Classical conditioning of the eye blink reflex

Motor learning which occurs during the classical conditioning of the eye blink reflex has been widely studied. In the eye blink reflex a puff of air delivered to the eye (US) will produce an eye blink (UR). The eye blink reflex can be conditioned if the air puff is paired with a tone (CS). The circuitry involved in motor learning in this reflex is shown in Figure.

Figure: Motor learning in the cerebellum; circuitry implicated in the conditioned eye blink reflex. pf, parallel  fiber; PC, Purkinje cell; CR, conditioned response; UR, unconditioned response; US, unconditioned stimulus;  cf, climbing  fiber; gc, granule cell; mf, mossy fiber.

The air puff (US) is sensed by neurons in the spinal nucleus of the trigeminal (5th) cranial nerve. The eye blink reflex (UR) is executed by connections among these cells and motor neurons in the facial (7th cranial) nerve. The Conditioning of the reflex needs the cerebellum. The US signal is transmitted through climbing fibers which arise from the inferior olivary nucleus. The tone (CS) signal goes by way of the pontine nucleus and ventral cochlear nucleus arriving at the cerebellum in mossy fibers.  Activation of the pf–PC synapse through the CS, 250 ms before the arrival of the US through the climbing fiber results in LTD of the pf–PC synapse. The effect of the LTD is which any subsequent arrival of the CS produces a smaller excitation of the PC. Therefore PC inhibition of the interpositus neurons is diminished so these cerebellar nucleus cells drive the eye blink by their connections with the red nucleus.