1. Physiology of synapses, interneuronal connections

2. What is a synapse? A synapse is the junction between 2 neurones.
A specialized junction that transfers nerve impulse information between neurons

4. Synapses
A junction that mediates information transfer from one neuron:
To another neuron
Called neuro-synapses or just synapse
To an effector cell
Neuromuscular synapse if muscle involved
Neuroglandular synapse if gland involve
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – transmits impulses away from the synapse
Two major types:
Electrical synapses
Chemical synapses

5. Anatomical Types of Synapses
Axo-dendritic – synapses between the axon of one neuron and the dendrite of another
Axo-somatic – synapses between the axon of one neuron and the soma of another
Other types of synapses include:
Axo-axonic (axon to axon)
Dendro-dendritic (dendrite to dendrite)
Dendro-somatic (dendrites to soma)

6. Functional classification or Types of comnication
A.Chemical synapse
Almost all synapses used for signal transmission in the CNS of human being are chemical synapses.
First neuron secretes a chemical substance called neurotransmitter at the synapse to act on receptor on the next neuron to excite it, inhibit or modify its sensitivity.

7. The chemical synapse is a specialized junction that transfers nerve impulse information from a presynaptic membrane to a postsynaptic membrane using neurotransmitters.
Axo-dendritic synapse Axo-somatic synanpse Axo-axonic synapse

8. The Chemical Synapse

9. Properties of neurotransmitters:
R.E.B, 4MedStudents.com, 2003
Neurotransmitters
Properties of neurotransmitters:
1) synthesized in the presynaptic neuron
2) Localized to vesicles in the presynaptic neuron
3) Released from the presynaptic neuron under
physiological conditions
4) Rabidly removed from the synaptic cleft by uptake or degradation
5) Presence of receptor on the post-synaptic neuron.
6) Binding to the receptor elicits a biological response 9

10. Neurotransmitters found in the nervous system
INHIBITORY
GABA
Glycine
EXCITATORY
Acetylcholine
Dopamine
Histamine
Nonepinephrine
Epinephrine
Glutamate
Serotonin 10

11. The Synapse

12. Structure of a synapse

13. Excitatory postsynaptic potential
Single stimuli applied to the sensory nerves in the experimental situation described above characteristically do not lead to the formation of a propagated action potential in the postsynaptic neuron. Instead, the stimulation produces either a transient, partial depolarization or a transient hyperpolarization.

14. If positive ion gates open (which allow more Na+ and Ca2+ to enter than K+ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.
If K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane becomes more depolarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.

15. Summary of Synaptic Transmission

16. Chemical Synapse Events at a chemical synapse
1. Arrival of action potential on presynaptic neuron opens volage-gated Ca++ channels.
2. Ca++ influx into presynaptic term.
3. Ca++ acts as intracellular messenger
stimulating synaptic vesicles to fuse with
membrane and release NT via exocytosis.
4. Ca++ removed from synaptic knob by
mitochondria or calcium-pumps.
5. NT diffuses across synaptic cleft and
binds to receptor on postsynaptic membran
6. Receptor changes shape of ion channel
opening it and changing membrane potential
7. NT is quickly destroyed by enzymes or
taken back up by astrocytes or presynaptic
membrane.
Note: For each nerve impulse reaching the presynaptic terminal, about 300 vesicles are emptied into the cleft. Each vesicle contains about 3000 molecules.

17. Postsynaptic Potentials
NT affects the postsynaptic membrane potential
Effect depends on:
The amount of neurotransmitter released
The amount of time the neurotransmitter is bound to receptors
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials

18. Inhibitory Synapses
Neurotransmitter binding to a receptor at inhibitory synapses:
Causes the membrane to become more permeable to potassium and chloride ions
Leaves the charge on the inner surface more negative (flow of K+ out of the cytosol makes the interior more negative relative to the exterior of the membrane
Reduces the postsynaptic neuron’s ability to produce an action potential

19. Electrical Synapses
Pre- and postsynaptic neurons joined by gap junctions
allow local current to flow between adjacent cells. Connexons: protein tubes in cell membrane.
Rare in CNS or PNS
Found in cardiac muscle and many types of smooth muscle. Action potential of one cell causes action potential in next cell, almost as if the tissue were one cell.
Important where contractile activity among a group of cells important.

20. The Synapse
The junction between two neurons is termed a synapse (synapsis = point of contact)
The narrow gap between the two neurons at the synapse is the synaptic cleft; the cleft is filled with extracellular fluid and spans an area of approximately 20 nm
= synapse
A neuron that conducts impulses toward a synapse is called a pre-synaptic neuron
A neuron that conducts impulses away from a synapse is called a post-synaptic neuron

21. The Synapse
A single neuron may have many thousands of synaptic junctions on its dendrites and cell body
axon terminal
synaptic
knob
mitochondrion
synaptic vesicle
(contains neurotransmitter)
pre-synaptic
membrane
synaptic
cleft
post-synaptic membrane (with receptors for neurotransmitter)

22. Events at the synapse Calcium ions diffuse into the synaptic knob
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
Voltage-gated calcium ion
channels open in the
pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane
calcium ions
in the extra-cellular fluid

23. Events at the synapse Calcium ions diffuse into the synaptic knob
An action potential travels down
the axon of the neuron to the
synaptic knob and depolarises
the pre-synaptic membrane
Voltage-gated calcium
channels open in the
pre-synaptic membrane
The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane
Calcium ions diffuse
into the synaptic knob
post-synaptic
membrane

24. Events at the synapse Calcium ions diffuse into the synaptic knob
The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane
Voltage-gated calcium
channels open in the
pre-synaptic membrane
Neurotransmitter is released
into the synaptic cleft
by EXOCYTOSIS
Calcium ions diffuse
into the synaptic knob
Neurotransmitter diffuses across the cleft and binds to specific protein receptors
embedded in the post-synaptic membrane
receptors in the post-synaptic membrane
post-synaptic
membrane

25. embedded in the post-synaptic membrane
Events at the synapse
Neurotransmitter diffuses across the cleft and binds to specific protein receptors
embedded in the post-synaptic membrane
Binding of neurotransmitter opens Na+ gates in the membrane and there is an influx of Na+ into the post-synaptic neuron
An excitatory post-synaptic potential (EPSP) builds up across the membrane and if this reaches threshold, an action potential is triggered in the post-synaptic neuron
sodium
ions
post-synaptic
membrane
Depolarisation of the post-synaptic membrane

26. Depolarisation of the post-synaptic membrane
Events at the synapse
The neurotransmitter, acetylcholine, is hydrolysed by the enzyme acetylcholinesterase, which is located at the surface of the post-synaptic membrane
Following activation of the post-synaptic membrane, neurotransmitter is removed from the synaptic cleft to enable further stimulation to occur
The neurotransmitter, noradrenaline, is actively transported back into the axon terminals
sodium
ions
post-synaptic
membrane
Depolarisation of the post-synaptic membrane

27. Unidirectionality synaptic vesicle
Unidirectionality describes the one-way transmission of nerve impulses between neurons
Neurotransmitter is stored and released only on the pre-synaptic side of the synaptic cleft
Receptors for neurotransmitter are only located on the post-synaptic membrane
synaptic vesicle
(contains neurotransmitter)
post-synaptic membrane (with receptors for neurotransmitter)
This arrangement allows for the transmission of impulses between neurons in one direction only

29. The nature of the neurotransmitter determines the response of the post-synaptic membrane
During hyperpolarisation, the post-synaptic membrane potential becomes more negative than its resting potential and results from either the efflux of positive charge or the influx of negative charge
Inhibition occurs at synapses where transmitter release results in the hyperpolarisation of the post-synaptic membrane

30. Occlusion
On account of divergence one neuron may pass excitive signals on the other neurons. Another neuron may excite several neurons. But if from both neurons which is divergented excitement will be simultaneously the total quantity of excited neurons will be decrease.

31. Opposite inhibition

32. Lateral inhibition
If in a neurons' chain, which secure opposite inhibition collaterals of axons of inhibition neurons form synaptic connection with neighboring excitive cells in these cells develop lateral inhibition

33. Spatial summation
The adding together of EPSPs generated simultaneously at many different synapses on a dendrite.
Two or more presynaptic inputs are active at the same time
A space (spatial) dependent process.
Occurs in a Convergent Synapse

34. Temporal summation
The adding together of EPSPs generated at the same synapse if they occur in rapid succession, within 1-15 msec of one another.
The same presynaptic fiber fires AP in quick succession
A Time (Temporal) dependent process
Occurs in a Divergent Synapse