Nervous Tissue

Introduction

Nervous tissue is a specialized tissue that controls and integrates all body activities within limits that maintain life. It is essential for communication, coordination, and regulation of bodily functions.

Functions of the Nervous System

Basic Functions

• Sensory Function: Detects changes in the internal and external environment using sensory receptors.

  • Example: Feeling the fullness of the stomach or the warmth of the sun on your skin.

• Integrative Function: Interprets and remembers sensory input, allowing for decision-making and learning.

• Motor Function: Reacts to sensory input by activating effectors such as muscles and glands.

  • Examples: Muscular contractions, glandular secretions.

Major Structures of the Nervous System

Components

• Brain

• Cranial nerves

• Spinal cord

• Spinal nerves

• Ganglia

• Enteric plexuses (in the gastrointestinal tract)

• Sensory receptors (in skin and other organs)

Divisions of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

• Responsible for processing and integrating information.

Peripheral Nervous System (PNS)

• Consists of cranial and spinal nerves containing sensory and motor fibers.

• Connects the CNS to muscles, glands, and sensory receptors.

Subdivisions of the PNS

• Somatic Nervous System (SNS): Voluntary control; sensory neurons from skin and special senses to CNS, motor neurons to skeletal muscle.

• Autonomic Nervous System (ANS): Involuntary control; sensory neurons from visceral organs to CNS, motor neurons to smooth and cardiac muscle and glands.

  • Sympathetic Division: Speeds up heart rate.

  • Parasympathetic Division: Slows down heart rate.

• Enteric Nervous System (ENS): Involuntary control of gastrointestinal tract; functions independently of ANS and CNS.

Neurons

Structure and Function

• Neurons are the functional units of the nervous system, capable of producing action potentials due to electrical excitability.

• Cell Body (Soma): Contains a single nucleus, Nissl bodies (for protein synthesis), neurofilaments (for shape/support), microtubules (for material transport), and lipofuscin pigment (aging marker).

• Processes:

  • Dendrites: Conduct impulses toward the cell body; typically short, highly branched, and unmyelinated; specialized for contact with other neurons.

  • Axons: Conduct impulses away from the cell body; long, thin, cylindrical; impulses arise from the initial segment (trigger zone); may branch (collaterals) and end in synaptic terminals containing neurotransmitters.

Structural Classification of Neurons

• Multipolar Neurons: Several dendrites and one axon; most common type.

• Bipolar Neurons: One main dendrite and one axon; found in retina, inner ear, and olfactory area.

• Unipolar Neurons: One process only (develops from a bipolar neuron); always sensory neurons.

Functional Classification of Neurons

• Sensory (Afferent) Neurons: Transport sensory information to the CNS.

• Motor (Efferent) Neurons: Transmit motor impulses to muscles and glands.

• Interneurons (Association Neurons): Connect sensory to motor neurons; comprise about 90% of all neurons.

Connective Tissue Coverings of Nerves

Organization

• Endoneurium: Wraps each individual nerve fiber (axon).

• Perineurium: Surrounds groups of nerve fibers (fascicles).

• Epineurium: Covers the entire nerve; blends with dura mater at the intervertebral foramen.

Neuroglial Cells

Types and Functions

• Neuroglia make up half the volume of the CNS; smaller and more numerous than neurons; capable of rapid mitosis (gliomas are tumors of glial cells).

• CNS Neuroglia:

  • Astrocytes: Star-shaped; form blood-brain barrier, metabolize neurotransmitters, provide structural support.

  • Oligodendrocytes: Most common glial cell; form myelin sheath around axons in CNS; analogous to Schwann cells in PNS.

  • Microglia: Small cells near blood vessels; phagocytic role—clear away dead cells and debris.

  • Ependymal Cells: Form epithelial membrane lining cerebral cavities and central canal; produce cerebrospinal fluid (CSF).

• PNS Neuroglia:

  • Schwann Cells: Encircle axons in PNS; produce myelin sheath.

  • Satellite Cells: Flat cells surrounding neuronal cell bodies in peripheral ganglia; support neurons in PNS ganglia.

Gray and White Matter

Organization in CNS

• White Matter: Composed of myelinated axons; appears white due to myelin.

• Gray Matter: Contains nerve cell bodies, dendrites, axon terminals of unmyelinated axons, and neuroglia; appears gray.

• In the spinal cord, gray matter forms an H-shaped inner core surrounded by white matter.

• In the brain, a thin outer shell of gray matter covers the surface and is found in clusters called nuclei inside the CNS.

Summary Table: Neuroglial Cells

The following table summarizes the main types of neuroglial cells, their location, and primary functions.

Additional info:

• Neuroglial cells are essential for maintaining the environment around neurons, providing support, and facilitating repair after injury.