synapse types and classification and .pdf
- 2. Chemical Synapses Synapse- Junction 1. Pre synaptic neuron 2. Post synaptic neuron 3. Synaptic cleft 4. Neurotransmitter
- 3. Types of synapses Neuron to neuron Neuron to muscle- Neuromuscular junction
- 4. Neuron to neuron synapse A neuron-to-neuron synapse involves a junction between an axon terminal of a presynaptic neuron and the dendrites or cell body of a postsynaptic neuron. The axon terminal of the presynaptic neuron, which conducts its action potentials toward the synapse, ends in a slight swelling, the synaptic knob.
- 5. 1. Action potential reaches axon terminal of presynaptic neuron. 2. Ca2+ enters synaptic knob (presynaptic axon terminal). 3. Neurotransmitter (NT) is released by exocytosis into synaptic cleft. 4. NT binds to receptors that are chemically gated channels on subsynaptic membrane of postsynaptic neuron. 5. Binding of NT to receptor opens that specific channel. Structure and function of single synapse (Neuron-neuron synapse)
- 6. Chemical signal Conversion of the electrical signal in the presynaptic neuron (an action potential) to an electrical signal in the postsynaptic neuron by chemical means takes time. This synaptic delay (the major disadvantage of chemical compared to electrical synapses) is usually about 0.5 to 1 msec neurotransmitter– receptor combination
- 7. EPSP and IPSP • Graded potentials and action potentials are two types of electric potentials that occur in the nervous system. • The graded potentials arise by the action of ligand-gated ion channel proteins. The action potentials arise by the voltage-gated sodium and potassium channels EPSP facilitates the firing of an action potential on the postsynaptic membrane whereas IPSP lowers the firing of the action potential. Two types of postsynaptic potentials - EPSP and IPSP. • EPSP- Excitatory Postsynaptic Potential • IPSP - Inhibitory Postsynaptic Potential. • EPSP is a temporary depolarization that is caused by the flow of positively-charged ions into the postsynaptic cell while IPSP is a hyperpolarization caused by the flow of negatively- charged ions into the postsynaptic cell.
- 8. EPSP- Excitatory Postsynaptic Potential • The non-propagated electrical potential • The binding of excitatory neurotransmitters, that are released from the presynaptic membrane, results in the formation of EPSP. • The common excitatory neurotransmitter is acetylcholine. At the synapses, acetylcholine acts by attaching to receptors and activating ligand-gated channels. • As a result, sodium ions with positive charges start to enter the postsynaptic cell. • This makes the inside of the membrane slightly less negative than resting potential, thus producing a small depolarization of the post synaptic neuron. EPSP is restricted to the synapse only. It typically increases the neurons’ membrane potential. The combined effect of multiple EPSPs on a single region of the postsynaptic membrane equals the sum of the individual EPSPs. EPSP possesses 2 main qualities: 1. It is non-propagated. 2. It disobeys the all-or-none law. EPSP results in the axon’s development of an action potential. Acetylcholine Glutamate Aspartate etc
- 9. • An electric charge on the postsynaptic membrane, which makes the postsynaptic membrane less likely to generate an action potential. • The IPSP is caused by the flow of negatively-charged chloride ions into the postsynaptic neuron. • The binding of the inhibitory neurotransmitters to the receptors of the postsynaptic membrane causes the opening of the ligand-gated chloride ion channels. This results in a hyperpolarization of the postsynaptic membrane. The hyperpolarization makes the postsynaptic membrane less likely to generate an action potential. • The most common inhibitory neurotransmitters are glycine and GABA. IPSP- Inhibitory Postsynaptic Potential Serotonin Dopamine Glycine etc
- 10. (a) If an excitatory presynaptic input (Ex1) is stimulated a second time after the first EPSP in the postsynaptic cell has died off, a second EPSP of the same magnitude will occur. (b) If, however, Ex1 is stimulated a second time before the first EPSP has died off, the second EPSP will add onto, or sum with, the first EPSP, resulting in temporal summation, which may bring the postsynaptic cell to threshold. (c) The postsynaptic cell may also be brought to threshold by spatial summation of EPSPs that are initiated by simultaneous activation of two (Ex1 and Ex2) or more excitatory presynaptic inputs. (d) Simultaneous activation of an excitatory (Ex1) and inhibitory (In1) presynaptic input does not change the postsynaptic potential, because the resultant EPSP and IPSP cancel each other out. Determination of the grand postsynaptic potential by the sum of activity in the presynaptic inputs
- 11. 1. An action potential in a motor neuron is propagated to the axon terminal (terminal button). 2. This local action potential triggers the opening of voltage-gated Ca2+ channels and the subsequent entry of Ca2+ into the terminal button. 3. Ca2+ triggers the release of acetylcholine (ACh) by exocytosis from a portion of the vesicles 4. ACh diffuses across the space separating the nerve and muscle cells and binds with receptor- channels specific for it on the motor end plate of the muscle cell membrane. 5. opening of these nonspecific cation channels, leading to a relatively large movement of Na+ into the muscle cell compared to a smaller movement of K+ outward. 6. The result is an end-plate potential. Local current flow occurs between the depolarized end plate and the adjacent membrane. 7. Opening of voltage-gated Na+ channels in the adjacent membrane. 8. The resultant Na+ entry reduces the potential to threshold, initiating an action potential, which is propagated throughout the muscle fiber. 9. ACh is subsequently destroyed by acetylcholinesterase Events at a neuromuscular junction
- 12. Neurotransmitter • Signaling molecule secreted by a neuron to affect another cell across a synapse. • Neurotransmitters are released from synaptic vesicles into the synaptic cleft where they are able to interact with neurotransmitter receptors on the target cell • Neurotransmitters are generally stored in synaptic vesicles, clustered close to the cell membrane at the axon terminal of the presynaptic neuron. • Small-molecule neurotransmitters are synthesized at nerve terminals. The enzymes necessary for neurotransmitter synthesis are made in the cell body of the presynaptic cell • Transported down the axon by slow axonal transport. • Precursors are taken up into the terminals by specific transporters. • Neurotransmitter synthesis and packaging take place within the nerve endings. • After vesicle fusion and release, the neurotransmitter may be enzymatically degraded. The reuptake of the neurotransmitter (or its metabolites) starts another cycle of synthesis, packaging, release, and removal
- 13. Small molecule neurotransmitters can be subdivided into groups based on chemical structure. • Amino acid transmitters include glutamate, GABA, and glycine. • The biogenic amines include serotonin and histamine, and the catecholamines, a subgroup of the biogenic amines, include dopamine, norepinephrine, and epinephrine. • Acetylcholine does not fit into a group. Small molecule neurotransmitters
- 15. Unlike the other small molecule neurotransmitters, norepinephrine is synthesized within the vesicles, not in the cytoplasm.
- 16. Neuropeptides • Neuropeptides are larger molecules made up of anywhere from 2 to about 40 amino acids. • They are synthesized in the neuronal cell body in the endoplasmic reticulum and Golgi complex and are subsequently moved by axonal transport along the microtubular highways to the axon terminal • Neuropeptides are not stored within small synaptic vesicles but instead are packaged in large dense-core vesicles, which are also present in the axon terminal. • The dense-core vesicles undergo Ca2+-induced exocytosis and release neuropeptides at the same time that the neurotransmitter is released from the synaptic vesicles. • An axon terminal typically releases only a single classical neurotransmitter, but the same terminal may also contain one or more neuropeptides that are co-secreted along with the neurotransmitter. Some known or suspected NP