Pia Mater
The pia mater, is a thin, richly vascularized layer of loose connective tissue that is firmly attached to the surface of the brain or spinal cord but is separated from the neurons by neuroglial cell processes.
Ramified, cuboidal epithelium-covered projections of the pia mater into the brain's ventricles are collectively termed the choroid plexus; they produce the cerebrospinal fluid by selective ultrafiltration of blood plasma.
Beneath the arachnoid mater and intimately applied to the spinal cord is the pia mater. Both the arachnoid and pia mater are continuous with the arachnoid and pia surrounding the brain, but unlike the arachnoid, which follows the dura mater, the pia essentially ends, with the caudal end of the spinal cord, at the level of the second lumbar vertebra.
A rope like extension of the pia mater, the filum terminale attaches the end of the spinal cord to the caudal end of the dura mater. In addition, the pia mater contains lateral projections called denticulate ligaments, which connect the spinal cord to the dura mater by projecting between the dorsal and ventral roots. The space between the arachnoid mater and pia mater is the subarachnoid space. It is normally filled with cerebrospinal fluid, which surrounds the entire brain and spinal cord.
The meninges are supplied by the general sensory branches of the cranial and spinal sensory nerves.
The cerebrospinal fluid is contained within the subarachnoid space between the pia and arachnoid.
Although the spinal coverings are generally similar to those of the brain, there are some differences.
The spinal dura is single layered and lacks the periosteal layer of the cranial dura. The spinal epidural space is an actual space in which venous plexuses are located and is used clinically for the administration of epidural anesthesia to produce a paravertebral nerve block. On the other hand, the cranial epidural space is a potential space that becomes filled with a fluid only in pathological conditions; normally, there is no space between the dura and the cranium. The spinal epidural space is located between the meningeal layer of the dura (there is no periosteal layer) and the periosteum of the vertebra, while the cranial epidural space (when present) is located between the periosteal layer of the dura and the cranium.
Dura Mater
As stated earlier, the spinal dura consists of only meningeal layer and lacks the periosteal layer of the cranial dura. Rostrally, the spinal dura joins the meningeal layer of the cranial dura at the margins of the foramen magnum. The spinal epidural space separates the spinal dura from the periosteum of the vertebra and is filled with fatty connective tissue and plexuses of veins. Caudally, the spinal dura ends at the level of the second sacral vertebra. At this level, the spinal dura becomes a thin extension (the coccygeal ligament or the filum terminale externum) and serves to anchor the spinal dura to the base of the vertebral canal.
Arachnoid
This membrane loosely invests the spinal cord and is connected to the dura via connective tissue trabeculae. Rostrally, it passes through the foramen magnum to join the cranial arachnoid, and caudally it surrounds the cauda equina (a bundle of nerve roots of all the spinal nerves caudal to the second lumbar vertebra) ). The subarachnoid space contains cerebrospinal fluid (CSF).
Pia Mater
The spinal pia is thicker as compared to the cranial pia. It is a vascular membrane and projects into the ventral fissure of the spinal cord. At intervals, toothed ligaments of pial tissue, called dentate ligaments, extend from the lateral surfaces of the spinal cord; these ligaments serve to anchor the spinal cord to the arachnoid and through it to the dura. As mentioned earlier, at caudal levels, spinal pia continues along the filum terminale that then anchors the spinal cord to the dura at the level of the second sacral vertebra. Rostrally, the spinal pia joins the cranial pia.
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A 44-year-old woman is suspected of having meningitis. To confirm the diagnosis, a lumbar puncture is ordered to collect a sample of the cerebrospinal fluid (CSF). Identify the last layer of tissue the needle will traverse in this procedure before reaching CSF.
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The correct answer is A. You answered D.
A lumbar puncture collects cerebrospinal fluid and, therefore, the needle has to enter the subarachnoid space, which is located between the arachnoid and pia mater. Therefore, the last layer of tissue the needle would traverse to enter the subarachnoid space is the arachnoid mater.
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