Motor Units and Muscle Spindles Flashcards
the somatic motor system consists of what?
skeletal muscles and the elements of the nervous system that control them
function of upper motor neurones?
within the brain
supply input to lower motor neurones to modulate their activity
function of lower motor neurones?
soma within the brain stem and ventral horn of spinal cord
receive input from UMNs, proprioceptors and interneurons
command muscle contraction and form the final common pathway
LMNs consist of what?
alpha motor neurones that innervate the bulk of fibres within a muscle that generate force
gamma motor neurones which innervate a sensory organ within the muscle known as a muscle spindle
describe 2 groups of synergistic muscles?
biceps and brachii and brachialis (flexors)
triceps brachii and anconeus (extensors)
these groups are antagonists to each other
axons of LMNs exit the spinal cord where?
in ventral roots (or via cranial nerves)
how are motor neurones distributed along the spinal cord?
not equally - greater number in cervical and lumbar enlargements
motor neurones that innervated distal and proximal muscles are mainly in the cervical and lumbar-sacral segments of the spinal cord
motor neurones innervating the axial musculature occur at all levels
what forms a motor unit?
alpha motor neurone and all the skeletal muscle fibres that it innervates
- the smallest functional component of the motor system
what is a motor neurone pool?
collection of alpha motor neurones that innervate a single muscle
the force of muscle contraction is graded by alpha motor neurones by which 2 principle mechanisms?
frequency of action potential discharge of the alpha motor neurone (each AP = twitch)
the recruitment of additional, synergistic motor units
how are cell bodies of LMNs organised in spinal cord?
somatotopic distribution in ventral horn
- LMNs innervating axial muscles = medial to those innervating distal muscles
- LMNs innervating flexors = dorsal to those supplying extensors
what are the 3 sources of input to an alpha motor neurone that regulate its activity?
central terminals of dorsal root ganglion cells who’s axons innervate the muscle spindles
UMNs in the motor cortex and brain stem
spinal interneurones
muscle strength depends on what 2 factors?
activation of muscle fibres
force production by innervated muscle fibres (fibre size and phenotype)
what determines amount of activation of muscle fibres?
firing rates of LMNs
number of LMNs that are simultaneously active
co-ordination of the movement (activation of agonist/antagonist)
a single action potential causes what in a muscle fibre and why is this important?
causes a twitch in a muscle fibre
summation of twitches causes a sustained contraction as the number of APs increases
types of muscle contraction depending on rate of motor neurone activity (AP firing)?
5Hz = twitch 10Hz = wave summation 20Hz = unfused tetanus 40Hz = fused tetanus
how does size of a motor unit relate to its function?
small = fine movement (extraocular eye muscles)
- innervated by small alpha motor neurones
large = large postural antigravity muscles (e.g legs)
- innervated by large alpha motor neurones
larger motor units = larger force of contraction
how are motor units divided into fast and slow types?
motor unit can contain only fibres of one type
alpha motor neurones innervating fast type tend to be larger and have faster conducting axons than those of slow units
what are the two major types of skeletal muscle and how do they differ?
slow twitch and fast twitch
differ in how quickly myosin ATPase splits ATP to provide energy for cross bridge cycling (reflected in time taken to reach peak tension)
also express different myosin heavy chains
describe slow (type 1) fibres?
ATP derived from oxidative phosphorylation
slow contraction and relaxation
resistant to fatigue
red fibres due to high myoglobin content
describe fast (type 2) fibres?
type 2a - ATP derived fom oxidative phosphorylation - fast contraction and relaxation - fatigue resistant - red and well vascularised type 2b - ATP derived from glycolysis - fast contraction - fatigues easily - white fibres and poorly vascularised
what are the 3 types of motor unit?
fast fatiguing
fatigue resistant
slow
describe fast fatiguing motor units?
very high tension fast fatigue large alpha motor neurones high threshold type 2b fibres used for burst power (e.g weight lifting)
describe fatigue resistant motor units?
high tension slow fatigue intermediate alpha motor neurones intermediate threshold type 2a fibres sustained locomotion (e.g distance running)
describe slow motor units?
low tension fatigue resistant small alpha motor neurones low threshold type 1 fibres antigravity, sustained movement (e.g yoga)
describe the henneman size principle?
the susceptibility of an alpha motor neurone to discharge an AP is a function of its size
- smaller ones have a lower threshold than larger ones, therefore slow motor units are more easily activated and trained by any training that activates that muscle
motor units (LMNs and the muscle they innervate) are recruited in order of their size (i.e small LMNs more easily excited than larger ones) what does this allow?
allows for fine control of muscle force across a wide range of tensions developed
recruitment in order slow (type 1) > fast fatigue resistant (type 2a) > fast fatigable (type 2b) results in increasing increments towards the maximal force that the muscle exerts
what is the myotatic reflex?
type of monosynaptic reflex arc where when a skeletal muscle is pulled, it pulls back (e.g knee jerk)
- change in length (and rate of change) is registered by a sensory organ within the muscle (muscle spindle)
a muscle spindle consists of what?
fibrous capsule
intrafusal muscle fibres
type Ia sensory afferents that innervate the intrafusal fibres
gamme motor neurone efferents that innervate the intrafusal fibres
how does the myotatic reflex work?
stretch of muscle spindle > activation of Ia afferents > excitatory synaptic transmission in spinal cord (monosynaptic) > activation of alpha motor neurone > contraction of homonymous muscle
intrafusal fibres consist of what?
non-contractile equatorial region innervated by the Ia sensory neurones
contractile polar ends that receive efferent input from gamma motor neurones with cell bodies in the ventral horn of the spinal cord (driven by higher centres, no Ia afferents)
stimulation of gamma motor neurons causes what in muscle spindle?
contraction
how are alpha and gamma motor neurones affected in voluntary movement?
they are normally co-activated by higher centres to that the intrafusal muscle fibres contract in parallel with the extrafusal fibres
this maintains the sensitivity of the spindle to stop it going slack when the extrafusal fibres contract
