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Basic Principles of Electricity
~ human body is electrically neutral, but there are areas where one charge dominates
~ work is done to separate opposite charges
~ there is potential energy in opposite charges that are separated
Voltage
~ measure of potential energy generated by separated charge
Potential difference
(Potential)
~ the difference in voltage between 2 points
Current
~ the flow of electrical charge from one point to another
Resistance
~ the hindrance to charge flow provided by substances through which the current must pass
Ohm's Law

Current (I) = V/R

~ the greater the voltage, the greater the current

~ the greater the resistance the smaller the current

~ no net current flow between points with equal potential
Membrane Ion Channels
~ large proteins with portions that form "gates"
Chemically Gated Channels
(ligand gated)
~ open when the appropriate chemical binds

~ in neurons, the chemical is a neurotransmitter
Voltage Gated Channels
~ open and close in response to changes in the membrane potential
Mechanically Gated Channels
~ open in response to physical deformation of the receptor

~ seen in sensory receptors for touch and pressure
Leakage Channels
(nongated)
~ always open

~ molecules freely diffuse along the [ ] gradient
Ions move along chemical ______ ______ when they diffuse passively from higher to lower [ ].
~ concentration gradients
Ions move along ______ ______ when they move toward an area of opposite electrical charge.
~ electrical gradients
The combination of concentration and electrical gradients is referred to as ______ ______.
~ electrochemical gradients
Resting Membrane Potential
(polarized)
~ the potential in a resting neuron
(approx. -70mV, but ranges from -40mV to -90mV)

~ cytosol contains lower [ ] of Na+ and higher [ ] of K+ than extracellular fluid

~ Na+/K+ pump stabilizes the RMP by maintaining the Na+ and K+ [ ] gradients
Graded Potentials
~ short lived, localized changes in membrane potential

~ magnitude varies directly with stimulus

~ triggered by a stimulus that causes gated ion channels to open

~ named depending on where they occur and functions they perform

~ essential in initiating APs (which occur at axon hillock)

~ decreases in strength over distances
Receptor
(generator)
Potential
~ a graded potential in which the receptor of a sensory neuron is excited by some form of energy
Postsynaptic Potential
~ a graded potential in which the stimulus is a neurotransmitter released by another neuron
(AP)
Action Potential
(nerve impulse)
~ principal way neurons send signals over long distances

~ only cells with excitable membranes (neurons and mucle cells) can generate

~ typically generated only in axons

~ all or none phenomenon (happens completely or not at all)



~ once initiated, is self-propagating and continues at a constant velocity

~ once generated,independent of stimulus strength and all are alike

~ stimulus intensity is coded for by the number of impulses generated per second (frequency of AP), rather than by increases in strength (amplitude)
Generation of an AP
1) Resting State
2) Depolarizing Phase
3) Repolarizing Phase
4) Hyperpolarization
Resting State
~ voltage gated Na+ and K+ channels are closed

~ only leakage channels are open, maintaining resting membrane potential
Depolarizing Phase
~ Na+ channels

~ Na+ enters cell and reverses (reduces) membrane potential

~ when threshold is reached, becomes self-generating and is urged on by positive feedback
Repolarizing Phase
~ Na+ channels begin to close

~ K+ channels open and K+ comes out of cell

~ restores electrical conditions but not ionic conditions
(ionic conditions are restored by Na+/K+ pump)
Hyperpolarization
~ K+ channels remain open longer than necessary and an "undershoot" is seen on an AP curve
Absolute Refactory Period
~ from opening of Na+ channels until Na+ channels begin to reset

~ neuron cannot respond to another stimulus

~ ensures AP is all or none and enforces one way transmission of AP
Relative Refractory Period
~ Na+ channels are at resting state

~ some K+ channels are still open and repolarization is occuring

~ axon's threshold is elevated and an exceptionally strong stimulus can reopen Na+ channels allowing another AP to be generated
Conduction velocity depends on _____ _______ and degree of __________.
~ axon diameter (larger diameter is faster)

~ myelination (faster than unmyelinated due to saltatory conduction)
Synapse
~ space between neurons or between a neuron and effector
Presynaptic Neuron
~ the neuron conducting the impulse toward the synapse
Postsynaptic Neuron
~ the neuron transmitting the signal away from the synapse
Electrical Synapses
~ less common

~ found in regions of the adult brain that are responsible for certain stereotyped movements and in axoaxonic neurons of the hippocampus

~ abundant in embryonic nervous tissue, where they direct neurons to make proper connections with one another
Chemical Synapses
~ either excitatory or inhibitory


consists of:
1) Axon Terminal
2) Receptor Region
Axon Terminal
~ knoblike portion of the presynaptic neuron which contains synaptic vesicles
Synaptic Vesicles
~ membrane-bound sacs filled with neurotransmitter
Receptor Region
~ region on the membrane of a dendrite or the cell body of the postsynaptic nerve that neurotransmitter bonds to
Information transfer across chemical synapses.
(3 cards)
1) Ca2+ channels open in the presynaptic axon terminal

(occurs simultaneously with the opening of Na+ channels upon reception of an AP)
2) Neurotransmitter is released
3) Neurotransmitter binds to postsynaptic receptors
4) Ion channels open in the postsynaptic membranes
5) Neurotransmitter effects are terminated by:
a) degradation of enzymes
b) reuptake by astrocytes or the presynaptic terminal
c) diffusion away from the synapse
Excitatory Synapses
~ neurotransmitter binding causes depolarization of the postsynaptic membrane

~ postsynaptic membranes do not generally generate APs
~EPSP~
Excitatory Postsynaptic Potential
~ local graded depolarization that occurs instead of APs at excitatory synapses

~ currents decline with distance

~ helps to trigger an AP distally at the axon hillock
Inhibitory Synapses
~ neurotransmitter binding reduces ability to generate an AP
~IPSP~
Inhibitory Postsynaptic Potential
~ postsynaptic neuron becomes less likely to "fire" and larger depolarizing currents are required to induce an AP
______ by the ____________ _______ can occur when 1000's of EPSPs are firing
(APs would never happen without this).
~ Summation

~ Postsynaptic Neuron
The 2 types of summation are:
1) Synaptic Potentiation
2) Presynaptic Inhibition
Synaptic Potentiation
~ repeated or continuous use of a synapse enhances the presynaptic neurons ability to excite the postsynaptic neuron, producing larger-than-expected postsynaptic potentials
Presynaptic Inhibition
~ the release of excitatory neurotransmitter by one neuron is inhibited by the activity of another neuron via an axoaxonic synapse
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