Agriculture Reference
In-Depth Information
There are several spaces inside of the brain, fi lled
with cerebrospinal fl uid. There are four ventricles and one
duct: two lateral ventricles (fi rst and second) within the
cerebral hemispheres; the third ventricle in the diencepha-
lon, the mesencephalic aqueduct (within the mesencepha-
lon); and the fourth ventricle located between the pons and
medulla ventrally, and the cerebellum dorsally. The fourth
ventricle communicates with the central canal of the spinal
cord.
All major parts of the cerebrum (and cerebellum) are
shown in Figure 6.37 and Figure 6.38 .
T
HE
C
RANIAL
N
ERVES
Some of the cranial nerves (CN) are sensory, some of them
motor, and the others are mixed. The sensory nerves bring
information from the sense organs to the brain to be pro-
cessed into sensations (CN I, II, and VIII). Five cranial
nerves are purely motor (CN III, IV, VI, XI, and XII). The
remaining four cranial nerves are mixed (CN V, VII, IX,
and X). Four pairs of cranial nerves have, in addition,
parasympathetic fi bers (CN III, VII, IX, and X). The origin
of the sensory nerves is inside of specifi c organs (olfactory
mucosa of the nasal cavity, the retina, and the vestibulo-
cochlear organ within the inner ear). The origins of the
other cranial nerves are in the brain. The cranial nerves
supply all of the structures of the head including most of
the head skin.
T
HE
S
PINAL
C
ORD
The second major component of the central nervous
system, the spinal cord, extends from the fi rst pair of cervi-
cal nerves to the end of the fi lum terminale, and is protected
within the vertebral canal. As the vertebral column is
divided into cervical, thoracic, lumbar, sacral, and caudal
vertebrae, there are fi ve parts of the spinal cord, corre-
sponding to the fi ve regions of vertebrae. There are two
enlargements, one in the cervical region and the other
between the lumbar and sacral regions. They are called
intumescences, and they represent concentrations of neuron
cells of the nerves supplying the thoracic and pelvic limbs.
The spinal cord ends as the medullary cone, located in
the sacral vertebral canal, and continues as the fi lum ter-
minale far beyond the sacral canal, within the caudal canal.
The fi lum terminale consists of supporting nervous cells
called glial cells.
The spinal cord is surrounded and protected by menin-
ges with the same names as those for the brain (pia mater,
arachnoid, and dura mater).
On a transverse section (Figure 6.39), the spinal cord
shows an H- or butterfl y-shaped grey matter surrounded
by white matter, and the central canal. The grey matter is
organized into three symmetrical horns (dorsal, middle,
and ventral) and the white matter into three symmetrical
columns (dorsal, middle, and ventral). The dorsal horns
and columns are sensory, the ventral horns and columns
are motor, the lateral horns are related to the autonomic
nervous system, and the lateral columns are both some
sensory and some motor.
T
HE
S
PINAL
N
ERVES
The nerves originating from the spinal cord are called
cervical, thoracic, etc., spinal nerves. They supply muscles,
joints, bones, skin, and other structures related to the body
wall and the limbs, except viscera and sense organs.
A spinal nerve originates by two roots, dorsal sensory,
and ventral motor. Each dorsal root is provided with a
spinal ganglion. The two roots join within the vertebral
canal and form the spinal nerve. Exiting the vertebral
canal, the spinal nerve sends four categories of branches:
dorsal, ventral, communicating, and meningeal. The dorsal
and ventral branches supply structures located dorsal to,
and ventral to, the vertebral column, respectively. The
communicating branch is part of the autonomic nervous
system, whereas the meningeal branch reenters the verte-
bral canal to supply the meninges. Both dorsal and ventral
branches are mixed, with sensory and motor fi bers. One of
the most important spinal nerves is the phrenic nerve,
which supplies the diaphragm. It originates from some of
the last cervical spinal nerves.
T
HE
A
UTONOMIC
N
ERVOUS
S
YSTEM
The autonomic nervous system consists of the sympathetic
and the parasympathetic nervous systems. They are pro-
vided with nerve fi bers and ganglia (sing. ganglion). From
the origin to the ganglia, the fi bers are called pregangli-
onic, and from the ganglia to the organ supplied they are
called postganglionic. As a rule, the sympathetic pregan-
glionic fi bers are short, and the postganglionic are long.
The parasympathetic preganglionic fi bers are long, and the
postganglionic are short. Two categories of ganglia are
associated with the sympathetic system: prevertebral and
paravertebral. The former are located ventral to the thora-
columbar vertebrae, whereas the latter are two symmetrical
The Peripheral Nervous System
The peripheral nervous system is represented by the cranial
nerves, the spinal nerves, and the autonomic nervous
system. Its function is to send stimuli to the central nervous
system, and to send orders from the central nervous system
to the effectors.
