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IV. Conduction Across Synapses synapses are gaps between neuron and target cell refer to diagram of a synapse identify: source of impulse: target: mechanism release of neurotransmitters that bind to receptors in target cell if neurotransmitter stimulates Na+ channels to open – impulse generated if neurotransmitter stimulates K+ channels to open – impulse suppressed (chemo)receptors can only recognize specific neurotransmitters can only respond to specific neurotransmitters thus, response is dependent on neurotransmitter respond to neurotransmitter as long as it is present. neurotransmitters usually one type per neuron effect usually due to type of receptor A. Components identify components listed in diagram “synaptic transmission” B. Steps go over briefly then students attempt to complete handout. “synaptic transmission” C. Termination of Synaptic Transmission if neurotransmitters are present in synaptic cleft post-synaptic membrane will be stimulated must remove neurotransmitters to 1- end transmission 2- ready synapse for next transmission 1. diffusion neurotransmitter floats out of cleft and into surrounding tissue slow – long lasting – more potent – long to make ex: neuropeptides like substance P endophorins will eventually be destroyed – takes minutes – hours 2. enzymatic destruction neurotransmitter split by a specific enzyme fragments re-absorbed by presynaptic neuron used to synthesize more neurotransmitter ex: acetylcholine (Ach) split by enzyme acetylcholinesterase (AChE) 3. transmitter re-uptake neurotransmitter transported back into pre-synaptic neuron for re-use ex: norepinephrine dopamine serotonin D. Neurotransmitters chemical messengers at synapses most are excitatory – depolarize post-synaptic membrane some are inhibitory – hyperpolarize post-synaptic membrane effect of neurotransmitter determined by receptor 1. acetylcholine (ACh) can be excitatory or inhibitory – depends on type of receptor and tissue neurons that produce ACh are called cholinergic neurons a) cholinergic neurons found in • all neuromuscular junctions (ie all motor neurons) • PNS neuron-neuron synapses • brain areas: basal nucleus (near limbic system) hippocampus (role in memory) cerebral cortex functions of ACh in brain memory creation and retrevial / learning pain conduction regulation of neuroendocrine regulation of REM cycles b) 2 classes of receptors • muscarine receptors in brain • nicotinic receptors in brain, other neuron receptors and neuromuscular junctions c) termination • with enzyme acetylcholinesterase (AChE) • ACh binds to receptor – channel opens – ACh falls off receptor- ACh degraded – receptor ready w/in milliseconds d) effects of some chemicalsclass activity nerve gas rattlesnake venom cobra venom BoTx curare black widow spider venom 2. serotonin a) produced by neurons in brain stem linked to and affects many areas affected by many areas limbic system hypothalamus cerebellum spinal cord cerebral cortex b) termination method re-uptake by transporters c) implicated in many behaviors mood hunger sleep fear learning and memory sexual arousal d) effects of some chemicals on serotonin synapses • SSRI = selective serotonin re-uptake inhibitor ex: Prozac; fluvoxamine, Paxil, Zoloft bind to serotonin transporter prevent termination serotonin remains in synapse longer • MDMA = ecstasy – an entacogen blocks serotonin re-uptake and causes release of serotonin (can deplete serotonin stores) 3. GABA gamma aminobutyric acid a) inhibits by increases flow of Cl- into target cells b) found in inhibitory neurons in brain and spinal cord cerebellum hippocampus nucleus accembens cerebral cortex amygdale c) associated with decreased anxiety, fear, stress d) termination – re-uptake many tranquilizers enhance GABA ethanol, Valium, Xanon 4. dopamine a) produced in ventral tegmental area (VTA) reward/pleasure pathway excites targets in frontal lobe produces feelings of euphoria b) termination – re-uptake by transporters 5. nitric oxide NO a gas produced on demand (not stored in vesicles) lasts 6-10 seconds after produced causes blood vessels to relax and widen roles: controls blood pressure controls blood flow to specific regions role in memory and learning 6. endorphins - opoids a) produced in many areas of nervous system in brain: basal ganglia, cortex, amygdale, pituitary, hypothalamus, parts of medulla b) role in: pain modulation released when stressed or injured if released in absence of pain – feelings of well-being/pleasure also role in : motor coordination learning and memory gastrointestinal function control of seizures hormonal regulation of reproductive system c) termination – diffusion d) effect of some drugs morphine, heroine, Demerol bind to endorphin receptors