Chapter 12
The Central Nervous System
447
12
fissures subdivide each hemisphere into
anterior
,
posterior
, and
flocculonodular lobes
(flok
0
u-lo-nod
9
u-lar). Te small propeller-
shaped flocculonodular lobes, situated deep to the vermis and
posterior lobe, cannot be seen in a surface view.
Like the cerebrum, the cerebellum has a thin outer cortex of
gray matter, internal white matter, and small, deeply situated, paired
masses of gray matter, the most familiar of which are the
dentate nu-
clei
. Several types of neurons populate the cerebellar cortex, includ-
ing
Purkinje cells
(see ±able 11.1 on p. 394). Tese large cells, with
their extensively branched dendrites, are the only cortical neurons
that send axons through the white matter to synapse with the cen-
tral nuclei of the cerebellum. Te distinctive pattern of white matter
in the cerebellum resembles a branching tree, a pattern fancifully
called the
arbor vitae
(ar
9
bor vi
9
te; “tree of life”) (Figure 12.15a, b).
Te anterior and posterior lobes of the cerebellum, which
coordinate body movements, have three sensory maps of the
entire body as indicated by the homunculi in Figure 12.15d. Te
part of the cerebellar cortex that receives sensory input from a
body region influences motor output to that region. Te medial
portions influence the motor activities of the trunk and girdle
muscles. Te intermediate parts of each hemisphere influence
the distal parts of the limbs and skilled movements. Te lateral-
most parts of each hemisphere integrate information from the
association areas of the cerebral cortex and appear to play a role
in planning movements rather than executing them. Te floc-
culonodular lobes receive inputs from the equilibrium appa-
ratus of the inner ears, and adjust posture to maintain balance.
Cerebellar Peduncles
As noted earlier, three paired fiber tracts—the cerebellar
peduncles—connect the cerebellum to the brain stem (see Fig-
ures 12.13 and 12.15b). Unlike the contralateral fiber distribu-
tion to and from the cerebral cortex, virtually all fibers entering
and leaving the cerebellum are
ipsilateral
(
ipsi
5
same)—from
and to the same side of the body.
Te
superior cerebellar peduncles
connecting cerebellum
and midbrain carry instructions from neurons in the deep
cerebellar nuclei to the cerebral motor cortex via thalamic re-
lays. Like the basal nuclei, the cerebellum has no
direct
con-
nections to the cerebral cortex.
Te
middle cerebellar peduncles
carry one-way commu-
nications from the pons to the cerebellum, advising the cer-
ebellum of voluntary motor activities initiated by the motor
cortex (via relays in the pontine nuclei).
Te
inferior cerebellar peduncles
connect medulla and cer-
ebellum. Tese peduncles convey sensory information to the
cerebellum from (1) muscle proprioceptors throughout the
body, and (2) the vestibular nuclei of the brain stem, which
are concerned with equilibrium and balance.
Cerebellar Processing
Cerebellar processing fine-tunes motor activity as follows:
1.
Te motor areas of the cerebral cortex, via relay nuclei in
the brain stem, notify the cerebellum of their intent to ini-
tiate voluntary muscle contractions.
which general somatic sensory information ascends from the
spinal cord to the somatosensory cortex.
Functions of the Medulla Oblongata
Te small size of the
medulla belies its crucial role as an autonomic reflex center
involved in maintaining body homeostasis. Te medulla con-
tains these important functional groups of visceral motor
nuclei:
Cardiovascular center.
Tis includes the
cardiac center
,
which adjusts the force and rate of heart contraction to meet
the body’s needs, and the
vasomotor center
, which changes
blood vessel diameter to regulate blood pressure.
Respiratory centers.
Tese generate the respiratory rhythm
and (in concert with pontine centers) control the rate and
depth of breathing.
Various other centers.
Additional centers regulate such ac-
tivities as vomiting, hiccuping, swallowing, coughing, and
sneezing.
Notice that many functions listed above are also attributed
to the hypothalamus (pp. 441–442). Te overlap is easily ex-
plained. Te hypothalamus controls many visceral functions by
relaying its instructions through medullary reticular centers,
which carry them out.
Check Your Understanding
10.
What are the pyramids of the medulla? What is the result of
decussation of the pyramids?
11.
Which region of the brain stem is associated with the
cerebral peduncles and the superior and inferior colliculi?
For answers, see Appendix H.
Cerebellum
Describe the structure and function of the cerebellum.
Te cauliflower-like
cerebellum
(ser
0
ĕ-bel
9
um; “small brain”),
exceeded in size only by the cerebrum, accounts for about 11%
of total brain mass. Te cerebellum is located dorsal to the pons
and medulla (and to the intervening fourth ventricle). It pro-
trudes under the occipital lobes of the cerebral hemispheres,
from which it is separated by the transverse cerebral fissure (see
Figure 12.4b).
By processing inputs received from the cerebral motor cor-
tex, various brain stem nuclei, and sensory receptors, the cer-
ebellum provides the precise timing and appropriate patterns
of skeletal muscle contraction for smooth, coordinated move-
ments and agility needed for our daily living—driving, typing,
and for some of us, playing the tuba. Cerebellar activity occurs
subconsciously—we have no awareness of it.
Cerebellar Anatomy
Te cerebellum is bilaterally symmetrical. Te wormlike
ver-
mis
connects its two apple-sized
cerebellar hemispheres
medi-
ally
(Figure 12.15)
. Its surface is heavily convoluted, with fine,
transversely oriented pleatlike gyri known as
folia
(“leaves”). Deep
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