lecture 39

The vagus has the most extensive distribution of any cranial nerve, supplying not only organs in the head and neck but also most
viscera of the thoracic and abdominal body cavities. It plays major roles in the control of cardiac, pulmonary, digestive, and urinary functions

Signals for proprioception below the head travel up the spinocerebellar
tracts to the cerebellum. Signals from the thoracic and
abdominal viscera travel to the medulla oblongata by way of sensory
fibers in the vagus nerve (cranial nerve X).

In a
sagittal section, the white matter shows a branching fernlike pattern
called the arbor vitae

Each hemisphere exhibits slender, parallel folds
called folia19 separated by shallow sulci

The cerebellum is connected to the brainstem by three pairs
of stalks called cerebellar peduncles21 (peh-DUN-culs): two inferior
peduncles connecting it to the medulla oblongata, two middle
peduncles to the pons, and two superior peduncles to the midbrain
(see fig. 15.6b). These consist of thick bundles of nerve fibers that
carry signals into and out of the cerebellum.

Origin or termination of cranial nerves IX–XII. Sensory nuclei receive input from the taste buds, pharynx, and thoracic and abdominal viscera.
Motor nuclei include the cardiac center (adjusts the rate and force of the heartbeat), vasomotor center (controls blood vessel diameter and
blood pressure), two respiratory centers (control the rate and depth of breathing), and centers involved in speech, coughing, sneezing,
salivation, swallowing, gagging, vomiting, sweating, gastrointestinal secretion, and movements of the tongue and head

Most
descending motor fibers decussate in the
medullary pyramids.

About 90% of these fibers cross over
(decussate) to the opposite side of the brainstem at a point
called the pyramidal decussation, near the caudal end of the pyramids, (see fig. 15.6a). As a result, muscles below the neck
are controlled by the contralateral side of the brain.

The pons10 measures about 2.5 cm long. Most of it
forms a broad anterior bulge in the brainstem just rostral to the
medulla (fig. 15.6a). Posteriorly, the pons consists mainly of two pairs of thick stalks called peduncles that attach it to the cerebellum
(fig. 15.6b); these are the cut edges of the upper half of figure
15.7b. The peduncles are further described with the cerebellum.
The pons contains several nuclei involved in basic physiological
functions including sleep, respiration, and bladder control

The embryonic mesencephalon produces just one mature brain
structure, the midbrain—a short segment of the brainstem that
connects the hindbrain and forebrain The
midbrain, too, contains continuations of the reticular formation and
medial lemniscus.

in midbrain -
a dark
gray to black nucleus pigmented with melanin, located between
the tegmentum and cerebral crura (discussed next). This is
a motor center that relays inhibitory signals to the thalamus
and basal nuclei (both discussed later). It improves motor
performance by suppressing unwanted muscle contractions.
Degeneration of the substantia nigra leads to the uncontrollable
muscle tremors of Parkinson disease

Parkinson57 disease (PD), also called paralysis agitans or
parkinsonism, is a progressive loss of motor function typically
beginning in a person’s 50s or 60s. It is due to degeneration of
dopamine-releasing neurons in the substantia nigra of the midbrain.
A gene has recently been identified for a hereditary form of
PD, but most cases are nonhereditary and of little-known cause.
Dopamine is an inhibitory neurotransmitter that normally prevents
excessive activity in the basal nuclei. Degeneration of the
dopamine-releasing neurons leads to hyperactivity of the basal
nuclei and, therefore, involuntary muscle contractions

the
conspicuous wrinkles, or gyri, of each hemisphere, separated by
grooves called sulci. The folding of the cerebral surface into gyri allows
a greater amount of cortex to fit in the cranial cavity. The gyri give the cerebrum a surface area of about 2,500 cm2, comparable to 4.5 pages of this book. If the cerebrum
were smooth-surfaced, it would have only
one-third as much area and proportionately
less information-processing capability.
This extensive folding is one of the
greatest differences between the human
brain and the relatively smooth-surfaced
brains of most other mammals.
Some gyri have consistent and predictable
anatomy; others vary from brain
to brain and from the right hemisphere to
the left. Certain unusually prominent sulci
divide each hemisphere into five anatomically
and functionally distinct lobes, listed
next.

have spheroidal somas with dendrites
projecting for short distances in all directions. They are concerned
largely with receiving sensory input and processing information on
a local level.

are tall and conical (triangular in tissue
sections). Their apex points toward the brain surface and has
a thick dendrite with many branches and small, knobby dendritic
spines. The base gives rise to horizontally oriented dendrites and an
axon that passes into the white matter. The axon also has collaterals that synapse with other neurons in the
cortex or in deeper regions of the brain.
Pyramidal cells are the output neurons of
the cerebrum—they are the only cerebral
neurons whose fibers leave the cortex and
connect with other parts of the CNS

The cerebral cortex27 is a layer about 2 to 3 mm thick
covering the surface of the hemispheres. It constitutes about 40% of
the mass of the brain and contains 14 to 16 billion neurons. It possesses
two principal types of neurons called stellate and pyramidal
cells

A
prominent C-shaped band of nerve tracts that
connect the right and left cerebral hemispheres to
each other, seen superior to the third ventricle in
a median section of the brain.

A
very deep groove, the longitudinal fissure, separates the right and
left hemispheres from each other. At the bottom of this fissure, the
hemispheres are connected by a thick bundle of nerve fibers called
the corpus callosum5—a prominent landmark for anatomical description