lecture 38

The nerve cord that passes through the
vertebral column and constitutes all of the central
nervous system except the brain

The spinal cord
exhibits longitudinal grooves on its ventral and dorsal sides—the
ventral median fissure and dorsal median sulcus, respectively.

The spinal cord has a central core of gray matter that looks somewhat
butterfly- or H-shaped in cross sections. The core consists mainly of
two dorsal (posterior) horns, which extend toward the dorsolateral
surfaces of the cord, and two thicker ventral (anterior) horns, which
extend toward the ventrolateral surfaces. The right and left sides are
connected by a gray commissure. In the middle of the commissure is
the central canal, which is collapsed in most areas of the adult spinal
cord, but in some places (and in young children) remains open, lined
with ependymal cells, and filled with CSF. The canal is a remnant of
the lumen of the embryonic neural tube

The white matter of the spinal cord surrounds the gray matter. It
consists of bundles of axons that course up and down the cord and
provides avenues of communication between different levels of the
CNS. These bundles are arranged in three pairs called columns,
or funiculi6 (few-NIC-you-lie)—a dorsal (posterior), lateral, and
ventral (anterior) column on each side. Each column consists of
subdivisions called tracts, or fasciculi

Multiple sclerosis and Tay–Sachs disease are degenerative disorders of
the myelin sheath. In multiple sclerosis22 (MS), the oligodendrocytes and
myelin sheaths of the CNS deteriorate and are replaced by hardened
scar tissue, especially between the ages of 20 and 40. Nerve conduction
is disrupted with effects that depend on what part of the CNS is
involved—numbness, double vision, blindness, speech defects, neurosis,
or tremors. Patients experience variable cycles of milder and worse
symptoms until they eventually become bedridden. The cause of MS
remains uncertain; most hypotheses suggest that it is an autoimmune
disease—a disorder in which one’s immune system turns against one’s
own tissues—perhaps triggered by a virus in genetically susceptible individuals.
There is no cure. There is conflicting evidence as to how much
it shortens a patient’s life expectancy, if at all. A few die within a year of
diagnosis, but many people live with MS for 25 or 30 years

The lumbar
enlargement and medullary cone give off a bundle of nerve roots
that occupy the vertebral canal from L2 to S5. This bundle, named
the cauda equina1 (CAW-duh ee-KWY-nah) for its resemblance to
a horse’s tail, innervates the pelvic organs and lower limbs.

C1-C8

T1-T12

L1-L5

S1-S5

1 - C0

Except in the thoracic region, the ventral rami branch and anastomose
(merge) repeatedly to form five weblike nerve plexuses:
the small cervical plexus in the neck, the brachial plexus near the
shoulder, the lumbar plexus of the lower back, the sacral plexus
immediately inferior to this, and finally, the tiny coccygeal plexus
adjacent to the lower sacrum and coccyx

In the simplest type of reflex arc, there is no interneuron. The
afferent neuron synapses directly with an efferent neuron, so this
kind of pathway is called a monosynaptic reflex arc Synaptic delay is minimal, and the response is especially quick.

The monosynaptic reflex
arc of the patellar tendon reflex.
• This diagram shows motor output to the quadriceps femoris muscle. For the
patellar tendon reflex to work, it is also necessary to inhibit contraction of the
ipsilateral hamstring muscles, using a pathway not illustrated here

Rostral1 means “toward the nose”

means “toward the tail.”

The cerebrum (seh-REE-brum or SEReh-
brum) constitutes about 83% of its volume and consists of a pair
of half-globes called the cerebral hemispheres. Each hemisphere is
marked by thick folds called gyri3 (JY-rye; singular, gyrus) separated
by shallow grooves called sulci4 (SUL-sye; singular, sulcus). 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

cerebrum
cerebellum
brainstem

The cerebellum6 (SER-eh-BEL-um) lies inferior to the cerebrum
and occupies the posterior cranial fossa. It is also marked by
gyri, sulci, and fissures. The cerebellum is the second-largest region
of the brain; it constitutes about 10% of its volume but contains
over 50% of its neurons

Authorities differ on how they define the brainstem. This book
treats it as that which remains of the brain if the cerebrum and
cerebellum are removed. Its major components, from rostral to
caudal, are the diencephalon, midbrain, pons, and medulla oblongata.
The most common alternative definition includes only the
last three of these.
In a living person, the brainstem is oriented like a vertical stalk
with the cerebrum perched on top of it like a mushroom cap. Postmortem
changes give it a more oblique angle in the cadaver and
consequently in many medical illustrations. Caudally, the brainstem
ends at the foramen magnum of the skull, and the central
nervous system (CNS) continues below this as the spinal cord

the main
information-processing tissue of the brain

part of the forebrain; sits between the midbrain and corpis callosum - consists of the thalamus, hypothalamus, epithalamus.

The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF) and located within the brain parenchyma. The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle (see the following images). The choroid plexuses located in the ventricles produce CSF, which fills the ventricles and subarachnoid space, following a cycle of constant production and reabsorption.

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