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Model Neuron

Structural Variations

Naming Neurons

Organelles

Nerve Ending

Click the identified structures on the model neuron to move to the related page.

The part of the plasma membrane of the nerve ending that is specialized to form functional contacts with other cells is the synapse.

Figure 8.6 Diagrammatic representation of an excitatory synapse, showing the spherical vesicles and the presynaptic density. View an EM of a nerve ending with spherical vesicles.

When neurons interact with muscle fibers, the region of functional contact is called the neuromuscular junction or motor endplate (Chapter 4). According to the classical definition of synapse, when a nerve ending synapses on a dendrite or soma of a second neuron it is termed either an axodendritic or an axosomatic synapse, respectively (Chapter 7, Part 2). However, almost all possible combinations of pre- and postsynaptic elements have been found in the central nervous system. These different types of synapse are designated by combining the name of the structure of the presynaptic element with that of the postsynaptic structure. For example, when the transfer of information occurs from an axon to axon or from one terminal to another, the synapse involved is called an axoaxonic synapse.

Cellular Elements at the Typical Nerve Terminal

Regions of functional contacts between neurons (synapses) have distinct morphological characteristics. Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following:

Figure 8.7 Diagrammatic representation of an excitatory synapse, showing the spherical vesicles and the presynaptic density. View an EM of a nerve ending with flat vesicles.

1. A presynaptic complement of membrane-bound synaptic vesicles exists. These are spherical vesicles in excitatory nerve endings, shown in Figure 8.6. In inhibitory neurons the synaptic vesicle are often flattened as shown in Figure 8.7.
2. The nerve ending often has aggregations of dense material in the cytoplasm immediately adjacent to the membrane on the pre- and postsynaptic side of the junction (these are known as presynaptic density or postsynaptic density, respectively.) This dense material on the presynaptic side is thought to be the site of vesicle attachment. The dense material on the postsynaptic side is a site where receptor proteins and channels are prevalent.
3. Many mitochondria are present, especially in the nerve terminal; and
4. There is a distinct synaptic cleft or intercellular space of approximately 20-40 nm.
5. An endoplasmic reticulum is present that regulates Ca²⁺ level.
6. Endosomal membrane that functions in the recycling of synaptic vesicles.