Skeletal Muscle Physiology Flashcards
muscle twitch
mechanical response to action potential
latent period
from AP initiation to cross bridge formation
-start of contraction
contraction time
beginning of contraction to beginning of relaxation
-until peak tension
active sites exposed to when they begin to be covered up
when Ca2+ is high enough to keep active sites exposed
relaxation time
peak tension to complete relaxation
Ca2+ sequestering into the SR
total force generated?
tension
sum of forces independently produced by many cycling cross-bridges
can vary with:
initial length of muscle fiber
pattern or frequency of muscle fiber stimulation
isometric contraction
muscle length constant
increase in tension, but no shortening
force production is equal to resistance
isotonic contraction
contraction occurs at constant load
-not really a constant force
length change occur
two phases of isotonic contraction
concentric and eccentric
concentric phase
muscle shortens as tension is produced
eccentric phase
muscle lengthens as tension is produced
length-tension relationship
for isometric contractions
-force production depends on initial fiber length
muscle length influences tension devleopment by determining region of overlap between actin and myosin
passive tension
tension prior to muscle contraction
increases as fiber is progressively lengthened because muscle becomes stiffer as it is distended
active tension
total tension - passive tension
cuased when cross-bridge cycling occurs in isometric contraction (fixed length)
when is active tension maximal?
near 100% of normal muscle length
what happens with increased fiber length?
ends of actin are pulled away from each other
greater than 150% - ends of actin are pulled beyond myosin
no interaction/overlaps occur and therefore no development of tension
what happens with decreased fiber length?
actin and myosin increase overlap
ends of actin filaments are pushed toward eachother
-tension can develop depending on degree of overlap
shortening to less than 70-85% of resting length
-opposing actin filaments slide over one another and hit Z disks
normal resting length?
of sarcomere
maximal overlap between actin and myosin filaments and maximal active tension
total tension = ?
passive + active tension
force-velocity relationship
in isotonic contractions
shortening velocity decreases as load increases
**lighter loads can be lifted faster
maximum velocity?
determined primarily by maximum velocity of myosin ATPase enzyme
Vmax also varies with fiber type
smaller the load?
greater the shortening velocity
larger the load?
the lower the shortening velocity
what is contraction at zero velocity?
ISOMETRIC!
at a given fiber length?
there is a hyperbolic relationship between shortening velocity and load
what does maximal velocity depend on?
maximal rate of cross-bridge turnover
-not on initial overlap of thin and thick filaments
therefore, it is independent of length
longer the initial fiber length?
the larger the maximal load under zero-velocity conditions
aka isometric conditions
work = ?
load x displacement
measureable mechanical work
-only when muscle displaces a load
power = ?
work/time
maximal at intermediate lads
-where both F and v are moderate
zero load F= 0
maximum load v = 0
power also = ?
load x displacement / time
aka load x velocity (Fv)
frequency summation?
tension of single fiber can be summed if APs fire rapidly
aka twitch summation
repetitive stimulation leads to increased tension
what causes frequency summation?
no fiber relaxation between stimuli due to sustained levels of Ca2+
tetanus?
twitches merge to a smooth, sustained, maximal contraction
result of high stimulation frequency
-muscle tension at a plateau
Calcium levels are sustained until tetanic stimuli ceases
tension increases very little at stimulation frequencies greater than the fusion frequency that causes tetanus
fusion frequency?
frequency of signals that lead to tetanus
motor unit?
single motor neuron and the muscle it innervates
whole muscle tension depends on?
size of muscle
number of motor units recruited
size of each motor unit recruited
muscles for refined, delicate movements?
few muscle fibers per motor unit
muscles performing stronger, coarser movements?
large number of fibers per motor unit
MMUS?
multiple motor unit summation
in skeletal muscle - increased force production with summation of multiple fibers
CNS can control how many individual fibers it stimulates
motor neuron pool
group of all motor neurons innervating a single muscle
asynchronous recruitment
some units develop tension while others relax
delays and prevents muscle fatigue during SUBMAXIMAL contraction
contractile strength can vary with?
number of active alpha-motor neurons in pool
frequency of firing of each alpha motor neuron
EMG
gross measure of electrical activity
hennemans size principle
size of cell body dictates excitability
smaller are more excitable
-threshold reached sooner
small recruited first, followed by larger
given excitatory stimulus…
will generate a larger EPSP in motor neurons with smaller cell bodies
slow-twitch motor unit
I
small amount of force, prolonged period of time
fast-twitch fatigue-resistant motor unit
FR
moderate amount of force, sustained for moderate amount of time
fast-twitch fatigable motor unit
FF
larger amount of force, brief period of time
order of recruitment of motor units?
I > FR > FF
type I motor units?
small cell diameter
fast conduction
high excitability
type II motor units?
large cell diameter
very fast conduction
low excitability
muscle fatigue
inability to maintain desired power output
decline in force production and shortening velocity
decline in maximal force production - from decreased number of active cross bridges
lower rates of force production and relaxation
-bc of impaired release and uptake of calcium from SR
role of fatigue?
protective
-allows contraction to occur at lower rates/forces while preventing extreme changes that can damage
muscle fatigue reversible?
yes, with rest
versus damage or weakness which compromise ability to develop force
factors contributing to fatigue
motivation, physical fitness, nutrition, type of motor unit recruited
central fatigue
changes in CNS
-brain > motor neuron cell bodies
can be opposed by cheering
-seriously though.
peripheral fatigue
motor neuron axon > NMJ > fiber
impaired APs, Ca release, depletion of metabolism substrates, accumulation of byproducts
peripheral fatigue and time required for recover depend on?
recruitment pattern and fiber type
anaerobic sources of ATP?
creatine phosphate (fast) glycolysis (pretty fast)
aerobic source of ATP?
oxidative phosphorylation (slow)
slow twitch muscles?
type I
fast twitch muscles?
type II
different fiber types how?
different myosin heavy chain isoforms
-difference in mATPase activity corresponds to rate of contraction
can be hybrid fibers with intermediate rates
type I fibers?
slow oxidative fibers
type IIA fibers?
fast-oxidative fibers
tyoe IIX fibers?
fast-glycolytic fibers
how are skeletal muscle fiber types classified?
pathway for ATP synthesis (ox vs. glycolytic)
rate of ATP hydrolysis (mATPase isoform)
contractile velocity (fast vs. slow)
slow-twitch fibers?
smaller cross section greater oxygen transport ability more capillaries appear red (myoglobin) low glycogen high mitochondria resistant to fatigue
type IIA fiber characteristics?
fatigue resistant oxidative metabolism -red (myoglobin) -mitochondria high -abundant glycogen** more capillaries
ensures adequate ATP generation for rapid depletion with rapid contraction
type IIX fiber characteristics?
fatigable rely on glycolysis few mitochondria white (low myoglobin) high glycolytic enzyme content high glycogen
slow-twitch fibers?
tetanize at lower stimulation frequencies
fast-twitch fibers?
develop larger maximal force due to greater twitch tesion and larger motor units
proprioception
detailed information sensed about location in space, direction, and speed of movement
2 main purposes of proprioception?
identify external objects
accurately guide movement
muscle proprioception?
afferent info to regulate skeletal muscle activity
muscle spindles
detect changes in muscle length and rate of stretch
golgi tendon organs
detect muscle tension in muscle tendon
muscle spindle structure?
intrafusal muscle fibers aligned in parallel with force generating extrafusal fibers
golgi tendon structure?
aligned in series with extrafusal fibers
function of muscle spindle
send proprioceptive info about muscle to CNS
respond to muscle stretch
two kinds of intrafusal fibers?
bag and chain
two kinds of sensory endings
primary and secondary
primary sensory endings?
of group Ia axons
innervate bag fibers (in addition to chain fibers)
sensitive to change in length
secondary sensory endings?
of group II axons
innervate mainly chain fibers
transduce static length
-slowly adapting receptors
what happens when muscle stretches?
firing rate of sensory fibers increased
gamma motor neuron
to contractile region of spindle fiber
alpha motor neuron
output to regular skeletal muscle fiber
stretch reflex pathway?
afferent from muscle spindle
alpha to skeletal muscle
why does muscle spindle also have motor innervation?
alpha motor contract extrafusal fiber and the spindle becomes slackened
gamma not neurons help to maintain the sensitivity of the spindle apparatus
sensory response of spindle depends on what?
length of whole muscle AND contractile state of intrafusal fiber
reflex
basic neural function, involves simple neural circuits
motor reflex
rapid, stereotyped motor response to a particular sensory stimulus
motor neurons receive many synaptic inputs within the brain and spinal cord
stretch reflex
type of monosynaptic reflex
-myotatic
group Ia sensory axons terminate monosynaptically on alpha motor neurons innervating same muscle
reciprocal innervation
as stretched muscle contracts, parallel circuits inhibit the alpha motor neurons of the antagonist
Ia sensory axons stimulate inhibitory interneurons that synapse with alpha motor neurons of antagonist
golgi tendon organs
autogenic inhibition
-protective refelx
group Ib axons in encapsulated collagen matrix
located at musculotendinous junction
increased muscle tension
located at musculotendinous junction
GTOs may respond to passive stretch, but especially respond during active muscle contractions
group Ib sensory axons?
in golgi tendon organs
autogenic inhibition
GTO circuit inhibits the muscle in which tension increased and excites the antagonist
response usually opposite the stretch reflex
in general, GTO mediated reflexes act to control muscle force and joint stability
