L6: Abnormal tone Flashcards
what is tone
- resistance offered by muscles to continuous passive stretch
- normal tone= prerequisite for normal mvmt
- resting level of tension in muscle - high enough to support against G but low enough for mvmt
- slight resistant when moving normal limb
non-neural and neural factors
non
-Passive stiffness of a joint & surrounding soft-tissue
Inherent visco-elastic properties of the muscle itself
Compliance of muscles, ligaments & joints (usually limiting factor under normal conditions)
Can vary with age, limb temperature, exercise state
neural Active tension set-up by stretch reflex
Neural
Activation of the contractile apparatus of the muscle
Can vary with age, emotional state
Output of alpha motor neuron, influenced by excitatory and inhibitory synapses from several systems*
peripheral and CNS - factor relating to tone
PNS CNS Muscle spindles (tonic component of stretch reflex)
Golgi tendon organ
Somatosensory receptors (skin, joints, connective tissue, muscles)
Sensory systems (visual, auditory, vestibular)
Limbic system (emotional state)
Motor systems
Interneurons in spinal cord
Higher centres via descending tracts
neural factors - stretch reflex
The stretch reflex is the body’s involuntary response to an external stimulus that stretches the muscles.
Proprioceptors/stretch receptors
muscle spindles - provide information concerning muscle length and the rate of change of length.
Muscle spindle located in the belly of the muscle.
This basic function of the muscle spindle helps to maintain muscle tone and to protect the body from injury. Also contributes to the performance of precise movements.
stretch reflex - cause
The stretch (myotatic) reflex is a direct result of stimulation of the muscle spindle (stretch receptor) – when a muscle is stretched, so is the spindle. Example of the stretch reflex is the knee jerk reflex.
sensory info to CNS
Sensory information is relayed to CNS via sensory neurons during a stretch - concerning the degree of stretch i.e. the change of length (type I a and type II ) and speed of the change (type I a only) on the involved muscle and the exact number of motor units needed to contract in order to overcome stretch
activation of stretch reflex
Activation of the stretch reflex attempts to resist the change in muscle length by causing the stretched muscle to contract
The more sudden the change in muscle length, the stronger the muscle contractions will be.
As fibres contract (shorten), the intrafusal muscle fibres which house the muscle spindle, shorten, so that the muscle spindle is deactivated and their activity ceases.
function of muscle spindle
This basic function of the muscle spindle helps to maintain muscle tone and to protect the body from injury. Also contributes to the performance of precise movements.
suprasinal control via:
Gamma motor neurones set sensitivity of muscle spindle.
Supraspinal control via: Pyramidal tracts Basal ganglia Cerebellum Vestibular system Reticular formation (RAF)
what alters stretch reflex?
Disruption to reflex arc at the level of:
Sensory input
Supraspinal control
Motor output
high tone
hypertonia
spasticity
rigidity
low tone
hypotonia
flaccidity
spasticity
‘Velocity-dependent increase in resistance to passive stretch of a muscle, with exaggerated tendon reflexes’ (Lance, 1990).
‘clasp knife’ phenomenon – abrupt cessation in movement (resistance greater at start of movement), followed by ‘a melting away’ of resistance.
Spasticity - Central
- loss of cortical inhibition
- imbalance in descending pathways
spasticity - peripheral
- altered biomechanical properties of muscle
Spasticity - pathophysiology mechanisms
maybe due to:
abnormal enhancement of the spinal stretch reflexes
Increased muscle spindle sensitivity (via gamma-motoneurone drive).
Increased excitability of central synapses involved in reflex arc.
Loss of inhibition of stretch reflex by descending supraspinal pathways .
causes:
Spasticity
upper motor neuron lesion - anywhere from motor cortex to spinal motor neurons.
common:
- stroke
- spinal cord compression,
- brain damage
- inflammatory lesions of SC
spasticity clincial features
Characteristic involvement of certain muscle groups – predominantly antigravity muscles e.g. UL flexors, LL extensors.
Increased responsiveness of muscles to stretch.
Hyper-reflexia – increased tendon reflexes.
Abnormal resistance to passive movement – the more rapid a limb is moved the greater the increase in tone.
Clasp knife – ‘catch’ followed by ‘melting’ away of resistance.
Clonus – rhythmic oscillations (usually of ankle & foot).
Factors influencing spasticity
positioning stress fatigue pain full bladder infection fear pressure sore constipation increased effort
assessment spasticity
Ashworth Scale (refer to Neurological Ax)
Modified Ashworth Scale
Movement grading:
Severe - no voluntary movement
Moderate - poor movement
Mild - good movement spasticity on resisted movement
Associated reactions:
Effort on the unaffected side can lead to an tone on spastic side / or effort of one spastic limb can lead to tone in the other spastic limb.
Involuntary movement e.g. yawning = AR
Tendon jerks
