Membrane potentials and action potentials
Electrical potentials exist across membranes.
Nerve and muscle cells are excitable - they generate their own electrochemical impulses at their membranes.
BASIC PHYSICS OF MEMBRANE POTENTIALS
A concentration difference of ions across a selectively permeable membrane can produce a membrane potential.
A concentration difference of ions across a selectively permeable membrane can produce a membrane potential.
Potassium diffusion potential
Potassium diffusion potential: The potential difference is - 94 mV
Sodium diffusion potential: +61 mV
Sodium diffusion potential: +61 mV
The Nernst equation describes the relation of diffusion potential to concentration difference.
EMF (millivolts = +- 61 log(concentration inside / concentration outside)
where EMF is the electromotor force.
The sign of the potential is positive if the ion is negative ion, and
negative if it is a positive ion.
The membrane potential that prevents net diffusion of another ion in either direction through the membrane is called the Nerst potential for that ion.
The Goldman equation is used to calculate the diffusion potential when the membrane is permeable to several different ions.
EMF (mV) = -61 log ((Ci Na, PNa )+ (Ci K, P K )+ (Co Cl , P Cl) ) / (Co Na, PNa )+ (Co K, P K )+ (Ci Cl , P Cl))
Sodium, Potassium and chloride ions are most importantly involved in the development of membrane potential in nerve and muscle fibres as well as neuronal cells and the central nervous system
The degree if importance of each ion in determining the voltage of is proportional to the membrane permeability for that particular ion.
A positive ion concentration gradient from inside the membrane to the outside of the membrane causes electronegativity inside the membrane.
Resting membrane potential of nerves.
The resting membrane potential of nerves is established by the difusion potentials, membrane permeability, and electrogenic nature of the Sodium Potassium pump.
Potassium diffusion potential: Nernst potential of -94 mV
Sodium Diffusion potential: Nernst potential of +64 mV
Membrane permeability: Potassium is 100 times more permeable and Sodium, so the diffusion of potassium contributes far more to the membrane potential. Goodman Potassium potential -86mV.
Electrogenic nature of the Sodium Potassium pump. Three sodium's are pumped out for every two potassium's pumped in inside - continuous loss of positive charge from the inside of the membrane. The Sodium Potassium pump is electrogenic causing a -4mV negative charge inside the cell.
NERVE ACTION POTENTIAL
NERVE ACTION POTENTIALNerve signals are transmitted by action potentials, which are rapid changes in the membrane potential.
Resting stage: The resting membrane potential before the action potential occurs.
Resting stage: The resting membrane potential before the action potential occurs.
Depolarisation stage: The membrane suddenly becomes permeable to Sodium ions, allowing tremendous numbers of positively charges sodium ions to flow to the interior of the axon, and the potential rises rapidly in the positive direction.
Repolarisation stage: Within a millisecond or two after the membrane becomes permeable to sodium ions, the sodium channels begin to close and the potassium channels are open. Potassium rapidly diffuses out of the cell to re-establish the normal negative resting membrane potential.
Voltage-gated sodium and potassium channels are activated and inactivated during the course of an action potential. Voltage-gated sodium channel causes both depolarisation and repolarisation of the nerve membrane during the action potential. The voltage gated potassium channel plays an important role in increasing the rapidity of repolarisation of the membrane. The two voltage gated channels are in addition to the sodium potassium pump and the Sodium Potassium leak channels
Events that cause action potential
During the resting stage: before the action potential the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This causes greater potassium leakage through the potassium channels.
At the onset of the action potential: the sodium channels become activated allowing the rapid conductance increase (5000 x) of sodium. This is followed by the voltage gating of the potassium channels.
At the end of the action potential: return of the membrane potential to the negative state causing channels to close back to their original state.
A positive feedback, vicious circle opens the sodium channels. Any event that causes the membrane potential to raise from -90mV up towards zero, may cause many if not all the sodium gated channels to open. This opening stimulates the opening of the remaining sodium gated channels till all the channels are open. This is positive feedback.
An action potential will not occur until the threshold potential has been reached. An increase in the number of sodium ions entering the nerve fiber becomes greater than the number of potassium leaving the nerve fiber - increases the membrane potential from -90 mV to -65 mV, usually causes an explosive release of all the sodium gates, the action potential.
A new action potential cannot occur as long as the membrane is still depolarised from the preceding action potential. Shortly after the action potential is initiated, the sodium channels become inactived, any any amount of excitement will not open these channels. The only condition that will open this channel is if the membrane potential returns to its baseline.
Absolute refractory period: An action potential cannot be elicited even with a strong stimulus.
Relative refractory period: A stronger than normal stimulus can excite the fiber, and an action potential can be initiated.
Propagation of the action potential
The transmission of the depolarisation process along a nerve or a muscle fiber is called a nerve or muscle impulse.
The transmission of the depolarisation process along a nerve or a muscle fiber is called a nerve or muscle impulse.
Direction of propogation: an excitable membrane has no single direction of propagation, but an action potential will travel in both directions away from the stimulus.
All-or-nothing principle: Once the action potential has been elicited at any point in the membrane, depolarisation will travel over the entire membrane.
Re-establishing sodium potassium ionic gradients after action - potentials - importance of energy metabolism
This is done with the Sodium Potassium pump.
Special Aspects of Signal Transmission in Nerve Trunks
Large nerve fibres are myelinated and the small ones are unmyelinated: the thich myelin sheath deposited by Schwann cells. The sheath sheath consists of multiple layers of cellular membrane containing the lipid substance sphingomyelin, which is an excellent insulator. At the juncture beween two successive Schwann cells there is a small non-insulated area called the node of Ranvier, where extracellular ions can contact the axon.
"Saltatory" conduction occurs in myelinated fibres.
The nerve impulse jumps from node to node, this is important because:
Increased velocity: 5 to 50 fold
Energy Conservation: Saves on energy required to re-establish the sodium potassium concentration difference across membranes after a series of nerve impulses. (less ATP)
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The velocity increases with the fiber diameter of the axon.
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This young woman presented with progressive left hemiparesis after having had a viral infection a week earlier. This MRI demonstrates bilateral asymmetric lesions with open ring enhancement characteristic of demyelination. Note that restricted diffusion is not seen centrally (usually seen in cerebral abscesses) but at the advancing rim of demyelination. She was treated with steroids, and rapidly improved. Acute disseminated encephalomyelitis is, as the name would suggest, acute inflammation and demyelination of white matter typically following a recent (1 - 2 weeks prior) viral infection or vaccination. Grey matter, especially that of the basal ganglia, is also often involved, but to a lesser extent, as is the spinal cord. It is usually a monophasic illness (c.f multiple sclerosis), although within the episode individual lesions may be of varying stages of evolution. Typically ADEM presents in children, however cases in all ages have been reported. To read more about ADEM and see more images of the same case, please visit Radiopaedia.org here. |
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