Skeletal muscles are composed of hundreds to thousands of individual cells called
one more neuron and all of the skeletal muscle fibers it innervates
The motor neuron and muscle fiber intersect at what is called
the neuromuscular junction
a skeletal muscle twitch is
one contractile response to a single action potential
When a muscle is repetitively stimulated, twitches can overlap eachother and result in a stronger muscle contraction than a stand alone twitch
Maximal tetanic tension
The stimulus frequency at which no further increase in force are generated by the muscle
Will changes to the stimulus voltage alter the duration of the latent period?
Yes, changing the stimulus voltage will change the latent period duration proportionality Voltage Latent Period 0 -- 4 2.8 5 2.4 6 2.4
the latent period
the period of time that elapses between the generation of an action potential and the start of muscle tension development in a muscle fiber
What occurs during the latent period of these isometric contractions?
All the steps of excitation-contraction coupling occur
An action potential in a motor neuron triggers the release of which neurotransmitter?
The term skeletal muscle fiber refers to
an individual skeletal muscle cell
An EPP (end plate potential) is
The graded depolarization in the skeletal muscle fiber that is elicited in response to one action potential from the motor neuron
A skeletal muscle twitch is
one contractile response to a single action potential
the contraction phase
the time between the end of the latent period and peak muscle tension
A strong electrical stimulus applied to an isolated, mounted skeletal muscle induces the development of muscle/muscle tension. The stimulus mimics
aceylcholine release at a neuromuscular junction
Skeletal muscle fibers are innervate (stimulated by)
A single action potential propagating down a motor axon results in
a single action potential and a single contractile event in the muscle fibers it innervates
In resting skeletal muscle, calcium is stored in
the sacroplasmic reticulum
During the latent period for an isometric contraction
the cellular events involved in excitation-contraction coupling occur
As the stimulus voltage is increased from 1.0 volt up to 10 voles, what will happen to the amount of active force generated with each stimulus?
the active force will first increase and then plateau at some maximal value as the stimulus voltage increases
What do you see in the active force display when the stimulus voltage is set to 0 and why does this observation make sense?
0.00 g; there was no activation of skeletal muscle fibers by this stimulus
What is the lowest stimulus voltage that induces active force in skeletal muscle?
Motor unit recruitment
an increase in the number of active muscle fibers to increase the force development in a muscle
Active tension (or force) in a skeletal muscle fiber results from
activation of cross bridge cycling via increased intracellular calcium levels
the minimal stimulus needed to cause depolarization of the muscle plasma membrane (sarcolemma)
the amount of stimulus required to recruit all the muscle fibers into developing active force
Why was a maximal voltage observed in this experiment?
At the maximal voltage, all the muscle fibers contained in this muscle are depolarized and they all develop active force (that is, they were all successfully recruited)
skeletal muscle fiber
What is the role of acetylcholine in a skeletal muscle contraction?
When Ach binds to its receptors, the affinity of ligand gated channels is changed; Na+ in and K+ out, produce EPP then AP
Describe the process of excitation-contraction coupling in skeletal muscle fibers
Describe the three phases of a skeletal muscle twitch
latent- from AP to tension development contraction- beginning of tension to peak tension
Does the duration of the latent period change with different stimulus voltages?
latent period remains the same
At the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane depolarization?
into the cell
Describe the effect of increasing stimulus voltage on isolated skeletal muscle.What happened to the muscle force with stronger electrical stimulation
kept increasing unit max was obtained
How is this change in whole muscle force achieved in vivo?
repetitive APs in neuron (summation)
What has happened in the isolated skeletal muscle when the maximal voltage was applied?
max active force
During a single twitch of skeletal muscle, maximal force is
when a skeletal muscle is repetitively stimulated, twitches can overlap each other and result in a stronger muscle contraction than a stand along twitch
Wave summation is achieved by
increasing the stimulus frequency (the rate of stimulus delivery to a muscle)
Wave summation increases the force produced in the muscle. Another way to increase the force produced by a muscle is to
increase the number of activated motor units
Was there any change in the force generated by the muscle during the second stimulated twitch?
Yes, the second twitch generated more muscle force
Is the total muscle force generated by the higher frequency stimulation greater than the force generated in previous stimulation?
Yes, it is greater than previous stimulations
Does the force generated by the muscle change with each additional stimulus?
As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increased, and a limiting maximum value was observed
Which of the following is not one of the ways that the body can increase the force produced by a skeletal muscle?
application of higher voltages to the whole muscle
In this experiment, the isolated skeletal muscle was repetitively stimulated such that individual twitches overlapped with each other and resulted in a stronger muscle contraction than a stand alone twitch
Wave summation is achieved by
increasing the rate of stimulus delivery (frequency) to the muscle
What is the difference between stimulus intensity and stimulus frequency?
stimulus intensity- increase voltage stimulus frequency- increase per unit time
both can increase force, but within limits
How does the frequency of stimulation affect the amount of force generated by the isolated skeletal muscle when the frequency of stimulation was increased such that the muscle twitches did not fully relax between subsequent stimuli?
increasing force with increasing the number of stimuli
Are these results called treppe or wave summation?
Compare and contract frequency dependent wave summation with motor unit recruitment
summating stimuli activate more motor units which is how the body produces more force
When the stimulus frequency reaches a value beyond which no further increases in force are generated by the muscle, the muscle has reach
maximal tetanic tension
Sketch what the phenomenon of maximal tetanic tension looks like on a graph
Force (grams) on y axis (0-6) max tetanic tension at 5 (120-200 msec) time (msec) on x axis 0-200 mscec
During an isometric contraction
the skeletal muscle is generating force, but it remains at a fixed length
The force that results from muscle being stretched
is determined by the amount of myosin bound to actin
passive force in skeletal muscle is largely caused by
the protein titin
Note the dip in total force at a muscle length of 90mm. Why does this occur?
At this muscle length, active force has decreased in value and passive force has not yet increased to a significant value
When a skeletal muscle is stimulated and generates force but remains at a fixed length,
the muscle is contracting isometrically
which protein is mostly responsible for the development of passive force in a muscle?
In skeletal muscle, active force stimulated through a range of muscle lengths will utilize...
ATP hydrolysis to drive the cross bridge cycle
Which of the following is not depicted in a typical skeletal muscle isometric length tension curve?
Maximal active tension will be produced in a skeletal muscle fiber when
the fiber is at its resting length
What happens to the amount of active, passive, and total force the muscle generates during the stimulated twitch?
in general, as length increases, force increases *but keep length-tension curve in mind
What is the key variable to an isometric contraction of a skeletal muscle?
no change in length
Based on the unique arrangement of myosin and actin in skeletal muscle sarcomeres, explain why active force varies with changes in muscle's resting membrane length
the overlap of myosin and actin is best at resing length is worst at minimum and maximum length
What skeletal muscle lengths generated passive force?
If you were curling a 7 kg dumbbell, when would your bicep muscles be contraction isometrically?
initial hold and when dumbbell at the shoulders (although some would argue that the hold at the shoulder is part of the concentric contraction, which is isotonic)
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