NERVOUS TISSUE

• Central Nervous System (CNS)


  A group of organs brought together for the purpose of making us react to our environment. Comprises the CNS and PNS
• CNS


  • Consists of the brain and the spinal cord
  • Tissue is characterized by cells closely packed with little extracellular substance, connected by frequent cell to cell junctions, with a special type of intercellular junction called the chemical synapse
• Gray matter


  Contains the perikaryon of mostly unmyelinated and some myelinated nerve, and neuroglial cells such as protoplasmic astrocytes, oligodendrocytes and microglia
• White matter


  Contains the myelinated nerve fibers and neuroglial cells like the fibrous astrocytes, oligodendrocytes and microglia
• Neuron


  • The structural and functional unit of the nervous system
  • Consists of perikaryon or somatic cell/cell body, dendrites, and axon
• Types of neurons according to function

  • Sensory/Afferent neurons
  • Motor/Efferent neurons
  • Interneurons
• Synapse


  A contact area of one axon with the dendrites or perikaryon of another, consisting of presynaptic membrane, synaptic cleft, and postsynaptic membrane
• Neuroglial cells


  • Supporting cells of the CNS that do not synapse with other cells
  • Include astrocytes, oligodendrocytes and microglia
• Astrocytes


  • Most important supporting element of CNS
  • Protoplasmic astrocytes found in gray matter, fibrous astrocytes found in white matter
• Oligodendrocytes


  • Smaller cells with round, dense nucleus and less numerous, shorter processes
  • In white matter, they are the precursor of myelin sheath
• Microglia


  • Smaller, denser and elongated cell bodies with thorny processes
  • Originate from the mesenchyme while other glial cells are derived from the ectoderm
• Ependymal cells


  • Supporting cells lining the central canal of spinal cord and ventricular cavities of the brain, with flattened to cuboidal shape and elongated nucleus
• Cerebrum


  • Consists of an outer cortex of gray matter and an inner medulla of white matter
  • Nerve cells in the cerebral cortex are arranged in six distinctive layers
• Cerebellum


  • Composed of an outer gray matter of cerebellar cortex and an inner cerebellar medulla
  • Cortical layers include molecular layer, Purkinje cell layer, and granular layer
• Spinal cord


  • Made up of an inner core of gray matter and an outer core of white matter
  • Gray matter includes anterior/ventral horns and posterior/dorsal horns joined by a central commissure
• Meninges of the CNS

  • Dura mater
  • Arachnoid mater
  • Pia mater
• Peripheral Nervous System (PNS)


  Composed of nerves and ganglia outside the CNS
• Peripheral nerves


  • Composed of nerve fibers that can be myelinated or unmyelinated
  • Myelinated fibers have a myelin sheath interrupted by nodes of Ranvier and clefts of Schmidt-Lantermann
• Nerve fibers


  Can either be axon or a dendrite
• Axis cylinder


  The conducting core of a nerve fiber
• Nerve fibers


  • Can be myelinated or unmyelinated
• Unmyelinated nerve fiber


  Devoid of myelin sheath, lined only by Schwann cells
• Myelination of a nerve fiber

  Schwann cells become spirally disposed along the membrane and compacted to form the lamellae of myelin sheath
• Myelin sheath


  • Interrupted at nodes of Ranvier
  • Shows cone shaped clefts called the cleft or incisures of Schmidt-Lantermann
• Neurolemma


  The sheath of Schwann cells between 2 nodes of Ranvier
• Sheath of Henle


  A fibrous connective tissue that covers a nerve either myelinated or unmyelinated
• Connective tissue investments in peripheral nerves

  • Endoneurium - loose connective tissue that encloses individual nerve fibers
  • Perineurium - dense connective tissue that covers bundles of nerve fibers
  • Epineurium - dense connective tissue that encloses several bundles of nerve fibers
• Ganglia


  Aggregations of nerve cell bodies outside the CNS, encapsulated with dense connective tissue
• Ganglia


  • Cranio-spinal ganglia and autonomic ganglia
• Ganglion cell


  Large oval or spheroidal shaped cell, with a large rounded nucleus and centrally located, lined by cellular fibrous connective tissue elements
• Satellite cells


  Flattened cells closely applied to the plasma membrane of the neuron
• Outer capsule cells


  Flattened fibroblast cells that make up the outer aspect of the satellite cell investment
• Autonomic ganglia


  • Contain synapses, where first neurons of the 2-neuron efferent system form a synapse with the second neuron
  • Neurons are multipolar type
  • Lipofuchsin granules are more frequent
• Sensory organs


  Receive information from both the external and internal environments and transmit them to the CNS
• Common or General sensory nerve endings

  • Touch - Meissner's corpuscles, Merkel's tactile disc, Hair follicle nerve endings, Golgi Mazzoni corpuscle
  • Pressure - Pacinian corpuscle
  • Pain free or naked nerve endings
  • Temperature - Krause end bulb, Ruffini's corpuscle
  • Proprioception - muscular spindle, tendon spindle
• Special sensory nerve endings

  • Taste - taste buds
  • Smell - bipolar cells of olfactory epithelium
  • Vision - rods and cones of retina
  • Hearing - hair cells or organ of Corti
  • Vestibular sense - Cristae ampullaris and maculae
• Merkel's tactile discs


  Free nerve endings that form associations with specialized tactile epidermal cells
• Golgi Mazzoni corpuscle


  Irregularly oval in shape, with thick capsule, and a big lumen
• Hair follicle nerve endings

  Break up into branches, some form spiral in the dermal root sheath while others proceed upward in the outer root sheath
• Pacinian corpuscle


  Large encapsulated nerve ending for pressure, resembles a sliced onion in transverse section