nervous system part 1 – section 2: membrane potentials and electrical signaling (from week 4 slides 31–end)

graded potential

Small localized change in a neurons membrane potential that varies in magnitude depending on the strength of the stimulus

graded potential characteristics

vary in size, can be depolarizing or hyperpolarizing, can summate

depolarizing graded potential

inside of the cell becomes less negative, moves toward threshold

hyperpolarizing graded potential

inside of the cell becomes more negative, moves away from threshold

where graded potentials occur

mainly on dendrites and cell bodies

graded potential decremental property

strength decreases as it travels along the membrane

action potential

Electrical signal that a neuron uses to send information down its accent to communicate with other neurons

where action potentials occur

on axons, typically starting at the axon hillock

resting membrane potential

A neurons baseline voltage usually around -70 mv , inside of the cell is more negative than outside

ions involved in action potential

sodium (na+) and potassium (k+)

voltage-gated sodium channels

open quickly in response to depolarization; cause rapid na+ influx

voltage-gated potassium channels

open more slowly; cause k+ efflux to repolarize membrane

phases of action potential

depolarization, repolarization, hyperpolarization

depolarization

na+ channels open, sodium enters the cell, membrane potential becomes positive

repolarization

na+ channels inactivate, k+ channels open, potassium exits the cell

hyperpolarization

k+ channels remain open longer than needed, membrane potential drops below resting

return to resting potential

na+/k+ pump restores ion distribution

threshold

for most neurons is around -55 mV

all-or-none property

action potentials always have same amplitude once threshold is reached

refractory period

time when neuron cannot or is less likely to fire again

absolute refractory period

na+ channels are inactive, no new action potential can occur

relative refractory period

k+ channels still open; stronger stimulus can trigger another action potential

purpose of refractory period

prevents backward propagation, ensures one-way transmission of impulses

frequency coding

strength of stimulus is indicated by frequency of action potentials, not amplitude

subthreshold stimulus

stimulus not strong enough to reach threshold, no action potential generated

suprathreshold stimulus

stimulus strong enough to cause repeated action potentials

propagation of action potential

movement of the signal along the axon

continuous conduction

unmyelinated axons; action potential travels along every part of membrane

saltatory conduction

myelinated axons; action potentials jump from node to node, faster transmission

nodes of ranvier

unmyelinated gaps in myelin sheath where ion channels are concentrated

advantages of myelination

increases conduction speed and efficiency, conserves energy

axon diameter effect on conduction

larger diameter = faster conduction velocity

factors affecting conduction velocity

myelination, axon diameter, and temperature

electrical synapse

connection via gap junctions allowing direct ion flow between cells

chemical synapse

connection using neurotransmitters across a synaptic cleft

synaptic cleft

space between presynaptic and postsynaptic membranes

presynaptic neuron

neuron sending signal through neurotransmitter release

postsynaptic neuron

neuron receiving neurotransmitter signal

synaptic delay

time between action potential arrival and postsynaptic response (~0.5–5 msec)

steps in synaptic transmission

action potential → ca2+ influx → vesicle docking → neurotransmitter release → binding to receptors → postsynaptic response → neurotransmitter removal

neurotransmitter removal

Via reuptake, enzymatic, derogation and diffusion

postsynaptic potential

change in membrane potential of postsynaptic cell due to neurotransmitter binding

excitatory postsynaptic potential (epsp)

depolarization of postsynaptic membrane that increases chance of firing

inhibitory postsynaptic potential (ipsp)

hyperpolarization of postsynaptic membrane that decreases chance of firing

ion movement in epsp

na+ influx or ca2+ entry causes depolarization

ion movement in ipsp

k+ efflux or cl– influx causes hyperpolarization

graded nature of epsp/ipsp

their strength depends on neurotransmitter amount and receptor activation

temporal summation

repeated stimulation by one neuron over time

spatial summation

simultaneous stimulation by several neurons

integration at axon hillock

summation of all epsps and ipsps determines whether threshold is reached

divergence

one neuron communicates with multiple target neurons

convergence

multiple neurons synapse on one target neuron

frequency coding in postsynaptic cell

increased presynaptic firing increases neurotransmitter release and postsynaptic depolarization

axoaxonic synapse

synapse between axon terminals of two neurons; modulates neurotransmitter release

presynaptic facilitation

increases neurotransmitter release from presynaptic neuron

presynaptic inhibition

decreases neurotransmitter release from presynaptic neuron

axoaxonic synapses effect

selective modulation of a single synapse rather than entire postsynaptic neuron

axon hillock

functional decision point where all synaptic inputs are integrated

neural integration

process of combining multiple signals to determine neuron output

threshold decision mechanism

if summed graded potentials reach threshold, an action potential is triggered

selective modulation advantage

allows fine control of signal transmission in neural circuits