Preshantha Govender

Roll #24

1. Definition

2. Structure

3. Function

4. Types

5. Synaptic TransmissionA. Electrical

B. Chemical

6. Transmission of Neurotransmitters

7. Excitatory & Inhibitory Neurotransmitters

8. Kiss & Run Docking

9. PropertiesA. One-way Conduction

B. Synaptic Delay

C. Fatigue

D. Summation

E. Facilitation

F. Excitatory & Inhibitory

G. Occlusion Phenomenon

H. Convergence & Divergence

10. Clinical

11. References

• The junction between two neurons is called a synapse.

• It is a specialized junction where transmission of

information takes place between a nerve fibre and

another nerve, muscle or gland cell.

• It is not the anatomical continuation. But, it is only a

physiological continuity between two nerve cells.

The synapse consists of:

1. A presynaptic ending that contains neurotransmitters,

mitochondria and other cell organelles.

2. A postsynaptic ending that contains receptor sites for

neurotransmitters.

3. A synaptic cleft or space between the presynaptic and

postsynaptic endings. It is about 20nm wide.

• The main function of the synapse is to transmit the

impulses, i.e. action potential from one neuron to another.

• They allow integration, e.g. an impulse travelling down a

neuron may reach a synapse which has several post

synaptic neurons, all going to different locations. The

impulse can thus be dispersed. This can also work in

reverse, where several impulses can converge at a

synapse

1. Synapse with another neuron

It is the junction between two nerve cells. They are of 3

types; axodendritic, axosomatic & axoaxonic

2. Neuromuscular

It is the synapse pf a motor neuron and a muscle

3. Neuroglandular

It is the synapse of a neuron and a endo/exocrine gland.

• It is the process which nerve cells communicate among

themselves or with muscles and glands.

• The synapse is the anatomic site where this

communication occurs.

• It can be of 2 types:

A. Electrical transmission

B. Chemical transmission

In these synapses the membranes of the two cells actually

touch, and they share proteins. This allows the action

potential to pass directly from one membrane to the next.

They are very fast, but are quite rare, found only in the

heart and the eye.

In a chemical synapse, electrical activity in the presynaptic

neuron is converted into the release of a chemical called a

neurotransmitter that binds to receptors located in the

plasma membrane of the postsynaptic cell.

I. At the end of the pre-synaptic neuron there are voltage-

gated calcium channels. When an action potential

reaches the synapse these channels open, causing

calcium ions to flow into the cell.

II. These calcium ions cause the synaptic vesicles to fuse

with the cell membrane, releasing their contents (the

neurotransmitter chemicals) by exocytosis.

III. The neurotransmitters diffuse across the synaptic cleft.

IV. The neurotransmitter binds to the neuroreceptors in the

post-synaptic membrane, causing the channels to

open.

• Inhibitory neurotransmitters produce a depolarization of the postsynaptic membrane called the inhibitory postsynaptic potential (IPSP). They reduce chances of a new impulse.

I. Serotonin

II. GABA (γ aminobutyric acid)

III. Glycine

• Excitatory neurotransmitters can cause the next cell to initiate a new impulse.

I. Acetylcholine

II. Norepinephrine

III. Dopamine

• Kiss-and-run docking is a type of synaptic vesicle release

where the vesicle opens and closes transiently.

• In this form of exocytosis, the vesicle docks and

transiently fuses at the presynaptic membrane and

releases its neurotransmitters across the synapse, after

which the vesicle can then be reused.

• Kiss-and-run differs from full fusion, where the vesicle

collapses fully into the plasma membrane.

(Bell-Magendie Law)

According to Bell-Magendie law, the impulses are

transmitted only in one direction in synapse, i.e. from

presynaptic neuron to postsynaptic neuron.

• During the transmission of impulses via the synapse,

there is a short delay in the transmission. It is due to the

time taken for:

i. Release of neurotransmitter

ii. Movement of the neurotransmitter from the axon

terminal to the postsynaptic membrane

iii. Action of the neurotransmitter to open the ionic

channels in the postsynaptic membrane

• During continuous muscular activity, the synapse forms

the seat of fatigue along with the Betz cells.

• The fatigue at the synapse is due to the depletion of

neurotransmitter substance, acetylcholine.

• Depletion of acetylcholine occurs by two factors:

I. Soon after the action, acetylcholine is destroyed by

acetylcholinesterase

II. Due to continuous action, new acetylcholine is not

synthesized.

• Synapses require the release of sufficient transmitter into

the cleft in order for enough of the transmitter to bind to

the postsynaptic receptors and the impulse to be

generated in the postsynaptic neuron.

• Summation is of 2 types:

I. Spatial summation occurs when excitatory potentials

from many different presynaptic neurons cause the

postsynaptic neuron to reach its threshold and fire.

II. Temporal summation occurs when a single

presynaptic neuron fires many times in succession,

causing the postsynaptic neuron to reach its threshold

and fire.

• When a pre-synaptic axon is stimulated with several

consecutive individual stimuli, each stimulus may evoke a

larger post-synaptic potential than that evoked by the

previous stimulus.

• Consequently, another excitatory signal entering the

neuron from some other source can excite the neuron

very easily.

• At an excitatory synapse, ionic currents flowing through

the ion channels cause a net depolarization of the

postsynaptic cell.

• At an inhibitory synapse, ionic currents flowing through

the ion channels cause a net hyperpolarization of the

postsynaptic cell.

• Convergence refers to the phenomenon of termination of

signals from many sources (i.e. many pre-synaptic

neurons on a single post-synaptic neuron).

• Divergence refers to one pre-synaptic neuron terminating

on many post-synaptic neurons. (i.e. single impulse is

converted into a number of impulses going to a number

of post-synaptic neurons.)

• The response to stimulation of 2 pre-synaptic neurons is

more than the sum total of the response obtained when

they are stimulated separately.

• This happens because some post-synaptic neurons are

common to both the pre-synaptic neurons.

• Thus occlusion is due to overlapping of afferent fibres

their central distribution.

A problem with communication between nerves at

synapses is often the basis for disease, like the following:

• Parkinson’s Disease

• Alzheimer's Disease

• Depression

• Anxiety

• Schizophrenia

Websites:

• www.wikipedia.org

• www.slideshare.net

• www.authorstream.com

Textbooks:

• Textbook of Medical Physiology, Guyton & Hall