Meninges Overview and Functions

Meninges

• The skull and vertebral column protect the central nervous system (CNS), but membranes of connective tissue called meninges lie between the bone and nervous tissue.
• Three layers are present: dura, arachnoid, and pia mater

Dura Mater

• The thick, external dura mater is tough and consists of dense, fibroelastic connective tissue.
• It's continuous with the periosteum of the skull.

Arachnoid

• The arachnoid has two components:
  • A sheet of connective tissue that contacts the dura mater.
  • A system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the pia mater.
• The subarachnoid space, which is filled with cerebrospinal fluid (CSF), surrounds the trabeculae.
• This space cushions and protects the CNS from trauma.
• The subarachnoid space is linked to the brain's ventricles, where CSF is produced.
• The arachnoid and pia mater are frequently considered a single membrane (the pia-arachnoid).

Pia Mater

• The innermost pia mater is composed of flattened cells derived from mesenchyme, and is closely attached to CNS tissue.
• It does not make direct contact with nerve cells or fibers.
• It is separated from neural elements by a thin layer of astrocytic processes (glia limitans).
• The glia limitans strongly adheres to the pia mater.

Blood-Brain Barrier (BBB)

• The BBB is a functional barrier that tightly controls the passage of substances from blood into CNS tissue, in contrast to most tissues.
• It is primarily composed of capillary endothelium, with tight junctions and little transcytosis activity.
• Astrocytic feet and the basal lamina of the capillaries also contribute to the BBB.
• The BBB protects neurons and glia from bacterial toxins, infectious agents, and exogenous substances.
• The BBB is absent in the choroid plexus, posterior pituitary, and hypothalamus.

Choroid Plexus

• The choroid plexus is highly specialized tissue with folds that protrude into brain ventricles.
• It is located in the roofs of the third and fourth ventricles and parts of the lateral ventricles.
• The ependymal cells that cover the choroid plexus produce cerebrospinal fluid (CSF).
• CSF is clear, colorless, and contains ions including Na⁺, K⁺, Cl^-, but with low protein content and sparse lymphocytes.
• CSF fills the ventricles, the central canal of the spinal cord, and subarachnoid spaces, helping to absorb shocks.

Peripheral Nervous System (PNS)

• The PNS comprises nerves, ganglia, and nerve endings.
• Nerves are bundles of nerve fibers (axons) surrounded by Schwann cells and connective tissue layers.

Nerve Fibers

• Nerve fibers are analogous to tracts in the CNS, with axons enveloped in glial cell sheaths that aid axonal function.

Myelinated Fibers

• Large-diameter axons in the PNS are enveloped by Schwann cells to develop myelin sheaths.

• The myelin sheath is composed mainly of lipid bilayers and membrane proteins.

• Schwann cell membranes wrap around the axon repeatedly, forming the myelin sheath.

• The myelin sheath acts as insulation and maintains ionic balance, essential for action potentials.

• Nodes of Ranvier, gaps in the myelin sheath, contain a high concentration of voltage-gated ion channels. The propagation of nerve impulses is typically saltatory (jumping).

Unmyelinated Fibers

• In the PNS, smaller-diameter axons are ensheathed within simple folds of Schwann cells, without forming a myelin sheath.
• Nodes of Ranvier are absent.

Nerve Organization

• Nerve fibers in the PNS are grouped into bundles (nerves).
• Nerves, except those containing solely unmyelinated fibers, are whitish due to myelin and collagen content.
• Connective tissue layers surround axons and Schwann cells:
  • Endoneurium: a thin layer surrounding individual axons.
  • Perineurium: a layer of connective tissue that bundles groups of axons into fascicles.
  • Epineurium: a layer of dense connective tissue that surrounds the entire nerve.

Explore the three layers of meninges: dura mater, arachnoid, and pia mater. This quiz will help you understand their structures, functions, and how they protect the central nervous system. Test your knowledge on the roles of each layer and the importance of cerebrospinal fluid.