Fast and Slow Muscle Flashcards
small motor units
less than 10 muscle fibres
large motor units
more than 100 muscle fibres
The force of muscle contraction is increased mainly by two
means:
• By activating more motor units i.e. recruitment
• By activating motor units more i.e. by increasing stimulus
frequency to cause summation or tetanus of individual motor
units
electromyography -EMG
The electrical activity of skeletal muscle can be recorded (electromyogram or EMG) using electrodes in or near the muscle
When will a clinician order an EMG
if a patient has signs or symptoms that may indicate a nerve or muscle disorder.
importance of EMG
often necessary to help diagnose or rule out a number of conditions such as:
myasthenia gravis
Diseases affecting the connection between the nerve and the muscle,
examples of disorders that affect motor units in brain or spinal cord
amyotrophic lateral sclerosis or polio
what are slow muscles adapted for
slow, sustained, tonic,
fatigue-resistant contractions e.g. postural muscles
fast muscles are adapted for
rapid, intense, phasic,
easily fatigued contractions e.g. gastrocnemius
muscle fibres are characterised based on
speed of contraction and ATP production
3 types of muscle fibres
– Type I: Slow-oxidative (small diameter)
– Type IIa: Fast-oxidative (intermediate diameter)
– Type IIb: Fast-glycolytic (large diameter)
fast muscle ATP production
= higher Myosin ATPase activity
oxidative
ATP produced by oxidative phosphorylation (Red Fibres)
– Much more efficient than glycolysis – more resistant to fatigue
glycolytic
generate ATP via anaerobic glycolysis (White Fibres)
– Inefficient method which also produces lactic acid
fast twitch speed
15-40 ms
slow twitch speed
50-100 ms
difference between cells and muscles
individual cells are either fast or slow
whole muscles have fast AND slow cells
whole muscle is seen as fast or slow depending on the majority of fast or slow cells
size of motor neurons in slow fibres in comparison to fast fibres
motor neurons in slow fibres are smaller than fast fibres
when will small cell bodies be recruited
Their cell bodies are more excitable and are the first
to be recruited during moderate contractions.
when will fast cell bodies be recruited
during more intense contractions
does training have an effect on fast and slow fibre numbers
Training has little effect on fast and slow fibre numbers
• Training regimens cannot alter fiber type or the
expression of myosin isoforms.
• Elite sprinters have high numbers of fast fibres
training can
Improve oxidative capacity (more resistant to fatigue)
• Increase blood supply/number of mitochondria
– Change muscle fibre diameter - hypertrophy - in short high intensity anaerobic exercise
atrophy
Muscle decreases in mass if not routinely used
lesion of motor nerves in skeletal muscle causes
A lesion of the motor nerves to skeletal muscle (lower motor neuron lesion) causes flaccid paralysis
Fasciculations
Visible twitching caused by release of Ach from
degenerating motoneurons
where does Type I and type II atrophy occur
peripheral neuropathies, disuse (bedrest,
spaceflight, immobilization), myasthenia gravis etc
where does type II atrophy mainly occur
with Duchenne muscular dystrophy and ageing
sarcopaenia
Type II hypertrophy occurs
with weight training
psuedohypertrophy
in calf muscles in Duchenne muscular dystrophy (“false
enlargement,” because the muscle tissue is abnormal.)
neuropathy
(disease affecting nerves) in the central or
peripheral nervous system or both (peripheral nerves,
example of neuropathy
multiple sclerosis
junctionopathy
disease affecting (Neuromuscular junctions,
example of junctionopathy
myasthenia gravis
myopathy
(disease affecting muscle,
examples of myopathy
muscular
dystrophies
