Classification and structure of synapses

Synapses can be classified by the type of cellular structures serving as the pre- and post-synaptic components. ... The axon can synapse onto a dendrite, onto a cell body, or onto another axon or axon terminal, as well as into the bloodstream or diffusely into the adjacent nervous tissue.

1. Classification And Structure Of Synapses Mechanisms Of Excitation Conduction In A Chemical Synapse Student name: ALKOLBEE ALAA RADHI MUHSSIN Lesson: Biochemistry of hormones Prof.name: Nina kanunnikava Yanka Kupala State University of Grodno

2. Synapse is the junction between two neurons. It is not an anatomical continuation . But , it is only a physiological continuity between two nerve cells. CLASSIFICATION OF SYNAPSE: Synapse is classified by two methods: A. Anatomical classification B. Functional classification.

3. Anatomical Classification Usually synapse is formed by axon of one neuron ending on the cell body, dendrite or axon of the next neuron. Depending upon ending of axon, synapse is classified into three types: 1 . Axo-axonic synapse in which axon of one neuron terminates on axon of another neuron 2 . Axo-dendritic synapse in which the axon of one neuron terminates on dendrite of another neuron 3. Axo-somatic synapse in which axon of one neuron ends on soma (cell body) of another neuron

4. Functional Classification Functional classification of synapse is on the basis of mode of impulse transmission 1. Electrical Synapse: is the synapse in which the physiological continuity between the presynaptic and the postsynaptic neurons is provided by gap junction between the two neurons. 2. Chemical synapse: Is the junction between a nerve fiber and a muscle fiber or between two nerve fibers, through which the signals are transmitted by the release of chemical transmitter

5. On The Basis Of Functions, Synapses Are Divided Into Two Types : 1. Excitatory synapses: which transmit the impulses (excitatory function) -Excitatory Postsynaptic Potential: Excitatory postsynaptic potential (EPSP) is the non propagated electrical potential that develops during the process of synaptic transmission. 2. Inhibitory synapses , which inhibit the transmission of impulses (inhibitory function) -Postsynaptic or Direct Inhibition: Postsynaptic inhibition (IPSP) is the type of synaptic inhibition that occurs due to the release of an inhibitory neurotransmitter from presynaptic terminal instead of an excitatory neurotransmitter substance. It is also called direct inhibition. Inhibitory neurotransmitters are gammaaminobutyric acid (GABA), dopamine and glycine

6. Chemical Synapse Terminal bouton is separated from postsynaptic cell by synaptic cleft . • Neurotransmitter (NT) are released from synaptic vesicles. Vesicles fuse with axon membrane and NT released by exocytosis. • Amount of NTs released depends upon frequency of action potentials AP.

7. Synaptic Transmission Neurotransmitter NT release is rapid because many vesicles form fusion-complexes at “docking site.” AP travels down axon to bouton. VG Ca2+ channels open. ◦ Ca2+ enters bouton down concentration gradient. ◦ Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs. Ca2 + activates calmodulin , which activates protein kinase. Protein kinase phosphorylates synapsins. ◦ Synapsins aid in the fusion of synaptic vesicles .

8. Synaptic Transmission NTs are released and diffuse across synaptic cleft . NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. Chemically-regulated gated ion channels open . ▫ EPSP: depolarization. ▫ IPSP: hyperpolarization . Neurotransmitter inactivated to end transmission .

9. Chemical Synapse EPSP ( excitatory postsynaptic potential ): ▫ Depolarization. IPSP ( inhibitory postsynaptic potential ): ▫ Hyperpolarization

10. Difference Between EPSP and IPSP EPSP IPSP EPSP: An EPSP is an electrical charge on the postsynaptic membrane, which is caused by the binding of excitatory neurotransmitters and makes the postsynaptic membrane generate an action potential. IPSP: An IPSP is an electric charge on the postsynaptic membrane, which is caused by the binding of inhibitory neurotransmitters and makes the postsynaptic membrane less likely to generate an action potential. EPSP is caused by the flow of positively-charged ions. IPSP is caused by the flow of negatively-charged ions. EPSP: EPSP is a depolarization. IPSP: IPSP is a hyperpolarization. EPSP: EPSP brings the postsynaptic membrane towards the threshold. IPSP: IPSP takes the postsynaptic membrane away from the threshold. EPSP makes the postsynaptic membrane more excited. IPSP makes the postsynaptic membrane less excited EPSP facilitates the firing of an action potential on the postsynaptic membrane. IPSP lowers the firing of an action potential on the postsynaptic membrane EPSP is the result of the opening of the sodium channels. IPSP is the result of the opening of the potassium or chloride channels. EPSP is generated by the flow of glutamate or aspartate ions. IPSP: IPSP is generated by the flow of glycine or GABA.

11. Acetylcholine (ACh) as NT ACh is both an excitatory and inhibitory NT, depending on organ involved. ▫ Causes the opening of chemical gated ion channels . • Nicotinic ACh receptors : Found in autonomic ganglia and skeletal muscle fibers . • Muscarinic ACh receptors : Found in the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands.

12. Acetylcholine (ACh) as NT Most direct mechanism. Ion channel runs through receptor . ▫ Receptor has 5 polypeptide subunits that enclose ion channel . ▫ 2 subunits contain ACh binding sites . Channel opens when both sites bind to ACh. ▫ Permits diffusion of Na + into and K+ out of postsynaptic cell. Inward flow of Na+ dominates . ▫ Produces EPSPs

13. GABA (gamma-aminobutyric acid): GABA (gamma-aminobutyric acid): ▫ major inhibitory NT in brain. Hyperpolarizes postsynaptic membrane. Motor functions in cerebellum . After excretion – GABA resorbed from synaptic cleft (neurons or surrounding glial cells); transport requires presence of extracellular Na + and Cl -

14. GABA (gamma-aminobutyric acid): The binding of GABA to receptor. -increases the flow of chloride (CI-) ions in the postsynaptic cells -raising its membrane potential and inhibiting it. The binding of GABA to receptors activates a second messenger opening K+ channels. produce -Inhibitory Postsynaptic Potential (IPSP) which counteracts the excitatory signals.

15. Drugs such as Phenobarbital, Valium, Librium, and other sedatives bind themselves to GABA receptors and enhance its inhibitory effect on the Central Nervous System. Amino acid is used at excitatory synapses in the Central Nervous System and is helpful in long term potentiation or memory. Serotonin and histamine also stimulate intestinal peristalsis. Neurotransmitters react differently to receptors in different areas of the brain. So while it can cause an excitatory effect in one area, it can cause an inhibitory effect in another.

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