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