The synapse

Presentation on the synapse for physiology, first year medicine.

1. The Synapse Preshantha Govender Roll #24

2. Contents Definition Structure Function Types Synaptic Transmission Electrical Chemical Transmission of Neurotransmitters Excitatory & Inhibitory Neurotransmitters Kiss & Run Docking Properties One-way Conduction Synaptic Delay Fatigue Summation Facilitation Excitatory & Inhibitory Occlusion Phenomenon Convergence & Divergence Clinical References

3. Definition 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 .

4. Structure The synapse consists of : A   presynaptic ending  that contains neurotransmitters, mitochondria and other cell organelles. A   postsynaptic ending  that contains receptor sites for neurotransmitters. A   synaptic cleft  or space between the presynaptic and postsynaptic endings.  It is about 20nm wide.

5. Function 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

6. Types Synapse with another neuron It is the junction between two nerve cells. They are of 3 types; axodendritic , axosomatic & axoaxonic Neuromuscular It is the synapse pf a motor neuron and a muscle Neuroglandular It is the synapse of a neuron and a endo /exocrine gland.

7. Synaptic Transmission 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: Electrical transmission Chemical transmission

8. Electrical 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. Chemical Transmission 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 .

9. Transmission of Neurotransmitters 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. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis. The neurotransmitters diffuse across the synaptic cleft. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open.

10. Excitatory & Inhibitory Neurotransmitters Inhibitory neurotransmitters produce a depolarization of the postsynaptic membrane called the inhibitory postsynaptic potential (IPSP). They reduce chances of a new impulse. Serotonin GABA ( γ aminobutyric acid) Glycine Excitatory neurotransmitters can cause the next cell to initiate a new impulse. A cetylcholine Norepinephrine Dopamine

11. Kiss & Run Docking 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.

12. Properties (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 . One Way Conduction

13. Synaptic Delay During the transmission of impulses via the synapse , there is a short delay in the transmission. It is due to the time taken for: Release of neurotransmitter Movement of the neurotransmitter from the axon terminal to the postsynaptic membrane Action of the neurotransmitter to open the ionic channels in the postsynaptic membrane

14. Fatigue 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: Soon after the action , acetylcholine is destroyed by acetylcholinesterase Due to continuous action , new acetylcholine is not synthesized.

15. Summation 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: Spatial summation occurs when excitatory potentials from many different presynaptic neurons cause the postsynaptic neuron to reach its threshold and fire.     Temporal summation occurs when a single presynaptic neuron fires many times in succession, causing the postsynaptic neuron to reach its threshold and fire.

16. Facilitation 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.

17. Excitatory & Inhibitory 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.

18. Convergence & Divergence 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.)

19. Occlusion Phenomenon 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- s ynaptic neurons are common to both the pre-synaptic neurons. Thus o cclusion is due to overlapping of afferent fibres their central distribution .

20. Clinical 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

21. References Websites: www.wikipedia.org www.slideshare.net www.authorstream.com Textbooks: Textbook of Medical Physiology, Guyton & Hall

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