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Chapter 8

Chapter 8

10 Pages

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Michelle French

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Chapter Eight: Synaptic Transmission & Neural Integration: two types of synapses in the nervous system: electrical synapses and chemical synapses + 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 between neurons and glial cells + at those synapses, plasma membranes of adjacent cells are linked together by gap junctions such that when an electrical signal is generated in one cell, it is directly transferred to the adjacent cell by means of ions flowing through the gap junctions. + second messenger molecules can also move through these junctions allow rapid communication between adjacent neurons that synchronizes the electrical activity in the cells + communication often bidirectional though some gap junctions allow current flow in only one direction - can be excitatory or inhibitory at same synapse, as either a depolarizing or a hyperpolarizing current can spread through these junctions + some are always active, whereas others seem to have gating mechanisms that allow current flow only under certain circumstances identified in the retina of the eye and certain areas of the cortex, believed to function in transmission of signals + found in areas of brainstem that regulate breathing + gap junctions believed to synchronize the neurons responsible for inspiration + hypothalamic neurons that release tropic hormones are connected to other hypothalamic neurons that release the same tropic hormones by gap junctions synchronizing their activity and resulting in bursts of tropic hormone release. Vast majority of synapses in the nervous system are chemical synapses Chemical Synapses: almost all neurons transmit messages to other cells at chemical synapses one neuron secretes a neurontransmitter into the extracellular fluid in response to an action potential arriving at its axon terminal + neurotransmitter then binds to receptors on the plasma membrane of a second cell, triggering in that cell an electrical signal that may or may not initiate an action potential depending on the number of circumstances a neuron can form synapses with other neurons or with effector cells such as muscle or gland cells + muscles and glands effector organs, and a synapse between a neuron and an effector cell neuroeffector junction + neuro-effector junctions operate according to the same basic principles presented for neuron-to- neuron synapses + non-neuronal cells can form synapses with neurons, for example, wiith taste receptor cells in taste buds on the tongue Functional Anatomy of Chemical Synapses: first neuron, which transmits signals to the second, is designated the presynaptic neuron + the second neuron which receives signals from the first is referred to as the postsynaptic neuron + narrow space between the presynaptic and post-synaptic neurons is called the synaptic cleft and is generally only 30-50nm wide (1nm = 10^-9m) + signalling across a synapse is unidirectional the presynaptic neuron communicates to the postsynaptic neuron most often the presynaptic neuron's axon terminal forms a synapse with either a dendrite or the cell body of the postsynaptic neuron synapses referred to as the axodendritic or axosomatic synapses + presynaptic neuron's axon terminal forms a synapse with the postsynaptic neuron's axon terminal synapse called an axoaxonic synapse axoaxonic synapseshave a special function in modulating communication at axodendritic and axosomatic synapses axon terminal of the presynaptic neuron releases neurotransmitters into the synaptic cleft + once released into the synaptic cleft, the neurotransmitters diffuse rapidly acorss the cleft and bind to receptors on the postsynaptic neuron + binding of the neurotransmitter to the receptors produces a response in the postsynaptic neuron by signal transduction mechanisms axon terminal of the presynaptic neuron contains numerous small, membrane-bound compartments synaptic vesicles which store neurotransmitter molecules + most neurotransmitters synthesized in the cytosol of the axon terminal where the enzymes for their synthesis are located + after synthesis, neurotransmitters are actively transported into synaptic vesicles where they are stored intil their eventual release by exocytosis cytosolic calcium triggers the release of neurotransmitter by exocytosis + membrane of a neuron contains ion channels of various types that depend on their location in the neuron + most abundant in the membrane of the axon terminal are voltage-gated calcium channels open when the axon terminal is depolarized which occurs upon arrival of an action potential at the axon terminal + when calcium channels open, they allow calcium to flow down its electrochemical gradient into the axon terminal, increasing the concentration of cytosolic calcium in the axon terminal, increasing the concentration of cytosolic calcium in the axon terminal + calcium then causes the membranes of synaptic vesicles to fuse with vesicle attachment sites on the inner surface of the axon terminal membrane and undergo exocytosis releasing the neurotransmitters into the synaptic cleft amount of neurotransmitter released depends on the concentration of calcium in the cytosol of the axon terminal, depends on the frequency of action potentials in the presynaptic neuron + following a single action potential, neurotransmitter release stops within a few miliseconds because the voltage-gated calcium channels close soon after opening, and because ions are actively transported out of the axon terminal on a continual basis, bringing the cytosolic calcium concentration back to its resting level + however, if a second action potential arrives before neurotransmitter is cleared from the synaptic cleft, then cytosolic calcium levels increase + increases in cytosolic calcium levels cause more neurotransmitter to be released from the presynaptic cell increasing the amount of neurotransmitter in the synaptic cleft + when series of action potentials arrives at an axon terminal in a short term, cytosolic calcium levels increase even more releasing even more neurotransmitter the concentration of neurotransmitter in the synaptic cleft increases as the frequency of action potentials increases once in the synaptic cleft, neurotransmitter molecules diffuse away from the axon terminal and toward the postsynaptic neuron where they bind to receptors inducing a response in the postsynaptic neuron + binding of a neurotransmitter molecule to a receptor is a brief and reversible process + if neurotransmitter molecules were to remain indefinitely in the synaptic cleft following their release, they would bind to receptors over and voer again, inducing a continual response in the postsynaptic neuron continual binding of neurotransmitter to receptor does not occur because a number of processes quickly clear that neurotransmitter from the cleft terminating the signal + some neurotransmitter molecules are degraded by enzymes, located either on postsynaptic neuron's plasma membrane, on the plasma membranes of nearby glial cells, in the interstitial fluid of the synaptic cleft, or even in the cytoplasm of the presynaptic neuron or glial cells + other neuron transmitter molecules can be actively transported back into the presynaptic neuron that released them reuptake + once inside the neuron, neurotramsitter molecules usually degraded and the breakdown products recycled to form new transmitter molecules. Other neurotransmitter molecules in synaptic cleft diffuse out of the cleft + as long as neurotransmitte molecules are not bound to receptors, they may be subject to any of the fates just described + result: neurotransmitter is usually present in the synaptic cleft for only a few miliseconds after its release from the presynaptic neuron takes 0.5-5msec from the time an action potential arrives at the axon terminal before a response occurs in the postsynaptic cell + time lag synaptic delay due mostly to the time required for calcium to trigger the exocytosis of neurotransmitter + once in synaptic cleft, diffusion of neurotransmitter to receptor is so rapid that the time is negligibleSignal Transduction Mechanisms at Chemical Synapses neurotransmitter released by a presynaptic neuron induces responses in the postsynaptic cell through the signal transduction mechanism + fast or slow response in a postsynaptic neuron fast response occurs whenever a neurotransmitter binds to a channel-linked receptor/ionotropic receptor + all channel-linked receptors are ligand-gated channels + binding of the neurotransmitter opens the ion channel, allowing a specific ion or ions to permeate the plasma membrane and change the electrical properties of the postsynaptic neuron + typical response is a change in membrane potential postsynaptic potential (PSP) occurs very rapidly and turns off rapidly (within a few miliseconds) because the channel closes as soon as the neurotransmitter leaves the receptor contrast: slow responses are mediated through G protein-linke
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