1. Nerve Impulse (pp )

2. Transmission of Nerve Impulses
Occurs due to an electrochemical change that moves in one direction along the length of a nerve fiber.
It is electrochemical because it involves changes in voltage as well as in the concentrations of certain ions.
Impulses travel from receptor (or dendrite) down the axon.
A neuron is either transmitting, or it is not (“all-or-none” response).
occurs at the nodes of Ranvier
When nerve impulse reaches the end of the node of Ranvier, it is able to “jump” to the next node of Ranvier.

3. Three Phases of Nerve Impulse
STEP 1: RESTING POTENTIAL
When axon is NOT sending a message, the inside of the axon is negative (-60 mV) compared to the outside
caused by the presence of large negative ions which are too large to cross the membrane
Na+ is more concentrated outside the axon
K+ is more concentrated inside the axon

4. This uneven distribution of K and Na ions is maintained by active transport across sodium-potassium pumps
Constantly working, because the membrane is partially permeable to these ions, & they tend to diffuse toward regions of lower concentration

5. STEP 2: ACTION POTENTIAL
If stimulated by electric shock, pH change, mechanical stimulation, a nerve impulse is generated (= action potential)
This is a rapid change in polarity across the axomembrane
Broken into an upswing and downswing:
1. During upswing
Sodium gates open & Na+ flows into the axon
This causes depolarization going from negative to positive (-65 mV to +40 mV)
2. During downswing
Potassium gates open & K+ flows out of the axon
This causes repolarization & axon charge returns to -65mV

7. STEP 3: PROPAGATION OF AN ACTION POTENTIAL
When an action potential reaches a large enough depolarization, it is called a threshold
This will make the next section undergo depolarization.
After a section has done its action potential then there is a short period of time for the Na+ & K+ pumps to reset (= refractory period). It makes sure that the action potential can only move in one direction.

8. Summary of Nerve Impulse Transmission
A. The 3 phases of nerve impulse.
1. Resting potential
2. Action potential – depolarization event
(Na+ gates opening, Na+ pouring in)
3. Action potential – repolarization event
(K + gates opening, K + pouring out)
4. Resting potential – neuron is ready to conduct again.

9. Myelin Sheath Composed of lipids
Composed of tightly packed spirals of the cell membrane of Schwann cells
gives nerves their characteristic shiny white appearance
Functions:
Insulation
Speeds up impulse transmission
Speed of transmission is ~200 m/s in myelinated fibers, but only 0.5 m/s in non-myelinated fibers.
nerve impulse can "jump" from node to node in myelinated fibers (= saltatory conduction)