Medical

I Classification of Neurons by Neuronal Processes ; Unipolar neurons

Unipolar neurons have one axon and no dendrites and probably occur only during development

Pseudounipolar neurons

Pseudounipolar neurons have a single process close to the perikaryon, which divides into two branches. One branch extends to a peripheral ending, and the other extends to the CNS. Pseudounipolar neurons are found in dorsal root ganglia and most cranial ganglia.

Bipolar neurons

Bipolar neurons have one axon and one dendrite. Bipolar neurons are found in the cochlear and vestibular ganglia as well as in the retina and olfactory mucosa.

^ Multipolar neurons

Multipolar neurons have one axon and multiple dendrites. Most neurons in the body are multipolar (e.g., ventral horn neurons in the spinal cord).

; Classification of Neurons by Functional Role | Motor neurons

Motor neurons control effector organs and muscle fibers.

| Sensory neurons

Sensory neurons receive sensory stimuli from the internal or external environment and relay | them to the CNS.

! Synapses

; Synapses are specialized membrane junctions designed for the unidirectional communication between neurons or between neurons and effector cells (Figure 1-2-3). The pre- and postsynaptic membranes are separated by only 20 nm; this space is called the synaptic cleft.

! Location

Synapses are either between an axon and a dendrite (axodendritic) or between an axon and a cell body (axosomatic). Synapses between dendrites (dendrodendritic) and between axons (axoaxonic) also occur.

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Figure 1-2-3. Axodendritic Synapse

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Figure 1-2-3. Axodendritic Synapse

Synaptic vesicles

Synapses contain synaptic vesicles. They consist of 30- to 50-|i.m spherical or ovoid structure« in the axoplasm that contain neurotransmitter (e.g., acetylcholine (ACh]). Neurotransmitter is released into the synaptic cleft at the synapse when synaptic vesicles fuse with the presynaptic membrane.

' Neurotransmitters may either excite (depolarize) or inhibit (hyperpolarize) the postsynaptic membrane, depending on the type of receptor to which it binds.

■ Certain neurotransmitters are inactivated in the synaptic cleft by enzymatic degradation (e.g., ACh is broken down by acetylcholinesterase (AChE)), whereas others are taken lip by the presynaptic cell (e.g., norepinephrine) in a process called reuptake.

Neuromuscular Junction

The neuromuscular junction occurs at the motor end plate. It is the synapse between neurons and muscle cells (Figure 1-2-4).

At the neuromuscular junction, the axon forms a number of small branches that fit into grooves on the muscle where the postsynaptic membrane is convoluted into numerous folds, called the subneural clefts.

ACh released from the axon depolarizes the sarcolemma via the acetylcholine nicotinic receptors.

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Axon terminal

Axon terminal

Figure 1-2-4. Portion of a Motor End Plate Along a Skeletal Muscle Fiber

Clinical Correlate

Myasthenia gravis is a disease characterized by weakness and easy fatigue of muscles. It can be life threatening if swallowing or breathing is affected.

It is caused by an autoimmune response to the ACh receptor. Normally, old receptors are constantly removed by endocytosis and transported to and degraded by the lysosomes. These are replaced by new receptors, which are manufactured by the Colgi apparatus and then inserted into the junctional folds. The normal half-life of a receptor is about 10 days. In myasthenia gravis, the half-life is reduced 1 to about 2 days, resulting in a marked decrease in the number of available receptors.

Administration of AChE inhibitors has both diagnostic and therapeutic value. By slowing the rate of ACh degradation, they increase the binding time of ACh to the remaining receptors. The usual response is prompt improvement in muscle power. An original clinical diagnosis of myasthenia gravis becomes questionable should no improvement be observed.

NEUROGLIA

Neuroglia (nerve glue) serve as the connective tissue cells of the nervous system. Although they do not generate or transmit neural impulses, they play an important role in the normal functioning of the nervous system. They form the myelin sheaths of axons and provide metabolic support to neurons. Neuroglia of the CNS include microglia, astrocytes, oligodendrocytes, and ependymal cells. In the PNS, neuroglia cells consist of Schwann cells.

Astrocytes

Astrocytes are the largest of the neuroglial cells. They have centrally located nuclei and numerous long processes with expanded vascular end-feet, or pedicels, which attach to the walls of blood capillaries.

Astrocytes are important in controlling the microenvironment of nerve cells and participate in the maintenance of the blood-brain barrier.

Oligodendrocytes

Oligodendrocytes have small nuclei and contain abundant mitochondria, ribosomes, and microtubules.

Oligodendrocytes myelinate axons in the CNS.

Microglia

Microglia are small, dense, elongated cells with elongated nuclei. They originate from the mesoderm, unlike other neuroglial cells, which originate from the neuroectoderm.

Microglia are phagocytic and are part of the mononuclear phagocyte system.

Ependymal Cells

Ependymal cells line the ventricular cavities of the brain and the central canal of the spinal cord. They are capable of mitosis and can develop long processes that deeply penetrate the neural tissue.

Cilia on the ependymal cells help move cerebrospinal fluid through the ventricles.

Schwann Cells

Schwann cells contain elongated nuclei that lie parallel to the axons of peripheral neurons. Schwann cells myelinate peripheral axons.

Chapter Summary

Neurons are composed of a cell body, dendrites, and an axon. They contain pigments such as melanin and lipofuscin. The cell body (soma or perikaryon) contains a nucleus, other cellular components, and rough endoplasmic reticulum. Microtubules and neurofilaments form the cytoskeleton. They are important for axonai transport. Dendrites receive and transmit information to the cell body. Axons arise from the perikaryon or proximal dendrite. They contain microtubules and neurofilaments. Rapid axonai transport utilizes microtubules. Kinesins promote anterograde transport whereas dynein promotes retrograde transport. Myelin is the covering of axons and is composed of phospholipids and cholesterol. Axons may be myelinated or nonmyelinated. Schwann cells myelinate a single peripheral nervous system axon.

They may also associate with several axons (unmyelinated) without forming myelin. Oligodendrocytes form myelin in the central nervous system. One oligodendrocyte myelinates many axons. The node of Ranvier is a collar of naked axon between a proximal and distal bundle of myelin that has myelinated the axon. Its purpose is to allow rapid signal transport by skipping from one node to the next, thus avoiding travel through the entire axon. This process is called saltatory conduction.

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