NeuroAnatomy 3
Trigeminal nerve - ophthalmic division innervates:
- Anterior cranial fossa - Tentorium cerebelli - Lining of superior aspect of cranial cavity - Falx cerebri Problems in the anterior cranial fossae tend to refer to the forehead, temples, and eyes
three (3) meningeal arteries
1. Anterior meningeal : This artery arises from the ophthalmic artery. 2. Middle meningeal : This is the largest of the meningeal arteries. It arises from the maxillary artery and enters the cranial vault through the foramen spinosum. 3. Posterior meningeal : There are usually more than one (1) artery that use this same name. These arteries may arise from the occipital and/or vertebral arteries. NB Since the meningeal arteries run in the periosteal layer of the dura mater, injuries to these arteries usually cause epidural hematomas.
dural folds
In several places the meningeal layer of the cranial dura is reflected inward to form septa called the dural folds that partially divide the cranial cavity into compartments.
diaphragma sellae
It is a round horizontal sheet that forms a roof over the sella turcica (or more correctly the pituitary fossa). It has a defect (i.e., hole) in its center that allows the pituitary stalk to run through the diaphragm sellae on its way from the hypothalamus to attach to the pituitary gland.
V agus nerve innervates:
Posterior cranial fossa. Problems in the posterior cranial fossa tend to refer to the neck and region behind the ear
C1-3 spinal nerves innervates:
posterior cranial fossa. Problems in the posterior cranial fossa tend to refer to the neck and region behind the ear
4th ventricle
rhombus-shaped (as seen from lateral aspect) and is located in the region of the pons and medulla. It, therefore, represents the remnants of the neural tube in the rhombencephalon (or both metencephalon and myelencephalon). The floor of the IV ventricle is considered it ventral (or anterior side while, its roof is considered its dorsal (or posterior) side. The floor consists of the posterior aspect of the pons and upper medulla (i.e., open portion) and is sometimes known as the rhomboid fossa.
Spinal nerves (segmentally) innervates:
spinal dura
The IV ventricle has five (5) openings :
a. The cerebral aqueduct - this is the superior opening through which cerebrospinal fluid enters the IV ventricle (note - remember all of its synonyms...) b. The central canal (of the spinal cord) is the inferior opening that represents the continuation of the neural tube cavity into the region of the spinal cord c. The foramen of Magendie (or median aperture) is a single midline opening in the posterior aspect of the roof of the IV ventricle (in the region of the inferior medullary velum). Cerebrospinal fluid can leave the IV ventricle (and the ventricular system) through this foramen to enter into the cisterna magna (of the subarachnoid space). d. The foramina of (von) Luschka (or lateral apertures) are a pair of openings (right and left) in the lateral aspects of the IV ventricle. Cerebrospinal fluid can leave the IV ventricle (and the ventricular system) through these foramina to enter into the pontine cistern (of the subarachnoid space).
The roof of the IV ventricle is made up of two (2) parts
a. The superior medullary velum (the upper 1⁄2 of the roof) is a thin layer of neural tissue that separates the cerebrospinal fluid of the IV ventricle from the cerebellum b. The inferior medullary velum (the lower 1⁄2 of the roof) is merely a glial membrane that separates the CSF of the IV ventricle from the cerebellum
cerebrospinal fluid performs several important functions :
1. It provides mechanical support of central nervous system (especially brain) by virtue of its buoyancy effect. 2. It acts as a shock absorber for central nervous system because of its fluid nature. 3. It acts as a "sink" for substances produced by the central nervous system. Some "waste" material can be "dumped" into the cerebrospinal fluid from the central nervous system parenchyma. NB Hydrocephalus ("water on the brain") occurs with obstruction of cerebrospinal fluid pathways and cerebrospinal fluid cannot be recycled back into the blood properly so that it builds up in the ventricular system. This causes ventricular dilation that results in pressure on (and often damage to) brain tissue. In children, where the skull has NOT completed development, the entire head can expand. Communicating hydrocephalus occurs when the flow of cerebrospinal fluid is NOT obstructed, but either cerebrospinal fluid is over produced (e.g., papilloma) or it cannot re- enter the blood (e.g., from blockage of arachnoid villi). Noncommunicating hydrocephalus occurs when there is a blockage to cerebrospinal fluid flow either within the ventricular system, where the ventricles communicate with the subarachnoid space (i.e., exits of the IV ventricle), or within the subarachnoid space (e.g., at the tentorial notch).
The general functions of the meninges include:
1. Protection of central nervous system (along with skull and vertebral column). This is especially accomplished by the dura mater and cerebrospinal fluid. 2. The cerebrospinal fluid helps keep the brain (and to a lesser extent the spinal cord) from Collapsing under its own weight. The central nervous system is very soft and gelatinous, since it has NO connective tissue in its substance. Since the brain and spinal cord "float" in the cerebrospinal fluid, their effective weights are greatly decreased. 3. The meninges are involved with the blood supply of the central nervous system. The dura mater helps form the dural venous sinuses (see previous lecture). Also, the pia mater is involved in conveying blood vessels into the substance of the central nervous system.
subarachnoid cisterns associated with the central nervous system :
1. The cisterna magna (or cerebellomedullary cistern) is the largest of these cisterns. It is located posterior to the medulla and inferior to the cerebellum. This places it just above the foramen magnum of the occipital bone. It is a single midline structure. It is a place that cerebrospinal fluid can be sampled, although this location is infrequently used. 2. The superior cistern (or cisterna vena magna cerebri) is located deep in the space between the cerebral hemispheres and the cerebellum. This puts it posterior to the midbrain and superior to the cerebellum. It is a single midline structure. As one of its names imply, it contains the great cerebral vein. 3. The mesencephalic cisterns (or cisternae ambiens) are NOT midline structures (and are NOT on the diagrams above). They are located on the lateral sides of the midbrain, so are paired - right and left. They connect the superior cistern (2) and the interpeduncular cistern (4). 4. The interpeduncular cistern (or cisterna basalis) is located between the cerebral peduncles just anterior to the midbrain. It is a single midline structure. The oculomotor nerves run through this cistern. 5. The chiasmatic cistern is a small dilation of the subarachnoid space located just above the optic chiasm. It is a single midline structure. 6. The pontine cistern is found just anterior to the pons. It contains the basilar artery. It is a single midline structure. 7. The cisterns of the lateral cerebral fissure (of Sylvius) are NOT midline structures (and are NOT on the diagrams above). They are located in the anterior aspect Sylvian fissures between the frontal and temporal lobes of the cerebral hemispheres, and so are paired - right and left. 8. The lumbar cistern is the only subarachnoid cistern that is NOT located in the head. It is in the lower lumbar region of the spine (below the point where the spinal cord ends). It contains the cauda equina. It is also the usual place where cerebrospinal fluid samples are taken in a procedure known as a lumbar puncture. It is a single midline structure NB In the spinal region, the subarachnoid space extends to the level of the 2nd sacral segment.
The are several specializations of the pia mater that are visible to the naked eye
1. The denticulate ligaments are folds of extra thick pia mater located on the lateral aspects of the spinal cord. They span the subarachnoid space to attach to the deep surface of the arachnoid. They help anchor the spinal cord in the vertebral canal. 2. The filum terminale (or filum terminale internum) is the single midline continuation of the pia mater that attaches the conus medullaris (the place where the spinal cord ends) to the inferior aspect of the dural sac (at the S2 vertebral level). It anchors the end of spinal cord in the vertebral canal. 3. Another specialization of the pia mater is tela choroidea. They are found in specific regions in each of the ventricles of the brain (see later). They are folds of pia mater (covered with typical ependyma) that support the choroid plexus, which is the modified ependymal cells (along with many blood vessels) that produce the cerebrospinal fluid.
subarachnoid space
Between the arachnoid and the pia mater. This is a real space that is filled with cerebrospinal fluid (CSF). The brain and spinal cord are basically suspended (i.e., float) in the cerebrospinal fluid.
flow of the cerebrospinal fluid
Lateral ventricles ↓ Formina of Monro ↓ 3rd ventricle ↓ Cerebral aqueduct ↓ 4th ventricle ↓↓ Foramen of Magendie and Foramina of von Luschka ↓↓ Cisterna magna (Subarachnoid space) from foramen of magendine and Pontine cistern from foramina of von Luschka
ventricles of the brain
The remnants of the neural tube cavity in the region of the brain expand to form the ventricles of the brain. These expanded regions are lined with ependyma and are filled with cerebrospinal fluid. The cerebrospinal fluid is produced by choroid plexus, which is supported by the tela choroidea. There is a specific region in each ventricle that develops choroid plexus/tela choroidea.
lateral ventricles
These are the largest of the ventricles. They are the remnants of the neural tube cavity in the region of the telencephalon and are, therefore, located in the cerebral hemispheres. Each lateral ventricle is horseshoe-shaped (or C-shaped) and has the following parts : a. Body (or pars centralis) located in the parietal lobe b. Anterior horn (or cornu) located in the frontal lobe c. Posterior horn (or cornu) located in the occipital lobe d. Inferior horn (or cornu) located in the temporal lobe
arachnoid villi (or granulations) or (pacchionian villi)
are evaginations of arachnoid that protrude mainly into the superior sagittal sinus (note - some of these can also be found in other dural venous sinuses). They act as one-way valves that allow cerebrospinal fluid to flow into the blood of the dural venous sinuses (thus "recycling" cerebrospinal fluid back into the blood).
septum pellucidum
double glial membrane (along with a sparse amount of gray matter) located in the midline, separates the two (2) anterior horns of the lateral ventricles from each other. The thalamus helps form the floor of the body. Cerebrospinal fluid leaves each lateral ventricle and enters the 3rd ventricle by it respective interventricular foramen (of Monro). There are several other C- shaped structures that are in relation to the lateral ventricles : a. Corpus callosum b. Caudate nucleus c. Hippocampus
Cerebrospinal fluid (CSF)
fluid that fills the ventricles of the brain and the subarachnoid space. At any give time there is a total of about 150 ml of CSF in the body, although about 500 ml of cerebrospinal fluid is continuously produced each day. Therefore, the CSF has a fairly rapid rate of flow and much of it gets reabsorbed each day (i.e., it gets totally replaced over 3x/day). Choroid plexus produces the cerebrospinal fluid. The choroid plexus is made of fenestrated (with diaphragms) capillaries, pia mater, and modified ependymal cells. Remember that the choroid plexus is supported by the tela choroidea and there is some choroid plexus in each of the ventricles. The choroid plexus produces CSF by active transport of Na+ from the blood into the ventricles. Recall that where Na+ goes, water follows. Therefore, cerebrospinal fluid is similar to blood plasma
arachnoid mater
is a very thin layer of loose connective tissue that lies deep to the dura mater. It is so thin that it is translucent to the naked eye. The arachnoid is avascular. It contacts the inner surface of the dura mater. Therefore there is a potential space between the dura mater and arachnoid known as the subdural space. With damage to (usually) external cerebral veins or dural venous sinuses, blood can gather in this space causing a subdural hematoma. The arachnoid villi (or granulations) or (pacchionian villi) are specialization of the arachnoid that recycle cerebrospinal fluid back into the blood. The arachnoid is attached to the underlying pia mater by trabeculae (strands), which give the arachnoid a spider web appearance and hence its name. These trabeculae span a true subarachnoid space. The subarachnoid space is filled with cerebrospinal fluid (CSF). This space also communicates with the ventricular system of the central nervous system (which is also filled with cerebrospinal fluid). Clinically the supratentorial and infratentorial regions of the subarachnoid space are connected by the narrow gap between the midbrain and the border of the tentorial incisure. This region can get obstructed, leading to an abnormal ventricular dilation. There are several places where the subarachnoid space is normally dilated. These expanded regions of the subarachnoid space serve as reservoirs of cerebrospinal fluid and are known as subarachnoid cisterns.
tentorium cerebelli
is also a single structure, but unlike the falx cerebri and falx cerebelli, it has more of a horizontal orientation. It is located between the occipital lobes of the cerebrum (above) and the cerebellum (below). Like its name implies, it is tent-shaped, such that its highest (most superior) portion is in the midline, where it attaches to the posterior inferior portion of the falx cerebri. It is directed inferolaterally from the midline on either side. Its concave anteromedial edge is free (i.e., does NOT attach to anything), except for the very anterior ends, which are attached to the anterior clinoid processes (of the sphenoid). Between this free margin and the dorsum sellae is a large curved hiatus (i.e., a hole) called the tentorial notch (or tentorial incisure). The midbrain is found in this notch. The peripheral attachments are from anterior to posterior : 1. Posterior clinoid processes (sphenoid) 2. Petrous portions of the temporal bones 3. Grooves (or sulci) for the transverse sinuses (on either side of the occipital bone). 4. Internal occipital protuberance (along with falx cerebri) -The oculomotor nerve runs between the attachment sites of the tentorium cerebelli with the anterior and posterior clinoid processes. This is a potential site of injury of that nerve. Several dural venous sinuses are associated with the tentorium cerebelli: 1. The transverse sinuses run in the posterior aspect of the peripheral attachment. 2. The superior petrosal sinuses run in the anterior aspect of the peripheral attachment. 3. The straight (or rectus) sinus runs in the midline region (where the tentorium cerebelli attaches to the falx cerebri). 4. The confluence of the sinuses is found where the tentorium cerebelli (and falx cerebri) attach to the skull at the internal occipital protuberance. The tentorium cerebelli divides the cranial cavity into two (2): 1. Supratentorial compartment that contains the forebrain 2. Infratentorial compartment that contains the hindbrain -The terms forebrain and hindbrain refer to the original primary brain vesicles (and their derivatives). Note that the midbrain is really in neither compartment since it sits in the tentorial incisure.
falx cerebelli
is another vertical fold in the midline (i.e., it is a single structure). It is much smaller than the falx cerebri and, as the name implies, lies between the cerebellar hemispheres. Its anterior border is a "free edge", while its posterior margin attaches to the midline of the skull in the region of the cerebellum. The falx cerebelli attaches superiorly to the falx cerebri and tentorium cerebelli in the region of the internal occipital protuberance (which is the same place as the confluence of the sinuses). The occipital sinus can be found in the attached margin of the falx cerebelli.
falx cerebri
largest dural fold. It is a sickle- shaped vertical partition that lies in the longitudinal fissure (between the right and left cerebral hemispheres). Therefore, it is a single midline structure. It has the following attachment sites: 1. Anteriorly - crista galli (ethmoid) 2. Superiorly - midline of cranial vault (i.e., on inside of sagittal suture) 3. Posteriorly - internal occipital protuberance 4. Inferiorly (about the posterior 1⁄4) - in the midline portion of the tentorium cerebelli Most of the inferior border of the falx cerebri (i.e., the anterior 3⁄4) is a "free edge" that does NOT attach to anything, but lies just above (and follows the contour of) the corpus callosum. There are several dural venous sinus structures associated with the falx cerebri: 1. Superior sagittal sinus runs in the superior border. 2. Inferior sagittal sinus runs in the free portion of the inferior border. 3. Straight sinus runs where the posterior portion of the inferior border that attaches to the midline meeting point with the tentorium cerebelli. 4. The confluence of the sinuses is found where the falx cerebri (and tentorium cerebelli) attach to the skull at the internal occipital protuberance.
What veins drain the dura mater?
meningeal veins
Trigeminal nerve - mandibular division innervates:
middle cranial fossa. Problems in the middle cranial fossa tend to refer to the cheeks, jaw, and mouth
Trigeminal nerve - maxillary division innervates:
middle cranial fossa. Problems in the middle cranial fossa tend to refer to the cheeks, jaw, and mouth
3rd ventricle
narrow cleft between the right and left halves of the thalamus (i.e., it is a single structure in the midline). It is the remnants of the neural tube cavity in the diencephalon, hence its relation to the thalamus. In about 50% of the population, the right and left halves of the thalamus meet each other in the midline. This is called the interthalamic adhesion and forms a solid bridge across the III ventricle. The III ventricle receives cerebrospinal fluid from the lateral ventricles by way of the foramina of Monro. Cerebrospinal fluid leaves the III ventricle through the a single midline opening known as the cerebral aqueduct (or iter or Sylvian aqueduct) located on the posteroinferior aspect of the III ventricle. The cerebral aqueduct represents the remnants of the neural tube cavity in region of the mesencephalon.
dura mater
outermost and thickest of the meninges. It is composed of dense irregular collagenous connective tissue. In the cranial region, the dura mater consists of two (2) layers that are firmly fused together (except at the dural venous sinuses) 1. Periosteal (or endosteal) - This is the outer layer that forms the periosteum on the inner side of the inner table of the skull bones that form the cranial vault. Therefore it is fused to the inside of compact bone of the parts of the skull that form the cranial cavity. This layer is technically an endosteum, but histologically is more like a periosteum so the term periosteal layer is used much more frequently than endosteal layer. 2. Meningeal - This is the inner layer of the dura mater surrounding the brain. It is thinner than the periosteal layer. The inner surface of this layer (i.e., the side facing the arachnoid) is lined with a simple squamous epithelium. This layer forms tubular sheaths for cranial nerves as they exit the skull. These sheaths transition into the epineurium of these nerves once they exit the skull. -The dural sheath of the optic nerve is continuous with the sclera The dural mater ends inferiorly at the level of the 2nd sacal segment and is anchored to the coccyx by the coccygeal ligament (aka filum terminale externum), which is a special condensation of all three (3) meningeal layers.
meninges
the connective tissue membranes that envelop the brain and spinal cord. There are three (3) layers of meninges: 1. Dura mater is the outermost and thickest layer. 2. Arachnoid mater is the delicate middle layer. It is often just called the arachnoid. 3. Pia mater is the very thin inner layer that is adherent to the central nervous system. -Sometimes the arachnoid and pia mater are considered together as the leptomeninges. The dura mater is then referred to as the pachymeninx.
Subdural space
the dura mater and arachnoid are NOT really (or VERY loosely) attached to each other, such that a potential space exists between the inner surface of meningeal layer of the dura mater and the arachnoid. Fluid can gather in this space (e.g., blood can gather here to form a subdural hematoma). Subdural hematomas usually result from damage to external cerebral veins or dural venous sinuses.
dural root sleeves
the dura mater surrounding the spinal cord forms tubular sheaths around the spinal nerve roots as they exit the cord and leave the vertebral canal via the intervertebral foramina. These dural root sleeves transition into the epineurium of the spinal nerves (in a similar fashion as the dural sleeves surrounding the cranial nerves).
Epidural space
the periosteal layer of the dura mater is fairly well attached to the bone, but this attachment can be torn if fluid is forced into the region between dura mater and the surrounding bone (e.g., blood from damage to the meningeal blood vessels can form an epidural hematoma). Therefore, the term - epidural space is a bit of a misnomer. It actually only "exists" abnormally. Also, this is NOT a potential space, since the dura and bone are attached together (and NOT merely next to each other). -The dura mater in the vertebral canal does NOT attach to the surrounding bone (unlike around the brain) such that there is a true epidural space in this region. The epidural space in the vertebral canal is filled with fat and the internal vertebral venous plexus.
pia mater
very delicate layer located deep to the arachnoid. It usually is made up of a single layer (or in some places several layers) of cells and is SO thin that it CANNOT be seen with naked eye. In the diagram below it is represented by the line that looks like it is the surface of the brain. The pia mater is intimately adherent to surface of central nervous system and follows all "nooks and crannies". Unlike the arachnoid, this layer is highly vascular. In fact a "sleeve" of pia mater surrounds blood vessels that penetrate into substance of central nervous system for a variable distance forming a perivascular space.
