ch03

THE NEURAL MEMBRANE

AT REST

ELECTRICAL PROPERTIES

Simple reflex : information needs to be quickly transmitted to the CNS and back Information is transmitted through action potentials (change in the electrical properties of the membrane)

Cells able to generate an AP have excitable membrane At rest, these cells have a inside negative electrical charge (resting membrane potential) that become positive during the AP

CYTOSOLIC AND EXTRACELLULAR FLUID

Water is the key ingredient in intracellular and extracellular fluid

Key feature – uneven distribution of electrical charge (O has a net negative

charge)

Ions are atoms or molecules with a net electrical charge dissolved in the water

Salz for example is a crystal of Sodium (Na+) and Chloride (Cl-)

Monovalent Ion: Difference between protons and electrons =1,

Divalent Ion: Difference between protons and electrons =2,

cation (+), anion (-)

When the crystal breaks down spheres of

hydration -layer of water are attracted to the ion

The orientation of the water molecules is

determined by the valence of the ion

IONS INVOLVED IN CELLULAR PHYSIOLOGY

Sodium

Potassium

+

+

Calcium

Chloride

2+

-

THE PHOSPHOLIPID MEMBRANE

Hydrophilic: Dissolve in water due to uneven electrical charge (e.g., salt,

proteins, carbohydrates)

Hydrophobic: Does not dissolve in water due to even electrical charge (e.g., oil,

lipids in general)

The Phospholipid Bilayer

Hydrophilic

Hydrophobic

Resting and Action potentials depend on special proteins that are inserted in the

membrane

THE PROTEIN

Proteins are molecules assembled by combination of different amino acids (20 types)

Central alpha

carbon

R group

Amino group Carboxyl group

THE PROTEIN STRUCTURE

Primary

Secondary

Tertiary

Quaternary

Peptide bond

CHANNEL PROTEINS

Ion Channels

They form a pore through the membrane that

is ion selective

They can be opened and closed (gated)

by changing in the local microenvironment

of the membrane

hydrophilic

hydrophobic

Ion Pumps

Formed by membrane spanning proteins

Uses energy from ATP breakdown

Neuronal signaling

THE MOVEMENT OF IONS

Diffusion: movement of ion due to concentration levels Dissolved ions tend to distribute evenly by following down concentration gradient Concentration gradient = difference of concentration of an ion across the membrane

Electricity Electrical current (I, measured in Amperes) represents ion movement.

It’s regulated by

1) electrical conductance (g, measured in Siemens) or electrical

resistance (R, measured in Ω): ability (or inability) of an electrical

charge to migrate from one point to another

2) electrical potential (V, measured in volts): difference in charge

between cathode and anode

THE MOVEMENT OF IONS

Electrical current flows across the membrane by

Ohm’s law relationship

I =gV or I =V/R

Membrane potential: Voltage across the

neuronal membrane.

The resting potential is typically -65 mV

…let’ see why…

EQUILIBRIUM POTENTIAL

Example 1

Example 2

Equilibrium is reached when

diffusional and electrical

forces are equal and opposite

(equilibrium potential, Eion)

MEMBRANE POTENTIAL

In the membrane ions have different concentration between inside and outside,

and this gradient is established by action of ionic pumps, that use energy in

order to move ions against concentration forces

Membrane permeability determines membrane resting and action potentials

MEMBRANE POTENTIAL

Membrane permeability determines membrane resting and action potentials

Membrane rest potential is determined by the higher number of K vs. Na channels

open (resting potential close to Ek potential)