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SYNAPSE between neurons and classification
- 1. SYNAPSE Dr Bishara PPushkar
- 2. • Synapse isthe junction between 2 neurons • Specialized junction between a neuron and a muscle cell, gland cell or receptor cell
- 3. Classification • Anatomical Axodendritic, axosomatic,axoaxonic • Physiological/ Functional Chemical, electrical, conjoint
- 4. Anatomical classification Axodendritic • 98%of cerebral cortical and 80% spinal cord synapse Axosomatic • 20% of spinal and 2% of cerebral cortical synapse Axoaxonal • Seen in spinal cord
- 5. Physiological / Functionalclassification Chemical • Synaptic cleft present • NT from presynaptic neuron excite/ inhibit postsynaptic neuron Electrical • Pre and post synaptic cells come closer to form gap junction • Ions pass through freely • In lateral vestibular nucleus Conjoint • Both electrical and chemical transmission occurs
- 6. Structure of chemicalsynapse • Components – Presynaptic nerve terminal, Synaptic cleft, Post synaptic membrane Presynaptic neuron • Synaptic knob • Mitochondria and synaptic vesicles • Thickened regions- active zones • Voltage gated Ca channels
- 7. Synaptic cleft • 20-40nm wide Postsynaptic neuron • Post synaptic density • Receptor proteins
- 8. Synaptic transmission Arrival ofaction potential in axon terminal ↓ Opening of voltage gated Ca channels ↓ Ca trigger fusion and exocytosis of vesicles ↓ “ Kiss and run” discharge of small clear vesicles containing ACh
- 9. ↓ NT pass throughsynaptic cleft ↓ NT bind with receptor ↓ Cause opening or closure of ion channels ↓ Cause depolarization(EPSP) or hyperpolarization(IPSP)
- 10. Synaptic delay • Whenan impulse reaches a presynaptic terminal an interval of at least 0.5 ms occurs before a response is obtained in post synaptic neuron • Time taken for NT release and its action on receptor • Determine whether the pathway is monosynaptic and polysynaptic
- 11. Electrical events inpost synaptic neuron • EPSP (Excitatory postsynaptic potential) • IPSP (Inhibitory postsynaptic potential) • AP ( Action Potential)
- 12. EPSP • A singlestimulus to an excitatory synaptic knob produces a transient partial depolarization of postsynaptic neuron • Ionic basis • Opening of Na channel - ↑in Na Influx • Opening of Ca channel - ↑in Ca Influx • Closing of K channel - ↓ in K efflux • Closing of Cl channel - ↓ Cl influx
- 13. Summation of EPSP •Spatial summation – • When > 1 synaptic knob is active at the same time their EPSP summate to reach firing level -> produce AP • Temporal summation • Repeated activity of a synaptic knob in quick succession cause temporal summation of EPSP -> produce AP
- 14. Action potential • EPSPsummate • When +10-+15mv of depolarization occurs it reaches firing level and AP is produced • AP is propagated
- 15. IPSP • When aninhibitory synaptic knob becomes active it cause hyperpolarization of post synaptic neuron • NT- GABA, Glycine • Ionic basis • Opening of Cl channel - ↑in Cl Influx • Opening of K channel - ↑in K efflux • Closing of Na channel - ↓ in Na influx • Closing of Ca channel - ↓ Ca influx
- 16. Synaptic inhibition • Presynapticand postsynaptic inhibition
- 17. Presynaptic inhibition • Inhibitionoccurs at the presynaptic terminals before the signal ever reaches the synapse • Inhibitory interneuron terminals form axoaxonal synapse on excitatory ending • Eg:- Gate control theory of pain
- 18. • Mechanism :- •Inhibitory NT is released (eg:-GABA) • Increase Cl influx or K efflux
- 19. Post synaptic inhibition •DIRECT Reciprocal inhibition • Afferent fiber from muscle spindle in skeletal muscle project directly to spinal motor neuron supplying same muscle • Produce EPSP AP muscle contraction • At the same time, IPSPs are produced in the antagonistic muscle via inhibitory interneuron interposed between afferent fiber and motor neuron • Antagonistic muscle relax
- 21. Renshaw cell inhibition •Spinal motor neuron gives off collateral that synapse with an inhibitory interneuron • Interneuron terminate on spinal motor neurons • Impulses in motor neuron activate interneuron to secrete inhibitory NT • This decreases discharge from spinal motor neuron
- 22. Feed forward inhibition •In cerebellum • Both Basket cells and Purkinje cells are excited by Parallel fibres • But Basket cells inhibit Purkinje cells • Limits the duration of excitation
- 23. • INDIRECT • Inhibitiondue to effect of previous discharge of post synaptic neuron • Post synaptic neuron is in its absolute refractory period • Or during after hyper polarization it is less excitable
- 24. PROPERTIES OF SYNAPSE Convergence •Many pre synaptic terminals converge on a single post synaptic neuron Divergence • A single pre synaptic neuron makes contact with many post synaptic neurons
- 25. One way conduction •Neurotransmitter vesicles are present only in presynaptic neuron Synaptic delay Synaptic inhibition Summation Fatigue • Repeated stimulation of presynaptic neurons leads to gradual decrease and final disappearance of postsynaptic response • d/t exhaustion of chemical transmitter
- 26. Occlusion • Response tostimulation of B & C together is smaller than the sum of responses to stimulation of B & C separately • This decrease in response due to pre synaptic fibers sharing post synaptic neurons is called occlusion
- 27. • Subliminal fringe •Post synaptic neurons are said to be in subliminal fringe if they are not discharged by the activity of presynaptic neurons, but their excitability is increased
- 28. • After discharge •A single instantaneous signal input can cause a sustained signal output (a series of repetitive discharges) – after discharge • Lasts for many ms to min after the input is over
- 29. Synaptic plasticity • Refersto capability of being easily moulded or changed • Synaptic transmission can be increased or decreased on the basis of past experience • Basis of learning and memory
Editor's Notes
- #4 Dendrodentritic between mitral and granule cells of olfactory bulb