• C11
• Explain the transmission of a nerve impulse through a neuron
• Relate the myelin sheath to impulse conduction
• Describe the components of a synapse
• Explain how an impulse travels across a synapse

• A nerve impulse is a series of electrochemical signals along a neuron, to transmit information
• Direction of travel: dendrite
 cell body  axon  dendrite of next neuron

• Sodium - potassium pumps in the neuron membrane cause changes in polarity (charge distribution) of the membrane
• membrane also contains channel proteins (gates) for passive transport of Na+ and K+ ions
Pump Animation

• When the neuron is not conducting an impulse:
– Na+ ions are concentrated outside the membrane (active)
– K+ ions are concentrated inside
– Negative ions inside result in a polarized state

Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+
-
K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+

• Sodium-potassium pump actively transports Na+ and
K+ ions
• Large negative ions
(proteins) are always inside the cell
• Outside of the membrane is more positive than the inside

• When an impulse is being transmitted, the ion distribution changes
• A stimulus from a sensory receptor or another neuron reaches the “threshold level ” (all or none response)
• Causes “gates” to open allowing ion movement
• Action Potential

1. Depolarization
2. Repolarization
3. Refractory period

• sodium gates open and Na+ ions move inside
• Charge inside changes from - to +

Na+ Na+ Na+
K+ Na+K+Na+ K+Na+ K+ Na+K+ K+
-
K+ Na+K+Na+ K+Na+ K+ Na+K+ K+
Na+ Na+ Na+

• Potassium gates open and K+ ions move to the outside
• Inside becomes negative again

K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+
-
Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+

• Sodium gates don’t re-open right away
• Prevents impulse from traveling backwards
• After the refractory period, resting potential is resumed
• Nerve Transmission

• Action potential moves along the neuron

• In myelinated fibers, the action potential jumps from node to node (can travel faster)

• When the impulse reaches the end of an axon, it must be transmitted across the synaptic cleft to the next neuron
• This is accomplished by chemicals called neurotransmitters, stored in synaptic vesicles in the axon bulb
• Examples of neurotransmitters: acetylcholine, norepinephrine, serotonin, dopamine, many others

• Excitatory neurotransmitters stimulate action potential in the post-synaptic membrane
• Inhibitory neurotransmitters inhibit action potential in the post-synaptic membrane

• Ca+ channels open in the axon bulb
• Ca+ ions enter the axon
• Ca+ stimulates synaptic vesicles to fuse with the membrane
• Vesicles release neurotransmitter molecules into the synaptic cleft

• Neurotransmitters bind to the post synaptic membrane (dendrite of the next neuron) and initiate action potential there
• Synapse

• Once transmission is complete, neurotransmitters are inactivated by enzymes in the synaptic cleft (ex.
Acetylcholinesterase) or reabsorbed into the axon to be re-used