Many of the terms used in this section are defined below.
Axolemma is the plasmalemma of the axon.
Endoplasmic reticulum is a labyrinthine, membrane bounded compartment in the cytoplasm where lipids are synthesized and membrane bound proteins are made. In some regions of the neuron ER is devoid of ribosomes and is termed smooth ER. The smooth ER is involved in Ca2+ buffering and in the biosynthesis and recycling of synaptic vesicles as will be discussed in Chapter 10.
Endosome is a membrane-bounded organelle that carries materials ingested by endocytosis and passes them to lysosomes and peroxisomes for degradation. It also functions in the nerve ending to recycle synaptic vesicles.
Golgi apparatus is a collection of stacked, smooth-surfaced membrane bound organelles where proteins and lipids made in the endoplasmic reticulum are modified and sorted.
Lysosomes contain enzymes that digest compounds that originate inside or outside the cells. They are involved in converting proteins to amino acids and glycogen to glucose, the basic nutrient of neurons. Their enzymes act at an acidic pH. As will be described later, they also serve as vesicles for reverse transport from axon terminals to the soma. Many lysosomes become degraded to lipofuscin granules, which accumulate as the organism ages and are regarded as neuronal refuse. Lysosomes form from the budding off the Golgi apparatus. They have a variety of membrane-bound shapes and sizes, ranging in size between 250 and 700 nm in diameter.
Microfilaments are 7 nm in diameter filaments arranged as a paired helix of two strands of globular actin. Microfilaments are especially prominent in synaptic terminals, in dendritic spines, and in association with the axolemma.
Microtubules are 20 to 25 nm diameter tubular structures that run in loose bundles around the nucleus and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally. They are made up of dimers of α and β tubulin subunits and contain associated proteins known as microtubule associated proteins (MAPS). The MAPS regulate the polymerization of tubulin subunits to form the microtubules. The dimers of α and β tubulin subunits polymerize to form proto-filaments arranged in an a helix such that 13 dimer subunits make up each full turn of the a helix. In addition, microtubules are not continuous, and each microtubule is composed of numerous 100 nm units. Microtubules are involved in axoplasmic transport (see below).
Mitochondria are distributed ubiquitously throughout the cytoplasm of the entire nerve cell and are especially plentiful at presynaptic specializations.
Neurofilaments are a type of intermediate filament found in nerve cells. Neurofilaments are involved in the maintenance of the neuron's shape and mechanical strength. Although neuronal neurofilaments are classified as intermediate filaments, their composition in neurons is different than that found in other cells. They are composed of three subunits that are arranged to form a 10-nm diameter tubule. It is the neurofilament that stains with heavy metal to permit the visualization of neuronal shape. Neurofilaments run in loose bundles around the cell nucleus and other organelles and funnel into the base of the axonal and dendritic processes where they form parallel arrays distributed longitudinally. Neurofilaments are more abundant than microtubules in axons, whereas microtubules are more abundant than neurofilaments in dendrites. It is the neurofilaments that undergo modification in the Alzheimer's disease to form neurofibrillary tangles.
Nucleolus is in the center of the nuclei of all neurons. It is a prominent, deeply stained spherical inclusion about one-third the size of the nucleus. The nucleolus synthesizes ribosomal RNA, which has a major role in protein synthesis.
Nucleus of the neuron is large and round and is usually centrally located. In some cells, masses of deeply staining chromatin are visible in the nucleus. The nuclear membrane of neurons is like that of other cells - a double membrane punctuated by pores (nuclear pores) which are involved in nuclear-cytoplasmic interactions. The nucleus in neurons is spherical and ranges in diameter from 3 to 18 micrometers depending on the size of the neuron. Neurons with long axons have a larger cell body and nucleus. As in other cells, the principal component of the nucleus is deoxyribonucleic acid (DNA), the substance of the chromosomes and genes.
Peroxisomes are small membrane bounded organelles that use molecular oxygen to oxidize organic molecules. They contain some enzymes that either produce or degrade hydrogen peroxide.
Plasmalemma of the neuron appears in the electron microscope as a typical bi-layered cellular membrane, approximately 10 nm thick.
Postsynaptic density is darkly staining material of postsynaptic cell adjacent to the synapse. Receptors, ion channels, and other signaling molecules are likely bound to this material.
Presynaptic density is the region of darkly staining material of the presynaptic membrane where synaptic vesicles are hypothesized to dock prior to fusion with the presynaptic membrane.
Ribosomes are particles composed of ribosomal RNA and ribosomal protein which associate with mRNA and catalyze the synthesis of proteins. When ribosomes are attached to the outer membranes of the ER, the organelle is termed rough ER. The rough ER, in laminae with interspersed ribosomes, is visible with the light microscope as Nissl substance. In light microscopic preparations, the appearance of Nissl substance varies in different types of neurons. It may appear as densely stained ovoids or as finely dispersed particles or aggregations of granules.
Synapse is the junction that allows signals to pass from a nerve cell to another cell or from one nerve cell to a muscle cell. The synaptic cleft is the gap between the membrane of the pre- and postsynaptic cell. In a chemical synapse the signal is carried by a diffusable neurotransmitter. The cleft between the presynaptic cell and the postsynaptic cells is 20 to 40 nm wide and may appear clear or striated. Recent studies have indicated that the cleft is not an empty space per se, but is filled with carbohydrate-containing material.
Synaptic vesicles are small spherical organelles in the cytoplasm of neurons that contain neurotransmitter and various proteins necessary for neurotransmitter secretion. Vesicles containing inhibitory neurotransmitter are often flat or elliptical whereas vesicles that contain excitatory neurotransmitter are usually more spherical.