D. Learned behaviors do not directly affect natural selection because they are not inherited. This is true because learned behaviors can be taught to the offspring but do not come within them the way instincts do.
Acetylcholine is a chemical that is found between the nerve synapses, or gaps, between nerve cells. When activated, it causes the contraction of skeletal muscles and activates glandular functions in the endocrine system. Think of acetylcholine as a mailperson; residents cannot receive their mail until he or she comes and delivers it to the mailbox. Like mailpersons who deliver the mail and move on to the next house, acetylcholine acts quickly and does not hang around. As a result, acetylcholine is rapidly broken down by another chemical substance called cholinesterase.
Acetylcholine was the first neurotransmitter scientists discovered, as well as the most abundant neurotransmitter in the body. A neurotransmitter is a chemical that is released by a neuron, or nerve cell, that sends a signal to another neuron across a synapse. The neurotransmitter binds to receptors to affect how the signal is received. The purpose of the neurotransmitter is to either amplify or inhibit the signals sent between the neurons.
Acetylcholine plays an important role in the signal of muscle movement, sensation of pain, learning and memory formation, the regulation of the endocrine system and rapid eye movement (REM) sleep cycles.
