The Meninges of the Brain and Spinal Cord
by Robert Tallitsch, PhD | November 12, 2021
Short video with 3D real patient anatomy on the cranial and spinal meninges with an explanation, patient case, and procedure example of an epidural injection!
Written by: Robert Tallitsch, PhD
The central nervous system (CNS), composed of the brain and spinal cord, needs to be protected from trauma. This protection is partially supplied by the cerebrospinal fluid (CSF) and the meninges surrounding the brain and spinal cord.
The meninges of the brain are continuous with those of the spinal cord and possess the same three layers. The most superficial meninge is the dura mater. Deep to the dura is the pia mater, and the deepest is the arachnoid mater.
The cranial dura mater has two layers and is composed of fibrous connective tissue. The outer layer of the dura, the periosteal dura, is fused to the periosteum lining the cranium. The inner layer is termed the meningeal cranial dura.
Dural sinuses are located between the layers of the dura. These sinuses contain interstitial fluid and blood. Veins of the brain drain into these sinuses which, in turn, drain into the internal jugular veins.
Four folds of the dura support different areas of the brain.
- The falx cerebri extends deep into the longitudinal fissure of the cerebrum, separating the two cerebral hemispheres.
- The diaphragma sellae lines the sella turcica of the sphenoid and supports the pituitary gland.
- The tentorium cerebelli supports and protects the occipital lobes of the cerebrum. It also separates the cerebellar hemispheres from the cerebrum.
- The falix cerebelli extends midsagitally, inferior to the tentorium cerebelli, separating the two hemispheres of the cerebellum.
The arachnoid mater lies deep to the dura and superficial to the pia mater. In a cadaver a subdural space may be seen between the arachnoid and the meningeal cranial dura. However, it is believed that such a space does not exist in a living individual.
Deep to the arachnoid mater is the subarachnoid space, which contains web-like extensions connecting the arachnoid to the pia mater. Cerebrospinal fluid (CSF) fills the subarachnoid space. CSF is a solution of dissolved oxygen, carbon dioxide, chemical messengers, and nutrients for the CNS. CSF also removes the waste from the CNS.
Thin, web-like extensions termed arachnoid granulations extend superficially from the arachnoid mater into the dural sinuses. These structures reabsorb CSF from the subarachnoid space into the venous circulation.
The cranial pia mater, which is highly vascular, is attached to the surface of the brain.
Spinal meninges surround the spinal cord and the roots of the spinal nerves. The spinal meninges are continuous with the cranial meninges at the foramen magnum. There are three spinal meninges: dura mater, arachnoid mater, and pia mater.
The dura mater is held in place by attachments to the rim of the foramen magnum and the vertebral arches of C2, C3, and sacrum. It also attaches to the posterior longitudinal ligament. Caudally the dura mater fuses with the filum terminale of the spinal cord, forming the coccygeal ligament.
Superficial to the dura mater is the epidural space. It is occupied by fat, connective tissue, and blood vessels.
The arachnoid mater is composed of a thin, simple squamous epithelium. Deep to this layer is the subarachnoid space, which separates the arachnoid mater from the pia mater. The subarachnoid space is filled with cerebrospinal fluid.
Deep to the subarachnoid space is the pia mater. Collagenous fibers of the pia mater are interwoven with those of the arachnoid mater.
The pia, which is firmly attached to the spinal cord, envelops blood vessels supplying the spinal cord with blood.
All three meninges of the spinal cord are interconnected by the denticulate ligaments, which are extensions of the pia mater.
Lumbar Punctures and Epidural Injections
Lumbar Punction (Spinal Tap)
A lumbar puncture is a procedure utilized to obtain a sample of CSF from the subarachnoid space.
The procedure is done following the administration of a local anesthetic at the area where the needle will be inserted. A needle is inserted between L4 and L5, passing through the spinal dura and arachnoid meninges and into the subarachnoid space. The CSF pressure is measured, and a small volume of CSF is then removed. The CSF pressure is measured a second time, ensuring that not too much CSF has been removed from the subarachnoid space.
Once the needle is removed the meninges seal themselves. Should spontaneous sealing not occur the physician may utilize a platelet plug to prevent CSF leakage.
Significant clinical information may be provided by a lumbar punction, such as the presence of multiple sclerosis, spinal or brain cancer, bacterial or fungal infections, or bleeding around the brain or spinal cord.
An epidural injection is often administered to manage pain during or after surgery, during childbirth, or to manage chronic pain from a pinched spinal nerve.
Anesthetic is administered into the epidural space through a catheter, preventing the transmission of sensory information by the spinal nerves at the site of the injection.
The anesthesiologist must use caution regarding the amount of anesthetic administered. Injection of too much anesthetic will cause the unintentional blockage of spinal nerve transmission more cranially and/or caudally than initially intended, and the effect of the anesthesia may persist longer than intended.
A car protects the driver in an accident, much like the skull protects the brain. However, a seatbelt protects the driver from an injury resulting from the driver hitting the steering wheel or dashboard of the car during the accident. The meninges and CSF protect the brain from injury that would result from the brain colliding with the skull, similar to the role played by the seatbelt of a car.
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