Muscles Flashcards
How many skeletal muscles does the body contain?
over 600
50% total body mass
Muscle functions
force production for locomotion and breathing
postural support
heat production
anendocrine organ
Muscle actions
flexors
extensors
attached to bones by tendon
Epimysium
surrounds entire muscle
connective tissue
Perimysium
surrounds fascicles
connective tissue
Endomysium
surrounds muscle fibres
connective tissue
Basement membrane
below endomysium
Sarcolemma
muscle cell membrane
- Myofibrils
contain contractile proteins
actin
myosin
Actin
thin filaments
Mysoin
thick filaments
- Sarcomere
z line
m line
h zone
a band
i band
- sarcoplasmic reticulum
storage site for calcium
terminal cisternae
- Transverse tubules
extend from sarcolemma to sarcoplasmic reticulum
Satellite cells
muscle growth and repair
divide and contribute nuclei to existing muscle fibres
undifferentiated cell
reside above sarcolemma
Satellite cells during growth
increase number of nuclei in mature muscle fibres
Muscle hypertrophy
increased myonuclei
more = greater protein synthesis
Muscle atrophy
decreased myonuclei
Neuromuscular junction
junction between motor neuron and muscle fibre
Motor end plate
pocket formed around motor neuron by sarcolemma
Neuromuscular cleft
short gap between neuron and muscle fibre
Role of Acetylcholine
neurotransmitter released from motor neuron
depolarization of muscle fibre
signal for muscular contraction to start
Sliding filament model of contraction
muscle shortening occur due to movement of actin filament over myosin filament
cross-bridge formation
actin and myosin form to create power stroke
reduction in distance between Z lines of sarcomere
head of myosin attach to actin-binding site
Energy for muscle contraction
release energy from ATP hydrolysis = energy for power stroke
myosin ATPase breaks down ATP as fibre contracts
ATP –> ADP + Pi
How much does a single contraction cycle shorten the muscle by?
~1% of resting length
Excitation-contraction coupling
sequence of events where nerve impulses reaches muscle membrane and leads to muscle shortening by cross-bridge activity
Fatigue
a decline in muscle power output due to:
decrease in muscle force production at cross-bridge level
decrease in muscle shortening velocity
depends on exercise intensity
Cause of fatigue very heavy exercise
decreased Ca2+ release from sarcoplasmic reticulum
accumulation of metabolites that inhibit myofilament sensitivity to Ca2+
Key metabolites contributing to fatigue
Pi and free radicals - modify crossbridge head + reduce number of crossbridge bound to actin
H+ - ions bind to Ca2+ binding sites on troponin = prevent binding/contraction
Causes of fatigue moderate intensity
increased radical production
glycogen depletion - decrease ATP production via oxidative phosphorylation
EAMCs
exercise-associated muscle cramps
spasmodic, involuntary muscle contraction
What are EAMS caused by?
hyperactive motor nerons in the spinal cord
high intensity exercise = alter muscle spindle and golgi tendon organ function
increased excitatory activity of muscle spindles
reduced inhibitory effect of golgi tendon organ
Strategies to alleviate EAMS
passive stretching
activating ion channels in mouth/throat = send inhibitory signals to spinal cord = inhibit overactive motor neurons
Concentric
shortens/decreases
muscle contracts with force greater than resistance
dynamic
Eccentric
lengthens/increases
muscle contracts with force less than resistance
dynamic
Isometric
muscle contracts but does not change length
static
Isotonic
muscle tension remains unchanged where muscle length decreases
Isokinetic
muscle length decreases with constant velocity
3 biochemical characteristics important to function:
oxidative capacity
type of myosin isoform expressed
abundance of contractile protein within fibre
How many fibres in arm/leg?
45-55%
type I
Distance runners
more slow twitch fibres
Track sprinters
more fast twitch fibres
What does muscle contraction speed depend on?
rate of crossbridge cycling
depends on myosin ATPase isoform
Maximal power output equation
= force x shortening velocity
high force/fast fibres = higher power output
What is shortening a results of?
changes in I band
not A band
Functional properties
max force production
contraction speed
max power output
fatigue resistance
muscle fibre efficiency
Oxidative capacity
number of capillaries, mitochondria and myoglobin
Immunohistochemical staining process
selective antibodies binds to unique myosin proteins
fibre types differentiated by colour difference
Muscle twitch
contraction resulting from single stimulus
After stimulation
short latent period exist-corresponds to depolarization of muscle fibre
contraction-released from SR
tension developed due to crossbridge binding
relaxation-reuptake of calcium in SR = crossbridge detachment
Where is speed of shortening greatest in?
fast twitch fibres
SR releases Ca2+ at faster rate
higher ATPase activity
More motor units =
greater force
Fast motor units =
greater force
Warmup exercise results in
postactivation potentiation
Each mature muscle fibre innervated by a
single axon
How is muscle force increased?
by recruiting more motor units
All muscle fibres that belong to same motor unit
same fibre type
Motorneurons supplying large fast motor units have:
larger cell bodies
larger diameter axon
greater number of axonal branches
sparse afferent innervation (less spindle excitatory input)
more complex and extensive motor end plate of neuromuscular junction
Henneman size principle
small slow unit = low force fatigue resistant
large fast unit = high force fatigue susceptible
Slow motor units
easily excited motor neurons
Fast motor units
higher threshold
harder to excite
Frequency of stimulation
simple twitch
summation
tetanus
When is speed of movement greater?
muscles with higher % fast-twitch fibres
When is max velocity of shortening greatest?
at lowest force
true for slow and fast fibres
Muscle force-power relationship
peak power generated greater in muscle with higher % fast-twitch fibres
peak power increases with velocity to 200-300 degrees/sec
decreases at higher velocities because force decreases with increasing movement speed
Formation of cross-bridges is triggered by
calcium binding to troponin
Amount of force exerted dependent o
type of motor unit recruited
initial length of muscle
nature of neural stimulation
Voluntary muscle contraction first step
innervating alpha-motor neuron reaches an action potnetial
Influence muscle fibre type on force velocity curve
peak power generated greater in muscle containing high % fast fibres
compared to muscle contain high % slow fibres
Myonuclear domain
region of the sarcoplasm surrounding an individual nucleus
importance = single nucleus responsible for gen expression for its surrounding sarcoplasm
NMJ trainable of fatigue
increase size
increase number synaptic vesicles
increase number ACH receptors on post-synaptic membrane
Shortening of the muscle will continue as long as?
ATP energy available
Ca2+ free to bind to troponin
