From my understanding, the brain sends electrical pulses through neurons to different part of the body to initiate a movement. My question is, how is the movement realized? For example, in a train, steam engine is used to convert heat energy to mechanical energy to move the engine. Similarly, in the body, I understand the energy is stored in chemical form. How is this converted to the mechanical motion of a body part?
The conversion from chemical to mechanical energy is in general done by molecular motors which perform a power-stroke, using the hydrolysis of adenosine triphosphate (ATP) or more rarely of GTP to power a change of their conformation while bound to cytoskeletal elements.
Molecular motors include myosins, which is the molecular family powering muscles in interaction with actin filaments but also other cells e.g. for their motility or for morphogenesis. animation of muscle contraction
There are also kinesins and dyneins, not used for muscle function, but which perform e.g. transport of "cargo" organelles along microtubules.
An alternative way to produce mechanical energy is via ion pumps, as is the case in flagellar rotors.