PHYSIOLOGY - Motor units and muscle spindles Flashcards
what is the somatic motor system
skeletal muscles and the elements of the nervous system that control them
LMNs receive input from ___ ___ and ___
UMNs
proprioceptors
interneurons
what 2 neurones comprise LMNs
alpha and gamma
what is an alpha motor neurone
innervates the bulk of the muscle fibres within a muscle that generates force
what is a gamma motor neurone
innervates a sensory organ within the muscle known as a muscle spindle
biceps brachii and ____ work as synergists
brachialis
triceps brachii and ____ work as synergists
anconeus
biceps brachii and brachialis oppose (antagonists) what muscles
triceps brachii and anconeus
what muscles control movement of the trunk and maintain posture
axial muscles
what muscles mediate locomotion and where are they found
proximal (girdle) muscles - found in shoulders, elbow, pelvis, knee
axons of LMNs exit the spinal cord in the ____ roots or via _____
vental roots
or via cranial nerves
each ventral root joins with ____ to form ____
each ventral root joins with a dorsal root to form a mixed spinal nerve containing motor and sensory fibres
are the motor neurones distributed equally in the spinal cord
no
where are greater numbers of motor neurones found in the spinal cord
the cervical enlargement and lumbar enlargement
what nerve levels is the cervical enlargement and what does it supply
C3 - T1
arm
what nerve levels is the spinal enlargement and what does it supply
L1 - S3
leg
motor neurones supplying the axial musculature occur at what levels
all
what is the smallest functional component of the motor system
motor unit
what is a motor unit
an alpha-MN and all of the skeletal muscle fibres that it innervates
what is a motor neurone pool
the collection of a-MNs that innervates a single muscle
what 2 mechanisms control force of contraction
frequency of AP discharge of the a-MN
the recruitment of additional synergistic motor units
each AP causes a muscle ____
twitch - rapid sequence of contraction followed by relaxation
in the ventral horn, LMN cell bodies innervating axial muscles are _____ to those innervating distal muscles
medial
in the ventral horn, LMN cell bodies innervating flexors are ____ to those innervating extensors
dorsal
what are the 3 sources of input into an a-MN that regulate its activity
central terminals of DRG cells whose axons innervate the muscle spindles
UMNs in motor cortex and brain stem
spinal interneurons
what 2 things does muscle strength depend on
activation of muscle fibres
force production by innervated muscle fibres
what 3 things influence the activation of muscle fibres
firing rate of the LMNs involved
number of LMNs that are simultaneously active
the coordination of the movement
what 2 things influence force production by innervated muscle fibres
fibre size fibre phenotype (fast or slow contracting muscle)
a single AP in an a-MN causes muscle fibre to ____.
_____ __ ______ causes a sustained contraction
twitch
summation of twitches
where would you see small motor units with a few fibres
fine movements e.g. extraocular eye muscles
where would you see large motor units with thousands of fibres
large antigravity postural muscles e.g. leg muscles
small motor units are innervated by ___ a-MNs
small (soma diameter)
- same true for converse
true/false
motor units contain muscle fibres of only one type of either fast or slow fibres
true
a-MNs innervating fast type tend to be smaller/larger
larger
the motor end plate is usually where on a muscle fibre
centre - some muscle fibres may have multiple endplates
true/false
motor unit types are dispersed within a muscle
true
what are the 2 major types of skeletal muscle fibre
fast twitch and slow twitch
in what 2 ways do fast and slow twitch muscle fibres differ
differ in how myosin ATPase splits ATP to provide energy for cross bridge cycling
express different myosin heavy chains
what is another name for type I fibres
slow oxidative / slow
where is ATP derived from in type I fibres
oxidative phosphorylation
type I fibres have ___ contraction and relaxation
slow
type I fibres fatigue quickly/slowly
fatigue resistant
what colour are type I fibres and why
red fibres
high myoglobin content
what are the 2 types of type II fibres
type IIa and IIb
where is ATP derived from in type IIa fibres
largely from oxidative phosphorylation
where is ATP derived from in type IIb fibres
mainly from glycolysis
are type IIa fibres or type IIb fibres fatigue resistant
IIa fatigue resistant
IIb fast fatiguing
type IIa fibres are ____ in colour and reasonably well _____
red
vascularised
type IIb fibres are ____ in colour and _____ vascularised
pale
poorly
what is another name for type IIa fibres
fast oxidative / fatigue resistant
what is another name for type IIb fibres
fast glycolytic / fast fatiguing
compare the mitochondria in types I IIa and IIb
I - high
IIa - high
IIb - low
compare the glycogen content in types I IIa and IIb
I - low
IIa - intermediate
IIb - high
compare the myosin ATPase activity in types I IIa and IIb
I - low
IIa - high
IIb - high
what is the Henneman size principle
the susceptibility of an a-MN to discharge APs is a function of its size
- smaller MNs have a lower threshold than larger ones
fast fatiguing (IIb) motor units are ___ a-MNs and have a ___ threshold
large
high
fatigue resistant (IIa) motor units are _____ a-MNs and have a ___ threshold
intermediate size and threshold
slow motor units are ___ a-MNs and have a ___ threshold
small
low
______ motor units are more easily activated and trained by any training
slow
muscles with small motor units have a ____ gradation of force
fine
muscles with large motor units have a ____ gradation of force
coarse
motor units are recruited by what order
order of their size - smaller more easily activated
what does recruitment of motor units by size allow
fine control of muscle force across a wide range of tensions developed
motor units are recruited in an order appropriate to the physical task that is being performed. what is the order? and what does this allow
type I before type IIa then IIb
increasing increments towards the maximal force that the muscle exerts
type I units are used for
standing
type IIa units are used for
running / walking
type IIb units are used for
jump / sprint
what is the myotatic reflex
monosynaptic reflex arc - when a skeletal muscle is pulled it pulls back
how is change of length and rate of change sensed by a muscle
sensory organ - muscle spindle
what do spindles consist of (4)
- fibrous capsule
- intrafusal muscle fibres
- sensory afferents that innervate the intrafusal fibres
- gamma motor neurone efferents that innervate the intrafusal fibres
what do extrafusal muscle fibres do
generate muscle force
what sensory afferents innervate the intrafusal muscle fibres
Ia - myelinated and very fast conducting
what do you strike to cause a knee jerk reflex
patellar tendon
biceps reflex
C5 C6
supinator reflex
C5 C6
nerve of biceps reflex
musculocutaneous
nerve and muscle of supinator reflex
radial nerve
brachioradialis
triceps reflex
C7
knee reflex
L3 L4
muscle of knee reflex
quads
ankle reflex
S1 S2
muscle of ankle reflex
gastrocnemius
describe the myotatic reflex
1) stretch of muscle spindle
2) activation of Ia afferent
3) excitatory synaptic transmission in spinal cord mediated by glutamate
4) activation of a-MN
5) contraction of muscle
monosynaptic reflex most prominent in flexor/extensor muscles
extensor
how can the myotatic reflex be reinforced
jendrassik manoeuvre - as patient to interlock fingers and try to pull them apart when instructed and illicit jerk immediatley
intrafusal fibres of muscle spindles are innervated by ___-MNs and ___ sensory afferents
gamma
1a
which parts of the intrafusal fibre do the gamma-MNs innervate and which do the Ia sensory afferents innervate
non-contractile equatorial region - 1a sensory afferent
contractile polar ends - gamma-MNs
gamma-MNs cell bodies are where
in the ventral horn of the spinal cord
gamman-MNs are driven by ___
higher centres not 1a afferents
stimulation of gamma-MNs causes
spindle to contract
in the reflex arc, a-MNs firing causes the muscle to contract which makes the spindle go taut/slack, increasing/decreasing 1a afferent firing
slack
decreasing
how is the sensitivity of the spindle maintained during voluntary movement
intrafusal fibres contract in parallel with extrafusal fibres - a and y MNs are co-activated by higher centres
in a reflex arc, when the spindle goes slack, if the muscle were stretched again what would happen
what happens to reset the tension
no 1a firing and no a-MN activation
Y-mn contraction of both ends to reset sensitivity
if both a-MNs and y-MNs are stimulated what happens
spindle under tension, 1a response is maintained during contraction
what forms the efferent portion of a reflex arc
a-MN
Y-mns have a key function in what (2)
maintaining muscle tone
non-conscious proprioception
LMNs arise in what laminae of the spinal cord
VIII and IX
what is the difference in control of a and y MNs
a-MN: under control of UMNs and 1a sensory afferents
y-MN: just under control of UMNs
why is it important for the y-MNs to keep the spindle taut
maintain tension to preserve sensitivity to muscle stretch by the spindle
Sensitivity to the stretching of muscle spindles allows for information on muscle length and velocity to be relayed to the ____ via ____ tracts
cerebellum
spinocerebellar tracts
how is tone increased in a muscle
increased y-MN firing as muscle spindle becomes hypersensitive to stretch –> greater activation and recruitment of a-MNs via reflex arc
stiff muscle on passive movement
what are the 2 types of intrafusal fibres
nuclear bag fibres
chain fibres
what are the 2 types of nuclear bag fibres
bag 1 - dynamic
bag 2 - static
describe the difference between the sensitivity of bag 1 and bag 2 fibres
bag 1 (dynamic) - very sensitive to rate of change of muscle length
bag 2 (static) - more sensitive to the absolute muscle length
describe the difference between the innervation of bag 1 and bag 2 fibres
bag 1 - innervated by dynamic Y-mns
bag 2 - innervated by static Y-mns
what are nuclear chain fibres sensitive to
absolute length of the muscle
nuclear chain fibres are innervated by
static y-MNs
the 3 types of intrafusal fibre are
dynamic nuclear bag fibre
static nuclear bag fibre
nuclear chain fibre
what are the 2 different afferent fibres that innervate the intrafusal fibres
1a fibres (Aa) II fibres (Ab)
___ fibres form a primary annulospiral nerve ending winding around the centre of all intrafusal fibres
1a fibres (Aa)
type II fibres which are more slow/fast conducting, form flowerspray endings on all intrafusal fibres except ___
slowly
bag 1 dynamic type (think: dynamic too fast for it)
both afferents respond to ___
stretch
which fibres are more sensitive to rate of change of stretch and which fibres are more sensitive to absolute length of intrafusal fibres
1a - rate of change of stretch
II - absolute length of intrafusal fibres
what do 1a fibres respond to
both rate of change (dynamic response) and absolute length (static response)
in activities in which muscle length changes slowly and predictably only ____ y-MNs are activated
static
when are dynamic y-MNs activated
behaviour in which muscle length is changing rapidly and unpredictably
where are golgi tendon organs located
junction of muscle and tendon
what do the golgi tendon organs monitor what by how
force generated by a muscle by measuring tension in its tendon
golgi tendon organs are in (parallel/series) with extrafusal fibres
in series
golgi tendon organs are innervated by group __ sensory afferents
1b
group 1b sensory afferents are slower/quicker, more/less myelinated than group 1a
slower
less
golgi tendon organs regulate muscle tension to (2)
protect muscle from overload (weight lifting)
regulate muscle tension to an optimal range
group 1b sensory afferents enter the spinal cord and synapse upon ____
inhibitory interneurones
group 1b sensory afferents enter the spinal cord and synapse upon inhibitory interneurones which synapse upon ___ to do what
a-MNs of homonymous muscle to form the reverse myotatic reflex - inhibition of a-MN supplying homoymous muscle to cause relaxation
the reverse myotatic reflex is mono/poly synaptic
poly
proprioceptive feedback of the reverse myotatic reflex is important for what
proper execution of fine motor acts - fragile objects require a steady but not too powerful grip
proprioceptive axons are found in ____
and respond to ____
and prevent
connective tissue of joints
changes in angle, direction and velocity of movement of a joint
excessive flexion or extension
what kind of proprioceptive axons are most numerous
free nerve endings
where are free nerve endings found
in capsule and connective tissue
free nerve endings are
HT/LT
SA/FA
function
HT
SA
nociceptive
golgi type endings are found
only in ligaments
golgi type endings are
HT/LT
SA/FA
function
HT
SA
protective role
paciniform endings are found
periosteum near articular attachments and fibrous part of joint capsule
paciniform endings are
HT/LT
SA/FA
function
LT
SA
acceleration detectors
ruffini endings are found
mainly in joint capsule
ruffini endings are
HT/LT
SA/FA
function
LT
SA
static position and speed of movements
proprioceptive information arises from what 3 things
muscle spindles
golgi tendon organs
joint receptors
what is the role of spinal interneurones
coordinated control of flexors and extensors for limb movements
what 4 things do spinal interneurones receive input from
primary sensory axons (1a and 1b fibres)
descending axons from brain
collateral branches of LMNs
other interneurones
what kind of interneurone mediates the inverse myotatic reflex
inhibitory
what else do inhibitory interneurones mediate
reciprocal inhibition between extensor and flexor muscles
outline an example of reciprocal inhibition
knee jerk reflex - quads extends - for this to happen hamstrings needs to relax
1a afferent from muscle spindle of extensor makes an excitatory monosynaptic contact with a-MN of homonymous muscle VIA a polysynaptic pathway involving inhibitory interneurone - the 1a fibre also inhibits the a-MN supplying the flexor muscle
how does the motor cortex use reciprocal inhibition in initiation of movement
when an extensor voluntarily contracts it stretches the opposing flexor which would stimulate the myotatic reflex however descending controls cause inhibitory interneurones to inhibit the a-MN of the opposing muscle allowing unopposed extension
what 2 things do excitatory interneurones mediate
flexor reflex
crossed extensor reflex
what is the flexor reflex
noxious stimuli causes limb to flex by contraction of flexor muscles via excitatory interneurones and relaxation of extensor muscles via excitatory and inhibitory interneurones
what is the crossed extensor reflex
noxious stimuli causes limb to extend by contraction of extensor muscles via excitatory interneurones and relaxation of flexor muscles via excitatory and inhibitory interneurones
which reflex enhances postural support during withdrawal of foot from a painful stimulus
crossed extensor reflex
what is the only monosynaptic reflex in humans
myotatic reflex
dynamic and static y-MNs form a ____ motor system that allows control of motor tasks of varying complexity
fusiform
