For skeletal muscle contraction, calcium binds to troponin to uncover actin binding sites.

In order for skeletal muscle contraction to occur, the protein myosin needs to bind to the protein actin and slide it to decrease the length of the sarcomere, which is the contractile unit of a muscle. At rest, the sites on actin that myosin needs to bind to are covered up by the protein tropomyosin. Troponin is a protein that is attached to tropomyosin, to which calcium can bind.

When an action potential reaches a muscle cell, it travels down T-tubules to reach the sarcoplasmic reticulum. The sarcoplasmic reticulum releases stores of calcium, which bind to troponin. This binding causes a conformational change in troponin that causes it to shift the position of the attached tropomyosin to uncover binding sites on actin. Myosin is then able to bind and contraction can occur.

Myosin binds to actin and slides it to shorten the sarcomere. Secretory vesicles release acetylcholine into the synaptic cleft upon stimulation of the presynaptic neuron. This stimulation causes an influx of calcium at the axon terminal, but is not directly linked to increases in intracellular calcium of the muscle cell. Dystrophin is the protein that stabilizes the cell membrane of skeletal muscles. Myelination of axons speeds up conduction of action potentials.

A deficiency of ATP is the main cause of skeletal muscle cramps.

During the anaerobic phase of exercise, ATP is quickly used up before cellular respiration can kick in. Muscle contraction requires myosin to bind to actin and slide it for sarcomere shortening. Attachment of ADP and inorganic phosphate to the myosin head causes it to bind to actin. Release of ADP and inorganic phosphate causes the myosin head to bend and pull on actin. Binding of ATP to myosin is needed for myosin to release its hold on actin. Hydrolysis of ATP to ADP and inorganic phosphate causes myosin to bind to actin again.

When ATP stores are depleted, myosin is unable to detach from actin so that skeletal muscle is "stuck" in its position. During this cramping period, the affected muscle cannot move and attempting to forcefully do so may tear it. As you rest and replete your ATP stores, myosin will be able to release from actin and the muscle can shorten or lengthen again.

While electrolyte deficiencies can also cause muscular weakness and cramping, this only happens when they are depleted during excessive sweating and rehydration with plain water. Sweating causes loss of electrolytes and intake of water will dilute the concentration of existing electrolytes. Hyponatremia and hypocalcemia are the main causes of electrolyte-deficiency cramps. Deficiency of acetylcholine can cause muscular weakness as seen in myasthenia gravis or botulism.

Actin houses binding sites for myosin that must be covered when a muscle is not contracting; otherwise myosin would constantly attach to actin, initiating unstimulated contraction. When calcium is released from the sarcoplasmic reticulum, it attaches to troponin. The troponin then causes a conformational change in tropomyosin. This change alters the orientation of tropomyosin away from the binding site on action. With the binding site revealed, myosin can adhere to the actin filament and contract the sarcomere.

Titin spans the length of the sarcomere and plays a key role in maintaining muscle elasticity. Collagen provides tensile strength around the muscles, and is mostly found in the extracellular matrix. ATP is not a protein, and is used to provide energy for the contraction process.

The sarcoplasmic reticulum within a muscle cells is where many calcium ions are stored and released. The release of calcium is a required step in muscle contraction. The ryanodine receptor pumps calcium ions from the intracellular fluid into the interior of the sarcoplasmic reticulum, this process keeps the intracellular calcium ions low, provides a quick store of calcium, and creates a concentration gradient. The sarcoplasmic reticulum membrane contains calcium ion/ATPase  pumps. This pump transports intracellular calcium ions into the sarcoplasmic reticulum.

Parvalbumin is a protein related to albumin, which is found in the blood. Parvalbumin plays a different role though, and is found in the muscle. Parvalbumin exists in muscle cytoplasm, and binds reversibly with calcium that is released from the sarcoplasmic reticulum during the muscle contraction process. Parvalbumin binding calcium is exothermic and produces labile heat. Parvalbumin then slowly releases calcium back into the cytoplasm. This makes parvalbumin a slow-releasing calcium buffer.

PCr or phosphocreatine works to maintain ATP concentrations during muscle contraction and use. It does so by transferring its phosphate group to ADP, which is the product of ATP breakdown. When ATP is broken down to ADP, PCr donates a phosphate to ADP to recreate an ATP. This reaction, PCr + ADP -> ATP + Cr, is catalyzed by the enzyme CPK.

Muscle contraction relies on elevations in the intracellular concentration of calcium. In order to sustain maximal contraction of a muscle fiber, intracellular calcium levels must remain higher than normal. Intracellular concentrations of sodium, potassium, and magnesium do not affect muscle contraction.

When ATP is used during the muscle contraction process, it is regenerated through the transfer of phosphate from PCr (phosphocreatine) to ADP (adenosine diphosphate). This reaction is catalyzed by the enzyme CPK (creatine phosphokinase).

 The net reaction that is catalyzed by CPK is: PCr + ADP -> ATP + Cr

T-tubules are small tunnels in the membrane of a muscle cell that allow an action potential to spread evenly. This allows for the whole muscle cell to contract smoothly and in unison.

A neuromuscular junction is the synaptic interface between a neuron and a muscle cell. When an action potential reached the junction, it causes depolarization of the sarcolemma (muscle cell membrane). This depolarization spreads to the T-tubules, which house proteins that directly interface with the sarcoplasmic reticulum. When depolarization of the T-tubules stimulates the sarcoplasmic reticulum, it releases calcium into the cytoplasm. The calcium bind to troponin to initiate the contraction of the sarcomere.

Acetylcholine is transported in vesicles, via the protein kinesin, in excitatory motor neurons. Muscle cells typically do not transport secretory vesicles, as they are not active secretors of most proteins. Neurons, in contrast, function to release neurotransmitters from secretory vesicles at synapses.

This logic leaves us with either type of neuron specified in the question. The best answer is motor neuron, as acetylcholine is the primary excitatory neurotransmitter at the neuromuscular junction. Had the question specified that the vesicle was filled with GABA or glycine, inhibitory neuron would have been the better answer.