Nervous Tissue

Nervous Tissue Overview

• Nervous tissue is of ectodermal origin
• Anatomically, the nervous system divides into:
  • Central nervous system (CNS):
    • Brain (inside the skull)
    • Spinal cord (inside the vertebral column)
  • Peripheral nervous system (PNS):
    • Nerves
    • Nerve ganglia
    • Nerve endings
• Structurally, nervous tissue consists of two cell types:
  • Nerve cells (neurons):
    • The human nervous system has over 100 million neurons
    • Neurons are the structural and functional unit
    • Functions:
      • Reception and transmission of stimuli
      • Triggering of cellular activities
  • Glial cells (neuroglia):
    • Support, protection, and nutrition of neurons in the CNS

Neuron Structure

• Neurons have two main parts:
  • Cell body (perikaryon): Contains the nucleus and surrounding cytoplasm
  • Processes:
    • Dendrites
    • Axon
• Internal structures of the neuron:
  • Nucleus: Single, spherical, central, fine dispersed euchromatin, prominent nucleolus
  • Cytoplasm: Mildly basophilic, contains cell organelles and inclusions
  • Nissl's bodies (granules): Highly developed rough endoplasmic reticulum (rER) with free ribosomes and polyribosomes, important for protein synthesis; not found in the axon
  • Golgi complex: Stacks of membranes around the nucleus
  • Mitochondria: Present in all parts of the neuron
  • Neurofilaments (intermediate filaments): Present in all parts of the neuron, stained with silver
  • Microtubules: Present in all parts of the neuron; involved in support, shaping, and impulse transport.
  • Lysosomes
  • Cell inclusions (e.g., lipid, glycogen, lipofuscin, melanin): Stored food and pigments

Axons and Dendrites

• Axon:
  • Arises from the perikaryon in a short pyramidal region called the axon hillock
  • Lacking Nissl's granules
  • Gives terminal arborizations (branches) in the PNS
  • Axolemma is the plasma membrane of the axon and axoplasm is the interior cytoplasm
  • Contains mitochondria, microtubules, neurofilaments, and axoplasmic vesicles
• Dendrites:
  • Usually multiple and short
  • Branch like tree branches, similar cytoplasm to perikaryon
  • Lack Golgi complexes but have mitochondria, Nissl bodies, neurofilaments and microtubules
  • Carry impulses towards the cell body

Classification of Nerve Cells (Neurons)

• Based on the number of processes:

  • Unipolar (in invertebrates)
  • Pseudo-unipolar (spinal ganglion neurons)
  • Bipolar (retina and olfactory mucosa neurons)
  • Multipolar (most neurons)

• Based on function:

  • Motor neurons (efferent): Carry impulses from CNS to effectors
  • Sensory neurons (afferent): Receive sensory stimuli and carry them towards the CNS
  • Interneurons: Connect other neurons in the CNS

Nerve Fibers

• Consist of axons enveloped by special sheaths
• Cellular sheath (neurolemmal sheath):
  • Formed by Schwann cells in the PNS
  • Important for nerve fiber regeneration
• Myelin sheath:
  • Formed by Schwann cells in PNS
  • Interrupted tube surrounding axons
  • Helps in conduction and insulation of nerve impulses

Synapse

• Site of contact between two neurons or neuron and effector cell
• Types of synapses:
  • Axodendritic
  • Axosomatic
  • Axoaxonic

Nerve Ganglia

• Collections of nerve cell bodies outside the CNS
• Ovoid structures surrounded by connective tissue capsule
• Contain neuronal cell bodies and glial cells

Craniospinal Ganglia and Autonomic Ganglia

• Craniospinal ganglia (sensory):
  • Unipolar neurons on spinal and cranial nerve roots
  • No synapses within the ganglia
• Autonomic ganglia:
  • Multipolar neurons
  • Impulse transmission from CNS to ganglia in the sympathetic and parasympathetic systems (intramural ganglia)
• Differences between the two types: (summarized)
  • Structure: Capsule thickness and shape differences, number and distribution of neurons and nuclei, and presence or absence of satellite cells; nerve fiber myelin features, and synapses.

Neuroglia/Glial Cells

• Types: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells
• Astrocytes:
  • Star-shaped cells with multiple processes
  • End foot processes at capillaries
  • Mildly basophilic cytoplasm with gliosomes (lysosomes)
• Oligodendrocytes:
  • Fewer processes than astrocytes
  • Form myelin sheaths in the CNS
  • Central, oval shaped nucleus
• Microglia:
  • Mesodermal origin, monocytes of the blood
  • Small and elongated with branching processes
  • Basophilic cytoplasm with lysosomes
• Ependymal cells: Ectodermal in origin; form epithelial lining of brain ventricles