CONDUCTION OF A

CTION

POTENTIA

L

CONDUCTION OF ACTION POTENTIAL (AP)Initially AP only occurs on a particular

area of the nerve cellIn order for nerve impulse to travel

along the neuron, the AP must be conducted along the neuron

When Na+ flows into the neuron as a result of open sodium gates, the positive charge inside causes the next sodium gate to open, causing another AP

This occurs over ad over again, until the end of the neuron

Once sodium gates have opened and closed, they cannot be activated for a short time (refractory period) thus impulse can only move forward, not backwards

The impulse moves even faster in myelinated fibres because the impulse (AP) jumps from node to node (node=exposed axon)

BRAIN BREAK!!!

The Schwann cells make up the myelin sheath

Most long fibers (both axons and dendrites) are covered by myelin sheath

Invertebrates do not have myelinated fibers, therefore move very slowly

THRESHOLD

Stimuli for initiation of AP can be anything, depending on the type of receptor

But, stimulus must have a certain intensity for nerve to fire – called the threshold

If stimulus is below the threshold nerve does not fire

(called “all-or-none” response)Therefore speed and intensity of

nerve impulse never vary

…. However, the frequency of the impulse and the number of nerves firing will vary

These are both dependent on the intensity of the stimulus

Also, different nerves will have different thresholds

Since nerves are in bundles this means that a warm plate will fire some nerves in the bundle but not others, whereas a hot plate will fire far more nerves

TRANSMISSION ACROSS A SYNAPSENerve impulses (AP’s) must be

transferred from one neuron to another

This occurs by way of the synapse

When a nerve impulse travels to a synaptic ending Ca2+ flows to ending

Causes synaptic vesicles to fuse with a pre-synaptic membrane and secrete neurotransmitter substances into the synaptic cleft (by exocytosis)

Neurotransmitter diffuses across the synaptic cleft

Transmitters (ex. Acetyl-choline: parasympathetic and Noradrenalin: sympathetic) bind to receptor sites on post-synaptic membrane

An enzyme (cholinesterase) is released after the neurotransmitter is released to destroy it

Otherwise, the Na+ gate wouldn’t close and the nerve could not fire

Insecticides block cholinesteraseThe neurotransmitter substances can

either excite or inhibit the next neuron

Whether or not the neuron “fires” depends on the amount of excitation compared to the amount of inhibition