Motor units and muscle force

Question 1

What is a motor neuron?

Answer 1

efferent neuron
motor cortex
brainstem
spinal cord
innervate muscles and glands to enable voluntary and involuntary motions

Question 2

What is a Motor unit?

Answer 2

1 motor neuron and its innervated muscle fibers

Question 3

What is a motor nucleus?

Answer 3

cluster of cell bodies of (hundred) motor neurons that innervate the same muscle

Question 4

What is an afferent/efferent impulse?

Answer 4

afferent: from sensory organ to nervous system
efferent: from nervous system to effector organs

Question 5

What are the different kinds of motor neurons?

Answer 5

alpha motorneuron: send info to skeletal muscles
gamma: send info to the muscle spindle fibers –> adjust the sensitivity of muscle spindle

Question 6

what is an agonist muscle?

Answer 6

contract to create force and enable a movement

Question 7

what is an antagonist muscle?

Answer 7

oppose the action of an agonist to control and module movements

Question 8

How does stepping spinal reflex/stretch-reflex work?

Answer 8

Stimulus: muscle is stretched –> muscle spindle –> afferent impulse –> spinal cord
Coordination of alpha MN and gamma MN
Alpha MN: Efferent impulses –> contraction of the stretched muscles + inhibit contraction of antagonistic muscles
gamma MN just the sensitivity of muscle spindles

Question 9

What is an innervation number?

Answer 9

number of muscle fibers innervated by a single motor neuron

Question 10

What is the innervation number used for?

Answer 10

indicate the finest control of the muscle
small number –> finer control

Question 11

What is the distribution of innervation numbers across the population of motor units?

Answer 11

Innervation number increases exponentially within a population of MUs –> non-linear
Most motor units innervate fewer fibers, while fewer large ones innervate many fibers.

Question 12

How much of the fibers have low innervation numbers?

within a muscle e.g. in the first dorsal interosseous muscle

Answer 12

50%
innervation nr = 0-900

smaller MUs innervate type I fibers, have low innervation numbers

Question 13

What are the 2 mechanisms used to control muscle forces, when the muscle develops a force?

Answer 13

recruitment - when
rate coding - speed

Question 14

what is the recruitment threshold?

Answer 14

level of stimulation required to activate a motor unit

Question 15

what is the discharge rate?

Answer 15

rate coding
the rate at which each of the active MN discharges APs

Question 16

What are the relationships between motorneurons’ size and the size of the muscle fibers that they innervate?

Answer 16

smaller MN – smaller, slow-twitch muscle fibers – lower forces but more fatigue-resistant
larger MN - larger, fast-twitch muscle fibers - higher forces but fatigue more quickly

Question 17

What is the Henneman’s size principle?

Answer 17

the size principle of MN recruitment
order: smaller - recruited before larger ones

Question 18

What is twitch contraction?

Answer 18

the menchanical response to a single AP

Question 19

What is contraction time?

Answer 19

time it takes the twitch to reach its peak force
1 measure of the contraction speed of the muscle fibers that compose a MU

Question 20

What happens with the action potential in the fast twitch and slow twitch at the motor unit?
regarding the contraction time and the force production

Answer 20

slow twitch MUs - weak + long contraction e.g. long-distance running, maintaining posture

fast-twitch MUs - strong + shorter contraction e.g. sprinting, weightlifting

Question 21

What is the difference between the peak force in fast- and slow-twitch motor units?

Answer 21

Absolute force: greater for fast-twitch MU at all frequency

Question 22

What is the relationship between the recruitment threshold of a motor unit and the twitch force it produces?

Answer 22

linear correlation

Question 23

What is explosive strength?

Answer 23

the ability to increase force or torque as quickly as possible during a rapid voluntary contraction realized from a low or resting level
e.g. athlete - sprinting, weightlifting, jumping

Question 24

What is the Rate of force development (RFD)?

Answer 24

is derived from the force- or torque-time curves recorded during explosive voluntary contractions (rapid or ballistic actions)

Question 25

What may affect the rate of force development?

Answer 25

neural factor/neural activation: the ability to rapid muscle activation e.g. MU discharge rate
early phase of RTD

Question 26

what does it mean by the neural drive to muscles?

Answer 26

The neural drive is the ensemble of motor neuron action potentials that reach the muscle per time unit.

Question 27

What is the early adaptation behavior?

Answer 27

The behavior of non-linear (~exponential) decrease in discharge rate happened in certain motorneurons

Question 28

What is the Nonnegative matrix factorization (NMF)?

Answer 28

a matrix V with nonnegative entries –> matrices W + H have only nonnegative entries, lower rank
W: columns as template vectors
H: rows as activations - indicate where these templates in W occur in V

Question 29

what is NMF for? in the context of muscular function investigation

Answer 29

Spike trains of MUs contractions factorization