3.1 MEASURE OF EXCITABILITY. CHANGES IN EXCITABILITY DURING EXCITATION. INHIBITION. CONDUCTION OF EXCITATION.

section

define excitation

process when there is an electrical stimulus

action potential

saltatory conduction

inhibition

define excitation

• excitable cells react to irritation or stimulation

• excitation: process of producing an action potential

process when there is an electrical stimulus

• when Na+ diffuse inward through the membrane in sufficient numbers can set off automatic opening of the sodium channels.

• This can be from mechanical disturbance of the membrane, chemical effects on the membrane, or passage of electricity through the membrane.

• A weak or negative electrical stimulus may not be able to excite a fibre.

• However, when the voltage of the stimulus is increased, excitation takes place when threshold reached → threshold or super threshold stimulus

• At point A – stimulus is too weak, causes membrane potential to change from –95 mv to –85 mv - subthreshold stimulus = local response

• At point B, the stimulus is greater, but the intensity is not enough. Causes local response.

action potential

Measure of excitability at rest there is no action potential stimulus above threshold causes depolarisation

• when nerve is resting there is no action potential between electrodes A and B, oscilloscope ray is on zero line

• Then electric stimulation of adequate power is applied

• when depolarisation reaches electrode A, outer surface of excited fibers in contact with it, become electronegative relative to resting fibers in contact with electrode B

• Due to resulting potential difference, ray moves upwards

• when excitation reaches electrode B, potential difference disappears and ray is on zero line

• when outer surface is in contact with electrode A is repolarised it becomes electropositive, relative to electrode B sectiona dn ray moves downards

action potential simply

• depolarisation = transient increase in membrane permeability to Na^+ so membrane becomes more positive Na^+ channels open

• Repolarisation = closing of Na^+ channels and opening of K channels, membrane becomes negative again → over shoots resting membrane potential = hyperpolarisation so Na/K pump brings back to RMP

saltatory conduction

• AP’s are unable to pass through myelin sheath of myelinated neurons

• there are gaps between myelin sheath that expose the membrane - nodes of Ranvier

• Impulse jumps across axons - saltatory conduction, increases velocity of nerve transmission

inhibition

• Membrane-stabilizing factors can decrease excitability.

• e.g., a high extracellular fluid calcium ion concentration

• decreases membrane permeability to sodium ions and simultaneously reduces excitability.

• Therefore, calcium ions are said to be a “stabilizer.”

• Tetrodotoxin is a sodium channel blocker-inhibits firing of AP