The brain and spinal cord are separated from the walls of their bone coverings (skull and vertebral column) by three membranes called the meninges. Meninges are classified as either spinal meninges, encasing the spinal cord, or cranial meninges, surrounding the brain.
However, the three meningeal layers that constitute the spinal and cranial meninges are continuous. The meninges, from surface to deep, are:
- Dura mater (pachymeninx)
- Arachnoid mater
- Pia mater
The meninges have three primary purposes: to support blood arteries, to shield the spinal cord and brain from mechanical injury, and to create a continuous channel through which cerebrospinal fluid (CSF) flows. The CSF flows explicitly between the two inner meningeal layers (pia and arachnoid), known as leptomeninges. These layers enclose the subdural, epidural, and subarachnoid spaces, three clinically significant areas.
Table of Contents
Structure
The arachnoid mater is one of the three meninges—the membranes surrounding the spinal cord and brain. It is situated between the two other meninges, the deeper pia mater and the more superficial dura mater, and the subarachnoid space separates it from the pia mater.
Two subgroups of arachnoid mater enclose the subarachnoid space, the ventral and dorsal layers. Internal cerebral veins are covered by the dorsal layer, which also holds them to the tela choroidea. On the other hand, the ventral layer of the arachnoid membrane forms an arachnoid envelope over the pineal area and is a direct anterior extension of that envelope.
The thin, spiderweb-like arachnoid layer, which is linked to the interior of the dura, shields the spinal cord and brain but does not coat the sulci (folds) of the brain.
The parts surrounding the spinal cord and brain are called arachnoidea spinalis and arachnoidea encephali.
The arachnoid and dura mater are relatively close throughout the skull until the S2 (human secondary somatosensory cortex), where they merge into one layer and end. A few veins connect the brain’s venous system with the dura mater’s venous system, present in the space between the arachnoid and the dura mater.
Anatomy
The subarachnoid space, filled with CSF, separates the pia mater from the arachnoid mater, a membrane in direct contact with the dura mater.
Although it reaches the superior surface of the pituitary fossa, it does not entirely enclose the pituitary gland. The entire brain is loosely encased in the arachnoid mater and is separated from the dura, except where it is attached to the internal carotid adventitia and vertebral arteries at the point where they reach the subarachnoid space. The cerebral and spinal arachnoid membranes are connected at the level of the foramen magnum. Along with the optic nerve-sheath complex, the arachnoid mater exits the skull.
Except for the sizeable longitudinal fissure between the cerebral hemispheres, it does not enter the sulci or fissures. Arachnoid trabeculae, arachnoid septae, and arachnoid membranes are a network of connective tissue that traverse the subarachnoid space and are loosely connected to the pia mater.
Arachnoid granulations are outpouchings of the arachnoid mater penetrating the dura in the venous dural sinuses and are associated with CSF reabsorption.
Function
The Pacchionian bodies, or arachnoid granulations, are protuberances of the arachnoid mater that penetrate the meningeal dura and project into the lumina of the dural venous sinuses.
The CSF diffuses into the dural venous sinuses through the arachnoid granulations’ lining. Therefore, the purpose of the arachnoid granulations is to allow continuous evacuation of the cerebrospinal fluid (CSF) from the subarachnoid into the vascular stream.
The subarachnoid space, also known as the subarachnoid cisterns, is created when the arachnoid mater entirely separates from the pia mater in specific locations. The brain has ten primary subarachnoid cisterns connected through the main subarachnoid space. These comprise the following:
- Ambient cistern
- Cerebellopontine cistern
- Chiasmatic cistern
- Crural and carotid cisterns
- Pontine cistern
- Quadrigeminal cistern
- Interpeduncular cistern
- Cistern of lamina terminalis
- Cistern of lateral cerebral fossa or Sylvian cistern
- Cisterna magna or cerebellomedullary cistern
Related Links:
- Difference between Brain and Spinal Cord Meninges
- Difference between Brain and Spinal Cord
- Human Nervous System
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