Steps in Nerve Impulse Transmission

NERVE IMPULSE TRANSMISSION I. Transmission Along a Neuron 1. RESTING POTENTIAL  Na K pump moves 3 Na out of the axon for every 2 K into the axon  ( -70mV is from the negatively charged proteins)

2. THE ACTION POTENTIAL Triggered by a stimulus strong enough to produce a depolarization through a special phenomenon called the (- 55mV) ALL OR NOTHING PHENOMENON Increasing the intensity of the stimuli above threshold will not produce an increased response. Neurons either fire maximally or not at all

STAGES OF THE ACTION POTENTIAL

A. DEPOLARIZATION PHASE (RISING PHASE)  Na channels open to let Na rush in  K channels stay closed (+30mV)

B. REPOLARIZATION PHASE (FALLING PHASE)  K channels open and let K rush out  Na stays in since gate is closed (down to -80mV) (extra dip is refractory period)

3. UNDERSHOOT (AKA REFRACTORY PERIOD)  Na and K channels close but NaK pump restores order (-70mV) after hyperpolarization II. Transmission Between Neurons Communication between neurons is accomplished by moving across a small gap called the synapse. Synapse: space between two neurons or between a neuron and an effector.

Presynaptic neuron: carries impulse toward synapse.

Postsynaptic neuron: carries impulse away from synapse.

Neurotransmitters (NT) are chemicals released from one neuron at the presynaptic nerve terminal. 1. NT then cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor 2. The action that follows activation of a receptor site may be either depolarization or hyperpolarization EXAMPLES OF NEUROTRANSMITTERS Acetylcholine (Ach) GABA Serotonin Dopamine Neurotransmitters can be destroyed in different ways if they are no longer needed Example-enzymatic degradationAcetylcholinesterase breaks Ach into choline (found in nerve gas) Summation: Effect produced by the accumulation of NTs from two or more neurons.

a. Excitatory postsynaptic potential (EPSP) Depolarization makes it MORE likely that an action potential will fire b. Inhibitory postsynaptic potential (IPSP) Hyperpolarization makes it LESS likely that an action potential will fire.