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CHAPTER
12
The Central Nervous System
Objectives
The Brain
1. Describe how space constraints affect brain development.
2. Name the major regions of the adult brain.
3. Name and locate the ventricles of the brain.
4. List the major lobes, fissures, and functional areas of the cerebral cortex.
5. Explain lateralization of hemisphere function.
6. Differentiate between commissures, association fibers, and projection fibers.
7. Describe the general function of the basal nuclei (basal ganglia).
8. Describe the location of the diencephalon, and name its subdivisions and functions.
9. Identify the three major regions of the brain stem, and note the functions of each area.
10. Describe the structure and function of the cerebellum.
11. Locate the limbic system and the reticular formation, and explain the role of each
functional system.
Lecture Outline
I. The Brain (pp. 429–452; Figs. 12.1–12.17; Table 12.1)
A. Embryonic Development (pp. 429–430; Figs. 12.1–12.2)
1. The brain and spinal cord begin as the neural tube, which rapidly differentiates into the
CNS.
2. The neural tube develops constrictions that divide the three primary brain vesicles:
the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon
(hindbrain).
3. Since the brain grows more rapidly than the developing skull, the brain forms folds,
allowing it to fit inside the space of the skull.
B. Regions and Organization (p. 430)
1. The medical scheme of brain anatomy divides the adult brain into four parts: the cerebral hemispheres, the diencephalon, the brain stem (consisting of the midbrain, pons,
and medulla oblongata), and the cerebellum.
2. The basic pattern of the CNS consists of a central cavity surrounded by a gray matter
core, external to which is white matter.
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3. In the brain, the cerebrum and cerebellum have an outer gray matter layer, which is
reduced to scattered gray matter nuclei in the spinal cord.
C. Ventricles (p. 430; Fig. 12.3)
1. The ventricles of the brain are continuous with one another and with the central canal
of the spinal cord.
a. The ventricles are lined with ependymal cells and are filled with cerebrospinal
fluid.
b. The paired lateral ventricles lie deep within each cerebral hemisphere and are
separated by the septum pellucidum.
c. The third ventricle lies within the diencephalon and communicates with the lateral
ventricles via two interventricular foramina.
d. The fourth ventricle lies in the hindbrain and communicates with the third ventricle
via the cerebral aqueduct.
e. The lateral and median apertures within the fourth ventricle connect the ventricles
with the subarachnoid space surrounding the brain.
D. Cerebral Hemispheres (pp. 430–441; Figs. 12.4–12.10, 12.12; Table 12.1)
1. The cerebral hemispheres form the superior part of the brain and are characterized by
ridges and grooves called gyri and sulci.
2. The cerebral hemispheres are separated along the midline by the longitudinal fissure
and are separated from the cerebellum along the transverse cerebral fissure.
3. The five lobes of the cerebral hemispheres separated by specific sulci are: frontal,
parietal, temporal, occipital, and insula.
a. The central sulcus separates the frontal and parietal lobes.
b. The precentral gyrus lies anterior to the central sulcus, while the postcentral gyrus
lies posterior to the central sulcus.
c. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.
d. The lateral sulcus separates the temporal lobe from the parietal and frontal lobes.
e. The insula forms part of the floor of the lateral sulcus and is covered by the temporal, parietal, and frontal lobes.
f. Each cerebral hemisphere has three regions: the superficial cortex of gray matter,
internal white matter, and areas of gray matter deep within the white matter, the
basal nuclei.
4. The cerebral cortex is the location of the conscious mind, allowing us to communicate,
remember, and understand, and comprises about 40% of the total brain mass.
5. The cerebral cortex has three kinds of functional areas: motor areas, sensory areas, and
association areas.
6. Each hemisphere has contralateral control over sensory and motor functions, meaning
that each hemisphere controls the opposite side of the body.
7. The hemispheres exhibit lateralization of function, meaning that there is specialization
of one side of the brain for certain functions.
8. Areas in the posterior part of the frontal lobes control voluntary movement.
a. The primary motor cortex allows conscious control of skilled voluntary movement
of skeletal muscles.
b. The premotor cortex is the region controlling learned motor skills.
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9.
10.
11.
12.
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c. Broca’s area is a motor speech area that controls muscles involved in speech
production.
d. The frontal eye field controls eye movement.
There are several sensory areas of the cerebral cortex that occur in the parietal,
temporal, and occipital lobes.
a. The primary somatosensory cortex allows spatial discrimination and the ability to
detect the location of stimulation.
b. The somatosensory association cortex integrates sensory information and produces
an understanding of the stimulus being felt.
c. The primary visual cortex and visual association area allow reception and interpretation of visual stimuli.
d. The primary auditory cortex and auditory association area allow detection of the
properties and contextual recognition of sound.
e. The vestibular cortex is responsible for conscious awareness of balance.
f. The olfactory cortex allows detection of odors.
g. The gustatory cortex allows perception of taste stimuli.
h. The visceral sensory areas are involved in conscious awareness of visceral
sensation.
Multimodal association areas are not connected to any specific sensory cortex, but are
highly interconnected areas throughout the cerebral cortex.
a. The anterior association area, or prefrontal cortex, is involved with intellect, cognition, recall, and personality and is closely linked to the limbic system.
b. The posterior association area aids in recognition of patterns and faces, as well as
understanding of written and spoken language, and includes Wernicke’s area.
c. The limbic association area deals with emotion surrounding situations and includes
the cingulate gyrus, parahippocampal gyrus, and hippocampus.
There is lateralization of cortical functioning, in which each cerebral hemisphere has
unique control over abilities not shared by the other half.
a. Often, the left hemisphere dominates language abilities, math, and logic, and the
right hemisphere dominates visual-spatial skills, intuition, emotion, and artistic and
musical skills.
b. The term “cerebral dominance” refers to which side of the cerebrum is dominant for
language, not which side of the brain is dominant overall.
Cerebral white matter is responsible for communication between cerebral areas and
the cerebral cortex and lower CNS centers.
a. Association fibers are tracts of cerebral white matter that run horizontally, connecting different parts of the same hemisphere.
b. Commissural fibers run horizontally and connect corresponding areas of gray
matter in the two hemispheres, allowing the hemispheres to function together as a
whole (includes the corpus callosum).
c. Projection fibers run vertically, and connect the cerebral cortex to the lower brain or
cord centers, tying together the rest of the nervous system to the body’s receptors
and effectors.
Basal nuclei consist of a group of subcortical nuclei that have overlapping motor
control with the cerebellum that regulate cognition and emotion.
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E. The diencephalon is a set of gray matter areas and consists of the thalamus,
hypothalamus, and epithalamus (pp. 441–443; Figs. 12.11, 12.13; Table 12.1).
1. The thalamus plays a key role in mediating sensation, motor activities, cortical
arousal, learning, and memory.
2. The hypothalamus is the control center of the body, regulating ANS activity, initiating
physical responses to emotions, and regulating body temperature, food intake, water
balance, thirst, sleep-wake cycles, and endocrine function.
3. The epithalamus includes the pineal gland, which secretes melatonin, and regulates the
sleep-wake cycle.
F. The brain stem, consisting of the midbrain, pons, and medulla oblongata, produces
rigidly programmed, automatic behaviors necessary for survival (pp. 443–447;
Figs. 12.12–12.14; Table 12.1).
1. The midbrain is comprised of the cerebral peduncles, corpora quadrigemina, and
substantia nigra.
2. The pons contains fiber tracts that complete conduction pathways between the brain
and spinal cord.
3. The medulla oblongata is the location of several visceral motor nuclei controlling vital
functions such as cardiac and respiratory rate.
G. Cerebellum (pp. 447–449; Fig. 12.15; Table 12.1)
1. The cerebellum processes inputs from several structures and coordinates skeletal
muscle contraction to produce smooth movement.
2. There are two cerebellar hemispheres consisting of three lobes each:
a. Anterior and posterior lobes coordinate body movements and the flocculonodular
lobes adjust posture to maintain balance.
b. Three paired fiber tracts, the cerebellar peduncles, communicate between the
cerebellum and the brain stem.
3. Cerebellar processing follows a functional scheme in which the frontal cortex communicates the intent to initiate voluntary movement to the cerebellum, the cerebellum
collects input concerning balance and tension in muscles and ligaments, and the best
way to coordinate muscle activity is relayed back to the cerebral cortex.
4. The cerebellum may also play a role in thinking, language, and emotion.
H. Functional brain systems consist of neurons that are distributed throughout the brain but
work together (pp. 449–452; Figs. 12.16–12.17; Table 12.1).
1. The limbic system is involved with emotions and is extensively connected throughout
the brain, allowing it to integrate and respond to a wide variety of environmental stimuli.
2. The reticular formation extends through the brain stem, keeping the cortex alert via the
reticular activating system and dampening familiar, repetitive, or weak sensory inputs.
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CHAPTER
15
The Special Senses
Objectives
The Eye and Vision
1. Describe the structure and function of accessory eye structures, eye layers, the lens, and
humors of the eye.
2. Outline the causes and consequences of cataracts and glaucoma.
3. Trace the pathway of light through the eye to the retina, and explain how light is focused
for distant and close vision.
4. Outline the causes and consequences of astigmatism, myopia, hyperopia, and presbyopia.
Lecture Outline
I. The Eye and Vision (pp. 545–565; Figs. 15.1–15.19)
A. Vision is our dominant sense; 70% of our body’s sensory receptors are found in the eye
(p. 545).
B. Accessory Structures of the Eye (pp. 545–549; Figs. 15.1–15.3)
1. Eyebrows are short, coarse hairs overlying the supraorbital margins of the eye that
shade the eyes and keep perspiration out.
2. Eyelids (palpebrae), eyelashes, and their associated glands help to protect the eye from
physical danger as well as from drying out.
a. Several glands are associated with the eyelid: tarsal glands and ciliary glands that
produce oily secretions.
3. The conjunctiva is a transparent mucous membrane that produces a lubricating mucus
that prevents the eye from drying out.
a. The palpebral conjunctiva lines the eyelids, and the bulbar conjunctiva covers the
anterior surface of the eyeball.
b. The conjunctival sac is the space between the bulbar and palpebral conjunctiva and
forms the space in which medications are dispensed into the eye.
4. The lacrimal apparatus consists of the lacrimal gland, which secretes a dilute saline
solution (tears), and small ducts that drain excess fluid into the nasolacrimal duct.
a. Tears contain mucus, antibodies, and lysozyme to cleanse, moisten, and protect the
eyes.
5. The movement of each eyeball is controlled by six extrinsic eye muscles that are
innervated by the abducens and trochlear nerves.
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a. Four rectus muscles superior, inferior, lateral, and medial, originate at the back of
the orbit, and run straight to their insertion on the eyeball.
b. Two oblique muscles, superior and inferior, run along the side of the eyeball and
insert on the eyeball at an angle.
c. The action of the oblique muscles offsets the action of the superior and inferior
rectus, allowing the eyeball to be directly elevated or depressed.
C. Structure of the Eyeball (pp. 549–553; Figs. 15.4–15.9)
1. Three layers form the wall of the eyeball:
a. The fibrous tunic is the outermost coat of the eye and is made of a dense avascular
connective tissue with two regions: the sclera and the cornea.
b. The cornea is avascular, but rich in nerve endings that detect pain.
c. The vascular tunic (uvea) is the middle layer and has three regions: the choroid that
supplies blood to all areas of the eyeball, the ciliary body, consisting of smooth
muscle and connective tissue that controls lens shape and anchors the lens in place,
and the iris, rings of smooth muscle that control dilation of the pupil.
d. The inner layer (retina) is made up of two layers: the outer pigmented layer absorbs
light; the inner neural layer contains millions of photoreceptors (rods and cones)
that transduce light energy.
e. The optic disc is the blind spot of the eye, occurs at the point where the optic nerve
exits the eyeball, and has no photoreceptor cells.
f. Lateral to the optic disc is the macula lutea, containing a central pit, the fovea centralis, which contains a very high density of cones, allowing precise color vision.
2. Internal Chambers and Fluids
a. The posterior segment (cavity) is filled with a clear gel called vitreous humor that
transmits light, supports the posterior surface of the lens, holds the retina firmly
against the pigmented layer, and contributes to intraocular pressure.
b. The anterior segment (cavity) is filled with aqueous humor that supplies nutrients
and oxygen to the lens and cornea while carrying away wastes.
3. The lens is an avascular, biconvex, transparent, flexible structure that can change
shape to allow precise focusing of light on the retina.
D. Optics and the Eye (pp. 553–556; Figs. 15.10–15.14)
1. Overview: Light and Optics
a. Electromagnetic radiation includes all energy waves from long waves to short
waves and includes the visible light that our eyes see as color.
b. Refraction, or bending, of a light ray occurs when it meets the surface of a different
medium at an oblique angle rather than a right angle.
c. A convex lens bends light so that it converges at a focal point, forming an image,
called a real image, which projects upside down and reversed from left to right.
2. Focusing of Light on the Retina
a. Light is bent three times: as it enters the cornea, upon entering the lens, and upon
leaving the lens.
b. The far point of vision is that distance beyond which no change in lens shape
(accommodation) is required and, in a normal eye, is at a distance of about
6 meters, or 20 feet.
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c. During distant vision, the ciliary muscles are completely relaxed, causing a
maximal flattening of the lens.
d. Focusing for close vision demands that the eye make three adjustments: accommodation of the lens, causing it to thicken and increase light refraction, constriction of
the pupils, which better directs light to the lens, and convergence of the eyeballs,
allowing the object to remain focused on the foveae.
e. The near point of vision occurs at the point of maximal thickening of the lens, and
is 10 cm, or 4 inches, from the eye.
f. Myopia, or nearsightedness, occurs when objects focus in front of the retina and
results in seeing close objects without a problem but distant objects are blurred.
g. Hyperopia, or farsightedness, occurs when objects are focused behind the retina and
results in seeing distant objects clearly but close objects are blurred.
h. Astigmatism results from an uneven curvature of the cornea or lens, which
produces blurred images.
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