<P> Nerves in the peripheral nervous system spread out to various parts of the body, including muscle fibers . A muscle fiber and the motor neuron to which it is connected . The spot at which the motor neuron attaches to the muscle fiber is known as the neuromuscular junction . When muscles receive information from internal or external stimuli, muscle fibers are stimulated by their respective motor neuron . Impulses are passed from the central nervous system down neurons until they reach the motor neuron, which releases the neurotransmitter acetylcholine (ACh) into the neuromuscular junction . ACh binds to nicotinic acetylcholine receptors on the surface of the muscle cell and opens ion channels, allowing sodium ions to flow into the cell and potassium ions to flow out; this ion movement causes a depolarization, which allows for the release of calcium ions within the cell . Calcium ions bind to proteins within the muscle cell to allow for muscle contraction; the ultimate consequence of a stimulus . </P> <P> The endocrine system is affected largely by many internal and external stimuli . One internal stimulus that causes hormone release is blood pressure . Hypotension, or low blood pressure, is a large driving force for the release of vasopressin, a hormone which causes the retention of water in the kidneys . This process also increases an individuals thirst . By fluid retention or by consuming fluids, if an individual's blood pressure returns to normal, vasopressin release slows and less fluid is retained by the kidneys . Hypovolemia, or low fluid levels in the body, can also act as a stimulus to cause this response . </P> <P> Epinephrine, also known as adrenaline, is also used commonly to respond to both internal and external changes . One common cause of the release of this hormone is the Fight - or - flight response . When the body encounters an external stimulus that is potentially dangerous, epinephrine is released from the adrenal glands . Epinephrine causes physiological changes in the body, such as constriction of blood vessels, dilation of pupils, increased heart and respiratory rate, and the metabolism of glucose . All of these responses to a single stimuli aid in protecting the individual, whether the decision is made to stay and fight, or run away and avoid danger . </P> <P> The digestive system can respond to external stimuli, such as the sight or smell of food, and cause physiological changes before the food ever enters the body . This reflex is known as the cephalic phase of digestion . The sight and smell of food are strong enough stimuli to cause salivation, gastric and pancreatic enzyme secretion, and endocrine secretion in preparation for the incoming nutrients; by starting the digestive process before food reaches the stomach, the body is able to more effectively and efficiently metabolize food into necessary nutrients . Once food hits the mouth, taste and information from receptors in the mouth add to the digestive response . Chemoreceptors and mechanorceptors, activated by chewing and swallowing, further increase the enzyme release in the stomach and intestine . </P>

Perception for which there are no appropriate external stimuli are called