What interactions can happen between two populations in a community

The answer is Neutralism, amensalism, commensalism, mutualism, and competition. 

Neutralism is the most common type of interactions between populations in which neither population affects the other one.
Amensalism is a relationship between two population in which only one of them suffers, and the other one is not affected. 
Commensalism is a relationship between two population in which only one of them has benefit, and the other one is not affected. 
Mutualism is a relationship between two different populations in which both species benefit. 
Competition is an interaction between populations in which they compete for limited resources. 

The contraction of the muscles (whether at the level of the arms or the legs) and more specifically the muscular fibers of the musculoskeletal system, that is to say organs, in the broad sense of the term, allowing the movement, is normally under the total dependence of the nerves which transmit a nervous command.

This command can be considered as a voluntary order (from the cerebral cortex). This nerve impulse then takes the direction of the spinal cord where it is directed by a series of nerves called relays to route the nerve impulse (order) to the muscles.

Then the nerve impulse propagates along the axon and when it reaches the motor plate it causes the release of a substance called neurotransmitter: acetylcholine. The neuroreceptor, in the motor plate, receives the nerve signal that the end of the axon transmits to it by a chemical mediator. Acetylcholine binds to the receptors, triggering a contraction of the muscle cell.

More precisely, acetylcholine is enclosed in vesicles (a kind of tiny sphere-shaped grains) located within the nervous corpuscles located at the end of each neuron. When nerve impulses (stimulation) reach the presynaptic membrane, acetylcholine is released and diffuses into the synaptic cleft (about 50 nanometers wide) filling it. Acetylcholine will at this time bind very briefly to receptors located after the synapse (postsynaptic) and trigger the opening of sodium channels (followed by their closure and an opening of potassium channels). These channels are tiny tubules allowing the passage of ions (atom having lost or gained an electron).

This results in the propagation of an "electric charge" action potential at the origin of the passage of the nerve impulse, in other words of the order given by the brain or by the autonomic nervous system.

After this first step acetylcholine is then released and degraded by an enzyme called acetylcholinesterase (AChE) located in the synaptic cleft but also on the postsynaptic membrane. The choline thus released is then recaptured by the presynaptic bodies and reused for the synthesis of new acetylcholine molecules.

Acetylcholine is involved in the control of muscles via neuromuscular terminations and viscera or glands and sometimes both. This is how it intervenes to make also work for certain organs like the heart, the salivary glands, the sweat glands, the bladder, the bronchi, the eyes, intestine etc.

A variety of enzymes called cholinesterases allow the rapid inactivation of acetylcholine. The chemical reaction that causes the contraction of the muscle fiber is a brief phenomenon. Indeed, acetylcholine is very rapidly degraded by cholinesterases. As a result, acetylcholine itself cannot be used in drug form. Nevertheless to circumvent these difficulties other drugs reproduce or prevent the effects of this neuromodulator. These are agonists or antagonists respectively.

The muscular fiber is an elongated cell used in the composition of the muscle, which is a fleshy organ with the property of contracting and relaxing. Each muscle cell is surrounded by a membrane containing a cytoplasm called sarcoplasm with myofibrils which are elongate filaments parallel to the major axis of the cell.