MENINGES

 

The meninges[1] of the encephalon are: one pachymeninx[2] and two leptomeninges[3]. They surround the encephalon and cranial nerve roots. They derive from the neural crest and the mesoderm. According to Lievre, the neural crest participates in the formation of the meninges of the forebrain but not in the meninges of the midbrain, hindbrain and spinal cord. These are derived only from the mesoderm[4].

 

The pachymeninx is the dura mater[5]. It consists of an outer layer fixed to the periosteum, and an inner layer that forms extensions that separate parts of the encephalon. These extensions are: the falx[6] cerebri (that extends into the longitudinal fissure), the membranous tentorium[7] cerebelli (that extends between the cerebrum and the cerebellum) and the diaphragm sellae (that projects over the dorsum of the pituitary fossa forming an incomplete septum around the pituitary stalk). At the level of the hypophysis, the space between the outer and the inner layers of the dura mater is occupied by veins that drain to the cavernous and intercavernous sinuses.

 

The leptomeninges are: the arachnoid[8] and the pia mater[9]. As the two meninges derive from a mesenchyme layer that undergoes cavitation, the two meninges are called pia-arachnoid. The arachnoid and pia mater are held together by thin trabeculae of collagen fibers and fibroblasts. The arachnoid is covered by the dura mater and the pia mater covers the nervous parenchyma.

 

The space between the arachnoid and the pia mater is the subarachnoid space. It contains cerebrospinal fluid (CSF) and blood capillaries. In certain areas, there are enlargements called cisterns. The biggest cisterns are: the interpeduncular cistern (located between the crus cerebri), the chiasmatic cistern (around the optic chiasm), the quadrigeminal cistern (dorsal to the mesencephalic colliculi) and the cisterna magna or cerebellomedullary cistern (located between the cerebellum and the medulla oblongata).

 

The velum interpositum is a triangular double layer of pia mater formed by an invagination of the pia located between the thalamus and the fornix. Within the velum interpositum, there are two arachnoid layers: the dorsal envelops the internal cerebral veins, and the ventral is an extension of the arachnoid envelope of the pineal region. The velum interpositum forms the roof of the third ventricle and contains the choroid plexuses of the third ventricle and the internal cerebral veins. It extends caudally to give rise to the quadrigeminal cistern. This an enlarged portion of the subarachnoid space located immediately superior to the tectum of the mesencephalon. It contains the great cerebral vein (of Galen), the distal part of the quadrigeminal artery, a segment of the posterior cerebral artery, and the exit of the trochlear nerve.

 

Innervation of the Meninges

The encephalic meninges are sensory innervated by the trigeminal, glossopharyngeal and vagal cranial nerves[10],[11],[12]. The three trigeminal nerve branches contribute to innervate the supratentorial meninges, and the glossopharyngeal and vagal nerves to the tentorial and infratentorial meninges. The leptomeningeal nerve fibers bundles always lack a perineural sheath[13].

 

 

[1] From the Greek plural meninx, “membrane”.

[2] From the Greek pachys, "thick".

[3] From the Greek lepto, "weak".

[4] Page 77 of "The neural crest" by Le Douarin, N. 1982

[5] From the Greek dura mater, "hard mother".

[6] From the Latin falx, "stickle".

[7] From the Latin tentorium, "tent".

[8] From the Greek arachne, "cobweb".

[9] From the Latin pia mater, "soft mother".

[10] Keller JT, Marfurt CF, Dimlich RVW, Tierney BE. 1989. Sympathetic innervation of the supratentorial dura mater of the rat. J Comp Neurol 290:310–321.

[11] Mayberg MR, Zervas NT, Moskowitz MA. 1984. Trigeminal projec- tions to supratentorial pial and dural blood vessels in cats demon- strated by horseradish peroxidase histochemistry. J Comp Neurol 223:46 –56.

[12] Mayberg M, Langer RS, Zervas NT, Moskowitz MA. 1981. Perivascu- lar meningeal projections from cat trigeminal ganglia: Possible pathway for vascular headaches in man. Science 213:228–230.

[13] Fricke, B., ANDRES, K.H., and VON DU ̈ RING, M. Nerve Fibers Innervating the Cranial and Spinal Meninges: Morphology of Nerve Fiber Terminals and Their Structural Integration. Microscopy research and technique 53:96–105 (2001)