The nerve fibers included in Figure on right are mostly myelinated, and because the nerve is cross-sectioned, the nerve fibers are also seen in this plane. They have a characteristic cross-sectional profile. Each nerve fiber shows a centrally placed axon (A); this is surrounded by a myelin space (M) in which some radially disposed precipitate may be retained, as in this specimen. External to the myelin space is a thin cytoplasmic rim representing the neurilemma. On occasion, a Schwann cell nucleus (SS ) appears to be perched on the neurilemma. As shown in the illustration, the upper edge of the nuclear crescent appears to occupy the same plane as that occupied by the neurilemma (N). These features enable one to identify the nucleus as belonging to a Schwann (neurilemma) cell. Other nuclei are not related to the neurilemma but, rather, appear to be between the nerve fibers. Such nuclei belong to the rare fibroblasts (F) of the endoneurium. The latter is the delicate connective tissue between the individual nerve fibers; it is extremely sparse and contains the capillaries (C) of the nerve bundle.
Peripheral nerve, longitudinal section, femoral nerve, H&E. ×200 and 640
The edge of a longitudinally sectioned nerve bundle is shown on left; a portion of the same nerve bundle is shown at higher magnification on right image. The boundary between the epineurium (Epn) and perineurium is ill-defined. Within the nerve bundle, the nerve fibers show a characteristic wavy pattern. Included among the wavy nerve fibers are nuclei belonging to Schwann cells and to cells within the endoneurium. Higher magnification allows one to identify certain specific components of the nerve. Note that the nerve fibers (NF) are now shown in longitudinal profile. Moreover, each myelinated nerve fiber shows a centrally positioned axon (A) surrounded by a myelin space (M), which, in turn, is bordered on its outer edge by the thin cytoplasmic band of the neurilemma cell (NI). Another diagnostic feature of myelinated nerve fibers is also seen in longitudinal section, namely, the node of Ranvier (NR). This is where the ends of the two Schwann cells meet. Histologically, the node appears as a constriction of the neurilemma, and sometimes, the constriction is marked by a cross-band, as in figure on right. It is difficult to determine whether the nuclei (N) shown here belong to Schwann cells or to endoneurial fibroblasts.
KEY A, axon AT, adipose tissue BNF, bundle of nerve fibers BV, blood vessels C, capillary Epn, epineurium F, fibroblast M, myelin N, nucleus of Schwann cell NF, nerve fiber Nl, neurilemma NR, node of Ranvier Pn, perineurium SS, Schwann cell nucleus arrows, septum formed by perineurium
The cerebrum is the principal portion of the brain and contains the cell bodies of nerves that receive and store sensory information, nerves that control voluntary motor activity, and nerves that integrate and coordinate the activity of other nerves, as well as the nerves and neural pathways that constitute memory.
Cerebral cortex, brain, human, Luxol fast blue-PAS ×65.