Ganong's Review of Medical Physiology, 23rd Edition

242 SECTION III Central & Peripheral Neurophysiology
Idea
GENERAL PRINCIPLES
ORGANIZATION
Cortical
association
areas
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 movements
are planned in the cortex as well as in the basal ganglia
and the lateral portions of the cerebellar hemispheres, as indicated
by increased electrical activity before the movement. The
basal ganglia and cerebellum funnel information to the premotor
and motor cortex by way of the thalamus. Motor commands
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. However,
collaterals from these pathways and a few direct connections
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 spinocerebellum.
The spinocerebellum projects in turn to the brain stem.
The main brain stem pathways that are concerned with posture
and coordination are the rubrospinal, reticulospinal,
tectospinal, and vestibulospinal tracts.
Plan Execute
Basal ganglia
Lateral
cerebellum
Premotor and
motor cortex
Intermediate
cerebellum
Movement
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.
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 concerned
with the control of skeletal muscles in the distal portions
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 movements.
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 lateral
corticospinal and rubrospinal tracts are concerned with
distal limb muscles and, particularly in the case of the lateral
corticospinal tract, with skilled voluntary movements. Phylogenetically,
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 lateral
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 monosynaptic
connections to motor neurons, especially those concerned
with skilled movements. Corticospinal tract neurons
also synapse on spinal interneurons antecedent to motor