Glial Cells in the CNS

Glial Cells

• Astrocytes:
  • Extend fibrous processes with expanded perivascular feet that cover capillary endothelial cells and modulate blood flow
  • Help move nutrients, wastes, and other metabolites between neurons and capillaries
  • Form a barrier layer of expanded protoplasmic processes called the glial limiting membrane, which lines the meninges at the external CNS surface
  • Fill tissue defects after CNS injury by proliferation to form an astrocytic scar
• Ependymal cells:
  • Line the fluid-filled ventricles of the brain and the central canal of the spinal cord
  • Have cilia and long microvilli that facilitate the movement of cerebrospinal fluid and absorption
  • Joined apically by apical junctional complexes, but lack a basal lamina
  • Have elongated basal ends that extend branching processes into the adjacent neuropil
• Microglia:
  • Origin: From circulating blood monocytes, belonging to the same family as macrophages and other antigen-presenting cells
  • Small cell bodies with many long, branched processes
  • Relatively static, but their processes continuously probe and interact with neuropil, synapses, and other cells in an area up to 10-fold that of the cell body
  • Function: Remove apoptotic bodies and debris from damaged or remodeled synapses by phagocytosis, and constitute the major mechanism of immune defense in the CNS

Glial Cells of the PNS

• Schwann cells (neurolemmocytes):
  • Origin: From precursors in the neural crest
  • Found only in the PNS, considered as the counterpart to oligodendrocytes of the CNS
  • Have trophic interactions with axons and form their myelin sheathes
  • Unlike oligodendrocytes, each Schwann cell forms myelin around a portion of only one axon
• Satellite cells of ganglia:
  • Origin: From the embryonic neural crest
  • Form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS
  • Exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments

Central Nervous System (CNS)

• Composed of three major structures: cerebrum, cerebellum, and spinal cord
• CNS tissue is covered with a connective tissue layer called meninges, which is relatively soft and easily damaged by injuries
• Arranged into two areas: white matter and gray matter
  • White matter: Main components are myelinated axons, myelin-producing oligodendrocytes, astrocytes, and microglia
  • Gray matter: Contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur

Nerve Organization

• Peripheral nerves have a dense, irregular fibrous coat called the epineurium, which extends deeply to fill the space between fascicles
• Nerves can be afferent/sensory or efferent/motor, and can be motor, sensory, or mixed
• Ganglia:
  • Ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells
  • Serve as relay stations to transmit nerve impulses
  • Divided into sensory or autonomic ganglia based on the direction of the nerve impulse
  • Associated with both cranial nerves (cranial ganglia) and the dorsal roots of the spinal nerves (spinal ganglia)

Autonomic Ganglia

• Autonomic nerves control the activity of smooth muscle, secretion of some glands, heart rate, and involuntary activities that maintain a constant internal environment (homeostasis)
• Autonomic ganglia:
  • Contain multipolar neurons
  • Located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia
  • Have two neuron circuits: preganglionic and postganglionic fibers
  • Autonomic nervous system has two parts: sympathetic and parasympathetic nervous systems

Neural Plasticity and Regeneration

• The nervous system exhibits neuronal differentiation and formation of new synapses even in adults
• Neural stem cells are present in the adult CNS, located in part among the cells of the ependymal
• Astrocytes can proliferate at injured sites, but can interfere with successful axonal regeneration
• In peripheral nerves, injured axons have a much greater potential for regeneration and return of function
• The onset of regeneration is signaled by changes in the perikaryon, including chromatolysis
• The new Schwann cells align to serve as guides for the regrowing axons and produce polypeptide factors that promote axonal outgrowth