WEEK 1 LECTURE NOTES

symporter vs antiporter

symporter - Moves two or more different molecules in the same direction across the membrane.

antiporter - Moves two or more different molecules in opposite directions across the membrane.

what do pumps do?

establish ion gradients!

higher extracellular concentrations of Na+, Ca2+, and Cl-

• higher intracellular concentrations of K+ and anions

what are the forces that drive ion movement?

chemical and electrical driving force

chemical driving force

diffuse down its concentration gradient (high to low)

electrical driving force

like charges repel, opposites attract

why does ion flow matter?

the flow of ions drives many important physiological processes, and predicting the directionality of ion flow allows us to infer function

membrane potential

a measure of the charge difference between the inside and outside of the cell.

what does a negative or positive membrane potential mean?

a negative membrane potential means the inside is more negative than the outside.

equilibrium potential

The membrane potential of an individual ion at which the chemical and electrical driving forces are equal and opposite

nernst equation

calculates the equilibrium potential (𝐸_ion) of an ion.

→ Assumes that the plasma membrane is ONLY permeable to the ion under consideration, and not to any others

standard physiological concentrations

Goldman-Hodgkin-Katz (GHK) equation

calculates the membrane potential (𝑉m) for a cell that is permeable to multiple ions at a time

nuerotransmitters and their importance

released by the presynaptic neuron typically cause gated ion channels to open

in the membrane of the postsynaptic neuron.

how to predict driving forces and ion movement

compare the electrical and chemical driving forces and ions

how to predict overall direction of ion flow

opening an ion channel will drive the membrane potential towards that ion’s equilibrium

flux

the number of ions that cross the membrane per unit time

ohms law

the equation(s) to predict flux

depolarization

Making the membrane potential less negative, also called an excitatory post-synaptic

potential (EPSP)

hyperpolarization

Making the membrane potential more negative, also called an inhibitory post-synaptic

potential (IPSP)

graded potentials

short-distance, variable-strength local signals that occur in the dendrites and cell body

EPSPs and IPSPs

what does initial segment of axon do?

integrates all the graded potentials

spatial vs temporal summation

temporal - epsp summation over time

spatial - epsp summation over space

features of graded potentials

→ as graded potentials spread throughout the neuron, their amplitude decays

→ Graded potentials closer to the initial segment will more strongly influence

whether a neuron fires.

current leak

some ions leak out of the cell through leak channels

graded potentials + inital segment =

→ Graded potentials are summated at the initial segment.

→ If the initial segment reaches threshold, the neuron fires an action potential.

→ strong or weak EPSPs don’t matter as long as threshold is met

where are all channels located

ligand → cell body and dendrites

leak → everywhere

voltage gated → axon and initial segment

AP phases: before threshold

at rest

AP phases: depolarization phase

first stage, Na + channel opens and Na + enters, towards AP peak

AP phases: repolarization phase

second stage: Na + channel inactivates, K + channel opens and K + leaves cell

AP phases: hyperpolarization/undershoot phase

last stage: Na+ channel resets back to closed, K+ channel remains open, until closes after delay

absolute refractory period

a window of time in which is it physically impossible for that neuron to generate another action potential, even if depolarized back to threshold

→ na+ channels open, and then inactive,

relative refractory period

It is possible to generate another action potential, but a larger-than-usual stimulus

is needed. during hyperpolarization phase

how are AP’s conducted

Action potentials are conducted along the length of the axon.

→ They spread down the axon without any decay in amplitude

How do action potentials retain the same amplitude?

through continous conduction! When one segment of the axon fires an action potential, it depolarizes the next segment along so voltage-gated Na⁺ channels in the next segment reach threshold and fire an action potential

continous conduction

action potential is continuously regenerated at every point.

Why donʼt action potentials go backwards?

After one segment of axon generates an action potential, it becomes refractory.

→ The action potential can only spread forward, because the region behind it is refractory

myelin

a sheath of fatty insulation that is wrapped around axons

where is myelin made in PNS and CNS

PNS - schwann cells
CNS - oligondendrocytes

why is myelin useful in AP propogation

speeds up process

prevents current and ions from leaking out

nodes of ranvier

segments of myelin separated by gaps that contain voltage-gated Na+ channels

how are nodes and myelin useful?

When one node fires an action potential, the depolarization spreads to the next node.

→ Myelin prevents ions from leaking out between the nodes.

saltatory conduction

Action potential is regenerated from node to node.