Exercise 15 Histology Of Nervous Tissue

Let's delve into the intricate world of nervous tissue, exploring its cellular components, structural organization, and functional significance. This exploration will serve as a practical guide for histology students and anyone interested in understanding the microscopic architecture of the nervous system.

Introduction to Nervous Tissue

Nervous tissue, the foundation of the nervous system, is responsible for coordinating and controlling bodily functions. Its primary function is to receive, process, and transmit information through electrical and chemical signals. This tissue is composed of two main cell types: neurons, which are responsible for communication, and neuroglia (glial cells), which support and protect neurons. Understanding the histological features of nervous tissue is crucial for comprehending its complex functions and the pathological changes that can occur in neurological disorders.

Cellular Components of Nervous Tissue

Neurons: The Communication Specialists

Neurons are the fundamental functional units of the nervous system. They are highly specialized cells capable of generating and transmitting electrical signals called action potentials. A typical neuron consists of three main parts:

• Cell Body (Soma): The cell body houses the nucleus and other essential organelles. It is the metabolic center of the neuron. Under a microscope, the soma appears as a prominent, often rounded structure with a large, centrally located nucleus. The cytoplasm, known as perikaryon, contains various organelles, including:
  • Nissl Bodies: These are large granular structures composed of rough endoplasmic reticulum (RER) and free ribosomes. Nissl bodies are responsible for protein synthesis, reflecting the high metabolic activity of neurons. They appear as basophilic clumps when stained with dyes like cresyl violet.
  • Golgi Apparatus: This organelle processes and packages proteins synthesized by the Nissl bodies. It is usually located near the nucleus.
  • Mitochondria: Neurons have a high energy demand, and mitochondria are abundant throughout the cell body and processes to supply ATP.
  • Neurofilaments and Microtubules: These cytoskeletal elements provide structural support and facilitate the transport of molecules within the neuron. Neurofilaments are particularly abundant and contribute to the neuron's shape and integrity.
• Dendrites: These are branching extensions of the cell body that receive signals from other neurons. Dendrites increase the surface area of the neuron, allowing it to receive multiple inputs simultaneously. They often contain dendritic spines, which are small protrusions that further increase the surface area for synaptic connections. Under a microscope, dendrites appear as tapering processes extending from the cell body.
• Axon: The axon is a single, long process that transmits signals away from the cell body to other neurons, muscles, or glands. It originates from a specialized region of the cell body called the axon hillock. The axon can be quite long, extending from the spinal cord to the tips of the toes in some cases. Key features of the axon include:
  • Axoplasm: The cytoplasm within the axon.
  • Axolemma: The plasma membrane of the axon.
  • Myelin Sheath: In many neurons, the axon is surrounded by a myelin sheath, a fatty insulation layer formed by glial cells (Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system). The myelin sheath increases the speed of signal transmission.
  • Nodes of Ranvier: These are gaps in the myelin sheath where the axon is exposed. Action potentials jump from one node to the next, a process called saltatory conduction, which significantly increases the speed of nerve impulse transmission.
  • Axon Terminals (Terminal Boutons): The axon ends in multiple branches called axon terminals. These terminals form synapses with other neurons or target cells. At the synapse, the neuron releases neurotransmitters, which transmit the signal to the next cell.

Neuroglia: The Supportive Cast

Neuroglia, also known as glial cells, are non-neuronal cells that provide structural and metabolic support to neurons. They are more numerous than neurons and play crucial roles in maintaining the microenvironment of the nervous system, protecting neurons from injury, and modulating synaptic transmission. There are several types of glial cells, each with distinct functions and histological characteristics:

• Astrocytes: These are the most abundant glial cells in the central nervous system (CNS). They have a star-shaped appearance and perform numerous functions, including:
  • Structural Support: Astrocytes provide a structural framework for neurons and help maintain the blood-brain barrier.
  • Metabolic Support: They regulate the chemical environment around neurons by taking up excess ions and neurotransmitters.
  • Nutrient Transport: Astrocytes transport nutrients from blood vessels to neurons.
  • Scar Formation: After injury to the CNS, astrocytes proliferate and form a glial scar, which helps to isolate the damaged area.
  • Types of Astrocytes: There are two main types of astrocytes: protoplasmic astrocytes, found in gray matter, and fibrous astrocytes, found in white matter. Protoplasmic astrocytes have many short, branching processes, while fibrous astrocytes have fewer, longer processes.
• Oligodendrocytes: These cells are responsible for forming the myelin sheath around axons in the CNS. A single oligodendrocyte can myelinate multiple axons. Under a microscope, oligodendrocytes appear as small cells with a dark, round nucleus and relatively little cytoplasm. The myelin sheath appears as a clear, lipid-rich layer surrounding the axon.
• Microglia: These are the resident immune cells of the CNS. They are small cells with irregular shapes and branched processes. Microglia are derived from monocytes and migrate into the CNS during development. Their primary function is to phagocytose cellular debris and pathogens, protecting the CNS from infection and injury. In histological preparations, microglia can be identified by their small size, irregular shape, and the presence of lysosomes and phagosomes in their cytoplasm.
• Ependymal Cells: These are epithelial-like cells that line the ventricles of the brain and the central canal of the spinal cord. They are ciliated, which helps to circulate cerebrospinal fluid (CSF). Ependymal cells also form the blood-CSF barrier, which regulates the passage of substances from the blood into the CSF. Histologically, ependymal cells appear as a simple cuboidal or columnar epithelium with cilia on their apical surface.
• Schwann Cells: These cells are the glial cells of the peripheral nervous system (PNS). They are similar to oligodendrocytes in that they form the myelin sheath around axons. However, unlike oligodendrocytes, a single Schwann cell only myelinates a single segment of one axon. Schwann cells also play a role in nerve regeneration after injury. In histological sections, Schwann cells appear as flattened cells wrapped around axons in the PNS.
• Satellite Cells: These are small cells that surround neuron cell bodies in ganglia of the PNS. They provide structural and metabolic support to the neurons in ganglia, similar to the role of astrocytes in the CNS. Satellite cells help regulate the microenvironment around neurons and provide a pathway for nutrients and waste products to be exchanged.

Organization of Nervous Tissue

Nervous tissue is organized into distinct structures that reflect its functional roles. These structures include the brain, spinal cord, nerves, and ganglia.

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, which are responsible for processing and integrating information.

• Brain: The brain is the control center of the nervous system, responsible for higher-level functions such as thought, memory, and emotion. Histologically, the brain is divided into several regions, including:
  • Cerebrum: The largest part of the brain, responsible for conscious thought, sensory perception, and voluntary movement. The cerebrum is composed of two hemispheres, each with an outer layer of gray matter called the cerebral cortex and an inner layer of white matter. The cerebral cortex is highly convoluted, with ridges called gyri and grooves called sulci, which increase its surface area. Histologically, the cerebral cortex is organized into six layers, each with a distinct cellular composition and function.
  • Cerebellum: Located at the back of the brain, the cerebellum is responsible for coordinating movement and maintaining balance. Like the cerebrum, the cerebellum has an outer layer of gray matter called the cerebellar cortex and an inner layer of white matter. The cerebellar cortex is organized into three layers: the molecular layer, the Purkinje cell layer, and the granule cell layer. Purkinje cells are large, distinctive neurons with extensive dendritic trees that extend into the molecular layer.
  • Brainstem: The brainstem connects the brain to the spinal cord and is responsible for regulating vital functions such as breathing, heart rate, and blood pressure. The brainstem consists of the midbrain, pons, and medulla oblongata. Histologically, the brainstem contains numerous nuclei (clusters of neuron cell bodies) and fiber tracts (bundles of axons) that connect different regions of the brain.
• Spinal Cord: The spinal cord is a long, cylindrical structure that extends from the brainstem to the lower back. It is responsible for transmitting sensory information from the body to the brain and motor commands from the brain to the body. Histologically, the spinal cord has a central core of gray matter surrounded by white matter. The gray matter is shaped like a butterfly and contains neuron cell bodies, dendrites, and synapses. The white matter contains myelinated axons that transmit signals between the brain and the body.

Peripheral Nervous System (PNS)

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

• Nerves: Nerves are bundles of axons that transmit signals between the CNS and the rest of the body. Nerves are classified as either cranial nerves (which originate from the brain) or spinal nerves (which originate from the spinal cord). Histologically, a nerve consists of multiple axons bundled together into fascicles. Each axon is surrounded by a myelin sheath formed by Schwann cells. The fascicles are surrounded by a connective tissue layer called the perineurium, and the entire nerve is surrounded by an outer connective tissue layer called the epineurium.
• Ganglia: Ganglia are clusters of neuron cell bodies located outside the CNS. They serve as relay stations for nerve signals. There are two main types of ganglia:
  • Sensory Ganglia: These ganglia contain the cell bodies of sensory neurons that transmit information from the periphery to the CNS. Sensory ganglia are located along the dorsal roots of spinal nerves and the sensory cranial nerves. The neurons in sensory ganglia are typically pseudounipolar, meaning they have a single process that branches into two axons.
  • Autonomic Ganglia: These ganglia contain the cell bodies of autonomic neurons that regulate the activity of smooth muscle, cardiac muscle, and glands. Autonomic ganglia are located in the sympathetic and parasympathetic nervous systems. The neurons in autonomic ganglia are multipolar, meaning they have multiple dendrites and a single axon.

Staining Techniques for Nervous Tissue

Several staining techniques are used to visualize the structures of nervous tissue under a microscope. Some common staining methods include:

• Hematoxylin and Eosin (H&E): This is a general-purpose stain that stains nuclei blue and cytoplasm pink. It is useful for visualizing the overall structure of nervous tissue and identifying major cell types.
• Nissl Stain: This stain uses dyes such as cresyl violet to stain Nissl bodies in the cytoplasm of neurons. It is useful for visualizing the distribution of neurons in different regions of the brain and spinal cord.
• Myelin Stain: This stain uses dyes such as Luxol fast blue to stain myelin sheaths. It is useful for visualizing the distribution of myelinated axons in white matter.
• Silver Stain: This stain uses silver salts to stain nerve fibers and cell bodies. It is useful for visualizing the detailed morphology of neurons and their processes.
• Immunohistochemistry: This technique uses antibodies to detect specific proteins in nervous tissue. It is useful for identifying specific cell types, neurotransmitters, and other molecules of interest.

Common Histological Features of Nervous Tissue

When examining histological sections of nervous tissue, there are several key features to look for:

• Neurons: Identify the cell body, dendrites, and axon. Look for Nissl bodies in the cytoplasm and the presence of a prominent nucleus.
• Neuroglia: Differentiate between astrocytes, oligodendrocytes, microglia, and ependymal cells based on their size, shape, and staining characteristics.
• Myelin Sheath: Identify myelinated axons in white matter and look for Nodes of Ranvier.
• Synapses: Although synapses are difficult to visualize with standard light microscopy, look for areas of close apposition between neurons, which may indicate the presence of synapses.
• Blood Vessels: Identify blood vessels within the nervous tissue and look for the presence of the blood-brain barrier.
• Meninges: Identify the layers of the meninges (dura mater, arachnoid mater, and pia mater) that surround the brain and spinal cord.

Clinical Significance

Understanding the histology of nervous tissue is essential for diagnosing and understanding 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 attacks the myelin sheath in the CNS, leading to demyelination and impaired nerve conduction.
• Alzheimer's Disease: A neurodegenerative disease characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain, leading to neuronal loss and cognitive decline.
• Parkinson's Disease: A neurodegenerative disease characterized by the loss of dopamine-producing neurons in the substantia nigra, leading to motor dysfunction.
• Stroke: A sudden interruption of blood flow to the brain, leading to neuronal damage and loss of function.
• Brain Tumors: Abnormal growths of cells in the brain, which can compress and damage surrounding nervous tissue.

Conclusion

The histology of nervous tissue is a complex and fascinating field that provides insights into the structure and function of the nervous system. By understanding the cellular components, structural organization, and staining techniques used to visualize nervous tissue, students and researchers can gain a deeper appreciation for the intricate workings of the brain and spinal cord. This knowledge is essential for diagnosing and treating neurological disorders and for advancing our understanding of the nervous system.