Neuromotor Control Part 2: Muscle Tone

Feb 23 Lecture

What is muscle tone?

The tension in the relaxed muscle

The resistance felt by the examiner during passive stretching of a joint when the muscles are at rest

May actually reflect a state of preparedness to a movement

What is hypertonicity vs hypotonicity?

Hypertonicity refers to increased muscle tone, leading to stiff or rigid muscles.

• Rigidity is the most extreme form of this

• Spasticity is more commonly seen

Hypotonicity describes decreased muscle tone, resulting in floppy or weak muscles

• Flaccidity = no tone at all, not even a flicker

What are the three mechanisms involved in regulating muscle tone?

1. Stretch reflex relationship to tone

2. Upper motor neuron mechanisms - UMN influences

3. Lower motor neuron mechanisms

describe the stretch reflex (reciprocal inhibition) pathway

1. The patellar tendon is tapped and stretched

2. 1A fibres in muscle spindles sense the stretching

3. Signal is sent to the SC, where the 1A fibres synapse directly with alpha MN neurons in the anterior horn of the SC

   1. Alpha MN to the extensor muscles (quads) are excited

4. 1A fibres also synapse with the inhibitory interneuron

   1. The inhibitory interneuron then goes to inhibit the alpha MN of the antagonist muscle (hamstrings)

How does the upper motor neuron system influence the stretch reflex?

You can consciously stop the knee-jerk reaction from happening if you are paying attention and decide you don’t want it to happen

This is possible because of UMN influences on the reflex

What tracts inhibit the stretch reflex?

Corticospinal tract (conscious voluntary control over stretch reflex) and Dorsal reticulospinal tract

Eg. If someone has a UMN lesion that impairs the corticospinal tract from the cortex, that results in impairment of an inhibitory signal on the stretch-reflex circuitry, allowing the facilitatory signal to take over and result in increased tone!!

Which tracts augment the spinal reflex?

Medial reticulospinal tract and Vestibulospinal tract

Damage to the corticospinal tract results in what to the muscle tone?

Damage to the corticospinal tract results in increased muscle tone due to the loss of inhibitory control over spinal reflexes, leading to hyperreflexia and spasticity.

Eg. stroke at the level of the cortex

How does the LMN system influence stretch reflex and tone?

The lower motor neuron (LMN) system directly mediates the stretch reflex by innervating skeletal muscles, allowing for muscle contraction in response to stretch. Damage to the LMN can lead to complete loss of tone (loss of both alpha and gamma motor neurons) and diminished stretch reflex activity, resulting in flaccidity.

Rule of thumb for tonicity and UMN vs LMN

UMN lesion usually results in increased tone

LMN lesion usually results in flaccidity

Lesions to the medial reticulospinal tract and/or vestibulospinal tract result in what muscle tone?

This will result in decreased tone or hypotonicity

How does impairment in the basal ganglia and cerebellum impact muscle tone?

These are modulating centres for fine tuning motor activity and movement regulation for output and coordination

Lesions here result in altered regulation of tone

Not necessarily impairment in the increase or decrease in tone

• Cerebellar Ataxia = on/off of muscle tone in an uncoordinated manner that results in shaky movement

Does decreased tone mean decreased strength?

Decreased tone does not necessarily indicate decreased strength; it primarily refers to the state of muscle tension at rest and may occur without weakness. However, these are often associated…

Weakness = decreased muscle fibre recruitment

Define hypotonia

Hypotonia is a condition characterized by decreased muscle tone, leading to reduced resistance to passive movement and often resulting in weakness and poor posture.

What is the clinical presentation of hypotonia? (3)

1. Diminished reflexes (at rest, they might not demonstrate the knee-jerk reaction)

2. Decreased responsiveness to weight-bearing surfaces

3. Loss of trophic factors (proteins and nutrients) that nourish muscles (leads to muscle atrophy)

   1. Decreased tone -> decreased circulation -> muscle atrophy

What is flaccidity? Etiology?

Extreme degree of hypotonicity

• often associated with paralysis

• Can be seen at rest and during PROM

Etiology:

• Congenital etiologies

• LMN lesion

• UMN lesion related to diaschisis

What is hypertonia? Etiology?

Hypertonia is a condition characterized by increased muscle tone, resulting in elevated resistance to passive movement and often leading to stiffness and reduced flexibility.

Etiology:

• Damage to descending pathways (basal ganglia pathology) - results in lack of modulation of inhibitory neurons and or alpha MN hyperexcitability

What are the two types of hypertonicity?

Rigidity and spasticity

What is rigidity?

resistance to PROM, irrespective of posture and velocity (not related to stretch reflex circuitry)

Commonly seen in extrapyramidal lesions (eg. Parkinson’s)

What are the two forms of rigidity?

Rigidity can be classified into two forms: lead-pipe rigidity and cogwheel rigidity.

• Lead-pipe rigidity involves constant resistance to movement throughout the range. Stiffness in all movements at all times (uniform resistance to passive stretching in all directions). Affects both flexors and extensors.

• Cogwheel rigidity features a ratcheting sensation during movement (intermittent catch and release pattern of muscle tension, resembling a cogwheel movement)

What is spasticity?

Velocity dependent increase in tonic stretch reflexes with exaggerated tendon jerks and clonus resulting from hyper excitability of the stretch reflex

What is the clinical presentation of spasticity?

1. Exaggerated stretch reflex (elicited by tapping a tendon)

2. Tone increases when person is given a quickly applied a passive stretch

3. Sometimes accompanied by clasp knife spasticity and clonus

What is clasp knife spasticity?

A sudden reduction in resistance to passive movement, resembling a closing knife, which occurs after initial resistance during a rapid stretch.

Resistance at first, and then gives way

What is clonus?

A series of involuntary, rhythmic muscle contractions and relaxations, often triggered by a sudden stretch.

Stretch of the flexor causes the flexors to contract, which causes the extensors to stretch and then contract as well

• pendulum effect, where opposing muscles keep getting their stretch reflexes triggered

What scale is important for measuring spasticity?

The Modified Ashworth Scale, which rates the resistance during passive stretch to assess muscle tone in patients.

0 = no increase in tone

1 = slight increase in tone, manifested by a catch followed by minimal resistance throughout the remainder of the range of motion.

1+ = slight increase in tone, manifested by a catch followed by some resistance throughout the remaining range of motion.

2 = more marked increase in tone, through most of the range of motion, but the affected part is easily moved.

3 = considerable increase in tone, making passive movement difficult.

4 = Rigidity in flexion or extension

What are the two other hyperactive reflexes?

1. Spasms - spontaneous muscle contraction or movement

   1. Can occur after SCI, MS, TBI, dystonia

   2. Can occur idiopathically (dehydration, muscle stiffness)

2. Hoffman Sign

   1. Flicking the nail of the middle finger elicits a reflexive finger flexion in the thumb and index finger.
      
      This sign is often used as a diagnostic tool for a CNS condition affecting C5 and C6

What are two reflexes not related to tone?

Cutaneous reflex (babinski response)

• Scrape along sole of foot from heel to toe and observe toes splaying

• This is a normal reflex in babies - should not see it in adults

Autonomic dyreflexia

• SC lesions above T6

• Sensory input increases sympathetic activity, arterial constriction, and increased BP

• Does not affect tone - can be life threatening

What are additional factors that influence tone?

1. Properties of the muscle and surrounding tissues

   1. muscle stiffness

   2. edema

   3. density of connective tissues surrounding

2. Reciprocal inhibition does not kick in

   1. Excessive co-activation of agonists and antagonists results in loss of reciprocal inhibition (Both agonist and antagonist are contracting simultaneously - get stuck in a movement)

3. Other factors such as:

   1. Activity level (Lot of effort - excessive energy output in one limb can result in tone deficiency in contralateral limb)

   2. Emotions (Stress = muscles tense up, depression = muscles let go, laughing = stomach hurts (not active contraction of abs, it happens as a result of emotions)

   3. Fatigue (time of day, sleep)

   4. Temperature (heat decreases muscle tone, cold increases it)

   5. Metabolic influences - medication/alcohol (Anything that influences neurotransmitter activity impacts fatigue level, energy level, muscle activity, etc…

A lesion in what part of the brain results in slowed/involuntary movement?

Basal ganglia