Dr. Ravikant Arjariya

Associate Professor

Dept. of Physiology

BMC, Sagar MP

Properties of Synapse

1) One way conduction (Law of forward conduction).

2) Synaptic delay.

3) Hypoxia.

4) Synaptic fatigue.

5) Summation.

6) Occlusion phenomenon.

7) Subliminal fringe effect.

8) Synaptic plasticity.

9) Facilitation.

10) Effect of acidosis and alkalosis.

11) Effect of drugs.

12) Divergence.

13) Convergence.

1) One way conduction (Law of forward conduction).

The chemical synapse allows only one way conduction i.e.

from presynaptic to postsynaptic neuron and never in opposite

direction.

This is also called Bell-Magendie Law.

It occurs because NT is present only at presynaptic area and

postsynaptic has specific receptor sites.

Hence antidromically conducted signal dies out at soma due to

lack of chemical substance.

Significance- for orderly neural function.

2.Synaptic Delay

During transmission of a neuronal signal from a presynaptic

neuron to a postsynaptic neuron, a certain amount of time is

consumed in the process of

(1) discharge of the transmitter substance by the presynaptic

terminal,

(2) diffusion of the transmitter to the postsynaptic neuronal

membrane,

(3) action of the transmitter on the membrane receptor,

(4) action of the receptor to increase the membrane

permeability,

(5) inward diffusion of sodium to raise the excitatory

postsynaptic potential.

2.Synaptic Delay

Significance of delay

1) Conduction along a chain of neurons is slow if there are

many synapses.

2) It is possible to know if reflex pathway is monosynaptic or

polysynaptic by measuring the delay in transmission of impulse

from dorsal to ventral root across the spinal cord.

3. 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.

This is observed when the brain’s blood flow is temporarily

interrupted, because within 3 to 7 seconds, the person becomes

unconscious.

4.Fatigue of Synaptic Transmission

When excitatory synapses are repetitively stimulated at a rapid

rate, the 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.

Fatigue is an exceedingly important characteristic of synaptic

function because when areas of the nervous system become

overexcited, fatigue causes them to lose this excess excitability

after awhile.

Fatigue is probably the most important means by which the

excess excitability of the brain during an epileptic seizure is

finally unresponsive so that the seizure stops.

Thus, the development of fatigue is a protective mechanism

against excess neuronal activity.

Fatigue of Synaptic Transmission

The mechanism of fatigue is mainly exhaustion or partial

exhaustion of the stores of transmitter substance in the

presynaptic terminals.

The excitatory terminals on many neurons can store enough

excitatory transmitter to cause only about 10,000 action

potentials, and the transmitter can be exhausted in only a few

seconds to a few minutes of rapid stimulation.

Part of the fatigue process probably results from two other

factors as well:

(1) progressive inactivation of many of the postsynaptic

membrane receptors and

(2) slow development of abnormal concentrations of ions

inside the postsynaptic neuronal cell.

5.Summation

Summation It is of two types,

1) Spatial Summation— When many presynaptic terminals

are stimulated simultaneously there is summation or fusion of

effects in postsynaptic neuron.

2) Temporal Summation— When one presynaptic terminal is

stimulated repeatedly.

Summation causes progressive increase in EPSP.

When the EPSP becomes great enough, the threshold for firing

will be reached and an action potential will develop

spontaneously in the initial segment of the axon.

Spatial Summation

Temporal Summation-

Temporal Summation-

Each time a presynaptic terminal fires, the released transmitter substance opens the membrane channels for at most a millisecond

But the changed postsynaptic potential lasts up to 15 milliseconds after the synaptic membrane channels have already closed.

Therefore, a second opening of the same channels can increase the postsynaptic potential to a still greater level,

and the more rapid the rate of stimulation, the greater the postsynaptic potential becomes.

Thus, successive discharges from a single presynaptic terminal, if they occur rapidly enough, can add to one another; that is, they can “summate.” - temporal summation.

Temporal Summation-

6. Occlusion phenomenon

It means that when there is simultaneous stimulation of two presynaptic neurons

the response is less than the sum total of the response obtained when they are stimulated separately.

For e.g. presynaptic neuron A & B upon stimulation separately each stimulates 10 post synaptic neurons making total of 20,

But when A & B are stimulated simultaneously they stimulate less e..g 15 postsynaptic neurons only.

This decrease in response is due to some post synaptic neurons being common to both presynaptic neurons.

7.Subliminal fringe 2+2 = 5

Opposite to occlusion.

Response obtained by the simultaneous stimulation of two

presynaptic neurons is greater than the sum total of the

response obtained when they are separately stimulated.

Suppose stimulation of neuron A causes stimulation of 5 post

synaptic neurons and stimulation of neuron B causes

stimulation of 5 postsynaptic neurons and then sum of neurons

stimulated is 10.

But when neuron A and B are stimulated simultaneously

number of the postsynaptic neuron stimulated is more then 10.

7. Subliminal fringe 2+2 = 5

Subliminal means below threshold

Fringe means border.

Thus the post synaptic neurons are said to be in a subliminal

fringe if they are not discharged by activity of pre synaptic

neurons but their excitability is increased.

Those which have discharged are in discharging zone and have

fired due to development of AP in them whereas

those in periphery (fringe) are excited up to sub threshold level

only and AP is not develop.

7.Subliminal fringe 2+2 = 5

8. Synaptic plasticity

Plasticity refers to the capability of being easily moulded or changed.

Synaptic transmission can easily be increased or decreased on the basis of past experience.

These changes can be presynaptic or postsynaptic in location

and play an important role in learning and memory.

Forms of synaptic plasticity-

1) Post tetanic Potentiation

2) Long term Potentiation

3) Long term Depression

4) Habituation

5) Sensitization

Forms of synaptic plasticity

1. Post tetanic Potentiation

Tetanizing stimuli in pre synaptic neuron results in increase

postsynaptic potentials lasting for minutes to hours.

Cause is increase Ca influx in pre synaptic neuron which

increases release of NTs.

2. Long term Potentiation

If post tetanic potentiation gets more prolonged and lasts for

days it is called as long term potentiation.

Cause is due to increase intracellular Ca in post synaptic

neuron rather than presynaptic.

This occurs in HIPPOCAMPUS.

Forms of synaptic plasticity

3. Long term depression

It is opposite of long term potentiation.

There is slower stimulation of presynaptic neurons, with decrease in synaptic conduction following decreased Ca influx.

Seen commonly in Hippocampus and Cerebellum.

4. Habituation

When a stimulus is benign and is repeated over and over, the response to the stimulus gradually disappears (habituation).

This is associated with decreased release of neurotransmitter from the presynaptic terminal because of decreased intracellular Ca2+.

The decrease in intracellular Ca2+ is due to a gradual inactivation of Ca2+ channels.

Forms of synaptic plasticity

5. Sensitization-

Sensitization is the presynaptic facilitation of an impulse. It

may occur as a transient response or there is prolong

occurrence of increased postsynaptic responses after a stimulus

is paired once or several times with a noxious stimulus.

It is due to Ca mediated changes - adenyl cyclase that results in

greater production of cAMP.

Sensitization may occur even if it is reinforced by pairing of

non- noxious stimulus.

For example, the mother who sleeps through many kinds

of noise but wakes promptly when her baby cries.

9. Facilitation

When presynaptic neuron is stimulated with several successive

individual stimuli,

each stimulus may evoke a larger post synaptic potential than

that evoked by the previous stimulus.

Often the summated postsynaptic potential is excitatory but has

not risen high enough to reach the threshold for firing of the

postsynaptic neuron.

When this happens, the neuron is said to be facilitated – its

membrane potential is nearer the threshold for firing than

normal, but not yet at the firing level.

Consequently, another excitatory signal entering the neuron

from some other source can then excite the neuron very easily.

10.Effect of Acidosis or Alkalosis Alkalosis greatly increases neuronal excitability - rise in

arterial blood pH from the 7.4 norm to 7.8 to 8.0 often causes

cerebral epileptic seizures because of increased excitability of

some or all of the cerebral neurons.

This can be demonstrated especially well by asking a person

who is predisposed to epileptic seizures to overbreathe,

overbreathing blows off carbon dioxide and therefore elevates

the pH of the blood momentarily, but even this short time can

often precipitate an epileptic attack.

Conversely, Acidosis greatly depresses neuronal activity;

A fall in pH from 7.4 to below 7.0 usually causes a comatose

state. For instance, in very severe diabetic or uremic

acidosis, coma virtually always develops.

11.Effect of Drugs

Many drugs are known to increase the excitability of neurons, and others are known to decrease excitability.

For instance, caffeine, theophylline, and theobromine,

which are found in coffee, tea, and cocoa, respectively, all

increase neuronal excitability, presumably by reducing the

threshold for excitation of neurons.

Most anesthetics increase the neuronal membrane threshold for

excitation and thereby decrease synaptic transmission at many

points in the nervous system.

Because many of the anaesthetics are especially lipid soluble -

change the physical characteristic of neuronal membrane.

12.Divergence of Signals An amplifying type of divergence - an input signal spreads to

an increasing number of neurons as it passes through

successive orders of neurons in its path.

Corticospinal pathway in its control of skeletal muscles, with

a single large pyramidal cell in the motor cortex capable of

exciting as many as 10,000 muscle fibers.

Divergence into multiple tracts - the signal is transmitted in two

directions information transmitted up the dorsal columns of the

spinal cord takes two courses in the lower part of the brain:

(1) into the cerebellum and

(2) on through the lower regions of the brain to the thalamus

and cerebral cortex. in the thalamus, almost all sensory

information is relayed both into still deeper structures of the

thalamus and at the cerebral cortex.

Divergence of Signals

Convergence means signals from multiple inputs uniting to

excite a single neuron.

convergence from a single source - multiple terminals from a

single incoming fiber tract terminate on the same neuron.

The importance of this is that neurons are never excited by an

action potential from a single input terminal.

But action potentials converging on the neuron from multiple

terminals provide enough spatial summation to bring the

neuron to the excited state.

13.Convergence of Signals

13.Convergence of Signals

Convergence can also result from input signals (excitatory or inhibitory) from

multiple sources

the interneurons of the spinal cord receive converging signals from

(1) peripheral nerve fibers entering the cord,

(2) propriospinal fibers passing from one segment of the cord to another,

(3) corticospinal fibers from the cerebral cortex, and

(4) several other long pathways descending from the brain into the spinal

cord.

Then the signals from the interneurons converge on the anterior motor

neurons to control muscle function.

Such convergence allows summation of information from different sources,

and the resulting response is a summated effect of all the different types of

information.

Convergence of Signals

Home work-

Q. Write about definition of Synapse, types of synapse, & describe briefly properties of synapse. And add a note on Neurotransmitters.

Thank you