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Physiology (114)
PSL201Y1 (46)
Yue Li (10)


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University of Toronto St. George
Yue Li

8 SYNAPTIC TRANSMISSION AND NEURAL INTEGRATION Electrical synapses: operate by allowing electrical signals to be transmitted from one neuron to another through gap junctions. Chemical synapses: operate through the release of neurotransmitters that activate signal transduction mechanisms. Electrical Synapses Exist between neurons, and neurons and glial cells. The plasma membrane of adjacent cells are linked together by gap junctions (electrical signal directly transferred to the adjacent cell by ions flowing). Allows rapid communication (usually bidirectional) between adjacent neurons that synchronizes the electrical activity in these cells. Communication can be excitatory or inhibitory at the same synapse, as either a depolarizing or a hyperpolarizing current can spread through the junctions. Chemical Synapses One neuron secretes a neurotransmitter into the extracellular fluid in response to an action potential arriving at its axon terminal. The neurotransmitter than binds to receptors on the plasma membrane of a second cell, triggering an electrical signal that may or may not initiate an action potential. Effector organs: muscles and glands. Neuroeffector junction: a synapse between a neuron and an effector cell FUNCTIONAL ANATOMY OF CHEMICAL SYNAPSES Presynaptic neuron: transmits signals to the second; Postsynaptic neuron: receives signals from the first; Synaptic cleft: the narrow space between the presynaptic and postsynaptic neurons (30-50nm wide). Signaling across a synapse is unidirectional, presynaptic neuron to the dendrite or cell body of the postsynaptic neuron (axodendritic or axosomatic synapses, respectively). In some cases, the presynaptic neurons axon terminal forms a synapse with the postsynaptic neurons axon terminal (axoaxonic synapse). Vital for modulating communication at axodendrite and axosomatic synapses. The axon terminal of the presynaptic neuron releases neurotransmitters into the synaptic cleft. Then the neurotransmitters diffuse rapidly across the cleft and bind to receptors on the postsynaptic neuron. The ligand binding produces a response in the postsynaptic neuron by signal transduction mechanisms. The axon terminal of the presynaptic neuron contains numerous small, membrane- bound compartments called synaptic vesicles, which store neurotransmitter molecules. Enzymes in the cytosol of the axon terminal synthesize neurotransmitters. After they are transported into synaptic vesicles. Cytosolic Ca triggers the release of neurotransmitter by exocytosis. The axon terminal 2+ 2+ has mostly voltage-gated Ca channels; open when depolarized. This allows Ca to go down its electrochemical gradient, increasing the [Ca ]in the axon terminal. Ca then causes the membranes of the synaptic vesicles to fuse with vesicle attachment sites on the inner surface of the axon terminal membrane and undergo exocytosis. 2+ Amount of neurotransmitters released depends on [Ca ] in the cytosol, which depends on the frequency of action potentials in the presynaptic neuron.8 SYNAPTIC TRANSMISSION AND NEURAL INTEGRATION Milliseconds after an action potential, neurotransmitter release stops because the voltage-gated Ca channels close rapidly, and because Ca ions are actively transported out of the axon terminal on a continual basis, hence cytosolic [Ca] back to resting level. If a 2 action potential arrives while there are neurotransmitters in the synaptic cleft, [Ca] increase and release more neurotransmitters. Thus the [neurotransmitter] in the synaptic cleft increases as the frequency of action potential increases. The neurotransmitter binds to receptors of the postsynaptic neuron (a brief and reversible process). Continual binding of neurotransmitter to receptor doesnt happen because many processes quickly clear the neurotransmitter in the synaptic cleft: molecules degraded by nearby enzymes, molecules can be actively transported back into the presynaptic neuron (reuptake) to be recycled, or can be diffused out of the cleft. It takes 0.5-5 msec from the time an action potential arrives at the axon terminal before a response occurs in the postsynaptic cell (synaptic delay). Mainly due to the time required for Ca to trigger the exocytosis of neurotransmitter. Diffusion of neurotransmitter to the receptor is so rapid. SIGNAL TRANSDUCTION MECHANISMS AT CHEMICAL SYNAPSES The fast response in the postsynaptic neuron occurs whenever a neurotransmitter binds to a channel-linked receptor (ionotropic receptor; ligand-gated channels). Postsynaptic potential (PSP): occurs very rapidly and turns off rapidly (milliseconds) because the channel closes as soon as the neurotransmitter leaves the receptor. Slow responses
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