Equilibrium Potential in Physiology

Study Notes

Electrochemical equilibrium is when there are equal and opposite forces acting on an ion across a membrane
The equilibrium potential is the membrane potential at which there is no net movement of an ion across a membrane
The Nernst Equation calculates the equilibrium potential for an ion and considers both the chemical and electrical driving forces
The simplified Nernst equation is: E = (RT/zF) * ln(Co/Ci) where E is the equilibrium potential, R is the gas constant, T is the temperature, z is the valence of the ion, F is Faraday’s constant, and Co and Ci are the concentrations of the ion outside and inside the cell respectively
If the membrane potential is at the equilibrium potential, there is no net movement of the ion across the membrane
If the membrane potential is higher than the equilibrium potential, the ion will move out of the cell
If the membrane potential is lower than the equilibrium potential, the ion will move into the cell
Typical values for equilibrium potentials in excitable cells are: +60mV for Na+, -90mV for K+, -70mV for Cl-, and +120mV for Ca++

Resting membrane potential is the membrane potential of a cell at rest
Goldman-Hodgkin-Katz Equation (GHK) is used to calculate the resting membrane potential
The GHK equation considers the permeability of the membrane to each ion
The resting membrane potential is typically -70mV in neurons
Changes in K+, Na+, or Cl- concentrations will change the resting membrane potential

Action potentials are rapid changes in membrane potential that are used for communication between neurons
The action potential is caused by the opening and closing of voltage-gated ion channels
The action potential has five phases: threshold, depolarization, overshoot, repolarization, and hyperpolarization
During the threshold phase, the membrane potential must reach a certain value to trigger an action potential
During the depolarization phase, the membrane potential rises rapidly due to the influx of Na+ ions
During the overshoot phase, the membrane potential becomes more positive than the equilibrium potential for Na+
During the repolarization phase, the membrane potential falls due to the efflux of K+ ions
During the hyperpolarization phase, the membrane potential becomes more negative than the resting membrane potential
The absolute refractory period is the time during which a new action potential cannot be generated, due to the inactivation of Na+ channels
The relative refractory period is the time during which a new action potential can be generated but only with a stronger stimulus, because of the increased permeability to K+

Graded potentials are localized changes in membrane potential that vary in size
Graded potentials are decremental, meaning they decay over distance
Graded potentials can be either depolarizing or hyperpolarizing
Graded potentials are important for sensory transduction, which is the process of converting different types of stimuli into action potentials
Some types of graded potentials are receptor potentials, generator potentials, post-synaptic potentials, pacemaker potentials, and passive conduction

Electrotonic conduction is the passive spread of electrical current along a membrane
Electrotonic conduction is decremental, meaning it decays over distance
Electrotonic conduction can be used to conduct signals over short distances
Electrotonic potentials can also lead to the generation of action potentials if the change in membrane potential is large enough to reach threshold

Action potentials are propagated along axons by a process called saltatory conduction
Saltatory conduction is the jumping of the action potential from one node of Ranvier to the next
Myelin sheath is a fatty substance that insulates the axon membrane
Nodes of Ranvier are gaps in the myelin sheath where the axon membrane is exposed
Action potentials are regenerated at the nodes of Ranvier, which allows them to travel faster and more efficiently
Demyelinating diseases, such as multiple sclerosis, damage the myelin sheath, which slows down action potential propagation

Electrotonic conduction is passive and decremental, while action potential conduction is active and non-decremental
Saltatory conduction is a special type of action potential conduction that occurs in myelinated axons, and is faster than non-myelinated axons
Electrotonic conduction occurs in all types of neurons, while action potential conduction occurs specifically in axons
Electrotonic conduction is important for short-distance signaling, while action potential conduction is important for long-distance signaling
Saltatory conduction is the most efficient way to conduct action potentials, but it requires the presence of myelin

Electrotonic conduction occurs in dendrites, cell body, and initial segment of the axon
Action potentials are generated at the axon hillock
Action potentials are propagated down the axon, and sometimes into dendrites
Axons are often myelinated, but dendrites are not

Demyelination slows down action potential propagation
Demyelination can cause a variety of neurological problems, including impaired sensation, muscle weakness, and paralysis
Demyelination diseases affect the speed and efficiency of action potential conduction.
Demyelination affects primarily action potential propagation.