Electrochemical Impulses

How do nerve cells send and receive messages? Dendrites Cell body Nucleus Axon hillock Axon Signal direction Synapse Myelin sheath Synaptic terminals Presynaptic cell Postsynaptic cell

Membrane Potential Every cell has a membrane potential or “voltage” across its plasma membrane Membrane Potential Separation of charge Ability to do work Resting Potential of a neuron is -70 mV

What causes membrane potential? Unequal distribution of positively charged ions inside and outside the cell Why only positively charged ions? Cells have large negative ions inside the cell that cannot cross the plasma membrane High concentration of K+ inside the cell High concentration of Na+ outside the cell

At rest, more permeable to K+ than it is to Na+ Unequal permeability (diffusion) of positively charged ions across the membrane At rest, more permeable to K+ than it is to Na+ More potassium ions diffuse out of the cell than sodium ions diffuse into the cell Cell looses more positive ions than it gains The inside of a cell is negative relative to the outside Negative and positive ions will accumulate along the inside of the membrane due to charge attraction

Maintaining the membrane potential at rest ADP Sodium Potassium Pump (Na-K Pump)

Resting Potential

Action Potential Fill in worksheet as you watch the following animation http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf

Action Potential Three Phases Resting potential – positive exterior, negative interior maintained by sodium-potassium pump Depolarization – negative exterior, positive interior Sodium channels open to allow sodium ions to rush into the cell Repolarization – positive exterior, negative interior Sodium channels close Potassium channels open Resting state restored by sodium-potassium pump

Action Potential – Three States resting potential: -70mV required before a signal can be detected Threshold: values vary If membrane potential reaches this value, will get an action potential because this triggers MANY Sodium channels to open = ALL OR NONE Principle action potential: +40mV change that occurs to transmit signal

Propagation of Action Potential

REPOLARIZATION (Refractory Period) – + Na+ Action potential K+ Axon An action potential is generated as Na+ flows inward across the membrane at one location. 1 2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, the action potential Is propagated along the length of the axon. RESTING DEPOLARIZATION REPOLARIZATION (Refractory Period) Refractory Period Na+ channels closed Refractory Period

Propagation of AP http://wps.aw.com/bc_martini_eap_5/141/36119/9246587.cw/content/index.html

Action Potential Factors refractory period threshold axon diameter saltatory conduction

Refractory Period Time is required to re-establish the resting potential after an action potential has been initiated. no other action potential may be initiated, no matter how strong the signal

Threshold Level Different neurons have differing threshold levels before an action potential will proceed. Sensitive neurons will have low threshold values. all-or-none response – neurons will or will not fire

greater axon diameter = greater axon surface area Larger surface area results in more ion channels and therefore less time to depolarize and repolarize. faster signals

Saltatory Conduction Na+ and K+ exchange can only occur where the axons are exposed to the extracellular fluid. allows for faster signal conduction along the axon Insulation

Signal Transmission Dendrites Cell body Nucleus Synapse Figure 48.5 Dendrites Cell body Nucleus Axon hillock Axon Signal direction Synapse Myelin sheath Synaptic terminals Presynaptic cell Postsynaptic cell

Signal Transmission postsynaptic neurons presynaptic neurons

Voltage-gated Ca2+ channel Synapse Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Presynaptic membrane Postsynaptic membrane Voltage-gated Ca2+ channel Synaptic cleft Sodium Channels Na+ K+ Ligand- gated ion channel Neuro- transmitter 1 Ca2+ 2 3 4 5 6 synapse – structure formed by two adjacent neurons action potentials cause axon terminals to release neurotransmitters into the synaptic cleft

Neurotransmitters acetylcholine – common neurotransmitter released upon neuron depolarization from presynaptic neuron causes Na+ channels to be opened in the postsynaptic neuron Animation

Neurotransmitters How does the postsynaptic neuron know when the signal has stopped? enzymes released will degrade the chemical cholinesterase – released by postsynaptic neuron

Signals positive (Excitatory) Negative (Inhibitory) causes action potential to proceed in postsynaptic cell Negative (Inhibitory) prevents action potential to proceed hyperpolarization – membrane is more negative; therefore stronger signal needed

Putting it all Together positive and negative signals collect (summation) on postsynaptic neuron Why are negative signals important? experience has told you what should be concentrated on and what can be ignored

Membrane potential (mV) Resting potential Threshold of axon of postsynaptic neuron (a) Subthreshold, no summation Terminal branch of presynaptic neuron Postsynaptic neuron –70 Membrane potential (mV) E1 + E2 Action potential (c) Spatial summation E1 E2 E1 E1 + I I (d) Spatial summation of EPSP and IPSP