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BIO204 Physio Lecture note 9.docx

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Ingo Ensminger

BIO204 Nov.24/2011 Chapter 45: Electric Signals in animals (pg.1068-1078) 45.3 Synapse  Neurotransmitters are molecules that transmit information from one neuron to another neuron or from a neuron to a target cell in a muscle or gland.  The interface between two neurons is called a synapse. Just inside the synapse, the axon contains synaptic vesicles that serve as storage sites for neurotransmitters (Figure 45.14).  The model of synaptic transmission is shown in Figure 45.15.  The sending cell is called the presynaptic neuron and the receiving cell is called the postsynaptic neuron.  Sequence of events begins when depolarization created by AP opens voltage gated Ca channels located near synapse in presynpatic membrane.  Electrochemical gradient for ca results in inflow of Ca ions though open channels.  In response to increase in Ca [] inside the axon, synaptic vesicles fuse with membrane and release a neurotransmitter into gap between cells. Gap is called synaptic cleft. What do neurotransmitters do?  There are several categories of neurotransmitters (Table 45.2).  To qualify as a neurotransmitter, the molecule must satisfy three criteria: (1) be present at the synapse and released in response to an action potential, (2) bind to a receptor on a postsynaptic cell, and (3) be taken up or degraded.  Many neurotransmitters function as ligands—molecules that bind to a specific site on a receptor molecule—that bind to ligand–gated ion channels.  Some other neurotransmitters bind to receptors that activate enzymes for production of a second messenger that may trigger changes in gene expression, enzyme activity, or membrane potential.  Two major types of synapses are distinguished by whether they lead to depolarization of hyperpolarization of the membrane.  Excitatory postsynaptic potentials (EPSPs) cause the membrane to depolarize; inhibitory postsynaptic potentials (IPSPs) cause the membrane to be hyperpolarized (Figure 45.16).  Synapses can be excitatory, inhibitory, or modulatory.  EPSPs and IPSPs are not all–or–none events but are graded in size and short–lived.  The size of an EPSP or IPSP depends on the amount of neurotransmitter that is released at the synapse. Both types of signals are short lived because neurotransmitters do not bind irreversibly to channels in post synaptic cells. Instead they are quickly activated and recycled.  Neurons typically make hundreds or thousands of synapses with other cells. At any instant, the EPSPs and IPSPs that occur at each of these synapses lead to short–lived surges of charge in the postsynaptic cell.  If an IPSP and an EPSP occur close together in space or time, the changes in membrane potential tend to cancel each other out.  If several EPSPs occur close together, they sum and make the neuron likely to fire an action potential (Figure 45.17). The additive nature of EPSPs and IPSPs is called summation.  The sodium channels that trigger action potentials in the postsynaptic cell are located near the start of the axon at a site called the axon hillock.  As IPSPs and EPSPs interact throughout the dendrites and cell body, charge spreads to the axon hillock.  If the membrane at the axon hillock depolarizes past a point termed the threshold, enough sodium channels open to trigger positive feedback and an action potential.  Once an action potential starts at the axon hillock, it propagates down the axon to the next synapse.  Summation that occurs prior to axon hillock is a crucial phenomenon. Determines whether AP begins is postsynaptic cell. Also explains why chemical transmission on chemical signals and synapses exists in 1 place. 45.4 The vertebrae NS  Examining the anatomy of the vertebrate nervous system allows us to look at electrical signalling at the level of tissues, organs, and systems. What does peripheral nervous system do?  The PNS consists of an afferent division, which transmits sensory informat
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