NERVOUS SYSTEM (SYSTEMIC HISTOLOGY)

Systemic Histology of the Brain

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Regions of the Brain

1. Brainstem
2. Cerebrum
3. Cerebellum

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Structure of Brain Tissue

1. White Matter:
   • Composed of numerous bundles of myelinated axons.
   • Myelin sheath in the CNS is thinner compared to the PNS.
2. Gray Matter:
   • Contains numerous neuronal soma (cell bodies), glial cells, and neuropil.
   • Lacks myelin.
   • In the cerebrum and cerebellum, gray matter is found on the surface, forming the cortex.

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Cerebral Cortex

• Composed of six distinct layers:
  1. Molecular or Flexiform Layer
  2. Outer Granular Layer
     • Composed of small neurons.
  3. Outer Pyramidal Layer
     • Contains medium and large neurons.
  4. Inner Granular Layer
     • Composed of small stellate neurons.
  5. Inner Pyramidal Layer
  6. Fusiform or Polymorphic Layer
     • Neurons vary in shape; spindle-shaped neurons are most prominent.

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Cerebral Medulla

• Located beneath the cortex.
• Composed of white matter, consisting of myelinated axons organized into tracts or fascicles.

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Cerebellum

• The surface of the cerebellum has narrow, lip-like regions.
• The cerebellar cortex consists of three layers:
  1. Outer Molecular Layer
     • Composed of neuropil.
  2. Middle Layer
     • Contains very large neurons located at the inner margin of the outer molecular layer.
  3. Inner Granular Layer
     • Contains densely packed, small neurons that are highly stained.
     • This layer appears dense and blue under the microscope.

Basket Cells

• Description:
  • A very large pyramidal neuron.
  • Located between the large pyramidal neurons of Purkinje cells in the brainstem.
• Structure:
  • Contains several nuclei and fiber track bundles of myelinated axons.

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Spinal Cord

• Structure:
  • A long tubular structure, outlined by a defined boundary.
  • Hollow, with a central canal (spinal canal) running through its center.
• Division:
  • The spinal cord is divided into two bilaterally symmetrical halves by:
    1. Dorsal Medial Septum
    2. Ventral Median Fissure
• Gray Matter:
  • Located internally, surrounding the central canal.

Gray Matter

• Arrangement:
  • Gray matter is arranged in the shape of an “H” or butterfly, with three columns or horns:
    1. Dorsal Horn
       • Long and narrow.
       • Contains small neurons and interneurons.
    2. Ventral Horn
       • The widest horn.
       • Contains large neurons, called somatic efferent neurons.
    3. Lateral Horn
       • Less distinct, but very prominent in the thoracolumbar segment.
       • Contains medium-sized neurons, referred to as visceral afferent neurons.

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Spinal White Matter

• Composition:
  • Made up of both myelinated and non-myelinated axons, as well as some glial cells.
  • The axons occur in bundles or fascicles, forming fiber tracts.
• Fiber Tracts:
  • Dorsal Funiculus: Located between the dorsal midline and the dorsal horn.
  • Ventral Funiculus: Located between the ventral midline and the ventral horn.
  • Lateral Funiculus: Located between the dorsal and ventral horns.

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Central Canal of Spinal Cord

• Shape:
  • The central canal is diamond-shaped.
• Lining and Contents:
  • Lined by ependymal cells.
  • Contains cerebrospinal fluid (CSF).

Meninges

The meninges are dense, collagenous, fibrous connective tissue coverings around the central nervous system (CNS). From the outside to the inside, the meninges are:

1. Dura Mater
2. Arachnoid Mater
3. Pia Mater

Dura Mater

The dura mater is the outermost covering of the CNS. Depending on the location, it is divided into two types:

1. Cranial Dura
   • Surrounds the brain.
   • Tough fibrous membrane made up of collagen fibers, fibroblasts, and some elastic fibers.
   • Contains definite blood vessels.
   • Consists of two layers:
     • Outer Periosteal Dura Mater: This is the periosteum lining the inner surface of cranial bones.
     • Inner Meningeal Dura Mater: Contains fewer blood vessels and remains in contact with the arachnoid mater. At most locations, it intimately fuses with the outer periosteal dura mater, except where dural sinuses are present.
2. Spinal Dura
   • Surrounds the spinal cord.
   • Consists of a single layer that remains separated from the periosteum of the vertebral column by the epidural space.
   • The epidural space is used for administering epidural anesthesia in animals.

Arachnoid Mater

The arachnoid mater is a thin, delicate, collagenous, or fibrous membrane lined by a layer of fibroblasts on both its inner and outer surfaces.

• It remains attached to the underlying pia mater by connective tissue strands through a space called the subarachnoid space, which houses cerebrospinal fluid (CSF).
• Arachnoid Granulations:
  • These are villous projections of the arachnoid into the dural sinuses through the meningeal layer of the dura mater.
  • They serve to return CSF into the vascular system.

Pia Mater

The pia mater is the innermost and most delicate collagenous membrane of the meninges.

• It closely follows the gyri (ridges) and sulci (grooves) of the brain and the septa and fissures of the spinal cord.
• In the brain, it lies external to the basal lamina.

Class Notes: Nervous System Overview

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Bissell Lamina and Related Structures

• Bissell Lamina:
  • Located outer to the clear limiting membrane.
  • Forms neurogliocytes and astrocytes in the spinal cord.
• Denticulate Ligament:
  • The spinal cord parameter condenses in the middle of its surface to form the denticulate ligament, which suspends the spinal cord within the dural covering.

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Peripheral Nervous System (PNS)

• Components:
  • Composed primarily of nerves, ganglia, dendritic terminals, and axonal terminals.
  • Structures include:
    • Receptors: Responsible for detecting stimuli.
    • Synapses: Facilitate communication between neurons.
• Types of Nerves:
  • Cranial Nerves
  • Spinal Nerves
• Ganglia:
  • Includes both autonomic and peripheral ganglia.
  • Covered by dense collagenous capsules.
• Plexuses:
  • Brachial Plexus
  • Lumbosacral Plexus

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Neuromuscular Junction (NMJ)

• Definition:
  • A junction between a neuron and a muscle cell.
• Components of Neuronal Terminals:
  • Neurons form dilated expansions in the dendritic zone.
  • These expansions terminate over a straw-like depression on the skeletal muscle cell surface.
• End Plate (Motor End Plate):
  • The depression is called the end plate or sole plate.
  • Contains synaptic vesicles carrying acetylcholine as the neurotransmitter.

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Motor Unit

• Definition:
  • A motor unit consists of:
    • One alpha motor neuron (located in the ventral horn of the spinal cord).
    • All the muscle cells innervated by this single alpha motor neuron.
• Structure:
  • One motor unit innervates 200–300 skeletal myocytes.

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Organization of Nerves

• Individual Nerve Fibers:
  • Origin surrounded by loose connective tissue (endoneurium) containing:
    • Reticular fibers.
    • Collagen.
• Nerve Bundles (Fascicles):
  • Multiple nerve fibers form a bundle or fascicle.
  • Covered by a thicker layer of connective tissue called the perineurium.
  • Perineurium Structure:
    • Composed of:
      • Collagen fibers.
      • Squamous fibroblasts with their own basal lamina (also known as epithelioid cells).
• Complete Nerve Structure:
  • Several fascicles join to form a complete nerve.
  • Surrounded by the epineurium.
• Vascular Supply:
  • Blood vessels that supply the nerves are called the vasa nervorum.

RECEPTORS

Receptors stimulate detectors formed by the dendritic zone of a neuron. Neurons associated with these detectors are called sensory neurons. These neurons are classified into two main categories:

1. Physiological
2. Morphological (Structural)

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1. Physiological Classification

Based on location, sensory neurons are classified as:

1. Exteroceptors
   • Location: Found on the body surface.
   • Function: Provide information about interactions with the external environment.
2. Interoceptors
   • Location: Found in visceral organs.
   • Function: Provide information about the internal environment of organs.
3. Proprioceptors
   • Location: Found in musculoskeletal structures, such as muscles, tendons, and joints.
   • Function: Provide information about mobility or motion of body parts.

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2. Classification Based on Stimulus Type

Depending on the type of energy trapped or the type of stimuli, receptors are classified as:

1. Mechanoreceptors
   • Function: Receive mechanical stimuli, such as touch, pressure, and sound.
2. Chemoreceptors
   • Function: Respond to chemical changes in a part, e.g., changes in the chemical nature of blood.
3. Thermoreceptors
   • Function: Receive stimuli from changes in temperature.

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3. Morphological (Structural) Classification

Based on structure, receptors are divided into two types:

1. Encapsulated Receptors
   • These receptors are surrounded by a connective tissue capsule.
2. Non-Encapsulated Receptors
   • These receptors are free nerve endings without a capsule.

1. Non-Encapsulated Receptors

• These are free nerve endings and are the most widely occurring receptors in the body.
• The dendritic branches are freely distributed among the cells of various organs.

2. Encapsulated Receptors

• In encapsulated receptors, the branches of dendrites are covered by a thin collagenous capsule.
• Encapsulated nerve endings (or dendritic nerve endings) are freely distributed among the cells of various organs.

Types of Encapsulated Receptors

1. Pacinian corpuscles
2. Meissner’s corpuscles
3. Golgi tendon organs