Ganong's Review of Medical Physiology, 23rd Edition

242
SECTION III
Central & Peripheral Neurophysiology

GENERAL PRINCIPLES

ORGANIZATION

There are two types of motor output: reflexive (involuntary)
and voluntary. A subdivision of reflex responses includes
some rhythmic movements such as swallowing, chewing,
scratching, and walking, which are largely involuntary but
subject to voluntary adjustment and control.
To move a limb, the brain must plan a movement, arrange
appropriate motion at many different joints at the same time,
and adjust the motion by comparing plan with performance.
The motor system “learns by doing” and performance improves
with repetition. This involves synaptic plasticity.
There is considerable evidence for the general motor control
scheme shown in Figure 16–1. Commands for voluntary
movement originate in cortical association areas. The move-
ments are planned in the cortex as well as in the basal ganglia
and the lateral portions of the cerebellar hemispheres, as indi-
cated by increased electrical activity before the movement. The
basal ganglia and cerebellum funnel information to the pre-
motor and motor cortex by way of the thalamus. Motor com-
mands from the motor cortex are relayed in large part via the
corticospinal tracts to the spinal cord and the corresponding
corticobulbar tracts to motor neurons in the brain stem. How-
ever, collaterals from these pathways and a few direct connec-
tions from the motor cortex end on brain stem nuclei, which
also project to motor neurons in the brain stem and spinal
cord. These pathways can also mediate voluntary movement.
Movement sets up alterations in sensory input from the special
senses and from muscles, tendons, joints, and the skin. This
feedback information, which adjusts and smoothes movement,
is relayed directly to the motor cortex and to the spinocerebel-
lum. The spinocerebellum projects in turn to the brain stem.
The main brain stem pathways that are concerned with pos-
ture and coordination are the rubrospinal, reticulospinal,
tectospinal, and vestibulospinal tracts.

CONTROL OF AXIAL & DISTAL MUSCLES

Within the brain stem and spinal cord, pathways and neurons

that are concerned with the control of skeletal muscles of the

trunk and proximal portions of the limbs are located medially

or ventrally, whereas pathways and neurons that are con-

cerned with the control of skeletal muscles in the distal por-

tions of the limbs are located laterally. The axial muscles are

concerned with postural adjustments and gross movements,

whereas the distal limb muscles mediate fine, skilled move-

ments. Thus, for example, neurons in the medial portion of

the ventral horn innervate proximal limb muscles, particularly

the flexors, whereas lateral ventral horn neurons innervate

distal limb muscles. Similarly, the ventral corticospinal tract

and medial descending brain stem pathways (tectospinal,

reticulospinal, and vestibulospinal tracts) are concerned with

adjustments of proximal muscles and posture, whereas the lat-

eral corticospinal and rubrospinal tracts are concerned with

distal limb muscles and, particularly in the case of the lateral

corticospinal tract, with skilled voluntary movements. Phylo-

genetically, the lateral pathways are newer. More details about

these motor pathways are provided below.

CORTICOSPINAL &

CORTICOBULBAR TRACTS

DESCENDING PROJECTIONS

The axons of neurons from the motor cortex that project to

spinal motor neurons form the

corticospinal tracts,

a large

bundle of about 1 million fibers. About 80% of these fibers

cross the midline in the medullary pyramids to form the

later-

al corticospinal tract

(Figure 16–2). The remaining 20%

make up the

ventral corticospinal tract,

which does not cross

the midline until it reaches the level of the spinal cord at which

it terminates. Lateral corticospinal tract neurons make mono-

synaptic connections to motor neurons, especially those con-

cerned with skilled movements. Corticospinal tract neurons

also synapse on spinal interneurons antecedent to motor

FIGURE 16–1
Control of voluntary movement.
Commands for voluntary movement originate in cortical association areas. The cortex,
basal ganglia, and cerebellum work cooperatively to plan movements. Movement executed by the cortex is relayed via the corticospinal tracts and
corticobulbar tracts to motor neurons. The cerebellum provides feedback to adjust and smooth movement.

Idea

Cortical

association

areas

Basal ganglia

Lateral

cerebellum

Premotor and

motor cortex

Intermediate

cerebellum

Movement

Plan Execute