Excitable Tissues- Nerve

Prof. K. Sivapalan

Nerve 2

Neuron

• There are many types of cells.

• Neuron is the main functional cell in the nervous system.

• Cell body and processes: axon and dendrites

• Axons transmit impulses from the cell body, covered by Schwan cells- Myalin sheeth and Nodes of Ranvier

• Cutaneous nerves- exceptionJune 2013

Nerve 3

Membrane Potential

• When two electrodes are placed, one inside and the other outside, a potential difference is observed.

• It is ‘resting membrane potential’

June 2013

Nerve 4

Recording Electrical Events- Oscilloscope

June 2013

Nerve 5

Basis of Resting Membrane Potential:Gibbs–Donnan effect• Cell membrane does

not conduct electricity

• Both sides of the membrane contains cations and anions

• Based on the permiability of the individual ions, imbalance of electrical and chemical neutrality is generated

June 2013

Nerve 6

Physics of Membrane Potential

• Diffusion potential is generated by ion concentration difference on both sides of the membrane

• Nernst equation:

June 2013

Nerve 7

EMF with Many Ions

• Goldman-Hodgkin-Katz equation:

• C- Concentration, P-Permiability

June 2013

Nerve 8

Role of Sodium Potassium Pump

June 2013

Nerve 9

Change in Resting Membrane Potential

• The resting membrane potential can be disturbed by external electrodes or mechanical / chemical factors that alter membrane permeability.

• If the potential changes towards zero, it is hypo polarization and if it becomes more negative, it is hyper polarization- Local response.

• This change is proportional to the strength of the stimulus

June 2013

Nerve 10

Local Response

June 2013

Nerve 11

Action Potential

• When the local response goes above -55 mV [firing or threshold level], the potential automatically goes to +35 [Depolarization] and immediately returns to the resting value [Repolarization]

• It is known as ‘Action Potential’

June 2013

Nerve 12

Ionic Basis of Action Potential• Membranes of nerves has voltage gated sodium and

potassium channels.

• When the threshold level is reached, voltage gated sodium channels open and close almost immediately.

• As the channels open, sodium ions rush inwards taking positive charge inwards reversing the membrane potential- Depolarization.

• Very soon voltage gated potassium channels open allowing the potassium ions out resulting in positive charge going out – Repolarization

• Sodium – Potassium (ATP ase) pump and diffusion of Potassium ions bring the ionic composition back to normal

June 2013

Nerve 13June 2013

Nerve 14

Components of Action Potential

• Resting membrane potential

• Firing level

• Depolarization

• Repolarization

• After hyperpolarization

• Refractory period

• Energy requirements

June 2013

Nerve 15

Spread of Action Potential• Local response affects only the

area stimulated.

• When the membrane has an area with +ive charge outside and –ive inside, it sets up flow of current which alters the potential difference of the adjacent area

• When it is due to action potential , it is sufficient to bring the potential to the threshold level and action potential is generated.

• A wave of depolarization spreads across the membrane by auto stimulation of the adjacent area followed by repolarization

June 2013

Nerve 16

Conduction in Myelinated Nerve

• The myelin sheeth is an insulator and it does not permit exchange of ions across the membrane.

• The nodes of Ranvier contains exposed membrane with voltage gated channels for sodium and potassium.

• When one node of Ranvier is depolarized, it stimulates the next node.

• This results in action potentials jumping from one node to the next by passing the membrane covered by myelin [saltatory conduction].

• When compared to non-myelinated nerve, myelinated nerve conducts the impulse faster

June 2013

Nerve 17

Myelination

• The Schwann cell envelops the axon and the cell raps round several times.

• Each cell is 1-3 mm in length

• The exposed axon in between is about 2-3

• Energy need- less

June 2013

Nerve 18

Velocity of Conduction

• Myelination- faster

• Larger diameter- faster

• Range- 0.25 to 100 M/Sec

June 2013

Nerve 19

Fiber Types in Mammalian Nerve

June 2013

Nerve 20

Susceptibility of Nerves

Susceptible to:

Most Intermediate Least

Hypoxia B A CPressure A B C

Local anaesthetics

C B A

June 2013