LAB EXAM 1: Properties of Skeletal Muscle

Question 1

a skeletal muscle is composed of 1)____

2) ___
3) _____

Answer 1

1. Contractile elements (actin and myosin)
2. Parallel elastic components (cell membranes and connective tissue)
3. Seires elastic components (tendons).

Question 2

Basic muscle unit

Answer 2

sarcomere

Question 3

what are the sriations

Answer 3

the contractile elements arranged in a distinct banded pattern.

Question 4

thousands of sarcomeres are connected end to end to make up a ____

Answer 4

myofibril

Question 5

skeletal muscle is composed of a number of ___ ___

Answer 5

motor units

Question 6

what is a motor unit

Answer 6

a SINGLE MOTOR NEURON and all the muscle fibers that neuron may innervate.

Question 7

is the response of an individual motor unit graded or all or none?

Answer 7

the reponse of each individual motor unit is an ALL or NONE response. the amount of contraction is dependent on how many muscle fibers the single motor neuron innervates (because the neuron itself will fire with the same intensity every time)

Question 8

the smaller the muscle unit, the ___ the control of movement in that muscle

Answer 8

FINER

Question 9

the muscles controlling the movements of the fingers and eyes have ____ motor units, and the muscles controlling the legs may have ____ motor units.

Answer 9

the muscles controlling the movements of the fingers and eyes have SMALL motor units, and the muscles controlling the legs may have LARGE motor units.

Question 10

In terms of motor units, why does the muscle generate a more forceful contraction if you increase the voltage?

Answer 10

as you increase the voltage, you recruit more motor units and thus more muscle fibers, which result in a harder contraction.

Question 11

When would you generate the max contraction?

Answer 11

when you are contracting all the individual muscle cells of that area.

Question 12

twitch contraction

Answer 12

brief contraction of ALL muscle fibers in a MOTOR UNIT in response to a SINGLE action potential of the motor unit’s motor neuron.

Question 13

factors that affect force generated by muscle contraction

Answer 13

1) more recruitment of active motor units
2) stimulating the motor units more frequently via APs arriving at the motor end plate in rapid succession. Allows the twitches to “add” together to maintain a muscle contraction until the APs stop or the mmuscle becomes fatigued.

3) max contraction is alos affected by the nutrients and oxygen availability and the amount of muscle stretch before contraction starts

Question 14

which neurotransmitter is involved in skeletal muscle contraction?

Answer 14

Ach

Question 15

When an AP terminates at the axon terminals, Ach is released from synaptic vesicles of the motor neuron and move through the synapse to bind to ____ ___ on the skeletal muscle ___ ____

Answer 15

nicotinic receptors on the skeletal muscle motor end plate.

Question 16

After Ach binds to the motor end plate, it causes a depolarization via ____ influx, which also triggers Ca2+ release from the ____

Answer 16

After Ach binds to the motor end plate, it causes a depolarization via Na+ influx, which also triggers Ca2+ release from the Sarcoplasmic reticulum

Question 17

What constitutes the latent period?

Answer 17

the time from the onset of the stimulus and the release of calcium

Question 18

What occurs during the contraction periof?

Answer 18

the Ca2+ binds to troponin, which exposes the binding sites on actin. Cross bridges form and the actin and myosin filaments merge by sliding against one another, causing the muscle to shorten and CONTRACT.

Question 19

What happens during the relaxation period?

Answer 19

when the nerve stimulation stops, Ca2+ is actively transported back into the sarcoplasmic reticulum and the myosin binding sites are covered by tropomyosin. Causes the muscle to relax.

Question 20

If action potentials are an “all or none” response in the motor neuron, why does the contraction strength increase with an increase in stimulus strength? Why is there a maximal response by the muscle?

Answer 20

each motor neuron may fire with the same intensity, but the contraction relies on HOW MANY motor units are recruited. If the stimulus is larger, there are more motor units recruited and thus more muscle cells contract

additionally, a large stimulus also causes the faster firing of action potentials (of the same intensity (all or none)), and quick succession of APs terminating at the motor end plate can cause the muscle contractions to summate together, creating a larger contraction.

There is a maximal reponse by the muscle when all the indivisual muscle cells are contracting at once.

Question 21

Explain the relationship between force of contraction and temperature

Answer 21

As the temperature increases, the force of contraction increases.

This is due to numerous reasons. Higher temps:

1) increase the fluidity of cytosol, allowing for more efficient electrical flow
2) Increase the speed at which protein ion channels can open
3) increase sensitivity to voltage
4) facilitate faster cross bridging.

Question 22

What happens to the duration of the latent, contraction and relaxation periods at different temperatures?

Answer 22

higher temps speeds up the all aspects of muscle contraction including the latent, contraction and relaxation periods. CP might get a bit higher because temp facilitates more binding of the cross bridges. Shorter relaxation period, shorter LP. Everything gets just faster.

Question 23

Why is a greater stimulus intensity required to elicit a muscle twitch when you stimulate the muscle directly rather than stimulating via nerve?

Answer 23

An applied voltage must navigate around more materials that are wrapping the muscle bundles. There is a lot of cytoplasm, connective tissue, and more cell membrane that make a muscle MORE RESISTANT to current flow.

The muscle also does not contain myelin, therefore, a muscle is NOT INSULATED.

A non-insulated muscle loses current faster and thus a larger voltage must be applied in order to create a muscle contraction.

With nerve stimulation, one nerve stimulates hundreds of muscle cells, and can facilitate a harder contraction to a lower stimulus.

Question 24

Would you expect the latent period to differ for the muscle when stimulated directly compared to stimulating the nerve directly?

Answer 24

The nerve cuts out travel time because the muscle has so much connective tissue and cytoplasm that the electrical stimulus must go through in order to trigger calcium release.

In a nerve, you still have to do some contraction excitation coupling, but there is practically no latent period.

Question 25

What do you conclude happened to the number of fibers contracting as the voltage was raised from threshold to that required to produce a maximal contraction?

Answer 25

the number of fibers increase as you progressively increase the voltage.

Question 26

At what pulse interval did the two muscle twitches summate?

Answer 26

Depends on the frog. Eventually signals will “piggy back.” Repeated action potentials back to back keeps the muscle “excited” by allowing calcium to remain in the sarcoplasm and keep the myosin binding heads exposed. Cross bridges can be maintained. multiple APs cause the contractions to add up and become stronger.

Question 27

Force ____ as you ___ time between stimuli

Answer 27

force increases as you decrease time between stimuli.

Question 28

T/F: Action potentials in the motor neuron can summate

Answer 28

false. they can fire faster and faster though so the muscle cells can summate.

Question 29

Briefly explain why summation and tetanus are possible in skeletal muscle

Answer 29

Muscles dont have a refractory period, thus as long as they are stimulated, the contraction can be sustained.

Frequent stimulation due to summaition will stretch SERIES ELASTIC COMPONENTS in the tendons as well. All the stimulus being generated can go into movement of the microfilaments.

Question 30

Muscles work on the principle that more or less motor units fire in order to generate :

Answer 30

a more FORCEFUL contraction.

Question 31

How is fatigue prevented over sustained contraction?

Answer 31

alternative motor units are being recruited at different times while others are being relieved in order to replenish their calcium stores.

Question 32

What is tetanus?

Answer 32

complete, sustained and smooth motor unit contraction of muscles caused by rapidly repeated stimuli. occurs in super stressful conditions.

cross bridges are maintained and connective tissue such as stretch elastic components in tendons are stretched out too, allowing for filaments to slide better.

Question 33

What mechanism terminates the action of acetyl choline at the motor endplate?

Answer 33

acetylcholinesterase. Ach gets degraded into choline and acetic acid.

Question 34

Would fatigue occur more readily in an isolated nerve-muscle preparation than in vivo?

Answer 34

Fatigue is pyschological, junctional, K+ buildup, loss of Ca2+ stores, lactic acid build up, no O2, etc.

It is WORSE in ISOLATED nerve muscle because there is NO CIRCULATORY system that can remove waste or replenish nutrients.

the muscle in isolated is relying on mere diffusion to remove waste from site. this is inefficient.

there is also no nutrients going to site. Glycogen stores cannot be replenished and there is no oxygen source besides SURFACE DIFFUSION.

In vivo, there are alternative body stores to help maintain contractions such as glycogen stores, creatine, body fat etc. there is also a circulatory system to remove waste and replenish the cite with O2.

Question 35

What is the benefit to warming up before exercise?

Answer 35

Warming up increases body temperature, which facilitates faster ion channels and increases membrane fluidity that will help facilitate fast electrical propagation.

stretching also helps align the thin and thick filaments to facilitate more cross bridge formation.