Synaptic Transmission PDF

Synaptic Transmission PROF. TARIG H MERGHANI Intended learning outcomes 1. Describe the organization of the synapse and enumerate the steps in synaptic transmission 2. List the properties of the synapse  The Synapse Junction between two neurons Occurs between an axon of a neuron with cell bod...

Synaptic Transmission PROF. TARIG H MERGHANI Intended learning outcomes 1. Describe the organization of the synapse and enumerate the steps in synaptic transmission 2. List the properties of the synapse  The Synapse Junction between two neurons Occurs between an axon of a neuron with cell body or dendrite of another neuron Synaptic Transmission Chemical conduction Unidirectional The synapse structure  The neurotransmitters Synthesis starts in the cell bodies of the neurons, stored in vesicles and then the vesicles are transported to the knob through the axon Mechanism of N.T. release Steps of neurotransmitter release 1- Arrival of an action potential to the terminal end of the axon Steps of neurotransmitter release 2- Opening of voltage-gated Ca++ channels, allowing Ca++ influx Steps of neurotransmitter release 3- Ca++ stimulates release of the NT from the vesicles by exocytosis into the synaptic cleft  Steps of neurotransmitter release 4- The NT binds its receptors on the postsynaptic membrane and generates local potentials (either excitatory or inhibitory postsynaptic potentials)  Steps of neurotransmitter release 5- Summation of excitatory postsynaptic potentials produces action potential in the postsynaptic membrane  Postsynaptic potentials Local potentials that occur due to increased permeability of postsynaptic membrane to ions following attachment of N.T. to its receptors  Postsynaptic potentials (PSPs) Are not action potentials because they are: Localized (not propagated as action potentials) Graded (not all or none like action potentials) Can be summated (unlike action potentials)  Postsynaptic potentials (PSPs) Two types: Excitatory post-synaptic potentials (EPSPs) Inhibitory post-synaptic potentials (IPSPs) EPSPs EPSPs are caused by opening of: ◦Sodium channels (Na influx) ◦Calcium channels (Ca influx)  IPSPs IPSPs are caused by: ◦Cl- influx ◦K+ efflux IPSPs hyperpolarize the postsynaptic membrane  Postsynaptic potentials A single EPSP can never generate an action potential unless summated  Summation There are two types of summation: ◦Spatial summation ◦Temporal summation Spatial summation Many synaptic knobs release their transmitter simultaneously (at the same time) on a single neuron Temporal summation One synaptic knob becomes stimulated repetitively (resulting in successive generation of EPSPs)  Inhibition at synapses Direct inhibition (or postsynaptic inhibition) ◦Action potential from a presynaptic neuron is not followed by discharge from the postsynaptic neuron due to direct inhibition (IPSP)  Inhibition at synapses Presynaptic inhibition ◦The inhibition occurs at the presynaptic neuron preventing it from releasing its excitatory NT Properties of synapses a- Transmission is uni-directional - From the presynaptic neuron (the source of the NT) to the postsynaptic neuron (the site of the receptor). b- Synaptic delay Minimum time required for synaptic transmission (= 0.5 ms). This can be used to estimate the number of synapses (from the response time of a reflex)  c- Synaptic fatigue - Failure or reduction in the frequency of conduction in a synapse following repetitive stimulation. - Occurs due to exhaustion of the neurotransmitter (release of all vesicles at the synaptic knob), or inactivation of the receptors at the postsynaptic membrane. d- Post-tetanic facilitation =Increased frequency of conduction in a synapse following repeated stimulation. - It is caused by the increased availability of calcium in the synaptic knob due to its repeated influx with the stimulation.  e- Convergence and divergence - Convergence: when multiple inputs synapse on a single postsynaptic neuron. - Divergence: when multiple outputs arise from a single presynaptic neuron. f- Summation - Multiple EPSPs (or IPSPs) are added together to produce a significant response (action potential or hyperpolarization, respectively). There are two types of summation: - Temporal - Spatial g- Synaptic excitation or inhibition The release of excitatory neurotransmitters results in excitation (action potentials) in the postsynaptic neurons, whereas the release of inhibitory neurotransmitters results in inhibition (hyperpolarization). Inhibitory N.T.: GABA, glycine, dopamine, serotonin Excitatory: A. choline, glutamate, norepinephrine, histamine (some may be excitatory and inhibitory based on the receptor)