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Properties of Synapse
Properties of Synapse. Dr Ghulam Mustafa. Learning objective’s. Discuss the properties of synapse Or Describe the factors affecting synaptic transmission. Decrement of Electrotonic Conduction in the Dendrites. Dales Law. Only one type of neurotransmitter in released at one synapse
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Properties of Synapse
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Properties of Synapse
Dr Ghulam Mustafa
Discuss the properties of synapse
Describe the factors affecting synaptic transmission
Decrement of Electrotonic Conduction in the Dendrites Dales Law Only one type of neurotransmitter in released at one synapse
One-way conduction Synapses generally permit conduction of impulses in one-way i.e. from pre-synaptic to post-synaptic neuron. Spatial Summation in Neurons Excitation of a single presynaptic terminal?? 10 to 20 millivolts - required to reach threshold Many presynaptic terminals are usually stimulated at the same time. Add to one another until neuronal excitation Spatial summation Summing simultaneous postsynaptic potentials by activating multiple terminals on widely spaced areas of the neuronal membrane Temporal Summation A presynaptic terminal fire changed postsynaptic potential lasts up to 15 milliseconds Second opening of the same channels -increase the postsynaptic potential to - still greater level Successive dischargesfrom a single presynaptic terminal Rapid enough- add to one another This type of summation is called Temporal summation. Facilitation of Neurons If the summated postsynaptic potential is excitatory…………. But has not risen high enough to reach the threshold The neuron is said to be facilitated. Another excitatory signal - excite the neuron very easily Fatigue of Synaptic Transmission. When excitatory synapses are repetitively stimulated at a rapid rate Number of discharges by the postsynaptic neuron is at first very great But the firing rate becomes progressively less in succeeding milliseconds or seconds. Fatigue of synaptic transmission. Protective mechanism Against excess neuronal activity Mechanism Of Fatigue Exhaustion or partial exhaustion of the stores of transmitter substance Progressive inactivation of many of the postsynaptic membrane receptors Slow development of abnormal conc. of ions inside the postsynaptic neuronal cell Effect of Acidosis or Alkalosis on Synaptic Transmission. Neurons are highly responsive to changes in pH Alkalosis greatly increases neuronal excitability 8.0 often causes cerebral epileptic seizures Acidosis greatly depresses neuronal activity; a fall in pH from 7.4 to below 7.0 Severe diabetic or uremic acidosis, Coma Effect of Hypoxia Neuronal excitability is also highly dependent on an adequate supply of oxygen. Cessation of oxygen for only a few seconds can cause complete inexcitability of some neurons If Brain’s blood flow is temporarily interrupted, Within 3 to 7 seconds, the person becomes unconscious. Effect of Drugs
Caffeine, Theophylline, and Theobromine, found in coffee, tea, and cocoa By reducing the threshold for excitation of neurons. Strychnine inhibits the action of someinhibitory transmitter substances
Most Anesthetics increase the neuronal membrane threshold for excitation Synaptic delayIs the minimum time required for transmission across the synapsethe synaptic delay 0.5 millisecond.
This time is taken by
Discharge of transmitter substance by pre-synaptic terminal Diffusion of transmitter to post-synaptic membrane Action of transmitter on its receptor Action of transmitter to membrane permeability Increased diffusion of Na+ to post-synaptic potential
Axons of most
pre-synaptic neurons divide into many branches that diverge
to end on many post-synaptic neurons.
Divergence Properties of synapse Dales Law One-way conduction Summation in Neurons Facilitation of Neurons Fatigue of Synaptic Transmission Effect of Acidosis or Alkalosis on Synaptic Transmission Effect of Hypoxia & Drugs Synaptic delay Convergence & Divergence