Motor neurons, units, and pools Flashcards
what defines a motor unit
single motor neuron and all the muscle fibres it innervates
what defines a motor pool
all of the motor units innervating a given muscle
do lower motor neurons span multiple spinal segments
yes - not all motor neurons for a certain muscle come from the same spinal segment (they’re distributed)
why is distribution of motor neurons in the spine beneficial
injury to spinal nerve coming out of the ventral horn means some strength is lost but the muscle is still able to contract (not complete loss of control)
what is hennemans size principle
MU fibre diameter relates to the number of fibres it innervates
- larger MUs innervate more fibres
is hennemans size pinciple absolute for all cases
no - holds up in most cases
what else does fibre diameter determine (other than # of muscle fibres innervated) according to hennemans size prinicple
the force level at which each MU becomes active
what order are muscle fibres recruited
small to large
- less force to most force output
what order are muscle fibres derecruited during relaxation
opposite order
- large to small (large derecruited first)
what is discharge rate
number of APs per second the motor neuron fires
when are the most powerful MUs recruited
when desired movement / contraction requires more force
given the same synaptic input, which size of MUs are more excitable (and why)
small MUs are more excitable
- larger are more leaky (more SA = more leak channels and therefore needs more stimulation to fire)
what is the effect of weight training on MNs
more activation in MNs = growth in size and more connections
- muscle fibres types can also evolve along a spectrum
why does a change happen with weight training
thought to be due to modulation in cortical drive onto MNs (more connections onto spinal interneurons / growth of spinal interneurons)
why is the firing of MNs synchronous
most of the time, MNs fire sycnhronously due to a common input at the level of the spine (interneuron connects to many MUs)
HOWEVER, sometimes one interneuron is not the only input onto the MNs
what is the effect of immobilisation on MNs
opposite of training effects - using muscle less = produces less force and less electrical activity
how do MN pools develop
initially overproduced and then kept or lost as NM junction begin (activity at a younger age = more MSK control for longer)
what is the definition of a doublet
when 2 MN APs occur in rapid succession causing an abrupt jump in output force
what does the force profile during a doublet look like and why
increases and then levels out
- due to the buildup of calcium (increased driving force from increased concen inside so it moves out at a higher rate)
- leveling point is when calcium in = calcium out
is conduction velocity higher in S type or FF type MNs
FF type - larger therefore decreased axoplasmic resistance
how do more APs result in higher force
more APs = more calcium dumped = more power strokes = continuous contractions = more force
what causes fused tetanus
temporal summation (APs very close together)
why do fast twitch fibres fatigue quicker when stimulated for an extended period of time
run out of ATP faster than other types due to higher force output by more fibres
