Nervous Tissue

Overview of Nervous Tissue

Nervous tissue is a specialized tissue found in the nervous system, composed primarily of two main cell types: neurons and neuroglial cells. It contains relatively little extracellular matrix compared to other tissues.

• Neurons: Excitable cells responsible for transmitting electrical signals throughout the body.

• Neuroglial Cells (Glia): Smaller, more numerous cells that provide support, protection, and nourishment to neurons. They do not transmit electrical signals.

Example: Figure 11.4 illustrates the basic structure of nervous tissue, showing neurons and neuroglial cells within the matrix.

Neuron Structure and Processes

Key Features of Neurons

Neurons are the functional units of the nervous system, specialized for communication.

• Longevity: Neurons are long-lived and generally do not regenerate, making them amitotic (do not undergo mitosis). They lack centrioles.

• Metabolic Rate: Neurons have a high metabolic rate and require a constant supply of energy; thus, they contain many mitochondria.

• Nissl Bodies: Clusters of free ribosomes and rough endoplasmic reticulum (ER) involved in protein synthesis.

• Golgi Bodies and Microtubules: Well-developed for processing and transporting cellular materials.

Neuron Processes

Neurons possess two main types of processes: dendrites and axons.

• Dendrites: Short, highly branched processes that receive information from other neurons and transmit it toward the cell body. Neurons may have many dendrites.

• Axons: Long processes that carry information away from the neuron. Each neuron typically has one axon, which may branch extensively at its terminal end.

Example: Figures 11.5 and the accompanying diagrams show the detailed structure of a neuron, including the cell body, dendrites, axon hillock, axon, myelin sheath, Schwann cells, nodes of Ranvier, and axon terminals.

Specialized Regions of the Neuron

• Axon Hillock: The region where the axon originates from the cell body; important for initiating action potentials.

• Axon Collaterals: Branches of the axon that allow communication with multiple target cells.

• Telodendria: Fine terminal branches of the axon and its collaterals.

• Axon Terminals (Synaptic Knobs): The ends of telodendria that form synapses with target cells.

Myelin Sheath and Nodes of Ranvier

The myelin sheath is a multilayered covering around axons, formed by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). It acts as an insulator and increases the speed of nerve impulse conduction.

• Nodes of Ranvier: Gaps between adjacent myelinated segments where the axon membrane is exposed; essential for rapid signal transmission via saltatory conduction.

Summary Table: Neuron Structure

Additional info:

• Neurons are classified structurally as multipolar, bipolar, or unipolar, depending on the number and arrangement of their processes.

• Functionally, neurons are categorized as sensory (afferent), motor (efferent), or interneurons.