Kin 1A03 - Nervous Tissue (Membrane Potentials and Neurotransmission)

electrical nature of neurons

electrical properties of neurons result from ionic concentration differences across plasma membrane and permeability of membrane

ionic concentration

ions dissolved inside and outside membrane

anion

negatively charged ion

cation

positively charged ion

voltage

created when oppositely charged ions are separated, ions attempt to move back together which creates electrical force

extracellular fluid

found outside cell membrane, has net positive charge

cytosol

found inside cell membrane, has net negative charge

difference between membrane potential inside and outside membrane

70 mV

concentration of ions in extracellular fluid (outside membrane)

has higher concentration of chloride (cl-) and sodium (Na+) ions

concentration of ions in cytosol (inside membrane)

has higher concentration of potassium ions (K+) and proteins (anions, negative charge)

electrical properties

result from ionic concentration differences across plasma membrane and permeability of membrane

ion concentrations

result from Na+/K+ pump and membrane permeability

membrane permeability

changes int he membrane that determine how easily things can move in/out of the cell, ease of movement is based on number of pumps present and whether they are open or closed

leak channels

substance specific channels that move with the concentration gradient (movement of ions from HIGH concentration to LOW concentration), essentially open all the time

leak channel function

usually based on size, shape, and charge, are K+ and Na+ leak channels

sodium potassium pump

active transport pump which moves ions against concentration gradient (movement of ions from LOW concentrations to HIGH concentrations), requires ATP

ATPase

enzyme that breaks down ATP

sodium potassium pump (ATPase) function

1. ATP binds to ATPase 2. ATP is converted into ADP and Pi by the sodium potassium pump ATPase - energy is released at this point from breaking the bond 3. pump is activated and 3Na+ exit and 2K+ enter the cell; overall 1ATP = 3Na+ out 2K+ in

two types of ion channels

leak (non-gated) ion channels, gated ion channels

gated ion channels

open and close (open in response to stimulus, close on demand)

gated ion channel types

ligand-gated, mechanically-gated, voltage-gated

ligand gated

opens in response to chemical stimulus

neurotransmitter

chemical that travels from one neuron to another

mechanically gated

physically opened

voltage gated

responds to change in membrane potential

factors that impact membrane permeability

number of open channels, size of ions, number of gated channels

leak channel locations

found in nearly all cells (including dendrites, cell bodies, and axons of all types of neurons)

leak channel types

K+ channels and Na+ channels, more K+ than Na+, K+ is more easily free-moving than Na+, concentration gradient dictates more of movement of ions

ligand gated channel locations

found on dendrites, cell bodies, and target tissues

ligand-gated channel function

responds to chemical stimuli (ligand binds to receptor)

acetylcholine

ligand gated channel example

mechanically gated channel locations

found on more specialized sensory receptors (e.g. touch, hearing)

mechanically gated channel function

respond to mechanical vibration or pressure stimuli

voltage gated channel locations

only found in axon of neurons (all types of neurons)

voltage gated channel function

allows ions to move through membrane, responds to direct changes in membrane potential, triggers a chain reaction (domino effect), one channel opens which causes a change in potential which triggers the next channel to open and so on

sodium

high concentration outside, low concentration inside

potassium

high concentration inside, low concentration outside

proteins

stuck inside

net charge inside cell

negative (because more Na+ outside than K+ inside)

net charge outside cell

positive (because more Na+ outside than K+ inside)

establishing resting membrane potential

K+ ions diffuse down their concentration gradient out of the cell, negatively charged proteins move towards membrane which attracts potassium to move back inside (creates electric charge)

resting membrane potential

reached when point of equilibrium is reached, number of potassium going out of cell due to concentration gradient equals number of potassium going in due to electrical gradient

electrical excitability of neurons

neurons are electrically excitable due to resting membrane potential

two types of electric signals

graded and action potentials

graded potentials

cause change in membrane potential that is localized to one area of the plasma membrane (doesn't move)

graded potential function

allow communication over short distances only, use ligand-gated and mechanically-gated ion channels (e.g. if you send a chemical to a single gate, the gate opens and causes a change in that one area, the more gates you open the more change occurs - size of change can vary)

action potentials

travel along axons

action potential function

allow communication over short and long distances, depends on length of axon, uses voltage gated channels (responds to change in membrane potential, once once is triggered all other will be triggered to open as well)

graded potential origin

occur from cell body and dendrite regions and specialized sensory receptors

effect of graded potentials on resting potential

cause small deviations form resting potential of -70mV, deviations can cause voltage to get closer or further away from zero (hyperpolarization or depolarizaton)

hyperpolarization

e.g. -70 mV to -75 mV, becoming more negative, making difference bigger

hyperpolarization causes

opening potassium channel (K+ ions flow out, takes away positive charge, membrane potential becomes more negative), opening a chloride channel (Cl- ions flow in, adding more negative, membrane potential becomes more negative)

depolarization

e.g. -70 mV to -60 mV, becoming more positive, moving closer to zero

depolarization causes

can occur by opening a sodium channel (Na+ ions rush into cell, brings in more positive, membrane potential becomes more positive), opening a calcium channel (Ca2+ ions rush in, brings in more positive, membrane potential becomes more positive)

amplitude of graded potential

depends on stimulus strength, if you open more gates the change in charge is greater

summation

adding together multiple graded potentials to become larger in amplitude, about timing of when signals are sent (the more frequent the signals are sent the bigger the graded potential can be)

ways to increase graded potentials

sending multiple fast signals of one type, sending many different signals at once

action potential origin

voltage gated channels start at trigger zone, where action potential starts

trigger zone

where voltage gated channels start

threshold

amount of change in membrane potential needed for voltage gated channels to open, when threshold is reached an action potential is created, graded potential must depolarize in order to reach threshold

first phase in action potential

depolarization, membrane potential goes from negative on inside to positive on inside

second phase in action potential

repolarization, membrane potential becomes negative again, initial conditions are reset

third phase in action potential

after hyperpolarizing phase, restores membrane potential

all or none principle

action potential either fires or doesn't fire, there is no in between, once one voltage-gated channel is triggerd to open all the channels are triggered

magnitude of action potential

action potential spreads over surface of cell without dying out, magnitude stays the same the whole time

sodium channel

has two gates

potassium channel

has one gate

resting membrane potential ion gate configurations

inactivation gate of sodium channel is open and activation is closed (Na+ cannot get in), voltage-gated K+ channel is closed

depolarizing phase of action potential

graded potential reaches threshold, voltage gated Na+ channels open and Na+ rushes into the cell (inside of cell now has positive charge)

repolarizing phase of action potential

Na+ inactivation gates close (no more sodium can enter), K+ channels open, K+ leaves and returns membrane potential back to -70 mV, Na+ activation gate closes and inactivation gate reopens (important because antoher action potential cannot be triggered again until it is in its resting condition)

after hyperpolarizing phase of action potential

if enough K+ ions leave the cell it will cause hyperpolarization, hyperpolarization makes it difficult to send another signal, membrane potential drops to approximately -90 mV, K+ channels close and resting potential of -70 mV will be restored (sodium potassium pump and leak channels restore resting potential)

refractory period of action potential

dictates how fast we can create another action potential, during this period neuron cannot generate another action potential

maximal stimulus

when stimulus are firing as fast as the absolute refractory period

absolute refractory period

comprised of depolarizing and repolarizing phases, no matter what happens another action potential will not be sent

why will a maximum stimulus not begin another action potential

Na+ activation gates must return to their resting state, if not back at resting position nothing can occur

relative refractory period

creates a larger than typical threshold, suprathreshold stimulus is requried to start an action potential, K+ channels open but Na+ channels closed

suprathreshold stimulus

any stimulus greater than threshold

when can action potentials occur

can only occur if membrane potential reaches threshold, signals can be ignored by hyperpolarizing a membrane (this will stop signals from being sent as graded potentials will not reach threshold)

subthreshold stimulus

will cause graded potential but not action potential

what affects signal speed

the closer the action potentials are to each other the faster the signals can go

propagation

spreading of action potential over surface of axon, signal doesn't physically move it spreads

propagation function

as Na+ flows into the cell during depolarization the voltage of adjacent areas is effected and their voltage-gated Na+ gates open, membrane potentials in surrounding axons are changed as well (signal is self-propagating)

continuous conduction process

action potential occurs at one spot (trigger zone) on a membrane but has ability to propagate by stimulating adjacent regions, once action potential stimulates another in an adjacent location (same initial action potential isnt moving it is stimulating other action potentials)

direction of action potentials

spread in one direction only due to refractory period (cannot move backwards)

continuous conduction

propagation in unmyelinated axons

local current

movement of positive ions, localized to where current is initially triggered, helps trigger neighboring voltage-gated ion channels to open

saltatory conduction

propagation in myelinated axons

saltatory conduction process

action potential is conducted from one node of Ranvier to another, current jumps from one node to the next (skips unmyelinated regions)

nodes of ranvier

where voltage gated Na+ channels are concentrated, more channels allow more to open at once which leads to mroe ions flowing and increased speed of depolarization

leaping effect

increases speed of flow of action potentials, only have to depolarize and nodes of ranvier so more energy efficient

factors that affect propagation speed

axon diameter, amount of myelination, temperature

axon diameter

larger diameter gives a larger surface area which means more voltage-gated ion channels and therefore faster depolarization

amount of myelination

more myelination = more leaping

temperature

voltage gated channels are proteins, protein configuration can be affected by temperature which can change speed of action potentials

type A nerve fiber

large diameter, myelinated, has fastest propagation speed (conducts as 12-130 m/s)

type A nerve fiber locations

found in motor neurons supplying skeletal muscle and most sensory neurons, in areas that require quick recognition and response

type B nerve fiber

medium diameter, lightly myelinated, conducts at 3-15 m/s

type B nerve fiber locations

part of ANS (skeletal muscle, heart, etc.)

type C nerve fiber

small diameter, unmyelinated, has slowest propagation speed (conducts at 2 m/s or less)

type C nerve fiber locations

part of ANS