The brain is protected by the rigid bone of the skull, the three membranes or meninges, and the cerebrospinal fluid (CSF). The cranial and facial bones are connected by sutures, which are relatively immovable joints consisting of fibrous tissue. If pressure inside the skull increases in infants before the sutures fuse or ossify, the cranial bones may separate, causing the head to enlarge. The skull contains a number of cavities, or fossae, as well as foramina (openings) and canals through which nerves and blood vessels pass. The largest opening, the foramen magnum, is located in the occipital bone at the base of the skull, where the spinal cord emerges.

Meninges

The meninges consist of three continuous connective tissue membranes covering the brain and spinal cord. The meninges and the contents of the spaces between the layers are as follows:

• The dura mater, the outer layer, is a tough, fibrous, double-layered membrane that separates at specific points to form the dural sinuses, which collect venous blood and CSF for return to the general circulation

• The subdural space, lying beneath the dura, is a potential space (i.e., normally empty, this space could fill with blood after an injury).

• The arachnoid (arachnoid mater), a loose, weblike covering, is the middle layer.

• The subarachnoid space, which contains the CSF and the cerebral arteries and veins, lies beneath the arachnoid.

• Arachnoid villi are projections of arachnoid into the dural sinuses at several places around the brain, through which CSF can be absorbed into the venous blood.

• The pia mater, a delicate connective tissue that adheres closely to all convolutions on the surface of the brain, is the inner layer. Many small blood vessels are found in the pia mater (Fig. 14-2).

FIGURE 14-2 **The Meninges**
The brain and the spinal cord are covered by three protective membranes, collectively called *meninges*. The outermost membrane is the dura mater, the middle layer is the arachnoid, and the innermost membrane is the pia mater. (From VanMeter K, Hubert R: *Microbiology for the Healthcare Professional,* St. Louis, 2010, Elsevier.)

Cerebrospinal Fluid

The cerebrospinal fluid (CSF) provides a cushion for the brain and spinal cord. Similar to plasma in appearance, it is a clear, almost colorless liquid, but it differs from plasma in the concentrations of electrolytes, glucose, and protein (Table 14-1), which remain relatively constant. A change in the characteristics of the CSF is a useful diagnostic tool (see Fig. 14-11). For example, the presence of significant numbers of erythrocytes in CSF indicates bleeding in the central nervous system.

TABLE 14-1

Characteristics of Normal Cerebrospinal Fluid

Appearance	Clear and colorless
Pressure	9-14 mmHg or 150 mm H₂O
Red blood cells	None
White blood cells	Occasional
Protein	15-45 mg/dL
Glucose	45-75 mg/dL
Sodium	140 mEq/L
Potassium	3 mEq/L
Specific gravity	1.007
pH	7.32-7.35
Volume in the system at one time	125-150 mL
Volume formed in 24 hours	500-800 mL

Cerebrospinal fluid is formed constantly in the choroid plexuses in the ventricles and then flows into the subarachnoid space, where it circulates around the brain and spinal cord and eventually passes through the arachnoid villi, returning into the venous blood. To maintain a relatively constant pressure within the skull (intracranial pressure), it is important for equal amounts of CSF to be produced and reabsorbed at the same rate.

Blood-Brain Barrier and Blood-Cerebrospinal Fluid Barrier

The blood-brain barrier is a protective mechanism provided primarily by relatively impermeable capillaries in the brain. The endothelial cells of the capillaries are tightly joined together by tight junctions rather than possessing pores. This barrier limits the passage of potentially damaging materials into the brain and controls the delicate but essential balance of electrolytes, glucose, and proteins in the brain. There is a similar blood-CSF barrier at the choroid plexus to control the constituents of CSF. The blood-brain barrier is poorly developed in neonates, and therefore substances such as bilirubin (see Chapter 22) or other toxic materials can pass easily into the infant’s brain, causing damage. When fully developed, the blood-brain barrier can be a disadvantage, because it does not allow the passage of many essential drugs into the brain (i.e., certain antibiotics and anticancer drugs). Lipid-soluble substances, including alcohol, pass freely into the brain.

14-1

Think about

a. List, in order, the brain coverings and spaces with their contents, from the brain tissue outward.

b. Explain the effect of the production of more CSF than can be reabsorbed.

Functional Areas

Cerebral Hemispheres

The cerebral hemispheres make up the largest and most obvious portions of the brain. The outer surface is covered by elevations, or gyri (sing., gyrus), that are separated by grooves, or sulci (sing., sulcus). The longitudinal fissure separates the left and right hemispheres. The corpus callosum consists of nerve fibers connecting the left and right hemisphere for the purpose of communication between the two hemispheres.

Each hemisphere is divided into four major lobes, each of which has specific functions (Fig. 14-3). Complex functions, such as language and memory, involve many areas of the brain. Each hemisphere is concerned with voluntary movement and sensory function in the opposite (contralateral) side of the body, and these areas of the cortex have been well mapped. In Figure 14-3, note the large number of nerve cells required to innervate the face compared with the amount of cortex allocated to the trunk. The cells of the motor cortex of the frontal lobe initiate specific voluntary movements, and these cells are often referred to as upper motor neurons (UMNs). Their axons form the corticospinal tracts in the spinal cord. Because the crossover of most of these tracts occurs in the medulla, damage to the motor cortex in the left frontal lobe adjacent to the longitudinal fissure (on top of the head) results in paralysis or paresis of the muscles of the right leg.

FIGURE 14-3 Functional areas of the brain showing the *left* side of the cerebral hemispheres.

Each special sensory area of the cortex has an association area surrounding the primary cortex, in which the sensory input is recognized and interpreted. For example, the occipital lobe contains the primary visual cortex, which receives the stimuli from the eye, and the surrounding association cortex identifies the object seen. If the primary cortex is damaged, the person is blind, but if the association area is damaged, the person can see an object but cannot comprehend its significance.

The right and left hemispheres are generally similar in structure but not necessarily in function (Table 14-2). The term dominant hemisphere refers to the side of the brain that controls language, which in most people is the left hemisphere. There are two special areas involved in language skills. Broca’s area is considered the motor or expressive speech area, in which the output of words, both verbal and written, is coordinated in an appropriate and understandable way. This area is located at the base of the premotor area of the left frontal lobe. Wernicke’s area is the integration center that comprehends language received, both spoken and written. This area is located in the posterior temporal lobe and has connecting fibers to the prefrontal, visual, and auditory areas. The left hemisphere also appears to be responsible for mathematical, problem-solving, and logical reasoning abilities. The right hemisphere has greater influence on artistic abilities, creativity, spatial relationships, and emotional and behavioral characteristics.

TABLE 14-2

Major Functional Areas of the Brain

Area	Function
Frontal lobe
Prefrontal area (left cortex)	Intellectual function and personality
Premotor cortex	Skilled movements
Motor cortex	Voluntary movements
Broca’s area (left cortex)	Speech (expression)
Parietal lobe
Somatosensory area	Sensation (e.g., touch, pain)
Occipital lobe
Visual cortex	Vision
Temporal lobe
Auditory cortex	Hearing
Olfactory cortex	Smell
Wernicke’s area (left cortex)	Comprehension of speech
	Memory
Cerebellum	Body balance and position, coordinated movement
Medulla oblongata	Control and coordination centers for respiration and cardiovascular activity
	Swallow reflex center, vomiting reflex, cough reflex
	Nuclei of five cranial nerves
Hypothalamus	Autonomic nervous system
	Link with endocrine system
	Control of body temperature, fluid balance
	Centers for thirst, hunger
Thalamus	Sensory sorting and relay center
Basal nuclei	Coordination and control of body movement
Reticular activating System	Arousal or awareness
Limbic system	Emotional responses

The prefrontal cortex lies anterior to the motor and premotor cortex and recent research indicates that it functions in coordinating complex cognitive behavior as well as providing components for expression of personality. It plays a very significant role in social relationships and impulse control.

The basal nuclei (previously called the basal ganglia) are clusters of cell bodies or gray matter located deep among the tracts of the cerebral hemispheres. These are part of the extrapyramidal system (EPS) of motor control, which controls and coordinates skeletal muscle activity, preventing excessive movements and initiating accessory and often involuntary actions, such as arm-swinging when walking. Two additional nuclei located in the midbrain, the substantia nigra and the red nucleus, are also connected to the basal nuclei and the EPS.

The limbic system consists of many nuclei and connecting fibers in the cerebral hemispheres that encircle the superior part of the brain stem. The limbic system is responsible for emotional reactions or feelings, and for this purpose, it has many connections to all areas of the brain. Part of the hypothalamus is involved with the limbic system. The hypothalamus provides the link for the autonomic responses, such as altered blood pressure or nausea, which occur when one experiences fear, excitement, or an unpleasant sight or odor. Any cognitive (intellectual) decision arising from the higher cortical centers may be accompanied by an emotional aspect mediated through the limbic system.

Diencephalon

The diencephalon is the central portion of the brain. It is surrounded by the hemispheres and contains the thalamus, the hypothalamus, and the epithalamus. The thalamus consists of many nerve cell bodies, the major function of which is to serve as a sorting and relay station for incoming sensory impulses. From the thalamus, connecting fibers transmit impulses to the cerebral cortex and other appropriate areas of the brain. The hypothalamus has a key role in maintaining homeostasis in the body, controlling the autonomic nervous system and much of the endocrine system through the hypophysis, or pituitary gland. It is responsible for the regulation of body temperature, intake of food and fluid, and the regulation of sleep cycles. The hypothalamus is also the key to the stress response and plays major roles in emotional responses through the limbic system and in biologic behaviors, such as the sex drive (libido).

Brain Stem

The inferior portion of the brain, called the brain stem, is the connecting link to the spinal cord. The pons is composed of bundles of afferent (incoming) and efferent (outgoing) fibers. Several nuclei of cranial nerves are also located in the pons. The medulla oblongata contains the vital control centers that regulate respiratory and cardiovascular function and the coordinating centers that govern the cough reflex, swallowing, and vomiting. The medulla is the location of the nuclei of several cranial nerves. It is distinguished by two longitudinal ridges on the ventral surface, termed the pyramids, marking the site of crossover (decussation) of the majority of fibers of the corticospinal (pyramidal) tracts, which results in the contralateral control of muscle function.

The reticular formation is a network of nuclei and neurons scattered throughout the brain stem that has connections to many parts of the brain. The reticular-activating system (RAS) is part of this formation and determines the degree of arousal or awareness of the cerebral cortex. In other words, these neurons decide which of the incoming sensory impulses the brain ignores and which it notices. Many drugs can affect the activity of the RAS, thus increasing or decreasing the input to the brain.

Cerebellum

The cerebellum is located dorsal to the pons and medulla, below the occipital lobe. It functions to coordinate movement and maintain posture and equilibrium by continuously assessing and adjusting to input from the pyramidal system, the proprioceptors in joints and muscles, the visual pathways, and the vestibular pathways from the inner ear.

14-2

Think about

a. Describe the specific location and function of each of the following: the prefrontal cortex, somatosensory area, the RAS, Wernicke’s area, the basal nuclei, and the visual association area.

b. Describe white matter—what it is and what its function is—and give an example.

c. Predict the effects of brain damage in the prefrontal cortex, left frontal lobe, the cerebellum, or the hypothalamus.

Blood Supply to the Brain

Blood is supplied to the brain by the internal carotid arteries and the vertebral arteries. Each internal carotid artery is a branch of a common carotid artery (right or left) and includes the carotid sinus, which is the location of the pressoreceptors, or baroreceptors, that signal changes in blood pressure and the chemoreceptors that monitor variations in blood pH and oxygen levels.

At the base of the brain, each internal carotid artery divides into an anterior and middle cerebral artery (see Fig. 14-14).

• The anterior cerebral artery supplies the frontal lobe.

• The middle cerebral artery supplies the lateral part of the cerebral hemispheres, primarily the temporal and parietal lobes, which comprise a high proportion of the brain tissue.

Posteriorly, the vertebral arteries join to form:

• The basilar artery, which supplies branches to the brain stem and cerebellum as it ascends.

At the base of the brain, the basilar artery divides into:

• The right and left posterior cerebral arteries, which supply blood to the occipital lobes.

• The anterior, middle, and posterior cerebral arteries follow a course over the surface of each hemisphere, with many branches penetrating into the brain substance (see Fig. 14-13A).

Anastomoses between these major arteries at the base of the brain are provided by the:

• Anterior communicating artery between the anterior cerebral arteries

• Posterior communicating arteries between the middle cerebral and posterior cerebral arteries

• This arrangement forms the circle of Willis and provides an alternative source of blood when the internal carotid or vertebral artery is obstructed. This circle of arteries surrounds the pituitary gland and optic chiasm.

Blood flow in the cerebral arteries is relatively constant because the brain cells constantly use oxygen and glucose (essential nutrients for neurons) and have little storage capacity. Autoregulation is a mechanism by which increased carbon dioxide levels or decreased pH in the blood, or decreased blood pressure, in an area of the brain results in immediate local vasodilation. The pressoreceptors (baroreceptors) and chemoreceptors protect the brain from damage related to abnormal blood pressure or pH levels in the systemic flow.

As mentioned, venous blood from the brain collects in the dural sinuses and then drains into the right and left internal jugular veins, to be returned to the heart.

Cranial Nerves

There are 12 pairs of cranial nerves. They originate from the brain stem and pass through the foramina in the skull to serve structures in the head and neck, including the eyes and ears. The vagus nerve (cranial nerve X) serves a more extensive area, branching to innervate many of the viscera. A cranial nerve may consist of motor fibers only (with associated sensory fibers from proprioceptors in the skeletal muscles) or of sensory fibers only, or it may be a mixed nerve, containing both motor and sensory fibers (Table 14-3). Four cranial nerves (III, VII, IX, X) include parasympathetic fibers.

14-3

Think about

a. Explain the function of the circle of Willis.

b. Describe the effect of an obstruction in the right anterior cerebral artery.

c. Explain how a lack of glucose or oxygen will affect brain function.

d. What are the different types of fibers and the functions of cranial nerves II, III, and IX? Describe the effects of damage to each.

TABLE 14-3

Major Components of Cranial Nerves

Number	Name	Type of Fibers	Function
I	Olfactory	Sensory	Special sensory—smell
II	Optic	Sensory	Special sensory—vision
III	Oculomotor	Motor	Eye movements
			Four extrinsic eye muscles
			Upper eyelid—levator palpebrae muscle
		PNS	Iris—pupillary constrictor muscle
		PNS	Ciliary muscle—accommodation
IV	Trochlear	Motor	Eye movements—superior oblique eye muscle
V	Trigeminal	Sensory	General sensory—eye, nose, face and oral cavity, teeth
V	Trigeminal	Motor	Muscles of mastication with sensory proprioceptive fibers; speech
VI	Abducens	Motor	Eye movements—lateral rectus eye muscle
VII	Facial	Sensory	Special sensory—taste, anterior two-thirds of tongue
		Motor	Muscles of facial expression
		Motor	Scalp muscles
		PNS	Lacrimal gland, nasal mucosa, salivary glands (sublingual and submandibular)
VIII	Vestibulocochlear	Sensory	Special sensory—hearing and balance (inner ear)
IX	Glossopharyngeal	Sensory	Special sensory—taste, posterior one-third of tongue
			General sensory—pharynx and soft palate (gag reflex)
			Sensory—carotid sinus for baroreceptors and chemoreceptors
		Motor	Pharyngeal muscles—swallowing
		PNS	Salivary gland (parotid)
X	Vagus	Sensory	Special sensory—taste, pharynx, posterior tongue
			General sensory—external ear and diaphragm
			Visceral sensory—viscera in thoracic and abdominal cavities
		Motor	Pharynx and soft palate—swallowing and speech
		PNS	Heart and lungs; smooth muscle and glands of digestive system
XI	Spinal accessory	Motor	Voluntary muscles of palate, pharynx, and larynx
XI	Spinal accessory	Motor	Head movements—sternocleidomastoid and trapezius muscles
XII	Hypoglossal	Motor	Muscles of tongue

PNS, parasympathetic nervous system.

Spinal Cord

The spinal cord is protected by the bony vertebral column, the meninges, and the CSF. The cord is continuous with the medulla oblongata and ends at the level of the first lumbar vertebra. Beyond this extends a bundle of nerve roots known as the cauda equina. This arrangement is significant because there is little risk of damaging the cord when a needle is inserted into the subarachnoid space below the first lumbar level (usually in the space between L3 and L4) to obtain a sample of CSF (see Fig. 14-11).

The spinal cord consists of the white matter (outer region) containing the spinal cord tracts, composed of myelinated fibers, and the gray matter (inner region) containing the nerve cell bodies, dendrites, and nonmyelinated axons. Within the gray matter three distinct areas can be identified:

• Ventral horn

• Contain motor neurons and their axons leave via the ventral root

• Dorsal horn

• Contain interneurons receiving information from sensory neurons of the dorsal root ganglia

• Lateral horns

• Contain visceral motor neurons

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Tags: Pathophysiology for the Health Professions 5e

Nov 27, 2016 | Posted by admin in PATHOLOGY & LABORATORY MEDICINE | Comments Off

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