modulation of spinal reflexes Flashcards
what are the numbers of bag and chain fibres in muscle spindles
2-3 bag
3-5 chain (variable)
what is the continuum of intrafusal fibres
not exclusively dynamic or static
- can have a mix of both to determine the sensitivty
- more dynamic = more sensitive to velocity of muscle stretch
- more static = more sensitive to amount of muscle stretch
why is proprioception faster than vision
no chemical cascades need to happen
all mechanical
how do spindle receptors sense muscle length
- muscle stretch deforms and opens stretch sensitive ion channels
- cytoskeletal strands connected to the channels on the inside of the membrane lengthen forcing the channels to open
- influx of ions excites the spindle receptor causing it to fire APs
what are the muscle spindle firing patterns (assuming no yMN activity) when the muscle is stretched, relaxed, and passively shortened
stretched
- causes spindle recptors to increase their firing rates
relaxed
- maintain a high tonic firing rate with regular APs
- slower firing rate than stretched
passive shortening
- reduces the firing rate of spindle receptors
what is reciprocal inhibition
stretch of agonist muscle produces reflex contraction of agonist/synergist muscles and inhibition of antagonists
what are the properties of short latency reflexes (SLRs)
- stimulus driven
- recruit only spinal circuits
- simple processing capabilities
what are the two ways SLRs scale
amount and rate of muscle stretch
- amplitude increases with larger stretch
background muscle activity (automatic gain scaling)
- amplitude increases with higher background muscle activity (even with same perturbation)
why do SLRs scale with higher background activity
need to recruit larger MUs because the small MUs are already recruited in the background
(hennemans and ohms law)
why do SLRs scale with amount and rate of stretch (larger responses to larger / longer stretches)
above threshold for a longer time = more APs = more NT released = more excitation of aMNs
(prolonged increases in receptor and membrane potential)
what is hennemans size principle
smaller, less fatiguable (slow twitch MUs are recruited first because they are higher resistance)
what is ohm’s law
membrane resistance is inversely proportional to the cell’s SA (number of parallel ion channels)
small MNs have higher membrane resistance (fewer parallel ion channels)
large MNs have low membrane resistance (leaky)
what is the difference in recruitment between small and large MUs based on ohm’s law
large
- hard to reach threshold for recruitment because the ions leak faster
- recruited by larger background muscle activity
small
- see larger changes in membrane potential with the same synaptic input because of higher membrane resistance (resistance to leak)
what is the difference in effect of the same EPSP between rest and increase in background muscle activity
at rest - series of EPSPs doesn’t elicit reflexive firing
increase BMA - tonic subthreshold increase in muscle activation causes the same EPSP to cause the aMN to fire and result in reflexive muscle contraction
(BMA pushes aMNs closer to the threshold membrane potential and increases likelihood of firing after receiving EPSP)
what is the effect of coactivation before contraction
increase the net force of contraction
- increase agonist
- increase inhibition of antagonist
what are the 3 main ways to modulate spinal reflexes
- alter excitability of spinal MNs
- alter excitability of pools of spinal interneurons
- increase sensitivity to sensory feedback in the spinal cord
what are the 3 ways to alter excitability of spinal MNs
- descending pathways from M1 and premotor (CSTs)
- supraspinal control of spinal MNs
- gain scaling of spinal stretch reflexes
what is supraspinal control of spinal MNs
brain has direct control over the excitability of the spinal cord (descending control)
how does supraspinal control of spinal MNs alter their excitability
by altering the tonic excitability or background activity of spinal MN, the brain has direct control over the sensitivity of spinal processing of sensory feedback
what is the relationship between background / reflex EMG and force applied
linear relationship between force production and amplitude of EMG
- higher forces = slower velocity
why does automatic gain scaling occur
caused by recruitment of larger MUs with larger amounts of background EMG
why does the timing of peaks (SLR and LLR) stay relatively constant regardless of MUs recruited
same circuit is used (same # of synapses, etc)
what are the 2 main factors in altering the excitability of pools of spinal interneurons
- supraspinal control of spinal MNs (renshaw cells)
- pre synaptic inhibition of spinal MNs
what are renshaw cells
- specific class of inhibitory interneurons
- located in the intermediate zones and ventral horn of the spinal cord
- can inhibit several populations of MNs
