Human Physiology, 14th edition (2016)

The Central Nervous System 233

for example, begin in the cerebral cortex and descend the
spinal cord.

Ascending Tracts

The ascending fiber tracts convey sensory information from
cutaneous receptors, proprioceptors (muscle and joint recep-
tors), and visceral receptors ( table  8.4 ). Most of the sensory
information that originates in the right side of the body crosses
over to eventually reach the region on the left side of the brain
that analyzes this information. Similarly, the information aris-
ing in the left side of the body is ultimately analyzed by the
right side of the brain. For some sensory modalities, this decus-
sation occurs in the medulla oblongata ( fig. 8.24 ); for others, it
occurs in the spinal cord. These neural pathways are discussed
in more detail in chapter 10, section 10.2.

Descending Tracts

The descending fiber tracts that originate in the brain consist of
two major groups: the corticospinal, or pyramidal, tracts, and
the extrapyramidal tracts ( table 8.5 ). The pyramidal tracts descend
directly, without synaptic interruption, from the cerebral cortex to
the spinal cord. The cell bodies that contribute fibers to these pyra-
midal tracts are located primarily in the precentral gyrus, forming
the primary motor cortex. However, the supplementary motor
complex, located in the superior frontal gyrus just anterior to the
“leg” region of the primary motor cortex, (see fig. 8.7 ), contributes
about 10% of the fibers in the corticospinal tracts.
From 80% to 90% of the corticospinal fibers decussate in the
pyramids of the medulla oblongata (hence the name “pyramidal
tracts”) and descend as the lateral corticospinal tracts. The remain-
ing uncrossed fibers form the anterior corticospinal tracts, which
decussate in the spinal cord. Because of the crossing over of fibers,

the right cerebral hemisphere controls the musculature on the left

side of the body ( fig.  8.25 ), whereas the left hemisphere controls

the right musculature. The corticospinal tracts are primarily con-

cerned with the control of fine movements that require dexterity.

Because of the decussation of descending motor tracts, people

who have damage to the right cerebral hemisphere (particularly of

the parietal lobe) have motor deficits mostly in the left side of the

body. However, patients with lesions in the left-hemisphere pari-

etal lobe often have impaired skilled motor activity of both hands.

These and other observations have led scientists to believe that

the left hemisphere is specialized for skilled motor control of both

hands. The left hemisphere appears to control the left hand indi-

rectly, via projections to the right hemisphere through the corpus

callosum. The right hemisphere is believed also to cross-talk with

the left in the control of motor behavior, although its contributions

are less well understood.

The remaining descending tracts are extrapyramidal

motor tracts. These originate in the brain stem ( table 8.5 ) and

are largely controlled by the motor circuit structures of the cor-

pus striatum—caudate nucleus, putamen, and globus pallidus

(see figs. 8.11 and 8.12 )—as well as by the substantia nigra and

thalamus. This is why the symptoms of Parkinson’s disease,

produced by inadequate dopamine released by the nigrostriatal

pathway (as previously discussed), are often referred to medi-

cally as “extrapyramidal symptoms.” These symptoms demon-

strate that the extrapyramidal system is needed for the initiation

of body movements, maintenance of posture, control of the mus-

cles of facial expression, and other functions.

The term extrapyramidal can be understood in terms of

the following experiment: If the pyramidal tracts of an experi-

mental animal are cut, electrical stimulation of the cerebral

cortex, cerebellum, and basal nuclei can still produce move-

ments. The descending fibers that produce these movements

must, by definition, be extrapyramidal motor tracts. The

Table 8.4 | Principal Ascending Tracts of the Spinal Cord

Tract Origin Termination Function

Anterior

spinothalamic

Posterior horn on one side of cord

but crosses to opposite side

Thalamus, then cerebral

cortex

Conducts sensory impulses for crude touch and

pressure

Lateral

spinothalamic

Posterior horn on one side of cord

but crosses to opposite side

Thalamus, then cerebral

cortex

Conducts pain and temperature impulses that

are interpreted within cerebral cortex

Fasciculus gracilis

and fasciculus

cuneatus

Peripheral afferent neurons;

ascends on ipsilateral side of

spinal cord but crosses over in

medulla

Nucleus gracilis and nucleus

cuneatus of medulla;

eventually thalamus, then

cerebral cortex

Conducts sensory impulses from skin, muscles,

tendons, and joints, which are interpreted as

sensations of fine touch, precise pressures,

and body movements

Posterior

spinocerebellar

Posterior horn; does not cross

over

Cerebellum Conducts sensory impulses from one side of

body to same side of cerebellum; necessary

for coordinated muscular contractions

Anterior

spinocerebellar

Posterior horn; some fibers cross,

others do not

Cerebellum Conducts sensory impulses from both sides

of body to cerebellum; necessary for

coordinated muscular contractions