Nervous System: Structure and Function

• Nervous tissues receive, transmit, and integrate information to control the body's activities.
• Highly specialized, interconnected neurons process information and generate response signals.

Central Nervous System (CNS)

• Consists of the brain and spinal cord.
• Protected within the skull and vertebral canal.
• Covered by meninges.
• Bathed by cerebrospinal fluid (CSF).

Peripheral Nervous System (PNS)

• Includes paired spinal and cranial nerves, and ganglia.

Functional Divisions

• Somatic Nervous System: Supplies the body wall and limbs, involved in voluntary functions.
• Autonomic Nervous System: Supplies viscera and glands, controls involuntary organ functions.

Cells of the Nervous System

• Neurons: Excitable cells that transmit electrical signals.
• Glial Cells (Neuroglia): Supporting cells including astrocytes, oligodendrocytes, ependymal cells, microglia, and Schwann cells.

Stains Used

• Hematoxylin and Eosin (H+E).
• Immunohistochemistry: Used to study neuron-specific proteins like neurofilament protein.
• Heavy Metal Impregnation Technique: Gold and silver are used to study neuron morphology.
• Nissl Method: Stains RNA for rough endoplasmic reticulum (Nissl Substance); the axon lacks Nissl Substance.

Neuron Structure

• Excitatory cell with a cell body (perikaryon) containing the nucleus, Nissl bodies, neurofilaments, and microtubules.
• Has a single axon and one or more dendrites.

Glial Cells (Neuroglia in CNS)

• Non-neuronal support cells in the nervous tissue.
• Oligodendrocytes
• Astrocytes
• Microglia
• Ependymal cells

Astrocytes

• Exhibit a small cell body, a large oval nucleus, and a dark-stained nucleolus.
• Have long, thin, radiating processes located between neurons and blood vessels.
• Perivascular fibrous astrocytes surround capillaries.
• Long processes extend to and terminate on the capillary as perivascular end feet.

Microglia

• Smallest neuroglial cells with elongated nuclei and fine branched processes.
• Originate from bone marrow precursors.
• Found throughout the CNS (white and gray matter).
• Act like macrophages, migrating to injury sites.
• Proliferate, become phagocytic, and remove dead or foreign tissue.

Oligodendrocytes and Schwann Cells

• Oligodendrocytes are smaller than astrocytes with fewer cytoplasmic processes.
• Oligodendrocytes myelinate axons in the CNS.
• A single oligodendrocyte can surround and myelinate several axons.
• Schwann cells myelinate axons in the PNS, forming one myelin sheath around the internode of a single myelinated axon.

Ependymal Cells

• Typically cuboidal-columnar.
• Often have cilia, originating from neuroepithelial cells of the neural plate.
• Line the ventricles of the brain and spinal canal.
• Play a role in the production of cerebrospinal fluid (CSF).
• Tumors of ependymal cells are called ependymomas.

Cerebrum

• Layers of neurons are arranged closely to form the gray matter or cerebral cortex.
• Neurons grouped deeply form nuclei (thalamus, basal ganglia).
• Inner white matter conveys axons of neurons (bundles of nerve fibers).
• The cerebral cortex has six layers: Molecular (plexiform), External granular, External pyramidal, Internal granular, Internal pyramidal, Multiform (fusiform).

Cerebellum

• Surface layer of gray matter (cortex) is arranged into folds (folia).
• Central core of nerve fibers forms the white matter.
• Outer molecular layer (ML) consists of unmyelinated nerve fibers.
• Middle Purkinje cell layer (PL) is made of Purkinje cells with large cell bodies in a single layer.
• Inner granular cell layer (GL) consists of small neurons.

Spinal Cord

• Neurons form the central core (gray matter). Fibers form the outer section (white matter).
• Gray matter is butterfly-shaped. It contains neurons and extends to form ventral, dorsal & lateral horns.
• White matter has nerve fibers forming ascending & descending tracts surrounding the gray matter.

Connective Tissue Layers of Nerve Fibers

• Nerves have nerve fibers in nerve fascicles.
• Endoneurium: Delicate connective tissue around each nerve fiber with Schwann cells.
• Perineurium: Connective tissue covering each nerve fascicle.
• Epineurium: Connective tissue covering the entire nerve.

Eye Layers

• Fibro-elastic outer coat (corneo-scleral coat): Includes sclera and cornea.
• Sclera: White posterior part of dense collagen fibers.
• Cornea: Anterior transparent part.
• Middle vascular coat (Uveal tract): Consists of the iris, ciliary body, and choroid.
• Choroid lies between the sclera and retina.
• Iris regulates light via the pupil aperture.
• Inner photosensitive nervous coat:The Retina.

Cornea Layers

• Corneal epithelium: Stratified squamous non-keratinized epithelium (5-6 layers).
• Bowman’s membrane or anterior limiting membrane: Compactly packed collagen fibrils.
• Stroma: 200 layers of collagen fibrils with fibroblasts in an organized manner.
• Descemet’s membrane: Compactly packed collagen fibrils.
• Endothelium: Simple squamous epithelium of the anterior chamber.

Retina Parts

• Pigmented cuboidal epithelium: Contains melanin to absorb light after photoreceptor stimulation, preventing back reflection.
• Multilayered nervous part: Contains photoreceptors (rods and cones), bipolar neurons, and ganglionic multipolar neurons. Association neurons and glial cells (horizontal cells, amacrine cells, and Müller’s cell) are present. Has 10 recognizable layers.

Retina Layers (Outside to Inside)

• Pigmented layer: Layer of pigmented simple cuboidal epithelium.
• Photoreceptor layer: Outer and inner segments of rods and cones.
• External limiting membrane.
• Outer nuclear layer.
• Outer plexiform layer.
• Inner nuclear layer.
• Inner plexiform layer.
• Ganglionic cell layer.
• Nerve fiber layer.
• Internal limiting membrane.

Eyelid

• Dense fibro-elastic plate (tarsal plate) covered externally by thin skin and lined internally by the conjunctiva.
• Orbicularis oculi muscle can be seen between the skin and tarsal plate.
• Contains 12-30 tarsal (Meibomian) glands within the tarsal plate.
• Ciliary sweat glands are associated with the eyelashes.
• Conjunctiva covers the sclera's exposed part and the inner eyelid surface. It contains non-keratinized, stratified columnar or cuboidal epithelium, and goblet cells.

Inner Ear

• Bony labyrinth and membranous labyrinth filled with perilymph and endolymph.
• Parts of bony labyrinth:
  • Vestibule: Contains maculae in the utricle and saccule for balance.
  • Semicircular canals: Crista ampullaris for balance.
  • Cochlea: Organ of Corti for hearing.

Cochlea

• Divided into three chambers: scala vestibuli, scala media (cochlear duct), and scala tympani.
• Scala vestibuli and scala tympani are filled with perilymph.
• Scala media is filled with endolymph.
• The organ of Corti is in the scala media, surrounded by endolymph.

Organ of Corti

• Rests on the basilar membrane
• Has inner and outer hair cells, as well as supporting cells.
• Apical part of hair cell show multiple cilia that attach to the tectorial membrane.
• Basal part of hair cells relate directly to the dendritic process of afferent sensory neuron.
• The fibrous tectorial membrane rests on top of the stereo cilia of the hair cells.

Vestibule

• Contains the utricle and the saccule (otolith organs).
• Each has a macula with sensory and supporting cells.
• Within each macula, stereocilia are embedded in the gelatinous otolithic membrane.
• Head movements bend the cilia due to gravity and position information is sent to the central nervous system.

Semicircular Canals

• Arranged in different planes; each contains a tube filled with endolymph.
• Each canal has an expanded end called the ampulla, which contains the crista ampullaris with hair cells, stereocilia, and kinocilium.
• Stereocilia and kinocilium are embedded in the cupula held by supporting cells, and innervated by vestibular nerve fibers.
• Head movements make the cupula and cilia in the cilia move, which stimulates or inhibits nerve fibers