Which type of receptor is involved in rapid responses of muscle cells and neurons?

ligand-gated ion channel

Explanation:

The ligand-gated ion channels are the complexes of transmembrane protein that conduct in the process of flow of ions through the channel pore in response to the process of binding to the neurotransmitter. These ion channels bind to the neurotransmitters and they also open in response to the ligand binding. These channels performs the role in controlling the synaptic transmission between the two neurons and also in between the a neuron and a muscle cell.    

blue light is scattered in different directions by the tiny molecules of air. blue is scattered more than other colours because it has a shorter distance to travel which results in smaller waves. this is why we see a blue sky most of the time.

a. spokes of a wheel.

as indicated in the question radial contains rad, also found in radius, so the fracture's would span the radius.