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Download Chap 28 – Nervous System Part 2 – Synaptic Transmission
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Chapter 28 Nervous Systems PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko Communication Between Neurons: Synaptic Transmission © 2012 Pearson Education, Inc. What happens when an action potential reaches end of neuron? Signal must be passed to another cell The synapse: – Relay point between neuron and an effector cell – Another neuron – Muscle cell – Endocrine cell © 2012 Pearson Education, Inc. 28.6 Neurons communicate at synapses Electrical synapses: – Electrical signals directly passes from neuron to receiving cell – Ex: heart, smooth muscle of digestive tract Chemical synapses: – Action potential of neuron converted to a chemical signal (neurotransmitter) – Neurotransmitter triggers response in effector cell Animation: Synapse © 2012 Pearson Education, Inc. Figure 28.6 SENDING CELL Axon of sending cell Synaptic vesicles Synaptic terminal of sending cell (Pre-synaptic cell) 1 Action potential arrives Synaptic terminal 2 3 Vesicle fuses with plasma membrane Dendrite of receiving cell Neurotransmitter is released into synaptic cleft SYNAPTIC CLEFT 4 Neurotransmitter binds to receptor RECEIVING CELL (Post-synaptic cell) Ion channels Neurotransmitter molecules Neurotransmitter Receptor Neurotransmitter broken down and released Ions 5 Ion channel opens 6 Ion channel closes Figure 28.6_1 AP reaching end of presynaptic cell triggers increase in intracellular Ca2+, which triggers release of NT 1 Sending cell Axon of sending cell Synaptic terminal of sending cell Action potential arrives Synaptic vesicles Synaptic terminal 2 Vesicle fuses with plasma membrane Dendrite of receiving cell 3 Neurotransmitter is released into synaptic cleft Synaptic cleft 4 Receiving cell Neurotransmitter binds to receptor Ion channels Neurotransmitter molecules Figure 28.6_2 Neurotransmitter either broken down by enzymes Or Recycled back to sending cell Neurotransmitter Receptor Neurotransmitter broken down and released Ions 5 Ion channel opens 6 Ion channel closes Neurotransmitters May be Excitatory or Inhibitory Excitatory – Trigger Na-gated channels to open and receiving cell starts to depolarize toward threshold – Stimulate action potential in receiving cell Inhibitory – Trigger K-gated channels to open and receiving cell hyperpolarizes – Inhibits action potential from occurring in receiving cell © 2012 Pearson Education, Inc. Figure 28.7 Synaptic terminals Dendrites Inhibitory Excitatory Myelin sheath Receiving cell body Axon Synaptic terminals Figure 28.7_1 Synaptic terminals Examples of Neurotransmitters Acetylcholine – in the brain and – at synapses between motor neurons and muscle cells. – Causes skeletal muscle contraction Serotonin and dopamine – are important neurotransmitters in the CNS – affect sleep, mood, and attention Epinephrine – Involved in involuntary nervous system – Flight or fright response © 2012 Pearson Education, Inc. Many drugs and toxins act at chemical synapses Many psychoactive drugs – act at synapses and – affect neurotransmitter action. Caffeine counters the effect of inhibitory neurotransmitters. Nicotine acts as a stimulant by binding to acetylcholine receptors. Alcohol is a depressant. © 2012 Pearson Education, Inc. Figure 28.9 Many drugs and toxins act at chemical synapses Botox – Produced by many bacteria such as Clostridium botulinum – Inhibits release of acetylcholine from synaptic vesicles – Prevents skeletal muscle contraction Snake Venoms – Inhibit acetylcholinesterase - the enzyme that breaks down acetylcholine – Can’t turn off induction of skeletal muscle contraction – Results in uncontrolled muscle contractions (tetany)