Motor system 1

Question 1

Define a motor unit

what happens when press against table with lightest touch

Answer 1

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

Question 2

Define size principle of recruitment of motor neurons

Answer 2

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)

Question 3

Describe graded tension in skletal muscle

Answer 3

change number of active motor neurons that innerv muscle

Question 4

key elements of stretch reflex

Answer 4

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

Question 5

ex knee jerk reflex what are components

what happens when activated

Answer 5

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

Question 6

what happens during a reflex

Answer 6

simultaneous activ of all muscle spindle sensory neuron and activ of all motor neurons

DRG neurons innerv muscle spindles = largest of DRG neurons, fastest

Question 7

what are stretch receptor neurons and what do they innerv

Answer 7

stretch receptor neuron = 1a afferents and innerv all motor neurons in that muscle

Question 8

what happens in cat calf

Answer 8

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

Question 9

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?

Answer 9

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

Question 10

define homonymous muscle

Answer 10

muscle that contains or is assoc directly with sense organ producing reflex

Question 11

define motor reflex

1) positive or neg feedback

2) what happens when muscle is stretch

Answer 11

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

Question 12

compare and contrast muscle spindles and golgi tendon organs

arrangement

Answer 12

muscle spindles = parallel with extrafusal fibers

golgi tendon = series with muscle; when muscle contract or passively stretched, 1b afferents fire AP

Question 13

compare and contrast muscle spindles and golgi tendon organs

info transmitted

Answer 13

muscle spindle = length

golgi tendon = force

Question 14

Describe gamma motor neurons

1) what do they innerv
2) what part of NS regulates these?
3) compare to alpha motor neurons

Answer 14

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

Question 15

what kinds of fibers are inside muscle spindle
what are those fibers innerv by?

what do gamma motor neurons do when activ

Answer 15

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

Question 16

what happens to intrafusal fibers as muscle shortens

purpose of this effect

Answer 16

as muscle shortens, intrafusal fibers excited by gamma motor neurons and contract

keeps afferent innerv stretched so response generated even when muscle shortened

Question 17

what happens when your expected contraction is greater than what object’s weight

Answer 17

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

Question 18

apply “what happens when your expected contraction is greater than what object’s weight”

to lifting a box you thought was heavy

Answer 18

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

Question 19

what happens as muscle shortens,

Answer 19

intrafusal fibers excited by gamma motor neurons and contract to keep afferent innerv stretched

therefore can respond even when muscle is shortened

Question 20

muscle spindle activation
1) muscle spindle aka ?

2) when muscle contracted what happens to spindle
3) mechanism of spindle activation

Answer 20

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

Question 21

where is golgi tendon organ located?

compare the 1b to 1a afferents

1b endings depol by?

Answer 21

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

Question 22

what is significance of series arrangement of golgi tendon organs

Answer 22

when muscle contracts, 1b afferents fire more AP

when passive stretch, golgi tendon organs fire less AP than basal level

Question 23

what is effect of 1b afferents on alpha motor neuron

Answer 23

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

Question 24

why doesn’t 1b reflex cancel 1a

Answer 24

1b is slower, has another synapse and not activated as much by passive stretching

Question 25

specify signaling involved in flexor withdrawal reflex and crossed extensor reflex

function of flexor withdrawal

Answer 25

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

Question 26

specify signaling involved in flexor withdrawal reflex and crossed extensor reflex

principle of reciprocal innerv

Answer 26

antagonists of intended movement will be inhibited such as when step on tack

Question 27

hierarchy of motor system

Answer 27

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

Question 28

alpha motor neuron define

degenerate in what disease

muscle cell receives how many neuronal input

Answer 28

neuron that innerv skeletal muscle, releases ACh

degenerate in ALS

muscle cell only receive input from one motor neuron

Question 29

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

Answer 29

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

Question 30

why are small motor units recruited first

Answer 30

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

Question 31

what happens if large motor units are recruited? what drives them to threshold?

when do you need it

Answer 31

if need volitional control for running = summation of current, causes large motor neuron to summate EPSP to cross threshold and recruit fast fatiguable

Question 32

motor unit properties are ___

reinnerv experiments for plasticitiy

Answer 32

use-dependent

cross innerv of small AMN on to fast twitch muscle fiber –> conversion MEANS MOTOR NEURON IDENTITY GOVERNS PHENOTYPE

Question 33

chronic stim of fast-fatiguable motor units at low rates for long time what happens

Answer 33

converts fast fatiguable fibers to small or fatigue resistant motor units

activity in motor neuron dictates phenotype of muscle

Question 34

how does exercise affect motor unit phenotype

compare endurance vs high intensity strength training

Answer 34

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

Question 35

motor neurons originate where?

Answer 35

in ventral horn of spinal cord

Question 36

somatotopic representation of body in spinal cord

Answer 36

gray matter
shoulder musc medial
hand musc lateral

white matter
lateral = corticospinal
medial = brainstem tracts

Question 37

location and function of descending motor pathways

cerebrospinal
rubrospinal
vestibulospinal
tectospinal
reticulospinal

Answer 37

lateral
corticospinal
rubrospinal = innerv limbs, cell bodie in midbrain

rmedial
= vestibulospinal = vestib innerv to axial for balance
= tectospinal = gaze reflexes
= reticulospinal =

Question 38

stretch reflex

Answer 38

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

Question 39

reciprocal innerv

Answer 39

via inhib interneuron in spinal cord promotes relax of motor neuron to innerv anatagonist muscle

so don’t get seizure everytime contract

Question 40

error correction of spinal cord circuitry

Answer 40

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

Question 41

what happens when lift heavier than expected

Answer 41

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.

Question 42

What happens when lift lighter than expected

Answer 42

1) spindle relax and extrafusal contracting too fast

2) less tone alpha motor neuron causes muscle to correctly relax to proper length

Question 43

voluntary shortening vs reflexive shortening

Answer 43

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

Question 44

how do golgi tendon organs detect tension

Answer 44

when tendon takes on load, collagen collapses to pinch nerve ending for tension detection

Question 45

negative feedback regulation of muscle tension

Answer 45

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)

Question 46

compare muscle spindle and GTO
what do they preferential signal

important for …

composed of …

signal via …

feedback system for…

Answer 46

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

Question 47

mechanism of crossed extensor-flexor coupling

Answer 47

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

Question 48

GTO innerv by

mechanism of GTO tension detection

Answer 48

1b afferent fibers

when tendon takes
on load via 1b afferents, collagen structure collapses to pinch nerve ending for tension
detection