Motor unit

Question 1

what is a motor unit composed of

Answer 1

Motor unit (MU) is
composed of alpha motor
neuron (alpha MN), axon
and muscle fibres

Question 2

how many muscle fibres does alpha MN innervate

Answer 2

<3000 muscle fibres

Question 3

where is the alpha MN located

Answer 3

spinal cord

Question 4

what is a motor neuron pool

Answer 4

group of alpha MNs that
innervates a muscle

Question 5

what nerves does the axon of a alpha MN reach

Answer 5

peripheral nerves

Question 6

what is the conduction velcity of APs

Answer 6

<120 m/s

Question 7

where do alpha MN receive APs ?

Answer 7

from dendrites

Question 8

what does EMG stand for

Answer 8

Electromyography

Question 9

how does the differential EMG measure the difference in electrical potential

Answer 9

Difference in electrical
potential is measured
between:
* 1) Muscle electrode 1
and reference electrode
(m1+n)
* 2) Muscle electrode 2
and reference electrode
(m2+n)

Question 10

what increases with the EMG signal

Answer 10

contraction force

Question 11

how to increase contraction force

Answer 11

increasing number of motor units

Question 12

what does the EMG consist of

Answer 12

1) muscle activity (m)
2) electric noise (n)

Question 13

what is the needle EMG used for

Answer 13

to record action
potentials of separate
motor units (MUs)

Question 14

describe when the shape of Motor units APs is abnormal

Answer 14

abnormalwhen muscles are denervated (nerve injury )

Question 15

describe the low recruitment threshold in motor units

Answer 15

Low threshold MUs need low effort (weak neural drive) to be activated

Question 16

describe the high recruitment threshold in motor units

Answer 16

An increase in effort leads to higher frequency of action potential in recruited
MUs, followed by recruitment of higher threshold MUs

Question 17

MU recruitment: Heneman’s size principle?

Answer 17

MUs are recruited according to their sizes in the following order
neural drive increases
low: S type
medium: S, FR types
high: S, FFR, FF types

Question 18

what are the 3 classifications of Motor units

Answer 18

S type
FR type
FF type

Question 19

what is a key characteristics of the S type

Answer 19

(slow, fatigue, resistant, small)
Easily activated even by weak neural inputs

Question 20

what is a key characteristics of the FR type

Answer 20

fatigue resistant, fast, medium size
Require stronger neural inputs for activation than S type

Question 21

what is a key characteristics of the FF type

Answer 21

• fast, fatigable, large size
• Recruited only by very strong neural inputs

Question 22

what MU does waking and jumping require

Answer 22

Waking: type S (slow) MUs
Jumping: type II (fast)
motor units

Question 23

which muscle fibres is needed during light exercise

Answer 23

TYPE i

Question 24

which muscle fibres is needed during medium exercise

Answer 24

Type I + type IIA (FR)

Question 25

which muscle fibres is needed during high exercise

Answer 25

Type I + Type IIA + Type IIX (FF)

Question 26

which type of fibre is always used in exercise

Answer 26

Type I fibres are always recruited during exercise

Question 27

describe the mechanisms behind increasing force with MU recruitment

Answer 27

A number of activated MUs increases with effort, which then increases force

Question 28

describe the central fatigue when there is a slope of decline in max voluntary effort

Answer 28

s steeper than that of the evoked contraction; hence, other mechanisms, in addition to the peripheral/muscle fatigue, contribute to the decline

Question 29

how is the isometric knee extension force induced

Answer 29

induced by stimulation and max voluntary effort (MVC)

Question 30

what evidence is there of central fatigue

Answer 30

Alpha-MN firing rate declines during sustained MVC contributing to decline in contraction force indicating central fatigue

Question 31

what is the function of Electromyography

Answer 31

to study the function of motor units

Question 32

the reflex arc; describe the 3 components and what is within them

Answer 32

receptors : e.g. proprioceptors , chemoceptors integration centre: sensory nerves, interneurons (not shown here), α Motor neurons effectors: skeletal muscle (or other organs, depending on the type of reflex)

Question 33

Muscle sensory receptors: proprioceptors components

Answer 33

muscle spindles, Golgi organs

Question 34

Proprioceptors: Muscle spindles key muscle fibres

Answer 34

extrafusal and intrafusal muscle fibres

Question 35

muscle spindles at work: describe and name the 2 types of sensory endings

Answer 35

Primary endings – respond to the rate of stretch (dynamic response) and the length of muscle (static response) - Secondary endings – static response only

Question 36

muscle spindles at work: function of γ motor neurons

Answer 36

γ motor neurons prevent “unloading” of muscle spindles and regulate their sensitivity of spindles to stretch

Question 37

Proprioceptors: Golgi tendon organ function and how are they activated

Answer 37

Activity of Golgi tendon organs signals muscle tension. They can be activated by muscle stretch or muscle contraction.

Question 38

how many synapses does this involve monosynaptic stretch reflex

Answer 38

Monosynaptic means ONE synapse is involved

Question 39

Monosynaptic stretch reflex: Sequence of events:

Answer 39

1. Tendon is stretched when hit by the hammer * 2. Muscle is stretched * 3. Action potential (AP) is generated in a muscle spindle (stretch receptor) * 4. AP travels by an afferent fibre of the sensory neuron 5. AP reaches the alpha motor neuron through the dorsal roots * 6. alpha motor neuron is depolarized and AP is generated * 7. AP travels by efferent fibre * 8. AP arrives at the neuromuscular junction and causes muscle contraction

Question 40

reciprocal inhibition: what inhibits knee flexors (hamstrings)

Answer 40

Stretch of the knee extensors (quadriceps muscle)

Question 41

reciprocal inhibition: what inhibits (hyperpolarisation) alpha motor neurons innervating hamstrings

Answer 41

a (fast conducting) afferents activate interneurons

Question 42

describe the autogenic inhibition reflex

Answer 42

neurons form Golgi tendon fires motor neuron in inhibited muscle relaxes and load is dropped

Question 42

reciprocal inhibition: what is the functional significance

Answer 42

coordination of antagonists - hamstring relaxed when quadriceps contract

Question 42

Employing Stretch Reflex: Countermovement jump, when is greater positive work generated

Answer 42

Muscles can do a greater positive work when they are stretched before the contraction

Question 43

Countermovement jump: name the 2 beneficial mechanisms

Answer 43

storage of elastic energy * Stretch reflex

Question 43

why do athletes stretch beforehand

Answer 43

athletes use ore stretching to increase power in movement

Question 44

Countermovement jump Beneficial mechanisms: what does the stretch reflex facilitate

Answer 44

Stretch reflex facilitates voluntary muscle activation

Question 45

Countermovement jump Beneficial mechanisms: what does the storage of elastic energy facilitate

Answer 45

Some energy is stored in elastic structures, for extension tendons, during the stretch phase and then released during the contraction phase

Question 46

what is plyometric exercise used for

Answer 46

improve power in sports

Question 47

what is a plymoetric exercise

Answer 47

explosive jumping training

Question 48

Plyometric exercise: what does stretch reflex assist

Answer 48

Stretch reflex assists recruitment of motor neurons by exciting or "facilitating” them

Question 49

what is the possible danger of plyometric exercise

Answer 49

muscle damage in stage 3