Histology of Nervous Tissue

Overview of the Nervous System

  • Interaction of Systems:

    • The endocrine system uses hormones to coordinate internal functions.
    • The nervous system employs electrical and chemical signals for rapid communication.

  • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.
    • Protected by the cranium and vertebral column.

  • Peripheral Nervous System (PNS):

    • Composed of nerves and ganglia.
    • Nerve: A bundle of nerve fibers (axons) encased in connective tissue.
    • Ganglion: A knot-like swelling that contains neuron cell bodies.

Coordination Steps in the Nervous System

  • Sense Organs: Detect internal and external changes.
  • Information Transmission: Sensory input sent via nerves to the CNS.
  • CNS Processing:
    • Analyzes information in relation to past experiences.
    • Issues commands to effectors (muscles and glands) to elicit responses.

Divisions of the Peripheral Nervous System

  • Sensory (Afferent) Division:

    • Relays signals from receptors to CNS.
    • Somatic Sensory: From skin, muscles, bones, joints.
    • Visceral Sensory: From internal organs in thoracic and abdominal cavities.

  • Motor (Efferent) Division:

    • Sends signals from CNS to effectors.
    • Somatic Motor: To skeletal muscles, allowing voluntary control.
    • Visceral Motor (ANS): To glands, cardiac, and smooth muscle, facilitating involuntary responses.
    • Sympathetic Division: Prepares body for action.
    • Parasympathetic Division: Calms the body.

Nervous Tissue Features

  • Neurons: Specialized cells for rapid communication via electrical and chemical signals.

    • Neurosoma: Cell body containing nucleus and organelles.
    • Dendrites: Branched structures that receive signals.

  • Neuroglia (Glial Cells): Support and protect neurons, providing a framework and insulation.

Structure of a Neuron

  • Soma: Generates dendrites for signal reception.
    • Axon Hillock: Initiates the axon, leading to signal transmission.
    • Axon: Conducts impulses away from the soma, enclosed in a myelin sheath for insulation and speed.
    • Synaptic Knobs: Terminate axons forming synapses with other cells.

Types of Neurons

  • Multipolar Neurons: One axon, many dendrites (most common).
  • Bipolar Neurons: One axon, one dendrite (e.g., in the retina).
  • Unipolar Neurons: Single process; carries sensory signals to the CNS.
  • Anaxonic Neurons: Multiple dendrites, no axon; communicate through dendrites.

Neuroglia Types and Functions

  1. Astrocytes: Form blood-brain barrier, structural support, fill gaps from damaged neurons.
  2. Microglia: Small macrophages from white blood cells, clean up debris.
  3. Ependymal Cells: Line ventricles, produce cerebrospinal fluid (CSF).
  4. Oligodendrocytes: Produce myelin sheaths around CNS axons.
  5. Schwann Cells: Myelin sheaths in PNS, aid in fiber regeneration.
  6. Satellite Cells: Surround neuron soma in PNS, regulate chemical environment.

Myelin Sheath and Nerve Conduction

  • Myelination: Insulation of nerve fiber enhances conduction speed; completed late adolescence.
  • Conduction Speed Factors:
    1. Diameter of Fiber: Larger diameters conduct faster due to increased surface area.
    2. Presence of Myelin: Myelinated fibers conduct signals faster than unmyelinated.

The Cerebrum

  • Accounts for 83% of brain volume, consists of left and right hemispheres, connected by the corpus callosum.
  • Features thick folds (gyri) separated by shallow grooves (sulci).

Lobes of the Cerebrum

  1. Frontal Lobe: Voluntary motor functions, planning, emotion, social judgment.
  2. Parietal Lobe: Receives sensations (touch, pain).
  3. Occipital Lobe: Visual processing center.
  4. Temporal Lobe: Involved in hearing, smell, learning, and emotion.
  5. Insula: Hidden deep cortical area for perception and emotion.

The Cerebellum

  • Located posterior to the cerebrum; coordinates movement and balance.
  • Distinctive features include folia and Purkinje cells.
  • Connected to the brainstem via cerebellar peduncles for sensory and motor integration.

Gray and White Matter

  • Gray Matter: Contains neuronal cell bodies and forms the outer cortex of the brain.
  • White Matter: Consists of myelinated axons, forming connections within the brain.

Meninges

  • Protective membranes consist of:
    1. Dura Mater: Tough outer layer.
    2. Arachnoid Mater: Middle layer with CSF between.
    3. Pia Mater: Delicate inner layer closely following the brain contours.

Meningitis

  • Inflammation of the meninges, often caused by infections leading to severe symptoms warranting a lumbar puncture for diagnosis.
  • Risk of rapid complications if untreated.

Cerebrospinal Fluid (CSF) Circulation

  • Ventricles: Four internal chambers of the brain where CSF is produced and circulates, maintaining CNS nutrient flow and protection.

Blood Supply

  • Neurons depend on consistent blood supply for oxygen and glucose; interruptions can lead to severe consequences within minutes.

Blood-Brain Barrier (BBB)

  • Comprised of epithelial tight junctions, regulates substance permeability; essential for CNS protection and homeostasis.

The Brainstem

  • Connects the brain to the spinal cord, composed of:
    1. Midbrain.
    2. Pons.
    3. Medulla Oblongata.

Key Functions of Brainstem Structures

  • Medulla Oblongata: Contains vital centers for breathing and heart rate regulation.
  • Pons: Connects cerebellum to various regions of the brain, involved in functions like sleep and respiration.
  • Reticular Formation: Network influencing motor control, consciousness, and cardiovascular control.

Diencephalon Components

  1. Epithalamus: Involved in circadian rhythms.
  2. Thalamus: Primary relay station for sensory and motor signals.
  3. Hypothalamus: Critical for homeostatic regulation, hormone release, and autonomic control.