Nervous Tissue Notes

Nervous Tissue

1. Location

  • Central Nervous System (CNS)
    • Comprises the brain and spinal cord.
  • Peripheral Nervous System (PNS)
    • Extends throughout the body as nerves that branch off from the brain and spinal cord, along with associated ganglia.

2. General Functions

  • Detect Changes
    • Senses changes in the internal and external environment and transmits this information to the CNS.
  • CNS Role
    • Interprets the received information, facilitates integration, memory, problem solving, and other cognitive functions.
  • Response Coordination
    • Sends messages to muscles and glands (effector organs) to elicit and coordinate appropriate responses to stimuli.

3. Cell Types

  • Neurons
    • Responsible for generating nerve impulses to transmit and receive messages.
  • Neuroglial Cells
    • Various support cells that assist neurons in functions and health.

4. General Characteristics of Neurons

  • Impulse Conducting Cells
    • Capable of generating graded and action potentials.
  • Cell Body (Soma)
    • Irregular shape filled with perikaryon (granular cytoplasm).
    • Contains Nissl Bodies:
    • Arrangement of rough endoplasmic reticulum and ribosomes surrounding the nucleus.
    • Contains Neurofibrils:
    • Bundles of intermediate filaments (neurofilaments) that constitute the cytoskeleton.
    • Provide internal support and intracellular transport of organelles and molecules.
    • Lacks centrioles, hence these cells are non-mitotic; all neurons present by 20 weeks in utero.
  • Neuron Processes
    • Axon (Nerve Fiber):
    • Forms tracts in the CNS and nerves in the PNS.
    • Oneaxonal per cell; can extend feet in length, connecting to the cell body at the axon hillock.
    • Action potential is generated at the axon hillock and conducted away from the cell body.
    • Typically enveloped by a Myelin Sheath:
      • In PNS, myelin sheaths are formed by Schwann Cells.
      • Schwann cells repeatedly wrap around axons, pushing their nuclei and cytoplasm outward.
      • Forms layers of phospholipid bilayer, contributing to myelin structure.
      • The living portion outside the myelin is called the Neurilemma.
      • Functions:
        • Facilitates nerve fiber regeneration by guiding new axon growth if damaged.
        • Acts as an insulator preventing ionic flow, enhancing impulse transmission speed.
      • Nodes of Ranvier:
      • Gaps in myelin between adjacent Schwann cells where ionic movement occurs, allowing for 'leaping' impulse transmission (saltatory conduction).
      • Oligodendrocytes:
      • Form myelin in the CNS around multiple axons (lack neurilemma).
      • These cells do not guide new growth after injury.
      • Myelin aids in brain and spinal cord growth from 5 months in utero to 20 years; 90% of brain mass reached by 6 years.

5. Neuron Process Classification

  • Classification based on myelin presence and diameter:
    • A Fibers:
    • Largest diameter and most myelin, leading to fastest impulse transmission (up to 250 mph).
    • Includes sensory neurons relaying messages from joints/senses (sharp pain, temperature) and motor neurons to skeletal muscle.
    • B Fibers:
    • Intermediate diameter and myelin, with slower speeds (up to 30 mph).
    • Convey sensory and motor impulses to internal organs.
    • C Fibers:
    • Smallest diameter, unmyelinated, therefore the slowest speed (1-5 mph).
    • Transport sensory and motor impulses to internal organs and sensory messages from the skin (throbbing pain, touch, itch, pressure).

6. Termination of the Axon

  • Consists of several Terminal Branches, each ending in a Terminal Knob (Axon Terminal):
    • Each knob houses Synaptic Vesicles with neurotransmitters that communicate with adjacent cells.
    • Terminal knobs release neurotransmitters across the Synaptic Cleft into the next neuron, inducing exocytosis of vesicles.

7. Dendrites

  • Varying from one to many per cell:
    • One in bipolar neurons; many in multipolar neurons.
  • Transmit graded potentials towards the cell body.
  • Do not possess a myelin sheath.
  • Contain receptors for neurotransmitters.

8. Classification of Neurons

  • By function:
    1. Motor (Efferent) Neurons
      • Carry impulses away from CNS to muscles or glands.
    2. Sensory (Afferent) Neurons
      • Carry impulses towards CNS.
    3. Interneurons (Association or Internuncial Neurons)
      • Facilitate messages between sensory and motor neurons; connect different CNS regions (99% of neurons).
  • By number of processes:
    1. Pseudounipolar (Unipolar)
      • One process (axon connecting to cell body).
      • Begins with receptive endings embedded in skin or organ walls to detect stimuli.
      • Sensory neurons carrying general sense impulses (hot, cold, touch, etc.).
    2. Bipolar Neurons
      • Two processes: single dendrite and an axon.
      • Responsible for special senses like sight, hearing, and smell.
    3. Multipolar Neurons
      • Multiple dendrites and one axon.
      • Comprise all motor neurons and interneurons in CNS (99% of all neurons).

9. Neuroglia (Glial Cells)

  • Supportive cells for neurons, outnumbering them by 20 to 1.
  • CNS Glial Cell Types:
    1. Microglia:
      • Act as macrophages, phagocytizing microbes and clearing debris.
    2. Astrocytes:
      • Most abundant; connect neurons to blood vessels, influencing blood-brain barrier.
      • Form scar tissue post-injury.
      • Control potassium levels and clean neurotransmitters.
    3. Oligodendrocytes:
      • Form myelin in CNS for A and B fiber neurons.
      • Multiple Sclerosis: immune system attacks these, replaced by astrocytes, forming scar tissue.
    4. Ependymal Cells:
      • Line brain and spinal cord cavities; assist in CSF production and movement.
  • PNS Glial Cell Types:
    1. Schwann Cells: