spinal mech of motor control Flashcards
- PRIMARY ENDINGS (Group Ia fiber)
- Sensory innervation of muscle spindle
–> one per muscle spindle
–> found on BOTH NUCLEAR BAG and CHAIN fibers
–> Shows both DYNAMIC (firing pattern signals RATE of muscle stretch/rate of change) and STATIC RESPONSE (firing pattern signals amount of stretch or muscle length
Secondary ending (group II fibers)
- Sensory innervation of muscle spindles
- Found on NUCLEAR CHAIN fibers ONLY (also static nuclear bag)
- SHOWS STATIC RESPONSE ONLY
- firing rate gives info about muscle length but does NOT emphasize changes in muscle length
Motor innervation of muscle spindle
- Gamma efferent fibers = provide fusimotor drive to muscle spindle
- Gamma-dynamic fibers = end on dynamic nuclear bag
- Gamma-static fibers = end on nuclear chain fibers and static nuclear bag
- Skeletofusimotor fibers = some motorneurons innervate BOTH extrafusal and intrafusal fibers (sometimes called beta-innervation)
describe ALpha motor neurons
- extrafusal motor neurons
- Large-caliber
- heavily myelinated = rapid speed
- Responsible for initiating contraction
Gamma motor neurons
- INtrafusal motor neurons
- Slender
- Lightly myelinated = less rapid transmitting
- control skeletal muscle by adjusting tension
explain function of COACTIVATION of alpha and gamma motorneurons
- When extrafusal muscle fibers contract, the muscle shortens and muscle spindles would become unloaded (not able to sense small lengthening of the muscle)
- To prevent unloading, gamma motor neurons are activated and they make intrafusal fibers contract causing any small lengthening of the muslce can be sensed immediately
- Ia fibers normally facilitate alpha motor neurons during muscle contraction. this facilitation would be lost if muscle spindles became unloaded during msucle contraction
describe static response to muscle spindle
- Firing pattern signals amount of stretch of muscle length
- Type Ia and Type II fibers
describe the dynamic response of muscle spindle
- firing pattern signals rate at which muscle is being stretched or rate of change in muscle length
- Type Ia ONLY
describe the role of gamma motorneurons in controlling spindle response
- the firing rate of muscle spindle afferents depends on both muscle length and the level of gamma activation of the intrafusal muscle fibers
- when the nervous system interprests the signals fro muscle spindles it must take into account the amount of fusimotor drive gamam neurons
describe the stretch reflex (myotatic reflex0
describe PHASIC stretch (myotatic) reflex
- Type Ia fibers synapse directly on alpha motor neuron of homonoymous and synergist muscles
- Initiated by tapping on tendon of muscle with reflex hammer
- Demonstrates evidence of LMN vs UMN problems
describe TONIC stretch (myotatic) reflex
- Type Ia fibers synapse directly on alpha motor neurons of homonymous and synegist muscles
- Type II fibers synpase directly on alpha motor neurons of homonymous muscles
- INTIATED BY passively bending a joint
- Patients with SPASTICITY, the stength of reflex in spastic muscles depends on the speed of muscle stretch
–> faster the stretch, the stronger the reflex (tonic reflex is exagerated in parkinsonism)
describe reciprocal innervation in spinal cord and how it is related to stretch reflex
- group Ia fibers excite homonymous muscles (agonist muscles) and its synergist (flexors)
- Group Ia fiber collaterals end on Ia inhibitory interneurons that act on antagonist muscles (extensors)
Define Muscle tone and relation to myotatic reflex
- Resting tone = slight tension that can be felt in relaxed muscles (properaly measured by testing the TONIC STRETCH REFLEX)
- if subject is deeply relaxed should be no EMG activity in muscle but many subjects cannot relax that much
Role of co-contraction
- Co-contraction of muscles at a joint means that both PRIME MOVERS at a joint and its ANTAGONISTS are contracted simultaneously
- THIS ACTION STIFFENS JOINT
- USEFUL when precision and joint stabilization are important
describe the GTO reflex
- REFLEX ARC for GROUP Ib REFLEX:
–> increased tension (force) on muscle increases firing in Ib fibers
–> Ib fibers synapses on inhibitory interneuons which inhibits alpha motor neurons to homonymous muslces resulting in decreased muscle tension (increased tension causes muscles to be inhibited)
–> conclusion: GTO FEEDBACK mechanisms monitors and maintains muscle force (opposite of myotatic reflex)
describe clasp-knife reflex
- Most easily domenstrated in the knee extensor of spastic patients
- clinician tries to quickly bend the patient knee
- resistance to flexion seems to build up gradually and then at a certain point, resistance suddenly decreases
- the hyperactive stretch reflexes of the spastic patient causes the increase in resistance to stretch found in the knee extensors
- the sudden DECREASE is the clasp-knife phenomenon
- the biceps is an anti-gravity muscle in humans and the clasp-knife reflex can demonstrated in the spastic biceps when you try to extend the elbow
