Myelin again

• Myelin speeds up the nerve impulse because nerve fibers have Schwann cells around them – Schwann cells restrict ion movement – So impulse “jumps” between the nodes of Ranvier

– This jumping is called saltatory transmission

Synaptic Transmission

• Saltatory transmission impulse along a neuron • Synaptic transmission impulse between

neurons – Happens at the end of an axon – No actual connection between the terminus and

the membrane of the next cell – Space between is called a synapse or synaptic

gap/cleft

Synaptic membranes

• Axon membrane is called the pre-synaptic membrane – Has Ca2+ gate – Has synaptic vesicles with neurotransmitters

inside • Membrane on the other side of synaptic cleft

is the post-synaptic membrane – Has protein receptor sites

Steps

1) Impulse arrives at end of axon 2) depolarization of pre-synaptic membrane 3) Ca2+ gates open - Ca2+ in the synaptic cleft moves

into the axon 4) Vesicles with neurotransmitters inside fuse with

the pre-synaptic membrane 5) Neurotransmitters are released (exocytosis) into

the synaptic cleft and diffuse across to the post synaptic membrane

axonterminal

1. After an action potential arrives at an axon terminal, Ca2+ enters, and synaptic vesicles fuse with the presynaptic membrane.

synaptic vesiclesenclose neurotransmitters

synaptic cleft

Ca2+

Ca2+

2. Neuro- transmitters are released and bind to receptors on the postsynaptic membrane.

axonterminal

synaptic vesiclesenclose neuro-transmitters

synaptic cleft

NT

6) Neurotransmitters bond to receptor sites on the post-synaptic membrane

7) Step (6) causes the Na+ gates to open on post synaptic membrane which starts the nerve impulse along that cell

8) Synaptic cleft is returned to normal - enzymes that destroy specific neurotransmitters - Ca2+ returned to the synaptic cleft (active

transport)

Ca2+

axonterminal

synaptic cleft

receptor

postsynapticneuron

NT

3. When an excitatory neurotransmitter binds to a receptor, Na+ diffuses into the postsynaptic neuron, and an action potential begins.

Na+

Synaptic Transmission in General

• Energy for synaptic transmission comes from mitochondria in the axon

• Synaptic transmission only occurs in one direction due to nature of the membranes on either side of the synaptic cleft

• 2 types of neurotransmitters – Inhibitory NT – makes it harder for depolarization of the

next membrane – Excitatory NT – promotes depolarization of the next

membrane

ANIMATION

• Transmission across a synapse