Chapter 4)Neuronal Physiology)Terminology+l )l )l )l Polarization Depolarization Repolarization HyperpolarizationIon)Gradients+l )l )l )l Resting Potential = -70 mV Excess Na+ in ECF Excess K+ in ICF Excess non-penetrating Anions in ICFIon)Channels+l )l )l )l ))))l Voltage Gated (triggered by charge) Ligand Gated (triggered by chemicals) Mechanically Gated (triggered by motion) Thermally Gated (triggered by temperature) )))When opened, one of two reactions occur – )– Graded Potentials Action PotentialsGraded)Poten5als+l )l ))))l ))))l )l Depolarization The stronger the trigger, the stronger the depolarization Occur in specific regions of the cell membrane Spread by passive current flow Die out over short distances (analogous to sound waves)Graded)Poten5al+Ac5on)Poten5als+l )l )l )l )l Rapid and Brief Large depolarization (100 mV) Nondecremental Require a threshold potential (-50 to -55 mV) All-or-None ResponseAc5on)Poten5al+Ion)Channels)and)APs+l ))))l The flux of Na+ and K+ create an action potential Controlled by voltage gated ion channels on the plasma membrane K+ ChannelIon)Channels)and)APs+Na+ ChannelsAc5on)Poten5al+Neuron+l )l )l )l )l Cell Body Dendrites (input) Axon Hillock Axon Axon TerminalCon5guous)Conduc5on+Refractory)Period+Myelina5on+Saltatory)Conduc5on+Synapses)• Occur at the end of an axon)• Two basic kinds)– Electrical (direct))• Not as common)• Simple)– Chemical (indirect))• More common)• complex)Electrical Synapse)• Electrical signal passes, nearly unperturbed)• Signal crosses through gap juncEons)– Direct Contact)• Fast acEon because of simplicity)– Good for escape responses (behaviors))– Especially in invertebrates)Chemical Synapses)• SynapEc CleJ)– No direct contact)– 20 to 40 nm, too wide for)electrical transmission)• Chemicals called)neurotransmiMers carry the)message across the synapse)• This acEon is slow compared to electrical)synapses)Chemical Synapses)• Advantages of chemical)synapses)– UnidirecEonal flow of informaEon)– Capability of various signaling)events)) ) )NeurotransmiMers))))• NeurotransmiMers can be from many different))chemical classes)) )– Most common are amines)• Examples)– – – – – – Acetylcholine)Dopamine)Epinephrine)Norepinephrine)Serotonin)Histamine)Chemical Synapses)• May sEmulate)– Another neuron)– Muscle Cell)– Gland)– Or have some other funcEon)• This sEmulaEon is called innervaEon)NeuronStoSNeuron Synapses)• Occurs between)– Axon terminal of a presynapEc neuron)– Dendrite of a postsynapEc neuron)• A neuron cell body typically receives many)inputs from many dendrites and associated)synapEc connecEons)• A single neuron in the)CNS may receive as)many as 100,000 inputs)Chemical Synapse)• Requires)– Ca2+)influx in SynapEc Knob)– Exocytosis)– Diffusion across the synapEc cleJ)– LigandSgated receptors on the postsynapEc)neuron)• All this takes Eme (0.5 to 1 millisecon d ))• If a chain of neurons is used, these Eme add)up – we call this reacEon Eme)ExcitaEon)• Excitatory synapses bring postsynapEc membranes closer)to an acEon potenEal)– This is done by opening nonSspecific caEon channels, allowing))Na+ and K+ to move)– Typically a single)innervaEon will not fire an acEon potenEal – it takes mulEple)innervaEons)– We call these EPSPs)• Excitatory PostsynapEc PotenEals)InhibiEon)• Inhibitory synapses push postsynapEc)membranes farther from an acEon potenEal)– Increased permeability to either K+ or CLS)– Creates a small hyperpolarizaEon)– We cal these IPSP)• Inhibitory PostsynatpEc PotenEals)– Lessens the likelihood that addiEonal innervaEons will)cause the membrane to reach threshold)ExcitaEon and InhibiEon )• Whether excitaEon or inhibiEon depends on)the neurotransmiMer and receptor)combinaEon)– A specific combinaEon with always have the same)response (either inhibitory or excitatory))– A given synapse is generally either excitatory or)inhibitory at a parEcular Eme)• But my change over long periods of Eme from one to)another (e.g. during development))Clearing the CleJ)• As long as the neurotransmiMer is bound to)the receptor, EPSPs or IPSPs conEunue)• Stopping will occur when)– NeurotransmiMers diffuse away)– NeurotransmiMers are deacEvated by enzymes)– NeurotransmiMers are taken back up through)reuptake carriers in the presynapEc membrane)Slow Synapses)• Those that mediate responses through second)messengers)– Chemicals created that may have longSterm)effects)– Example: Serotonin induces the the acEvaEon of)cAMP)– In turn this leads to longSterm opening of specific)ion channels that can be inhibitory or excitatory)– These neurotransmiMers are oJen called)neuromodulators)Slow)Synapse)Neuromuscular Synapses)• AcEon potenEals are conducted from the CNS)through large myelinated axons in)motorneurons to special juncEons)– Neuromuscular juncEons)– Each muscle cell (muscle fiber) has only one)neuromuscular juncEon)Neuromuscular)Synapse)SynapEc IntegraEon)SynapEc IntegraEon)• GPSPs)– The composite of all IPSPs and EPSPs)– GPSP = sum of all EPSPs and)IPSPs)• Two types of summaEon)– Temporal (e.g. same synapse firing quickly))– SpaEal (e.g. different synapses firing)simultaneously))Temporal SummaEon)SpaEal