Motor unit Flashcards
what is a motor unit composed of
Motor unit (MU) is
composed of alpha motor
neuron (alpha MN), axon
and muscle fibres
how many muscle fibres does alpha MN innervate
<3000 muscle fibres
where is the alpha MN located
spinal cord
what is a motor neuron pool
group of alpha MNs that
innervates a muscle
what nerves does the axon of a alpha MN reach
peripheral nerves
what is the conduction velcity of APs
<120 m/s
where do alpha MN receive APs ?
from dendrites
what does EMG stand for
Electromyography
how does the differential EMG measure the difference in electrical potential
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)
what increases with the EMG signal
contraction force
how to increase contraction force
increasing number of motor units
what does the EMG consist of
1) muscle activity (m)
2) electric noise (n)
what is the needle EMG used for
to record action
potentials of separate
motor units (MUs)
describe when the shape of Motor units APs is abnormal
abnormalwhen muscles are denervated (nerve injury )
describe the low recruitment threshold in motor units
Low threshold MUs need low effort (weak neural drive) to be activated
describe the high recruitment threshold in motor units
An increase in effort leads to higher frequency of action potential in recruited
MUs, followed by recruitment of higher threshold MUs
MU recruitment: Heneman’s size principle?
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
what are the 3 classifications of Motor units
S type
FR type
FF type
what is a key characteristics of the S type
(slow, fatigue, resistant, small)
Easily activated even by weak neural inputs
what is a key characteristics of the FR type
fatigue resistant, fast, medium size
Require stronger neural inputs for activation than S type
what is a key characteristics of the FF type
- fast, fatigable, large size
- Recruited only by very strong neural inputs
what MU does waking and jumping require
Waking: type S (slow) MUs
Jumping: type II (fast)
motor units
which muscle fibres is needed during light exercise
TYPE i
which muscle fibres is needed during medium exercise
Type I + type IIA (FR)
which muscle fibres is needed during high exercise
Type I + Type IIA + Type
IIX (FF)
which type of fibre is always used in exercise
Type I fibres are always
recruited during exercise
describe the mechanisms behind increasing force with MU recruitment
A number of activated
MUs increases with
effort, which then increases force
describe the central fatigue when there is a slope of decline in max voluntary effort
s steeper than that of the
evoked contraction; hence, other mechanisms, in addition to the peripheral/muscle fatigue, contribute to the decline
how is the isometric knee extension force induced
induced by stimulation
and max voluntary effort (MVC)
what evidence is there of central fatigue
Alpha-MN firing rate
declines during sustained MVC contributing to decline in contraction force indicating central fatigue
what is the function of Electromyography
to study the function of motor units
the reflex arc; describe the 3 components and what is within them
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)
Muscle sensory receptors: proprioceptors components
muscle spindles, Golgi organs
Proprioceptors: Muscle spindles key muscle fibres
extrafusal and intrafusal muscle fibres
muscle spindles at work: describe and name the 2 types of sensory endings
Primary endings – respond to the rate of stretch (dynamic response) and the length of muscle (static response)
- Secondary endings – static
response only
muscle spindles at work: function of γ motor neurons
γ motor neurons prevent
“unloading” of muscle spindles and regulate their sensitivity of spindles to stretch
Proprioceptors: Golgi tendon organ function
and how are they activated
Activity of Golgi tendon organs signals
muscle tension. They can be activated by
muscle stretch or muscle contraction.
how many synapses does this involve monosynaptic stretch reflex
Monosynaptic means ONE synapse is involved
Monosynaptic stretch reflex: Sequence of events:
- 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 - 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
reciprocal inhibition: what inhibits knee flexors (hamstrings)
Stretch of the knee extensors (quadriceps muscle)
reciprocal inhibition: what inhibits (hyperpolarisation) alpha motor neurons innervating hamstrings
a (fast conducting) afferents activate interneurons
describe the autogenic inhibition reflex
neurons form Golgi tendon fires
motor neuron in inhibited
muscle relaxes and load is dropped
reciprocal inhibition: what is the functional significance
coordination of antagonists - hamstring relaxed when quadriceps contract
Employing Stretch Reflex:
Countermovement jump, when is greater positive work generated
Muscles can do a greater positive work when they are stretched before the contraction
Countermovement jump: name the 2 beneficial mechanisms
storage of elastic energy
* Stretch reflex
why do athletes stretch beforehand
athletes use ore stretching to increase power in movement
Countermovement jump
Beneficial mechanisms: what does the stretch reflex facilitate
Stretch reflex facilitates voluntary muscle activation
Countermovement jump
Beneficial mechanisms: what does the storage of elastic energy facilitate
Some energy is stored in elastic structures, for extension tendons, during the stretch phase and then released during the contraction phase
what is plyometric exercise used for
improve power in sports
what is a plymoetric exercise
explosive jumping training
Plyometric exercise: what does stretch reflex assist
Stretch reflex assists recruitment of motor neurons by exciting or “facilitating” them
what is the possible danger of plyometric exercise
muscle damage in stage 3
