Nervous Tissue.

Lecture 4: Nervous Tissue

Learning Objectives

  • Understand the main function of nervous tissue.
  • Identify the two main components of nervous tissue.
  • Name the various cells/subunits for each component.
  • Explain how nerve impulses are transmitted.

Nervous Tissue Function

  • Information processing
    • Sensory neurons: Sensory input (external or internal cues).
    • Interneurons: Integration of signals.
    • Motor neurons: Motor output.
  • Coordination and control
    • Mainly via the nervous system (nerve impulses) and endocrine system (hormones).

Two Main Components of Nervous Tissue

  • Neurons
    • Generate electrical signals (conducted nerve impulses / action potentials).
  • Glia (Glia Cells/Neuroglia)
    • Non-neuronal cells that mainly support neuronal function.

Neuron Structure & Glia Types

The following glial cell types are found in the CNS and PNS:

  • Astrocytes (A)
    • CNS
    • Functions:
      • Information transfer.
      • Regulate extracellular ion concentrations.
      • Promote blood flow to neurons.
      • Help form the blood-brain barrier.
      • Act as stem cells to replenish certain neurons.
  • Oligodendrocytes (B)
    • CNS
    • Function: Myelinate axons in the CNS; myelination increases the conduction speed of action potentials.
    • Myelin: Insulates axons.
  • Microglia (C)
    • CNS
    • Function: Immune cells in the CNS that protect against pathogens.
  • Schwann Cells (D)
    • PNS
    • Function: Myelinate axons in the PNS.
  • Ependymal Cells (E)
    • CNS
    • Function: Line the ventricles of the brain and promote circulation of the cerebrospinal fluid.

Neuron Structure & Function

  • Cell Body (Soma): Contains the nucleus and other cellular organelles.
  • Dendrites:
    • Receive messages from other neurons at specialized junctions called synapses.
  • Axon Hillock:
    • Serves as the junction between the cell body and axon.
    • Integrates signals from multiple synapses.
  • Axon:
    • Propagates the integrated signal to axon terminals.
  • Axon Terminals (Synaptic Terminals):
    • Synapse on other neurons, muscles, or target organs.
    • Chemicals (neurotransmitters) are released to communicate signals to cells of the target tissue.
  • Nodes of Ranvier:
    • Gaps between the myelin insulation of Schwann cells.
    • Sites where the signal is recharged.

Nerve Impulse: Action Potential & Neural Transmission

  • Ion Channel: A protein complex penetrates a cell membrane and allows specific ions to pass through the membrane.
  • Membrane Potential: The difference in electrical potential across the membrane.
  • Resting Potential: The membrane potential of an inactive cell.
    • Negative potential: More sodium ions outside the cell than potassium ions inside the cell.
    • Potassium ions diffuse out at a faster rate than sodium ions diffuse into the cell due to more potassium leakage channels.
    • Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential.
  • Action Potential:
    • Forms when a stimulus causes the cell membrane to depolarize, causing all sodium ion channels to open.
    • Potassium ion channels open, and sodium ion channels close: the cell membrane becomes hyperpolarized as potassium ions leave the cell; the cell cannot fire during this refractory period.
    • The action potential travels down the axon as the membrane of the axon depolarizes and repolarizes.
    • Myelin insulates the axon to prevent leakage of current as it travels down the axon.
    • Nodes of Ranvier contain sodium and potassium ion channels, allowing the action potential to travel quickly down the axon by jumping from one node to the next.

Sporadic Amyotrophic Lateral Sclerosis (sALS)

  • Progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord.
  • "Sporadic" - cause of disease is unknown.
  • Motor neurons die, leading to a loss of the brain's ability to initiate and control muscle movement.
  • Microscopic changes include:
    • Neuronal and axon loss.
    • Loss of myelinated axons in lateral and anterior columns of the spinal cord.
    • Decrease in size of the anterior horn of the spinal cord.

Classic ALS Histopathology

  • A: Loss of motor neurons in the anterior horn of the spinal cord
  • I: Bunina bodies in the cytoplasm of motor neurons
  • C: Loss of motor neurons in the motor cortex
  • E: Shrinkage and contraction of the motor cortex
  • G: Vacuolization and spongiosis in the motor cortex
  • K: Bunina bodies are positive for cystatin c