Ganong's Review of Medical Physiology, 23rd Edition

246
SECTION III
Central & Peripheral Neurophysiology

undamaged part of the cortex. Thus, the maps of the motor cor-
tex are not immutable, and they change with experience.

BRAIN STEM PATHWAYS

INVOLVED IN POSTURE AND

VOLUNTARY MOVEMENT

As mentioned above, spinal motor neurons are organized
such that those innervating the most proximal muscles are lo-
cated most medially and those innervating the more distal
muscles are located more laterally. This organization is also
reflected in descending brain stem pathways (Figure 16–6).

MEDIAL BRAIN STEM PATHWAYS

The medial brain stem pathways, which work in concert with
the ventral corticospinal tract, are the
pontine and medullary
reticulospinal, vestibulospinal,
and
tectospinal tracts.
These

pathways descend in the ipsilateral ventral columns of the spi-

nal cord and terminate predominantly on interneurons and

long propriospinal neurons in the ventromedial part of the

ventral horn to control axial and proximal muscles. A few me-

dial pathway neurons synapse directly on motor neurons con-

trolling axial muscles.

The medial and lateral vestibulospinal tracts were briefly

described in Chapter 13. The medial tract originates in the

medial and inferior vestibular nuclei and projects bilaterally

to cervical spinal motor neurons that control neck muscula-

ture. The lateral tract originates in the lateral vestibular nuclei

and projects ipsilaterally to neurons at all spinal levels. It acti-

vates motor neurons to antigravity muscles (eg, proximal limb

extensors) to control posture and balance.

The pontine and medullary reticulospinal tracts project to

all spinal levels. They are involved in the maintenance of pos-

ture and in modulating muscle tone, especially via an input to

γ

-motor neurons. Pontine reticulospinal neurons are primar-

ily excitatory and medullary reticulospinal neurons are pri-

marily inhibitory.

FIGURE 16–6
Medial and lateral descending brain stem pathways involved in motor control. A)
Medial pathways (reticulospinal, ves-
tibulospinal, and tectospinal) terminate in ventromedial area of spinal gray matter and control axial and proximal muscles.
B)
Lateral pathway (ru-
brospinal) terminates in dorsolateral area of spinal gray matter and controls distal muscles.
(From Kandel ER, Schwartz JH, Jessell TM [editors]:
Principles of
Neural Science,
4th ed. McGraw-Hill, 2000.)

Rubrospinal

tract

B Lateral brain stem pathways

Red nucleus

(magnocellular part)

A Medial brain stem pathways

Tectum

Medial

reticular

formation

Reticulospinal

tract

Tectospinal

tract

Vestibulospinal

tracts

Lateral and medial

vestibular nuclei