I. Skeletal muscles are controlled by their motor neurons. All
the motor neurons that control a given muscle form a motor
Motor Control Hierarchy
I. The neural systems that control body movements can be
conceptualized as being arranged in a motor control hierarchy.
a. The highest level determines the general intention of an
b. The middle level establishes a motor program and speciﬁ es
the postures and movements needed to carry out the
intended action, taking into account sensory information
that indicates the body’s position.
c. The local level ultimately determines which motor neurons
will be activated.
d. As the movement progresses, information about what the
muscles are doing feeds back to the motor control centers,
which make program corrections.
e. Almost all actions have conscious and unconscious
Local Control of Motor Neurons
I. Most direct input to motor neurons comes from local
interneurons, which themselves receive input from peripheral
receptors, descending pathways, and other interneurons.
II. Muscle spindle stretch receptors monitor muscle length and
the velocity of changes in length.
a. Activation of these receptors initiates the stretch reﬂ ex,
which inhibits motor neurons of ipsilateral antagonists and
activates those of the stretched muscle and its synergists.
This provides negative feedback control of muscle length.
b. Tension on the stretch receptors is maintained during
muscle contraction by gamma efferent activation to the
spindle muscle ﬁ bers.
c. Alpha and gamma motor neurons are generally coactivated.
III. Golgi tendon organs monitor muscle tension. Through
interneurons, they activate inhibitory synapses on motor
neurons of the contracting muscle and excitatory synapses on
motor neurons of ipsilateral antagonists. This provides negative
feedback control of muscle tension.
IV. The withdrawal reﬂ ex excites the ipsilateral ﬂ exor muscles and
inhibits the ipsilateral extensors. The crossed-extensor reﬂ ex
excites the contralateral extensor muscles during excitation of
the ipsilateral ﬂ exors.
The Brain Motor Centers and the Descending
Pathways They Control
I. Neurons in the motor cortex are anatomically arranged in a
II. Different areas of sensorimotor cortex have different functions,
but much overlap in activity.
III. The basal nuclei form a link in a circuit that originates in
and returns to sensorimotor cortex. These subcortical nuclei
facilitate some motor behaviors and inhibit others.
IV. The cerebellum coordinates posture and movement and plays a
role in motor learning.
V. The corticospinal pathways pass directly from the sensorimotor
cortex to motor neurons in the spinal cord (or brainstem, in
the case of the corticobulbar pathways) or, more commonly, to
interneurons near the motor neurons.
a. In general, neurons on one side of the brain control muscles
on the other side of the body.
b. Corticospinal pathways control predominately ﬁ ne, precise
c. Some corticospinal ﬁ bers affect the transmission of
information in afferent pathways.
VI. Other descending pathways arise in the brainstem and are
involved mainly in the coordination of large groups of muscles
used in posture and locomotion.
VII. There is some duplication of function between the two
I. Hypertonia, as seen in spasticity and rigidity, usually occurs
with disorders of the descending pathways.
II. Hypotonia can be seen with cerebellar disease or, more
commonly, with disease of the alpha motor neurons or muscle.
Maintenance of Upright Posture and Balance
I. Maintenance of posture and balance depends upon inputs from
the eyes, vestibular apparatus, and somatic proprioceptors.
II. To maintain balance, the body’s center of gravity must be
maintained over the body’s base.
III. The crossed-extensor reﬂ ex is a postural reﬂ ex.
I. The activity of interneuron networks in the spinal cord brings
about the cyclical, alternating movements of locomotion.
II. These pattern generators are controlled by corticospinal and
brainstem descending pathways and affected by feedback and
Additional Clinical Examples
is a disease of muscle rigidity caused when a bacterial
toxin blocks the release of an inhibitory neurotransmitter.
alpha motor neuron
crossed-extensor reﬂ ex
extrafusal ﬁ ber
gamma motor neuron
Golgi tendon organ
intrafusal ﬁ ber
lower motor neurons
motor neuron pool
nuclear bag ﬁ ber
nuclear chain ﬁ ber
postural reﬂ ex
primary motor cortex