232 Chapter 8
8.5 Spinal Cord Tracts
Sensory information from most of the body is relayed to
the brain by means of ascending tracts of fibers that con-
duct impulses up the spinal cord. When the brain directs
motor activities, these directions are in the form of nerve
impulses that travel down the spinal cord in descending
tracts of fibers.neurotransmitters that promote arousal. Some of these neurons
have been shown to be involved in narcolepsy, a neurological
disorder (affecting about 1 in 2,000 people) in which the person
will fall asleep inappropriately during the day despite having
adequate amounts of sleep. Near the end of the twentieth cen-
tury, scientists demonstrated that people with narcolepsy have a
loss of LHA neurons that release a particular polypeptide neu-
rotransmitter that promotes wakefulness. This neurotransmit-
ter was discovered by two research groups who proposed two
names for it: orexin and hypocretin-1. Orexin neurons promote
wakefulness by activating neurons of the RAS that release his-
tamine, as well as other neurons that elicit arousal of the cere-
bral cortex.
Narcolepsy has a genetic basis that promotes autoimmune
destruction of the approximately 70,000 orexin neurons in the
lateral hypothalamus. Through extensive connections between
the lateral hypothalamus and other hypothalamic areas, as well
as with the limbic system and other cortical areas, orexin neu-
rons function to promote wakefulness, the craving for food (and
for abused drugs), and physical activity. Orexin also promotes
an increase in metabolic rate and, through activation of the
sympathoadrenal system, an elevation of blood pressure and
heart rate.
CLINICAL APPLICATION
Hypnotic drugs are those that promote sleep by acting on
the RAS. The antihistamine Benadryl, for example, promotes
drowsiness by crossing the blood-brain barrier and inhibit-
ing histamine-releasing neurons of the RAS. (The nondrowsy
antihistamine Claritin, by contrast, cannot cross the blood-
brain barrier.) Barbiturates, anesthetic gases, and the ben-
zodiazepines (such as Valium and Xanax ) act to enhance the
effectiveness of GABA, thereby inhibiting the RAS to reduce
arousal and promote sleep. The benzodiazepines and the
newer nonbenzodiazepine hypnotic drugs (such as Ambien ),
which act like the benzodiazepines but are chemically differ-
ent, are the most frequently prescribed drugs for promoting
sleep. The amphetamines, by contrast, promote wakefulness
by inhibiting the dopamine reuptake transporter, thereby
enhancing dopamine action in the monoamine arousal path-
ways of the RAS.Clinical Investigation CLUES
After his accident, Kevin took Ambien to help him sleep.- What type of drug is Ambien, and how does it
promote sleep? - What drugs use a different mechanism to promote
sleep?
| CHECKPOINT
8a. List the structures of the midbrain and describe their
functions.
8b. Describe the functions of the medulla oblongata and
pons.- Identify the parts of the brain involved in the reticular
activating system. What is the role of this system?
How is it inhibited?
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the sensory and motor pathways to and
from the cerebral hemispheres. - Describe the structure and function of the pyramidal
and extrapyramidal motor tracts.
The spinal cord extends from the level of the foramen magnum
of the skull to the first lumbar vertebra. Unlike the brain, in
which the gray matter forms a cortex over white matter, the
gray matter of the spinal cord is located centrally, surrounded
by white matter. The central gray matter of the spinal cord is
arranged in the form of an H, with two dorsal horns and two
ventral horns (also called posterior and anterior horns, respec-
tively). The white matter of the spinal cord is composed of
ascending and descending fiber tracts. These are arranged into
six columns of white matter called funiculi.
The fiber tracts within the white matter of the spinal cord
are named to indicate whether they are ascending (sensory) or
descending (motor) tracts. The names of the ascending tracts
usually start with the prefix spino- and end with the name of
the brain region where the spinal cord fibers first synapse. The
anterior spinothalamic tract, for example, carries impulses
conveying the sense of touch and pressure, and synapses in
the thalamus. From there it is relayed to the cerebral cortex.
The names of descending motor tracts, conversely, begin with
a prefix denoting the brain region that gives rise to the fibers
and end with the suffix- spinal. The lateral corticospinal tracts,