NERVOUS TISSUE

Nervous tissue is the fourth and last basic tissue to be studied. It is found within the most highly specialized system of the body, the nervous system, which is composed of an intercommunicating network of specialized cells (neurons) and support cells (glia). The nervous system serves the following functions: (1) reception of information from the external and internal environment, (2) integration and analysis of the incoming information, (3) generation of new signals, and (4) conduction of these neural messages to special responding tissue (effectors). Nervous tissue is enabled to accomplish these tasks due to its unique properties of excitation and conduction. These properties also allow individuals to constantly adapt to environmental changes or stimuli.

The anatomical and functional unit of the nervous system is the neuron. All neurons have the same basic structure, though they vary greatly in size and shape within the nervous system (see Figure 44 below). A neuron consists of a large cell body, which contains a nucleus and cytoplasm known as perikaryon, as well as protoplasmic processes (one axon and one or more dendrites). Dendrites are typically short, branching processes that form the major part of the receptor area of the neuron and conduct impulses toward the cell body (informational input). Each nerve cell also has a single axon. This process, which varies greatly in length, conducts impulses away from the cell body. A collection of neurons outside the central nervous system is called a ganglion. A cluster of neurons in the gray matter of the brain or spinal cord is called a nucleus. This type of "nucleus" refers to a functional aggregation of neurons and should not be confused with the nucleus of a cell. There is no regeneration of neurons.

Figure 44: Schematic depiction of a neuron. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 117, Figure 7.1.

Divisions of the nervous system

1. Central nervous system - The central nervous system (CNS) consists of the brain and spinal cord. The histology of the CNS will be covered in great detail during the neuroanatomy class held in the upcoming fall semester.

2. Peripheral nervous system - The peripheral nervous system (PNS) consists of all the nervous tissue lying outside the brain and spinal cord. It consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. This PNS is further divided into the somatic nervous system, which supplies motor fibers to skeletal muscles that are under conscious control, and the autonomic nervous system (ANS), which supplies motor fibers to smooth muscle, cardiac muscle, and glands. The ANS consists of the sympathetic (thoracolumbar) nervous system and the parasympathetic (craniosacral) nervous system. Consult your gross anatomy notes for more information on the ANS.

Peripheral Nerves

Peripheral nerves may contain any combination of afferent or efferent fibers of either the somatic or autonomic nervous system. The morphological definition of a nerve is one or more bundles of nerve fibers enclosed by connective tissue sheaths. The connective tissue in peripheral nerves is arranged similarly to the epi-, peri-, and endomysium seen in muscle cells. Each individual nerve fiber (with its enveloping Schwann cell, discussed later) is surrounded by a loose vascular connective tissue called the endoneurium. A large collection of nerve fibers is called a nerve fascicle, and this structure is invested by a dense, lamellated connective tissue called the perineurium. If the peripheral cell has more than one fascicle, the epineurium surrounds the fascicles and condenses peripherally to form a strong sheath. Peripheral nerves are supplied by penetrating blood vessels that travel longitudinally within these three layers of connective tissue. See Figure 45 below when studying the CT coverings of nerve fibers and fascicles.

Figure 45: Left photo, micrograph of a peripheral nerve depicting 8 fascicles (F) containing nerve fibers. Each fascicle is surrounded by collagen fibers and epithelial cells – or perineurium (P). The entire peripheral nerve is surrounded by a CT sheath – Epineurium (E). Taken from Wheater’s Functional Histology, a text and colour atlas, p. 130, Figure 7.13. Right photo, higher magnification of a peripheral nerve showing the 3 CT layers. Taken from Junquiera and Carneiro, Basic Histology, a text and atlas, p. 184, Figure 9-34.

Study slide A51, peripheral nerve, cross-section, human (H&E). Can you identify the three connective tissue sheaths present in the nerve seen in this cross-section? Blood vessels of various sizes can be identified within these layers.

In the PNS, all axons are encased by Schwann cells, which are derived from the neuroectoderm and function in metabolic and structural support. Small, unmyelinated axons are simply surrounded by the cytoplasm of Schwann cells. However, axons with a larger diameter are wrapped concentric layers of the Schwann cells plasma membrane, which forms a myelin sheath. These myelinated axons are capable of conducting action potentials at a higher velocity as the rate of conduction is directly related to the diameter of the axon. In the CNS, Schwann cells do not myelinate axons; a different type of support cell called an oligodendrocyte (discussed in neuroanatomy) accomplishes the process of myelination.

Study slide A51, peripheral nerve, cross-section, human (H&E). During the process of fixing and staining tissue, myelin is poorly preserved and unstained due to its high lipid content. Therefore, large, myelinated fibers are seen (at high magnification) as an unstained ring around a nerve fiber. Can you locate both myelinated and unmyelinated nerve fibers? Remember that myelinated and unmyelinated axons are peripheral processes of cell bodies located in the CNS, spinal ganglia (like the dorsal root ganglion), and autonomic ganglia (discussed below). Do you see the nuclei of the Schwann cells among these nerve fibers? The flattened nuclei of fibroblasts can also be seen in the perimysium of this section.

Continue studying slide A51, peripheral nerve, longitudinal section, human (H&E). Note the longitudinal section of the nerve on this slide. Identify both the Schwann cell nuclei that mark the course of individual nerve fibers and the slender, more condensed nuclei of fibroblast. How can you tell the difference between this tissue and dense, regular connective tissue?

Since, nervous tissue is one of the four basic tissues. Nerves can be located in almost every histology slide utilized in this class. They are often seen within connective tissue in close association with arteries and veins. These are considered part of a neurovascular bundle.

Study slide A90, artery, vein, & nerve, monkey (H&E). The nerves can be seen as circular profiles in proximity to large blocks of acidophilic skeletal muscle. Note the blood vessels in the surrounding connective tissue and the Schwann cells of the nerve. In what connective tissue layer can you see fibroblasts?

Study slide A91, cross-section of artery, human (elastin). The nerves can be seen in cross-section within the loose connective tissue that is surrounding large sections of lymphatic tissue. This elastin stain clearly stains portions of the walls of a large artery, and the nerves are located peripherally around this structure.

Spinal ganglia

Ganglia are aggregations of neuron cell bodies outside of the CNS. These cell bodies are associated with pseudo-unipolar (somatic sensory) neurons and are located within dorsal root ganglions (no slide available; utilize a recommended atlas). Fasicles of nerve fibers course through the middle of the spinal ganglion while the ganglion cells are grouped together on the periphery. The ganglion cells have centrally-located nuclei and a surrounding capsule, which consists of an inner portion of flattened satellite cells with round (or ovoid) nuclei and an outer portion of fibroblasts and connective tissue fibers. The satellite cells (called capsule cells) are derived from neuroectoderm (like the Schwann cells). They also serve similar functions as the Schwann cells: structural and metabolic support. The spinal ganglion is encapsulated by a condensed tissue, which is continuous with the epineurium and perineurium of the sensory neuron.

Sympathetic ganglion

This type of autonomic ganglia is very similar to the somatic sensory ganglia previously described, with a few subtle variations (see Figure 46 below). These include: (1) eccentric nuclei within the ganglion cells, (2) ganglion cells are rather evenly distributed, (3) the capsule surrounding the ganglion cells is less distinct, (4) some ganglion cells are binucleated, and (5) brown-stained lipofuscin granules are located with the cytoplasm of the ganglion cells. Although the neurons are pseudo-unipolar in spinal ganglia versus multipolar in sympathetic ganglia, this difference is difficult to detect in a single, thin section without the use of special stains. The nerve fibers, scattered between the cell bodies, have little to no myelin. The large myelinated fibers seen in spinal ganglia are absent.

Figure 46: Micrograph of sympathetic ganglion. Along the periphery of the cytoplasm are deposits of lipofuscin granules containing cellular debris. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 133, Figure 7.18.

Parasympathetic ganglia

Due to their short, postganglionic axons, these ganglia are often seen within or in proximity to their effector region. This is evident within the walls of the gastrointestinal tract (as parasympathetic ganglia are located within the submucosa or between the circular and longitudinal layers of smooth muscle). See Figure 47 below to compare with slides under your microscope.

Figure 47: Micrograph of parasympathetic ganglia between two smooth muscle layers in the wall of the GI tract. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 133, Figure 7.19.

Study slide B47, pyloric stomach, monkey (H&E). Neurons of these parasympathetic ganglia generally have large, pale-staining nuclei with prominent nucleoli. The cytoplasm is not always visible, but it may stain basophilic in H&E sections.

These ganglia are also in the connective tissue surrounding the seminal vesicle and prostate.

Study slide C26, seminal vesicle, human (H&E). Look for other examples of these parasympathetic ganglia.

Study slide C28, seminal vesicle & prostate, monkey (H&E)

Study slide C32, prostate, young human (H&E).