The second major division of the forebrain is the diencephalon (Fig. 10-3), which consists of the thalamus and the hypothalamus. The diencephalon also contains the third ventricle and is almost completely covered by the cerebral hemispheres. This portion of the brain has a primary role in sleep, emotion, thermoregulation, autonomic activity, and endocrine control of ongoing behavioral patterns.

FIG. 10-3 The diencephalon and its boundaries. CC, Corpus callosum.

(From Fitzgerald MJT, et al: Clinical neuroanatomy and neuroscience, ed 6, St. Louis, 2011, Saunders.)

The thalamus consists of right and left egg-shaped masses, which compose the greatest bulk of the diencephalon and form the lateral wall of the third ventricle. Each thalamus serves as a relay center for all incoming sensory stimuli, except for taste. These impulses are then grouped and transmitted to the appropriate area of the cerebral cortex. Because of its interconnections with the hypothalamus, the limbic system, and the frontal, temporal, and parietal lobes, this structure is also integrally involved with emotional activities, instinctive responses, and attentive processes.

The hypothalamus is a group of bilateral nuclei that forms the floor and part of the lateral walls of the third ventricle. Extremely complex in function, the hypothalamus has extensive connections with the autonomic nervous system and with other parts of the CNS. It also influences the endocrine system by virtue of direct and indirect connections with the pituitary gland and the release of its own hormones. In association with these other structures, the hypothalamus participates in the regulation of appetite, water balance, carbohydrate and fat metabolism, growth, sexual maturity, body temperature, pulse rate, blood pressure, sleep, and aspects of emotional behavior. Because of the connection of the hypothalamus with the thalamus and cerebral cortex, emotions can influence visceral responses on certain occasions.⁶

Midbrain

The midbrain, or mesencephalon, is a short narrow segment of nervous tissue that connects the forebrain with the hindbrain. The midbrain is vital as a conduction pathway and as a reflex control center. Passing through the center of the midbrain is the cerebral aqueduct, a narrow canal that serves to connect the third ventricle of the diencephalon with the fourth ventricle of the hindbrain for the circulation of CSF. In addition, cranial nerves III (oculomotor) and IV (trochlear) originate in the ventral aspect of the midbrain.⁷

Hindbrain

The hindbrain, or rhombencephalon, consists of the pons, the medulla oblongata, the cerebellum, and the fourth ventricle (Fig. 10-4).

FIG. 10-4 Ventral view of the brainstem in situ.

(From Fitzgerald MJT, et al: Clinical neuroanatomy and neuroscience, ed 6, St. Louis, 2011, Saunders.)

Pons

The pons is literally the bridge between the midbrain and the medulla oblongata as it lies in front of the fourth ventricle and separates it from the cerebellum. It receives many ascending and descending fibers en route to other points in the CNS. The pons also contains the motor and sensory nuclei of cranial nerves V (trigeminal), VI (abducens), VII (facial), and VIII (acoustic). The roof of the pons contains a portion of the reticular formation, and the lower pons assists in the regulation of respiration.

Medulla oblongata

The medulla oblongata is an expanded continuation of the spinal cord and is located between the base of the skull, the foramen magnum, and the pons. It is anatomically complex and not usually amenable to surgery. Many of the white fiber tracts between the brain and spinal cord decussate as they pass through the medulla. Centers for many complex reflexes are located in the medulla oblongata and include those for swallowing, vomiting, coughing, and sneezing. The originating nuclei of cranial nerves IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal) are found in the medulla oblongata (Table 10-1). Because of these originating nuclei, the medulla has an essential role in the regulation of cardiac, respiratory, and vasomotor reflexes. Injuries to the medulla, such as those that accompany basal skull fracture, often prove fatal.

Table 10-1 Cranial Nerves and Their Functions

Cerebellum

The cerebellum overlaps the pons and the medulla oblongata dorsally and is located just below the occipital lobes of the cerebrum. It is separated from the cerebrum by the tentorium, a folded layer of the dura mater. Structurally, the cerebellum comprises two hemispheres with a constricted central portion with a bilayered cortex composed of gray matter. Beneath the gray matter are white fiber tracts that extend like branches of a tree to all parts of the cerebellar cortex. Deep within the white matter are masses of gray matter called the cerebellar nuclei. These nuclei connect the cerebellar hemispheres with each other and with areas in the cerebrum, the hindbrain, and the spinal cord.

The cerebellum has no sensory function and does not initiate movement as the cerebrum does. Functionally, it coordinates muscle tone and voluntary movements through important connections via the spinal cord with the proprioceptor nerve fibers in skeletal muscles, tendons, and joints. In addition, the cerebellum is involved in reflexes necessary for the maintenance of equilibrium and posture, through its connections with the vestibular apparatus of the inner ear. The cerebellum also receives optic and acoustic information, but the specifics of the anatomic pathways involved have not yet been discerned.

Damage to the cerebellum does not result in paralysis or sensory loss. The outcome of damage depends on which portion of the structure is involved. Damage to one part can result in loss of balance, nystagmus, and a reeling gait (cerebellar ataxia). Damage to another area can cause disturbances in the postural reflexes. Posterior lobe disturbances result in changes in voluntary movements, such as discrepancies in force, direction, and range of movements; lack of precision in movements; and, possibly, intention tremors.

Fourth ventricle

The fourth ventricle is a diamond-shaped space located between the cerebellum posteriorly and the pons and medulla oblongata anteriorly. The ventricle contains CSF.

Brainstem

Authors disagree to some extent as to what structures collectively constitute the brainstem. All agree that it includes the midbrain, the pons, and the medulla oblongata. Some believe that the diencephalon rightly belongs in the group. Whichever grouping is used, all functions of each structure within it may be considered to be basic activities of the brainstem. All the cranial nerves are attached to the brainstem (if the diencephalon is included), with the exception of the olfactory nerve and the spinal portion of the accessory nerve.

Reticular formation

The reticular formation lies within the brainstem (including the diencephalon). An important function of the reticular formation is its action as an intermediary between the upper and lower motor neurons of the extrapyramidal system. In this way, the reticular formation facilitates or augments reflex activity and voluntary movements. Its motor neurons can be excitatory or inhibitory in action. For example, with inhibition of extensor muscles, it facilitates the action of flexor muscles.

Every pathway that carries information to the brain also contributes afferent fibers to the reticular formation, so that it is kept well informed about conditions of both the outside world and the internal organs. Efferent impulses that leave the reticular formation travel to the cerebral cortex and to the spinal cord. By virtue of its location in and connections with the brainstem and diencephalon, the reticular formation participates integrally in their activities.

Another important function of the reticular formation is the activation and regulation of those brain activities related to attention arousal and consciousness. For this reason, it is often called the reticular activating system.

Damage to the reticular formation results in greatly decreased levels of consciousness. When the cerebral cortex is isolated from the reticular activating system by disease or injury of the upper portion of the midbrain, decerebrate posturing occurs. This stereotypical posturing involves the arms and legs being held straight out, the toes being pointed downward, and the head and neck being arched backwards. This abnormal posturing results from the dominant effect of the extensor muscles and a lack of inhibition from opposing motor neurons and flexor muscles. The rigidity is accompanied by a profoundly reduced level of consciousness.

Protection of the brain

The brain is protected by the cranial bones, the meninges, and the CSF (Figs. 10-5 and 10-6).

FIG. 10-5 Lateral view of skull, showing relationship of skull and cervical vertebrae to face.

FIG. 10-6 Interior of cranial cavity.

Cranial bones

Eight cranial bones encase the brain and support and protect it from most ordinary bumps and jarring. In the adult, immovable fibrous joints, or sutures, fuse these bones together to form the rigid walls of the box known as the cranium. The base of the cranium is both thicker and stronger than its roof or walls.

The bones of the cranium are the frontal, right and left parietal, occipital, sphenoid, ethmoid, and right and left temporal bones. The frontal bone forms the anterior roof of the skull and the forehead. Within the frontal bone are the frontal sinuses, which communicate with the nasal cavities. The parietal bones form much of the top and sides of the cranium. The occipital bone forms the back and a large portion of the base of the skull. The two temporal bones are complicated and form part of the sides and a part of the base of the skull. Their inner surfaces are not as smooth and regular as the bones previously mentioned. Parts of the temporal bones articulate with the condyles of the lower jaw, and air cells in the mastoid portions of the temporal bones communicate with the middle ear. The sphenoid bone occupies a central portion of the floor of the skull. It alone articulates with each of the other cranial bones. Its middle portion contains the sphenoid sinuses, which open into the nasal cavity. The upper portion of the sphenoid bone has a marked saddlelike depression, the sella turcica, which holds the pituitary gland. The ethmoid bone is light and has a spongy structure. It is located between the orbital cavities and is a cribriform plate that forms the roof of the nasal cavity and part of the base of the cranium. The ethmoid sinuses open into the nasal cavities.

Several features of the cranial bones are particularly noteworthy for the PACU nurse. Among these features is the fact that the air cells in the mastoid portion of the temporal bone may become infected from otitis media or after surgery on the middle or inner ear. This mastoiditis can cause severe complications if it extends through the thin plate of bone that separates it from the cranial meninges. Another point of interest is that surgical access to the pituitary gland is commonly accomplished through the sphenoid bone via the nostrils; one example is transsphenoidal hypophysectomy. Finally, nasal suctioning is absolutely contraindicated in the patient with cranial surgery because of the danger of perforation of the cribriform plate of the ethmoid bone, which results in leakage of CSF and permits direct access to the brain by infectious organisms.

One main opening is located at the base of the skull and is called the foramen magnum. It marks the point at which the brainstem changes structure and becomes identified inferiorly as the spinal cord. Many smaller openings in the skull allow the cranial nerves and some blood vessels to pass through it to and from the face, the jaw, and the neck. The atlas of the vertebral column (C₁) supports the skull and forms a moveable joint with the occipital bone.⁷

Meninges

The meninges (Fig. 10-7) are three fibrous membranes between the skull and the brain and between the vertebral column and the spinal cord. The outer membrane is the dura mater and the inner is the pia mater; between them lies the arachnoid mater.

FIG. 10-7 Arrangement of meninges in the cranial cavity.

(From Drake RL, et al: Gray’s anatomy for students, ed 2, Philadelphia, 2010, Churchill Livingstone.)

Dura mater.

The dura mater is a shiny, tough, inelastic membrane that envelops and supports the brain and spinal cord and, by various folds, separates parts of the brain into adjoining compartments. The portion within the skull differs from the dura of the spinal cord in three ways. First, the cranial dura is firmly attached to the skull. The spinal dura has no attachment to the vertebrae. Second, the cranial dura consists of two layers; it covers the brain (meningeal dura) and lines the interior of the skull bones (periosteal dura). Third, the two layers of the cranial dura are in contact with each other in some places but separate in others where the inner layer dips inward to form the protective partitions between parts of the brain. In addition, the spaces or channels formed by these separations of dural layers are filled with venous blood that is leaving the brain; these spaces are called cranial venous sinuses and are an elaborate network unique to the brain (see Fig. 10-7).

There are three major partitioning folds of the meningeal dura. The falx cerebri separates the right and left hemispheres of the cerebrum. The tentorium cerebelli supports and separates the occipital lobes of the cerebrum from the cerebellum. The falx cerebelli separates the two cerebellar hemispheres. The tentorium separates the posterior cranial chamber from the remainder of the cranial cavity and serves as a line of demarcation for describing the site of a surgical procedure or a lesion as either supratentorial or infratentorial.

Encased between the two dural layers are two major groups of venous channels that drain blood from the brain. None of these vascular channels possesses valves, and their walls are extremely thin because of the absence of muscular tissue. The superior-posterior group consists of one paired and four unpaired sinuses. The anterior-inferior group consists of four paired sinuses and one plexus. The sinuses function to drain venous blood into the internal jugular veins, which are the principal vessels responsible for the return of the blood from the brain to the heart (Fig. 10-8).

FIG. 10-8 Venous drainage of brain, head, and neck.

Arachnoid mater.

The arachnoid is a fine membrane between the dura mater and the pia mater. Between the arachnoid and the dura is the subdural space, a noncommunicating space filled with CSF. The cerebral blood vessels that traverse this space have little supporting structure, which makes them particularly vulnerable to injury at this point.

The arachnoid forms a type of roof over the pia mater, to which it is joined by a network of trabeculae in the subarachnoid space. It does not follow the depressions of the surface architecture. The arachnoid sends small, tuftlike extensions through the meningeal layer of the dura into the cranial venous sinuses. These extensions are called the arachnoid granulations or arachnoid villi. The arachnoid villi serve as a pathway for the return of CSF to the venous blood system. Subarachnoid CSF is most abundant in the grooves between the gyri, particularly at the base of the brain, where the more freely communicating compartments form six subarachnoid cisternae, or reservoirs.

Pia mater.

The inner layer of the meninges, the pia mater, is a fine membrane rich in blood (choroid) plexuses and mesothelial cells. This layer is closely associated with the arachnoid and covers the brain intimately, following the invaginations and convolutions of the brain surface. The veins of the brain lie between threadlike trabeculae in the subarachnoid space. Branches of the cortical arteries in the subarachnoid space are carried with the pia mater and enter the brain substance itself.⁵

Cerebrospinal fluid system

The CSF is a clear colorless watery fluid with a specific gravity of 1.007. A principal function of this fluid is to act as a cushion for the brain. Because both brain tissue and CSF have essentially the same specific gravity, the brain literally floats within the skull. CSF also serves as a medium for the exchange of nutrients and waste products between the blood stream and the cells of the CNS.

Cerebrospinal fluid is found within the ventricles of the brain, in the cisterns that surround it, and in the subarachnoid spaces of both the brain and the spinal cord (Figs. 10-9 and 10-10). The largest of the cisterns is the cisterna magna, which is located beneath and behind the cerebellum.

FIG. 10-9 Circulation of cerebrospinal fluid.

(From Fitzgerald MJT, et al: Clinical neuroanatomy and neuroscience, ed 6, St. Louis, 2011, Saunders.)

FIG. 10-10 Ventricular system, lateral and superior views.

Although some CSF is formed by filtration through capillary walls throughout the brain’s vascular bed, its primary site of formation is in the choroid plexuses within the ventricles. This formation is achieved with a system of secretion and diffusion. The choroid plexuses are highly vascular, tufted structures composed of many small granular pouches that project into the ventricles of the brain. CSF is formed continuously and is reabsorbed at a rate of approximately 750 mL per day. The net pressure of the CSF is regulated in part by a balance between formation and reabsorption.

The four ventricles of the brain communicate directly with each other. The first and second (lateral) ventricles are elongated cavities that lie within the cerebral hemispheres. The third ventricle is a slitlike cavity beneath and between the two lateral ventricles. The fourth ventricle is a diamond-shaped space between the cerebellum posteriorly and the pons and medulla anteriorly.

The main route of reabsorption of excess CSF is through the arachnoid villi that project from the subarachnoid spaces into the venous sinuses of the brain, particularly those of the superior sagittal sinus. The arachnoid villi provide highly permeable regions that allow free passage of CSF, including protein molecules and some small particulate matter contained within it. The process of osmosis is believed to be mainly responsible for the reabsorption of the fluid.⁶

Blood-brain barriers of the central nervous system

The amount and rate of diffusion of fluids and dissolved substances across capillary membranes from the plasma into the extracellular fluid surrounding cells differs from tissue to tissue. This difference is largely a function of the structural differences between capillaries found in the different tissues and reflects the physiologic function of the tissue. For example, capillaries within the liver are highly porous, creating little hindrance to the movement of fluid, and dissolve substances such as drugs and proteins between plasma in the extracellular environment of the liver tissue; this reflects the function of the liver as an organ responsible for the synthesis of many substances and the metabolism of many others.

On the other extreme of the spectrum is brain tissue. Brain tissue is highly sensitive to changes in its extracellular environment and the introduction of foreign or unusual substances. To facilitate the maintenance of this uniquely balanced environment, the body has evolved a blood-brain barrier to tightly regulate what may enter the extracellular environment of the brain from the capillaries.

The site of the blood-brain barrier is not at the surface of the neurons themselves. Rather, it is a series of special adaptations to the capillaries present in the cerebral circulation. These adaptations form a series of physical barriers that act together to prevent the normally rapid transport of substances from the blood to the nervous tissue. These barriers include the tight intercellular junctions between the epithelial cells of the capillaries that appear to effectively reduce permeability. A substantial basement membrane surrounds the capillaries, and an external membrane is provided by the end-feet of the astrocytes between the neurons and the capillaries. This series of physical barriers plays a major role in retarding or preventing the passage of foreign substances into the brain tissue. The integrity of the blood-brain barrier and its ability to control what enters the CNS can break down in areas of the brain that are infected, traumatized, or irradiated or contain tumors.

The speed with which substances penetrate the blood-brain barrier is inversely proportional to their molecular size and directly proportional to their lipid solubility. Only water, carbon dioxide, and oxygen cross the blood-brain barrier rapidly and readily, whereas glucose crosses more slowly and by a facilitated transport mechanism. Water-soluble compounds, electrolytes, and large protein molecules generally cross very slowly or not at all. Most general anesthetics effectively cross the blood-brain barrier because of their high lipid solubility.⁸^,⁹

Arterial blood supply to the brain

The entire arterial blood supply to the brain, with the exception of a small amount that flows in the anterior spinal artery to the medulla, is carried through the neck by four vessels: the two vertebral arteries and the two carotid arteries (Figs. 10-11 and 10-12).

FIG. 10-11 Arterial supply to neck and head.

FIG. 10-12 Major arteries as seen on the base of brain.

The two vertebral arteries supply the posterior portion of the brain. They ascend in the neck through the transverse foramina on each side of the cervical vertebrae, enter the skull through the foramen magnum, and join near the pons to form the basilar artery of the hindbrain. A relatively small volume of the total blood flow to the brain is carried by the vertebral or basilar artery. The circle of Willis, in turn, is formed by the union of the basilar artery and the two internal carotid arteries. Before they join the circle of Willis, these arteries send essential branches to the brainstem, cerebellum, and falx cerebelli.

The circle of Willis is a ring of blood vessels that surrounds the optic chiasm and the pituitary stalk. The circle of Willis gives rise to three pairs of arteries: the anterior, the middle, and the posterior cerebral arteries. Each pair of arteries supplies specific areas of the brain: (1) the anterior cerebral arteries supply approximately half of the frontal and parietal lobes, including much of the corpus callosum; (2) the middle cerebral arteries perfuse most of the lateral surfaces of the hemispheres and send off branches to the corpus striatum and the internal capsule; and (3) the posterior cerebral arteries supply the occipital lobes and the remaining portions of the temporal lobes that are not supplied by the middle cerebral arteries.⁷

Regulation of cerebral blood flow

The brain has almost no ability to store nutrients. For this reason, it is dependent on a continuous flow of blood to supply the glucose and oxygen required for normal neuronal cell functioning. Even a brief interruption of blood flow, for as short as few seconds, can result in loss of consciousness. The high rate of metabolic demand associated with neuronal cell functioning also requires a relatively large amount of blood flow. In fact, 15% of the total resting cardiac output goes to the brain, which represents approximately 2% of the total body weight.

The body has several mechanisms to ensure the uninterrupted high rate of blood flow required by the brain. As in other tissues in the body, the brain’s requirement varies with metabolic activity. The more active the brain, the greater the blood supply it requires. Regulation of this blood supply is primarily based upon the concentration of carbon dioxide in the brain tissue. Carbon dioxide is a normal byproduct of neuronal cellular metabolism. The concentration of carbon dioxide in the brain tissue increases as neuronal cellular metabolism increases with an increase in brain activity. This increase in carbon dioxide causes the cerebral vasculature to dilate, increasing blood flow to the brain. The opposite occurs when carbon dioxide concentration declines in the brain. This mechanism is highly sensitive, and even small changes from the normal carbon dioxide levels in the brain can cause significant changes in the volume of cerebral blood flow.

The vessels of the brain will also react to changes in the oxygen concentration of the brain tissue. As the oxygen concentration level falls in the brain, the vasculature will vasodilate to increase cerebral blood flow. An increase in the oxygen level will cause a degree of vasoconstriction. This mechanism is not as sensitive as the one regulated by carbon dioxide. The vasculature changes triggered by alterations in the oxygen concentration do not occur until oxygen levels are significantly increased or decreased from their normal levels.

A third mechanism to maintain blood flow to the brain is called autoregulation. Autoregulation ensures a constant cerebral blood flow to the brain despite the normal fluctuations that occur in blood pressure in the body. It achieves this effect by changing the diameter of the vasculature in response to changes in blood pressure. An increase in the blood pressure causes the cerebral vessels to constrict, whereas a decrease in blood pressure causes them to dilate. Autoregulation is effective over the normal range of blood pressures seen in the body, but when blood pressures become very low (a mean arterial pressure [MAP] less than 50 mm Hg) or very high (MAP greater than 150 mm Hg) the mechanism loses its effectiveness and blood flow to the brain becomes pressure dependent. In addition, autoregulation, like the blood-brain barrier, can be lost in cases of trauma, infection, or tumors in the brain.

Intracranial pressure dynamics

The bones of the skull form a rigid box that surrounds and contains the brain, CSF, and blood that is perfusing the brain. Intracranial pressure (ICP) is the pressure created within the skull by the contents; it usually ranges from 4 to 15 mm Hg. Compensatory mechanisms exist that control for minor variations in the changes of the volumes of blood, CSF, and brain tissue within the skull, keeping the ICP nearly constant. If ICP should increase because of an increase in one or more of the contents of the skull, such as a brain tumor or swelling because of a blow to the head, significant injury can result to the brain from lack of blood flow or death owing to herniation of the brain through the base of the skull.

Cerebral perfusion pressure

Cerebral perfusion pressure (CPP) is essentially the blood pressure of the brain; it is a reflection of the flow of blood to the brain and usually ranges from 90 to 100 mm Hg. In normal conditions, CPP and resultant blood flow are determined by the difference between the inflow and the outflow pressures. Inflow pressures are represented by the MAP, and in normal conditions the mean outflow pressure is equivalent to the central venous pressure. In situations in which ICP is greater than venous pressure, the following equation applies:

From this formula, it is obvious that an increase in ICP or a decrease in MAP can compromise CPP and blood flow to the brain. This situation is a common concern for neurosurgical patients during the perioperative period and a critical concern for PACU nurses. Efforts must be simultaneously made to maintain mean arterial pressure while at the same time control the intracranial pressure. Common steps to control intracranial pressure include decreasing brain activity by administering sedative drugs, controlling blood flow to the brain by decreasing carbon dioxide concentration in the brain by hyperventilating the patient, and maintaining a patient in a semi-Fowler’s position to optimize venous drainage from the brain. Care must always be taken, though, not to sacrifice mean arterial pressure in an attempt to decrease intracranial pressure, as can happen with the overzealous use of diuretics.⁵^,¹⁰

Spinal cord

Protection of the spinal cord

Bones of the spine

The spine is composed of a series of irregular bony vertebrae “stacked” one atop the other to form a strong but flexible column. They are joined by a series of ligaments and intervening cartilages and have two primary functions. Together these structures support the head and trunk. The spine also protects the spinal cord and its 31 pairs of spinal nerve roots by encasing them in a long canal formed by openings in the center of each vertebra. This vertebral canal extends the entire length of the spine and conforms to the various spinal curvatures and to the variations in size of the spinal cord itself.

There are 7 cervical, 12 thoracic, and 5 lumbar vertebrae. In the adult, the sacrum consists of five vertebrae fused to form one bone. Similarly, the coccyx results from the fusion of four or five rudimentary vertebrae.

Despite variations in their structure, all but two vertebrae share certain anatomic and functional aspects. With the exception of the first and second cervical vertebrae (C1 and C2), all have a solid drum-shaped body that serves as the weight-bearing segment. The posterior segment of the vertebra is called the arch, and each one comprises two pedicles, two laminae, and seven processes (four articular, two transverse, and one spinous). Projecting from the upper part of the body of each vertebra is a pair of short thick pedicles. The concavities above and below the pedicles are the four intervertebral notches. When the vertebrae are articulated, the notches in each adjacent pair of bones form the oval intervertebral foramina, which communicate with the vertebral canal and transmit the spinal nerves and blood vessels.

Arising from the pedicles are two broad plates of bone, the laminae, that meet and fuse at the midline posteriorly to form an arch. Projecting backward and downward from this junction is the spinous process, a knobby projection easily palpated under the skin of the back. Lateral to the laminae, near their junction with the pedicles, are paired articular processes, which facilitate movement of the vertebral column. The two superior processes of each vertebra articulate with the inferior processes of the vertebra immediately above it. The small surfaces where they meet are called facets. The transverse processes are located somewhat anterior to the junction of the pedicles and the laminae; they are between the superior and inferior articular processes. These and the spinous processes provide sites for the attachment of muscles and ligaments. The hollow opening formed by the body of the vertebra and the arch is termed the vertebral foramen, a protected space through which the spinal cord passes.

Between each of the vertebrae and atop the sacrum is an intervertebral disk composed of compressible tough fibrous cartilage concentrically arranged around a soft pulpy substance called the nucleus pulposus. Each disk acts as a cushionlike shock absorber between the vertebrae. When the intervertebral disk is ruptured, the soft nucleus pulposus can protrude into the vertebral canal, where it can exert pressure on a spinal nerve root and can cause significant, debilitating pain and motor function impairment. This herniated nucleus pulposus may require surgical excision through a laminectomy if the herniation is severe enough.

Many important variations exist among the regional vertebrae. For example, the first cervical vertebra, or atlas, is ring shaped and supports the cranium. It has no body or spinous process and allows for a nodding motion of the head. The second cervical vertebra, or axis, is most striking because of the odontoid process, or dens, that arises perpendicularly to meet with the atlas and allows rotation of the head. The cervical spine as a whole is extremely mobile and is therefore particularly susceptible to acceleration-deceleration and twisting injuries that hyperflex or hyperextend the neck. In addition, the spinal cord is relatively large in this area and therefore sustains damage fairly easily after injury to the cervical spine (Fig. 10-13).

FIG. 10-13 Types of vertebrae. A, Cervical vertebrae. B, Atlas and axis.

The 12 thoracic vertebrae increase in size as they approach the lumbar area. They are distinctive in that they have facets on their transverse processes and bodies for articulation with the ribs. The thoracic spine is fixed by the ribs, but the lumbar spine is not, which creates a vulnerability that is responsible for an increased incidence rate of fracture or dislocation at T12, L1, and L2. These injuries are typically found in motor vehicle crash victims who were wearing lap seatbelts without shoulder restraints.

The five lumbar vertebrae are large and massive because of their prominent role in weight bearing. They have no transverse foramina. The sacrum, with its five fused vertebrae, is large, triangular, and wedge-shaped. It forms the posterior wall of the pelvis and articulates with L5, the coccyx, and the iliac portions of the hips. The triangular coccyx is formed by four small segments of bone, the most rudimentary part of the vertebral column.¹

Spinal meninges

In addition to the bony vertebral column, the spinal cord is covered and protected by the continuous downward projection of the three meninges that perform the same protective function for the brain. The dura mater is the outermost membrane and is a strong but loose and expandable sheath of dense fibrous connective tissue that ends in a sac at the end of the second or third segment of the sacrum and protects the cord and the spinal nerve roots as they leave the cord. The dura does not extend beyond the intervertebral foramina. As noted previously, the spinal dura differs from the cranial dura in that the spinal dura is not attached to the surrounding bone, consists of only one layer, and does not send partitions into the fissures of the cord.

The epidural space is located between the outer surface of the dura and the bones and ligaments of the vertebral canal. It contains a quantity of loose connective tissue, fat, and a plexus of veins. The subdural space is a potential space that lies below the inner surface of the dura and the arachnoid membrane; it contains only a limited amount of CSF.

The middle meningeal layer is the arachnoid membrane, which is thin, delicate, and nonvascular; it is continuous with the cranial arachnoid and follows the spinal dura to the end of the dural sac. For the most part, the dura and arachnoid are unconnected, although they are in contact with each other.

The arachnoid is attached to the pia mater by delicate filaments of connective tissue. The considerable space between these two meningeal layers is called the subarachnoid space. It is contiguous with that of the cranium and is largest at the lower end of the spinal canal, where it encloses the masses of nerves that form the cauda equina. The spinal subarachnoid space contains an abundant amount of CSF and is capable of expansion to the point of completely filling the entire space included in the dura mater. This subarachnoid space plays a vital role in the regulation of ICP by allowing for the shunting of CSF away from the cranium. When spinal anesthesia is used, the local anesthetic agent is deposited into the subarachnoid space. Because the CSF in the subarachnoid space bathes the spinal nerves as they emerge from the cord, the local anesthetic effectively blocks spinal nerve conduction.

The third and innermost meningeal layer of the spine is the delicate pia mater. Although it is continuous with the cranial pia mater, it is less vascular, thicker, and denser in structure than the pia mater of the brain. The pia mater intimately invests the entire surface of the cord, and, at the point where the cord terminates, it contracts and continues down as a long slender filament (filum terminale) through the center of the bundle of nerves of the cauda equina and anchors the cord at the base of the coccyx.¹¹

Lumbar puncture

The examination of CSF and determination of CSF pressure are frequently of great value in the diagnosis of neurologic and neurosurgical conditions. The collection of CSF is ordinarily accomplished through the insertion of a long spinal needle between the open spaces between L3 and L4 or L4 and L5, through the dura and arachnoid into the subarachnoid space. Because the spinal cord in adults ends at the level of the disk between L1 and L2, danger of injuring the cord with this procedure is minimal. In children, the spinal cord may extend below L3 so that the subarachnoid space is usually safely entered in the areas between L4 and L5. In both adults and children, flexion of the spine by assuming the fetal position raises the cord superiorly somewhat farther, thus further decreasing the risk of damage to the cord. Because the most superior points of the iliac crests are at the level of the upper border of the spine of L4, they are used as anatomic reference points in selection of the site for lumbar puncture. For a complete description of spinal and epidural anesthesia, see Chapter 25.¹²^,¹³

Structure and function of the spinal cord and the spinal nerve roots

The lowest level of the functional integration of information in the CNS takes place in the spinal cord. Here, information is received in the form of afferent (sensory) nerve impulses from a variety of sensory receptors from the periphery of the body. This information may be processed locally within the cord, but more often is relayed to higher brain centers for additional processing and modification, thus resulting in sophisticated and elaborate motor (efferent) responses. A discussion of the spinal cord primarily involves the consideration of its function as a relay system for both afferent and efferent impulses.

The spinal cord is the elongated slightly ovoid mass of central nervous tissue that occupies the upper two thirds of the vertebral canal. In the adult, it is approximately 45 cm (17 inches) long, although this length varies somewhat among individuals depending on the length of the trunk. The cord is actually an inferior extension of the medulla oblongata and begins at the level of the foramen magnum of the occipital bone.

From that point, the cord continues downward to the upper level of the body of L2, where it narrows to a sharp tip called the conus medullaris. From the end of the conus, an extension of the pia mater known as the filum terminale continues to the first segment of the coccyx, where it attaches (Fig. 10-14).

FIG. 10-14 Vertebral column showing structure of vertebrae, filum terminale, and termination of dura mater.

The small central canal of the spinal cord contains CSF. This cavity extends the entire length of the cord and communicates directly with the fourth ventricle of the medulla oblongata.

The spinal cord (Fig. 10-15) is composed of 31 horizontal segments of varying lengths. It comprises 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segment, each with a corresponding pair of spinal nerves attached.