Nerve Impulse (pp. 320-321).

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Nerve Impulse (pp. 320-321)

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.

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

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

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

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.

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.

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)