The Vestibular System • generates compensatory responses to head motion – postural responses – ocular-motor responses – visceral responses • To achieve this the vestibular system measures – Head rotation – Head acceleration • Einstein’s equivalency theorem states that an accelerometer cannot distinguish between translational accelerations and tilts Vestibularly driven eye movements, or the VESTIBULO- OCULAR REFLEX (or VOR) is a very basic and evolutionarily old neural response designed to maintain clear and stable vision in the presence of head movements. From Leigh and Zee, The Neurology of Eye Movement Life is hard for those who don’t have a VOR During a walk I found too much motion in my visual picture of the surroundings to permit recognition of fine detail. I learned that I must stand still in order to read the lettering on a sign --J.C, 1952 M.D. with no vestibular system His vision was disturbed by head movements as small as those induced by the beat of his heart while at rest. Why do we need a vestibular system to hold gaze stable during head movement when we have a variety of ocular following mechanisms that cause the eyes to track a target that moves with respect to the head? •Visual latencies are on the order of ~70 msec, while VOR responses occur an order of magnitude faster. •If images on our retina slipped across photodetectors by more than about 4°/sec, visual acuity goes down. We cannot achieve this level of gaze stability by using long- latency visually mediated responses. •Because of this, the VOR must operate in the absence of feedback, and is thus open loop!! Why Study the VOR? Answer: While having the same basic goals of other motor reflexes, the VOR is comparatively simple: • Relatively easy to control head motion •There are no changes in load in the effector organ (i.e., the eye) •The eye moves with relatively few degrees of freedom •Head and eye movements are easily measured •Every cell involved in the VOR resides within the cranial vault, and are thus relatively easy to record from in animal preparations The Magnetic Scleral Search Coil Eye movements can be measured with very high precision using only slightly invasive procedures From Leigh and Zee Vestibular End Organs From Leigh and Zee Semicircular canals lie orthogonal to each other. Further, canals are located on both sides of the head, forming push-pull pairs with their contralateral counterparts. IV abducens III oculomotor From Leigh and Zee From Kandel and Schwartz Mechanical Model of Semicircular Canal • Circular lumen of overall radius R and lumen radius r. The lumen is filled with fluid with moment of inertia I. The moment of viscous friction per relative angular velocity of the fluid is B. • The lumen is rotated through angle q. A piston lags behind the lumen (due to inertia and friction, and thus only rotates through angle p, and the relative displacement of the piston is Q (Note that q=Q+p). • Further, the piston has a spring-like restorative force, K. (s) T1T2 s q(s) T1s 1T2 s 1 •Canal is a high- pass transducer of rotational velocity. •10°/sec rotation causes roughly 0.03° of cupula deflection!! 1 Hz Low freq stimulus results in diminished gain and a phase lead. 0.01 Hz Input/Output Relationship of Semicircular Canal Vestibular Nystagmus If the vestibular system responded to large head movements with a smooth eye movement, the eye position would rapidly exceed the physiological range. For this reason, the vestibular system produces a sawtooth-like waveform known as vestibular nystagmus, in which quick phases rapidly bring the eye back into the physiological range, while the slow phases are compensatory for head movement. Velocity Storage (positive feedback) effectively raises the time constant of the VOR to above that of canals (by about a factor of three, improving low- frequency performance. Visual information combines with vestibular information to further supplement low- frequency behavior. The resulting eye movements are known as optokinetic nystagmus (OKN) and optokinetic afternystagmus (OKAN). This information apparently enters ocular motor pathways through the velocity storage loop. From Leigh and Zee Otolith Organs Contribute to: •Image stabilization during head translation and tilt •Perception of motion •Orientational mechanisms •Postural Stability Translational VOR • The translational VOR is more complex than the angular VOR • Size of response is highly dependent on the distance of the target LVOR During Interaural Translation (Monkey) Naso-Occipital Translational Motion • The kinematics to naso-occipital motion are even d more complex Y • Response X magnitude is highly Given dependent on tan 1 Y X Motion Then both target Y X Y 2 distance and X 2 1 2 X gaze angle Y X Y2 2 sin ( in rads / unit d ) d Horizontal NO-LVOR Response 10 Head Velocity (cm/sec) 0 -10 1.25 A B Eye Velocity (deg/sec) 0.00 -1.25 -2.50 10 Eye Position 5 (deg) 0 -5 0.5 sec -10 Otolith Ambiguity Translation Tilt Acceleration Both Angles are equal. •The otolith organs transduce acceleration •Thus, translational accelerations and tilts of the head cause similar activity on otolith afferents. •Otolith information is therefore ambiguous (Einstein’s equivalency principle) and must be resolved in order to provide proper stabilizing and orientation responses. EYE MOVEMENT Trials 100 100 Sled Velocity Eye Velocity 50 50 (cm/sec) (deg/sec) 0 0 -50 -50 -100 -100 0 10 20 30 40 50 100 50 Sled Velocity Eye Velocity 50 25 (cm/sec) (deg/sec) 0 0 -50 -25 -100 -50 0 10 20 30 40 50 20 50 Sled Velocity 10 Eye Velocity (cm/sec) (cm/sec) 0 0 -10 -50 -20 0 10 20 30 40 50 60 Time (sec) Sled Velocity Sled Velocity Sled Velocity (cm/sec) (cm/sec) (cm/sec) 50 -100 50 100 -100 50 100 0 0 0 -50 -50 -50 0 0 0 10 10 10 20 20 20 Time (sec) 30 30 30 40 40 40 Translation PERCEPTION 50 50 50 0 0 0 20 40 50 50 -40 -20 -50 -50 -100 100 -100 100 Percept Velocity Percept Velocity Percept Velocity (cm/sec) (cm/sec) (cm/sec) Translational-LVOR 1 Sensitivity (°/cm) 0.1 0.01 Translational VOR Response is UP-IA Horizontal HIGH PASS, even though otolith ND-IA Horizontal 0.001 RD-DV vertical organs are ALL PASS LD-DV vertical Average 0.0001 270 225 Phase (deg) 180 135 90 45 0 -45 0.01 0.1 1 Frequency (Hz) Squirrel Monkey: Avg. Values Tilt PERCEPTION 3 0 0 0 z .5 0 H 5 1 0 1 -5 3 -0 3 0 1 0 z .0 0 H 5 1 0 1 -5 3 -0 Positon(degres) 3 0 2 0 z .5 0 H 5 1 0 1 -5 3 -0 0c 0s 1e . 4 1 . 0 00 . 2 1 11 . -25 PHASE(Degrees) . 0 1 22 . -50 GAIN . 8 0 33 . -75 . 6 0 45 . -00 .5 0 00 0 . 01 .5 0 02 re ec qu Fny Tilt responses are LOW PASS Translational VOR Responses High frequency Translational Perception Responses Otolith information Low Tilt VOR frequency Responses Tilt Perception Responses Input/Output Relationship of the Eye Motoneuron Activity From Robinson,Handbook of Physiology, 1981 •Ocular motoneurons vary firing rate by both eye position and eye velocity. •Recall that muscles require mostly position commands, but require some velocity information to overcome viscous forces. •Since the vestibular endorgans generate velocity information, we clearly have a communication problem between the vestibular system and the eye muscles Neural Integration Problem: Eye muscles require a combination of position and velocity information, while the vestibular system (and the saccadic system) generate only velocity information. Solution: Mathematically integrate velocity information, then sum velocity and position information in correct proportion to satisfy the needs of the ocular motor plant. From Leigh and Zee Easiest place to study neural integration is in the saccadic system, which changes gaze during normal “looking around. Neural integration of the vestibular signal entails many of the same features and neural substrates. From Leigh and Zee Neural integration is essentially a summing operation, and can be achieved using a simple positive feedback neural circuit. However, such a circuit will be inherently unstable, and will saturate in the presence of tonic nerve activity. Lateral Inhibition These drawbacks can be overcome by using a laterally-inhibitory net. In lateral inhibition, each cell projects in an inhibitory fashion to its neighbors. Thus, activity on a cell causes inhibition of neighboring cells, thus disinhibiting itself. From Robinson, 1981 Motor Learning (Adaptive Plasticity) Because the vestibular system must be fast, the VOR functions in an open loop manner. Despite this, the VOR can correct long-term errors. The cerebellum is particularly influential in this process. When climbing fibers (cf) fire in response to retinal slip (poor VOR gain) during head movement, the b of the cerebellar loop changes. It is not clear whether this change takes place From Robinson, 1981 in the synapse between the parallel fiber (gc) and the Purkinje cell (pc), or between the Purkinje cell and the vestibular nucleus.
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