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.