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Physiology of Synapses in the CNS- L2-L4

Faisal I. Mohammed, MD, PhD

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Objectives

Students should be able to: Define synapse and list the types of synapse Describe the mechanism of neurotransmitter release List the major types of neurotransmitters (NT) Compare the small molecules NT and Neuropeptides Describe the resting membrane potential and Nernst

Equation Determine the how EPSP, IPSP and Presynaptic inhibition

develops Describe summation of EPSP and IPSP Describe the characteristics of synapse (Fatigue and Delay)

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The Synapse Structures important to the function of the synapse:

presynaptic vesicles: contain neurotransmitter substances to excite or inhibit postsynaptic neuron

mitochondria: provide energy to synthesize neurotransmitter

Membrane depolarization by an action potential causes emptying of a small number of vesicles into the synaptic cleft which excites or inhibits the postsynaptic neuron.

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Signal Transmission at the Synapse 2 Types of synapses

Electrical ionic current spreads to next cell through gap junctions faster, two-way transmission & capable of synchronizing

groups of neurons Chemical

one-way information transfer from a presynaptic neuron to a postsynaptic neuron axodendritic -- from axon to dendrite axosomatic -- from axon to cell body axoaxonic -- from axon to axon

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Electrical Synapse

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Neuronal Synapse and Transmitter release

Chemical synapse

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Mechanism of Neurotransmitter Release Presynaptic membranes contain voltage - gated

calcium channels. depolarization of the presynaptic membrane by an

action potential opens voltage-gated Ca2+ channels influx of Ca2+ induces the release of the

neurotransmitter substance the exact mechanism is unknown but it results in

the fusion of the synaptic vessel to the membrane and release of transmitter by exocytosis

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Signal transmission at a chemical synapse

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Action of Neurotransmitter on Postsynaptic Neuron

Postsynaptic membrane contains receptor proteins for the transmitter released from the presynaptic terminal.

These receptors contain a binding component and an ionophore component.

The binding component binds to the transmitter.

The ionophore component either opens an ion channel or activates a second messenger system.

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Ion Channels Cation channels most often allow sodium ions to pass

(some allow potassium or calcium).

Anion channels allow chloride to pass.

Transmitters that open sodium channels excite the postsynaptic neuron.

Transmitters that open chloride channels inhibit the postsynaptic neuron.

These channels open and close rapidly providing a means for rapid activation or rapid inhibition of postsynaptic neurons.

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Second Messenger Activators (SMAs) Cause prolonged changes in the activity of neurons

(seconds to months) some processes like memory require long term

changes in neuronal activity and function

About 75% of the SMAs are transduced with G-protein coupled receptors G-protein activation initiates a cascade of events

that leads to an increase in cAMP which causes protein phosphorylation which leads to alterations in the cellular activity.

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Neurotransmitter “Second Messenger” System

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Neurotransmitters Chemical substances that function as synaptic transmitters

1. Small molecules which act as rapidly acting transmitters acetylcholine, norepinephrine, dopamine, serotonin,

GABA, glycine, glutamate, NO 2. Neuropeptides

more potent than small molecule transmitters, cause more prolonged actions

endorphins, enkephalins, VIP, ect. hypothalamic releasing hormones TRH, LHRH, ect. pituitary peptides ACTH, prolactin, vasopressin, ect.

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Neurotransmitters

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