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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? Synaptic Cleft Presynaptic membrane Postsynaptic membrane PRESYNAPTIC MEMBRANE Encloses synaptic vesicles filled with neurotransmitters (n.t.) = a chemical message manufactured by the axon POSTSYNAPTIC MEMBRANE Contains protein receptor sites within the membrane to recognize specific n.t.. 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: At the same time, the CALCIUM GATES OPEN and calcium (Ca2+) also moves into the axon bulb / terminal of the presynaptic neuron Calcium ions Calcium channel with a calcium ion 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 presynaptic membrane. Requires ATP STEP 4: EXOCYTOSIS occurs as the vesicles release neurotransmitters into the synaptic gap. The n.t. diffuses across the gap. STEP 5: Neurotransmitters bond with the receptor sites on the postsynaptic membrane. STEP 6: When an excitatory n.t. attaches to the receptors, the voltage of the post-synaptic membrane changes to cause the sodium gates to open. o This depolarizes the membrane. Sodium & potassium ions (sodium moves in and potassium moves out) Neurotransmitter If an inhibitory neurotransmitter is released and attaches to the receptors, the post-synaptic membrane will be hyperpolarized Sodium & potssium ions (sodium moves in and potassium moves out) Neurotransmitter “Hyperpolarization” makes it more difficult for the threshold to be reached less likely an action potential will occur Chemical Synapse Animation Ch 17 http://highered.mcgrawhill.com/classware/ala.do?alaid =ala_1039849 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. ENZYME THAT DESTROYS NEUROTRANSMITTER Causes the sodium gate to close 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 mitochon/dria that can be found in abundance in the synaptic/axon knob EXCELLENT SYNAPSE ANIMATION: http://faculty.plattsburgh.edu/donald.slish/NMJ.htm DO NOT WRITE THIS DOWN: Acetylcholine is involved in arousal, attention, memory, motivation, and movement. Involved in muscle action. Degeneration of neurons that produce ACh have been linked to Alzheimer‟s disease. Too much can lead to spasms and tremors; too little, to paralysis or torpor. Dopamine is involved in many behaviors and emotions, including pleasure. It has been implicated in schizophrenia and Parkinson‟s disease. Serotonin is involved in the regulation of sleep, dreaming, mood, eating, pain, and aggressive behavior. Implicated in depression. Noradrenalin affects arousal, wakefulness, learning, memory, and mood. DO NOT WRITE THIS DOWN: Endorphins are involved in the inhibition of pain. Released during strenuous exercise. May be responsible for “runner‟s high.” Glutamate is involved in long-term memory and the perception of pain. GABA is a largely inhibitory neurotransmitter distributed widely throughout the CNS. Implicated in sleep and eating disorders. Low levels of GABA have also been linked to extreme anxiety. Glycene is mainly responsible for inhibition in the spinal cord and lower brain centers. The 2 neurotransmitters you need to know! 1) Acetylcholine: * •This is responsible for promoting all responses in a relaxed state •Also involved in controlling skeletal muscles. *it is destroyed by the enzyme acetylcholinesterase 2) Noradrenalin: •This is the excitatory transmitter (also known as norepinephrine). •It almost always increases the activity of the receiving cell/tissue/organ. •It is involved in „fight or flight‟ situations (stress). *it is destroyed by the enzyme monoamine oxidase Drugs have different ways of acting on the synaptic transmission system: 1. Some drugs hold the receptors open for a longer time. Alcohol causes the GABA neurotransmitters to work for a longer amount of time, thus quieting the brain ALCOHOL: more than normal. 2. Some drugs block the enzymes from destroying the neurotransmitters. When you are depressed, your serotonin is usually reabsorbed before it can do its job. Prozac stops this from happening. http://www.p bs.org/wnet/ closetohome /animation/g aba-animmain.html 3. Some drugs cause increased secretions of n.t.. • Cocaine increases dopamine secretion and causes pleasure sensations • Ecstasy increases serotonin secretion and produces a sense of intimacy with others and diminished feelings of fear and anxiety. 4. Some drugs imitate or mimic the neurotransmitters and take their place on the receptors. • Morphine binds to the receptors that endorphins naturally would and cause a sense of well being (like you normally get after exercise). • Nicotine binds to the receptors for acetylcholine and cause arousal and reward sensations. • Caffeine counters inhibitory neurons to increase alertness 5. Some drugs stop the neurotransmitters from joining the receptors. Pain killers occupy the receptor sites so that the sensation of pain cannot be transmitted between the nerve cells. Drug Addiction (ex. Cocaine) Normally at a synapse, the neurotransmitter is broken down and recycled so receptors do not continually fire. Some drugs, like cocaine, bind to “recyclers” so the neurotransmitter cannot be removed from the synapse which causes neurons to increase their rate of depolarization To compensate, the postsynaptic membrane can decrease the number of protein receptors If you remove the drug, the level of neurotransmitter returns to normal BUT with fewer receptors Result with usage? Decreased number of receptors will make it difficult to depolarize the neurons which makes the “high” more difficult to get You crave the drug! And can experience withdrawal symptoms without it Complete Synaptic Transmission WS