Exam 3 (chapter 12 part 2) Flashcards
sequence of events whereby an action potential in the sarcolemma causes contraction
- dependent of neural input from the motor neuron
- requires Ca++ release from the sarcoplasmic reticulum
excitation contraction coupling (skeletal muscle)
what is excitation contraction coupling in skeletal muscle dependent on and what does it require?
neural input (from motor neuron) Ca++ release (from SR)
SR =
sarcoplasmic reticulum
point of contact between a motor neuron and a skeletal muscle cell
neuromuscular junction
- each motor neuron innervates ___________ muscle cells
- each muscle fiber receives input from ___________ motor neuron
several, one
what neurotransmitter is released at the neuromuscular junction?
acetylcholine
The region of a sarcolemma adjacent to the axon terminals at a neuromuscular junction is called the _________ ________ __________.
- high density of acetylcholine receptors
- highly folded
motor end plate
does the motor end plate have high or low surface area? why?
high (folded), more acetylcholine receptors
Synaptic potential at Neuromuscular junction
end plate potential
T/F: motor neuron action potentials always create muscle cell action potentials
True
what happens if there is no Ca++?
troponin holds tropomyosin over myosin binding sites on actin (no cross bridges form, muscle relaxed)
what are the 6 steps of excitation-contraction coupling in skeletal muscle?
action potential in sarcolemma, action potential down T tubules, Ca++ channels open on SR, Ca++ increases in cytosol, Ca++ binds to troponin shifting tropomyosin, crossbridge cycling occurs
action potential in sarcolemma, action potential down T tubules, Ca++ channels open on SR, Ca++ increases in cytosol, Ca++ binds to troponin shifting tropomyosin, crossbridge cycling occurs
the 6 steps of excitation-contraction coupling in skeletal muscle
SR Ca++ channels are _________ gated and coupled to _____ ____________
voltage, T tubules
how is a contraction terminated in skeletal muscle (3 steps)?
Ca++ leaves troponin, tropomyosin covers myosin binding sites on actin, Ca++ removed from cytoplasm
how is Ca++ removed from the cytoplasm?
Ca++ ATPase actively transports to the SR
contraction produced in a muscle fiber in response to a single action potential
- all or nothing event for a muscle fiber at rest
- can be defined for a muscle fiber, motor unit, or whole muscle
Twitch
what are the 3 phases of the twitch?
latent period, contraction phase, relaxation phase
time from action potential in muscle cell to onset of contraction
- only a few milliseconds
- excitation-contraction coupling
latent period
time that tension is increasing
- 10-100 msec
- crossbridge cycling
contraction phase
what occurs during the latent period of a twitch?
excitation contraction coupling
what occurs during the contraction phase of a twitch?
crossbridge cycle
time that tension is decreasing back to zero
- longer than contraction phase
- Ca++ reuptake
relaxation phase
what occurs during the relaxation phase of a twitch?
Ca++ reuptake
contractile elements
- repeating units of myosin and actin (myofibrils)
sarcomeres
series of elastic elements
connective tissue, tendons
force exerted by contracting muscle
tension
force opposing contraction
Ex: weight to be moved
load
length of muscle is constant
- contractile elements contract, generating tension
- LOAD > TENSION
- stretches series of elastic elements
- does NOT shorten (load not lifted)
isometric twitch contraction
LOAD > TENSION
isometric twitch contraction
tension of muscle is constant
- TENSION > LOAD
- load is lifted as muscle shortens
isotonic twitch contraction
TENSION > LOAD
isotonic twitch contraction
what is an example of isometric twitch contraction?
Trying to lift a building, pushing a wall
what is an example of isotonic twitch contraction?
lifting a 10 pound weight
purely isotonic contractions are __________
rare
why are purely isotonic contractions rare?
load changes as limb position changes (isometric continues until tension exceeds load)
what 2 factors do graded muscle contractions depend on?
tension produced by each fiber (number of cross bridges that bind to actin), number of fibers contracting
more cross bridges that bind = _____________ force
more
T/F: tension remains constant as muscle shortened (isotonic)
True
increases in _____ of action potentials increase tension in two ways
- treppe
- summation
frequency
stepwise increase in force when the frequency of stimulation is increased
- one motor unit
- tension goes back to 0 before increasing again
treppe
contractions overlapping and summing due to repeated action potentials
- one muscle fiber
- not all Ca++ removed, so tension does not go back to 0
contraction summation (recruitment)
A smooth sustained muscle contraction, such as occurs in skeletal muscle when stimulation frequency is high enough
tetanus
Latent period is _____ the twitch and has __ slope
before, no slope
The contraction phase is at the _____ of the twitch, and has a ____ slope
beginning
positive
The relaxation phase is ____ of the twitch and has a _____ slope
the second half
negative
one nerve stimulus arrives at a muscle fiber so soon that the fiber has only partially relaxed from the previous twitch
incomplete tetanus
When a muscle is stimulated repeatedly at a high rate, the amount of tension gradually increases to a sustained maximum tension.
complete tetanus
____ _____ ______: inherent ability of a muscle to generate force
- depends on the number of cross bridges in each sarcomere and the geometric arrangement of sarcomere
force generating capacity
more crossbridges/sarcomere = __________ force
more
more sarcomeres in parallel (not in series) = ___________ force
more
number of thick and thin filaments per area is ___________ even though fiber diameter varies
constant
larger diameter leads to more filaments which leads to ___________ force
more
lower diameter leads to less filaments which leads to _____ force
less
length of a fiber at the onset of a contraction affects ___________ generated
force
resting length of muscle at which the fiber can develop the greatest amount of tension
- due to maximum overlap of thick filament cross bridges and thin filaments
optimal length (for force generation)
when are most muscles at optimal length?
normal body positions
what is optimal fiber length due to?
maximum overlap of thick filament cross bridges and thin filaments
does muscle growth increase the number of cells? if not, what happens?
no, they grow larger (increase diameter (hypertrophy)
______: Increase in muscle size
Hypertrophy
what is the work of a muscle cell?
generate force
length that decreases cross bridge overlap
longer than normal body position (longer than optimal length)
thin filaments overlap each other
less than optimal length
amount of tension developed depends on the amount of _______ bound to troponin
Ca2+
at high frequencies, release exceeds _____________
- Ca++ increase in cytosol
reuptake
- all troponin has Ca++ bound to it
- Crossbridge cycling maxed out
- Maximum tetanic contraction
saturation
