Histology Of Nervous Tissue Review Sheet

Nervous tissue, the cornerstone of our intricate nervous system, orchestrates communication throughout the body. Understanding its microscopic structure, or histology, is crucial for comprehending how it functions. This review sheet provides a comprehensive overview of the histology of nervous tissue, covering its cellular components, organization, and key features.

Introduction to Nervous Tissue

Nervous tissue is primarily responsible for receiving, processing, and transmitting information. This complex task is accomplished through two main types of cells: neurons and glial cells. Neurons are the functional units of the nervous system, specialized for electrical signaling. Glial cells, also known as neuroglia, provide support, insulation, and protection for neurons. Together, they form the intricate network that allows us to think, feel, and act.

Cellular Components: Neurons

Neurons are highly specialized cells characterized by their ability to generate and transmit electrical impulses called action potentials. A typical neuron consists of the following components:

• Cell Body (Soma): The control center of the neuron, containing the nucleus and other essential organelles. It integrates signals received from other neurons. The cytoplasm of the cell body is called the perikaryon.
• Dendrites: Branch-like extensions that receive signals from other neurons or sensory receptors. They increase the surface area available for receiving signals.
• Axon: A single, long extension that transmits signals away from the cell body to other neurons, muscles, or glands. The axon arises from a specialized region of the cell body called the axon hillock.
• Axon Terminals (Synaptic Terminals): Branched endings of the axon that form synapses with other neurons or target cells. At the synapse, the neuron releases chemical messengers called neurotransmitters to communicate with the next cell.

Neuron Classification

Neurons can be classified based on their structure and function:

• Structural Classification:

  • Multipolar Neurons: Have one axon and multiple dendrites. This is the most common type of neuron in the central nervous system (CNS).
  • Bipolar Neurons: Have one axon and one dendrite. They are found in specialized sensory organs such as the retina and olfactory mucosa.
  • Unipolar (Pseudounipolar) Neurons: Have a single process that branches into two axons. These are typically sensory neurons.

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit signals from sensory receptors to the CNS.
  • Motor (Efferent) Neurons: Transmit signals from the CNS to muscles or glands.
  • Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS. They are involved in complex processing and reflexes.

Key Histological Features of Neurons

• Nissl Bodies: Large clusters of rough endoplasmic reticulum (rER) and free ribosomes found in the cell body and dendrites. They are responsible for protein synthesis.
• Neurofilaments: Intermediate filaments that provide structural support to the neuron and maintain its shape.
• Axon Hillock: A cone-shaped region of the cell body where the axon originates. It is devoid of Nissl bodies.
• Myelin Sheath: A fatty insulation layer that surrounds the axons of some neurons, increasing the speed of signal transmission. It is formed by glial cells (oligodendrocytes in the CNS and Schwann cells in the PNS).
• Nodes of Ranvier: Gaps in the myelin sheath where the axon is exposed. These gaps allow for saltatory conduction, a process that speeds up signal transmission.

Cellular Components: Glial Cells

Glial cells, or neuroglia, are non-neuronal cells that provide essential support and protection to neurons. They are more numerous than neurons and play a crucial role in maintaining the proper functioning of the nervous system. There are four main types of glial cells in the CNS and two types in the PNS:

Glial Cells in the Central Nervous System (CNS)

• Astrocytes: The most abundant glial cells in the CNS. They have a star-shaped morphology and perform a variety of functions, including:

  • Providing structural support to neurons
  • Regulating the chemical environment around neurons by absorbing excess ions and neurotransmitters
  • Forming the blood-brain barrier, which protects the brain from harmful substances
  • Participating in synapse formation and plasticity

• Oligodendrocytes: Responsible for forming the myelin sheath around axons in the CNS. One oligodendrocyte can myelinate multiple axons.

• Microglia: The resident immune cells of the CNS. They are phagocytic cells that remove cellular debris and pathogens.

• Ependymal Cells: Line the ventricles of the brain and the central canal of the spinal cord. They produce cerebrospinal fluid (CSF) and help to circulate it.

Glial Cells in the Peripheral Nervous System (PNS)

• Schwann Cells: Form the myelin sheath around axons in the PNS. Each Schwann cell myelinates only one segment of one axon.
• Satellite Cells: Surround neuron cell bodies in ganglia. They provide support and regulate the microenvironment around neurons.

Key Histological Features of Glial Cells

• Astrocytes: Characterized by their star-shaped morphology and numerous processes that extend to neurons and blood vessels. They stain positively for glial fibrillary acidic protein (GFAP).
• Oligodendrocytes: Have a small, round nucleus and fewer processes than astrocytes. They are often found near axons.
• Microglia: Small cells with elongated nuclei and branched processes. They are difficult to distinguish from other glial cells in routine histological preparations.
• Ependymal Cells: Columnar or cuboidal cells with cilia and microvilli on their apical surface. They form a single layer that lines the ventricles and central canal.
• Schwann Cells: Flattened cells that wrap around axons in the PNS. The myelin sheath appears as a series of concentric layers around the axon.
• Satellite Cells: Small, flattened cells that surround neuron cell bodies in ganglia. They are difficult to distinguish from other cells in the ganglion.

Organization of Nervous Tissue

Nervous tissue is organized into distinct structures in both the central and peripheral nervous systems.

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It is characterized by the presence of gray matter and white matter.

• Gray Matter: Contains primarily neuron cell bodies, dendrites, and unmyelinated axons, as well as glial cells. It is the site of synaptic integration and information processing.
  • In the brain, gray matter forms the outer layer called the cortex (e.g., cerebral cortex, cerebellar cortex) and is also found in deeper structures called nuclei.
  • In the spinal cord, gray matter is located in the central region and has a butterfly or H-shape.
• White Matter: Contains primarily myelinated axons and glial cells. It is responsible for transmitting signals between different regions of the CNS.
  • White matter forms the bulk of the brain and spinal cord, surrounding the gray matter.
  • The myelin sheath gives white matter its characteristic white appearance.

Peripheral Nervous System (PNS)

The PNS consists of all nervous tissue outside the brain and spinal cord, including nerves and ganglia.

• Nerves: Bundles of axons that transmit signals between the CNS and the periphery. Nerves are surrounded by connective tissue layers:

  • Endoneurium: Surrounds individual axons.
  • Perineurium: Surrounds bundles of axons called fascicles.
  • Epineurium: Surrounds the entire nerve.

• Ganglia: Clusters of neuron cell bodies located outside the CNS. They are associated with both sensory and motor nerves.

  • Sensory ganglia (e.g., dorsal root ganglia) contain the cell bodies of sensory neurons.
  • Motor ganglia (e.g., autonomic ganglia) contain the cell bodies of motor neurons.

Key Features of Nervous Tissue Histology

• Neurons: Large cells with a prominent nucleus and Nissl bodies.
• Glial Cells: Smaller cells that support and protect neurons. Different types of glial cells have distinct morphological features.
• Gray Matter: Contains primarily neuron cell bodies and unmyelinated axons.
• White Matter: Contains primarily myelinated axons.
• Nerves: Bundles of axons surrounded by connective tissue.
• Ganglia: Clusters of neuron cell bodies located outside the CNS.

Staining Techniques for Nervous Tissue

Several staining techniques are used to visualize nervous tissue under the microscope. Some common stains include:

• Hematoxylin and Eosin (H&E): A general-purpose stain that stains nuclei blue and cytoplasm pink. It is useful for identifying basic tissue structures.
• Nissl Stain: Stains Nissl bodies in neurons, allowing for visualization of neuron cell bodies and dendrites.
• Myelin Stain: Stains the myelin sheath, allowing for visualization of myelinated axons.
• Immunohistochemistry: Uses antibodies to detect specific proteins in nervous tissue, such as GFAP for astrocytes or neurofilament proteins for neurons.

Clinical Significance

Understanding the histology of nervous tissue is essential for diagnosing and treating neurological disorders. Many diseases affect the structure and function of neurons and glial cells, leading to a variety of symptoms. Some examples include:

• Multiple Sclerosis (MS): An autoimmune disease that damages the myelin sheath in the CNS, leading to impaired signal transmission.
• Alzheimer's Disease: A neurodegenerative disease characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain, leading to cognitive decline.
• Parkinson's Disease: A neurodegenerative disease characterized by the loss of dopamine-producing neurons in the brain, leading to motor dysfunction.
• Brain Tumors: Abnormal growths of cells in the brain. They can be derived from neurons, glial cells, or other types of cells.

Nervous Tissue Under the Microscope: A Closer Look

To truly grasp the histology of nervous tissue, it's essential to visualize these structures under a microscope. Here's a guide to what you might observe:

Neuron Cell Body (Soma)

• Nucleus: A large, prominent, and typically spherical nucleus is a hallmark of neurons. It often contains a distinct nucleolus.
• Nissl Substance: Look for intensely stained, granular clumps scattered throughout the cytoplasm. These are the Nissl bodies, representing the rough endoplasmic reticulum and ribosomes involved in protein synthesis. Their abundance reflects the high metabolic activity of neurons.
• Cytoplasm: The cytoplasm surrounding the nucleus should appear relatively clear, but with careful observation, you may discern fine neurofibrils contributing to the cell's cytoskeleton.

Neuron Processes: Axons and Dendrites

• Dendrites: These branching extensions emerging from the cell body are usually shorter and thicker than axons. They taper as they extend outward and are often studded with small protrusions called dendritic spines, the sites of synaptic connections.
• Axons: Typically, a single, slender axon arises from the axon hillock, a cone-shaped region devoid of Nissl substance. Axons can be quite long and are responsible for transmitting signals over considerable distances.

Glial Cells

• Astrocytes: These star-shaped cells are difficult to distinguish in standard H&E-stained sections but are easily identified using immunohistochemistry for glial fibrillary acidic protein (GFAP). Their nuclei are generally smaller and darker than those of neurons.
• Oligodendrocytes: These cells are responsible for myelination in the CNS. They have small, round, intensely stained nuclei and are often observed closely associated with axons.
• Microglia: These small, irregularly shaped cells are the resident immune cells of the CNS. Their nuclei are elongated and dense, and their cytoplasm is sparse.
• Ependymal Cells: These epithelial-like cells line the ventricles of the brain and the central canal of the spinal cord. They are often ciliated to aid in the circulation of cerebrospinal fluid.
• Schwann Cells: These cells myelinate axons in the PNS. Their nuclei are flattened and elongated, and they wrap around the axon in a characteristic spiral pattern.
• Satellite Cells: These small cells surround the cell bodies of neurons in ganglia. They provide support and protection to the neurons.

Gray and White Matter

• Gray Matter: In the CNS, gray matter appears darker due to the high concentration of neuron cell bodies, dendrites, and synapses. It is the site of information processing and integration.
• White Matter: White matter appears lighter due to the abundance of myelinated axons. It is responsible for transmitting signals between different regions of the CNS.

Nerves

• Nerve Fibers: In cross-sections of nerves, individual nerve fibers (axons) appear as small, round structures surrounded by myelin sheaths.
• Connective Tissue Layers: The endoneurium, perineurium, and epineurium provide structural support and protection to the nerve.

Ganglia

• Neuron Cell Bodies: Ganglia contain clusters of neuron cell bodies, which are similar in appearance to those found in the gray matter of the CNS.
• Satellite Cells: The neuron cell bodies are surrounded by satellite cells, which provide support and protection.

Common Pitfalls in Identifying Nervous Tissue Histology

• Distinguishing Neurons from Glial Cells: Novices often confuse neurons with glial cells. Focus on the key differences: neurons are larger, possess prominent nuclei with nucleoli, and exhibit Nissl substance. Glial cells are typically smaller with darker, more condensed nuclei.
• Differentiating Axons from Dendrites: This can be tricky, especially in routine stains. Axons are generally longer and thinner than dendrites and arise from the axon hillock. Dendrites are more branched and may display dendritic spines.
• Recognizing Different Types of Glial Cells: This requires familiarity with their specific morphological features and, often, the use of immunohistochemical stains.
• Identifying Myelin Sheaths: Look for the characteristic concentric layers surrounding axons in myelinated fibers. Myelin stains can enhance visualization.
• Understanding Gray and White Matter: Remember that gray matter is rich in cell bodies, while white matter is dominated by myelinated axons. This difference in composition leads to their distinct staining patterns.

Frequently Asked Questions (FAQ)

• What is the main function of nervous tissue?

  • Nervous tissue is responsible for receiving, processing, and transmitting information throughout the body.

• What are the two main types of cells in nervous tissue?

  • Neurons and glial cells.

• What is the function of neurons?

  • Neurons are specialized for generating and transmitting electrical signals.

• What is the function of glial cells?

  • Glial cells provide support, insulation, and protection for neurons.

• What are Nissl bodies?

  • Large clusters of rough endoplasmic reticulum and free ribosomes found in the cell body and dendrites of neurons.

• What is the myelin sheath?

  • A fatty insulation layer that surrounds the axons of some neurons, increasing the speed of signal transmission.

• What are the nodes of Ranvier?

  • Gaps in the myelin sheath where the axon is exposed.

• What are the different types of glial cells in the CNS?

  • Astrocytes, oligodendrocytes, microglia, and ependymal cells.

• What are the different types of glial cells in the PNS?

  • Schwann cells and satellite cells.

• What is the blood-brain barrier?

  • A protective barrier that prevents harmful substances from entering the brain. It is formed by astrocytes.

• What is gray matter?

  • Contains primarily neuron cell bodies, dendrites, and unmyelinated axons.

• What is white matter?

  • Contains primarily myelinated axons.

• What are nerves?

  • Bundles of axons that transmit signals between the CNS and the periphery.

• What are ganglia?

  • Clusters of neuron cell bodies located outside the CNS.

Conclusion

A firm grasp of nervous tissue histology is paramount for anyone venturing into neuroscience, medicine, or related fields. By mastering the identification of cellular components, understanding the organization of gray and white matter, and appreciating the nuances of staining techniques, you’ll be well-equipped to interpret microscopic images and delve deeper into the fascinating world of the nervous system. Remember that diligent observation, coupled with a solid theoretical foundation, is the key to success in histology. This review sheet serves as a foundation, encouraging further exploration and a lifelong pursuit of knowledge in this ever-evolving field.