Synapse , physiology of synapses in very easy way
- 1. RASHID KHAN MOHMAND ANESTHESIA LECTURER
- 2. • The junction b/w two neurons • It is not an anatomical continuation • It is only a physiological continuity • Synapse is classified by two methods: 1. Anatomical classification 2. Functional classification
- 3. • Usually synapse is formed by axon of one neuron and soma, dendrite or axon of the next neuron • Depending upon ending of axon, synapse is classified into three types: 1. Axo-axonic synapse 2. Axo-dendritic synapse 3. Axo-somatic synapse
- 5. • This is based on mode of impulse transmission • According to this, synapse is classified into two categories: 1. Electrical synapse 2. Chemical synapse • However, generally the word synapse refers to a chemical synapse
- 6. • The synapse in which the physiological continuity b/w two neurons is provided by gap junction • There is direct exchange of ions b/w the two neurons • b/c of this, the action potential reaching the terminal portion of one neuron directly enters to other neuron • Synaptic delay is very less b/c of the direct flow of current • Impulse transmission is unidirectional • Examples are, cardiac muscle fibers, smooth
- 8. • Junction between a nerve fiber and a muscle fiber or b/w two nerve fibers • Signals are transmitted by the release of chemical transmitter • There is no continuity b/w the two neurons because of the presence of a space called synaptic cleft • Action potential reaching the presynaptic terminal causes release of neurotransmitter substance • Neurotransmitter reaches the postsynaptic neuron through synaptic cleft and causes the production of potential change
- 9. • Neuron from which the axon arises is called the presynaptic neuron • Neuron on which the axon ends is called postsynaptic neuron • Axon of the presynaptic neuron divides into many small branches before forming the synapse • These branches are known as presynaptic axon terminals
- 10. 1. Terminal knobs • Terminals which are enlarged slightly like knobs • Terminal knobs are concerned with excitatory function of the synapse 2. Terminal coils or free endings • Terminals which are wavy or coiled with free ending without the knob • These terminals are concerned with inhibitory function
- 11. • It has a definite intact membrane known as presynaptic membrane • Axon terminal has two important structures: 1. Mitochondria, which help in the synthesis of neurotransmitter 2. Synaptic vesicles, which store neurotransmitter • Membrane of the postsynaptic neuron is called postsynaptic membrane, contains some receptor proteins • Small space in between the presynaptic membrane and the postsynaptic membrane is called synaptic cleft • The cleft contains cholinesterase, which destroys acetylcholine
- 13. • On the basis of functions, synapses are divided into two types: 1. Excitatory synapses, which transmit the impulses 2. Inhibitory synapses, which inhibit the transmission of impulses
- 15. • Inhibition of synaptic transmission is classified into five types: 1. Postsynaptic or direct inhibition 2. Presynaptic or indirect inhibition 3. Negative feedback or Renshaw cell inhibition 4. Feed forward inhibition 5. Reciprocal inhibition
- 17. • It occurs due to the failure of presynaptic axon terminal to release sufficient quantity of excitatory neurotransmitter • It is mediated by axo-axonal synapses • It is prominent in spinal cord and regulates the propagation of information to higher centers in brain • This inhibitory neuron inhibits the presynaptic neuron and decreases the magnitude of action potential in presynaptic neuron
- 18. • The smaller action potential reduces calcium influx • This in turn decreases the quantity of neurotransmitter released by presynaptic neuron • So the magnitude of EPSP in postsynaptic neuron is decreased resulting in synaptic inhibition
- 20. • Renshaw cells are small motor neurons present in anterior gray horn of spinal cord • Some nerve fibers leave the spinal cord and reach the effector organ, muscles • Of these, some fibers called collaterals fibers terminate on Renshaw cells
- 21. • When motor neurons send motor impulses, some of the impulses reach the Renshaw cell by passing through collaterals • Now, the Renshaw cell is stimulated and sends inhibitory impulses to motor neurons so that, the discharge from motor neurons is reduced • In this way, Renshaw cell inhibition represents a negative feedback mechanism
- 23. • It occurs in cerebellum and control its neuronal activity • During the process of neuronal activity in cerebellum, stellate cells and basket cells inhibit the Purkinje cells by releasing GABA
- 24. • Inhibition of antagonistic muscles when a group of muscles are activated is called reciprocal inhibition • It is because of reciprocal innervation Significance of Synaptic Inhibition • Synaptic inhibition in CNS limits the number of impulses going to muscles and enables the muscles to act properly and appropriately • Thus, the inhibition helps to select exact number of impulses and to omit or block the excess ones
- 25. • Bell- Magendie Law • Synaptic Delay • Fatigue • Summation • Electrical Property
- 26. “The impulses are transmitted only in one direction in synapse” • From presynaptic neuron to postsynaptic neuron
- 27. • A short delay that occurs during the transmission of impulses through the synapse • It is due to the time taken for: 1. Release of neurotransmitter 2. Passage of neurotransmitter from axon terminal to postsynaptic membrane 3. Action of the neurotransmitter to open the ionic channels in postsynaptic membrane • Normal duration of synaptic delay is 0.3 to 0.5 millisecond
- 28. • During continuous muscular activity, synapse becomes fatigue • Fatigue at synapse is due to the depletion of neurotransmitter substance, acetylcholine • Depletion of acetylcholine occurs because of two factors: 1. Soon after the action, acetylcholine is destroyed by acetyl cholinesterase 2. Due to continuous action, new acetylcholine is not synthesized
- 29. • The fusion of effects or progressive increase in the excitatory postsynaptic potential when many presynaptic excitatory terminals are stimulated simultaneously or when single presynaptic terminal is stimulated repeatedly • Summation is of two types: 1. Spatial Summation • It occurs when many presynaptic terminals are stimulated simultaneously 2. Temporal Summation • It occurs when one presynaptic terminal is
- 31. • Electrical properties of the synapse are the EPSP and IPSP
- 32. 1. Convergence • The process by which many presynaptic neurons terminate on a single postsynaptic neuron 2. Divergence • The process by which one presynaptic neuron terminates on many postsynaptic neurons