Pia Mater: Ligamentous Structures, Functions, and Clinical Relevance

What is the ligament of pia mater?

While there isn't a single, universally recognized "ligament of pia mater," the pia mater, the innermost of the three meninges, gives rise to several crucial ligamentous structures that anchor and stabilize the central nervous system, most notably the denticulate ligaments of the spinal cord.

Understanding the Pia Mater

The central nervous system (CNS), comprising the brain and spinal cord, is encased and protected by three layers of connective tissue known as the meninges: the dura mater (outermost), arachnoid mater (middle), and pia mater (innermost). The pia mater is a delicate, highly vascularized membrane that adheres directly to the surface of the brain and spinal cord, following every contour, sulcus, and gyrus. Its close association with neural tissue allows it to supply blood vessels directly to the CNS parenchyma and contribute to the formation of other vital structures.

The Primary Ligamentous Structures of the Pia Mater

While the pia mater itself is a membrane, it forms extensions that serve ligamentous functions, providing stability and preventing excessive movement of the delicate neural tissue within the vertebral canal.

Denticulate Ligaments

The denticulate ligaments are the most prominent and well-known ligamentous structures derived from the pia mater.

• Anatomy and Location: These are 21 pairs of triangular, tooth-like (denticulate) extensions of the pia mater, found along the lateral aspects of the spinal cord. They arise midway between the dorsal and ventral nerve roots. Each ligament extends laterally from the pia mater, piercing the arachnoid mater, and firmly attaches to the inner surface of the dura mater.
• Function: Their primary role is to anchor and stabilize the spinal cord within the vertebral canal. By attaching to the dura mater, they prevent the spinal cord from shifting excessively from side to side, protecting it from injury during movement, flexion, and extension of the trunk. This stabilization is crucial for maintaining the integrity of neural pathways and preventing mechanical stress on the spinal cord and exiting nerve roots.

Filum Terminale

Another significant pial derivative with a ligamentous function is the filum terminale.

• Anatomy and Location: This is a slender, fibrous strand of pia mater that extends inferiorly from the conus medullaris (the tapered end of the spinal cord, typically around the L1-L2 vertebral level). It consists of two parts:
  • Filum Terminale Internum (Pial Part): This upper part, about 15 cm long, is composed of pial tissue and descends within the dural sac, surrounded by cerebrospinal fluid and the cauda equina nerve roots.
  • Filum Terminale Externum (Dural Part/Coccygeal Ligament): At the S2 vertebral level, the filum terminale internum pierces the dura mater and acquires a layer of dura, becoming the filum terminale externum. This part continues inferiorly to attach to the posterior surface of the coccyx.
• Function: The filum terminale serves to anchor the spinal cord inferiorly to the coccyx, preventing superior displacement of the spinal cord during movement and maintaining its longitudinal stability within the vertebral column.

Functional Significance in the Central Nervous System

The ligamentous structures derived from the pia mater, particularly the denticulate ligaments and the filum terminale, are critical for the mechanical stability and protection of the spinal cord.

• Mechanical Stability: They ensure that the spinal cord remains centrally located and relatively fixed within the cerebrospinal fluid-filled subarachnoid space. This prevents undue tension or compression on the neural tissue during changes in body position or impact.
• Protection: By limiting excessive movement, these ligaments help protect the delicate spinal cord from direct trauma against the surrounding bony vertebral column. This is fundamental to preserving neurological function.

Clinical Relevance

Understanding these pial ligaments is important in various clinical contexts:

• Spinal Cord Integrity: Their role in anchoring and stabilizing the spinal cord is crucial for its overall health and function. Any pathology affecting their integrity could potentially lead to spinal cord instability.
• Surgical Considerations: Surgeons operating on the spinal cord must be aware of these structures as they provide key anatomical landmarks and may need to be carefully managed during procedures. For instance, the denticulate ligaments are sometimes used as a point of reference or even for gentle retraction of the spinal cord during certain neurosurgical interventions.
• Tethered Cord Syndrome: While not directly a pathology of these ligaments, conditions like tethered cord syndrome involve an abnormally low attachment of the conus medullaris, often due to a thickened or shortened filum terminale, which can cause traction on the spinal cord and lead to neurological deficits.

Conclusion

While the term "ligament of pia mater" is not a singular anatomical entity, the pia mater's role in forming the denticulate ligaments and the filum terminale highlights its vital contribution to the structural integrity and protection of the spinal cord. These pial derivatives are essential for anchoring the spinal cord, preventing excessive movement, and safeguarding the delicate neural tissue within the vertebral column, underscoring the sophisticated biomechanical design of the human central nervous system.