Neuronal Signaling and the Structure of the Nervous System
185
responses usually occur without conscious control or aware-
ness, as though they were indeed autonomous (in fact, the
autonomic nervous system has been called the “involuntary”
nervous system). However, it is wrong to assume that this is
always the case, for some visceral or glandular responses can be
learned and thus, to an extent, voluntarily controlled.
Blood Supply, Blood-Brain Barrier,
and Cerebrospinal Fluid
As mentioned earlier, the brain lies within the skull, and the
spinal cord lies within the vertebral column. Between the soft
neural tissues and the bones that house them are three types of
membranous coverings called
meninges:
the thick
dura mater
next to the bone, the
arachnoid mater
in the middle, and
the thin
pia mater
next to the nervous tissue (
Figure 6–47
).
The
subarachnoid space
between the arachnoid and pia is
fi l l e d w i t h
cerebrospinal fl
uid (CSF).
The meninges and
their specialized parts protect and support the central ner-
vous system, and they circulate and absorb the cerebrospinal
uid.
Meningitis
is an infection of the meninges that origi-
nates in the CSF of the subarachnoid space and that results in
increased intracranial pressure and, in some cases, seizures and
loss of consciousness.
As described previously, CSF is produced by ependymal
cells, which make up a specialized epithelial structure called
the
choroid plexus.
The black arrows in Figure 6–47 show
the fl
ow of CSF. It circulates through the interconnected ven-
tricular system to the brainstem, where it passes through small
openings out to a space between the meninges on the surface of
the brain and spinal cord. Aided by circulatory, respiratory, and
Right lateral
ventricle
Third ventricle
Fourth ventricle
Choroid plexus of
fourth ventricle
Choroid plexus
of third ventricle
Scalp
Skull bone
Dura mater
Dura mater
Pia mater
Pia mater
Meninges
Brain (cerebrum)
Arachnoid mater
Venous blood
Arachnoid
mater
Subarachnoid
space of brain
Subarachnoid
space of brain
Cerebrum
Cerebrospinal fluid
Brainstem
Lateral
ventricle
Central canal
Spinal cord
Cerebellum
Venous
blood
Vein
Figure 6–47
The four interconnected ventricles of the brain. The lateral ventricles form the fi rst two. The choroid plexus forms the cerebrospinal fl
uid
(CSF), which fl ows out of the ventricular system at the brainstem (arrows).
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