Action Potential Brain and Behavior

Information Transmission within a Neuron

W HAT IS AN I ON ? Atoms are made of: –P rotons  P ositively charged particles –N eutrons  Particles with N o charge – Electrons  Negatively charged particles Ion = a charged molecule –Charge may be positive or negative

W HAT IS AN I ON ? Some important ions: –Sodium (Na+) –Potassium (K+) –Chloride (Cl-) –Calcium (Ca2+)

T HE N EURON AT R EST Membrane of a neuron maintains an electrical gradient –Difference in electrical charge between inside and outside of the cell Neuron is surrounded by a membrane –Protein molecules embedded in the membrane form channels

Resting Potential of Neuron Polarization –Difference in electrical charge between two locations Slightly negative electrical potential inside the membrane compared to the outside (approx. -70 mV) –Due to negatively charged proteins inside the cell

H OW IS THE R ESTING P OTENTIAL M AINTAINED ? Membrane is selectively permeable –Some chemicals can pass freely across membrane Oxygen, carbon dioxide, urea, water NOT ions!  Can only cross when protein channels are open –Ion channels when membrane is at rest: Sodium (Na+) channels  Closed Potassium (K+) channels  Mostly closed (K+ leaks out slowly)

H OW IS THE R ESTING P OTENTIAL M AINTAINED ? Sodium-Potassium Pump –Made of proteins –Repeatedly moves: 3 Na+ ions out of the cell 2 K+ ions into the cell –Resulting distribution 10x more Na+ outside the membrane More K+ ions inside the membrane

Membrane potential Diffusion=concentration gradient Electrostatic pressure=electrical gradient Sodium-potassium pump pushes 3 Na+ out for every 2K+ in

I MPORTANCE OF THE R ESTING M EMBRANE P OTENTIAL Where would ions move if given the chance? – Na+  Rush into the cell Electrical and Concentration gradients “pulling it in” – K+  Flow slowly out of the cell Electrical gradient “pulling it in” Concentration gradient “pushing it out”

I MPORTANCE OF THE R ESTING M EMBRANE P OTENTIAL From resting potential (-70 mV), three options: –Maintenance of the resting potential –Hyperpolarization (“increased polarization”) Increase negative charge inside the neuron (e.g., -85mV) –Depolarization (“decreased polarization”) Decrease negative charge inside the neuron (towards zero…ex: -65mV)

Electrical Potential of Neuron Membrane potential - difference in electrical charge inside and outside cell: inside “-”, outside “+” Resting potential – membrane potential at rest -70mV Action potential – brief reversal of membrane potential: inside “+”, outside “-”, electrical impulse

Electrical Potential of Neuron (cont.) Depolarization – “decreased polarization”, decrease “-” charge inside neuron, membrane potential to 0 Excitation threshold – level of depolarization to produce action potential Repolarization – return to resting potential Hyperpolarization – “increased polarization”, increase “-” charge inside neuron, increase of membrane potential

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Action Potential External stimulus Excitation threshold reached Na+ in DepolarizationK+ out Na+ channels close Action potential K+ channels close HyperpolarizationRepolarization ube.com/watch?v =U0NpTdge3aw

Action Potential

T HE A CTION P OTENTIAL Threshold of excitation (depolarization level) –Stimulation beyond this level produces sudden, massive depolarization of the membrane –Result: Na+ channels open suddenly Causes rapid, massive influx of Na+ ions –Results in more depolarization Reduces negative charge (to approx. +50 mV) This rapid depolarization = ACTION POTENTIAL

T HE A LL - OR -N ONE L AW Sub-threshold stimulation –Produces a small amount of depolarization Proportional to the amount of current applied Stimulation above the threshold of excitation –Stimulation that reaches the threshold produces the action potential –The neuron either “fires” or it doesn’t… …and all action potentials, once firing, are the same

R EPOLARIZATION Return to resting membrane potential (-70 mV) Achieved by voltage-activated K+ channels… Shortly after Na+ channels open, the K+ channels open K+ ions flow out of the membrane –Due to concentration gradient –Also, now due to electrical gradient Membrane potential decreases to slightly below normal resting potential –Temporary hyperpolarization results in refractory period

P OST A CTION P OTENTIAL Membrane has returned to resting potential Post-action potential ion distribution: –More Na+ ions in the cell –Fewer K+ ions in the cell Sodium-Potassium Pump reactivates –Restores original ion distribution

All-or-None Law Revisited All action potentials are equal: – Amplitude (intensity) – Velocity Strength of stimulus communicated in FREQUENCY of action potentials All-or-none law applies to axon only

Refractory Period Revisited Hyperpolarization after action potential – Additional APs are extremely difficult (if not impossible) for a brief period Absolute refractory period – AP not possible Relative refractory period – AP is possible with stronger than usual stimulus

Conduction of Action Potential All or none law Action potential always remains the same Rate law Saltatory conduction

Role of Myelin Nodes of Ranvier –Short (1 mm) unmyelinated gaps of axon –Where membrane depolarization occurs Saltatory conduction –APs “jump” down the axon from node to node Why have saltatory conduction? –Conservation of energy Less work for sodium-potassium pumps

Multiple Sclerosis Loss of myelination in the central nervous system Myelinated axon develops sodium channels almost exclusively at its nodes When myelination is lost, no new sodium channels are formed Result: APs die out between nodes