Chapter 8: Skeletal Muscle Structure and Function Flashcards
from bigger picture to smaller details, describe muscle structure
muscle —> fascicles —> muscle fibers (cells) —> myofibrils —> sarcomeres —> thick and thin filaments
how many nuclei do skeletal muscle fibers have
many nuclei among the entire length of the muscle fiber
where are subsarcolemmel (SS) mitochondria located? purpose?
directly beneath the cell membrane; produce cellular energy needed to maintain active transport of ions across sarcolemma (to establish a concentration gradient for action potentials)
where are intermyofibrillar mitochondria (IMF) located? purpose?
located near the myofibrillar (contractile) proteins; provide energy needed to sustain muscle contraction
purpose of satellite cells
increase number of nuclei in muscle fibers during muscle growth
what does an increase in # of nuclei allow for muscle fibers?
greater protein synthesis, which is important for muscle growth in response to strength training
non-endocrine functions of skeletal muscle
-force production for locomotion and breathing
-force production for postural support
-heat production during cold stress (shivering thermogenesis)
endocrine functions of skeletal muscle
produce myokines and IL-6 during contraction, promote anti-inflammatory environment
purpose of myokines
stimulate glucose uptake and fatty acid oxidation, promote blood vessel growth in muscle, promote liver glucose production and triglyceride breakdown
purpose of IL-6 during exercise
anti-inflammatory effects which reduces chronic inflammation
what is advantageous about an anti-inflammatory environment induced by exercise?
reduces chronic inflammation, reduces risk of heart disease, type 2 diabetes and certain cancers
muscles that decrease joint angles
flexors
muscles that increase joint angles
extensors
describe static exercise
muscle exerts force without changing length (i.e. pulling against immovable object or postural muscles)
describe concentric dynamic exercise
muscle shortens during force production
describe eccentric dynamic exercise
muscle produces force but length increases, this is associated with muscle fiber injury and soreness
define motor unit
motor neuron and all the muscle fibers it innervates
role of somatic motor neurons of the PNS
carry neural messages from spinal cord to skeletal muscles
define neuromuscular junction
junction between motor neuron and muscle fiber
define motor end plate
pocket formed around motor neuron by the sarcolemma of the muscle fiber
describe the sliding filament model
muscle shortening occurs due to the movement of the thin filament over the thick filament
during muscle contraction, there is a reduction in distance between the __ lines of the sarcomere
Z lines
5 stages of excitation-contraction coupling
1) action potential stimulates Ach release from alpha motor neuron at the neuromuscular junction
2) Ach induces an AP in muscle fiber; AP spreads down sarcolemma and T-tubules
3) AP stimulates Ca2+ release from sarcoplasmic reticulum
4) Ca2+ initiates tropomyosin displacement, revealing myosin-binding sites
5) myosin forms cross-bridges with actin, producing force
7 stages of cross-bridge cycling
1) myosin binding sites on the actin molecule are covered in resting fibers
2) the AP releases calcium from SR
3) Ca2+ binds troponin; tropomyosin moves, revealing binding sites
4) cross bridge forms when myosin heads bind actin
5) myosin head pivots, moving actin, ADP and Pi dissociate from myosin head
6) myosin head releases actin when a fresh ATP binds
7) myosin head bends next actin; ATP is cleaved to ADP + Pi and the cycle repeats
what is a muscle cramp
spasmodic, involuntary muscle contractions often associated with prolonged, high intensity exercise
why does the electrolyte depletion and dehydration theory fall short in explaining muscle cramps?
dealing with electrolyte imbalance/dehydration is a full body phenomenon that doesn’t necessarily have to do with one single muscle, also stretching/massaging helps relieve cramps
describe the altered neuromuscular control theory for muscle cramps
there is abnormal spinal reflex activity due to fatigue which results in increased excitatory activity of muscle spindles and reduced inhibitory effect of Golgi tendon organ
role of muscle spindles, specifically in regards to muscle cramps
muscle spindles detect stretch of the muscle, then sensory neurons conduct APs to the spinal cord and synapse with alpha motor neurons, stimulation of alpha motor neurons causes the muscle to contract to resist being stretched to far (muscle spindle activity is increased during a muscle cramp)
role of Golgi tendon organ, specifically with regard to muscle cramps
Golgi tendon organs detect tension applied to a tendon, sensory neurons conduct APs to spinal cord and synapse with inhibitory interneurons that synapse with alpha motor neurons, inhibition of the alpha motor neurons causes muscle relaxation, relieving tension (decreased Golgi tendon activity during a muscle cramp)
what relieves muscle cramps and why does this work?
passive stretching, because it activates the Golgi tendon organ and inhibits motor neurons in the spinal cord, resulting in muscle relaxation
type I fibers
slow-twitch fibers, slow-oxidative fibers
type IIa fibers
intermediate fibers, fast-oxidative glycolytic fibers
type IIx fibers
fast-twitch fibers, fast-glycolytic fibers
3 biochemical properties differing between muscle fiber types
1) oxidative capacity
2) type of myosin ATPase isoform
3) abundance of contractile proteins
what is meant by oxidative capacity? which muscle fiber type has the highest?
number of capillaries, mitochondria, and amount of myoglobin; type I has the highest oxidative capacity
what does the type of myosin ATPase isoform determine? which muscle fiber type has the fastest isoform?
speed of ATP degradation; type IIx
which muscle fiber type has the most abundance of contractile proteins?
type IIx
4 contractile properties differing between muscle fiber types
1) speed of contraction (Vmax)
2) maximal force production
3) maximal power output
4) muscle fiber efficiency
what is speed of contraction dependent on? which muscle fiber type has the fastest speed of contraction?
dependent on rate of SR releasing calcium and ATPase activity, type IIx has the fastest speed of contraction
what is maximal force production dependent on? which fiber type has the highest max force production?
dependent on the force per unit of cross-sectional area; type IIx has the highest max force production
how do men and women differ in terms of muscle mass?
men have significantly more skeletal muscle mass than women
how do men and women differ in absolute force production?
men are stronger than women in terms of absolute force production
how to men and women differ in terms of relative force production?
there is no apparent sex or age differences in fiber distribution, so men and women have the same relative force production
what does maximal power output depend on? what type of muscle fibers have the highest maximal power output?
power = force x velocity of shortening
type IIx
what is muscle fiber efficiency dependent on? which muscle fiber type is the most efficient?
how much ATP is used to generate force (lower ATP, more efficient); type I are the most efficient
power athletes such as sprinters generally have higher percentage of
fast fibers
endurance athletes such as distance runners generally have a higher percentage of
slow fibers
3 factors that determine the amount of force produced during a muscle contraction
1) types and number of motor units recruited
2) initial muscle length
3) nature of the neural stimulation of motor units
describe Type S (slow) motor neurons
smallest, innervate type I muscle fibers
describe Type FR (fast, fatigue resistant) motor neurons
innervate type IIa muscle fibers
describe type FF (fast, fatigable) motor neurons
largest of the motor neurons, innervate type IIx muscle fibers
describe the recruitment pattern of motor neurons during graded exercise
type S —> type FR —> type FF
describe the muscle fiber type recruitment pattern
type I —> type IIa —> type IIx
what is the size principle with regard to motor neuron recruitment?
progressive recruitment of motor units begins with the smallest motor unit and progressively gets to larger motor units
how does increasing stimulus affect the motor units recruited?
recruits more motor units and thus produces more force
describe the length-tension relationship in skeletal muscle
there is an “ideal” length for force generation which allows for maximal cross bridge formation; not too much overlap and not too little
describe how frequency of stimulation affects the force produced in muscles
increasing frequency results in summation of simple twitches until tetanus is finally reached
how can force production be additive?
increased Ca2+ availability, not enough time to fully resequester it within the sarcoplasmic reticulum
what is delayed onset muscle soreness (DOMS) due to? what is it NOT due to?
microscopic tears in muscle fibers or connective tissue, not due to buildup of lactic acid
timeline of DOMS
sarcomere damage (hours to days) —> immune cell infiltration (days) —> satellite cell activation (days to weeks)
mechanism of DOMS
1) structural damage to muscle fibers
2) membrane damage
3) calcium leaks out of SR
4) protease activation- breakdown of cellular proteins
5) inflammatory response
6) edema and pain
describe the repeated bout effect on DOMS
following recovery, another bout of the same exercise results in minimal injury (soreness)
