Encyclopedia of Health and Medicine

232 The Nervous System
and HORMONE functions of basic survival (such
as body temperature regulation)
• thalamus, a small structure that filters and sorts
(modulates) sensory impulses that enter and
the motor impulses that leave the brain
The diencephalon incorporates the olfactory
and optic tracts (origins and pathways of the first
and second cranial nerves, respectively). Also
within the diencephalon are the PITUITARY GLAND
and PINEAL GLAND.
The Midbrain
The midbrain, also called the brainstem, is the
point of origin for the third cranial nerve (oculomotor
nerve) and the fourth cranial nerve
(trochlear nerve). It is the neuronal bridge that
joins the forebrain, hindbrain, and spinal cord.
The midbrain controls primitive survival functions
such as BREATHING and heartbeat. It also contains a
cluster of cells called the substantia nigra which
secrete DOPAMINE, a NEUROTRANSMITTER essential for
movement. The death of cells in the substantia
nigra causes PARKINSON’S DISEASE.
The Hindbrain
The hindbrain is beneath and to the back of the
forebrain. Its two substructures are the metencephalon
and the myelencephalon, which control
numerous bodily functions essential for survival.
The fourth ventricles are also located within the
hindbrain.
The metencephalon is the point of origin for
the fifth (trigeminal), sixth (abducens), seventh
(facial), and eighth (vestibulocochlear) cranial
nerves. The metencephalon contains the:
• cerebellum, which directs and coordinated voluntary
MUSCLE function; it receives sensory
input from the vestibular structures of the
inner EAR (balance) and from peripheral proprioceptors
(specialized sensory nerve endings in
the limbs) that report the body’s spatial orientation
within its environment
• pons, which connects the medulla oblongata
and the cerebellum with the cerebrum and
from which the fifth (trigeminal), sixth
(abducens), seventh (facial), and cochlear segment
of the eighth (vestibulocochlear) cranial
nerves originate
The myelencephalon contains the medulla
oblongata, which connects the brainstem and the
spinal cord. The ninth (glossopharyngeal), tenth
(vagus), eleventh (spinal accessory), and twelfth
(hypoglossal) cranial nerves arise from the
medulla oblongata. The fourth ventricle is within
the medulla oblongata. The medulla oblongata
regulates BLOOD PRESSURE, HEART RATE, RESPIRATORY
RATE, digestion, and elimination (URINATION and
defecation), as well as reflexive actions such as
sneezing and coughing.
Neuron Communication in the Brain
Brain neurons communicate with one another
through electrical impulses and biochemical facilitators
called neurotransmitters. Neurotransmitters
conduct or block the impulse’s travel across a
synapse. Each brain neuron has up to 10,000
synapses, which make up its neuronal pathways;
the brain overall has 50 to 200 trillion synapses.
The brain sends and receives nerve impulses contralaterally—that
is, the brain’s right hemisphere
handles functions dealing with the left side of the
body and its left hemisphere handles functions
dealing with the right side of the body. The brain
receives sensory nerve signals from the body,
which its various regions and areas process and
assimilate. The brain sends motor nerve signals to
the body in response.
Recent research suggests the brain appears to
continually adapt and adjust its neuronal, or
synaptic, pathways by extending and branching
existing axons and shutting down axon branches
it no longer uses. This process of continual pruning
seems aimed at keeping the brain’s neuronal
communications streamlined and efficient and
perhaps also at compensating for diminishment
that may occur through aging. Though the brain is
most receptive to learning during the childhood
years when the establishment of synaptic pathways
is at its peak, the brain remains capable of
learning for the duration of the lifespan.
Health Conditions and the Brain
The brain is vulnerable to the effects of health
conditions that affect other body systems as well