2 NERVE IMPULSESThe nerve impulse involves the movement of ions through the axon membraneWhen a neurone is not conducting an impulse it is in the Resting State and the inside has a slight negative chargeWhen positively charged Na ions enter, the inside is briefly positive; Depolarised, generating an Action Potential.
3 The ions causing the resting state include sodium, potassium, chloride and large negatively charged organic ionsThe concs of these vary across the axon membrane due to its relative permeability to the ionsDuring the resting state K+ ions can move out through protein channels (gates); but negative organic ions can’t move out so, negative inside keeps K+ insideNa+ ions are in high conc outside but membrane has low permeability with few sodium channels, those that do enter are expelled by ion pumps.This all results in a negative (-70mV) charge inside the axon during its resting state
4 Action PotentialWhen nerve impulse generated, more sodium channels are opened so Na+ ions move in faster than can be expelled by pumpsThe charge inside briefly becomes positive (+40mV)and an action potential caused
5 The permeability to sodium ions is quickly decreased again; simultaneously potassium channels open allowing K+ ions to flood outThis restores the negative charge inside the axon; RepolarisedThe potassium channel opening causes a slight “overshoot” causing a slightly too negative charge inside the axon; Hyperpolaristaion, but soon charge stabilises at resting state
6 Transmission speeds vary from 0.5msec-1 to over 100 msec –1 So the nerve impulse is caused by a wave of depolarisation travelling along the axon from each region of membrane to the nextThe ion channels are “voltage dependent” opening in response to depolarisationImpulses jump from one Ranvier node to the next in myelinated neurones, greatly increasing the speed of transmission; Saltatory ConductionTransmission speeds vary from 0.5msec-1 to over 100 msec –1Controlled by:Diameter of the axon (greater=faster)Myelin sheath (myelinated=faster)
7 Resting potentialThe inside of a resting nerve fibre is about 60 mV negative compared to the surface of the cellThis electrical potential difference across the fibre membrane (axolemma) is called the resting potentialThe resting potential is maintained by differences in the concentration of ions and other charged particles between the cytoplasm and the tissue fluid around the fibre
8 Resting potentialUnder normal conditions there is a greater concentration of sodium ions in the extracellular fluid than in the axon, and a greater concentration of potassium ions in the axon than outside.As a result there is a very slow inward diffusion of sodium ions, and a slightly faster outward diffusion of potassium ions.At rest this is balanced by the action of the sodium-potassium pump.As a result there is a very slow inward diffusion of sodium ions, and a slightly faster outward diffusion of potassium ions... but a greater concentration of negatively charged protein molecules in the axoplasm keeps the inside negative with respect to the outside.There is also a very slow inward diffusion of chloride ions ...The membrane of the resting axon contains channels (gates) specific for sodium, potassium and chloride ions. At rest, more K+ channels are open than Na+ channels. Like most animal cell membranes, it also contains the sodium-potassium pump (Na+/K+-activated ATPaseThe result of all of these differences in concentration and permeability is the resting potential of -60 mV.
10 The action potentialThe action potential is followed by a refractory period, when the sodium gates are inactivated. During the refractory period no stimulus, however strong, can trigger another action potential.The action potential is an all-or-nothing event: if the initial depolarisation reaches threshold value, a full action potential is triggered; if it does not, there is no action potential.When the initial depolarisation reaches threshold, Na+ gates open: Na+ ions flood in, causing rapid depolarisation1 ms later Na+ gates are inactivated, and K+ gates open: K+ ions flood outThe axon is hyper-polarised and refractoryOscilloscope traceResting potentialResting potential restored1234Time / msSodium gates inactivatedK+ gates openSodium gates closedSome K+ gates openSodium gates reactivated and closedSodium gates closedEvents in the axon-60+45-70-60Some K+ gates openMost K+ gates open
11 The action potential: how much have you understood? What causes this sharp rise in potential?What causes this sharp fall in potential?What is this potential called?Oscilloscope traceWhat is this potential called?What is the state of the axon here?1234Time / msEvents in the axon
12 Propagation of the action potential RepolarisationThresholdLocal currents occur between the depolarised and resting regions of the membraneInitial depolar-isationAt the trailing edge of the impulse local currents cannot open sodium gates because they are inactivated for the refractory period.At the leading edge of the impulse local currents cause initial depolarisation, which opens sodium gates when it reaches threshold.Hyperpolarised and refractoryRestingDirection of impulseSodium inPotassium out: refractory
13 Propagation of the action potential Na+ gates behind a.p. inactivated (refractory period), preventing backward propagation.Threshold depolarisation opens Na+ gates ahead of a.p., moving it forward.Action potential initiated by opening sodium gatesResting axon: sodium gates closed, some potassium gates openLocal currents propagated, causing initial depolarisation
14 Propagation of the action potential The sodium inflow and potassium outflow at each action potential are restored by the sodium-potassium pump: the ion gradients maintained by the pump represent the energy store for action potential transmissionThe sodium and potassium ions exchanged at each action potential represent about one ten-millionth of those available: even if the sodium-potassium pump is stopped by metabolic inhibitors, the axon can still transmit thousands of action potentials
15 Properties of the action potential Action potentials are all-or-nothing: there either is one, or there isn’t; this is therefore a digital signalling systemAction potentials are self-propagating: once initiated, an action potential propagates itself along the axon using the ion gradients maintained by the sodium-potassium pumpAction potentials do not decay: unlike analogue signalling systems, there is no loss of signal strength with distance travelled
16 Properties of the action potential Frequency of action potentials conveys information about different intensities of stimulationSensory neurones commonly have a resting transmission frequency: raising or lowering this frequency can therefore convey opposite kinds of information. E.g. hair cells in the inner ear:Hair bent to rightResting frequencyHair bent to left
17 Properties of the action potential The absolute refractory period following an action potential lasts about 1 ms, giving a theoretical maximum frequency of transmission approaching 1000 per second. In practice maximum frequencies observed are about 200 per second.Unmyelinated human nerve fibres conduct action potentials at about 1-2 ms-1; squid giant axons conduct at about 20 ms-1; the largest myelinated fibres in mammals conduct at over 100 ms-1. Myelination speeds up by transmission by allowing saltatory conduction (see next slide)
18 Saltatory conduction… so the action potential ‘jumps’ from one node to the next, greatly speeding up conduction.Insulation by myelin allows local currents to flow only between nodes …
19 Comprehension checkSuppose that two action potentials have been generated at opposite ends of a fibre, and are travelling toward each other. What will happen when they meet?The membrane on both sides is refractory and incapable of being depolarised: both action potentials will therefore fade out.When the actions potentials merge, what is the state of the membrane on both sides?Make your prediction now …
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