The Cerebellum: Brain for an Implicit Self by Masao Ito

Author:Masao Ito [Ito, Masao]
Language: eng
Format: epub
Tags: Science, Life Sciences, Medical, Biology, Neurology, Neuroscience
ISBN: 9780132623087
Google: mntMnsG4c3EC
Amazon: 0137050682
Publisher: FT Press
Published: 2011-08-01T05:00:00+00:00

12-7. Adaptive Control of Somatic Reflexes

Similar control system models can be applied to somatic reflexes. Deiters neurons, via the lateral vestibulospinal tract, function as the controller of the controlled object provided by the segmental circuit for extensor muscles (Figure 32). In this system, the peripheral proprioceptive and cutaneous sensory signals are sent to Deiters neurons via spinocerebellar tracts. Deiters neurons, in turn, send descending signals to excite extensor motoneurons and inhibit their inhibitory interneurons. As a result, a limb can remain extended during a standing posture or during the supporting (stance) phase of stepping. Attached to this long loop reflex, the B-zone receives mossy fiber afferents from the spinocerebellar tracts and, in turn, projects Purkinje cell axons directly to Deiters neurons. Climbing fiber responses occur in B-zone Purkinje cells when an error is sensed (Chapter 10).

For the withdrawal reflex of a forelimb, the anterior interpositus nucleus acts as a controller, whereas the magnocellular red nucleus neurons, forelimb flexor motoneurons, and associated segmental neurons are nested as a controlled object (Figure 33). A microcomplex involving the C1/C3-zone and the anterior interpositus nucleus constitutes an adaptive controller.

The unconditioned eye-blink reflex is evoked by an air puff, which drives trigeminal premotor neurons and motoneurons and induces eyelid closure. The magnocellular red nucleus forms a long loop pathway superposed on the eye-blink reflex pathway. Based on the neuronal circuit so far dissected (Figure 35), the block diagram in Figure 41 is formulated. There, the anterior interpositus nucleus, together with the HVI area of the C1- and C3-zones, form an adaptive controller of the eye-blink reflex, whereas the magnocellular red nucleus is nested to the controlled object together with trigeminal premotor neurons and motoneurons. Normally, tone pip stimuli do not drive the eye-blink reflex, but after classic eye-blink conditioning, they evoke an eye blink in a timely and predictive manner in order to protect the cornea from the air puff. This adaptive mechanism is now explained by a combination of conjunctive LTD that is induced by tone-pip-induced simple spikes, and air-puff-induced climbing fiber spikes in Purkinje cells. Another form of learning is also involved. It occurs in those synapses supplied by the tone-pipe-mediating mossy fibers to the anterior interpositus nuclear neurons (see Chapter 9, Section 7 and Chapter 11, Section 5).

Figure 41. Control system scheme for eye-blink conditioning.

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