Neuronal Signaling PDF

Neuronal Signaling  Cells of the Nervous System  The nervous system:  Central nervous system (CNS): the brain and spinal cord.  Peripheral nervous system (PNS): the nerves that connect the CNS with the body’s organs and tissues.  The functional unit of the nervous sys...

Neuronal Signaling  Cells of the Nervous System  The nervous system:  Central nervous system (CNS): the brain and spinal cord.  Peripheral nervous system (PNS): the nerves that connect the CNS with the body’s organs and tissues.  The functional unit of the nervous system is the individual cell, the neuron.  Neurons generate electrical signals which cause the release of chemical messengers—neurotransmitters—to communicate with other cells.  Most neurons serve as integrators: their output reflects the balance of inputs they receive from other neurons.  Glial cells: non-neuronal cells of the nervous system.  Perform various supportive functions; retain the capacity to divide throughout life.  Structure of Neurons  Cell body (soma): contains the nucleus, ribosomes and other organelles.  Dendrites: highly branched outgrowths that receive incoming information from other neurons; Branching dendrites increase a cell’s surface area.  Axon: extends from the cell body and carries outgoing signals to its target cells.  Axon hillock (or initial segment): the location where propagated electrical signals are generated.  Collaterals: branches of the axon.  Near their ends, both the axon and its collaterals undergo further branching; each branch ends in an axon terminal.  Myelination of axons  The axons of many neurons are myelinated.  Myelin: 20-200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell.  The myelin sheaths are formed by oligodendrocytes (glial cell) in the CNS and Schwann cells in the PNS.  The spaces between adjacent sections of myelin where the axon’s plasma membrane is exposed to extracellular fluid are called the nodes of Ranvier.  The myelin sheath speeds up conduction of the electrical signals along the axon and conserves energy.  Axonal transport  Axonal transport: movement of organelles and materials between the cell body and the axon terminals.  It depends on microtubules and on specialized motor proteins kinesins and dyneins.  The “double-headed” motor proteins bind on one end to their cellular cargo, and the other end uses energy (from ATP) to “walk” along the microtubules.  Kinesin (anterograde) transport: from the cell body toward the axon terminals; moves nutrients, enzymes, mitochondria, neurotransmitter-filled vesicles.  Dynein (retrograde) transport: carries in the opposite direction recycled membrane vesicles, growth factors, and other chemical signals. Kinesin Walking  Functional Classes of Neurons  Afferent neurons: convey information from the tissues and organs towards the CNS.  Efferent neurons: convey information away from the CNS to effector cells (muscle, gland, etc).  Interneurons: connect neurons within the CNS.  For each afferent neuron entering the CNS, there are ~ 10 efferent neurons and ~200,000 interneurons.  Afferent neurons have peripheral sensory receptors, which respond to stimuli; the receptor may be a specialized portion of the neuron or a separate cell closely associated with the neuron ending.  Afferent neurons have only a single process, an axon with a peripheral process and a central process.  Both the cell body and the long axon are outside the CNS and only a part of the central process enters the CNS.  Functional Classes of Neurons  Efferent neurons have their cell bodies in the CNS, and the axons extend out to the periphery.  Groups of afferent and efferent neuron axons, together with myelin, connective tissue, and blood vessels, form the nerves of the PNS.  Interneurons lie entirely within the CNS; account for >99% of all neurons and have a wide range of physiological properties, shapes, and functions. A tract is a collection of nerve fibers (axons) in the central nervous system. A nerve is a collection of nerve fibers (axons) in the peripheral nervous system.  Synapses  Synapse: the junction between two neurons where one neuron alters the electrical and chemical activity of another.  The signal is transmitted from one neuron to another by neurotransmitters, which bind to a specific protein receptors on the membrane of the receiving neuron.  Most synapses occur between an axon terminal of one neuron (presynaptic neuron) and a dendrite or the cell body of a second neuron (postsynaptic neuron).  A postsynaptic neuron may have thousands of synaptic junctions on the surface of its dendrites and cell body, so that signals from many presynaptic neurons can affect it.  Glial Cells  Astrocytes: help regulate the composition of the extracellular fluid in the CNS by removing K+ and neurotransmitters around synapses.  Stimulate the formation of tight junctions between the cells that make up the walls of capillaries found in the CNS (the blood–brain barrier).  Sustain the neurons metabolically (Ex. Provide glucose and remove secreted metabolic waste).  Astrocytes have many neuron-like features (ion channels, receptors, capability of generating weak electrical responses) → may take part in information signaling in the brain.  Glial Cells  Microglia: a CNS glial cell, are specialized, macrophage-like cells that perform immune functions in the CNS, and may also contribute to synapse remodeling and plasticity.  Ependymal cells: line the fluid-filled cavities within the brain and spinal cord and regulate the production and flow of cerebrospinal fluid.  Schwann cells: are the glial cells of the PNS, have most of the properties of the CNS glia; they produce the myelin sheath of the axons of the peripheral neurons.  Basic principles of electricity  The main solutes in the extracellular fluid: Na+ and Cl- ; main solutes in the intracellular fluid: K+ and ionized non-penetrating molecules (PO4- compounds and proteins with negatively charged side chains).  Separated electrical charges of opposite sign have the potential to do work if they are allowed to come together (electrical potential, potential difference, potential).  The potential differences are small and are measured in millivolts.  The electrical potential between charges tends to make them flow, producing a current.  The amount of charge that moves (i.e. the magnitude of the current) depends on the potential difference and the nature of the material or structure through which they are moving.  The Resting membrane potential  At rest, neurons have a potential difference across their plasma membranes, with the inside of the cell negatively charged with respect to the outside → the resting membrane potential ( Vm)(RMP).  Note: volts are a measure of the difference in charge across a membrane and not absolute number of negative and positive charges.  Nature and magnitude of the Vm  Vm of neurons: −40 to −90 mV.  The Vm exists because of a tiny excess of negative ions inside the cell and an excess of positive ions outside.  The excess negative charges inside are electrically attracted to the excess positive charges outside the cell, and vice versa.  → The excess charges (ions) collect in a thin layer on both sides of the PM.  Note: In the bulk of the intracellular and extracellular fluid the number of positive and negative charges is balanced → each solution is indeed electrically neutral.  Major ions across the PM Distribution of Major Mobile Ions Across the  Na+ and K+ generally make the most Plasma Membrane of a Typical Neuron important contributions in generating the Vm.  The concentration differences for Na+ and K+ are established by the action of the sodium/potassium- ATPase pump (Na+/K+-ATPase) that pumps Na+ out of the cell and K+ into it.  The magnitude of the resting membrane potential depends mainly on two factors:  (1) differences in specific ion concentrations in the intracellular and extracellular fluids;  (2) differences in membrane permeability to the different ions, which reflects the number of open channels for the different ions in the PM.  Contribution of ion concentration differences  Hypothetical situation: membrane is open only to K+  K+ will diffuse from compartment 2 into compartment 1 → a potential difference is created across the membrane.  Electrical potential: as Comp. 1 becomes increasingly positive and Comp. 2 increasingly negative, the membrane potential difference begins to influence the movement of the K+.  The negative charge of Comp. 2 tends to attract K+ back into their original compartment, and the positive charge of Comp. 1 tends to repel them out of Comp. 1.  A net flux of K+ will occur from Comp. 2 to Comp. 1.  Equilibrium is reached: no net movement of K+  At this point, the membrane potential = the equilibrium potential for K+.  Contribution of ion concentration differences  Hypothetical situation: membrane is open only to Na+ → same process as for K+ ion.  Equilibrium potential: the movement of ions due to the concentration gradient is equal but opposite to the movement due to the electrical gradient → no net movement at Equilibrium potential.  The equilibrium potential for one ion can be different in magnitude and direction from those for other ions.  The Nernst equation describes the equilibrium potential for any ion (the electrical potential necessary to balance a given ionic concentration gradient across a membrane so that the net flux of the ion is zero).  Contribution of ion concentration to membrane potential  The Nernst equation describes the equilibrium potential for any ion (the electrical potential necessary to balance a given ionic concentration gradient across a membrane so that the net flux of the ion is zero.  Eion = equilibrium potential for a particular ion, in mV  Cin = intracellular concentration of the ion  Cout = extracellular concentration of the ion  Z = the valence of the ion  61 = a constant value that takes into account the universal gas constant, the temperature (37°C), and the Faraday electrical constant.  Thus, at these typical concentrations, Na+ flux through open channels will tend to bring the membrane potential toward +60 mV, whereas K+ flux will bring it toward −90 mV.  If the concentration gradients change, the equilibrium potentials will change.  Contribution of ion permeability to membrane potential  When channels for more than one type of ion are open in the membrane at the same time, the permeabilities and concentration gradients for all the ions must be considered when accounting for the membrane potential.  For a given concentration gradient, the greater the membrane permeability to one type of ion, the greater the contribution that ion will make to the membrane potential.  Given the concentration gradients and relative membrane permeabilities (Pion) for Na+, K+, and Cl−, the resting membrane potential of a membrane (Vm) can be calculated using the Goldman- Hodgkin-Katz (GHK) equation:  The GHK equation can be used to determine the resting membrane potential of any cell if the conditions are known.  Example: if the relative permeability values of a cell were PK = 1, PNa = 0.04, and PCl = 0.45 and the ion concentrations were equal to those listed in Table 6.2, the resting membrane potential would be  Development of a Resting Membrane Potential  In mammalian neurons, the K+ permeability is x100 greater than that for Na+ and Cl− → neuronal resting membrane potentials (RMP) are close to the equilibrium potential for K+.  The RMP is generated across the PM largely because of the movement of K+ out of the cell down its concentration gradient through constitutively open K+ channels (leak channels).  A small number of open Na+ leak channels pull the RMP slightly toward the Na+ equilibrium potential.  Development of a Resting Membrane Potential  Despite the leakage of Na+ and K+ ions, the concentrations of intracellular Na+ and K+ do not change, however, because of the action of the Na+/K+-ATPase pump.  In a resting cell, the number of ions the pump moves equals the number of ions that leak down their electrochemical gradient.  Graded Potentials and Action Potentials  Cells have a RMP due to the presence of ion pumps, ion concentration gradients, and leak channels in the cell membrane.  All neurons and muscle cells also have ion channels that can be gated (opened or closed) under certain conditions → produce electrical (excitability).  Two types of electrical signals:  Graded potentials: are important in signaling over short distances.  Action potentials: are long-distance signals that are particularly important in neuronal and muscle cell membranes.  Gated ion channels in a membrane may be opened or closed by mechanical, electrical, or chemical stimuli. AP Dr. Mike  Graded Potentials  Graded potentials: are changes in membrane potential that are confined to a relatively small region of the PM.  The magnitude of the potential change can vary (is “graded”).  A chemical signal depolarizes a small region of a membrane by briefly opening membrane cation channels and producing a potential less negative than that of adjacent areas.  Charge flows between the place of origin of this potential and adjacent regions of the PM, which are still at the resting potential.  Graded Potentials  Graded potentials can be depolarizing or hyperpolarizing.  The magnitude depends on the magnitude of the initiating event.  Change in membrane potential decreases as the distance increases from the initial site of the potential change → graded potentials can function as signals only over very short distances.  If additional stimuli occur before the graded potential died away, summation can occur.  Action Potentials (APs)  APs are large and very rapid (1–4 milliseconds) changes in the membrane potential.  Are mediated by Voltage-Gated Na+ and K+ Channels.  Depolarization opens both channels: Na+ channels respond faster than K+ channels; if the membrane is then repolarized to negative voltages, the voltage-gated K+ channels are also slower to close.  Voltage gated Na+ channels have an inactivation gate (“ball and chain”) which limits the flux of Na+ by blocking the channel shortly after depolarization opens it.  When the membrane repolarizes, the channel closes, forcing the inactivation gate back out of the pore and allowing the channel to return to the closed state.  Action Potential Mechanism 1) Resting membrane potential is near EK 2) Local membrane is brought to threshold voltage by a depolarizing stimulus. 3) Current through opening voltage-gated Na+ channels rapidly depolarizes the membrane, causing more Na+ channels to open. 4) Inactivation of Na+ channels and delayed opening of voltage-gated K+ channels halt membrane depolarization. 5) Outward current through open voltage-gated K+ channels repolarizes the membrane back to a negative potential. 6) Persistent current through slowly closing voltage-gated K+ channels hyperpolarizes membrane toward EK; Na+ channels return from inactivated state to closed state. 7) Closure of voltage-gated K+ channels returns the membrane potential to its resting value.  Action Potential Mechanism  APs occur only when the initial stimulus plus the current through the Na+ channels it opens are sufficient to elevate the membrane potential beyond the threshold potential.  Stimuli strong enough to depolarize the membrane are threshold stimuli (usually ~15 mV less negative than the resting membrane potential).  Stimuli stronger than threshold stimuli generate APs identical to those caused by threshold stimuli → APs are all-or-none.  An AP cannot convey information about the magnitude of the stimulus; the information depends upon the frequency of the APs.  Refractory Periods  During the AP, a second stimulus will not produce a second AP → the membrane is in its absolute refractory period.  Relative refractory period: a 2nd AP can be produced but only if the stimulus strength is considerably greater than usual.  The refractory periods limit the number of APs an excitable membrane can produce in a given period of time; most neurons respond at frequencies of up to 100 APs/second.  Refractory periods contribute to the separation of these APs so that individual electrical signals pass down the axon.  Action Potential Propagation  The current entering during an AP is sufficient to easily depolarize the adjacent membrane to the threshold potential.  The difference between the potentials causes current to flow, and this local current depolarizes the adjacent membrane where it causes the voltage-gated Na+ channels located there to open.  Because a membrane area that has just undergone an AP is refractory and cannot immediately undergo another, the only direction of AP propagation is away from a region of membrane that has recently been active.  Speed of AP propagation  The larger the fiber diameter, the faster the AP propagates (less resistance to local current).  Myelinated regions have few Na+ channels → APs occur only at the nodes of Ranvier, where the myelin coating is interrupted and the concentration of voltage-gated Na+ channels is high.  APs appear to “jump” from one node to the next (saltatory conduction) → is faster than propagation in non-myelinated fibers of the same axon diameter.  Because ions cross the membrane primarily at the nodes of Ranvier, the membrane pumps need to restore fewer ions. Saltatory Conduction  Generation of Action Potentials  “Stimuli” initiate APs by bringing the membrane to the threshold potential, and voltage-gated Na+ channels initiate the action potential.  In afferent neurons, the initial depolarization to threshold is achieved by a graded potential—here called a receptor potential. For example, mechanoreceptors might open Na+ channels in response to pressure, causing a depolarization.Photoreceptors (like in the retina) may have their ion channels open in response to light, resulting in a hyperpolarization in the absence of light and depolarization when light hits the receptor.  In all other neurons, the depolarization to threshold is due either to a graded potential generated by synaptic input to the neuron, known as a synaptic potential, or to a spontaneous change in the neuron’s membrane potential, known as a pacemaker potential.  Synapses  Hundreds or thousands of synapses from many different presynaptic cells can affect a single postsynaptic cell (convergence), and a single presynaptic cell can send branches to affect many other postsynaptic cells (divergence).  Types of Synapses  Electrical synapse: gap junctions connect the two neurons → currents (APs) flow directly across the junction, leading to the depolarization of the 2nd neuron.  Rapid communication, helps to synchronize electrical activity of neurons in CNS.  The conductance of some gap junctions is regulated by membrane voltage, intracellular pH, and Ca2+ concentration.  Chemical Synapse: the synaptic cleft (10-20 nm) separates the presynaptic and postsynaptic neurons.  Signals are transmitted by means of a chemical messenger (a neurotransmitter) concentrated in synaptic vesicles and released from the presynaptic axon terminal.  The neurotransmitters bind on receptors in specialized area - the postsynaptic density.  Mechanisms of Neurotransmitter Release  After fusion, vesicles can undergo at least two possible fates:  Some vesicles completely fuse with the membrane and are later recycled by endocytosis from the membrane at sites outside the active zone.  At synapses where AP firing frequencies are high, vesicles may fuse only briefly while they release their contents and then reseal the pore and withdraw back into the axon terminal.  Activation of the Direct neurotransmitter action: Postsynaptic Cell Ionotropic receptors  Are ion channels and feature at fast chemical  Neurotransmitters rapidly and synapses. reversibly bind to receptors on the  Mediating excitatory or PM of the postsynaptic cell. inhibitory signaling that is generally immediate,  The activated receptors may be ion simple, and brief. channels or indirectly linked to ion  Examples: acetylcholine, channels. glycine, glutamate, GABA.  To terminate the signal, unbound neurotransmitters are removed by  1) active reuptake into the Indirect neurotransmitter action: presynaptic axon Metabotropic receptors  (2) transport into nearby glial cells  Are G-protein-coupled and degradation receptors (GPCRs).  Signaling tends to be far  (3) diffusion out of the receptor slower, more complex and site; longer-lasting in its  (4) enzymatically transformed into consequences. inactive substances  Examples: neurotransmitters (also acetylcholine, glutamate, and GABA).  Excitatory Chemical Synapses  In excitatory chemical synapse, the postsynaptic response is a depolarization, which opens non-selective channels permeable to Na+ and K+, which move according to the electrical and concentration gradients across the membrane.  Both electrical and concentration gradients drive Na+ into the cell, whereas for K+, the electrical gradient opposes the concentration gradient.  → simultaneous movement of a small number of K+ out of the cell and a larger number of Na+ into the cell → net: a slight depolarization, an excitatory postsynaptic potential (EPSP).  The EPSP is a depolarizing graded potential that decreases in magnitude as it spreads away from the synapse by local current.  Inhibitory Chemical Synapses  An inhibitory chemical synapses generates hyperpolarizing graded potential, an inhibitory postsynaptic potential (IPSP).  At an inhibitory synapse, the activated receptors on the postsynaptic membrane open Cl− or K+ channels.  As Cl− channels open, Cl− enters the cell, producing a hyperpolarization (IPSP).  An increased K+ permeability also produces an IPSP, since with increased K+ permeability, more K+ leave the cell and the membrane moves closer to the K+ equilibrium potential (-90 mV), causing a hyperpolarization  Synaptic Integration  A single excitatory signal by itself is not enough to reach threshold in the postsynaptic neuron.  An AP can be initiated only by the combined effects of many excitatory synapses.  Temporal summation: the second EPSP of two consecutive EPSPs arrives before the first EPSP dies away creating a greater depolarization than from one input alone.  Spatial summation: 2 EPSPs from 2 separate neurons occur closely in time, resulting in a greater degree of depolarization.  The interaction of multiple EPSPs through spatial and temporal summation can bring the postsynaptic membrane to threshold so that APs are initiated.  EPSPs and IPSPs might cancel each other.  Synaptic Integration  Depending on the amount of charge moving through the ion channels, the synaptic potential will spread to a greater or lesser degree across the PM.  A large area of the membrane becomes slightly depolarized (excitatory synapse) or slightly hyperpolarized (inhibitory synapse).  These graded potentials will decrease with distance from the synaptic junction.  The axon hillock has a more negative threshold (i.e., much closer to the RMP) than the rest of the cell, due to its high density of Na+ channels.  → Is most responsive to small changes in the membrane potential → is the first region to reach threshold → generate AP.  Modification of Synaptic Transmission by Drugs  Drugs (therapeutic, illicit, “recreational”) act on the nervous system by altering synaptic mechanisms.  Specific agonists and antagonists can affect receptors on both presynaptic and postsynaptic membranes.  Diseases and toxins can also affect synaptic mechanisms.  Neurotransmitters and Neuromodulators  Neuromodulators: chemical messengers that elicit complex responses that cannot be described as simply EPSPs or IPSPs generated by neurotransmitters.  Distinctions between neuromodulators and neurotransmitters are not always clear.  Certain neuromodulators are often co-released with the neurotransmitter.  Many hormones, paracrine factors, and messengers used by the immune system serve as neuromodulators.  Neuromodulators often modify the postsynaptic cell’s response to neurotransmitters; they may also change the presynaptic cell’s synthesis, release, reuptake, or metabolism of a transmitter.  Receptors for neurotransmitters influence ion channels (excitation or inhibition) within milliseconds; those of neuromodulators often cause changes in metabolic processes, and include alterations in enzyme activity or protein synthesis.  → Neuromodulators are associated with slower events (Ex. learning, development, and motivational states).  Acetylcholine (ACh)  Is a major neurotransmitter in the PNS at the neuromuscular junction and in the brain. Neurons that release ACh are called cholinergic neurons.  Acetylcholine is synthesized from choline (found in many foods) and acetyl coenzyme A.  After receptor activation, Acetylcholinesterase rapidly destroys Ach and releases choline which is transported back into the presynaptic axon terminals. Nicotinic Acetylcholine Receptors  Are ligand-gated ion channel permeable to Na+ and K+; net effect: depolarization due to Na+ influx.  Are present at the neuromuscular junction; several receptor antagonists (toxins) induce paralysis.  Are important in cognitive functions and behavior (attention, learning, and memory) in the brain. Muscarinic Acetylcholine Receptors  Are metabotropic and couple with G proteins; are prevalent at some cholinergic synapses in the brain and PNS innervating glands, tissues, and organs (salivary glands, smooth muscle cells, the heart).  Alzheimer’s Disease  Affects 10% - 15% of people over age 65, and 50% of people over age 85.  Degeneration of cholinergic neurons decreases the amount of ACh in certain areas of the brain, additionally, there is loss of the postsynaptic neurons.  → Declining language and cognitive abilities, confusion, and memory loss.  Several genetic mechanisms have been identified as potential contributors to increased risk of developing Alzheimer’s disease.  Biogenic Amines  Are small, charged molecules synthesized from amino acids and contain an amino group. Ex. Dopamine, norepinephrine, serotonin, and histamine. Catecholamines  Dopamine (DA), norepinephrine (NE), and epinephrine all contain a catechol ring and are formed from the amino acid tyrosine.  These neurotransmitters have essential functions in states of consciousness, mood, motivation, directed attention, movement, blood pressure regulation, and hormone release.  Serotonin  Serotonin (5-hydroxytryptamine, or 5-HT): is produced from tryptophan, an essential amino acid; works as a neuromodulator via at least 16 different receptor subtypes.  Serotonin has an excitatory effect on pathways that are involved in the control of muscles, and an inhibitory effect on pathways that mediate sensations.  The activity of serotonergic neurons is lowest or absent during sleep and highest during states of alert wakefulness.  Serotonergic pathways also function in the regulation food intake, reproductive behavior, and emotional states such as mood and anxiety.  Selective serotonin reuptake inhibitors such as paroxetine (Paxil) are thought to aid in the treatment of depression by increasing the synaptic concentration of the neurotransmitter.  Serotonin is found in both neural and non-neural cells, with 90% of the body’s total serotonin found in the digestive system and only 1-2% found in the brain.  Amino acids: GABA  GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the brain; is a modified form of glutamate.  GABA neurons in the brain are small interneurons that dampen activity in neural circuits.  GABA increases Cl− flux into the cell → hyperpolarization (an IPSP) of the postsynaptic membrane.  The GABA receptor can also bind other compounds, including steroids, barbiturates, and benzodiazepines.  Benzodiazepine drugs such as alprazolam (Xanax) and diazepam (Valium) reduce anxiety, guard against seizures, and induce sleep by increasing Cl− flux through the GABA receptor.  Ethanol stimulates GABA synapses and inhibits excitatory synapses → global depression of the electrical activity of the brain → progressive reduction in overall cognitive ability, along with sensory perception inhibition (hearing, balance), loss of motor coordination, impaired judgment, memory loss, and unconsciousness.  Neuropeptides  Are composed of two or more amino acids linked together by peptide bonds.  Many neuropeptides have been previously identified in non-neural tissue where they function as hormones or paracrine substances.  Neurons that release one or more of the peptide neurotransmitters are collectively called peptidergic; in many cases, neuropeptides are co-secreted with another type of neurotransmitter and act as neuromodulators.  Neuropeptides can diffuse away from the synapse and affect other neurons at some distance, in which case they are referred to as neuromodulators.  Neuropeptides can interact with either ionotropic or metabotropic receptors and are eventually broken down by peptidases located in neuronal membranes.  Endogenous opioids: a group of neuropeptides that includes beta-endorphin, the dynorphins, and the enkephalins.  Their receptors are the sites of action of opiate drugs such as morphine and codeine.  Gases  Nitric oxide (NO) is the best understood gas neurotransmitter (in addition to CO and H2S).  Gases are produced by enzymes in axon terminals (in response to Ca2+ entry), diffuse from their sites of origin into the intracellular fluid of other neurons or effector cells, where they bind to and activate proteins.  Example: NO activates guanylyl cyclase in recipient cells, increasing the concentration of cyclic GMP, which in turn can alter ion channel activity in the postsynaptic cell.  NO functions in learning, development, drug tolerance, penile and clitoral erection, and sensory and motor modulation. Graded Potential