Page 337 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Motor System Cortical afferents. The cortex receives
motor input from (a) the body periphery (via
Coordinated muscular movements (walking, thalamus ! S1 [! p. 323 A] ! sensory asso-
grasping, throwing, etc.) are functionally de- ciation cortex ! PMA); (b) the basal ganglia
pendent on the postural motor system, which is (via thalamus ! M1, PMA, SMA [! A2] ! pre-
responsible for maintaining upright posture, frontal association cortex); (c) the cerebellum
balance, and spatial integration of body move- (via thalamus ! M1, PMA; ! A2); and (d)
sensory and posterior parietal areas of the cor-
ment. Since control of postural motor function
Central Nervous System and Senses pulses from the periphery, this is also referred put from the cortex is mainly projected to (a)
and muscle coordination requires the simul-
tex (areas 1–3 and 5–7, respectively).
Cortical efferents. (! C, D, E, F) Motor out-
taneous and uninterrupted flow of sensory im-
the spinal cord, (b) subcortical motor centers
to as sensorimotor function.
(see below and p. 328), and (c) the con-
α motoneurons in the anterior horn of the
spinal cord and in cranial nerve nuclei are the
tralateral cortex via commissural pathways.
terminal tracts for skeletal muscle activation.
The pyramidal tract includes the corticospi-
Only certain parts of the corticospinal tract
nal tract and part of the corticobulbar tract.
Over 90% of the pyramidal tract consists of thin
and type Ia afferents connect to α mo-
fibers, but little is known about their function.
toneurons monosynaptically. Other afferents
tract (! C) project to the spinal cord from
sors, mechanosensors), other spinal cord seg-
ments, the motor cortex, cerebellum, and
areas 4 and 6 and from areas 1–3 of the sensory
motor centers of the brain stem connect to α
cortex. Some of the fibers connect mono-
12 from the periphery (propriosensors, nocisen- The thick, rapidly conducting corticospinal
motoneurons via hundreds of inhibitory and
synaptically to α and γ motoneurons re-
stimulatory interneurons per motoneuron. sponsible for finger movement (precision
Voluntary motor function. Voluntary move- grasping). The majority synapse with inter-
ment requires a series of actions: decision to neurons of the spinal cord, where they in-
move ! programming (recall of stored sub- fluence input from peripheral afferents as well
programs) ! command to move ! execution as motor output (via Renshaw’s cells) and
of movement (! A1–4). Feedback from affer- thereby spinal reflexes.
ents (re-afferents) from motor subsystems and
information from the periphery is constantly Function of the Basal Ganglia
integrated in the process. This allows for ad- Circuitry. The basal ganglia are part of multiple
justments before and while executing volun- parallel corticocortical signal loops. Associ-
tary movement. ative loops arising in the frontal and limbic cor-
tex play a role in mental activities such as
The neuronal activity associated with the first two
phases of voluntary movement activates numerous assessment of sensory information, adapta-
motor areas of the cortex. This electrical brain activ- tion of behavior to emotional context, motiva-
ity is reflected as a negative cortical expectancy tion, and long-term action planning. The func-
potential, which can best be measured in associa- tion of the skeletomotor and oculomotor loops
tion areas and the vertex. The more complex the (see below) is to coordinate and control the
movement, the higher the expectancy potential and velocity of movement sequences. Efferent pro-
the earlier its onset (roughly 0.3–3 s). jections of the basal ganglia control
The motor cortex consists of three main areas thalamocortical signal conduction by (a) at-
(! C, top; ! see p. 311 E for area numbers): tenuating the inhibition (disinhibiting effect,
(a) primary motor area, M1 (area 4), (b) premo- direct mode) of the thalamic motor nuclei and
tor area, PMA (lateral area 6); and (c) sup- the superior colliculus, respectively, or (b) by
plementary motor area, SMA (medial area 6). intensifying their inhibition (indirect mode).
The motor areas of the cortex exhibit somato- The principal input to the basal ganglia
topic organization with respect to the target comes from the putamen and caudate nucleus,
muscles of their fibers (shown for M1 in B) and which are collectively referred to as the stri-
324
their mutual connections. atum. Neurons of the striatum are activated by
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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