained contraction of these fibers as long as it is stretched.

This is the physiological basis of maintaining muscle tone

Mechanism

Muscle Stretches → Sensory info send to spinal cord → Muscle contracts to prevent overstretch damage

  1. Muscle stretches
  2. Intrafusal fibers sense stretch
  3. Ia + II afferents send signals to SC
  4. Synapses onto agonist Alpha motor efferents
  5. Agonist muscle contracts

Patellar Tendon Reflex

Alpha motor neurons → Extrafusal fiber contracts to prevent overstretching Quadriceps contract

Reciprocal Inhibition

Within spinal cord, Ia & II afferents also synapse onto:

  1. Inhibitory Interneurons of alpha motor neurons
  2. Inhibits alpha motor neurons of antagonist muscles while agonist contracts
  3. Result: Antagonist muscle cannot contract preventing further stretch of the agonist

How Do They work during muscle stretch

Patellar Tendon Reflex

  1. Hammer hit at tendon
  2. Quadricep muscle is stretched → Stretching both the intrafusal and extrafusal fibers
  3. Intrafusal fibers detect stretch (length/velocity)
  4. Type Ia and II afferent fibers
  5. Signals carried to DRG
  6. To dorsal horn of spinal cord and synapses on:

From dorsal horn to α-efferent

  1. Motor neuron (alpha/efferent)
  2. quad extrafusal fiber and cause it to contract to protect the muscle from overstretching (a monosynaptic reflex)

From dorsal horn to interneuron

  1. Interneuron → reciprocal inhibition
  2. Reciprocal inhibition
  3. inhibits alpha-motor efferents of antagonist muscles (Hamstrings relax)

Recurrent inhibition

  • Motor neuron fires → activates inhibitory interneuron
  • Inhibits the same neuron or a group of neurons.
  • Mediated by Renshaw cells in spinal cord

This feedback loop helps regulate motor neuron activity, preventing excessive firing and contributing to the fine-tuning of motor control.

Found to be increased in those with SCI - Overexcitation1.

Important

VERY DEBATED IF INVOLVED IN SPASTICITY

Alpha-Gamma Coactivation

Body needs to maintain stretch sensation when a muscle is contracting:

  1. Muscle contraction →
  2. Extrafusal fibers AND intrafuMuscle contraction → sal fibers contract→
  3. Muscle spindle tension is maintained due to the intrafusal fiber contraction →
  4. Maintaining the firing rate for the type Ia fibers and keeps it in an optimal range for detecting a change in muscle length

Tip

HYPOTHETICALLY if ONLY alpha-motor efferents are excited and Extrafusal fibers contract → reduces tension on muscle spindle → desensitization → decreased stretch proprioception info

HOWEVER this is not exactly what happens because the body needs to have stretch sensation during contraction/shortening!

Intrafusal fibers

  • Muscle stretches
  • Intrafusal fibers sense stretch
  • Ia + II afferents send signals to SC
    • Synapses onto agonist Alpha motor efferents
    • Synapses onto inhibitory interneurons
    • Synapse on Agonist gamma-motor efferent
      • Stimulates agonist intrafusal fiber contraction → tensioning muscle spindle to maintain stretch sensitization
    • Synapse on Inhibitory Interneuron of antagonist gamma-motor efferent
      • Inhibits gamma motor neuron to antagonist muscle, decreasing tension on antagonist muscle spindle (desensitized to stretch)

Intrafusal and Extrafusal fibers’ role in proprioception

To provide proprioception, the intrafusal fibers must contract and stretch in parallel with the extrafusal fibers MM Stretch → both intrafusal and extrafusal are stretched → type Ia/II → sensitive to stretch During a stretch, the stretch on the intrafusal fibers provide proprioceptive information of a muscle stretch MM Contraction → you would expect extrafusal contraction and intrafusal fibers to not contract However, not exactly the case;alpha gamma co-activation Alpha and gamma motor neurons contract in parallel in order to maintain tension on the intrafusal sensory fibers to provide proprioception during contraction

GTOS: Prevents tendon avulsion

Golgi tendon organs (GTOs) are sensory organs found within tendons

These organs detect tendon stretch and send sensory signals via Type 1b afferents → spinal cord.

GTO Example: Ego bicep curling

Muscle contraction → tendon lengthens → GTOs are proprioceptive/sense length → type 1b afferents → SC → DRG → synapses onto 2 interneurons:

Inhibitory interneuron

The inhibitory interneuron is activated which inhibits the alpha motor neuron to the bicep → Decreasing tension of agonist (bicep)

Excitatory Interneuron

Antagonist contraction to pull forearm in opposite direction Interneuron → alpha motor neuron → Excites antagonist (triceps)

Supraspinal influences

CST and RST tracts

Corticospinal Tract Cerebral cortex → crosses at pyramidal decussation → motor neurons Inhibits lower motor neurons Gives off corticoreticular pathway on way down → medullary reticulospinal nuclei → reticulospinal (RST) tract Inhibits lower motor neurons

1.
Shefner JM, Berman SA, Sarkarati M, Young RR. Recurrent inhibition is increased in patients with spinal cord injury. Neurology. 1992;42(11):2162-2168. doi:10.1212/wnl.42.11.2162