Block 2 Module 3: Neurophysiology - PNB 2774

insulator

membranes typically prevent charge passage

conductor

membranes CAN allow charge passage

active/passive behaviors

membrane electrical behavior—whether “active” (channels opening and closing, pumps working) or passive (membrane behaving like components of a circuit)—is responsible for signaling in neurons, muscle, & other cell types

Vm

the potential inside the cell (typically negative) relative to the “reference” potential outside the cell (always assumed to be relatively zero); contributing factors include unequal ion distribution and unequal ion permeability

equilibrium potential

the electrical potential (voltage) at which the diffusional flow of an ion one way is balanced by electrostatic attraction in the other (zero net flux); same concept as Nernst potential and reversal potential

repolarization

Returning membrane potential towards rest potential (from either a depolarized OR a hyperpolarized state)

neural action potential limits

in neuron APs, upper barrier for depolarization set by Ena, lower barrier for hyperpolarization set by Ek

equilibrium

an unchanging state that remains so without manipulation

steady-state

an unchanging state that remains so only by constant input/output

permeability at rest

the membrane is permeable to more than one ion at rest; K+ efflux dominates at rest; Vm is the cell membrane potential at which the net flux of ALL ionic current together equals zero - membrane is in steady state and can only be at one potential

GHK equation

If a cell is permeable to multiple ions, Vm is a weighted average of the Ex for each permeable ion; equation predicts membrane potential from contribution of all membrane-permeant ions (so, ions with greater permeability have greater influence on Vm)

dendrite

receive incoming signals (INPUTS) at synapses in the form of passive, “electrotonic” graded potentials

cell body

“integrates” (integrator) multiple incoming signals via summation

axon

carries the output signal; an “all-or-none” action potential

graded potentials

passive, degrade with distance and time, variable amplitude, duration, and sign (i.e. hyper OR depolarizing); SUB-threshold

action potentials

active; self-regenerating, no decrement with distance (same amplitude); all-or-none depolarization; SUPRA-threshold

action potential steps

resting membrane potential, with leak channels active and voltage gated closed; depolarizing stimulus depolarizes the membrane to threshold, opening VG Na - Na enters, depolarizing cell; VG K channels slowly start to open and VG Na channels slowly start to block; K leaves cell thru VG and leak channels, re and hyperpolarizing membrane; repolarization leaves VG Na channels closed and blocked; VG K channels slowly close and VG Na channels slowly unblock → return to resting membrane potential and resting channel states

trigger zone in neuron

the axon hillock and axon initial segment - have high density of voltage-gated sodium channels that trigger action potentials - AP or no AP is the ultimate output of dendritic integration

AP propagation

action potentials propagate as a result of local circuit current flow; after one segment of axon membrane is very depolarized, nearby membrane patches are also depolarized (action potential is self-regenerating) - then, neighboring patch of membrane goes through action potential steps

refractory

membrane sections that just finished the action potential; they are resistant to depolarization because the Na+ channels need time to recover from inactivation

unmyelinated axons

each segment must go through all steps of action potential

myelination

10-160 concentric “wrappings” of glial membrane around axon (distance between nodes of ranvier is internodes); results in saltatory conduction - can increase conduction speed by a factor of 100; done by glial cell types called oligodendrocytes in CNS and schwann cells in PNS

post-synaptic receptor classes

ligand-gated ion channels, G protein-coupled receptors

ligand-gated ion channels

fast synaptic transmission, opens ion channels (usually), receptor and channel are part of the same protein, little amplification, ionotropic (ion movement response) receptor

EPSP

if a neurocrine signal results in net more sodium influx or less potassium efflux

IPSP

if a neurocrine signal results in net less sodium influx or more potassium or chloride efflux

G protein-coupled receptor

slow synaptic transmission, opens or closes ion channels, among other things; receptor and channel (if used) are separate proteins, involves amplification process, metabotropic (uses second messengers) receptor

post-synaptic graded potentials

summation of several subthreshold signals can either result in an action potential, or if signal is not strong enough or inhibitory neuron signals in combination no AP is fired

temporal summation

if two subthreshold graded potentials are far apart in time, an AP will not be initiated - if they are close enough, they can add up over time and initiate an AP

electrical synapse

bi-directional signaling (connected by gap junctions) - direct electrical coupling; second cell mirrors the first one

chemical synapse

anterograde signaling (forward only); presynaptic vesicles hold NTs, which act on postsynaptic receptors provoking a response (on ex. ionotropic, or metabotropic receptors)

pre-synaptic chemical synapse steps

action potential depolarizes axon terminal membrane, voltage gated calcium channels open and calcium enters cell, calcium entry sensed by SYNAPTOTAGMIN triggers exocytosis of synaptic vesicle contents, NT diffuses across synaptic cleft, binds to receptors on postsynaptic cell provoking a response (in that cell)

vescicle exocytosis steps

building vesicles: 1) filler with neurotransmitter using transporters and pumps 2) vesicle translocation using cytoskeleton and pumps 3) docking using SNARe proteins 4) priming using SNARes 5) fusion with membrane (requires calcium)

vescicle endocytosis steps

recycling vesicles: 1) vesicle membrane translocation 2) coating with clathrin (usually) 3) fission of coated vesicle from membrane via protein called dynamin, which assists with this pinching off 4) uncoating from clathrin 5) recycling (by several paths)