A & PNeurophysiologyCh 11
Neurophysiology - how neurons become excited or inhibited and how they
communicate with other cells in the body
Basic Principles of electricity: 1. Human bodies are electrically neutral.
2. There are some areas where one type of charge predominates, making them
positively or negatively charged.
3. Opposite charges attract one another: energy must be used to separate them.
4. When opposite charges come together, energy is liberated.
5. Liberated energy can be used to do work.
6. Separated electrical charges of opposite sign have potential energy, which is
measured in volts (V) or millivolts (mV).
* Voltage is always measured between two points and is called the ‘potential’
between them.
* In a body, electrical currents reflect the flow of charged ions, not free electrons.
The Resting Membrane Potential:
-The potential difference between two points is measured by connecting two
electrodes to a voltmeter.
- When one electrode is inside a neuron and the other is outside its surface, a
measure can be made of potential energy.
- Value of Vr can vary from -40 mV to - 90 mV, but averages - 70 mV
(minus sign means the inside of the neuron is negatively charged)
- This potential is measured in a membrane at rest
- The differences in charge are due to a lower conc. of Na+ and a higher conc. of
K+ inside the cell than outside
-The sodium-potassium pump maintains this chemical and electrical gradient and
accounts for 1/3 of the energy you expend when at rest.
* A change in membrane potential can be produced by anything that changes
membrane permeability to any type of ion, or anything that alters ion
concentrations on the two sides of the membrane.
Depolarization - a reduction in membrane potential, the inside of the cell becomes
less negative Ex: from - 70 to - 50 mV
Hyperpolarization - occurs when the membrane potential or voltage increases,
becoming more negative than the resting potential Ex: from - 70 to -90 mV
Graded Potentials: short-lived, local changes in membrane potential that can be
either depolarizations or hyperpolarizations
Graded potentials (cont.):
- magnitude varies directly with the intensity or strength of the stimulus.
- the more intense the stimulus, the more voltage changes and the farther the
current flows
- current flow dies out within a few millimeters of its origin
- act only over very short distances, but are essential in initiation action potentials
* Not all local depolarization events produce action potentials!
Since magnitude increases with stimuli:
1. Brief, weak stimuli (subthreshold stimuli) produce depolarizations that are not
translated into nerve impulses.
2. Threshold stimuli produce depolarizing currents that push the membrane
potential toward and beyond the threshold voltage.
3. Critical factor is total amount of current (electrical charge x time)
- strong stimuli depolarize the membrane to threshold quickly
- weaker stimuli must be applied for longer periods to achieve threshold
- very weak stimuli are unable to trigger action potentials at all
Action Potentials (APs):
- only cells with excitable membranes (nerve & muscle) can generate APs
- The principal way neurons communicate is by generating and propagating them
- a brief reversal of membrane potential with a total change of about 100 mV
( - 70 mV to +30 mV)
- whole event is over in a few milliseconds
- APs in nerves are also called ‘nerve impulses’, a nerve impulse will occur only
when the nerve is adequately stimulated
* Only axons are capable of generating APs!
- the transition from the local graded potential to AP take place at the axon hillock
Generation of an action potential:
1. Resting state: Active channels closed. Two gates respond to depolarizations,
but they do so at different rates. Activation gates respond by rapidly opening Na+
channels and Inactivation gates respond by closing Na+ channels more slowly.
2. Depolarizing phase: increase in sodium permeability and reversal of the
membrane potential
3. Depolarizing phase: decrease in sodium permeability due to inactivation gates
4. Repolarizing phase: increase in potassium permeability due to voltageregulated gates. Membrane potential moves back to resting value.
5. Sodium-potassium pump restores original ionic distributions quickly.
* Remember: action potentials are all-or-none, they never go halfway!