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2 Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply

2 Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply

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CHAPTER 14



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517



Telencephalon

Prosencephalon



Optic vesicle

Diencephalon



Mesencephalon

Metencephalon

Rhombencephalon

Myelencephalon



Spinal cord



(a) 4 weeks



(b) 5 weeks



Telencephalon

Forebrain

Diencephalon

Mesencephalon



Midbrain



Pons

Metencephalon

Cerebellum



Hindbrain



Myelencephalon

(medulla oblongata)

Spinal cord

(c) Fully developed



FIGURE 14.4 Primary and Secondary Vesicles of the Embryonic Brain. (a) The primary vesicles at 4 weeks. (b) The secondary vesicles at

5 weeks. (c) The fully developed brain, color-coded to relate its structures to the secondary embryonic vesicles.



of the brain are more fully described and pictured in

chapter 20.

In certain places, the meningeal layer of the dura

folds inward to separate major parts of the brain from

each other: the falx14 cerebri (falks SER-eh-bry) extends

into the longitudinal fissure as a tough, crescent-shaped

wall between the right and left cerebral hemispheres;

the tentorium15 (ten-TOE-ree-um) cerebelli stretches like

a roof over the posterior cranial fossa and separates the

14

15



falx = sickle

tentorium = tent



sal78259_ch14_511-560.indd 517



cerebellum from the overlying cerebrum; and the falx

cerebelli partially separates the right and left halves of the

cerebellum on the inferior side.

The arachnoid mater and pia mater are similar to those

of the spinal cord. The arachnoid mater is a transparent

membrane over the brain surface, visible in the caudal

half of the cerebrum in figure 14.1c. A subarachnoid space

separates it from the pia below, and in some places, a

subdural space separates it from the dura above. The pia

mater is a very thin, delicate membrane that closely follows all the contours of the brain, even dipping into the

sulci. It is not usually visible without a microscope.



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Skull

Dura mater:

Periosteal layer

Meningeal layer

Arachnoid granulation

Arachnoid mater

Blood vessel



Subdural space

Subarachnoid

space

Superior sagittal

sinus

Falx cerebri

(in longitudinal

fissure only)



Pia mater

Brain:

Gray matter

White matter



FIGURE 14.5 The Meninges of the Brain. Frontal section of the head.



DEEPER INSIGHT 14.1



Clinical Application



Meningitis

Meningitis—inflammation of the meninges—is one of the most serious diseases of infancy and childhood. It occurs especially between 3

months and 2 years of age. Meningitis is caused by a variety of bacteria and viruses that invade the CNS by way of the nose and throat,

often following respiratory, throat, or ear infections. The pia mater

and arachnoid are most often affected, and from here the infection

can spread to the adjacent nervous tissue. Meningitis can cause swelling of the brain, cerebral hemorrhaging, and sometimes death within

mere hours of the onset of symptoms. Signs and symptoms include

high fever, stiff neck, drowsiness, intense headache, and vomiting.

Meningitis is diagnosed partly by examining the cerebrospinal

fluid (CSF) for bacteria and white blood cells. The CSF is obtained

by making a lumbar puncture (spinal tap) between two lumbar vertebrae and drawing fluid from the subarachnoid space. This site is

chosen because it has an abundance of CSF and there is no risk of

injury to the spinal cord, which does not extend into the lower lumbar vertebrae.

Death from meningitis can occur so suddenly that infants and

children with a high fever should therefore receive immediate medical attention. Freshman college students show a slightly elevated

incidence of meningitis, especially those living in crowded dormitories

rather than off campus.



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Ventricles and Cerebrospinal Fluid

The brain has four internal chambers called ventricles

(fig. 14.6). The largest and most rostral ones are the lateral

ventricles, which form an arc in each cerebral hemisphere. Through a tiny pore called the interventricular

foramen, each lateral ventricle is connected to the third

ventricle, a narrow median space inferior to the corpus

callosum. From here, a canal called the cerebral aqueduct

passes down the core of the midbrain and leads to the

fourth ventricle, a small triangular chamber between the

pons and cerebellum. Caudally, this space narrows and

forms a central canal that extends through the medulla

oblongata into the spinal cord.

On the floor or wall of each ventricle is a spongy mass

of blood capillaries called a choroid (CO-royd) plexus,

named for its histological resemblance to a fetal membrane called the chorion. Ependyma, a type of neuroglia

that resembles a cuboidal epithelium, lines the ventricles

and canals and covers the choroid plexuses. It produces

cerebrospinal fluid.

Cerebrospinal fluid (CSF) is a clear, colorless liquid

that fills the ventricles and canals of the CNS and bathes

its external surface. The brain produces about 500 mL of

CSF per day, but the fluid is constantly reabsorbed at the

same rate and only 100 to 160 mL is normally present at

one time (but see Deeper Insight 14.2). About 40% of it is



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CHAPTER 14



Caudal



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519



Rostral



Lateral ventricles



Cerebrum



Interventricular

foramen



Lateral ventricle

Interventricular

foramen



Third ventricle



Third ventricle



Cerebral

aqueduct



Cerebral

aqueduct



Fourth ventricle

Lateral aperture



Fourth ventricle



Median aperture



Lateral aperture

Median aperture



Central canal



(a) Lateral view



(b) Anterior view



Rostral (anterior)

Longitudinal

fissure

Frontal lobe

Gray matter

(cortex)

White matter



Corpus callosum

(anterior part)



Lateral ventricle



Caudate nucleus

Septum

pellucidum

Sulcus

Gyrus



Temporal lobe

Third ventricle

Lateral sulcus

Insula



Thalamus

Lateral ventricle



Choroid plexus

Corpus callosum

(posterior part)



Occipital lobe



Longitudinal

fissure



(c)



Caudal (posterior)



FIGURE 14.6 Ventricles of the Brain. (a) Right lateral aspect. (b) Anterior aspect. (c) Superior view of a horizontal section of the cadaver brain,

showing the lateral ventricles and some other features of the cerebrum.

formed in the subarachnoid space external to the brain,

30% by the general ependymal lining of the brain ventricles, and 30% by the choroid plexuses. CSF production begins with the filtration of blood plasma through



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the capillaries of the brain. Ependymal cells modify the

filtrate as it passes through them, so the CSF has more

sodium and chloride than blood plasma, but less potassium, calcium, and glucose and very little protein.



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DEEPER INSIGHT 14.2



CSF is not a stationary fluid but continually flows

through and around the CNS, driven partly by its own

pressure, partly by the beating of ependymal cilia, and

partly by rhythmic pulsations of the brain produced by

each heartbeat. The CSF secreted in the lateral ventricles

flows through the interventricular foramina into the third

ventricle and then down the cerebral aqueduct to the

fourth ventricle (fig. 14.7). The third and fourth ventricles

and their choroid plexuses add more CSF along the way.

A small amount of CSF fills the central canal of the spinal

cord, but ultimately, all of it escapes through three pores

in the walls of the fourth ventricle—a median aperture

and two lateral apertures. These lead into the subarachnoid space on the brain and spinal cord surface. From

this space, CSF is reabsorbed by arachnoid granulations,

extensions of the arachnoid meninx shaped like little

sprigs of cauliflower, protruding through the dura mater

into the superior sagittal sinus. CSF penetrates the walls

of the granulations and mixes with blood in the sinus.



Clinical Application



Hydrocephalus

Hydrocephalus16 is the abnormal accumulation of CSF in the brain,

usually resulting from a blockage in its route of flow and reabsorption.

Such obstructions occur most commonly in the interventricular foramen, cerebral aqueduct, and apertures of the fourth ventricle. The

accumulated CSF expands the ventricles and compresses the nervous

tissue, with potentially fatal consequences. In a fetus or infant, it can

cause the entire head to enlarge because the cranial bones are not yet

fused. Good recovery can be achieved if a tube (shunt) is inserted to

drain fluid from the ventricles into a vein of the neck.



16



hydro = water; cephal = head



Arachnoid

granulation



8



Superior

sagittal

sinus

Arachnoid mater

1 CSF is secreted by

choroid plexus in

each lateral ventricle.

2 CSF flows through

interventricular foramina

into third ventricle.

3 Choroid plexus in third

ventricle adds more CSF.



Subarachnoid

space

Dura mater

1

2

Choroid plexus

Third ventricle



3



7



4

4 CSF flows down cerebral

aqueduct to fourth ventricle.



Cerebral

aqueduct

Lateral aperture



5 Choroid plexus in fourth

ventricle adds more CSF.



Fourth ventricle

6



5



6 CSF flows out two lateral apertures

and one median aperture.



7 CSF fills subarachnoid space and

bathes external surfaces of brain

and spinal cord.



8 At arachnoid granulations, CSF is

reabsorbed into venous blood of

dural venous sinuses.



FIGURE 14.7 The Flow of Cerebrospinal Fluid.

● Locate the sites of obstruction that cause hydrocephalus.



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Median aperture

7



Central canal

of spinal cord

Subarachnoid

space of

spinal cord



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Cerebrospinal fluid serves three purposes:

1. Buoyancy. Because the brain and CSF are similar in

density, the brain neither sinks nor floats in the CSF.

It hangs from delicate specialized fibroblasts of the

arachnoid meninx. A human brain removed from the

body weighs about 1,500 g, but when suspended in

CSF its effective weight is only about 50 g. By analogy,

consider how much easier it is to lift another person

when you are standing in a lake than it is on land.

This buoyancy allows the brain to attain considerable

size without being impaired by its own weight. If the

brain rested heavily on the floor of the cranium, the

pressure would kill the nervous tissue.

2. Protection. CSF also protects the brain from striking

the cranium when the head is jolted. If the jolt is

severe, however, the brain still may strike the inside

of the cranium or suffer shearing injury from contact

with the angular surfaces of the cranial floor. This is

one of the common findings in child abuse (shaken

child syndrome) and in head injuries (concussions)

from auto accidents, boxing, and the like.

3. Chemical stability. The flow of CSF rinses metabolic wastes from the nervous tissue and homeostatically regulates its chemical environment. Slight

changes in CSF composition can cause malfunctions of the nervous system. For example, a high

glycine concentration disrupts the control of body

temperature and blood pressure, and a high pH

causes dizziness and fainting.



Blood Supply and the Brain Barrier System

The blood vessels that supply and drain the brain are

detailed in chapter 20. Although the brain is only 2% of

the adult body weight, it receives 15% of the blood (about

750 mL/min.) and consumes 20% of the oxygen and

glucose. Because neurons have such a high demand for

ATP, and therefore glucose and oxygen, the constancy of

blood supply is especially critical to the nervous system.

A mere 10-second interruption in blood flow can cause

loss of consciousness; an interruption of 1 to 2 minutes

can significantly impair neural function; and 4 minutes

without blood causes irreversible brain damage.

Despite its critical importance to the brain, the blood

is also a source of antibodies, macrophages, bacterial toxins, and other potentially harmful agents. Damaged brain

tissue is essentially irreplaceable, and the brain therefore

must be well protected. Consequently, there is a brain

barrier system that strictly regulates what substances can

get from the bloodstream into the tissue fluid of the brain.

There are two potential points of entry that must be

guarded: the blood capillaries throughout the brain tissue and the capillaries of the choroid plexuses. At the

former site, the brain is well protected by the blood–brain

barrier (BBB), which consists of tight junctions between



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The Brain and Cranial Nerves



521



the endothelial cells that form the capillary walls. In the

developing brain, astrocytes reach out and contact the capillaries with their perivascular feet, inducing the endothelial cells to form tight junctions that completely seal off the

gaps between them. This ensures that anything leaving the

blood must pass through the cells and not between them.

The endothelial cells are more selective than gaps between

them would be, and can exclude harmful substances

from the brain tissue while allowing necessary ones to pass

through. At the choroid plexuses, the brain is protected

by a similar blood–CSF barrier formed by tight junctions

between the ependymal cells. Tight junctions are absent

from ependymal cells elsewhere, because it is important to

allow exchanges between the brain tissue and CSF. That is,

there is no brain–CSF barrier.

The brain barrier system (BBS) is highly permeable to

water, glucose, and lipid-soluble substances such as oxygen,

carbon dioxide, alcohol, caffeine, nicotine, and anesthetics.

It is slightly permeable to sodium, potassium, chloride, and

the waste products urea and creatinine. While the BBS is an

important protective device, it is an obstacle to the delivery

of medications such as antibiotics and cancer drugs, and

thus complicates the treatment of brain diseases.

Trauma and inflammation sometimes damage the BBS

and allow pathogens to enter the brain tissue. Furthermore,

there are places called circumventricular organs (CVOs)

in the third and fourth ventricles where the barrier is

absent and the blood has direct access to brain neurons.

These enable the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables.

Unfortunately, CVOs also afford a route of invasion by the

human immunodeficiency virus (HIV).



Before You Go On

Answer the following questions to test your understanding of the

preceding section:

5. Name the three meninges from superficial to deep. How does

the dura mater of the brain differ from that of the spinal cord?

6. Describe three functions of the cerebrospinal fluid.

7. Where does the CSF originate and what route does it take

through and around the CNS?

8. Name the two components of the brain barrier system and

explain the importance of this system.



14.3 The Hindbrain and Midbrain

Expected Learning Outcomes

When you have completed this section, you should be able to

a. list the components of the hindbrain and midbrain and

their functions; and

b. describe the location and functions of the reticular

formation.



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The study of the brain in the following pages will be organized around the five secondary vesicles of the embryonic

brain and their mature derivatives. We will proceed in a

caudal to rostral direction, beginning with the hindbrain

and its relatively simple functions and progressing to the

forebrain, the seat of such complex functions as thought,

memory, and emotion.



The Medulla Oblongata

As noted earlier, the embryonic hindbrain differentiates

into two subdivisions: the myelencephalon and metencephalon (see fig. 14.4). The myelencephalon becomes just

one adult structure, the medulla oblongata (meh-DULLuh OB-long-GAH-ta).

The medulla (figs. 14.2 and 14.8) begins at the foramen magnum of the skull and extends for about 3 cm rostrally, ending at a groove between the medulla and pons.

It looks superficially like an extension of the spinal cord,

but slightly wider. Significant differences are apparent,

however, on closer inspection of its gross and microscopic

anatomy. Externally, the anterior surface features a pair of

ridges called the pyramids. Resembling side-by-side baseball bats, these are wider at the rostral end, taper caudally,

and are separated by a groove, the anterior median fissure

continuous with that of the spinal cord. Lateral to each

pyramid is a prominent bulge called the olive. Posteriorly,

the gracile and cuneate fasciculi of the spinal cord continue as two pairs of ridges on the medulla.

All nerve fibers connecting the brain to the spinal cord

pass through the medulla. As we saw in the cord, some

of these are ascending (sensory) and some are descending

(motor) fibers. The ascending fibers include first-order

sensory fibers of the gracile and cuneate fasciculi, which

end in the gracile and cuneate nuclei seen in figure 14.9c,

a cross section of the medulla. Here, they synapse with

second-order fibers that decussate and form the ribbonlike medial lemniscus17 on each side. The second-order

fibers rise to the thalamus, synapsing there with thirdorder fibers that complete the path to the cerebral cortex

(compare fig. 13.5a, p. 485). In the cross section near the

cuneate nucleus, we also see a continuation of the spinal

posterior spinocerebellar tract, which carries sensory signals to the cerebellum.

The largest group of descending fibers is the pair of

corticospinal tracts filling the pyramids on the anterior

surface. These carry motor signals from the cerebral cortex

on the way to the spinal cord, ultimately to stimulate the

skeletal muscles. Any time you carry out a body movement

below the neck, the signals en route to your muscles pass

through here. About 90% of these fibers cross over at the

pyramidal decussation, an externally visible point near

the caudal end of the pyramids (fig. 14.8a). As a result,

muscles below the neck are controlled by the contralateral

17



lemn = ribbon; iscus = little



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side of the brain. A smaller bundle of descending fibers,

the tectospinal tract, originates in the midbrain, passes

through the medulla, and controls muscles of the neck.

The medulla contains neural networks involved in a

multitude of fundamental sensory and motor functions.

The former include the senses of touch, pressure, temperature, taste, and pain; the latter include chewing, salivation, swallowing, gagging, vomiting, respiration, speech,

coughing, sneezing, sweating, cardiovascular and gastrointestinal control, and head, neck, and shoulder movements. Signals for these functions enter and leave the

medulla not only by way of the spinal cord, but also by

four pairs of cranial nerves that begin or end here: the

glossopharyngeal (cranial nerve IX), vagus (X), accessory

(XI), and hypoglossal (XII) nerves. At the level of the section in figure 14.9c, we see the origins of two of them, the

vagus and hypoglossal. (The trigeminal nerve, V, belongs

to the pons but has parts that extend into the medulla

and show in this cross section as well.) Functions of the

individual nerves are detailed in table 14.1 (pp. 548–555).

Another feature seen in cross section is the wavy inferior olivary nucleus, a major relay center for signals going

from many levels of the brain and spinal cord to the cerebellum. The reticular formation, detailed later, is a loose

network of nuclei extending throughout the medulla, pons,

and midbrain. In the medulla, it includes a cardiac center,

which regulates the rate and force of the heartbeat; a vasomotor center, which regulates blood pressure and flow

by dilating and constricting blood vessels; two respiratory

centers, which regulate the rhythm and depth of breathing; and other nuclei involved in the aforementioned

motor functions.



The Pons

The metencephalon develops into two structures, the pons

and cerebellum. We will return to the cerebellum after

finishing the brainstem. The pons18 measures about 2.5 cm

long. Most of it appears as a broad anterior bulge rostral

to the medulla (figs. 14.2 and 14.8). Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar

peduncles, the cut edges in the upper half of figure 14.9b.

They connect the cerebellum to the pons and midbrain

(fig. 14.8b) and will be discussed with the cerebellum.

In cross section, the pons exhibits continuations of the

previously mentioned reticular formation, medial lemniscus, and tectospinal tract. We also see extensions of the

spinal cord’s anterolateral system and anterior spinocerebellar tract (see p. 486). The anterior half of the pons

(lower half of fig. 14.9b) is dominated by tracts of white

matter, including transverse fascicles that cross between

left and right and connect the two hemispheres of the

cerebellum, and longitudinal fascicles that carry sensory

and motor signals up and down the brainstem.

18



pons = bridge



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CHAPTER 14



Diencephalon:

Thalamus

Infundibulum

Mammillary body



The Brain and Cranial Nerves



523



Optic tract

Cranial nerves:



Midbrain:

Cerebral peduncle



Optic nerve (II)

Oculomotor nerve (III)

Trochlear nerve (IV)

Trigeminal nerve (V)

Abducens nerve (VI)



Pons



Facial nerve (VII)

Vestibulocochlear nerve (VIII)

Glossopharyngeal nerve (IX)

Vagus nerve (X)

Accessory nerve (XI)



Medulla oblongata:

Pyramid



Hypoglossal nerve (XII)



Anterior median fissure



Regions of the brainstem

Diencephalon

Midbrain



Pyramidal decussation



Spinal nerves



Spinal cord



Pons

Medulla oblongata



(a) Ventral view



Diencephalon:

Thalamus

Lateral geniculate body

Pineal gland

Medial geniculate body



Optic tract



Midbrain:

Superior colliculus

Inferior colliculus

Cerebral peduncle

Pons



Superior cerebellar

peduncle

Middle cerebellar

peduncle



Fourth ventricle



Inferior cerebellar

peduncle

Olive



Medulla

oblongata



Cuneate fasciculus



Gracile fasciculus

Spinal cord

(b) Dorsolateral view



FIGURE 14.8 The Brainstem. (a) Anterior view. (b) Posterolateral view. Color-coded to match the embryonic origins in figure 14.4. The boundaryy

between the middle and inferior cerebellar peduncles is indistinct. Authorities vary as to whether to include the diencephalon in the brainstem.



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Posterior

Superior colliculus

Tectum



Cerebral aqueduct

Medial geniculate nucleus



Reticular formation



Central gray matter



Cerebral peduncle:

Tegmentum



Oculomotor nucleus

Medial lemniscus

Red nucleus



Substantia nigra

(a) Midbrain

Cerebral crus

Oculomotor nerve (III)



(b) Pons



Anterior

(a) Midbrain

(c) Medulla

Fourth ventricle



Vermis of cerebellum

Superior cerebellar

peduncle



Ventral

spinocerebellar tract



Middle cerebellar

peduncle



Tectospinal tract



Trigeminal nerve nuclei



Anterolateral system



Sensory root of

trigeminal nerve

Trigeminal nerve

Reticular formation



Transverse fascicles

Medial lemniscus

Longitudinal fascicles



(b) Pons



Nucleus of

hypoglossal nerve

Gracile nucleus



Fourth ventricle



Cuneate nucleus



Nucleus of

vagus nerve



Dorsal spinocerebellar

tract



Trigeminal nerve:

Nucleus

Tract



Reticular formation

Tectospinal tract



Medial lemniscus



Inferior olivary

nucleus

Olive

Hypoglossal nerve



Pyramids of medulla



Corticospinal tract



FIGURE 14.9 Cross

Sections of the

Brainstem. The level of

each section is shown in the

figure on the right. (a) The

midbrain, cut obliquely to

pass through the superior

colliculi. (b) The pons. The

straight edges indicate cut

edges of the peduncles where

the cerebellum was removed.

(c) The medulla oblongata.

● Trace the route taken

through all three of these

figures by fibers from the

gracile and cuneate fasciculi

described in chapter 13.



(c) Medulla oblongata



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CHAPTER 14



Cranial nerves V to VIII begin or end in the pons,

although we see only the trigeminal nerve (V) at the level

of figure 14.9b. The other three emerge from the groove

between the pons and medulla. The functions of these

four nerves, detailed in table 14.1, include sensory roles

in hearing, equilibrium, and taste, and in facial sensations

such as touch and pain; as well as motor roles in eye movement, facial expressions, chewing, swallowing, urination,

and the secretion of saliva and tears. The reticular formation in the pons contains additional nuclei concerned with

sleep, respiration, and posture.



tectum = roof, cover

corpora = bodies; quadrigemina = quadruplets

21

colli = hill; cul = little

22

tegmen = cover

23

substantia = substance; nigra = black



525



center that relays inhibitory signals to the thalamus and

basal nuclei (both of which are discussed later), preventing unwanted body movement. Degeneration of the neurons in the substantia nigra leads to the muscle tremors

of Parkinson disease (see Deeper Insight 12.4, p. 473).

The cerebral crus (pronounced “cruss”; plural, crura) is

a bundle of nerve fibers that connect the cerebrum to the

pons and carry the corticospinal nerve tracts.

The cerebral aqueduct is encircled by the central

(periaqueductal) gray matter. This is involved with the

reticulospinal tracts in controlling awareness of pain, as

further described in chapter 16.



The Midbrain

The mesencephalon becomes just one mature brain structure, the midbrain—a short segment of brainstem that

connects the hindbrain and forebrain (figs. 14.2 and 14.8).

It contains the cerebral aqueduct, continuations of the

medial lemniscus and reticular formation, and the motor

nuclei for two cranial nerves that control eye movements:

cranial nerves III (oculomotor) and IV (trochlear). Only

the first of these is seen at the level of the cross section in

figure 14.9a. (The medial geniculate nucleus seen in this

figure is not part of the midbrain, but a part of the thalamus

that happens to lie in the plane of this section.)

The part of the midbrain posterior to the cerebral

aqueduct is a rooflike tectum.19 It exhibits four bulges, the

corpora quadrigemina.20 The upper pair, called the superior colliculi21 (col-LIC-you-lye), functions in visual attention, visually tracking moving objects, and such reflexes

as blinking, focusing, pupillary dilation and constriction,

and turning the eyes and head in response to a visual

stimulus (for example, to look at something that you catch

sight of in your peripheral vision). The lower pair, called

the inferior colliculi, receives signals from the inner ear

and relays them to other parts of the brain, especially

the thalamus. Among other functions, they mediate the

reflexive turning of the head in response to a sound, and

one’s tendency to jump when startled by a sudden noise.

Anterior to the cerebral aqueduct, the midbrain consists mainly of the cerebral peduncles—two stalks that

anchor the cerebrum to the brainstem. Each peduncle has

three main components: tegmentum, substantia nigra,

and cerebral crus. The tegmentum22 is dominated by the

red nucleus, named for a pink color imparted by its high

density of blood vessels. Fibers from the red nucleus form

the rubrospinal tract in most mammals, but in humans

its connections go mainly to and from the cerebellum,

with which it collaborates in fine motor control. The substantia nigra23 (sub-STAN-she-uh NY-gruh) is a dark gray

to black nucleus pigmented with melanin. It is a motor



The Brain and Cranial Nerves



Apply What You Know

Why are the inferior colliculi shown in figure 14.8b but not

in figure 14.9a? How are these two figures related?



The Reticular Formation

The reticular24 formation is a loosely organized web of gray

matter that runs vertically through all levels of the brainstem, appearing at all three levels of figure 14.9. It occupies

much of the space between the white fiber tracts and the

more anatomically distinct brainstem nuclei, and has connections with many areas of the cerebrum (fig. 14.10). It

consists of more than 100 small neural networks defined

less by anatomical boundaries than by each network’s

use of a different neurotransmitter. The functions of these

networks include the following:













19



Somatic motor control. Some motor neurons of the

cerebral cortex send their axons to reticular formation

nuclei, which then give rise to the reticulospinal

tracts of the spinal cord. These tracts adjust muscle

tension to maintain tone, balance, and posture, especially during body movements. The reticular formation also relays signals from the eyes and ears to the

cerebellum so the cerebellum can integrate visual,

auditory, and vestibular (balance and motion) stimuli

into its role in motor coordination. Other motor nuclei

include gaze centers, which enable the eyes to track

and fixate objects, and central pattern generators—

neural pools that produce rhythmic signals to the

muscles of breathing and swallowing.

Cardiovascular control. The reticular formation

includes the previously mentioned cardiac and vasomotor centers of the medulla oblongata.

Pain modulation. The reticular formation is one

route by which pain signals from the lower body

reach the cerebral cortex. It is also the origin of

the descending analgesic pathways mentioned in

the description of the reticulospinal tracts on page

486. Under certain circumstances, the nerve fibers



20



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24



ret = network; icul = little



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526



PART THREE



Integration and Control



Radiations to

cerebral cortex

Thalamus



Auditory input

Visual input

Reticular formation

Ascending general

sensory fibers

Descending motor

fibers to spinal cord



FIGURE 14.10 The Reticular Formation. The formation consists

of over 100 nuclei scattered throughout the brainstem. Red arrows

indicate routes of input to the reticular formation; blue arrows indicate

the radiating relay of signals from the thalamus to the cerebral cortex;

and green arrows indicate output from the reticular formation to the

spinal cord.

● Locate components of the reticular formation in all three parts of

figure 14.9.











in these pathways act in the spinal cord to deaden

one’s awareness of pain (see chapter 16).

Sleep and consciousness. The reticular formation

has projections to the thalamus and cerebral cortex

that allow it some control over what sensory signals

reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness

such as alertness and sleep. Injury to the reticular

formation can result in irreversible coma.

Habituation. This is a process in which the brain

learns to ignore repetitive, inconsequential stimuli

while remaining sensitive to others. In a noisy

city, for example, a person can sleep through

traffic sounds but wake promptly to the sound of

an alarm clock or a crying baby. Reticular formation nuclei that modulate activity of the cerebral

cortex are called the reticular activating system or

extrathalamic cortical modulatory system.



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The Cerebellum

The cerebellum is the largest part of the hindbrain and

second-largest part of the brain as a whole (fig. 14.11).

It consists of right and left cerebellar hemispheres connected by a narrow wormlike bridge called the vermis.25

Each hemisphere exhibits slender, transverse, parallel

folds called folia26 separated by shallow sulci. The cerebellum has a surface cortex of gray matter and a deeper

layer of white matter. In a sagittal section, the white matter exhibits a branching, fernlike pattern called the arbor

vitae.27 Each hemisphere has four masses of gray matter

called deep nuclei embedded in the white matter. All

input to the cerebellum goes to the cortex and all of its

output comes from the deep nuclei.

Although the cerebellum is only about 10% of the

mass of the brain, it has about 60% as much surface area

as the cerebral cortex and it contains more than half of all

brain neurons—about 100 billion of them. Its tiny, densely

spaced granule cells are the most abundant type of neuron in the entire brain. Its most distinctive neurons, however, are the unusually large, globose Purkinje28 (pur-KINjee) cells. These have a tremendous profusion of dendrites

compressed into a single plane like a flat tree (see fig. 12.5,

p. 444, and the photograph on p. 439). The Purkinje cells

are arranged in a single file, with these thick dendritic

planes parallel to each other like books on a shelf. Their

axons travel to the deep nuclei, where they synapse on

output neurons that issue fibers to the brainstem.

The cerebellum is connected to the brainstem by three

pairs of stalks called cerebellar peduncles29 (peh-DUNculs): a pair of inferior peduncles connected to the medulla oblongata, a pair of middle peduncles to the pons, and a

pair of superior peduncles to the midbrain (see fig. 14.8b).

These consist of thick bundles of nerve fibers that carry

signals to and from the cerebellum. Connections between

the cerebellum and brainstem regions are very complex,

but overlooking some exceptions, we can draw a few generalizations. Most spinal input enters the cerebellum by

way of the inferior peduncles; most input from the rest

of the brain enters by way of the middle peduncles; and

cerebellar output travels mainly by way of the superior

peduncles.

The function of the cerebellum was unknown in

the 1950s. By the 1970s, it had come to be regarded as

a center for monitoring muscle contractions and aiding

in motor coordination. People with cerebellar lesions

exhibit serious deficits in coordination and locomotor

ability; more will be said later in this chapter about

the role of the cerebellum in movement. But cerebellar

lesions also affect several sensory, linguistic, emotional,

verm = worm

foli = leaf

27

“tree of life”

28

Johannes E. von Purkinje (1787–1869), Bohemian anatomist

29

ped = foot; uncle = little

25

26



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CHAPTER 14



The Brain and Cranial Nerves



527



Superior colliculus

Inferior colliculus



Pineal gland

Posterior commissure

Cerebral aqueduct

Mammillary body

Midbrain



White matter

(arbor vitae)



Oculomotor nerve



Gray matter



Fourth ventricle

Pons

Medulla oblongata



(a) Median section



Anterior



Vermis



Anterior lobe



Posterior lobe



Cerebellar

hemisphere



Folia

Posterior



FIGURE 14.11 The Cerebellum. (a) Median section, showing

(b) Superior view



and other nonmotor functions. Recent studies by positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) (both described on p. 24),

and behavioral studies of people with cerebellar lesions,

have created a much more expansive view of cerebellar

function. It appears that its general role is the evaluation

of certain kinds of sensory input, and monitoring muscle

movement is only part of its broader function.

The cerebellum is highly active when a person

explores objects with the fingertips, for example to compare the textures of two objects without looking at them.

(Tactile nerve fibers from a rat’s snout and a cat’s forepaws

also feed into the cerebellum.) Some spatial perception

also resides here. The cerebellum is much more active

when a person is required to solve a pegboard puzzle than

when moving pegs randomly around the same puzzle

board. People with cerebellar lesions also have difficulty

identifying different views of a three-dimensional object

as belonging to the same object.

The cerebellum is also a timekeeper. PET scans show

increased cerebellar activity when a person is required to



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relationship to the brainstem. (b) Superior aspect.



judge the elapsed time between two stimuli. People with

cerebellar lesions have difficulty with rhythmic fingertapping tasks and other tests of temporal judgment. An

important aspect of cerebellar timekeeping is the ability to predict where a moving object will be in the next

second or so. You can imagine the importance of this to a

predator chasing its prey, to a tennis player, or in driving

a car in heavy traffic. The cerebellum also helps to predict

how much the eyes must move in order to compensate for

head movements and remain fixed on an object.

Even hearing has some newly discovered and surprising cerebellar components. Cerebellar lesions impair

a person’s ability to judge differences in pitch between

two tones and to distinguish between similar-sounding

words such as rabbit and rapid. Language output also

involves the cerebellum. If a person is given a noun such

as apple and told to think of a related verb such as eat, the

cerebellum shows higher PET activity than if the person

is told merely to repeat the word apple.

People with cerebellar lesions also have difficulty

planning and scheduling tasks. They tend to overreact



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