Identify the Passageway Found in the Spinal Cord That is Continuous With the Ventricles

Ventricles and Coverings of the Brain
by Annie Burke-Doe, PT, MPT, PhD
Practicing Physical Therapist and Associate Professor, University of St. Augustine for Health Sciences, San Diego, California Slide 1: Ventricles and Coverings of the Brain

Hi, and welcome to neural anatomy and physical therapy. I'm Dr. Annie Burke-Doe, a practicing physical therapist and an associate professor at the University of St. Augustine for Health Sciences in San Diego, California. This lecture series has been developed for physical therapists embarking on the study of neurology. In this lecture, we will be looking at the brain in relation to coverings and protection including the bony cranial vault, or skull; the meninges, which are the three layers of connective tissue membranes; and the formation of cerebrospinal fluid that provides support and nutrition. We will also look at the important clinical disorders of this system and the implication for the rehabilitation professional. Slide 2: Cranial Vault

Here in Slide 2, we are looking at the basal view of the skull from the outside or external aspect. Remember, our brain is encased in protective layers that cushion it from trauma. These layers include the skin, the subcutaneous tissue, the bony cranium itself, and the meninges. Beneath the skin and the subcutaneous tissue lies the hard bones that form the skull or cranium. The skull has many foramina, or openings, that allow cranial nerves, the spinal cord, and blood vessels all to enter and leave that intracranial cavity. We will cover the foramina in more detail in our lecture on cranial nerves, but here depicted are just a few. The largest foramena is at the base of the skull, and it's called the foramen magnum. It can be easily seen in imaging of the skull. The foramen magnum is the point where the spinal cord meets the medulla, and as we have discussed earlier, it's called the cervicomedullary junction. We can also see here the jugular foramena. It can be divided into three compartments, and it carries a number of vessels and a number of cranial nerves. We can also see the foramena ovale, which carries a branch of the fifth cranial nerve or the trigeminal nerve, and finally, the carotid canal carrying the internal carotid artery. Slide 3: Base of Skull, Internal Aspect

On the inner surface of the skull, there are several ridges of bone that divide the base of the cranial cavity into different compartments. We call those fossa. The anterior fossa on each side contains the frontal lobe. The middle fossa on each side contains the temporal lobe. The posterior fossa contains the cerebellum and the brain stem. The anterior fossa is divided from the middle fossa by the lesser wing of the sphenoid bone. The middle fossa is divided from the posterior fossa by a ridge of the temporal bone, as well as by a sheet of meninges, which we will describe next. Slide 4: The Cranial Meninges

The final layers of protection within the skull and surrounding the brain are the meninges and the cerebrospinal fluid. The three layers of the meninges from the inside to the outside are pia, arachnoid, and dura. There is a pneumonic that can be used for the meningeal layers: it's PAD or pad. You may sometimes see the term "mater," meaning mother, to accompany the meningeal layer names, for example, the pia mater, the arachnoid mater, and the dura mater. When looking at each layer, the pia mater sticks to the surface of the brain, anchored by astrocytes. It will extend into every fold, and it also accompanies the branches of cerebral blood vessels as they penetrate the surface of the brain. The arachnoid mater is unique in that it consists of an arachnoid membrane and an epithelial layer; and the cells and fibers of the arachnoid trabeculae that cross the subarachnoid space extend between the arachnoid membrane and the pia mater. This arachnoid layer is very wispy and is similar to a spider's web. This layer adheres to the inner surface of the dura. Within the arachnoid, the cerebrospinal fluid will bathe and percolate over the surface of the brain. And finally, the dura mater: dura, which means "hard," consists of an outer and an inner fibrous layer. Dura feels very thick and tough. The outer layer itself is fused to the periosteum of the cranial bones. As a result, there is no epidural space superficial to the dura mater, which does occur along the spinal cord. The outer or endosteal and inner or meningeal layers of the cranial dura mater are typically separated by a slender gap that contains tissue fluids and blood vessels, including several large venous sinuses. The veins of the brain open into these sinuses, which will deliver the venous blood to the internal jugular veins of the neck. Slide 5: The Cranial Meninges (Cont.)

As we continue to examine the cranial meninges, we see that in several locations the inner layer of the dura mater will extend into the cranial cavity forming sheets called dural folds. The sheets will dip inward and then return to the surface. These dural folds provide additional stabilization and support the brain; much like a seat belt provides a support structure for a person in a car. There are also dural sinuses, such as the transverse sinus pictured here on the right lower section of the slide. Sinuses are large collecting veins that are located inside dural folds. The three largest dural folds are the falx cerebri, the tentorium cerebelli, and the falx cerebelli. When you are in your wet lab, you will be able to easily see these large dural folds attached to the dura mater. We will begin with the falx cerebri, meaning curve or sickle shape. It is a fold of dura mater that is going to project between the right and left halves of the cerebral hemispheres in the longitudinal fissure. Its inferior portion, the deep portion, is attached anteriorly to the crista galli. The crista galli is a median ridge of the ethmoid bone, which separates the nasal cavity from the brain. It also attaches posteriorly to the internal occipital crest, which is a ridge along the inner surface of the occipital bone. The tentorium cerebelli—tentorium meaning "a covering"—protects the cerebellar hemispheres, which are in the back of the brain, and separates them from those of the cerebrum. It extends across the cranium at right angles to the falx cerebri. The transverse sinus, which is depicted, lies within the tentorium cerebelli. And finally, the falx cerebelli will divide the two cerebral hemispheres along the midsagittal line inferiorly or below the tentorium cerebelli. Slide 6: The Cranial Meninges (Cont.)

As we continue to look at the cranial meninges, these meninges form three important spaces that carry critical blood vessels that can potentially, if damaged, cause bleeding or hemorrhaging into each of these spaces. The epidural space is considered a potential space, because typically the dura would tightly adhere to the cranium. The epidural space is located between the inner surface of the skull and the dura. The middle meningeal artery is an example of a vessel that enters the skull through a foramina, specifically, the foramen spinosum, and it runs into the epidural space between the dura and the skull. Often, you can see grooves on the inner surface of the skull that are formed by this artery and its many branches. The subdural space is the space between the inner layer of the dura and the loosely adherent arachnoid mater. There are veins that bridge this subdural space. These veins drain the cerebral hemispheres and pass through the subdural space en route to several large dural venous sinuses. Those dural sinuses are large venous channels that lie enclosed within the dural layers. The subarachnoid space is the space between the arachnoid and the pia, and it is filled with our cerebrospinal fluid. So, as you can imagine, some kind of trauma to the head can cause bleeding into these spaces, creating pressure and damage in this enclosed vault of the cranium. Slide 7: Ventricles of the Brain

As you may remember, during embryologic development the neural tubes form several cavities within the brain that we call ventricles. The ventricles contain our cerebrospinal fluid or CSF, which is produced by a specialized vascular structure called choroid plexus, meaning "vascular coat." The choroid plexus lies inside the ventricles. For example, in the lateral ventricles, the choroid plexus lies on the floor of the ventricles. The septum pellucidum is a thin partition in the middle of the two lateral ventricles that separates them from each other. Because there are two lateral ventricles, the ventricle in the diencephalon is called the third ventricle. The two lateral ventricles are not directly connected but each communicates with the third ventricle through a passageway called the interventricular foramen or foramen of Monro. The mesencephalon has a slender canal know as the aqueduct of the membrane, also called the mesencephalic aqueduct or the aqueduct of Sylvius. This passageway connects the third ventricle to the fourth ventricle. The surrounding portion of the fourth ventricle lies between the posterior surface of the pons and the anterior surface of the cerebellum. The fourth ventricle extends into the superior portion of the medulla oblongata, and this ventricle will then become narrower and continuous with the central canal of the spinal cord. Now may be a good time for review. What composes the diencephalon, or what composes the mesencephalon? Slide 8: Ventricles (Cont.)

Here we are looking at a sagittal view of the ventricles of the brain. Remember that the largest of the ventricles are the two lateral ventricles, and the lateral ventricles have extensions called horns that are named after the lobes or after the direction in which they extend. The frontal or anterior horn of the lateral ventricles extends anteriorly from the body of the lateral ventricles into the frontal lobe. By definition, the ventral horn begins anterior to the intraventricular foramen of Monro. The body of the lateral ventricle merges posteriorly with the atrium or trigone. The atrium connects three parts of the lateral ventricle: the body; the occipital or posterior horn, which extends back into the occipital lobe; and the temporal or inferior horn, which extends inferiorly or anteriorly into the temporal lobe. The lateral ventricles communicate with the third ventricle, here depicted via that intraventricular foramen of Monro. The third ventricle communicates with the fourth ventricle via the cerebral aqueduct, also called the aqueduct of Sylvius, which travels through our midbrain. And finally, the fourth ventricle is formed by the cerebellum, and the floor is formed by the pons and the medulla. Slide 9: Cerebrospinal Fluid (CSF)

Let's take a look at the function of cerebrospinal fluid. Remember that cerebrospinal fluid completely surrounds and bathes the exposed surfaces of the central nervous system, and cerebrospinal fluid has several important functions including the following: It cushions neural structures, which are typically very delicate. It helps support the brain. In essence, the brain is suspended in the cranium and floating in the cerebrospinal fluid. A human brain weighs about 1400 grams or 3.9 pounds in air, but only about 50 grams or 1.8 ounces when supported by our cerebrospinal fluid. Another function of CSF is to transport nutrients, chemical messengers, and waste products. This will occur except at the choroid plexus where CSF is produced. That ependymal lining is freely permeable, and the CSF is in constant chemical communication with interstitial fluid that surrounds the neurons and the neuroglia. Because free exchange occurs between the interstitial fluid of the brain and the CSF, any changes in central nervous system function can produce changes in the composition of our cerebrospinal fluid. Clinically, a spinal tap can be used by injecting a needle into our subarachnoid space to determine the contents of the cerebrospinal fluid. Slide 10: CSF Formation and Circulation

Here you can see the direction of flow for cerebrospinal fluid. Remember that the choroid plexus, depicted in red, consists of a combination of specialized ependymal cells and permeable capillaries involved in the production of CSF. There are two extensive folds of choroid plexus; one which covers the floor of the lateral ventricles, and the other originates in the roof of the third ventricle which also extends through the interventricular foramina. In the inferior brain stem, a region of choroid plexus in the roof of the fourth ventricle projects between the cerebellum and the pons. The specialized ependymal cells are interconnected by tight junctions, which surround the capillaries of the choroid plexus. These ependymal cells secrete CSF into the ventricles, about 500 ml over a 24-hour period, and at any given moment, we have about 150 ml surrounding our brain. The entire volume of CSF is replaced every eight hours, and removing waste products and adjusting its composition over time are essential. There is a difference between the composition of CSF and blood plasma. Remember, blood plasma is blood with cellular elements removed. These differences are quite pronounced. For example, blood contains higher concentrations of soluble proteins but CSF does not. The concentrations of individual ions, amino acids, lipids, and waste products are also different between CSF and blood plasma. CSF leaves the ventricular system of the brain via several foramina in the roof of the fourth ventricle; these are called the lateral aperture or foramen of Luschka and the middle aperture or foramen of Magendie. The CSF then bathes the brain, the spinal cord, and the cauda equina in that subarachnoid space and is ultimately reabsorbed by the arachnoid granulations, or fingerlike extensions into the dural venous sinuses, and thus back into the bloodstream. Slide 11: Arachnoid Granulations

We can see here the fingerlike extensions of the arachnoid membrane called arachnoid villi. These villi penetrate the meningeal layer of the dura mater and extend into our superior sagittal sinus. In adults, these clusters of villi form large arachnoid granulations, and CSF is absorbed into the venous circulation at these arachnoid granulations. Slide 12: The Blood-Brain Barrier

Anatomists have known for nearly a century that when a colored dye is injected into the arterial bloodstream of an animal, all of its organs become stained except the brain. This indicated that the neural tissue was somehow isolated from circulation. The reason is that capillary endothelial walls of most of the body are separated by larger clefs or fenestration allowing relatively free permeability of fluid. In the brain, however, capillary endothelial cells, which we can see here on the left, are linked by tight junctions, and substances entering or leaving the brain must travel through those endothelial cells but, in order to do so, they need to use an active transport process. These endothelial cells and the tight junctions between them form what we call the blood-brain barrier. A similar barrier also exists between choroid plexus and the CSF, referred to as the blood-CSF barrier. Remember that the capillaries in the choroid plexus are freely permeable, but choroid epithelial cells form a barrier between the capillaries and the CSF, depicted here on the right. Lipid-soluble substances including oxygen and carbon dioxide can permeate readily across the cell membrane of the blood-brain barrier and the CSF barrier; however, most substances must be conveyed in both directions through specialized transport systems. These may include active transport, facilitated transport, ion exchange, and ion channels. Clinically, if this normal circulation or reabsorption of CSF is interrupted, a variety of clinical problems can occur. Some examples include: extravasation or leakage of fluid out into the extracellular space producing vasogenic edema; compression of the ventricular system causing hydrocephalus or water in the brain; and lesions that can create abnormal electrical discharge in the cortex and provoke seizures. Slide 13: Disorders

Let's go ahead and take a look at some important clinical abnormalities that can involve these structures. These may include headache, intracranial masses, elevated intracranial pressure, brain herniation, hemorrhaging, hydrocephalus, tumors, and infections. Slide 14: Intracranial Masses

Anything that abnormally occupies the volume within the cranial vault functions as an intracranial mass. Mass lesions can produce both local tissue damage and remote effects through the mechanical distortion of adjacent structures. We use the term "mass effect" as a descriptive for any distortion of normal brain geometry due to a mass lesion. Mass effect can be as subtle as a mild flattening or thinning of sulci next to a lesion, and it may produce no symptoms. Depending on the location or size of a mass, it can produce neurologic abnormalities. For example, a mass lesion in the primary motor cortex will cause contralateral or opposite-sided weakness; a mass lesion may compress blood vessels, which can cause an ischemic infarct; or a mass lesion may lead to an erosion through blood vessels and can cause hemorrhage. As we look at our picture here on the slide, a tumor, hemorrhage, or edema can cause a mass displacement of intracranial structures. It can be severe enough to push structures from one compartment to another. Brain herniations represent a shift of the normal brain through or across regions to another site, due to a mass effect. These are generally complications of mass effect, whether from tumor, trauma, or infections. Herniations of the brain can be divided into categories, and some examples are shown here: One is a cingulate herniation under the falx cerebelli; two, a downward transtentorial herniation or a central herniation, which is a downward displacement of the brain; three, an uncal herniation over the edge of the tentorium; or four, a cerebellar tonsillar herniation downward into the foramen magnum. Coma and ultimately death can result when a downward transtentorial herniation, an uncal herniation, or a cerebellar tonsillar herniation produces brain stem compression. Slide 15: Elevated Intracranial Pressure

The contents of the intracranial space are confined by the hard walls of the bony cranium. Increasing any one of the contents, such as brain tissue, blood, cerebrospinal fluid, or other brain fluids, will produce increased intracranial pressure. Whenever there is a space-occupying lesion within the skull, something must leave the skull to accommodate that extra volume. Small lesions will often be accommodated by a decrease in intracranial cerebrospinal fluid pressure and blood without causing a rise in the intracranial pressure, but larger lesions overcome this compensatory mechanism, and the intracranial pressure will steeply rise. Here on this slide we are seeing a list of some common signs and symptoms that a person may complain of as intracranial pressure rises—things like a change in mental status, having a headache, nausea, or vomiting. Slide 16: Treatment

The treatment of increased intracranial pressure is varied and would depend on the causative nature of the damage and the response of the patient to treatment. Here is a list of some examples, such as elevating the head of the bed to promote venous drainage; intubating and hyperventilating thus increasing CO₂ to promote vasoconstriction; medication to decrease edema; starting ventricular drainage to decrease pressure; inducing a coma to reduce metabolic demand; and performing a craniectomy to decompress the intracranial cavity. These are just some examples of what can be done by the medical profession to reduce damage to the brain. Slide 17: Hydrocephalus

Hydrocephalus, meaning "water on the brain," is caused by excessive cerebrospinal fluid in the intracranial cavity. The condition can result from abnormal production of CSF that occurs in rare tumors; a tumor or a bleeding that caused obstruction of flow of the cerebrospinal fluid; or a decrease in the reabsorption of CSF when arachnoid granulations are damaged or clogged, causing increased water on the brain. Sometimes, seen in the elderly, normal-pressure hydrocephalus is characterized by chronically-dilated ventricles. Patients with normal pressure typically present with a clinical triad of difficulties: gait abnormalities, urinary incontinence, and mental decline. Measurements of CSF pressure are not usually elevated or are intermittently elevated. It's really the clinical signs and symptoms and outward change that potentially could indicate normal pressure hydrocephalus, and these should be on a rehabilitation professional's list of differential diagnoses. Treatment of hydrocephalus usually involves a procedure that allows the CSF to bypass the obstruction and drain from the ventricles. In an external ventricular drain, also called a ventriculostomy, the fluid from the lateral ventricles is drained via a bag outside the head. A more permanent treatment is a ventriculoperitoneal shunt, in which the shunt tubing passes from the lateral ventricle out of the skull and is then tunneled under the skin to drain into the peritoneal cavity of the abdomen. A valve prevents flow of fluid in the reverse direction from the abdomen to the ventricle. Slide 18: Traumatic Brain Injury

Traumatic brain injury or head trauma is a common cause of morbidity and mortality, especially in young adults and the adolescent population. Mild head trauma will cause a concussion, which is defined as a reversible impairment of neurologic function, minutes to hours after the injury. The mechanism of concussion is unknown, but it's thought to be a transient neuronal dysfunction. Clinical features would include loss of consciousness, seeing "stars," followed by headache, dizziness, and occasionally nausea and vomiting. We might see this on a football field when two players hit heads; one goes down, we see them not move for a few seconds, and then they wake up. Diffuse axonal injury—more widespread and patchy damage to white matter and cranial nerves—would be another cause of traumatic brain injury. Of course, with hemorrhaging or bleeding, you can have petechial or small spots of blood in white matter or larger intracranial hemorrhages. They can be traumatic or atraumatic, maybe due to some medical condition. A cerebral contusion and direct tissue injury usually is a result of some kind of penetrating trauma: open skull fractures and gunshot wounds as examples. Cerebral edema can also occur with or without other injuries and will contribute to elevated intracranial pressures. Epidural hematomas, subdural hematomas, subarachnoid hemorrhages, and intracranial or intraparenchymal hemorrhages can all cause problems for brain tissue as mass effects that we discussed previously. Slide 19: Infections

Infections of the central nervous system can also cause debilitating effects. Bacterial infections caused by cocci and bacilli include bacterial meningitis, brain abscess, and epidural abscess. Bacteria most often gain access to the CNS through the bloodstream, and frequently they originate from infections elsewhere in the body, such as the lungs or the heart. In addition, infections can spread by a direct extension from the oronasal passages and from trauma or surgery. Infectious meningitis is an infection of the CSF in the subarachnoid space. It can be caused by bacteria, viruses, fungi, or parasites. Except for in the elderly, very young, or immunosuppressed patients, infectious meningitis is usually heralded by marked signs and symptoms of meningeal irritation or meninges. Meningism is a triad of nuchal rigidity or neck stiffness, photophobia or intolerance of light, and headache. Meningismus is a term used when the above listed symptoms are present without actual infection or inflammation. It is usually seen with other acute illnesses in the pediatric population. Depending on the cause of meningeal irritation, the onset of symptoms can be gradual—over weeks to months, as in the case of fungal or parasitic infections—or sometimes quite rapid, occurring within hours in the case of many bacterial infections. Diagnosis will be made by clinical evaluation, as well as sampling of the CSF by lumbar puncture or spinal tap. Brain abscess is another important bacterial infection of the nervous system. It presents as an expanding intracranial mass lesion, much like a brain tumor but often more rapid. Common presenting features include headache, lethargy, fever, nuchal rigidity, nausea, vomiting, seizures, and focal signs determined by the location of the abscess. Viral meningitis tends to be less fulminant than bacterial, and the person will usually recover in a shorter time. That patient will present with headache, fever, nuchal rigidity, and other signs of meningeal irritation. Common causes include enteroviruses, such as coxsackievirus and mumps virus. Parasitic infections that involve the nervous system include toxoplasmosis, malaria, and African sleeping sickness, as examples. In recent years, a novel protein-based infectious agent called the prion has been identified in certain neurological disorders. Prions are unique in their ability to transmit illnesses from one animal to another, despite the fact they apparently do not contain DNA or RNA. Pathologically, diffuse degeneration of the brain and spinal cord occurs with multiple vacuoles, resulting in a spongy appearance. Human prion diseases include Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker disease, Kuru, and fatal familial insomnia. These disorders are all relatively rare. Treatment for infections is typically based on the cause or etiology of the disorder. Slide 20: Lumbar Puncture

Lumbar puncture is an important procedure that provides direct access to the subarachnoid space of the lumbar cistern. It can be used to obtain CSF samples, measure CSF pressure, remove CSF in the case of suspected normal pressure hydrocephalus, and occasionally to introduce drugs. Lumbar puncture is performed with sterile technique under local anesthesia. A hollow needle is introduced through the skin with a stylus occluding the opening or the lumen to prevent the introduction of skin cells into the CSF during needle insertion. The needle will pass through the subcutaneous tissues, the ligaments of the spinal column, the dura, and the arachnoid to finally encounter the CSF in the subarachnoid space of the lumbar cisterna. The bottom portion of the spinal cord, or conus medullaris, ends at about L1 or L2 level of the vertebral bones, and the nerve roots continue down into the lumbar cistern forming the cauda equina, meaning "horse's tail." To avoid hitting the spinal cord, the spinal needle is generally inserted at the spaces between L4 and L5 vertebral bones. As the tip of the needle enters the subarachnoid space, the nerve roots are usually harmlessly displaced. The posterior iliac crest serves as a landmark to determine the approximate level of L4/L5 interspace. Slide 21: Clinical Case 1

The following clinical cases have been developed for your review. They contain subject matter that is clinically related and will reinforce the lecture content in each slide series. The questions for the case follow the introductory case slide, and the discussion for the case is in the slide notes. I recommend not looking for the answer in the discussion section until you have attempted to answer the question on your own. Good luck, and I will see you in the next topic. Slide 22: Clinical Case 1 (Cont.)