Page 87 - Headlines Histology2024_Neat

• Axodendritic synapse: occurs between axon and dendrite.
• Axosomatic synapse: occurs between axon and cell body.
• Axoaxonic synapse: occurs between tow axons.
II) According to the presence of electron-dense granules on the cytoplasmic side
of the postsynaptic membrane:
• Asymmetrical synapse: with postsynaptic density.
• Symmetrical synapse: has no postsynaptic density.
III) According to the way by which nerve impulses are transmitted:
• Chemical synapses: characterized by the presence of synaptic vesicles

containing neurotransmitter substance within their telodendria. Nerve
impulses induce the release of the neurotransmitter that diffuses across the
intercellular space (synaptic cleft) to stimulate receptors in the postsynaptic
membrane. Interaction of the neurotransmitter with postsynaptic receptors
induces either excitation or inhibition of the other neurons.
• Electrical synapses: do not have synaptic vesicles and are similar to gap
junctions with an intercellular cleft of about 2nm wide.
• Mixed synapses: use both electrical and chemical transmission.
Structure of synapse

• A synapse is made up of three main elements: presynaptic element, synaptic
cleft and postsynaptic membrane.

Presynaptic element

• It consists of non-myelinated, dilated terminal portion of the telodendria.
• It contains mitochondria and membrane-bound vesicles of neurotransmitter

known as synaptic vesicles.
• The synaptic vesicles are of two types; electron-dense (nor-epinephrine-

containing) type and electron lucent (acetylcholine containing) type. They are
derived from the sER of the axon.

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