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
2
Inhibitory
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
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 presynaptic 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 postsynaptic 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