Week 3 - Spinal Reflexes and Electromyogram Flashcards
what are characteristics of sensory receptors acting on spinal motoneurons?
- excitatory or inhibitory
- movement quality/efficiency
- safety and protection
- monosynaptic (direct synaptic contacts) or polysynaptic (indirect)
what is a stretch receptor?
- how are they arranged?
- what are they targets for?
component of a muscle spindle that sense changes in muscle length
- only afferent in CNS that receives a nerve supply
- arranged in parallel with extrafusal (regular) muscle fibers
- target for gamma motoneurons
how do gamma motoneurons compare with alpha motoneurons?
gamma have smaller soma and lower conduction velocities
- innervate intrafusal muscle fibers
- smaller diameter
- no force
- affect stiffness of sensory regions
- nuclear bag and nuclear chain types
what do intrafusal fibers include? what causes afferent firing, and what are the characteristics?
2 types of sensory zones that fire from spinal stretch
- central (bag) region: Ia receptors
- -dynamic, intense firing with stretch onset, then slows down (non-linear)
- distal regions: II receptors
- -steady firing, increasing with increasing stretch (linear)
what is perturbation in terms of the arm stretch reflex?
increased load (torque) about elbow causes rotation
- spindle (intrafusal fibers) in biceps are stretched
- afferent signals sent to spinal cord, activating (recruiting) biceps alpha motoneurons (stretch reflex)
what is correction in terms of the arm stretch reflex?
restoration of equilibrium
- original (homonymous) and synergist (heteronymous) muscles excited
- antagonist inhibited via Ia inhibitory interneuron and reciprocal inhibition
- biceps contracts and shortens, elbow flexes, resisting perturbation
what happens when extrafusal (muscle) fibers contract?
shortening unloads spindle, eliminating ability to signal further changes in muscle length
- a silent (unloaded) stretch receptor is no use to CNS
- solve by firing gamma-motoneurons to shorten intrafusal fibers, so afferent endings remain sensitive to further changes in muscle length
how does alpha motoneuron activation differ with and without gamma motoneuron?
w/o: stimulating extrafusal muscle fibers (via alpha) will cause afferent activity with “silence” where the muscle contracted
w/: gamma stimulated causes Ia response to be “filled in”, and remains sensitive to further stretch
what is a golgi tendon organ?
type Ib receptor in series with muscle extrafusal fibers (VS spindle in parallel)
- mechanical deformation of receptor endings opens channels, causing APs
- active muscle contraction causes strong GTO firing, sensing force generation
- passive force has little effect on GTO activity
how do muscle spindle (intrafusal muscle fibers, Ia) and golgi muscle tendon (Ib) differ when extrafusal muscle fibers stretch?
Ia - steady activity after stretch, none after contract
Ib - activity before/after stretch/contraction, but faster after the event
what is the golgi tendon organ action?
activity in Ib afferent (checks and balances)
- fires Ib inhibitory interneuron
- inhibits homonymous muscle
- excites antagonist muscle
what is the flexion crossed extension reflex?
protective role - faster limb withdrawal than voluntary RXN time (if step on tack with one leg, and shift weight to another)
- muscles working in agonist/antagonist pairs (reciprocal organization)
- opposite activity ACROSS midline
- reflexes are layered (growing complexity)
- interneurons serve multiple roles:
- -amplify/distribute excitatory signal
- -change signal polarity
how would the muscles work if you stepped on a tack with your right foot?
- cutaneous afferent fiber from nociceptor (A-delta) sends info to 4 different receptors
- R extensor inhibited, R flexor stimulated so R leg flexes to withdraw
- L extensor stimulated, L flexor inhibted so L leg extends to support
what is the central pattern generator? how can it be activated?
CPG - distributed network of neurons in spinal cord that can produce coordinated movements in the absence of higher input
- activated by e-stimulation in brainstem (mesencephalic lomotor region) or via pharmacologic input (noradrenaline)
- can be modified by training
what are implications of CPG training?
- spinal cord plasticity
- activity-dependent
- rehabilitation (?)
what are electromyograms used for?
to measure muscle units of a single motor unit, as they serve as natural biologic amplifiers of a nerve AP
how easy/hard is it to activate and monitor/record muscle fibers?
hard to activate with direct electrical stimulation -long pulse duration needed -burns are common easy to monitor/record activity -commonly used for diagnosis if NMD
what does the electrode design influence?
- volume of muscle recorded from
- signal characteristics
- frequency response
- sensitivity to noise (more selective = more noise)
what are types of electrode designs?
- surface (non-selective; have conductive paste since skin is an insulator; can also use for stimulation)
- needle (non-selective sub-dermal; used in operating room)
- concentric bipolar (selective; standard for EMG exams)
- hollow cannula (selective; insulated along length, except at tip; used for Botox injections)
why is standardization important?
allows comparison of findings between labs and across subjects
what does it mean if the patient…
- is unable to recruit any motor units?
- can only recruit a few motor units with very slow firing rates?
- has APs that are unusually low amplitude?
- has APs that are unusually large amplitude?
- spinal cord injury/stroke
- incomplete spinal cord injury
- myasthenia gravis (unmyelinated)
- denervated then partially innervated
