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. 
Potassium diffusion potential
Potassium diffusion potential: The potential difference is - 94 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 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.
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.

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)


Acute Demyelinating Encephalomyelitis (ADEM) 
with associated optic neuritis: 
9 year-old girl presents to an outside hospital 
with fatigue, poor appetite, and 
decreased activity for 3 weeks.

Conduction velocity is greatest in large myelinated nerve fibres
The velocity increases with the fiber diameter of the axon. 




acute disseminated encephalomyelitis
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.

Comments

Post a Comment

Popular posts from this blog

The Nervous System, the Cerebrum

-
Image
Cerebral Cortex Basal Ganglia Limbic System Cingulate Gyrus Thalamus Hippocampus Amygdala Brain function Ability to generate information and output signals in the absence of external input. Three systems that influence output by motor systems: the sensory system monitors internal and external environments and initiates reflex responses a cognitive system voluntary responses a behavioural state system sleep-wake cycles and intrinsic behaviours The Cerebral cortex is organised into functional areas There are three functional specialisations: Sensory areas - sensory inputs Motor areas - direct muscle movement Association areas - integrate sensory and motor input and direct voluntary behavious Cerebral lateralization of function or left-right brain dominance. Neurons display a level of plasticity.  Sensory information is Integrated in the Spinal Cord and Brain Primary Somatic Sensory Cortex touch, temperature, pain, itch, and body position Visual Cortex...
Read more

The Nervous System, The Sense of Hearing

-
Image
The Tympanic Membrane and the Ossicular System Sound from the tympanic Membrane to the Cochlea Tympanic Membrane, Malleus, Incus, Stapes, Oval Window - the ossicular chain. Malleus is also attached to the tensor tympani muscle. Impedance matching between sound waves in air and sound waves in the cochlear fluid is mediated by the ossicular chain. Sound is not amplified by increasing movement at the stapes, but increasing force of movement  at the stapes. Increasing pressure, increases the vibration of the fluid in the membranous labyrinth - via the oval window.  Impedance matching between air waves and fluid waves.  Contraction of the stapedius and the tensor tympani muscles attenuates sound conduction This is the same mechanism used to diminish the sensitivity to ones own speech Increases the rigidity of the stapes, and therefore the ossicular chain. Sound Transmission Through Bone Conduction The cochlea is embedded in bone, vibration of the ...
Read more

AN UNEXPECTED CHEST XRAY

-
Image
AN UNEXPECTED CHEST XRAY Mrs MacLeod was a rather prudent 54 year old women, that presented with a shortness of breath, a chronic cough and recurrent pneumonia. She had an obstructive lung pattern on pulmonary function tests. She come in for an xray of the chest. The chest XRAY was rather different. There was unequal lung volumes, with the right lung volume being larger than the left. There was also decreased left lung vessels noted. There were no pleural effusions and no pneumothorax present either. The right lung was perfectly normal. Upon further investigation she reported to have had a significant childhood viral infection that resulted in distal airspace obstruction, probably secondary to an adenovirus or and RSV viral infection. A case of Sweyer-James syndrome was diagnosed. She was treated with influenza vaccinations, but in this case no lobectomy was performed as the no specific lung segment was identified.
Read more