FIGURE 12.2 • Diagram illustrating different types of neurons. The cell bodies of pseudounipolar (unipolar), bipolar, and postsynaptic autonomic neurons are located outside the CNS. Purkinje and pyramidal cells are restricted to the CNS; many of them have elaborate dendritic arborizations that facilitate their identification. Central axonal branch and all axons in remaining cells are indicated in green.
Cell Body surfaced endoplasmic reticulum (rER) and free ribosomes when observed with the transmission electron microscope
The cell body of a neuron has characteristics of a protein (TEM), a feature consistent with its protein synthetic activity.
producing cell.
In the light microscope, the ribosomal content appears as small The cell body is the dilated region of the neuron that contains bodies called Nissl bodies that stain intensely with basic dyes a large, euchromatic nucleus with a prominent nucleolus and and metachromatically with thionine dyes (see Fig. 12.4a). surrounding perinuclear cytoplasm (Fig.12.4a, Plate 27,
Each Nissl body corresponds to a stack of rER. The perinupage 390). The perinuclear cytoplasm reveals abundant rough- clear cytoplasm also contains numerous mitochondria, a large node of Ranvier nucleus of Schwann cell spinal nerve Pacinian corpuscle cell body of motor neuron cell body of sympathetic neuron smooth muscle and striated enteroceptors of ANS muscle
FIGURE 12.3 • Schematic diagram showing arrangement of motor and sensory neurons. The cell body of a motor neuron is located in the ventral (anterior) horn of the gray matter of the spinal cord. Its axon, surrounded by myelin, leaves the spinal cord via a ventral (anterior) root and becomes part of a spinal nerve that carries it to its destination on striated (skeletal) muscle fibers. The sensory neuron originates in the skin within a receptor (here, a Pacinian corpuscle) and continues as a component of a spinal nerve, entering the spinal cord via the dorsal (posterior) root. Note the location of its cell body in the dorsal root ganglion (sensory ganglion). A segment of the spinal nerve is enlarged to show the relationship of the nerve fibers to the surrounding connective tissue (endoneurium, perineurium, and epineurium). In addition, segments of the sensory, motor and autonomic unmyelinated neurons have been enlarged to show the relationship of the axons to the Schwann cells.
perinuclear Golgi apparatus, lysosomes, microtubules, neurofilaments (intermediate filaments), transport vesicles, and inclusions (Fig. 12.4b). Nissl bodies, free ribosomes, and occasionally the Golgi apparatus extend into the dendrites but not into the axon. This area of the cell body, called the axon hillock, lacks large cytoplasmic organelles and serves as a landmark to distinguish between axons and dendrites in both light microscope and TEM preparations.
The euchromatic nucleus, large nucleolus, prominent Golgi apparatus, and Nissl bodies indicate the high level of anabolic activity needed to maintain these large cells.
Neurons do not divide; however, in some areas of the brain, neural stem cells are present and are able to differentiate and replace damaged nerve cells.
Although neurons do not replicate, the subcellular components of the neurons turn over regularly and have life spans measured in hours, days, and weeks. The constant need to replace enzymes, neurotransmitter substances, membrane components, and other complex molecules is consistent with the morphologic features characteristic of a high level of synthetic activity. Newly synthesized protein molecules are transported to distant locations within a neuron in a process referred to as axonal transport (page 363).