NERVOUS TISSUE

• Nervous tissue

  • made up of erosely packed cells that are separated by very little amount of intercellular substance
  • high cellularity
  • arises from embryonic ectoderm
  • comprise the nervous system
  • in CNS is devoid of connective tissue, except for those associated with blood vessels
  • in PNS, there is some amount of intercellular material, mainly connective tissue, albeit minimal
• two divisions of nervous system:

  • central nervous system - brain and the spinal cord
  • peripheral nervous system - relates to all other nervous tissue in the body
• cells of nervous tissue:

  • neurons (nerve cells)
  • supporting cells (neuroglial or glial cells)
• Neurons (Nerve cells)

  • functional units of nervous tissue
  • exhibit irritability (ability to react to stimulus) and conductivity (ability to transmit stimulus)
  • arranged in close apposition to, and communicate extensively with, each other
  • between 14 billion to 1 trillion
  • large cells (up to 150 um in diameter)
  • small neurons (only 5 um in diamter)
  • incapable of cell division
  • if it dies, it is not replaced
  • has cytoplasmic organelles, a cytoskeleton, and inclusions
• neurons in sorts of shapes:

  • Stellate neurons
  • Pyramidal neurons
• Stellate neurons

  • ventral gray matter of the spinal cord and the motor nuclei of the brain stem
• Pyramidal neurons

  • present in the cerebral cortex
  • and flask-shaped neurons called Purkinje cells
• Purkinje cells

  • give off a dendrite which arborizes like a tree are seen in the middle layer of the cerebellar cortex
• parts of neuron that is common

  • perikaryon or soma
  • processes
• processes of neurons

  • draw out from nerve cell body
• two kinds of processes of neurons: can regenerate when damaged

  • axon
  • dendrite - largely determine the shape of neuron
• Perikaryon

  • cell body of neuron
  • has nucleus that is surrounded by basophilic cytoplasm (neuroplasm) and enclosed by a cell membrane (neurolemma)
• Neurolemma

  • is structurally the same as the cell membrane of other cell types
• Nucleus (neuron)

  • usually one, but some have more
  • large, spherical or ovoid, and centrally located
  • nuclear envelope - structurally similar to that of other cells
  • chromatin - finely dispersed
  • LM preparations - nucleus appear pale and nucleolus very prominent
• Cytoplasmic granules (neuron)

  • rER - highly developed, seen as deeply basophilic, granular masses that are referred to as Nissl bodies (chromophilic substances; tigroid bodies)
• Nissl bodies (neuron)

  • abundant throughout the perikaryon and are also found in dendrites
  • absent in the axon and axon hillock
  • usually larger and more numerous in the bigger neurons
• axon hillock

  • area of the perikaryon where the axon originates
• Golgi complex (neuron)

  • present but is confined to the perikaryon
• what cytoplasmic organelles are involved in the synthesis of proteins?

  • rERs (Nissl bodies)
  • Golgi complex
• Parts of a Myelinated Neuron

  A) dendrites

  B) Nissl granules

  C) axon

  D) axon hillock

  E) myelin sheath

  F) nodes of ranvier

  G) axon terminals

  
• Mitochondria (neuron)

  • abundant
  • generally smaller
  • particularly profuse in axon terminals (i.e., bulb-like swellings along or at the ends of axons)
• Lysosomes (neuron)

  • abundant
  • long-lived
  • bandy in recycling proteins (dealing with abnormal and foreign proteins)
• peroxisomes (neuron)

  • present in significant numbers
  • found in the main body
  • smaller (about 0.25 to 0.5 um in diamter)
  • help in preventing the degeneration of the neuron by not allowing the accumulation of strong oxidizing agents and by playing a role in detoxifying noxious substance.
• centrosome (MTOC) (neuron)

  • present even neurons are incapable of cell division
  • located in the peripheral area of the perikaryon
  • does not contain centrioles
  • source of the microtubules that the cells need
• Inclusions of neurons:

  • Fat droplets
  • lipochrome or lipofuschin granules - increases with age
  • pigment granules - sometimes
• pigment granules:

  • melanin
  • iron
• melanin granules

  • present in
  • substantia nigra of the midbrain
  • locus coeruleus near the fourth ventricle,
  • spinal ganglia
  • sympathetic ganglia
• iron granules

  • present in
  • globus pallidus
  • increases with age
• three types of fibrillar elements (neurofibrils) in cytoskeleton of neuron:

  • microfilaments
  • intermediate filaments
  • microtubules
• neurofibrils

  • present in all neurons
  • well-developed in large one
  • extend into the axon and the dendrites
• Microfilaments (neuron)

  • fines of the fibrillar elements in neuron
  • average 5.0 nm in diameter
  • made up of the fibrillar type of actin (F-actin) that consists of two strands of helically-arranged, polymerized G-actin filaments
• Neuron

  A) dendites

  B) nucleus

  C) axon

  D) axon hillock

  
• intermediate filament (neuron)

  • also called as neurofilaments
  • 10 nm in diameter
  • present in the cell body and the cell processes
  • provide internal support for the cell and fix the diameter of dendrites and axons
• microtubules (neuron)

  • also called as neurotubules
  • provide internal support for the neurons
  • strengthen synapses
  • play a role in the intracellular transport of organelles and secretory vesicle
• more than 90% of the cytoplasm is in the processes
• axon

  • type of neuron processes that conducts impulses away from the cell body
• dendrite

  • type of neuron processes which carries impulses towards the cell body
• 1:1 - axon and neuron
• classifications of neurons (morphological)

  • Unipolar
  • Pseudounipolar
  • Bipolar
  • Multipolar
• Unipolar

  • only one process, axon is present
  • exists in early embryonic life but is rarely present in adults