to ion channels responsible for chemotransduction. such is the case in olfactory cells. depolarization in these cells result from opening of non-selective cation channels upon binding of the odorant to the specific receptor. g protein-coupled receptors in the plasma membrane of these cells can initiate second messenger pathways that cause cation channels to open. in response to stimuli, the sensory receptor initiates sensory transduction by creating graded potentials or action potentials in the same cell or in an adjacent one. sensitivity to stimuli is obtained by chemical amplification through second messenger pathways in which enzymatic cascades produce large numbers of intermediate products, increasing the effect of one receptor molecule. systematic response nervous-system response though receptors and stimuli are varied, most extrinsic stimuli first generate localized graded potentials in the neurons associated with the specific sensory organ or tissue. in the nervous system, internal and external stimuli can elicit two different categories of responses: an excitatory response, normally in the form of an action potential, and an inhibitory response. when a neuron is stimulated by an excitatory impulse, neuronal dendrites are bound by neurotransmitters which cause the cell to become permeable to a specific type of ion; the type of neurotransmitter determines to which ion the neurotransmitter will become permeable. in excitatory postsynaptic potentials, an excitatory response is generated. this is caused by an excitatory neurotransmitter, normally glutamate binding to a neuron's dendrites, causing an influx of sodium ions through channels located near the binding site. this change in membrane permeability in the dendrites is known as a local graded potential and causes the membrane voltage to change from a negative resting potential to a more positive voltage, a process known as depolarization. the opening of sodium channels allows nearby sodium channels to open, allowing the change in permeability to spread from the dendrites to the cell body. if a graded potential is strong enough, or if several graded potentials occur in a fast enough frequency, the depolarization is able to spread across the cell body to the axon hillock. from the axon hillock, an action potential can be generated and propagated down the neuron's axon, causing sodium ion channels in the axon to open as the impulse travels. once the signal begins to travel down the axon, the membrane potential has already passed threshold, which means that it cannot be stopped. this phenomenon is known as an all-or-nothing response. groups of sodium channels opened by the change in membrane potential strengthen the signal as it travels away from the axon hillock, allowing it to move the length of the axon. as the depolarization reaches the end of the axon, or the axon terminal, the end of the neuron becomes permeable to calcium ions, which enters the cell via calcium ion channels. calcium causes the release of neurotransmitters stored in synaptic vesicles, which enter the synapse between two neurons known as the presynaptic and postsynaptic neurons; if the signal from the presynaptic neuron is excitatory, it will cause the release of an excitatory neurotransmitter, causing a similar response in the postsynaptic neuron. these neurons may communicate with thousands of other receptors and target cells through extensive, complex dendritic networks. communication between receptors in this fashion enables discrimination and the more explicit interpretation of external stimuli. effectively, these localized graded potentials trigger action potentials that communicate, in their frequency, along nerve axons eventually arriving in specific cortexes of the brain. in these also highly specialized parts of the brain, these signals are coordinated with others to possibly trigger a new response. if a signal from the presynaptic neuron is inhibitory, inhibitory neurotransmitters, normally gaba will be released into the synapse. this neurotransmitter causes an inhibitory postsynaptic potential in the postsynaptic neuron. this response will cause the postsynaptic neuron to become permeable to chloride ions, making the membrane potential of the cell negative; a negative membrane potential makes it more difficult for the cell to fire an action potential and prevents any signal from being passed on through the neuron. depending on the type of stimulus, a neuron can be either excitatory or inhibitory. muscular-system response 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. vasopressin 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 individual's 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. epinephrine 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. cephalic phase 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. enteric nervous system the digestive system is also able to respond to internal stimuli. the digestive tract, or enteric nervous system alone contains millions of neurons. these neurons act as sensory receptors that can detect changes, such as food entering the small intestine, in the digestive tract. depending on what these sensory receptors detect, certain enzymes and digestive juices from the pancreas and liver can be secreted to aid in metabolism and breakdown of food. research methods and techniques clamping techniques main articles: voltage clamp and current clamp intracellular measurements of electrical potential across the membrane can be obtained by microelectrode recording. patch clamp techniques allow for the manipulation of the intracellular or extracellular ionic or lipid concentration while still recording potential. in this way, the effect of various conditions on threshold and propagation can be assessed. noninvasive neuronal scanning main articles: pet scan and mri scan positron emission tomography (pet) and magnetic resonance imaging (mri) permit the noninvasive visualization of activated regions of the brain while the test subject is exposed to different stimuli. activity is monitored in relation to blood flow to a particular region of the brain. other methods hindlimb withdrawal time is another method. sorin barac et al. in a recent paper published in the journal of reconstructive microsurgery monitored the response of test rats to pain stimuli by inducing an acute, external heat stimulus and measuring hindlimb withdrawal times (hlwt).