Neurobiology Quiz: Glial Cells and Myelin Sheath

PNS

• 5-6 capsular or satellite cells are found in craniospinal and autonomic ganglia, forming a one-cell thick covering over the cell body of neurons with spherical nuclei.
• Schwann cells accompany the PNS fibers, produce myelin in myelinated fibers, and still ensheath unmyelinated fibers.

Myelin Sheath

• Myelin sheath is formed by two types of glia: Oligodendrocytes (in CNS) and Schwann cells (in PNS).
• Myelinated fibers can conduct electrical signals much faster than unmyelinated fibers (about 150 times faster).
• Gaps between myelin sheath cells are the Nodes of Ranvier.

Nerve Fiber

• Consists of an axon enveloped by a special sheath that protects the axon and electrically isolates it, increasing the rate of action potential transmission.
• In PNS, unmyelinated fibers are covered by a sheath of Schwann cell (neurilemma).

Formation of Myelin Sheath (PNS)

• Cytoplasm of Schwann cell wraps around the axon several times, forming a lamellate membrane that unites to form a lipo-protein complex called MYELIN.
• The enfolding are called mesaxon (internal and external).

Schwann Cell

• Each Schwann cell myelinates a single internode.
• Internode length can be up to 1.5 mm in the largest nerve fibers.
• Myelination of axons is not continuous along its length, leaving gaps called Node of Ranvier, where no myelin is present, but still covered by folds from Schwann cell.

Myelination in CNS

• There are no Schwann cells; the myelin sheath is formed by the processes of oligodendrocytes.
• The differences from Schwann cells are:
  1. Different branches of one cell can envelope segments of several axons in CNS.
  2. The cell body is not directly opposite to the myelin sheath.
  3. Neurolemma is not found.

Nodes of Ranvier

• Nodes of Ranvier are gaps between myelin sheath cells.
• They are covered by folds from Schwann cell.

Ganglia

• Ganglia are collections of neurons outside the CNS (in PNS) or inside the white matter of CNS (called Nuclei).
• Two types of ganglia: Sensory and Motor.
• Ganglia are ovoid structures containing neuronal cell bodies, glial cells, and supported by connective tissue.
• They act as a station to transmit nerve impulses.

Sensory Ganglia

• Receive afferent impulses that go to the CNS.
• Two types: spinal ganglia (associated with spinal nerves) and cranial ganglia (associated with cranial nerves).
• Type of neuron is pseudounipolar, with a centrally located nucleus surrounded by several capsular glial cells (satellite cells).
• The center of the ganglia is occupied by nerve fibers, while the neurons are placed at the periphery.
• Surrounded by a capsule, they have a sensory function.

Autonomic Ganglia

• Sympathetic ganglia are located just ventral and lateral to the spinal cord.
• They are called paravertebral ganglia.

Neuroglial Cells

• Functions: support, protection, neural nutrition, and viability, removing debris (phagocytosis), formation of myelin, defense, and participating in the Blood-Brain Barrier.
• Six types of neuroglial cells are present: four in CNS (Astrocyte, Oligodendrocyte, Microglia cell, and Ependymal cell) and two in PNS (Schwann cell and Satellite cell).

CNS Neuroglial Cells

• Astrocytes: the largest, with a large spherical and granulated nucleus, star-shaped, and multiple processes.

  • Functions: support, maintaining the integrity of the Blood-Brain Barrier, regulating blood flow in the brain, and maintaining synapses.
  • Two types: protoplasmic (found mainly in gray matter) and fibrous (found mainly in white matter).

• Oligodendrocytes: much smaller, less processes than astrocyte, and produce myelin in CNS.

• Microglia: small, elongated cells with a dense oval or elongated nucleus, short irregular processes, and less numerous; involved in inflammation and repair in the adult CNS.

• Ependymal cell: epithelium-like cuboidal-columnar, lining the cavities of the brain and the central canal of the spinal cord, with apical microvili and cilia; controlling the production and flow of cerebrospinal fluid (CSF), brain metabolism, and waste clearance.