Monday April 7, 2014.

Introduction to the nervous system and biological electricity

1. Pre-lecture quiz2. A word about prelecture readings3. Introduction to the nervous system4. Neurons and nerves5. Resting membrane potential

A word about the readings

• Today’s readings were fine. (section 45.1 – Principles of electrical signaling is good background)

• Wed: pp. 891-898 (section 45.2 – dissecting action potentials & 45.3 The synapse).

• Fri: pp. 899-904 (section 45.4 The vertebrate nervous system)

The ability of animals to respond RAPIDLY to the environment and to move is due to the electrical properties of neurons and muscles.

Venus flytrap can send a signal to close that travels 1 to 3 cm/s.

Action potentials along neurons travel up to 100 meters per second (or 10,000 cm/s).

Mary Shelley wrote Frankensteinin 1818 long before we knew about neurons.

Why did she choose to use electricity to bring Frankenstein to life?

She knew about the work byGalvani on frog legs.

Galvani &frogs legs

Galvani showed the applying a current to a frog nerve could make the muscles twitch. Previously, folks thought that nerves were pipes or tubes. Galvani introduced the idea “biological electricity”. Many of his speculations were incorrectbut he is credited with the important insight that animals use electricity in nerveand muscle cells.

Central vs. peripheral nervous system

The brain integrates sensory information and sends signalsto effector cells.

Sensory neuron

Sensory receptor

CNS (brain spinal cord)

Interneuron

Motor neuron(part of PNS)

Effector cells

Examples of sensory receptors in vertebrates

• Nocirecptors = pain stimuli• Thermorecptors = changes in temperature• Mechanoreceptors = changes in pressure• Chemoreceptors = detection of specific molecules• Photoreceptors = detection of light• Electroreceptors = detection of electric fields• Magnetoreceptors = detection of magnetic fields

Information flow through neurons

Nucleus

DendritesCollectelectricalsignals

Cell bodyIntegrates incoming signalsand generates outgoingsignal to axon

AxonPasses electrical signalsto dendrites of anothercell or to an effector cell

Nerves are bundles of neurons surrounded by connective tissue

Neurons form networks for information flow

An introduction to membrane potentials

• A difference of electrical charge between any two points creates a difference in electrical potential, or a voltage.

• Ions carry a charge, and in virtually all cells, the cytoplasm and extracellular fluid contain unequal distributions of ions. Therefore, there is a separation of charge across the membrane called a membrane potential.

• Membrane potentials are a form of electrical potential and are measured in millivolts (mV). In neurons, membrane potentials are typically about 70–80 mV.

• A flow of charged ions is an electric current.

Electrical Properties of CellsAll cells maintain a voltage difference across their membranes (Emembrane):

Two factors a required to establish a membrane potential

1.There must be a concentration gradient for an ion

2.The membrane must be somewhat permeable to that ion

Outside of cell

Inside of cell

Microelectrode0 mV

– 65 mV

K channel

A quick lesson from physics . . .

[Na+] low

[Na+] high

freely permeable membrane

With a permeable membrane, it takes force to keep the distribution of ions.

How much force (voltage) is required tomaintain the imbalance?

Answer: Nernst Equation

A quick lesson from physics . . . (see Box 45.1)

[Na+] low

[Na+] high

freely permeable membrane

Nernst Equation

E=voltageR=gas constantT=temperature in KelvinF=faraday’s constant (charge carried by mole of an ion)Z = valance (1 for Na+, -1 for Cl-)X1 and X2 are concentrations in thetwo sides.

A quick lesson from physics . . . (see Box 45.1)

[Na+] low

[Na+] high

freely permeable membrane

Nernst Equation

E=voltageR=gas constantT=temperature in KelvinF=faraday’s constant (charge carried by mole of an ion)Z = valance (1 for Na+, -1 for Cl-)X1 and X2 are concentrations in thetwo sides.

Altered under physiological conditions

Unaltered under physiological conditions

[Na+] low

[Na+] high

freely permeable membrane

compartment 2 compartment 1

Assume the only thing changing are the concentrations of Na+ in the two compartments and consider the following scenarios.

Scenario 1: [Na+] in compartment 1 = 500mM, [Na+] in compartment 2 = 50mM

Scenario 2: [Na+] in compartment 1 = 700mM, [Na+] in compartment 2 = 50mM

Which one of the two scenarios results in a larger value for E?

[Na+] low

[Na+] high

freely permeable membrane

compartment 2 compartment 1

Scenario 1: [Na+] in compartment 1 = 500mM, [Na+] in compartment 2 = 50mM

log (500 / 50) = 1

Scenario 2: [Na+] in compartment 1 = 700mM, [Na+] in compartment 2 = 1.46

log (700 / 50) = 1.146

Outside of cell

Inside of cell

Microelectrode 0 mV

– 65 mV

K channel

Outside of cell

Inside of cell

Microelectrode 0 mV

– 65 mV

K channel

Increasingly negative charge inside the neuron

Increasing [K+] outside the neuron

Equilibrium!

Animation of resting potential

• https://www.youtube.com/watch?v=YP_P6bYvEjE

at 20° C

Homework: Calculate the membrane potential for each of the three ions below.

Inside cell Outside cell

[K+] 400 mM 20 mM

[Na+] 50 mM 440 mM

[Cl-] 51 mM 560 mM

[ ] [ ] [ ]58 *log

[ ] [ ] [ ]

K o Na o Cl i

membrane

K i Na i Cl o

P K P Na P ClE mV

P K P Na P Cl

at 20° C

The Goldman Equation extends the Nernst Equation to consider the relativepermeabilities of the ions (P): Ions with higher P have a larger effect on Emembrane

Calculating the total resting potential – the Goldman Equation

Permeabilities change during an action potential and how this allows neurons to “fire”.