Fundamentals of the Nervous System and Nervous Tissue (Chapter 11 Part A) - Study Notes

Chapter 11 Part A: Fundamentals of the Nervous System and Nervous Tissue

THECB Learning Outcomes

• Anatomical terminology: Used to identify and describe locations of major organs of the nervous system.

• Interrelationships: Explains how molecular, cellular, tissue, and organ functions interact in the nervous system.

• System interactions: Describes interdependency and interactions among all body systems.

• Homeostasis: Explains how nervous system organs contribute to maintaining homeostasis.

• Homeostatic imbalances: Identifies causes and effects of disruptions in homeostasis.

11.1 Functions and Organization of the Nervous System

Basic Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to communicate rapidly and specifically, usually causing almost immediate responses.

• Sensory input: Information gathered by sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input.

• Motor output: Activation of effector organs (muscles and glands) produces a response.

Structural and Functional Divisions

The nervous system is divided into two principal parts:

• Central nervous system (CNS): Consists of the brain and spinal cord located in the dorsal body cavity. It is the integration and control center.

• Peripheral nervous system (PNS): Consists mainly of nerves that extend from the brain and spinal cord.

  • Spinal nerves: To and from spinal cord.

  • Cranial nerves: To and from brain.

Functional Divisions of the PNS

• Sensory (afferent) division:

  • Somatic sensory fibers: Convey impulses from skin, skeletal muscles, and joints to CNS.

  • Visceral sensory fibers: Convey impulses from visceral organs to CNS.

• Motor (efferent) division: Transmits impulses from CNS to effector organs (muscles and glands).

  • Somatic nervous system: Voluntary control of skeletal muscles.

  • Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

    • Sympathetic division: Mobilizes body systems during activity.

    • Parasympathetic division: Conserves energy, promotes housekeeping functions during rest.

11.2 Neuroglia Support and Maintain Neurons

Nervous Tissue Histology

Nervous tissue consists of two principal cell types:

• Neurons (nerve cells): Excitable cells that transmit electrical signals.

• Neuroglia (glial cells): Small cells that surround and wrap delicate neurons.

Neuroglia of the CNS

• Astrocytes: Most abundant and versatile. Support and brace neurons, guide migration, absorb leaked K+ ions, recycle neurotransmitters, and participate in information processing.

• Microglial cells: Small, ovoid cells with thorny processes. Monitor neuron health, migrate toward injured neurons, and can transform to macrophages to phagocytize microorganisms or debris.

• Ependymal cells: May be ciliated; line central cavities of brain and spinal column. Form permeable barrier between cerebrospinal fluid (CSF) and tissue fluid bathing CNS cells.

• Oligodendrocytes: Branched cells; processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers.

Neuroglia of the PNS

• Satellite cells: Surround neuron cell bodies in PNS; function similar to astrocytes of CNS.

• Schwann cells (neurolemmocytes): Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers; vital to regeneration of damaged peripheral nerve fibers.

11.3 Neurons: Structural Units of the Nervous System

Neuron Characteristics

• Extreme longevity: Lasts a person's lifetime.

• Amitotic: With few exceptions, neurons do not divide.

• High metabolic rate: Requires continuous supply of oxygen and glucose.

Neuron Structure and Function

• All neurons have a cell body and one or more processes.

• The plasma membrane is the site of electrical signaling.

• Three functional parts:

  • Receptive (input) region

  • Conducting component

  • Secretory (output) region

Neuron Cell Body

• Also called perikaryon or soma.

• Biosynthetic center: synthesizes proteins, membranes, chemicals.

• Contains rough ER (chromatophilic substance or Nissl bodies), free ribosomes, Golgi apparatus, spherical nucleus with nucleolus, mitochondria, and cytoskeletal elements.

Neuron Cell Bodies in CNS vs PNS

• Nuclei: Clusters of neuron cell bodies in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

Neuron Processes

• Processes that extend from cell body: dendrites and axons.

• CNS contains both neuron cell bodies and their processes; PNS contains chiefly neuron processes.

Dendrites

• Motor neurons can contain hundreds of short, tapering, diffusely branched dendrites.

• Receptive (input) region of neuron; convey incoming messages toward cell body as graded potentials (short distance signals).

• Finer dendrites may be highly specialized to collect information; contain dendritic spines (appendages with bulbous or spiky ends).

Axon

• Starts at cone-shaped area called axon hillock.

• Long axons are called nerve fibers.

• Tracts: Bundles of nerve fibers in CNS.

• Nerves: Bundles of nerve fibers in PNS.

• Distal endings are called axon terminals or terminal boutons.

Axon Functional Characteristics

• Conducting region of neuron; generates nerve impulses along axolemma (neuron cell membrane) to axon terminal.

• Neurotransmitters are released by axon terminals.

• Movement of molecules occurs in both directions:

  • Anterograde: Away from cell body (e.g., mitochondria, cytoskeletal elements, membrane components, enzymes).

  • Retrograde: Toward cell body (e.g., organelles to be degraded, signal molecules, viruses, bacterial toxins).

• Axons quickly decay if cut or damaged.

Myelinated vs. Nonmyelinated Fibers

• Myelinated fibers: Segmented sheath surrounds most long or large-diameter axons; composed of myelin (whitish, protein-lipid substance) that protects and electrically insulates axon, increasing speed of nerve impulse transmission.

• Nonmyelinated fibers: Do not contain sheath; conduct impulses more slowly.

Myelination in the PNS

• Formed by Schwann cells wrapping around axon in jelly roll fashion; one cell forms one segment of myelin sheath.

• Outer collar of perinuclear cytoplasm (formerly neurilemma): peripheral bulge containing nucleus and most of cytoplasm.

• No channels or carriers, so good electrical insulators.

• Myelin sheath gaps: Gaps between adjacent Schwann cells (formerly called nodes of Ranvier).

Myelin Sheaths in the CNS

• Formed by processes of oligodendrocytes, not whole cells.

• Each cell can wrap up to 60 axons at once.

• Myelin sheath gap is present; no outer collar of perinuclear cytoplasm.

• Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia.

Gray and White Matter

• White matter: Regions of brain and spinal cord with dense collections of myelinated fibers (usually fiber tracts).

• Gray matter: Mostly neuron cell bodies and nonmyelinated fibers.

Structural Classification of Neurons

• Multipolar: Three or more processes (one axon, others dendrites); most common, major neuron type in CNS.

• Bipolar: Two processes (one axon, one dendrite); rare, found in special sense organs (e.g., retina, olfactory mucosa).

• Unipolar: One T-like process (two axons); also called pseudounipolar.

  • Peripheral (distal) process: Associated with sensory receptor.

  • Central (proximal) process: Enters CNS.

Table: Comparison of Structural Classes of Neurons

Neuron Type	Processes	Location
Multipolar	1 axon, many dendrites	Most neurons in CNS
Bipolar	1 axon, 1 dendrite	Special sense organs (retina, olfactory mucosa)
Unipolar (Pseudounipolar)	1 T-like process (2 axons)	Mainly in PNS, sensory neurons

Relationship of Anatomy to Functional Regions

• Multipolar: Trigger zone at axon hillock.

• Bipolar: Trigger zone between cell body and axon.

• Unipolar: Trigger zone at junction of peripheral and central processes.

Additional info: The above notes are expanded and clarified for academic completeness, including definitions, examples, and table formatting for structural neuron classification.