Neurophysiology Ch3 Lectures PDF

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Ch3 Lectures...

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Neurophysiology & Action Potentials

Neurophysiology: the study of electrical and chemical signaling in neurons Action Potential: a rapid electrical signal that travels along the axon of a neuron Neurotransmitter: a chemical messenger between neurons A neuron at rest is a balance of electrochemical forces 

Ions: electrically charged molecules o

Anions: negatively charged (Cl-)

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Cations: positively charged (Na+)

o

Ions are dissolved in intracellular fluid, separated from the extracellular fluid by the cell membrane

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Microelectrode: inserted into a resting cell shows that it is more negative than the extracellular fluid

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Resting membrane potential: -50 to -80 millivolts (mV) and shows the negative polarity of the cells interior

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Cell membrane: lipid bilayer (two layers of lipid molecules)

o

Ion channels: are proteins that span the membrane and allow ions to pass 

They open and close in response to voltage changes, chemicals, or mechanical action

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Some ion channels are open all of the time and allow only potassium (K+) to cross

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Selective permeability: the neuron allows K+ to enter or leave the cell freely, but restricts the flow of other ions

Two opposing forces drive ion movement 

Diffusion: causes ions to flow from high -> low areas of concentration, along their concentration gradient

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Electrostatic Pressure: causes ions to flow towards oppositely charged areas o

Ex. Potassium ions have the same charge so they repel, but potassium and chlorine don’t mind each other

Sodium Potassium Pump: maintains resting potential, stable balance 

Pumps 3 Na+ ions out for every 2 K+ ions pumped in

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At rest, K+ ions move into the negative interior of the cell because of electrostatic pressure

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As K+ ions build up inside the cell, they also diffuse out through the membrane, along the concentration gradient

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K+ reaches equilibrium when the movement out is balanced by the movement in

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This corresponds to the resting membrane potential of about -60 mV (range of -50 to -80)

Nernst Equation: predicts the voltage needed to counterbalance the diffusion force pushing an ion across a membrane 

Predicts the equilibrium potential of an ion, usually K+

Goldman Equation: predicts voltage potentials that are quite close to observed resting potentials 

It takes into account the intracellular and extracellular concentrations of several ions and the degree of membrane permeability to each

Action Potentials: brief but large changes in the membrane potential 

Originate at the axon hillock and are propagated along the axon

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Patterns of action potentials carry info to postsynaptic targets

Hyperpolarization: an increase in membrane potential - the interior of the membrane becomes even more negative, relative to the outside 

A hyperpolarizing stimulus produces an immediate response that passively follows the stimulus

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The greater the stimulus, the greater the response o

Graded response: is the change in potential

o

The distortions at the beginning and end of the neuron’s response are caused by the membrane’s ability to store electricity, known as capacitance

Depolarization: a decrease in membrane potential - the interior of the cell becomes less negative 

Depolarizing stimuli produce local, graded responses

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If the membrane potential reaches the threshold (about -40 mV), and action potential is triggered

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The membrane potential reverses and the inside of the cell becomes positive

Local Potential: an electrical potential that spreads passively across the membrane, diminishing as it moves away from the point of stimulation

All or None Property of Action Potential 

Neuron fires at full amplitude or not at all

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Does not reflect increased stimulus strength

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Info is coded in the frequency of action potentials o

Increased frequency = increased stimulus strength

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Afterpotentials: changes in membrane potential after action potentials

Action Potentials are produced by the movement of Na+ ions into the cell 

At the peak of an action potential, the concentration gradient pushing Na+ ions into the cell equals the positive charge driving them out

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Membrane shifts briefly from a resting state to an active state and back

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Voltage-gated Na+ channels: open in response to the initial depolarization o

More voltage-gated channels open and more Na+ ions enter

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This continues until the membrane potential reaches the threshold/ Na+ equilibrium potential of +40 mV

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As the inside of the cell becomes more positive, voltage-gated K+ channels open

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K+ moves out and the resting potential is restored

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Refractory Period: time when no stimuli can produce an action potential

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Absolute Refractory Period: time when no action potentials are produced

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Relative Refractory Period: time when only string stimuli can produce an action potential

Action potentials are regenerated along the axon - each adjacent section is depolarized and a new action potential occurs 

Action potentials travel in one direction because of the refractory period of the membrane after a depolarization

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Conduction velocity: the speed of propagation of action potentials - varies with diameter

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Nodes of Ranvier: small gaps in the insulating myelin sheath

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Saltatory conduction: the axon potential travels inside the axon and jumps from node to node

Animal toxins selectively block certain channels: 

Tetrodotoxin (TTX) and Saxitoxin (STX) block voltage-gated Na+ channels

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Batrachotoxin forces Na+ channels to stay open

Optogenetics 

Uses genetic tools to insert light-sensitive ion channels into neurons

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Stimulating the brain with light, delivered by fiber-optic cables, can excite or inhibit those targeted neurons

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Some algae and bacteria produce light-sensitive proteins called opsins, which resemble the mammalian opsins found in light-receptor cells in our eye

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Channelrhodopsin responds to blue light by allowing Na+ ions to enter the cell, depolarizing it

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Halorhodopsin responds to yellow light by allowing Cl- ions into the cell, hyperpolarizing it...