Neurons

Neurons in the CNS are located in the gray matter, while in the peripheral nervous system (PNS) they are located in the ganglia. They can be broken into three types:

• Sensory (aka afferent)neurons convey impulses from receptors to the CNS
• Motor (aka efferent) neurons convey impulses from the CNS or ganglia to effector cells
• Interneurons (aka intercalated) are found between sensory and motor neurons, completing the arc between them. They make up 99.9% of neurons

In the CNS, neurons have the greatest variety of size (4-135μm in diameter) and shape (e.g. spherical and pyramidal). Components of the neuron including the following:

• Cell body (perikaryon) contains nucleus and organelles, maintaining the nerve cell. They contain an euchromatic nucleus with a prominent nucleolus (linked to owl’s eye in LM appearance), nissl bodies (sacks of rough ER and free polysomes), numerous mitochondria, multiple perinuclear golgi complexes, and a cytoskeleton made of microtubules (neurotubules) and intermediate filaments (neurofilaments).
• One axon, which is the longest process extending from the cell (up to 1 meter long). It transmits impulses away from the cell body. They originate from conical projections of the cell body. They are called axon hillocks and are devoid of nissl and Golgi. They propagate impulses away from the cell body and branch near target organs. Golgi type I neurons in motor nuclei of the CNS have axons more than 1 meter long (which innervate distant muscle fibers), and Golgi type II neurons, or interneurons of the CNS, have short axons. The cytoplasm (aka axoplasm) contains neurotubules, neurofilaments, mitochondria, and vesicles, and is surrounded by the axolemma. They are myelinated in white matter of the CNS. The initial (proximal) segment lies in gray matter and is devoid of myelin. Not all axons are myelinated (some axons in the PNS are unmyelinated).
• Many dendrites, which are shorter and thicker than the axon. They receive stimuli from other nerve cells or from the environment and transmit impulses towards the cell body. They are unmyelinated, contain nissl bodies and other cytoplasmic organelles (such as the occasional Golgi). Arborizations are called dendritic trees, which greatly increase receptor surface of the neuron
• Synaptic junctions, which allows contact with other neurons or an effector cell (e.g. muscle cell and glands cells). The number of synapses on one neuron range from a few to tens of thousands. Classified morphologically as axodendritic (axons terminate on dendrites), axosomatic (axons terminate on cell body), axoaxonic (axons terminate at unmyelinated regions on other axons, i.e. proximal and end segments), and dendrodendritic (between dendrites and dendrites, very rare).

Neurons are classified on the basis of the number of processes. These include:

• Multipolar, which is one axon and two or more dendrites. Most neurons in the CNS are multipolar, being either motor neurons or interneurons. Direction of impulses go from dendrite to cell body to axon, or from cell body to axon. Functionally, the dendrites and cell body are the receptors.
• Bipolar, which is one axon and one dendrites. They are not very numerous, and are limited to the retina, ganglia of the vestibulocochlear nerve (CN VIII - spiral ganglion in the inner ear), and olfactory epithelium.
• Pseudounipolar (aka unipolar), which is one process that divides close to the cell body into two processes. One long process extends peripherally (serving as a dendrite) and one short one extends towards the CNS (serving as an axon). Perikarya of unipolar neurons are located in the dorsal root ganglia and cranial nerve ganglia.

Elements of the Chemical Synapses:

• Presynaptic bouton (also called presynaptic component or presynaptic knob) is the end of the neuron process where neurotransmitters are released. Neurotransmitters are contained in synaptic vesicles, and are released when these vesicles fuse with the presynaptic plasma membrane. The cytoplasmic side of the plasma membrane is dense. The bouton contains numerous small mitochondria.
• Synaptic cleft is a 20-30nm space that separates the pre and post components
• Postsynaptic membrane has receptors for the neurotransmitters, and has postsynaptic density

Peripheral neuroglia:

Schwann cells have a neural crest origin and support myelinated and unmyelinated nerve cells in the PNS as their main function by producing a lipid rich layer (myelin sheath, or the neurilemma). Neurilemma insulates axons from the extracellular compartment, allowing rapid conduction of impulses. The axon hillock and terminal arborizations are free of myelin. The myelin sheath is segmented, formed by numerous Schwann cells. Junctions between adjacent Schwann cells are called nodes of Ranvier (also myelin free). Myelin associates with small amounts of cytoplasm, including the following:

• Inner collarof Schwanna cytoplasm, which is between axon and myelin
• Schmidt-Lanterman clefts, which are small islands/channels of cytoplasm within the lamellae of myelin. They connect the inner and outer cytoplasmic collars
• Perinodal cytoplasm, which is at the node of Ranvier
• Outer collar of perinuclear cytoplasm, which is around myelin. It contains the nucleus and other organelles and is surrounded by external lamina

Satellite cells have a neural crest origin and surround ganglionic neuronal cell bodies. They are cuboidal in shape, and form complete layers around the cell body (with only the nuclei visible). They are found in paravertebral and peripheral ganglia, where dendrites penetrate the satellite cell layer to form synapses. Sensory ganglia have NO synapses. They are analogous to Schwann cells, providing insulation and nutrition.

Central neuroglia:

Oligodendrocytes form myelin in the CNS by concentric layers of the oligodendrocyte plasma membrane. They appear as small cells with scant cytoplasm that appear as a rim around the nucleus. The cells align in rows between axons. Each oligodendrocyte gives off several (3-50) tongue-like processes, each of which around around an axon forming and internodal segment.

Astrocytes provide physical and metabolic support for CNS neurons. There are two kinds seen:

• Protoplasmic, prevalent in gray matter
• Fibrous, more common in white matter

Both have prominent bundles of intermediate filaments, consisting of glial fibrillary acidic protein (GFAP). They have elaborate processes extending between vessels and neurons. The ends of the processes form end feet that cover large areas of the outer surface of blood vessels or axolemma. They play a role in movement of metabolites and waste to and from neurons, and regulate ionic concentrations in intracellular compartments by taking up K+.They may also have a role in regulating tight junctions of the blood brain barrier.

Microglia are phagocytic cells of the CNS, part of MPS. They are present in small numbers, and proliferate during injury/disease. They originate in the bone marrow and remove debris of cells. They are the smallest of the glia, with an elongated nucleus and short, twisted processes covered with ruffled spikes.

Ependymal cells line brain ventricles and central canal of the spinal cord and are responsible for secreting cerebrospinal fluid (CSF). They are made of simple cuboidal or low columnar epithelium that rest on basal lamina. They have basal folds and their free surfaces are covered with cilia and microvilli.