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.    

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.

f=ma newton's second law 7x8=56n