Schwann Cells and Neurons

Schwann Cells (Neurolemmocytes)

• Function and Structure

  • Wrap around all nerve fibers in the Peripheral Nervous System (PNS).

  • Functionally similar to oligodendrocytes found in the Central Nervous System (CNS).

  • Play a vital role in repairing damaged nerve fibers.

• Myelination Process

  • Schwann Cell Structure:

  • Composed of Schwann cell plasma membrane and cytoplasm.

  • Envelops an axon to form a myelin sheath.

  • Myelination of a Nerve Fiber (Axon):

  • The Schwann cell rotates around the axon, wrapping its plasma membrane around it in successive layers.

  • The Schwann cell cytoplasm is forced from between the membranes during the wrapping process.

  • The tight membrane wrappings surrounding the axon create the myelin sheath.

  • Nodes of Ranvier:

  • Gaps in the myelin sheath; important for the propagation of nerve impulses.

• Related Cell Types:

  • Satellite Cells:

  • Schwann cells and satellite cells surround neurons in the PNS.

Neurons

• Overview

  • Neurons are the primary cells of the nervous system, conducting electrical impulses.

  • There are approximately 100 billion neurons in the CNS.

  • Neurons can normally fire at a frequency of about 80 times per second.

  • During conditions like epilepsy, firing rates can escalate to 500 times per second.

• Characteristics:

  • Longevity: Neurons can live over 100 years.

  • Amitotic: Neurons do not divide. Neurogenesis has been documented in some other mammals but remains unproven in humans.

  • High metabolic rate: Neurons require a constant supply of glucose and oxygen to function properly.

• Structure of Neurons:

  • Neurons exhibit diverse shapes and sizes; are typically large and complex.

  • Components of a Neuron:

  • Receptive Region (Dendrites): Receives signals.

  • Cell Body (Soma): Contains the nucleus and biosynthetic machinery; functions as a receptive region.

  • Conducting Region (Axon): Transmits impulses away from the cell body.

  • Output Region (Nerve Terminal): Releases neurotransmitters (NT) to communicate with other cells.

  • Detailed Components:

  • Dendrites: Receptive regions of the neuron.

  • Cell Body: Contains the nucleus, nucleolus, and rough endoplasmic reticulum (chromatophilic substance).

  • Axon: Impulse-generating and conducting region, terminating in axon terminals (secretory region).

  • Axon Hillock: Initial segment of the axon; triggers action potentials.

  • Direction of Impulse: Defined path of electrical signals from dendrites to terminal branches.

• Plasticity:

  • Defined as the ability of neurons to change and adapt structurally and functionally in response to experience or injury.

Visual Representations

• Illustrative graphics may include:

  • Diagram of Schwann cells wrapping around an axon.

  • Structural overview of a neuron with labeled components (dendrites, cell body, axon, axon hillock).

  • Depiction of myelination and Nodes of Ranvier.