Generality
The dural sac is the dura mater of the spinal cord, that is, the outermost meninge of the nervous organ housed inside the spinal column.
The dural sac begins just before the foramen magno, where the spinal cord also begins, and ends at the level of the II sacral vertebra.
In addition to enclosing the spinal cord, the dural sac also envelops the cauda equina; this occurs at the end of the spinal cord, from the II lumbar vertebra to the II sacral vertebra.
Together with the other two underlying meninges (arachnoid and pia mater), the dural sac has the task of protecting the spinal cord.
The dural sac is involved in various medical conditions (including the anchored marrow syndrome) and diagnostic / therapeutic procedures (lumbar puncture and epidural anesthesia).
Short review of the Meninges
The meninges are the 3 sheets (or membranes) of fibrous connective tissue, arranged one above the other, which cover the inside of the skull and spinal canal, and which at the same time cover the outer surface of the brain and spinal cord with protective purposes, that is the two organs constituting the so-called central nervous system .
From the most external to the most internal, the 3 meninges are: dura mater, arachnoid (or arachnoid mother ) and pia mater .
What is Dural Bag?
The dural sac, or thecal sac, is the dura mater of the spinal cord.
The dural sac is therefore the first of the three sheets of fibrous connective tissue that cover the inside of the vertebral canal and protect the spinal cord.
The dural sac is the outermost meninge of the spinal cord.
To understand: Spinal Cord and Vertebral Canal
- The spinal cord is the complex nervous organ which, together with the brain, constitutes the so-called central nervous system (or CNS ).
Consisting of several groups of neurons, the spinal cord is a cylindrical nerve structure, which arises directly from the brainstem (one of the four main components of the brain) and, after being introduced into the spinal canal of the spine, runs through it up to shortly before the II lumbar vertebra (L2), the place where its course ends.
On average 45 centimeters in man and 43 centimeters in women, the spinal cord is known to give rise to 31 pairs of spinal nerves .
- The often mentioned vertebral canal (or spinal canal) is the long duct of the vertebral column, which results from the superposition of the so-called vertebral holes present in each vertebra.
The vertebral canal begins at the height of the foramen magno (or occipital foramen), with the I cervical vertebra (vertebra C1), and ends at the level of the V sacral vertebra (vertebra S5).
The vertebral canal houses the spinal cord, which - as stated in the previous point - extends up to the II lumbar vertebra.
Importance of the central nervous system
The spinal cord, together with the brain, forms the central nervous system.
The central nervous system has the vital task of analyzing in detail all the information coming from the internal and external environment to the organism, and to elaborate the most suitable answers to the aforementioned information.
Anatomy
The dural sac is the outermost meninge of the spinal cord; this means that it borders on the bony walls of the vertebral canal, on the one hand (to be precise the external one), and with the meninge arachnoid, on the opposite side (hence the inner one).
Similar to a cylinder, the dural sac is made up of a thick lamina of fibrous connective tissue with flat cells, which gives it a marked resistance.
The thecal sac is in continuity with the dura mater of the brain (or dura mater encephalic), from which however it differs in some aspects.
Throughout its extension within the vertebral canal, the dural sac never really adheres to the vertebral hole wall, but is held at a distance to generate a very important empty space, whose name is epidural space or peridural space ; in the epidural space, takes place of the adipose tissue and there are the arteries and veins destined, the first ones, to nourish the spinal cord with oxygen-rich blood and, the second, to direct the oxygen-poor blood to the heart which is circulated in the spinal cord.
In addition to covering the spinal cord, the dural sac also envelops the so-called cauda equina ; with development below the spinal cord, the cauda equina is the particular nerve structure similar to a bundle, which groups together the last 10 pairs of spinal nerves, before they come out of the spinal column.
Origin and Course of the Dural Bag
Not forgetting that it is in continuity with the dura mater, the dural sac begins its course just before the foramen magno, in the same point (and it is not a case) in which the spinal cord begins.
From the foramen magno, therefore, it extends downwards, obviously within the vertebral canal, up to the II sacral vertebra (vertebra S2); this means that the dural sac is longer than the spinal cord, to which it must offer protection, of different vertebrae (remember that the spinal cord terminates at the level of the II lumbar vertebra, so between it and the dural sac there is a gap total of 5 vertebrae).
From where the spinal cord (vertebra L2) ends up to the vertebra S2, the dural sac is not an empty cylinder ; inside, in fact, takes place the already mentioned cauda equina, which originates and extends below the spinal cord, and brings together the last 4 pairs of lumbar spinal nerves (of the 5 existing), the 5 sacral spinal nerves and the coccygeal spinal nerve.
Summary of the salient points regarding the origin and course of the dural sac:
- The dural sac is, in fact, an extension of the encephalic dura mater;
- The dural sac initiates its course at the level of the foramen magno, at the same point where the spinal cord originates;
- The dural sac runs along the vertebral canal up to the II sacral vertebra;
- Up to the II lumbar vertebra, the dural sac envelops the spinal cord; from the II lumbar vertebra onwards, where the spinal cord ends, it covers the cauda equina.
Sacco Durale and Nervi Spinali
From where it begins to where it ends, the dural sac has characteristic pairs of perforations, from which the spinal nerves oriented towards the intervertebral holes emerge.
The intervertebral holes are the lateral openings of the vertebral column, which result from the particular arrangement of the vertebrae and from which the spinal nerves destined to the various organs and cutaneous regions of the human body escape.
At the level of the perforations, the thecal sac covers part of the spinal nerves coming out, merging, in a certain sense, with the epinevrio of the aforementioned nerves.
The epineurium is the outer most connective tissue lining of the peripheral nerves, including the spinal nerves.
Dural Sack and Hard Mother Encephalic: the Differences
Although both play the role of external meninges, the dural sac and the dura mater are, as anticipated, somewhat different from one another.
Indeed:
- The dural sac is a single-layer meninge; the encephalic dura mater, on the other hand, is a double-layer (bi-lamellar) meninge, in which the outer layer acts as a lining of the inner surface of the skull (and is called " endosteal layer " or " periosteal dura mater "), while the inner layer acts as a coating of the outer surface of the brain (and is called " meningeal layer " or " dura mater meningea ");
- The dural sac is a cylinder with no particularity, except for the perforations reserved for the exit of the spinal nerves; the encephalic dura mater, on the other hand, has characteristic folds, called reflection folds, which derives from the adaptation of the meningeal layer to the typical furrows, convolutions and cavities of the brain and cerebellum.
Among these reflective folds of the encephalic dura mater, include: the cerebral sickle, the tentorium of the cerebellum, the cerebellar sickle and the sellar diaphragm .
- While the dura mater adheres to the inner wall of the skull, the dural sac is separated from the inner wall of the vertebral canal.
From the separation between the dural sac and the inner wall of the vertebral canal derives the epidural space, in which adipose tissue and blood vessels take place.
Brief study of the relationship between dural sac and dura mater of the brain
At this point in the reading, there are all the elements to clarify that the dural sac is in continuity with the meningeal layer of the encephalic dura mater, ie with the innermost layer of the meninge preceding the meninge arachnoid and the meningal pia mater.
Bleeding spraying of dural bag
The dural sac is the only one of the 3 meninges of the spinal cord to receive a significant quantity of oxygenated blood (the blood supply to the arachnoid and the pia mater is scarce).
In detail, the so-called vertebral artery meningeal branches (NB: the branches of an artery are its ramifications) deal with the blood supply of the thecal sac.
Histology
The dural sac consists of a very dense connective tissue, rich in collagen and elastin ; moreover, it presents an important layer of flat cells, which give it a certain impermeability .
Thanks to its impermeability, the dural sac manages to perform perfectly one of the functions for which it is intended: the containment of the cerebrospinal fluid within the subarachnoid space (to learn more, see the chapter dedicated to the function of the dural sac).
Function
With the support of the other two underlying meninges (arachnoid and pia mater), the dural sac has the task of protecting the spinal cord from physical insults (eg: trauma to the spine) and from other threats that can reach, by blood, this component of the central nervous system.
Furthermore, with the help of the other two meninges, the dural sac contributes to the containment in the subarachnoid space (space between arachnoid and pia mater) of the cephalorachidian liquor (or cephalorachidian liquor ).
What is the CSF liquor and what is it used for?
Resulting from a process of ultrafiltration of blood plasma, the CSF is a transparent fluid, free of red blood cells, rich in white blood cells and poor in plasma proteins, which has the task of:
- Protect brain and spinal cord,
- Create an ideal environment for nerve cells to function,
- Provide nourishment to the central nervous system,
- Adjust intracranial pressure,
- Promote the removal of waste products from the central nervous system.
diseases
The dural sac may be involved in various medical conditions; Among these medical conditions, they deserve a particular note:
- Injuries to the connective tissue that makes up the dural sac. Result of trauma to the spine or surgical complications, lesions to the dural sac may result in the loss of CSF.
- Anchored spinal cord syndrome . This pathology arises, when there is an anatomical anomaly that prevents the spinal cord from moving inside the meninges (normally, the spinal cord enjoys a certain mobility within the spinal canal, limited by the presence of particular ligaments).
Clinical meaning
The dural sac also plays a role of some importance in the clinical-diagnostic and clinical-therapeutic fields; the dura mater of the spinal cord is in fact involved in the execution of the so-called lumbar puncture and in the practice of an anesthetic technique known as epidural anesthesia .
Lumbar puncture
The lumbar puncture consists in taking, through a special needle, a portion of the CSF contained in the subarachnoid space of the spinal cord and in the subsequent laboratory analyzes of the aforementioned quota.
Lumbar puncture is a fundamental test to detect the presence of infectious agents in the spinal cord (and in the central nervous system in general) and to understand if a local inflammation is in progress.
The lumbar puncture involves the dural sac, as the latter is the first meninge to be the object of perforation, during the withdrawal.
Did you know that ...
On the occasion of the lumbar puncture, the collection takes place at the level of the cauda equina, in a point of the latter in which the so-called lumbar cistern (of cephalorachidian liquor) resides.
Epidural anesthesia
Epidural anesthesia (or epidural anesthesia) is a type of local anesthesia, which involves the injection of anesthetics and analgesics into the epidural space of the spinal cord, in order to eliminate the painful sensation in the lower back and along both lower limbs .
Epidural anesthesia involves the dural sac, because, as previously reported, the epidural space is the space between the dural sac and the bony wall of the vertebral canal.
