Histology: Nervous Tissue

Overview of Nervous Tissue

Nervous tissue is a specialized tissue that forms the majority of the brain, spinal cord, and nerves. It is essential for the transmission and processing of information throughout the body, enabling rapid communication and coordination of bodily functions.

• Nervous tissue consists of two main cell types: neurons and neuroglia.

Neurons

Neurons are the primary functional cells of nervous tissue, responsible for receiving and transmitting electrical signals. They are typically larger and less numerous than neuroglia, and are amitotic (do not divide after maturity).

• Excitable cells: Capable of generating and propagating electrical impulses.

• Soma (cell body): Contains the nucleus and most organelles; serves as the metabolic center.

• Dendrites: Multiple short, branched processes that receive signals from other neurons; part of the receptive region.

• Axon: A single, long process that conducts electrical signals away from the soma; the conducting region.

• Axon hillock: Cone-shaped region of the soma where action potentials are initiated.

• Axon terminals: The secretory region at the end of the axon; stores and releases neurotransmitters via exocytosis.

• Neurofibrils: Cytoskeletal elements that provide structural support within the neuron.

Example: Multipolar neurons in the spinal cord have many dendrites and a single axon, allowing integration of multiple signals.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in nervous tissue. They are smaller, more numerous, and capable of mitosis. Neuroglia perform various functions that maintain the health and efficiency of neurons.

• Electrical insulation: Some neuroglia form myelin sheaths that insulate axons.

• Cerebrospinal fluid circulation: Certain glial cells help circulate CSF in the brain and spinal cord.

• Anchoring: Neuroglia anchor neurons and blood vessels in place.

• Synapse formation: Aid in the development and maintenance of synaptic connections.

• Monitoring tissue health: Respond to injury or disease within nervous tissue.

Example: Schwann cells in the peripheral nervous system form myelin sheaths around axons, while astrocytes in the central nervous system provide structural and metabolic support.

Myelin Sheath and Nodes of Ranvier

The myelin sheath is a multilayered covering formed by glial cells (Schwann cells in the PNS, oligodendrocytes in the CNS) that wraps around axons. It provides electrical insulation and greatly increases the speed of action potential propagation.

• Schwann cells: Form myelin sheath in the peripheral nervous system.

• Myelin sheath gaps (Nodes of Ranvier): Spaces between adjacent Schwann cells where the axon is exposed; these gaps facilitate rapid signal transmission via saltatory conduction.

Example: In myelinated axons, action potentials "jump" from node to node, increasing conduction velocity.

Structural Identification on Models and Slides

When examining nervous tissue under a microscope or on anatomical models, the following structures should be identified:

• Soma (cell body): Central region containing the nucleus.

• Dendrites: Short, branched extensions from the soma.

• Axon hillock: Area where the axon emerges from the soma.

• Axon: Long, thin process extending from the soma.

• Axon terminals: Endings of the axon where neurotransmitters are released.

• Neurofibrils: Threadlike structures within the soma and processes.

• Schwann cells: Cells wrapping around the axon in the PNS.

• Myelin sheath: Layered covering around the axon.

• Nodes of Ranvier: Gaps between myelinated segments.

Table: Comparison of Neurons and Neuroglia

Key Terms and Definitions

• Neuron: Excitable cell that transmits electrical signals.

• Neuroglia: Supporting cells of nervous tissue.

• Myelin sheath: Insulating layer around axons.

• Node of Ranvier: Gap in the myelin sheath.

• Axon hillock: Site of action potential initiation.

Relevant Equations

While histology focuses on structure, understanding the propagation of action potentials is important in physiology:

• Action potential propagation speed: Increases with myelination and axon diameter.

• Ohm's Law (for membrane potential):

• Nernst Equation (for equilibrium potential):

Additional info: Equations provided for academic context; not directly referenced in the original notes but relevant for understanding nervous tissue physiology.