Page 341 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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connections to the cortex. Input: a. Via the neurons (Golgi, stellate and basket cells) heighten
pontine nuclei and mossy fibers, the lateral the contrast of the excitatory pattern on the cerebel-
cerebellum receives input from cortical cen- lar cortex by lateral and recurrent inhibition.
ters for movement planning (e.g., parietal, pre- Postural Motor Control
frontal and premotor association cortex; sen-
sorimotor and visual areas). b. It also receives Simple stretch reflexes (! p. 316) ) as well as
input from cortical and subcortical motor cen- the more complicated flexor reflexes and
crossed extensor reflexes (! p. 320) are con-
ters via the inferior olive and climbing fibers
Central Nervous System and Senses areas of the cortex. tial loss of peripheral reflexes below the lesion
trolled at the level of the spinal cord.
(see below). Output from the lateral cerebel-
lum projects across motor areas of the
Spinal cord transection (paraplegia) leads to an ini-
thalamus from the dentate nucleus to motor
(areflexia, spinal shock), but the reflexes can later be
provoked in spite of continued transection.
Lesions of the median cerebellum lead to distur-
The spinal reflexes are mainly subordinate to
bances of balance and oculomotor control (vertigo,
supraspinal centers (! E). Postural motor
nausea, pendular nystagmus) and cause trunk and
gait ataxia. Lesions of the lateral cerebellum lead to
function is chiefly controlled by motor centers
of the brain stem (! E1), i.e., the red nucleus,
disturbances of initiation, coordination and termina-
tion of goal-directed movement and impair the rapid
vestibular nuclei (mainly lateral vestibular nu-
ment (diadochokinesia). The typical patient exhibits
These centers function as relay stations that
tremor when attempting voluntary coordinated
pass along information pertaining to postural
movement (intention tremor), difficulty in measuring
and labyrinthine postural reflexes required to
the distances during muscular movement (dys-
12 reprogramming of diametrically opposing move- cleus), and parts of the reticular formation.
metria), pendular rebound motion after stopping a maintain posture and balance (involuntary).
movement (rebound phenomenon), and inability to Postural reflexes function to regulate muscle
perform rapid alternating movements (adiado- tone and eye adaptation movements (! p.
chokinesia ). 343 C). Input is received from the equilibrium
The cerebellar cortex exhibits a uniform neural organ (tonic labyrinthine reflexes) and from
ultrastructure and circuitry. All output from propriosensors in the neck (tonic neck reflexes).
the cerebellar cortex is conducted via neurites The same afferents are involved in postural re-
of approximately 15 ! 10 Purkinje cells. These flexes (labyrinthine and neck reflexes) that
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GABAergic cells project to and inhibit neurons help to maintain the body in its normal posi-
of the fastigial, emboliform, dentate, and tion. The trunk is first brought to its normal
lateral vestibular nuclei (Deiter’s nucleus; ! F, position in response to inflow from neck pro-
right panel). prioceptors. Afferents projecting from the
cerebellum, cerebral motor cortex (! C), eyes,
Input and circuitry: Input from the spinal cord ears, and olfactory organ as well as skin recep-
(spinocerebellar tracts) is relayed by the inferior olive
and projected via stimulatory (1 : 15 diverging) tors also influence postural reflexes. Sta-
climbing fibers that terminate on a band of Purkinje tokinetic reflexes also play an important role in
cells extending across the folia of the cerebellum, the control of body posture and position. They
forming the sagittal excitatory foci. The climbing play a role e.g. in startle reflexes and nystag-
fibers use aspartate as their transmitter. Serotoniner- mus (! p. 360).
gic fibers from the raphe nuclei and noradrenergic
fibers from the locus caeruleus terminate also on the Descending tracts to the spinal cord arising from
excitatory foci. Mossy fibers (pontine, reticular and the red nucleus and medullary reticular formation
spinal afferents) excite the granular cells. Their axons (rubrospinal and lateral reticulospinal tracts) have a
form T-shaped branches (parallel fibers). In the generally inhibitory effect on α and γ motoneurons
molecular layer, they densely converge (ca. 10 : 1) (! p. 316) of extensor muscles and an excitatory ef-
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on strips of Purkinje cells that run alongside the fect on flexor muscles (! E2). Conversely, the tracts
folium; these are called longitudinal excitatory foci. It from Deiter’s nucleus and the pontine areas of the re-
is assumed that the climbing fiber system (at the ticular formation (vestibulospinal and medial reti-
“crossing points” of the perpendicular excitatory culospinal tracts) inhibit the flexors and excite the α
328 foci) amplify the relatively weak signals of mossy and γ fibers of the extensors. Transection of the brain
fiber afferents to Purkinje cells. Numerous inter- stem below the red nucleus leads to decerebrate rigid-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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