Transmission of nerve impulses

Uploaded byMd. Atai Rabby

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OUR MAJOR CONCERN----

• Nerve Impulse• Role of Na+ & K+ ions • Membrane Potential & Resting Potential• Polarization & Depolarization• Action Potential• Repolarisation & Hyperpolarisation

Transmission of nerve impulses• Neurones transmit impulses as electrical signals• These signals pass along the cell surface membrane

of the axon as a nerve impulse• It is NOT the same as an electric current passing

down a wire (which is much faster)• The mechanisms is the same throughout the animal

kingdom.

Na+ & K+ ions---

• The two important ions in a nerve cell (neurone or neuron) are K+ and Na+

• Both are cations (positively charged ions)• Sodium ions and potassium ions are transported

across the membrane against their concentration gradients by active transport

• The Na+ ions are passed out faster (active transport by Na+-pump) than the K+ ions are bought in.

• Approx. three Na+ ions leave for every two K+ ions that enter.

Active transport via Na+-K+ pump

Axon membrane

Inside axon

outside axon

Continue---• K+ ions can also diffuse back out quicker than Na+

ions can diffuse back in. Na+ ions move more slowly across the membrane than K+ or Cl- ions. This is because although the Na+ ion is smaller than the K+ ion, Na+ has a larger coating of water molecules giving it a bigger diameter. This makes the plasma membrane 25 times more permeable to K+ than Na+

• Net result is that the outside of the membrane is positive compared to the inside.

Membrane Potential/ Resting Potential/ Depolarisation• K+ ions slowly leak through K+ pore channels

• The membrane has a poor permeability to Na+ ions so they cannot get in to the neurone

• This brings about the membrane potential of neurones

• As the K+ leaks out, the inside of the resting cell becomes more negatively charged

• Neurones like other cells are more negatively charged inside than outside

Continue----

• This results in a membrane potential .• This difference in potential is called the resting

potential and is typically about between -70 mV to -80 mV

• In this resting state the axon is said to be polarised.

Getting Excited• As the neurone’s membrane at rest is more negative

inside than outside, it is said to be polarised • Neurones are excitable cells • The cells are excited when their membranes become

depolarised (An Action Potential is generated) • Depolarising membranes may be achieved by:• a stimulus arriving at a receptor cell

(e.g. vibration of a hair cell in the ear)• a chemical fitting into a receptor site

(e.g. a neurotransmitter)

Generation of Action Potential

• A nerve impulse travelling down a neurone. A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation.

• The action potential is the state of the neurone membrane when a nerve impulse passes by

• A small change in the membrane voltage will depolarise the membrane enough to flip open Na+ channels

• These are called voltage-gated Na+ channels • As Na+ moves into the cell more and more Na+ channels

open

Generation of Action Potential

• A small change in the membrane permeability to Na+ results in a big change in membrane potential.

• This is because the volume of the axon is minute compared to the volume of the extracellular fluid

• When the axon is stimulated the resting potential changes.

• It changes from –70 mV inside the membrane to +30 mV.

• For a very brief period the inside of the axon becomes positive and the outside negative.

Generation of Action Potential

• This change in potential is called the Action Potential and lasts about 0.5 to 3 milliseconds.

• When an action potential occurs, the axon is said to be Depolarised.

• When the resting potential is re-established the axon membrane is said to be Repolarised

depolarisation

depolarisation

reploarisation

depolarisation

reploarisation

‘overshoot’

depolarisation

reploarisation

‘overshoot’

direction of impulse

Repolarisation • Potassium channels open in the membrane and K+

ions diffuse out along their concentration gradient.• This starts of repolarisation • At the same time, sodium channels in the

membrane close preventing any further influx of Na+ ions.

• The resting potential is re-established as the outside of the membrane becomes positive again compared to the inside.

Repolarisation

• So many K+ ions leave that the charge inside becomes more negative that it was originally.

• This shows up as an ‘overshoot’. • The potassium channels close and the sodium-

potassium pump starts again.• Normal concentrations of sodium and potassium

ions is re-established.• The membrane is once again at its resting potential.

Hyperpolarisation• The membrane potential falls below the

resting potential of –70mV • It is said to be hyperpolarised (overshoot)• Gradually active pumping of the ions (K+ in

and Na+ out) restores the resting potential• During this period no impulses can pass along

that part of the membrane• This is called the refractory period

direction of impulse

+ + + + + + + +

+ + + + + + + +

- - - - - - - -

- - - - - - - -

a) In the resting axon, there is a high conc. of Na+ ions outside and a high conc. of K+ ions inside. But the net effect is that the outside is positive compared to the inside giving the resting potential

+ + + + + + + -

+ + + + + + + -

- - - - - - - +

- - - - - - - +

b) The axon is stimulated producing an action potential, setting up local circuits on the axon membrane

Leading edge of impulse

+ + + + + + - -

+ + + + + + - -

- - - - - - + +

- - - - - - + +

c) Sodium ions rush into the axon along a diffusion gradient depolarising the membrane causing an action potential

direction of impulse

Na+

Na+

+ + + + + + - -

+ + + + + + - -

- - - - - - + +

- - - - - - + +

d) As the action potential passes along the axon potassium ions diffuse out along a concentration gradient, starting off the process of repolarisation

direction of impulse

K+

K+

+ + + + + - - +

+ + + + + - - +

- - - - - + + -

- - - - - + + -

e) The sodium-potassium pump is re-established, fully repolarising the membrane

direction of impulse

K+

K+

K+

K+

Na+

Na+