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Transcript
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 axon terminal Ca2+ 1. After an action potential arrives at an axon terminal, Ca2+ enters, and synaptic vesicles fuse with the presynaptic membrane. synaptic vesicles enclose neurotransmitters synaptic cleft axon terminal 2. Neurotransmitters are released and bind to receptors on the postsynaptic membrane. NT Ca2+ synaptic vesicles enclose neurotransmitters synaptic cleft 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) axon terminal Ca2+ 3. When an excitatory neurotransmitter binds to a receptor, Na+ diffuses into the postsynaptic neuron, and an action potential begins. NT receptor Na+ synaptic cleft postsynaptic neuron 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