2NeuronThe basic structural unit of the nervous system
3The job of the neuronsNeurons transfer long-distance information via electrical signals and usually communicate between cells using short-distance chemical signals.
4The higher order processing of nervous signals may involve clusters of neurons called ganglia or most structured groups of neurons organized into a brain.
5Types of neurons Sensory (afferent) – receive stimulus Motor (efferent) stimulate effectors which are target cells, muscles, sweat glands, stomach, etc.Association (interneurons) located in spinal cord or grain integrate or evaluate impulses for appropriate responses.
7The transmitting cell is called the presynaptic cells The receiving cell is the postsynaptic cell
8Neuron StructureCell body which contains the nucleus and organelles and numerous extensionsDendrites receive signalsAxon longer, transmits signalsEnds of axons end in synaptic terminals which release neurotransmitters across a synapseGlial cells nourish and support the neurons
9Direction of impulse Dendrites Stimulus Presynaptic Nucleus cell Axon Fig. 48-4DendritesStimulusNucleusPresynapticcellAxonhillockCellbodyDirection of impulseAxonSynapseSynaptic terminalsPostsynaptic cellNeurotransmitter
10Glial Cells Nourish neurons Insulate axons Regulate the extracellular fluid around the neuron
11Nerve conductionIn order to conduct an electrical nerve impulse, a voltage or membrane potential, exists across the plasma membrane of all cells.For a typical non-transmitting neuron, this is called the resting potential and is between -60 and -80 mV.
12Membrane Potential Inside is NEGATIVE! Principal cation inside of cell KPrinciple anion inside of cell:negatively-charged proteins, aminoacids, PO4 and SO4. Symbol is A-.Inside is NEGATIVE!
13Outside of the cellPrincipal ion is Na+Outside is positive!
20What causes the generation of a nerve signal? Neurons and muscle cells are excitable cells – they can change their membrane potentials due to gated ion channels* – can be chemically gated which respond to neurotransmitters or voltage-gated which respond to a change in membrane potential.* Found only in nerve cells
21Upon receiving a stimulus, Na+ channels open and Na+ flows into the cells and thus they become more positive inside andmore negative outside and the charge on the membrane becomes depolarized.The stronger the stimulus, the more Nagated Ion channels open.
22Production of an Action Potential Once depolarization reaches a certain membrane voltage called the threshold level (-50 mv), more Na gates open and an action potential is triggered that results in complete depolarization.This stimulates neighboring Na gates, further down the neuron, to open. The action potential is an all or none event, always creating the same voltage spike once the threshold is reached.
23Notice, gates are closed! FigKeyNa+K++50Actionpotential3Membrane potential(mV)24Threshold–50115Resting potentialDepolarization–100TimeExtracellular fluidSodiumchannelPotassiumchannelPlasmamembraneNotice, gates are closed!CytosolInactivation loopUndershoot1Resting state
24Notice, gates are closed! FigKeyNa+K+Some Na+ gates open!+50Actionpotential3Membrane potential(mV)24Threshold–50115Resting potential2Depolarization–100TimeExtracellular fluidSodiumchannelPotassiumchannelNotice, gates are closed!PlasmamembraneCytosolInactivation loopUndershoot1Resting state
25A lot of Na+ gates open! Fig. 48-10-3 Key Na+ Na+ gates open! K+ Rising phase of the action potential+50Actionpotential3Membrane potential(mV)24Threshold–50115Resting potential2Depolarization–100TimeExtracellular fluidSodiumchannelPotassiumchannelPlasmamembraneCytosolInactivation loopUndershoot1Resting state
26In response to the inflow of Na, the gated K channels begin to open, allowing K to rush to the outside of the cell. Na gates close. This creates a reverse charge polarization, (neg outside, positive inside) called repolarization.
27Na closes, K opens Fig. 48-10-4 Key Na+ K+ 3Rising phase of the action potential4Falling phase of the action potential+50Na closes, K opensActionpotential3Membrane potential(mV)24Threshold–50115Resting potential2Depolarization–100TimeExtracellular fluidSodiumchannelPotassiumchannelPlasmamembraneCytosolInactivation loopUndershoot1Resting state
28In fact more K ions go out than is actually needed to return to threshold, resulting in an increased negative charge inside called a hyperpolarization or undershoot.This keeps the direction of the nerve impulse going one way and not backing up.
29K just keeps flowing out. Hyperpolarization Fig. 48-10-5 Key Na+ K+ 3Rising phase of the action potential4Falling phase of the action potential+50Actionpotential3Membrane potential(mV)24Threshold–50K just keepsflowing out.115Resting potential2Depolarization–100TimeExtracellular fluidSodiumchannelPotassiumchannelPlasmamembraneCytosolInactivation loop5Undershoot1Resting stateHyperpolarization
30Refractory PeriodAfter the impulse, the Na channels remain inactivatedSince the neuron cannot respond to another stimulus with the reversal of charges, the Na-K pump has to restore the original charge location. This is called the refractory period.Action Potentials Video | DnaTube.com - Scientific Video Site
36Properties of an Action Potential Are all or none depolarization – once threshold is reached (-50 mV) – always creates the same voltage spike regardless of intensity of the stimulus.The frequency of the action potentials increases with intensity of stimulus.Action potentials travel in only ONE direction!The greater the axon diameter, the faster action potentials are propagated.
38Importance of myelin Acts as insulators. Gaps in the myelin are called nodes of Ranvier and serve as points along which the action potential is propagated, increasing the speed.This is called saltatory conduction.
39The myelin sheath is composed of Schwann cells (PNS) or oligodendrocytes (CNS) that encircle the axon in vertebrates.
40Saltatory ConductionVoltage channels concentrated at the nodes of Ranvier - jumping action potentials
42The SynapseArea between two neurons, between sensory receptors and neurons or between neurons and muscle cells or gland cells
43What happens at the synapse? FigWhat happens at the synapse?5Na+K+Synaptic vesiclescontainingneurotransmitterPresynapticmembraneVoltage-gatedCa2+ channelPostsynapticmembrane1Ca2+426Synapticcleft3Figure A chemical synapseLigand-gatedion channels
44Types of synapsesElectrical – via gap junctions such as in giant axons of crustaceans**Chemical – electrical impulses changed into chemical signalsArrival of action potential opens Ca+ channels (membrane signaling cAMP), causes synaptic vesicles full of NT’s to fuse with membrane and pop open
45Post-synaptic Responses EPSP - excitatory post-synaptic potential --> open Na channels --> inside +May generate an APIPSP - inhibitory post-synaptic potentialopens Cl channels - Cl-in -> more neg > no AP --> opens K channels - K-out -> more neg > no AP
51Temporal summation occurs with repeated release of nt’s from one or more synaptic terminals before RPSpatial summation occurs when several different presynaptic terminals release NT’s simultaneously
52Assume a single IPSP has a negative magnitude of -0 Assume a single IPSP has a negative magnitude of -0.5 mV at the axon hillock and that a single EPSP has a positive magnitude of +0.5 mV, for a neuron with initial membrane potential of -70 mV, the net effect of 5 IPSP’s and 2 EPSPs spatially would be to move the membrane potential to? Would the impulse continue?-85 mV
53Neurotransmitters Affect ion channels Affect signal transduction pathwaysHow? Involve cAMP, cAMP protein kinases, GTP, GTP binding proteins
54After release, the neurotransmitter May diffuse out of the synaptic cleftMay be taken up by surrounding cellsMay be degraded by enzymes
55NeurotransmittersThe same neurotransmitter can produce different effects in different types of cellsThere are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases
56a. ACETYLCHOLINE Found in vertebrate neuromuscular junctions - excitatory at skeletal muscles- inhibitory at heart
57b) Biogenic Amines (derived from amino acids) epinephrine, norepinephrine (fight or flight),dopamine, serotonin (involved in sleep, mood, attention, and learning).
61e) Gaseous signalsGases such as nitric oxide and carbon monoxide are local regulators in the PNS
62How do drugs work?Agonists – mimic drugs such as in nicotine mimicking acetycholineAntagonists – block action of NT’s such as atropine and curare (poisons) – block acetylcholine and thus prevent nerve firing in muscles – leads to paralysis and deathCocaine and amphetamines block the reuptake of NT’s at adrenergic synapsesMany antidepressants block reuptake of serotonin so serotonin lingers longer in synaptic cleft.