Smooth pursuit involves tracking a moving target with the eyes (Fig. 9.29 ). Visual information about target velocity is processed in a series of cortical areas, including the visual cortex in the occipital lobe, several temporal lobe areas, and the frontal eye fields. In the past, the frontal eye fields were thought to be related only to control of saccades, but more recent evidence has shown that there are distinct regions within the frontal eye fields dedicated to either saccade production or smooth pursuit. Indeed, there may be two distinct cortical networks, each specialized for one of these types of eye movement. Cortical activity from multiple cortical areas is fed to the cerebellum via parts of the pontine nuclei and nucleus reticularis tegmenti pontis. Specific areas in the cerebellum—namely, parts of the posterior lobe vermis, the flocculus, and the paraflocculus—receive this input, and they in turn project to the vestibular nuclei. From the vestibular nuclei, activity can then be forwarded to the oculomotor, abducens, and trochlear nuclei, as was described for the VOR earlier.
The neural circuits underlying vergence movements are not well known. There are premotor neurons (neurons that feed onto motor neurons) located in the brainstem areas surrounding the various oculomotor nuclei. In some cortical visual areas and the frontal eye fields, there are neurons whose activity is related to the disparity of the image on the two retinas or to the variation of the image during vergence movements. How vergence signals in these cortical areas feed into the brainstem premotor neurons is not clear. The cerebellum also appears to play a role in vergence movements because cerebellar lesions impair this type of eye movement. Note that lesions of the medial longitudinal fasciculus that result in a loss of the VOR do not compromise vergence.
•Fig. 9.28 Horizontal Saccade Pathways. A, Circuitdiagramofthemajorpathways.EBN,excitatoryburstneuron;FEF,frontaleyefield;IBN,inhibitoryburstneuron;LBN,longleadburstneuron;OPN,omnipauseneuron;PPRF,paramedianpontinereticularformation.B, Firingpatternsofsomeoftheneuronsinvolvedinmakingsaccades.Excitationofburstneuronsoftherighthorizontalgazecentercausesabducensmotorneuronsontherightandmedialrectusmotorneuronsonthelefttobeactivated.Theascendingpathwaytotheoculomotornucleusisthroughthemediallongitudinalfasciculus.Thelefthorizontalgazecenterissimultaneouslyinhibited.
CHAPTER 9 Organization of Motor Function •Fig. 9.29 Smooth Pursuit Pathways. Thestimulusforsmoothpursuiteyemovementisamovingvisualtarget.Thiscausesactivitytoflowthroughthecircuitrydiagramedinthefigureandleadstomaintenanceofthefoveaonthetarget.FEF,frontaleyefield;LGN,lateralgeniculatenucleus;MSTandMT,higherordervisualassociationareas;MVN,medialvestibularnucleus;NRTP,nucleusreticularistegmentipontis;SEF,supplementaryeyefield;V1,primaryvisualcortex.Retina LGN V1 MT MST Pontine nuclei FEF SEF NRTP Flocculus paraflocculus Vermis MVN y-group Fastigial nucleus Oculomotor nuclei Cortical areas involved in processing object motion Brainstem and cerebellar areas involved in transforming sensory-related signal into motor command Motor commands to eye muscles 1.
The extrafusal skeletal muscle fibers are innervated by α motor neurons. A motor unit consists of a single α motor neuron and all the muscle fibers with which it synapses. Motor unit size varies greatly among muscles; small motor units allow finer control of muscle force.
2.