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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
- Only one type of neurotransmitter in released at one synapse
- 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
- 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,
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.
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