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 1 Excitatory 2Inhibitory

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?? –0.5 to 1 millivolt 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 discharges from 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 Prevent over excitation

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 Stimulants: Caffeine, Theophylline, and Theobromine, –found in coffee, tea, and cocoa By reducing the threshold for excitation of neurons. Strychnine inhibits the action of some inhibitory transmitter substances Inhibitory Most Anesthetics increase the neuronal membrane threshold for excitation

Is the minimum time required for transmission across the synapse Synaptic delay Is the minimum time required for transmission across the synapse the 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

Convergence When many pre-synaptic neurons converge on any single post-synaptic neuron

Divergence Axons of most pre-synaptic neurons divide into many branches that diverge to end on many post- synaptic neurons.

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