Motor system 1 Flashcards
Define a motor unit
what happens when press against table with lightest touch
alpha neuron + all muscle fiber it innerv
every time alpha motor neuron fibers, all muscle fibers connected contract
light touch = recruit motor units with small # of muscle fibers harder, recruit more units with more # muscle fibers
Define size principle of recruitment of motor neurons
as incr force generating, you start recruiting motor units with larger and larger sizes (varied force)
ex: if push table soft or firm, you have to recruit right size muscle fibers (smallest motor neurons recruit first, largest last)
Describe graded tension in skletal muscle
change number of active motor neurons that innerv muscle
key elements of stretch reflex
sensory 1a neuron with cell body in DRG
sensory receptor = receive input from muscle spindle stretch receptor
synapses on dendrites of alpha motor neuron in gray matter
ex knee jerk reflex what are components
what happens when activated
sensory neuron cell body in DRG
motor neuron
sensory receptor = muscle spindle
target muscle = quadriceps
all 1a afferents in quad activ
spatial sum of EPSP
each 1a afferent has train of high frq AP so EPSP summate = temporal summation
what happens during a reflex
simultaneous activ of all muscle spindle sensory neuron and activ of all motor neurons
DRG neurons innerv muscle spindles = largest of DRG neurons, fastest
what are stretch receptor neurons and what do they innerv
stretch receptor neuron = 1a afferents and innerv all motor neurons in that muscle
what happens in cat calf
60 1a afferent
300 alpha motor neuron to gastrocs
each 1a synapse on all alphascausing small EPSP in motor neuron for spatial summation
since lots of NT, also temporal summation
activ motor neuron –> fire AP
in addition to motor neurons innerv muscle in which spindle resides what are two other sets of neurons innerv by 1a affernt ends
alpha motor neuron innerv what?
alpha motor neuron = LMN with branches
1) excitatory synapse on motro neurons of synergistic muscles
2) via interneurons, inhib motor neurons of antagonistic muscles
3) motor neurons of muscle where spindle located
define homonymous muscle
muscle that contains or is assoc directly with sense organ producing reflex
define motor reflex
1) positive or neg feedback
2) what happens when muscle is stretch
1) neg feedback
2)
1) muscle stretch
2) 1a afferent distorted, initiate AP
3) AP travel in cord to muscle’s motor neuron
4) alpha motor neuron fire AP for contraction
relieves stress so 1a neurons silenced
compare and contrast muscle spindles and golgi tendon organs
arrangement
muscle spindles = parallel with extrafusal fibers
golgi tendon = series with muscle; when muscle contract or passively stretched, 1b afferents fire AP
compare and contrast muscle spindles and golgi tendon organs
info transmitted
muscle spindle = length
golgi tendon = force
Describe gamma motor neurons
1) what do they innerv
2) what part of NS regulates these?
3) compare to alpha motor neurons
1) afferent, innerv small muscle fibers inside muscle spindle (stretch receptor)
2) regulates activity of primary somatosensory receptors
3) smaller and slower conducting than alpha motor neuron
what kinds of fibers are inside muscle spindle
what are those fibers innerv by?
what do gamma motor neurons do when activ
contain intrafusal fibers
innerv by gamma motor neuron
gamma motor neuron provides afferent signal to stretch spindle ends and cause sensory response in mid spindle where efferent innerv
what happens to intrafusal fibers as muscle shortens
purpose of this effect
as muscle shortens, intrafusal fibers excited by gamma motor neurons and contract
keeps afferent innerv stretched so response generated even when muscle shortened
what happens when your expected contraction is greater than what object’s weight
motor neurons compensate for expected shortening of muscle
1) intrafusal fibers stretch so 1a afferents don’t change activity during shortening
2) if discrepancy in muscle contraction for moving object, stretch receptor output signals to brain to compensate
apply “what happens when your expected contraction is greater than what object’s weight”
to lifting a box you thought was heavy
1) you exert more force than necessary
2) muscle shortens faster than expected
3) stretch of spindle by gamma motor neuron not large enough for sensory response
4) signal from 1a afferents decr to decr excitation of alpha motor neurons and decr excitation of muscle
5) adjust force matching
what happens as muscle shortens,
intrafusal fibers excited by gamma motor neurons and contract to keep afferent innerv stretched
therefore can respond even when muscle is shortened
muscle spindle activation
1) muscle spindle aka ?
2) when muscle contracted what happens to spindle
3) mechanism of spindle activation
muscle spindle = intrafusal fibers; when muscle contracted, spindle not working
1) periph DRG neuron enter spindle and wrap around intrafusal fiber
2) fiber stretched, nerve ending deformed, membane depol –> AP to cord and afferent fire
where is golgi tendon organ located?
compare the 1b to 1a afferents
1b endings depol by?
in tendon of muscles where sensory neuron endings (1b afferents) ramify (many in single tendon)
these are 1b, don’t conduct AP as rapid
1b depol by stretch or deformation
what is significance of series arrangement of golgi tendon organs
when muscle contracts, 1b afferents fire more AP
when passive stretch, golgi tendon organs fire less AP than basal level
what is effect of 1b afferents on alpha motor neuron
1) 1b synapse in spinal cord on interneurons that inhib alpha motor neurons of homonymous muscle
(FEEDBACK INHIB TO LIMIT STRONG CONTRACTS THAT DAMAGE MUSCLE LIKE SPASTIC PARALYSIS)
2) responds to small change in tension to monitor muscle force and tell us how much tension muscle exerting
why doesn’t 1b reflex cancel 1a
1b is slower, has another synapse and not activated as much by passive stretching
specify signaling involved in flexor withdrawal reflex and crossed extensor reflex
function of flexor withdrawal
flexor withdrawal = cause you to withdraw limb when activ flexors
1) Adelta fibers excite flexors via synapse on flexor/extensor neurons innervating both leg muscles via polysynaptic connections in spinal interneurons
2) simultaneously excite contralateral extensors
specify signaling involved in flexor withdrawal reflex and crossed extensor reflex
principle of reciprocal innerv
antagonists of intended movement will be inhibited such as when step on tack
hierarchy of motor system
1) motor neurons
2) spinal cord
3) brainstem = movement and
postural control
4) motor cortex= plan, initatie and direct voluntary movement
5) basal ganglia = gating initiation of movement in motor cortex
6) cerebellum = sensory motor coord modulating brainstem
alpha motor neuron define
degenerate in what disease
muscle cell receives how many neuronal input
neuron that innerv skeletal muscle, releases ACh
degenerate in ALS
muscle cell only receive input from one motor neuron
describe size principle
compare slow, fast fatigue-resistant, and fast fatiguable units
- size alpha motor neuron, caliber axons
- size of forces
- fatigue speed
- recruited when
1) slow motor units = small AMN with fine caliber axons
- innerv SMALL # of slow oxid muscle fibers
- generate small forces
- fatigue slowly
- recruited first DUE TO HIGH INPUT RESISTANCE
2) fast fatigue-resistant motor units = intermed AMN with medium axons
- innerv intermed # fast oxidative glycolytic muscle fibers
- generate large forces
- fatigue slowly
3) fast fatiguable motor units
- large AMN with thick fibers
- innerv LARGE # fast glycolytic muscle fibers
- large force
- fatigue quick
- recruit last
orderly recruit of fine to fast fatiguable = based on motor neuron size
why are small motor units recruited first
resistance of small neuron higher than large neuron because can’t pack as many
ion channels
so larger neurons = leakier so require more input to change voltage
what happens if large motor units are recruited? what drives them to threshold?
when do you need it
if need volitional control for running = summation of current, causes large motor neuron to summate EPSP to cross threshold and recruit fast fatiguable
motor unit properties are ___
reinnerv experiments for plasticitiy
use-dependent
cross innerv of small AMN on to fast twitch muscle fiber –> conversion MEANS MOTOR NEURON IDENTITY GOVERNS PHENOTYPE
chronic stim of fast-fatiguable motor units at low rates for long time what happens
converts fast fatiguable fibers to small or fatigue resistant motor units
activity in motor neuron dictates phenotype of muscle
how does exercise affect motor unit phenotype
compare endurance vs high intensity strength training
endurance exercise slows the contractile properties of motor unit and incr endurance and strength (fatigue-resistant)
high intensity strength training = incr contractile protein
also central changes alter recruitment
motor neurons originate where?
in ventral horn of spinal cord
somatotopic representation of body in spinal cord
gray matter
shoulder musc medial
hand musc lateral
white matter
lateral = corticospinal
medial = brainstem tracts
location and function of descending motor pathways
cerebrospinal rubrospinal vestibulospinal tectospinal reticulospinal
lateral
corticospinal
rubrospinal = innerv limbs, cell bodie in midbrain
rmedial
= vestibulospinal = vestib innerv to axial for balance
= tectospinal = gaze reflexes
= reticulospinal =
stretch reflex
when stretch fiber
1) stretch of spindle —> many AP down 1a sensory afferents (very fast)
2) reach spinal cord ventral horn
3) many AP then come down alpha motor neuron to contract
extrafusal fibers
reciprocal innerv
via inhib interneuron in spinal cord promotes relax of motor neuron to innerv anatagonist muscle
so don’t get seizure everytime contract
error correction of spinal cord circuitry
1) if muscle shortens unexpectedly then spindle longer than length of extrafusal fibers (undercontracted)
2) 1a afferent decr firing rate
3) alpha motor neuron has decr drive
4) lengthens muscle
what happens when lift heavier than expected
when lift heavier,
1) spindles stretch causing 1a
afferent to incr rate
2) feedback causes stabilizing incr in contraction of extrafusal
spindles would be over contracted and Ia afferents decr firing rates
decr α motor neuron drive.
What happens when lift lighter than expected
1) spindle relax and extrafusal contracting too fast
2) less tone alpha motor neuron causes muscle to correctly relax to proper length
voluntary shortening vs reflexive shortening
voluntary
AP down gamma motor neurons everytime voluntary contraction, contract both extra and intrafusal to maintain gain and sensitivity to stretch
reflexive
no AP down gamma neurons so gamma uncontracted and less sensitive to stretch
how do golgi tendon organs detect tension
when tendon takes on load, collagen collapses to pinch nerve ending for tension detection
negative feedback regulation of muscle tension
GTO decr activation of muscle when really large forces generate to protect muscle by stabilizing muscle contraction
at intermediate levels, it maintains steady level of force to counteract decr effects that decr muscle force (fatigue)
compare muscle spindle and GTO
what do they preferential signal
important for …
composed of …
signal via …
feedback system for…
muscle spindle = signal muscle stretch important for maintain tone composed of muscle fibers signals via 1a afferent feedback for maintain muscle length
GTO signal muscle tension impt for stabilizing contraction composed of ocllagen fibers and capsule signals via 1b afferent feedback to maintain muscle force
mechanism of crossed extensor-flexor coupling
1) step on tack
2) c fiber carry input to cord
3) other leg = contralat inhib interneuron inhib alpha motor neurons of contralat flexor and excite extensor
when lift injured leg, inhib extensor and activ flexor of ipsil leg
GTO innerv by
mechanism of GTO tension detection
1b afferent fibers
when tendon takes
on load via 1b afferents, collagen structure collapses to pinch nerve ending for tension
detection
