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 DrugsStimulants:• 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 substancesInhibitory • Most Anesthetics increase the neuronal

membrane threshold for excitation

Synaptic delayIs the minimum time required for Is the minimum time required for transmission across the synapsetransmission 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