Lecture 6:Nervous System Anatomy and Physiology

Definition and Classification of Nervous System

• Nervous tissue receives, transmits, and integrates information from outside and inside the body to control the activities of the body
• Classified into Central Nervous System (CNS) and Peripheral Nervous System (PNS)

Divisions of Nervous System

• Central Nervous System (CNS):
  • Brain and Spinal cord
  • Protected and enclosed within skull and vertebral canal respectively
  • Covered by meninges
  • Cerebro-spinal fluid (CSF) baths surface of brain and spinal cord
  • Composed of neurons and supporting cells (glial cells)
• Peripheral Nervous System (PNS):
  • Paired spinal and cranial nerves
  • Ganglia

Functions of Nervous System

• Somatic nervous system: supplies the body wall and limbs, involved with voluntary functions
• Autonomic nervous system: supplies the viscera and glands, exerts control over many involuntary organs

Cells of Nervous Tissue

• Neurons: excitable cells that transmit electrical signals
• Glial cells/Neuroglia: supporting cells that include:
  • Astrocytes
  • Oligodendrocytes
  • Ependymal cells
  • Microglia
  • Schwann cells

Structure of a Neuron

• Consists of:
  • Cell body containing the nucleus surrounded by cytoplasm (perikaryon)
  • Nissl bodies (H+E)
  • Neurofilaments and microtubules
  • Processes of two types:
    • Dendrites (receive signals)
    • Axon (conducts action potentials)

Types of Neurons

• Multipolar neurons: multiple dendrites and a single axon (e.g. motor neurons, interneurons)
• Bipolar neurons: single dendrite and a single axon (e.g. sensory neurons of special sensory organs)
• Pseudo-unipolar neurons: single axon and dendrite arise from a common stem of the cell (e.g. primary spinal sensory neuron)

Glial Cells

• Functions:
  • Supply nutrients and oxygen to neurons
  • Insulate one neuron from another
  • Surround neurons and hold them in place
  • Destroy and remove the debris of dead neurons
• Types:
  • Oligodendrocytes
  • Astrocytes
  • Microglia
  • Ependymal cells

Stains Used to Study Nervous Tissue

• Hematoxylin and Eosin (H+E)
• Heavy Metal Impregnation Technique: gold and silver are used to study neuron morphology
• Immunohistochemistry: used to study neuron-specific proteins (e.g. neurofilament protein)
• Nissl Method: stains RNA, identifying the rough endoplasmic reticulum (Nissl Substance)

Organization of Neurons in CNS

• Cerebrum:
  • Layers of neurons arranged close to the surface of the brain to form the gray matter or cerebral cortex
  • Neurons grouped together deep in the brain form nuclei (thalamus, basal ganglia)
  • Inner white matter conveys axons of the neurons (bundles of nerve fibers)
• Cerebellum:
  • Surface layer, grey matter, cortex arranged in folds called the folia
  • Central core of nerve fibers, white matter
  • Three layers of cerebellar cortex: molecular layer, Purkinje cell layer, granular cell layer
• Spinal cord:
  • Neurons form central core (gray matter) of the spinal cord with fibers outside (white matter)
  • Gray matter, butterfly-shaped central mass of neurons that extends to form ventral, dorsal, and lateral horns
  • White matter: nerve fibers forming ascending and descending tracts surrounding the gray matter

Peripheral Nervous System

• Consists of:
  • 31 pairs of spinal nerves
  • 12 pairs of cranial nerves
  • Ganglia: primary sensory spinal ganglia, ganglia associated with cranial nerves, and autonomic ganglia

Connective Tissue Layers of Nerve Fibers

• Each nerve fiber with investing Schwann cell is surrounded by delicate layer of connective tissue, the endoneurium
• Each nerve fascicle in a nerve is wrapped by a connective tissue covering, the perineurium
• Each nerve is externally covered by connective tissue, the epineurium

Myelinated and Non-Myelinated Nerve Fibers

• Non-myelinated nerve fibers: small axons (ANS, pain fibers) are only enveloped by cytoplasm of Schwann cells
• Myelinated nerve fibers: large axons are wrapped by concentric layers of Schwann cell plasma membrane called myelin sheath