Motor neurons, units, and pools

Question 1

what defines a motor unit

Answer 1

single motor neuron and all the muscle fibres it innervates

Question 2

what defines a motor pool

Answer 2

all of the motor units innervating a given muscle

Question 3

do lower motor neurons span multiple spinal segments

Answer 3

yes - not all motor neurons for a certain muscle come from the same spinal segment (they’re distributed)

Question 4

why is distribution of motor neurons in the spine beneficial

Answer 4

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)

Question 5

what is hennemans size principle

Answer 5

MU fibre diameter relates to the number of fibres it innervates

• larger MUs innervate more fibres

Question 6

is hennemans size pinciple absolute for all cases

Answer 6

no - holds up in most cases

Question 7

what else does fibre diameter determine (other than # of muscle fibres innervated) according to hennemans size prinicple

Answer 7

the force level at which each MU becomes active

Question 8

what order are muscle fibres recruited

Answer 8

small to large

• less force to most force output

Question 9

what order are muscle fibres derecruited during relaxation

Answer 9

opposite order

• large to small (large derecruited first)

Question 10

what is discharge rate

Answer 10

number of APs per second the motor neuron fires

Question 11

when are the most powerful MUs recruited

Answer 11

when desired movement / contraction requires more force

Question 12

given the same synaptic input, which size of MUs are more excitable (and why)

Answer 12

small MUs are more excitable

• larger are more leaky (more SA = more leak channels and therefore needs more stimulation to fire)

Question 13

what is the effect of weight training on MNs

Answer 13

more activation in MNs = growth in size and more connections

• muscle fibres types can also evolve along a spectrum

Question 14

why does a change happen with weight training

Answer 14

thought to be due to modulation in cortical drive onto MNs (more connections onto spinal interneurons / growth of spinal interneurons)

Question 15

why is the firing of MNs synchronous

Answer 15

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

Question 16

what is the effect of immobilisation on MNs

Answer 16

opposite of training effects - using muscle less = produces less force and less electrical activity

Question 17

how do MN pools develop

Answer 17

initially overproduced and then kept or lost as NM junction begin (activity at a younger age = more MSK control for longer)

Question 18

what is the definition of a doublet

Answer 18

when 2 MN APs occur in rapid succession causing an abrupt jump in output force

Question 19

what does the force profile during a doublet look like and why

Answer 19

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

Question 20

is conduction velocity higher in S type or FF type MNs

Answer 20

FF type - larger therefore decreased axoplasmic resistance

Question 21

how do more APs result in higher force

Answer 21

more APs = more calcium dumped = more power strokes = continuous contractions = more force

Question 22

what causes fused tetanus

Answer 22

temporal summation (APs very close together)

Question 23

why do fast twitch fibres fatigue quicker when stimulated for an extended period of time

Answer 23

run out of ATP faster than other types due to higher force output by more fibres