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3. Synapses.pdf

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York University
PSYC 2240

3. Synapses Wednesday, June 12, 20137:09 PM MainIdeas 1) At a synapse, a neuron releases neurotransmitters (chemicals) that excite or inhibit another cell or alter its response to other input. 2) In most cases, a single release of neurotransmitter produces only a subthreshold response in the receiving cell. This response summates with other subthreshold responses to determine whether or not the cell produces an action potential. 3) Transmission at synapses goes through many steps, and interference at any of them can alter the outcome. 4) Nearly all drugs that affect behavior or experience do so by acting at synapses. 5) Nearly all abused drugs increase the release of dopamine in certain brain areas. 6) Addiction changes certain brain areas, increasing the tendency to seek the addictive substance and decreasing the response to other kinds of reinforcement. 3.1 The Concept of theSynapse The Properties of Synapses • Synapse-- a specialized gap between neurons ○ First discovered by Charles Scott Sherrington in 1906 • Reflexes-- automatic muscular responses to stimuli • Reflex arc-- the circuit from sensory neuron to muscle response • Several properties of reflexes suggest special processes at the junctions between neurons (Sherrington's evidence for synaptic delay): 1) Reflexes are slower than conduction along an axon 2) Several weak stimuli presented at slightly different times or slightly different locations produce a stronger reflex than a single stimulus does 3) When one set of muscles becomes excited, a different set becomes relaxed • Speed of a Reflex and Delayed Transmissionat the Synapse ○ The speed of conduction through the reflex arc varies but is never more than 15 m/s (slower than conduction along the axon) because information must be transferred at the synapse • Temporal Summation ○ Temporal summation-- repeated stimuli within a brief time have a cumulative effect  A single stimuli may not be enough to reach the threshold for a synaptic transmission ○ Postsynaptic neuron-- the cell that receives the message ○ Presynaptic neuron-- the neuron that delivers the synaptic transmission ○ Excitatory postsynaptic potential (EPSP)-- a graded potential (e.g., a potential that can be either depolarizations (excitatory) or hyperpolarizations (inhibitory))  an action potential is always a depolarization • Spatial Summation ○ Spatial summation-- synaptic inputs from separate locations combine their effects on a neuron (e.g., pinching simultaneously in two separate places)  Pinching two points activated two sensory neurons, whose axons converged onto one neuron in the spinal cord • Inhibitory Synapses • Inhibitory Synapses ○ Interneuron-- an intermediate neuron ○ A pinch on the foot sends a message along a sensory neuron to an interneuron in the spinal cord, which in turn excites the motor neurons connected to the flexor muscles of that leg ○ The interneuron also sends a message to block activity of motor neurons to the extensor muscles in the same leg and the flexor muscles of the three other legs ○ Inhibitory postsynaptic potential (IPSP)-- occurs when synaptic input selectively opens the gates for potassium ions to leave the cell (carrying a positive charge with them) or for chloride ions to enter the cell (carrying a negative charge) in order to decrease the probability of an action potential Relationship Among EPSP, IPSP, and Action Potentials • Most neurons have a spontaneous firing rate-- a periodic production of action potentials even without synaptic input ○ EPSPs increase the frequency of action potentials above the spontaneous rate (depolarizing); IPSPs decrease it below that rate (hyperpolarizing) • The summation of EPSPs and IPSPs determine whether or not the postsynaptic cell fires an action potential 3.2 Chemical Eventsat theSynapse The Discovery of Chemical Transmission at Synapses • Synaptic transmission depends on a chemical process, not an electrical process as Sherrington believed • The sympathetic nervous system accelerates the heartbeat, relaxes the stomach muscles, dilates the pupils of the eyes, and regulates other organs ○ Adrenaline produces these effects • What was Loewi's evidence that neurotransmission depends on the release of chemicals? ○ When Loewi stimulated a nerve that increased or decreased a frog's heart rate, he could withdraw some fluid from the area around the heart, transfer it to another frog's heart, and thereby increase or decrease its rate also. The Sequence of Chemical Events at the Synapse • Major chemical events at the synapse: 1) The neuron synthesizes chemicals that serve as neurotransmitters 2) The neuron transports the neuropeptides that were formed in the cell body to the axon terminals or to the dendrites 3) Action potentials travel down the axon. At the presynaptic terminal, an action potential enables calcium to enter the cell. Calcium releases neurotransmitters from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neuron. 4) The neurotransmitter molecules separate from their receptors 5) The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling (reuptake) or may diffuse away 6) Some postsynaptic cells send reverse messages to control further release of neurotransmitter by presynaptic cells • Types of Neurotransmitters ○ Neurotransmitters-- chemicals released by one neuron that affect a second neuron  Major categories of neurotransmitters: amino acids, neuropeptides (chains of amino acids), acetylcholine, monoamines (e.g., serotonin, dopamine, norepinephrine), purines, and gases ○ Almost all neurotransmitters are amino acids, derivatives of amino acids, or chains of amino acids  One exception is nitric oxide (a gas)-- dilates blood vessels increasing blood flow to certain areas of the brain of the brain • Synthesis of Transmitters ○ Like any other cell in the body, a neuron synthesizes the chemicals it needs from substances in the diet ○ Catecholamines-- epinephrine, norepinephrine, and dopamine  Synthesized from phenylalanine (from diet) ○ Tryptophan-- the precursor to serotonin (common in foods such as soy) • Transport and Storage of Transmitters ○ The most abundant neurotransmitters are synthesized in the presynaptic terminal, near the point of release ○ Neuropeptides are synthesized in the cell body and then transported down the axon or into the dendrites (neuropeptides are released from many sites in the cell) ○ Vesicles-- tiny nearly spherical packets in the presynaptic terminal that store high concentrations of neurotransmitter molecules ○ Neurons that release dopamine, serotonin, or norepinephrine contain an enzyme, MAO (monoamine oxidase), which breaks down these neurotran
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