Neurophysiology
I.
Principles of electricity
a. The human body is electrically neutral
b. Some areas have higher concentrations of positive or negative charges
c. Separated electrical charges of opposite sign have potential energy
d. Voltage – measure of potential energy
i. Measured between two points – the potential
ii. The greater the difference at the two points – higher the voltage
e. Current – the flow of electrical charge from one point to another
f. Resistance – the hindrance to charge flow
II.
Ion channels
a. Made of membrane proteins
b. Control the movement of ions across the membrane of nerves
c. Passive (leakage) – always open
i. Located in all places on the neuron
d. Active (gated) – have a molecular “gate” of one or more protein molecules that
open or close the channel in response to various signals
i. Chemically-gated – open when the appropriate neurotransmitter binds
1. Located on the dendrites and cell body
ii. Voltage-gated – open and close in response to changes in the membrane potential
1. Located at the axon hillock and the unmylinated portions of the axon
iii. Each type of channel is selective to which ion it will allow to pass
III.
Membrane Potential
a. The inside of the membrane is negative
b. The outside of the membrane is positive
c. The potential difference in charges at rest is called the resting membrane potential
i. Generated by the difference in ionic composition of the intracellular and
extracellular fluid
ii. Na concentration is higher outside the cell, K is higher inside the cell
d. The cell membrane is selectively permeable to different ions
i. Use ion channels
ii. Depends on the number of ion channels
IV.
Membrane potential & signals
a. Some cells use the changes in the membrane potential as communication
b. Graded potentials – signal over short distances
c. Action potentials – long distance signals
d. Depolarization – reduction in membrane potential
i. The inside of the membrane becomes less negative than the resting potential
ii. Increases the probability of producing nerve impulses
e. Hyperpolarization – when the membrane potential or voltage increases
i. Becomes more negative
ii. Decreases the probability of producing nerve impulses
V.
Action Potentials
a. Principle way neurons communicate
b. Only used with cells that have excitable membranes
c. When action potential is transmitted – called nerve impulse
d. Neuron only transmits an impulse when adequately stimulated
e. Stimulus changes the permeability of the membrane by opening specific channels on
the axons
f. Channels open and close due to changes in membrane potential and are activated by
gated potentials
g. Only axons can generate action potentials
h. The transmission from the graded potential to the action potential happens at the
axon hillock
i. Generating and action potential
i. Changes in membrane permeability
ii. Increase in Na permeability
iii. Restoration of Na impermeability
iv. Increase in K permeability
j. Propagation
i. Polarity reverses and the positive ions move laterally from the area of polarity
reversal
ii. Impulse always propagates away from the point of origin
iii. Self-propagating – process continues along the axon at a constant velocity
iv. Once it is started it can not be stopped
k. Threshold
i. Minimal amount of stimulation to cause an impulse
ii. The stronger the stimulus, the quicker the membrane reaches threshold
l. All or none
i. The nerve impulse (action potential) either happens completely or it does not
happen at all
m. Conduction velocities
i. Diameter
1. the larger the axon’s diameter, the faster it conducts impulses
ii. Myelin sheath
1. Presence of a myelin sheath increases the rate of propagation