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- 1. Neuron to neuron Synapses L.Lumbuka
- 2. SYNAPTIC INTEGRATION Neurons inthe brain receive thousands of synaptic inputs from other neurons. Synaptic integration is the term used to describe how neurons ‘add up’ these inputs before the generation of a nerve impulse, or action potential. The ability of synaptic inputs to effect neuronal output is determined by a number of factors
- 3. Size, shapeand relative timing of electrical potentials generated by synaptic inputs the geometric structure of the target neuron, the physical location of synaptic inputs within that structure expression of voltage‐gated channels in different regions of the neuronal membrane.
- 4. SYNAPTIC INTEGRATION AND ITSMECHANISM Neurons within a neural network receive information from, and send information to, many other cells, at specialised junctions called synapses. Neurons are specialised for electrical signalling, with the main neuronal input signal (synaptic potentials) and the main neuronal output signal (action potentials).
- 5. Synaptic potentials occurwhen neurotransmitter binds to and opens ligand‐operated channels in the dendritic membrane, allowing ions to move into or out of the cell according to their electrochemical gradient. Synaptic potentials can be either excitatory or inhibitory depending on the direction and charge of ion movement. Action potentials occur if the summed synaptic inputs to a neuron reach a threshold level of depolarisation and trigger regenerative opening of voltage‐gated ion channels. Synaptic potentials are often brief and of small amplitude, therefore summation of inputs in time (temporal summation) or from multiple synaptic inputs (spatial summation) is usually required to reach action potential firing threshold.
- 8. SYNAPTIC KNOB OR BUTTON. ■ ■ ■ ■ ■ ■ Axonloses myelin sheath. End into small swellings as knobs Contains Mitochondria & Neurotransmitters in vesicles Circular – excitatory Flat – inhibitory. Transport –by Microtubules.
- 9. Overview of Neuronal Structural Synapses Principlecell – cell Communication Anatomical or structural synapses
- 10. TYPES OF SYNAPSES Typesof synapses there are two types of physiological synapses: electrical synapses chemical synapses
- 11. Electrical synapse • electricalsynapses are a direct electrical coupling between two cells – mediated by gap junctions, which are pores (as shown in the electron micrograph) constructed of connexin proteins – essentially result in the passing of a gradient potential (may be depolarizing or hyperpolarizing) between two cells • very rapid (no synaptic delay) • passive process --> signal can degrade with distance-> may not produce a large enough depolarization to initiate an action potential in the postsynaptic cell • bidirectional – i.e., "post"synaptic cell can actually send messages to the "pre"synaptic cell
- 13. Chemical Synapse Thevast majority of synapses in the nervous system are chemical synapses. A chemical synapse, in contrast, converts an electrical signal into a controlled chemical signal, so there is no loss of strength. ● Synaptic vesicles. The axon terminal of the presynaptic neuron of every chemical synapse houses synaptic vesicles. These vesicles contain chemical messengers called neurotransmitters that transmit signals from the presynaptic to the postsynaptic neuron. ● Synaptic cleft. Whereas the cells of an electrical synapse are electrically connected by gap junctions, the cells of a chemical synapse are separated by a larger but still microscopic space called the synaptic cleft. The synaptic cleft measures 20–50 nm and is filled with extracellular fluid. ● Neurotransmitter receptors. In chemical synapses the postsynaptic neuron must have receptors for the neurotransmitters that the presynaptic neuron releases or it cannot respond to the signal being transmitted. Receptors are generally linked either directly or indirectly to ion channels.
- 15. RECEPTO RS–-A chemical synapse may haveeither an ionotropic receptor or metabotropic receptor Ionotropic receptors are single protein complexes that combine two functions. They have recognition sites on their surfaces extending into the extracellular fluid that allow them to interact with neurotransmitter molecules. They also have the ability to open and close, allowing ions to move across the neural membrane. ionotropic receptors respond very quickly.
- 16. RECEPTORS- METABOTROP IC Metabotropic receptorsare made up of multiple protein complexes embedded in the neural membrane. One complex has the capacity to recognize neurotransmitter molecules but cannot open and close. Instead, the metabotropic receptor binds molecules of neurotransmitter It releases a G protein, or "second messenger," from its surface extending into the intracellular fluid of the neuron
- 17. METABOTROPIC RECEPTOR This Gprotein travels away from the receptor where it can interact with adjacent ion channels, which can then open and close like the ionotropic receptor. The eventual opening of ion channels is slower than it is with ionotropic receptors.
- 18. in contrast,chemical synapses are slow active (require ligand-gated channels) pseudo- unidirectional
- 20. OUTCOMES OF SYNAPTIC TRANSMISSION-PostsynapticPotentials The out comes of the transmission at a synapse are either excitatory or inhibitory. Excitatory potentials caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neuron to generate an action potential. An inhibitory postsynaptic potential caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more difficult for a postsynaptic neuron to generate an action potential.
- 23. EPSP The membranepotential of the postsynaptic neuron moves closer to threshold. A small, local depolarization called an excitatory postsynaptic potential (EPSP) occurs, which brings the membrane of the postsynaptic neuron closer to threshold. If the membrane potential reaches threshold, an action potential is triggered. EPSPs typically result from the opening of ion channels such as those for sodium or calcium ions and the entrance of positive charges into the postsynaptic neuron. A single EPSP produces only a very small, local potential. However, each successive EPSP makes the membrane more depolarized and so more likely to reach threshold and fire an action potential.
- 24. EPSP • The restingpotential of neurons is approximately -70 mV. • The resulting EPSP will depolarize the post synaptic membrane potential, bringing it toward 0 mV.
- 25. IPSP The membranepotential of the postsynaptic neuron moves away from threshold. A small, local hyperpolarization known as an inhibitory postsynaptic potential (IPSP) occurs, moving the membrane of the postsynaptic neuron farther away from threshold, and so tending to inhibit an action potential from firing. This usually involves the opening of chloride or K+ ions resulting in hyperpolarization of the post synaptic membrane.
- 27. IPSP • Assume thatthe postsynaptic neuron has a resting potential of -60 mV and an action potential threshold of -40 mV. • If ECl is -70 mV, transmitter release at this synapse will inhibit the postsynaptic cell. • Since ECl is more negative than the action potential threshold. • It reduces the probability that the postsynaptic cell will fire an action potential.
- 28. • Some typesof neurotransmitters, such as glutamate, consistently result in EPSPs • Others, such as GABA, consistently result in IPSPs. • The action potential lasts about one msec, or 1/1000th of a second. • In contrast, the EPSPs and IPSPs can last as long as 5 to 10 msec. This allows the effect of one postsynaptic potential to build upon the next and so on.
- 29. SYNAPTIC INTEGRATION ■ Itis phenomenon of summation of both EPSP or IPSP at the post synaptic membrane i.e algebraically summated potential will determine transmission.
- 30. POST-SYNAPTIC INHIBITION. ■ Directpost synaptic inhibition by development of inhibitory post synaptic potential – by releasing inhibitory NT. ■ Post synaptic inhibition due to refractory period.
- 31. PRE-SYNAPTIC INHIBITION. ■ ■ ■ By actionof inhibitory neuron (C) – releases GABA By opening Cl- channels of pre synaptic terminals produces hyperpolarization. By activation of G protein. ■ ■ By opening K+ channels By directly blocking Ca channels.
- 32. FEEDBACK INHIBITION Renshaw cellinhibition. ■ It occurs in spinal alpha motor neuron ■ Neuron inhibits those neuron which excite it. ■ It serves to limit excitability of motor neurons.
- 33. RECIPROCAL INHIBITION. ■ Afferentsignal activates excitatory neurons to group of muscles & simultaneously inhibitory neurons to antagonistic muscles.
- 34. SIGNIFICANCE OF SYNAPTIC INHIBITION. ■Important for restriction over neurons & muscles to react properly & appropriately.
- 35. PROPERTIES OF SYNAPTIC TRANSMISSION. ■Facilitation. ■ Synaptic fatigue. ■ Synaptic plasticity & learning. ■ Reverberation. ■ Reciprocal inhibition. ■ After discharge. ■ Effect of acidosis & Hypoxia ■ One way conduction. ■ Synaptic delay. ■ Summation property of synapse. ■ Conversions & divergence. ■ Occlusion phenomenon. ■ Subliminal fringe effect.
- 36. ONE WAY CONDUCTION. ■ Lawof dynamic polarity or Bell Magendie law – synapse allow only one way conduction from pre to post synaptic neuron. ■ Significance – For orderly conduction of impulse in one direction only.
- 37. SYNAPTIC DELAY ■ Timelapse between arrival of nerve impulse at the pre synaptic terminal & its passage to post synaptic membrane. ■ 0.5 ms
- 38. CAUSES OF SYNAPTICDELAY ■ Release of neurotransmitter. ■ Diffusion through cleft ■ Binding with post synaptic receptors & opening ion channels. ■ Diffusion of ions causing RMP.
- 39. SUMMATION PROPERTY OFSYNAPSE. ■ Property of summation is essential for stimulation of post synaptic membrane either by stimulations of large no of Presynaptic terminals or repeated stimulation of single terminal.
- 40. SYNAPTIC FATIGUE. ■ Presynaptic neuron when stimulated continuously there is Gradual Decrease & finally disappearance of post synaptic response. ■ Cause – ■ Gradual inactivation of Ca ■ Accumulation of waste.
- 41. SYNAPTIC PLASTICITY & LEARNING. ■Synaptic transmission can be increased or decreased on the basis of past experience ■ Post tetanic potentiation. ■ Long term potentiation ■ Sensitization ■ Long term depression.
- 42. REVERBERATION. ■ Passage ofimpulse from pre synaptic neuron and again back to presynaptic neuron to cause continuous stimulation of Pre synaptic Neuron.
- 43. EFFECT OF ACIDOSIS& HYPOXIA ■ Synaptic transmission is vulnerable to acidosis & Hypoxia.
- 44. Neuronal Pools Neuronalpools are groups of interneurons within the CNS. These pools typically are a tangled mat of neuroglial cells, dendrites, and axons in the brain, while their cell bodies may lie in other parts of the CNS. Each neuronal pool begins with one or more neurons called input neurons that initiate the series of signals. The input neuron branches repeatedly to serve multiple neurons in the pool; however, it may have different effects on different neurons. For some neurons, it may generate EPSPs that trigger an action potential, and for others, it may simply bring the neuron closer to threshold.
- 45.
- 46. Neural Circuits Theseform either divergent or convergent synapses. Diverging circuit begins with one axon of an input neuron that branches to make contacts with multiple postsynaptic neurons. The axons of these postsynaptic neurons then branch to contact more neurons. Converging circuit is essentially the opposite of a diverging circuit. In converging circuits, axon terminals from multiple input neurons converge onto a single postsynaptic neuron, allowing for spatial summation of synapses. Diverging circuits are important in transmitting incoming sensory information, which is sent from neurons in the spinal cord to different neuronal pools in the brain for processing. Converging circuits are useful in control of skeletal muscle movement—the interneurons in the spinal cord receive input from neurons in different regions of the brain, which then converges to synapse on the motor efferent neurons that stimulate skeletal muscle contraction. Converging circuits also allow the nervous system to respond to the sensory information that it collects and processes.
- 49.