Peripheral Nervous Tissue EM Slides

Motor Neuron Cell Body: In this electron micrograph, note some of the features you saw in ventral horn motor neurons with the light microscope, such as the large, pale nucleus, prominent nucleolus, Nissl bodies, dendrites and axon. Adjacent to the neuron, note myelinated axons of various sizes and also that there are no spaces between cell processes. All spaces are occupied either by the processes of neurons or glia or by capillaries (these capillaries are somewhat swollen here because the tissue was fixed by perfusion).

Node of Ranvier (longitudinal section): Remember that the node of Ranvier is actually a short segment of the axon that is bare at the junction between two Schwann cells, making "saltatory conduction" possible. Note the manner in which the myelin ends in each Schwann cell at the junction, by a "peeling off" of successive myelin layers, which come to lie against the axon as small cytoplasmic swellings.

Myelinated Nerve Fibers (longitudinal section): This image shows the typical appearance of a myelinated nerve, consisting of parallel bundles of axons (light areas) wrapped with sheaths of myelin (dark areas). As in the peripheral nervous system each Schwann cell myelinates only one axon, the discontinuous appearance of the axon labeled 5 in a Schwann cell is probably due to its curvature around the nucleus.

Schwann Cell with Myelinated Nerve Fiber (cross section): In this cross section of a myelinated nerve process, note the axon, containing microtubules and neurofilaments and bounded by a plasma membrane ("axolemma"). Outside the plasma membrane of the axon is the myelin sheath, which you will remember is composed of tightly wrapped plasma membranes of the Schwann cell. Also, note the nucleus and cytoplasmic organelles of the Schwann cell. Remember that the myelin is part of the Schwann cell, not of the axon.

Unmyelinated Nerve Fibers (cross section): The axons seen in this electron micrograph are all non-myelinated. They are embedded in grooves in the Schwann cell surface (in some cases there may be more than one axon per groove), with each Schwann cell thus supporting a considerable number of these small, unmyelinated axons. Although the axons are very close together, you will observe thin partitions of Schwann cell between them.

Multipolar Neurons (Celiac Ganglion): The celiac ganglia are autonomic ganglia. Note the large ganglion cells with somewhat eccentrically placed nuclei in several cells, a characteristic feature of autonomic ganglion cells.

Myelinated Nerve with Endoneurium and Perineurium (cross section): Between the axons you will see delicate connective tissue and an occasional fibroblast, which constitute the endoneurium. At the periphery of the fascicle, observe the perineurium, made up of several layers of flattened cells; it is a highly specialized layer that acts as a barrier and protects the nerve from the environment.

Neuromuscular Junction: This is a motor end plate. Note that the nerve axon loses its myelin sheath as it approaches the motor end plate and it terminates as a bulbous expansion in a trough of the muscle cell surface. The bulbous knob reveals numerous mitochondria and small synaptic vesicles, which contain cholinergic neurotransmitter substances. When these vesicles fuse the cell membrane of the axon bulb (the presynaptic membrane) and the content is released into the synaptic clefts to be taken up by the postsynaptic membrane (muscle cell membrane).

Neuromuscular Spindle: Note the two types of intrafusal muscle fibers, the nuclear bag fibers and nuclear chain fibers are enclosed by a delicate internal capsule. This arrangement serves as a muscle stretch receptor. The sensory nerve endings are activated by the stretching of the intrafusal fibers and the nerve impulse generated excites the somatic motor neurons in the spinal cord to evoke a reflex contraction of extrafusal fibers.