Question

Which of the following is important for muscle contraction and nerve impulse transmission?
A. $C{a^{2 + }}$ ions
B. $C{a^{2 + }}$ and $M{g^{2 + }}$ ions
C. $M{g^{2 + }}$ ions
D. $F{e^{2 + }}$ ions

Answer 1

Muscle contraction refers to the reflex that allows the muscles of the body parts to contract which causes the motion in them. Nerve impulse transmission is the phenomenon of the transfer of the nerve signal from one neuron to another via synapses. Moreover, muscle contraction is a result of a nerve impulse.

Complete answer: The mechanism of muscle contraction is explained by the sliding filament theory. The contraction of muscle fibers occurs by the sliding of the actin filament over the myosin filament.
The contraction of the muscle fibers is initiated by the nerve signals that are sent from the CNS through a motor neuron. After traveling through the motor neuron, the nerve impulse reaches the axon terminals and the neuromuscular junction. The synaptic knobs here contain the synaptic vesicles that have the neurotransmitters. When a wave of depolarization reaches the presynaptic membrane then the voltage-gated calcium channels are opened. Due to this, the calcium ions diffuse in the cytoplasm of the synaptic knobs of the axon terminals from the ECF (Extracellular fluid). The calcium ions stimulate the fusion of the synaptic vesicles and the presynaptic membrane and release of the neurotransmitter into the synaptic cleft via exocytosis. The neurotransmitters bind with their specific receptor molecules of the postsynaptic membrane. This binding opens the sodium ion channels that generate new potential.
After the action potential is generated in the sarcolemma, it spreads to the T-tubules. The impulse is related to the Sarcoplasmic reticulum, which stimulates it to release the calcium ions in the sarcoplasm. The increased calcium ion concentration in the sarcoplasm initiates the sliding of the filaments.
The magnesium ions are critical for the regulation of muscular activity. This is because magnesium is required for the uptake of the calcium ions in the sarcoplasmic reticulum. Other than that, it also plays an important role in the regulation of the speed of muscular contraction. This is done by modulation of the actin-binding and the ADP release in myosin.
Thus, based on the above information we can conclude that the $C{a^{2 + }}$ and $M{g^{2 + }}$ ions are important for muscle contraction and nerve impulse transmission.

Hence, the correct answer is an option (B).

Note: Whenever the muscle fiber is relaxed then the concentration of the calcium ions in the sarcoplasm is low. This is because the SR membrane contains the $C{a^{2 + }}$ active transport pumps that move the calcium ions into the SR. Also, calcium ions are stored and sequestered inside the sarcoplasmic reticulum.