C h a p t e r 3 6
Disorders of Neuromuscular Function
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lowest level of the hierarchy occurs at the spinal cord,
which contains the basic reflex circuitry needed to
coordinate the function of the motor units. Above
the spinal cord is the brain stem, and above the brain
stem are the cerebellum and basal ganglia, structures
that modulate the actions of the brain stem systems.
Overseeing these supraspinal structures are the motor
centers in the cerebral cortex. The highest level of func-
tion, which occurs at the level of the frontal cortex,
is concerned with the purpose and planning of motor
movements. The efficiency of movement depends on
input from sensory systems that operate in parallel
with the motor systems.
The Spinal Cord
The spinal cord is the lowest level of motor hierarchi-
cal organization.
2
It contains the neuronal circuits that
mediate a variety of reflexes and automatic rhythmic
movements. Similar circuits governing reflex movements
of the face and mouth are located in the brain stem.
The simplest circuits are monosynaptic, containing only
a primary motor neuron. However, most reflexes are
polysynaptic, involving more than one interposed inter-
neurons. Interneurons and motor neurons also receive
input from axons descending from higher centers.
These supraspinal signals can modify reflex responses
to peripheral stimuli by facilitating or inhibiting differ-
ent populations of interneurons. They also coordinate
movements through these interneurons.
The Brain Stem
The next level of motor hierarchy is in the brain stem.
The brain stem contains many groups of neurons that
project from the spinal gray matter. These projections
are grouped into two main systems—the medial and
lateral brain stem systems—which receive input from
the cerebral cortex and subcortical nuclei and proj-
ect to the spinal cord.
2
The medial descending systems
contribute to the control of posture by integrating
visual, vestibular, and somatosensory information.
The lateral descending systems control more distal
limb muscles and are thus more concerned with goal-
directed movements, especially of the arm and hand.
Brain stem circuits also control movements of the eyes
and head.
The Motor Cortex
The motor cortex represents the highest level of motor
function. Precise, skillful, and intentional movements
of the distal and especially flexor muscles of the limbs
and speech apparatus are initiated and controlled by
the primary, premotor, and supplementary motor cor-
tices located in the posterior part of the frontal lobe
1,3
(Fig. 36-1). These motor areas receive information from
the thalamus and somatosensory cortex and, indirectly,
from the cerebellum and basal ganglia.
The primary motor cortex (Brodman area 4) is
located on the medial surface of the brain, and adjacent
portions of the central sulcus. It controls specific muscle
movement sequences and is the first level of descending
control for precise motor movements. The neurons in
the primary motor cortex are arranged in a somatotopic
array or distorted map of the body called the
motor
homunculus
4
(Fig. 36-2). The areas of the body that
require the greatest dexterity have the largest cortical
areas devoted to them. More than half of the primary
motor cortex is concerned with controlling the muscles
of the hands, of facial expression, and of speech.
The premotor cortex (areas 6 and 8), which is located
just anterior to the primary motor cortex, sends some
fibers into the corticospinal tract but mainly innervates
the primary motor cortex. Nerve signals generated by
the premotor cortex produce much more complex “pat-
terns” of movement than the discrete patterns generated
by the primary motor cortex. For example, the move-
ment pattern to accomplish a particular objective, such
as throwing a ball or picking up a fork, is programmed
by the prefrontal association cortex and associated tha-
lamic nuclei.
The supplementary motor cortex, which contains
representations of all parts of the body, is located on the
Supplementary
(6,8)
Premotor (6)
Premotor
cortex
Motor (4)
Motor
cortex
(4)
Somatosensory
(3,1,2)
Somatosensory
cortex
(3,1,2)
Primary
visual
(17)
Primary
visual
cortex
(17)
Primary
auditory
cortex
(44)
Frontal
eye fields
(part of
area 8)
Broca
area
(45,44)
Vestibular
cortex
(8) (6)
FIGURE 36-1.
Primary motor cortex.
(
Top
)
The location of the
primary, premotor, and supplementary cortex on the medial
surface of the brain. (Bottom)The location of the primary and
premotor cortex on the lateral surface of the brain.
