Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 16

Control of Posture & Movement 243

neurons; this indirect pathway is important in coordinating
groups of muscles.
The trajectory from the cortex to the spinal cord passes
through the corona radiata to the posterior limb of the inter-
nal capsule. Within the midbrain they traverse the cerebral
peduncle and the basilar pons until they reach the medullary
pyramids on their way to the spinal cord.
The
corticobulbar tract
is composed of the fibers that pass
from the motor cortex to motor neurons in the trigeminal,
facial, and hypoglossal nuclei. Corticobulbar neurons end
either directly on the cranial nerve nuclei or on their anteced-
ent interneurons within the brain stem. Their axons traverse
through the genu of the internal capsule, the cerebral pedun-
cle (medial to corticospinal tract neurons), to descend with
corticospinal tract fibers in the pons and medulla.

The motor system can be divided into lower and upper

motor neurons.

Lower motor neurons

refer to the spinal and

cranial motor neurons that directly innervate skeletal mus-

cles.

Upper motor neurons

are those in the cortex and brain

stem that activate the lower motor neurons. The pathophysio-

logical responses to damage to lower and upper motor neu-

rons are very distinctive (see Clinical Box 16–1).

ORIGINS OF CORTICOSPINAL &

CORTICOBULBAR TRACTS

Corticospinal and corticobulbar tract neurons are pyramidal

shaped and located in layer V of the cerebral cortex (see Chap-

ter 11). The cortical areas from which these tracts originate

were identified on the basis of electrical stimulation that pro-

duced prompt discrete movement. Figure 16–3 shows the ma-

jor cortical regions involved in motor control. About 31% of

the corticospinal tract neurons are from the

primary motor

cortex (M1; Brodmann’s area 4).

This region is in the precen-

tral gyrus of the frontal lobe, extending into the central sulcus.

The

premotor cortex

and

supplementary motor cortex

(Brodmann’s area 6)

account for 29% of the corticospinal

tract neurons. The premotor area is anterior to the precentral

gyrus, on the lateral and medial cortical surface; and the sup-

plementary motor area is on and above the superior bank of

the cingulate sulcus on the medial side of the hemisphere. The

other 40% of corticospinal tract neurons originate in the

pari-

etal lobe (Brodmann’s area 5, 7)

and

primary somatosenso-

ry area (Brodmann’s area 3, 1, 2)

in the postcentral gyrus.

MOTOR CORTEX & VOLUNTARY

MOVEMENT

PRIMARY MOTOR CORTEX

By means of stimulation experiments in patients undergoing

craniotomy under local anesthesia, it has been possible to out-

line most of the motor projections from M1. These have been

confirmed in unanesthetized unoperated humans by PET scan

and fMRI (Figure 16–4). The various parts of the body are rep-

resented in the precentral gyrus, with the feet at the top of the

gyrus and the face at the bottom (Figure 16–5). The facial area

is represented bilaterally, but the rest of the representation is

generally unilateral, with the cortical motor area controlling

the musculature on the opposite side of the body. The cortical

representation of each body part is proportionate in size to the

skill with which the part is used in fine, voluntary movement.

The areas involved in speech and hand movements are espe-

cially large in the cortex; use of the pharynx, lips, and tongue

to form words and of the fingers and apposable thumbs to ma-

nipulate the environment are activities in which humans are

especially skilled. A somatotopic organization continues

throughout the corticospinal and corticobulbar pathways.

FIGURE 16–2
The corticospinal tracts.
This tract originates in
the precentral gyrus and passes through the internal capsule. Most fi-
bers decussate in the pyramids and descend in the lateral white matter
of the spinal cord to form the lateral division of the tract which can
make monosynaptic connections with spinal motor neurons. The ven-
tral division of the tract remains uncrossed until reaching the spinal
cord where axons terminate on spinal interneurons antecedent to mo-
tor neurons.

Corticospinal tract

Decussation of

the pyramids

Anterior

horn cell

Internal capsule

Lateral cortico-

spinal tract

(80% of fibers)

Spinal nerve

Pyramids

Ventral cortico-

spinal tract

(20% of fibers)

Interneuron

Precentral

gyrus

(area 4,

etc)

Distal

muscle

Proximal

muscle