212 Introduction to Anatomy and Physiology
Cerebellum
The cerebellum (SER-eh-BEHL-um),
found below the occipital lobe, looks similar
to the cerebrum with its outer gray cortex,
convolutions, and dual hemispheres (Figure 6.9).
The cerebellum serves the important role of
coordinating body movements, including balance.
To coordinate body movements and balance,
the cerebellum receives input from the eyes, inner
ears, and sensory receptors throughout the body. It
also continuously monitors body segment positions
and motions. If the body’s positions and motions
are not what the cerebellum intended them to be, it
sends out signals to make adjustments.
The primary functions of the different
structures of the brain are summarized in
Figure 6.13.
Meninges
Three protective membranes, the meninges
(meh-NIN-jeez), surround the brain and spinal
cord (Figure 6.14). The outer membrane, the
dura mater (DOO-rah MAY-ter), meaning “hard
mother,” is a tough, double-layered membrane
that lies beneath the skull and surrounds
the brain. The inner layer of the dura mater
continues down to enclose the spinal cord.
The middle membrane, the arachnoid mater,
is composed of weblike tissue. Beneath this
membrane is the subarachnoid space, fi lled with
cerebrospinal (seh-REE-broh-SPIGH-nal) fl uid,
which cushions the brain and spinal cord.
The innermost layer of the meninges
attaches directly to the surface of the brain and
spinal cord. This layer is the delicate pia mater
(PIGH-ah MAY-ter), meaning “gentle mother.”
Blood-Brain Barrier
A rich network of blood vessels supplies
the brain. Like all tissues of the body, the brain
depends on a circulating blood supply to provide
nutrients and carry away the waste products
of cell metabolism. At any given time, roughly
20%–25% of the blood in your body is circulating
in the region of the brain.
The capillaries supplying the brain,
however, are different from other capillaries in
the body. Specifi cally, they are impermeable to
many substances that freely diffuse through the
walls of capillaries in other body regions. This
property of impermeability has given rise to the
term blood-brain barrier.
The blood-brain barrier protects the brain
against surges in concentrations of hormones,
ions, and some nutrients. Substances allowed
to pass through the capillaries include water,
glucose, and essential amino acids. Other
substances that can penetrate the blood-brain
barrier are blood-borne alcohol, nicotine, fats,
respiratory gases, and anesthetics.
Spinal Cord
The spinal cord extends from the brain stem
down to the beginning of the lumbar region
of the spine. It serves as a major pathway for
relaying sensory impulses to the brain and motor
impulses from the brain. It also provides the
Figure 6.12 The diencephalon includes the thalamus
(exterior), and the hypothalamus and epithalamus
(both interior). The brain stem includes the midbrain,
pons, and medulla oblongata. Which area of the brain
identified above controls your sneezing reflex?
Diencephalon
(thalamus
in view)
Midbrain
Pons
Medulla
oblongata
Spinal cord
Anterior view
Brain
stem