the motor unit Flashcards
what are the 4 components of the control of movement?
spinal cord and brainstem ( lower motor neuron)
descending systems ( upper motor neuron)
cerebellum( error correction of ongoing movements)
basal ganglia (movement initiation and suppression of unwanted movements)
primary afferents are in the?
periphery
interneurons are in the?
spinal cord
what are the 2 enlargements in the spinal cord and what do they innervate?
cervical ( innervates arms)
lumbar ( innervates the legs)
what does the cauda equina contain?
nerves to and from the legs and pelvic floor
what are 5 characteristics of lower motor neurons?
they have large cell bodies, they have an extensive dendritic tree, large axons, myelinated axons, rapid conduction velocity ( 60 m/s)
what is the distribution of lower motor neurons?
lower motor neurons are distributed along the spinal cord in the ventral horn.
what is the somatotopy of motorneurons?
proximal muscles are in the medial ventral horn ( e.g biceps), distal muscles are in the lateral ventral horn (e.g. hand muscles)
over how many levels is a lower motor neuron typically distributed and what are the implications of this?
2 or 3 sometimes 4. (e.g. T1, C8,C7,C6 for abductor policis brevis)
This is good because if one level doesn’t work well the person won’t be paralyzed because they have input from other levels.
what is the motor unit made up of?
cell body, it’s axon, and all the muscle fibers the axon innervates
can an axon innervate both the biceps and brachials?
no an axon can only innervate ONE muscle
what is the collection of muscle fibers innervated by one axon called?
muscle unit
are properties of muscle fibers within a muscle unit the same or different?
they are identical
do muscle fibers in a muscle unit contract simultaneously or separately?
simultaneously unless a disease (e.g. mysthenia gravis), trauma, anesthetics
what is the typical size of a muscle unit?
it varies from 6-10 ( extraocular eye muscle motor unit to 1500+ fibers for the quadriceps
when is a small motor unit needed?
when precise movements need to be made like in the eye or for dexterity in the hands
when are large motor units needed?
when you need to generate a lot of force like in the quadriceps
what is meant when we say the neuromuscular junction is reliable?
every time an axon fires the muscle fiber will contract
where do lower motor neruons synapse?
on a muscle fiber
what is the functional unit of a muscle fiber?
sarcomere
how is muscle force generated?
when filaments slide past one another
force generation in a muscle requires what?
ATP, Ca2+ binding and rotation of myosin cross bridge
If there is not enough ATP what happens to the muscle?
it remains in rigor
What are the groups which motor units can be grouped into?
slow, fast-fatigue resistant, fast- fatiguable
describe slow motor units:
they have a slow rate of force increase during twitch, and they are very slow to fatigue ( really don’t fatigue)
contraction time is 60-120 ms
describe fast fatigue resistant motor units:
they fatigue but over a short period of time they won’t fatigue and they are relatively fast.
contraction time is 30-80 ms
describe fast fatiguable motor units:
they have the fastest rate of force increase during twitch and rapid force loss with repeated twitches
contraction time is 20-40 ms
more rapid rates of stimulation of motor units results in ?
higher force ( tetany)
in a fatigue test in which motor units are electrically stimulated for 2 seconds and then relieved for 2 seconds which motor units see a force drop
fast fatiguable see a rapid force drop and fast fatiguable eventually see a force drop whereas slow motor units don’t really see any drop
what happens if motor units are constantly stimulated without and termination of stimulation?
after 10 minutes of stimulation 100% of muscle fibers will die.
what are 2 ways to modulate force generation?
recruitment of a previously silent motor unit
and rate coding of an already firing motor unit ( partially fused output in humans i.e. not full tenany)
what is the size principle?
smallest motor units are recruited before larger motor units
what size motor neurons have lower excitability?
bigger motor neurons,
smaller motor neurons are more excitable hence they are recruited earlier
bigger motor neuron units have more complex dentrites, more ( name 3 things) :
short term ESPS potentiation w/ repeated activation and axonal muscle fibers
what would happen if motor unit recruitment during movements was random?
bad outcomes: loss of movement precision and loss of function
when we stand we are using mostly what type of motor unit?
slow
when we walk or run we are mainly using what type of motor unit?
fast-fatigue resistant
when we jump we are using mainly what type of motor unit?
fast fatiguable
what determines motor unit type?
activity pattern ( availability of trophic factors) influences motor unit type
and genetics
when are muscle fibers poly-innervated?
at birth
when can muscle fiber properties change?
after injury
