Question

2.Put these events in the correct chronological sequence:

1. Upper motor neuron fires an action potential

2. T-tubules convey action potentials into the interior of the cell.

3. Acetylcholine is released from the lower motor neuron.

4. Ca2+ is released from the sarcoplasmic reticulum.

A) 1, 2, 3, 4

B) 3, 1, 2, 4

C) 2, 1, 4, 3

D) 1, 3, 2, 4

E) 4, 3, 2, 1

4.Which of the following sequences represent muscle contraction?

A)Action Potential leads to Ca++ release- troponin pulls troposmyosin -Ca++ binds to troponin- away the myosin binding site- myosin heads pull actin toward the M line - myosin head binds to myosin binding site on actin

B)Action Potential leads to Ca++ release- - myosin head binds to myosin binding site on actin- Ca++ binds to troponin--troponin pulls troposmyosin away from the myosin binding site - myosin heads pull actin toward the M line

C)Action Potential leads to Ca++ release- Ca++ binds to tropomyosin-tropomyosin pulls troponin away from the myosin binding site- myosin head binds to myosin binding site on actin- myosin heads pull actin toward the M line

D)Action Potential leads to Ca++ release- Ca++ binds to troponin-troponin pulls troposmyosin away the myosin binding site- myosin head binds to myosin binding site on actin- myosin heads pull actin toward the M line

E)Ca++ binds to troponin -Action Potential leads to Ca++ release- -troponin pulls

Answer 1

Answer:

Answer 2. Option D; (1,3,2,4)

Explanation:

Statement 1: Upper motor neuron fires an action potential

Statement 3: Acetylcholine is released from the lower motor neuron

Statement 2: T-tubules convey action potentials into the interior of the cell

Statement 4: Ca2+ is released from the sarcoplasmic reticulum

All these above mentioned points are the sequential steps in the skeletal muscle contraction phenomenon.

For a muscle to contract, initially information gets generated in the brain which then gets transmitted by the motor neurons till the effectors/target for its contraction. Thus, the motor neurons are set under action which leads to their depolarization. The action is said to have been executed by releasing a neurotransmitter; acetylcholine which gets cleaved by the plasma membrane of the skeletal muscle; also called as the sarcoplasm. The plasma membrane of the skeletal muscle bears T tubules (which are invaginated plasma membrane with a sacroplasmic reticulum on the either sides of it).

In response to the information received, the sarcoplasmic reticulum releases calcium ions which are known to initiate the process of muscle contraction.

Answer 4. OptionD; Action Potential leads to ca2+ release-ca2+ binds to troponin-troponin pulls tropomyosin away from the myosin binding site-myosin head binds to myosin binding site on actin –myosin heads pull action towards the M line

Explanation:

A muscle is said to be contracted when the formation of actin-myosin takes place and then due to their association, the action is brought in close association to the M line ( middle line) located in the middle of the dark bands ( A band) of the myofibrils.

The complete sequence of the muscle contraction is as given below:

a) Initially the transmission of the action potential propagates through the motor neurons and are then perceived by the T tubules and the sarcoplasmic reticulum

b) In response to the information received, the sarcoplasmic reticulum releases calcium ions

c) The calcium ions released then thus get bound to the troponin C (a protein present in the thin myofilaments)(NOTE: in a relaxed muscle, the active sites present on the actin gets covered by Tropomyosin and other Troponin proteins thus preventing the attachment of the myosin)

d) In response to the calcium ions thus received, the Troponin C gets activated and it thus gets bound to the surface of tropomysoin thereby altering its structure so that it can no longer then get bound to the actin thereby freeing the active sites on the actin surface

e) The myosin head then thus gets attached to the active sites on the actin protein utilizing the energy thus leading to the formation of the actin-myosin complex. Due to this the actin protein is thus brought closer to the middle of the A(=dark) band called as the M line. This leading to the shortening of the sarcomere; and hence the muscle contraction takes place. The process is called as the sliding filament theory