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Transcript
SYNAPTIC TRANSMISSION The Synapse: How does the impulse travel from one neuron to another? How does an impulse travel from a neuron to a muscle or gland (effector)? How can they do this when they are not physically connected (there’s a GAP)? • • • When an impulse arrives at the end of an axon, it must make a connection to the next nerve cell, or to the muscle or gland. BUT neurons do NOT directly contact each other There is a small space, termed the SYNAPTIC GAP / CLEFT, which the impulse must cross. How does this work? PRESYNAPTIC MEMBRANE POSTSYNAPTIC MEMBRANE Encloses synaptic vesicles filled with neurotransmitters = n.t. (a chemical message manufactured by the axon). Contains protein receptor sites within the membrane to recognize specific neurotransmitters Synaptic transmission: STEP 1: When an impulse arrives at the end of an axon, the sodium gates open and sodium floods into the axon bulb / terminal. STEP 2: 2+ At the same time, the CALCIUM GATES OPEN and calcium (Ca ) also moves into the axon bulb / terminal of the presynaptic neuron. STEP 3: The calcium binds with CONTRACTILE PROTEINS (microfilaments) attached to the vesicles and this causes them to contract, thus pulling the vesicles towards the pre-synaptic membrane o This requires ATP STEP 4: EXOCYTOSIS occurs as the vesicles release neurotransmitters into the synaptic gap. The neurotransmitter diffuses across the gap. STEP 5: Neurotransmitters bond with the receptor sites on the postsynaptic membrane. STEP 6: When an excitatory neurotransmitters attaches to the receptors, the voltage of the post-synaptic membrane changes to cause the sodium gates to open. o This depolarizes the membrane. If an inhibitory neurotransmitter is released and attaches to the receptors, the post-synaptic membrane will be hyperpolarized which makes it more difficult for the threshold to be reached (less likely an action potential will occur). STEP 7: If the synapse is between an axon and dendrite, then the Action Potential will continue down the next neuron If the synapse is between an axon and a muscle cell, then the muscle will contract. If the synapse is between an axon and a gland, then the gland will release a hormone. STEP 8: The synaptic gap contains enzymes that will destroy the neurotransmitters, thus returning the synapse to its original condition prior to the arrival of the impulse STEP 9: The calcium ions are returned to the synaptic gap by active transport. Because only the axon bulb has the neurotransmitters and only the dendrite has the receptors, synaptic transmission may only occur in one direction. The energy for this entire process comes from the mitochondria that can be found in abundance in the synaptic / axon bulb.