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Chapter 5Lec.docx

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University of Toronto Mississauga
Ayesha Khan

Chapter 5 • Communication Between Neurons The Concept of Synapse • Neurons communicate by transmitting chemicals at junctions, called “synapses” In 1906, Charles Scott Sherrington (1857 - 1952) coined the term synapse to describe the specialized gap that existed between neurons Sherrington’s discovery was a major feat of scientific reasoning Properties of Synapses • Sherrington investigated how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli) • Sherrington observed that repeated stimuli over a short period of time produced a stronger response • This led to the idea of temporal summation Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak • Presynaptic neuron: neuron that delivers the synaptic transmission • Postsynaptic neuron: neuron that receives the message • Excitatory postsynaptic potential (EPSP): graded potential that decays over time and space • The cumulative effect of EPSPs are the basis for temporal and spatial summation • Sherrington also noticed that several small stimuli on a similar location produced a reflex when a single stimuli did not • This led to the idea of spatial summation or that synaptic input from several locations can have a cumulative effect and trigger a nerve impulse • Sherrington also noticed that during a reflex, the leg of his test subject that was pinched retracted while the other three legs were extended Inhibitory postsynaptic potential (IPSP) or the temporary hyperpolarization of a membrane An ISPS occurs when synaptic input selectively opens the gates for positively charged potassium ions to leave the cell or negatively charged chloride ions to enter the cells Serves as an active “brake” that suppresses excitation Relationship Among EPSP, IPSP, and Action Potentials The Discovery of Chemical Transmission at Synapses • German physiologist Otto Loewi (1873 – 1961) was the first to convincingly demonstrate that communication across the synapse occurs via chemical means • Neurotransmitters are chemicals that travel across the synapse and allow communication between neurons • Chemical transmission predominates throughout the nervous system Types of Synapses • Axodendritic Axon - dendrite • Axomuscular Axon - muscle • Axosomatic Axon - cell body • Axosynaptic Axon - terminal • Axoaxonic Axon - axon • Axoextracellular Axon - extracellular fluid • Axosecretory Axon - capillary
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