512
UNIT 3
Regulation and Integration of the Body
13
(CPGs)
. CPGs consist of networks of oscillating inhibitory and
excitatory neurons, which set crude rhythms and alternating
patterns of movement.
The Projection Level
Te spinal cord is under the direct control of the
projection
level
of motor control. Te projection level consists of neurons
acting through the direct and indirect motor pathways (see
±able 12.3 and pp. 469–473):
Upper motor neurons
of the motor cortex initiate the
direct
(
pyramidal
)
pathways
. Axons of direct pathway neurons pro-
duce discrete voluntary movements of the skeletal muscles.
Brain stem motor nuclei oversee the
indirect pathways
. Ax-
ons of these pathways help control reflex and CPG-controlled
motor actions, modifying and controlling the activity of the
segmental apparatus.
Projection motor pathways convey information to lower
motor neurons, and send a copy of that information as
inter-
nal feedback
to higher command levels, continually informing
them of what is supposed to happen. Te direct (pyramidal)
and indirect pathways provide separate and parallel pathways
for controlling the spinal cord, but these systems are interrelated
at all levels.
The Precommand Level
±wo other systems of brain neurons, located in the cerebellum
and basal nuclei, regulate motor activity. Tey precisely start
or stop movements, coordinate movements with posture, block
Reflex activity
Motor
output
Sensory
input
(a) Levels of motor control and their interactions
(b) Structures involved
Precommand level
Cerebellum
Basal nuclei
Projection level
Primary motor cortex
Brain stem nuclei
Precommand Level (highest)
Cerebellum and basal nuclei
Programs and instructions
(modified by feedback)
Projection Level (middle)
Motor cortex (pyramidal pathways)
and brain stem nuclei (vestibular,
red, reticular formation, etc.)
Conveys instructions to spinal cord
motor neurons and sends a copy of
that information to higher levels
Segmental Level (lowest)
Spinal cord
Contains central pattern generators
(CPGs)
Segmental level
Spinal cord
Figure 13.14
Hierarchy of motor control.
Dashed lines in (a) indicate feedback.
unwanted movements, and monitor muscle tone. Collectively
called
precommand areas
, these systems
control the outputs
of
the cortex and brain stem motor centers and stand at the highest
level of the motor hierarchy.
Te key center for “online” sensorimotor integration and
control is the
cerebellum
. Remember the cerebellum is a tar-
get of ascending proprioceptor, tactile, equilibrium, and visual
inputs—feedback that it needs to rapidly correct “errors” in mo-
tor activity. It also receives information via branches from de-
scending pyramidal tracts, and from various brain stem nuclei.
Te cerebellum lacks direct connections to the spinal cord.
It acts on motor pathways through the projection areas of the
brain stem and on the motor cortex via the thalamus to fine-
tune motor activity.
Te
basal nuclei
receive inputs from
all
cortical areas and
send their output back mainly to premotor and prefrontal cor-
tical areas via the thalamus. Compared to the cerebellum, the
basal nuclei appear to be involved in more complex aspects of
motor control. Under resting conditions, the basal nuclei inhibit
various motor centers of the brain. When the motor centers are
released from inhibition, coordinated motions can begin.
Cells in both the basal nuclei and the cerebellum are involved
in this unconscious planning and discharge
in advance
of willed
movements. When you actually move your fingers, both the
precommand areas and the primary motor cortex are active.
At the risk of oversimplifying, it appears that the cortex says, “I
want to do this,” and then lets the precommand areas take over
to provide the proper timing and patterns to execute the desired
movements. Te precommand areas control the motor cortex
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