2 Peripheral nervous system (PNS) Central nervous system (CNS) OverviewSensorEffectorSensory inputMotor outputIntegrationPeripheral nervous system (PNS)Central nervous system (CNS)
3 Types of neuronsSensors detect external stimuli and internal conditions and transmit information along sensory neuronsSensory information is sent to the brain or ganglia, where interneurons integrate the informationMotor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity
4 Parts of a neuron Dendrites Stimulus Axon hillock Nucleus Cell body Presynaptic cellSignal directionAxonSynapseNeurotransmitterSynaptic terminalsPostsynaptic cellParts of a neuron
5 Evolutionary Adaptation of Axon Structure The speed of an action potential increases with the axon’s diameterIn vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increaseMyelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS
6 AxonMyelin sheathSchwann cellNodes of RanvierNode of RanvierLayers of myelinNucleus of Schwann cell
7 Nerve SignalsMembrane potential - the electrical charge difference across a membraneDue to different concentrations of ions in & out of cellAnions more concentrated inside, Cations more concentrated outsideResting potential – the membrane potential of an unstimulated neuronAbout -70 mV (more negative inside)
8 Changes in membrane potential act as signals Concentration of Na+ highest outside, concentration of K+ highest inside cell.Sodium –potassium pumps use ATP to maintain concentration gradientsEnergy is potential chemical energy
9 A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cellThe resulting buildup of negative charge within the neuron is the major source of membrane potential
11 KeyNaKSodium- potassium pumpPotassium channelSodium channelOUTSIDE OF CELL
12 Maintenance of Resting Potential - Note gradients for sodium and potassium- To maintain resting potential, you need to keep concentration gradients constant- ions have a charge, so need facilitated diffusion through membrane (is this with or against the gradient?)- there are selective ion channels that allow “leakage”
13 Maintenance of Resting Potential To keep sodium & potassium in right gradients, sodium-potassium pump uses ATP to maintain gradientsThe sodium-potassium pump pumps 2K+ in and 3Na+ out each time.
14 What is the result of changes in membrane potential? Graded potentials – based on magnitude of stimulusAction potentials – all or nothing depolarization
15 Types of ion channels: Ungated ion channels – always open Gated ion channels – open or close in response to stimuliLigand gated ion channels (chemically gated) ion channels–in response to binding of chemical messenger (i.e. neurotransmitter)Voltage gated ion channels – in response to change in membrane potentialStretch gated ion channels – in response to mechanical deformation of plasma membrane
16 Graded potentialsThe magnitude of the change (in polarization) varies with the strength of the stimulusThese are not the nerve signals that travel along axons, but they do have an effect on the generation of nerve signals
17 HyperpolarizationStimulusThresholdResting potentialHyperpolarizations5050Membrane potential (mV)When gated K+ channels open, K+ diffuses out, making the inside of the cell more negativeThis is hyperpolarization, an increase in magnitude of the membrane potential
18 DepolarizationStimulusThresholdResting potentialDepolarizations505010012345Membrane potential (mV)Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potentialFor example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell
19 Graded potentials occur up to a particular voltage, the threshold voltage If depolarization reaches the threshold voltage, then an action potential is triggered.
20 Action PotentialsSignals conducted by axons, transmitted over long distancesOccur as the result of gated ion channels that open or close in response to stimuliA specific dynamic change in the charge across the membrane of a cell, one that occurs either totally or not at all (“all or nothing”
22 Resting stateMost voltage gated sodium(Na+) and potassium gated (K+) ion channels are closed, membrane potential is at -70 mV
23 ThresholdAn electric stimulus causes the voltage-gated Na+ channels open first and Na+ flows into the cellThe flow of Na+ causes more voltage – gated Na+ channels to open, so more depolarization(what kind of feedback is this)If this depolarization reaches threshold potential, then more Na+ gates open
24 Depolarization phaseMembrane potential rises rapidly as sodium ions rush into cellAlso known as “rising” phase
25 Repolarization phaseThe Na+ voltage gates close shortly after opening, so Na+ inflow stopsThe K+ voltage gates open, so K+ rushes outThere is a loss of positive charge, so cell returns to resting state“Falling” phase
26 UndershootThe membranes permeability to K+ is higher than at rest, so membrane potential dips down lower than resting potentialK+ gates close, so membrane potential returns to normal
27 Refractory periodAfter an action potential occurs, there is a period of time when another action potential cannot be triggeredDuring undershoot – sodium channel inactivation gates are closed – haven’t had enough time to reopen yet
28 Membrane potential (mV) 50 OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelAction potentialThresholdResting potentialTimeMembrane potential (mV)5010050NaKKey21345Resting stateUndershootDepolarizationRising phase of the action potentialFalling phase of the action potential
30 How do action potentials “travel” along a neuron? Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membraneAction potentials travel in one direction – towards synaptic terminals
31 Action potentials are self-propagating across the axon. Plasma membraneAction potential1CytosolNaAction potentials are self-propagating across the axon.
34 Why doesn’t it travel backwards? The refractory period is due to inactivatedNa+ channels, so the the depolarization can only occur in the forward direction.
35 Speed of action potentials Speed is proportional to diameter of axon, the larger the diameter, the faster the speedSeveral cm/sec – thin axons100 m/sec in giant axons of invertebrates such as squid and lobsters
36 http://www.youtube.com/watch?v=omXS1bjYLMI Nerves with giant axons GangliaBrainArmNerveEyeMantleNerves with giant axons
37 Speeding up Action potential in vertebrates Myelination (insulating layers of membranes) around axonMyelin is deposited by Schwann cells or oligodendrocytes.Cell bodySchwann cellDepolarized region (node of Ranvier)Axon
38 Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are foundAction potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction