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Monday April 7, 2014. Introduction to the nervous system and biological electricity 1. P re -lecture quiz 2. A word about prelecture readings 3. Introduction to the nervous system 4. Neurons and nerves 5. Resting membrane potential. A word about the readings.

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slide1

Monday April 7, 2014.

Introduction to the nervous system and biological electricity

1. Pre-lecture quiz

2. A word about prelecture readings

3. Introduction to the nervous system

4. Neurons and nerves

5. Resting membrane potential

a word about the readings
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)
slide3

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

slide4

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).

slide5

Mary Shelley wrote Frankenstein

in 1818 long before we knew

about neurons.

Why did she choose to use

electricity to bring Frankenstein

to life?

She knew about the work by

Galvani on frog legs.

slide6

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 incorrect

but he is credited with the important insight that animals use electricity in nerve

and muscle cells.

slide8

The brain integrates sensory information and sends signals

to effector cells.

Sensory neuron

CNS (brain

 spinal cord)

Sensory receptor

Interneuron

Motor neuron

(part of PNS)

Effector cells

examples of sensory receptors in vertebrates
Examplesof 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
slide10

Information flow through neurons

Nucleus

Dendrites

Collect

electrical

signals

Cell body

Integrates incoming signals

and generates outgoing

signal to axon

Axon

Passes electrical signals

to dendrites of another

cell or to an effector cell

an introduction to membrane potentials
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 cells
Electrical Properties of Cells
  • All cells maintain a voltage difference across their membranes (Emembrane):
  • Two factors a required to establish a membrane potential
  • There must be a concentration gradient for an ion
  • The membrane must be somewhat permeable to that ion

Outside of cell

Microelectrode

0 mV

K channel

–65 mV

Inside of cell

slide16

A quick lesson from physics . . .

freely permeable membrane

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

[Na+]

high

[Na+]

low

How much force (voltage) is required to

maintain the imbalance?

Answer: Nernst Equation

slide17

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

freely permeable membrane

Nernst Equation

[Na+]

high

[Na+]

low

E=voltage

R=gas constant

T=temperature in Kelvin

F=faraday’s constant (charge carried by mole of an ion)

Z = valance (1 for Na+, -1 for Cl-)

X1 and X2 are concentrations in the

two sides.

slide18

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

freely permeable membrane

Nernst Equation

[Na+]

high

[Na+]

low

E=voltage

R=gas constant

T=temperature in Kelvin

F=faraday’s constant (charge carried by mole of an ion)

Z = valance (1 for Na+, -1 for Cl-)

X1 and X2 are concentrations in the

two sides.

Altered under physiological conditions

Unaltered under physiological conditions

slide19

freely permeable membrane

compartment 1

[Na+]

high

compartment 2

[Na+]

low

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?

slide20

freely permeable membrane

compartment 1

[Na+]

high

compartment 2

[Na+]

low

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

slide21

Outside of cell

Microelectrode

0 mV

K channel

–65 mV

Inside of cell

slide23

Outside of cell

Increasing [K+] outside the neuron

Microelectrode

0 mV

Equilibrium!

K channel

Increasingly negative

charge inside the neuron

–65 mV

Inside of cell

animation of resting potential
Animation of resting potential
  • https://www.youtube.com/watch?v=YP_P6bYvEjE
slide26

Calculating the total resting potential – the Goldman Equation

The Goldman Equation extends the Nernst Equation to consider the relative

permeabilities of the ions (P): Ions with higher P have a larger effect on Emembrane

at 20° C

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