Which of the following best describes a motor unit? O All the nerve fibers and m
ID: 3508580 • Letter: W
Question
Which of the following best describes a motor unit? O All the nerve fibers and muscle fibers in a single muscle bundle O A single motor neuron and all the muscle fibers that it innervates. O One muscle fiber and its single nerve fiber. O The neuron that provides the CNS with information on the state of contraction of the muscle. Check A Which statement about muscle contractions is false? O An action potential in the muscle cell activates contraction by releasing Ca?t within the muscle. O A single action potential at the neuromuscular junction is sufficient to cause a muscle to twitch. O Summation of twitches leads to a graded increase in the tension that can be generated by a single muscle fiber O The tension generated by a muscle can be varied by controlling how many of its motor units are active. O When maximum muscle tension is achieved, no ATP is required to maintain that level of tension. & Check Ans How can muscle fibers show a range of responses to different levels of stimulation? O A new contraction can occur only after the resting condition is reached O Each muscle fiber contraction is all or none. O Individual twitches in the same fiber can summate. O Following stimulation, the fiber stays contracted.Explanation / Answer
Answer to first question:
A motor unit is defined as - A single nerve fiber and the muscle it innervates.(the second option).
Explanation:
A single motor nerve with its axon terminals together with all the muscle fibers innervated by it constitute a motor unit. The stimulation of a motor nerve causes all the muscle fibers to contract associated with the nerve.
The number of muscle fibers varies in each motor unit. Those with deal with fine and precise movements, like the eye muscles, have only 3-6 muscles per motor unit. But, those muscles which take part in crude movements for example the muscles of the leg and the back, are made of around 120 to 165 muscle fibers.
How an action potential leads to muscle contraction:
An action potential is defined as a series of electrical changes that occur when a muscle or nerve is stimulated.
And this particular signal( electrical change) is transfered from the motor nerve ending to the muscle fiber through the neuromuscular junction. It begins by opening of the voltage gated calcium channels in the axon terminal of the nerve.The axon terminal contains acetylcholine( the neurotransmitter substance) which is stored here in sacs/vescicles. This calcium causes bursting of sacs containing acetylcholine to a space /cleft between the nerve ens and the muscle.( synaptic cleft) thereby releasing the acetylcholine. This leads to an opening of ligand gated sodium channels which lead to their opening and the entry of sodium ions, generating another potential which transmits the signal to the muscle fiber.
Events that occur in the muscle:
This action potential causes calcium ions to be released from the sarcoplasmic reticulum in the muscle cell.This calcium forms the link between the excitation and contraction of muscle, by cross bridging the actin and myosin heads present in the muscle fibers, sliding the actin filaments over the myosin filaments, making the muscle contract.This requires energy expenditure from ATP. The muscles relax, when calcium is pumped back into the sarcoplasmic reticulum.
Answer to the second question is:
The false statement is:
When maximum muscle tension is achieved, no ATP is required to maintain that level of tension.(the last option).
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