Neurohistology

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Like the microscopic anatomy of other tissues, this approach focuses on nerve cells: their types, internal and external features, structural, functional, and chemical properties, and interrelationships. But the great size, extent, and diversity of neurons, their interdependence, and their complex spatial relationships can be daunting to the histologist.

Let us examine a motor neuron. Figure 15 is a schematic illustration showing a motor neuron innervating a muscle just above the letter B. We begin with the cell body, noting its size, organelles, and inclusions, and then trace its dendrites (100 terminal ones), spines, and relationships to neuroglial processes and capillaries. We identify the axon, note its features (axon hillock, initial segment, side branches if any), and follow its course through the gray matter into the white matter (the two are not delineated), where it becomes myelinated by oligodendrocytes (OI) for rapid conduction and protection.

We follow the axon into the PNS. Here it is myelinated by Schwann cells (S1), for the same reasons. Outside the myelin sheath, three connective tissue sheaths (endoneurium, perineurium, epineurium; not shown) provide vascular, isolating, and protective support for the delicate nerve fibers. We trace the axon for a meter or so to a skeletal muscle and inspect its preterminal branches and axon terminals on muscle fibers at the neuromuscular junction: the nerve-muscle synapse.

We cannot examine one motor neuron in all respects. For the total picture, we piece together these parts from many motor neurons. The hardest thing to grasp is the relative size of a neuron and its parts.

Figure 15 Neuronal and neuroglial cell types in the nervous system: the end feet of protoplasmic astrocytes (PA) form a leaky membrane around the CNS and others in the gray matter about either neurons (the dark cells) or capillaries (Cap). Fibrous astrocytes (FA) lie among axons in the white matter, many of which are myelinated by oligodendrocytes (Ol). Microglia (M) scavenge debris after injury, and ependymal cells (E) line the ventricles (V). Schwann cells are the glial cells of the PNS, myelinating peripheral nerve fibers (S1), surrounding unmyelinated axons (S2), and satelliting dorsal root and autonomic ganglion cells (S3). Direction of impulses in various axons is indicated by arrows. Other abbreviations: skin (A), skeletal muscle (B), gastrointestinal tract, containing intramural smooth muscle and glands (C), autonomic ganglion (AG). See also text. From J. Nolte, The Human Brain. An Introduction to Its Functional Anatomy, 4thed.,Mosby, St. Louis, 2000 (illustration by Prof. Dr. Radivoj Krstic).

Figure 15 Neuronal and neuroglial cell types in the nervous system: the end feet of protoplasmic astrocytes (PA) form a leaky membrane around the CNS and others in the gray matter about either neurons (the dark cells) or capillaries (Cap). Fibrous astrocytes (FA) lie among axons in the white matter, many of which are myelinated by oligodendrocytes (Ol). Microglia (M) scavenge debris after injury, and ependymal cells (E) line the ventricles (V). Schwann cells are the glial cells of the PNS, myelinating peripheral nerve fibers (S1), surrounding unmyelinated axons (S2), and satelliting dorsal root and autonomic ganglion cells (S3). Direction of impulses in various axons is indicated by arrows. Other abbreviations: skin (A), skeletal muscle (B), gastrointestinal tract, containing intramural smooth muscle and glands (C), autonomic ganglion (AG). See also text. From J. Nolte, The Human Brain. An Introduction to Its Functional Anatomy, 4thed.,Mosby, St. Louis, 2000 (illustration by Prof. Dr. Radivoj Krstic).

Neuroanatomist Jack Nolte offers this perspective: If the cell body of a motor neuron were the size of a tennis ball, its dendrites would fill a room and its axon would extend, like a 0.5-in. garden hose, nearly 0.5 mile.

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