acetylcholine receptor (achr) proteins are found at the synapse between neurons and skeletal muscle cells. acetylcholine released from neurons binds to a specifi c site on the receptor proteins, which causes an ion channel in the receptors to open and allow sodium ions (na ) to enter muscle cells. the resulting depolarization of muscle cells initiates muscle contractions. another molecule, nicotine, can also bind to certain types of achr proteins and activate the receptors.