By Dr Irum Junaid

• Cell Body

• Dendrites (input structure) receive inputs from other

neurons

perform spatio-temporal integration of inputs

relay them to the cell body

• axon (output structure) a fiber that carries messages

(spikes) from the cell to dendrites of other neurons

The cell membranes of all the excitable body cells in the resting condition are, polarized which means that they show an electrical potential difference.

Membrane potential refers to a separation of charges across the membrane or a difference in the relative number of cations and anions in the ICF and ECF.

Diffusion potential Equilibrium potential

Ion Inside Outside Cross PM

K+ 140 4 yes

NA+ 14 142 no

Cl- 5 125 yes

H2O 55,000 55,000 yes

Anion- 108 0 no

Nernst equilibrium? For any univalent ion at body

temperature of 37° C EMF (mV)= ±61log (Conc. inside/Conc.

outside) Calculate for K+ and Na+

K= -61log(140/4) Na= -61log(14/142) Sign is –ve shows the polarity inside the cell.

If Ko = 5 mM and Ki = 140 mM

EK = -61 log(140/4)

EK = -61 log(35)

EK = -94 mV

If Nao = 142 mM and Nai = 14 mM

EK = -61 log(14/142)

EK = -61 log(0.1)

EK = +61 mV

a. Concept of ‘Selective membrane’ b. How permeable the membrane is to proteins,

K+, and Na+c. Diffusion and electrostatic forces and how they

act on K+ and Na+d. Concept of ‘Dynamic equilibrium’e. Concept of ‘Membrane potential’ f. ATP Na/K pump and its role in maintaining the

membrane potential

Polarity of each ion Membrane permeability of the ions Concentrations of respective ions on

both sides: (i= inside), (o= outside)

12

OUTSIDE

INSIDE

K+ = Potassium; Na+ = Sodium; Cl- = Chloride; Pr- = proteins

Na+

Na+

K+

K+

Force of Diffusion

Electrostatic Force

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - -

Cl-

Force of Diffusion

Cl-

Electrostatic Force

Pr-

Closedchannel

openchannel

openchannel

nochannel

3Na/2Kpump

Resting Membrane Potential

- 65 mV

Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-).

The resting potential exists because ions are concentrated on different sides of the membrane. Na+ and Cl- outside the cell. K+ and organic anions inside the cell.

inside

outsideNa+Cl-Na+

K+

Cl-

K+

Organic anions (-)

Na+Na+

Organic anions (-)

Organic anions (-)

Electrogenic pump Maintains Concentrationgradient Contributes -4mV.

These are rapid transient changes in the membrane potential that spread in the form of a chain reaction rapidly along the nerve fiber membrane .

Action PotentialsCan travel up to100 meters/second

Usually 10-20 m/s0.1sec delay between muscle and sensory neuron action potential

When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open.

Sodium ions rush in. The membrane potential changes from -70mV to

+40mV.

Na+

Na+

Na+

-

+

+

-

Sodium ion channels close and become refractory. Depolarization triggers opening of voltage-gated

potassium ion channels. K+ ions rush out of the cell, repolarizing and then

hyperpolarizing the membrane.

K+ K+

K+Na+

Na+

Na+

+

-

#1 Triggered by depolarization

a less negative membrane potential that occurs transiently

Threshold depolarization needed to trigger the action potential

10-20 mV depolarization must occur to trigger action potential

Are all-or- none event Amplitude of AP is the same regardless of

whether the depolarizing event was weak (+20mV) or strong (+40mV).

Propagates without decrement along axon

The shape (amplitude & time) of the action potential does not change as it travels along the axon

At peak of action potential the membrane potential reverses polarity

Becomes positive inside as predicted by the Ena Called OVERSHOOT

Return to membrane potential to a more negative potential than at rest

Called UNDERSHOOT

Absolute refractory period follows an action potential. Lasts 1 msec

During this time another action potential CANNOT be fired even if there is a transient depolarization.

Limits firing rate to 1000AP/sec

The number of voltage gated sodium channels per square micrometer of the membrane in mylinated neuron is;

A. cell body 50-75B. initial segment 350-500C. on the surface of the myelin <25D. Node of the ranvier 2000-12000E. axon terminal 20-75 Unmylinated neuron: 110

Copyright © Allyn & Bacon 2004

Membrane Potentials

1. Resting Potential (just described)

2. Excitatory Post-synaptic potential

threshold4. Inhibitory Post-synaptic potential

3. Action Potential

Increase internal diameter of axon which decreases the internal resistance to ion flow

Increase the resistance of the plasma membrane to charge flow by insulating it with myelin.

Sodium:Decreasing the external Na+ concentration reduces the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted, since the permeability of the membrane to Na+ at rest is relatively low.

Potassium:Conversely, increasing the external K+ concentration decreases the resting membrane potential.

Calcium:

It is possible to determine the minimal intensity of stimulating current (threshold intensity) that, acting for a given duration, will just produce an action potential.

Action potential fails to occur if the stimulus is subthreshold in magnitude,produces graded potentials.

Suprathreshold stimuli produce action potential during relative refractory period.