Reflex control of the retractor bulbi muscle in the cat

Neurons located in the trigeminal sensory complex and the lateral pontine tegmentum project to the oculomotor nucleus in the rabbit

Neurons located in the trigeminal sensory complex (TSC) and the lateral pontine tegmentum (LPT) have been reported to project to both the accessory abducens and the facial nuclei, which innervate the retractor bulbi and orbicularis oculi muscles respectively, in order to control the nictitating membrane (NM) and eyelid defensive reflex. Since muscles innervated by the oculomotor nucleus (OCM) also appear to be involved in this reflex, retrograde and anterograde tracers were used in this study to determine whether there are projections from the TSC and LPT to the OCM in the rabbit. Injections of horseradish peroxidase (HRP) in the OCM nucleus labeled neurons in the LPT surrounding the trigeminal motor nucleus dorsally, laterally and ventrally. Only a few scattered neurons were found in the principal and spinal trigeminal nuclei. Injection of biocytin in the LPT area containing most of the HRP-labeled neurons caused anterograde labeling of fibers that crossed the midline and ascended just dorsal to the contralateral medial lemniscus. A proportion of these fibers coursed in a dorsal direction to enter and terminate within the OCM contralateral to the injection site. The location of the motoneuronal groups innervating the different extraocular muscles was studied by retrograde transport of HRP, and compared with the distribution of biocytin-labeled terminals. It was found that the terminals were located in the superior rectus and the levator palpebrae zone of the nucleus. We discuss the functional significance of this projection for the eyelid and NM response.

Muscle activity during unconditioned and conditioned eye blinks in the rabbit

EMGs were recorded from the orbicularis oculi, retractor bulbi and superior rectus muscles in rabbits to investigate the time course of muscle activation during unconditioned and conditioned eye blinks. EMGs from the three muscles showed two responses, with the responses of the orbicularis oculi and retractor bulbi showing the same latency, and the responses of the superior rectus lagging. The latency of responses to periorbital electrostimulation was about 5 ms, and to air puff stimulation about 10 ms. Results showed a tight coupling of activity between muscles, with cross-correlograms peaking at 0.65 to 0.85 and showing little time shift. Stimulus-response curves showed clear non-linearities in the response of the muscle to changes in stimulus strength. Local anesthesia of the cornea had little effect on unconditionally evoked responses. The form of unconditionally evoked responses was similar with periorbital electrostimulation and air puff stimuli but differed in latency. These results show the form of the eye blink reflex response and will be of importance in interpreting electrophysiological studies of the classically conditioned eye blink of rabbits.

Changes in the motor pattern of learned and unlearned responses following cerebellar lesions: a kinematic analysis of the nictitating membrane reflex

Kinematic and dynamic analyses were employed to study the effects of cerebellar lesions on conditioned and unconditioned nictitating membrane responses in the rabbit. It was found that conditioned responses acquired to an auditory stimulus accelerated in two bursts as indicated by two distinct peaks of acceleration. The second peak of acceleration was very weak during the early portions of conditioning but became a prominent feature of the conditioned response over 16 sessions of conditioning. The second peak of acceleration in the conditioned response was more sensitive to cerebellar damage than was the first peak. When lesions of the cerebellum permanently reduced the amplitude of conditioned responses, but did not affect their frequency, the second peak of acceleration was nearly abolished while the first peak was unaffected. When cerebellar lesions profoundly impaired both the amplitude and frequency of conditioned responses, large and permanent impairments occurred in both peaks of acceleration. Lesions of the anterior interpositus nucleus most severely impaired both peaks of acceleration in the conditioned response and significantly reduced the acceleration of unconditioned responses across a wide range of intensities of corneal air puff. The deficit in the acceleration of unconditioned responses became manifest only after membrane extension exceeded 0.12 mm. The impairment in the amplitude of the unconditioned response after cerebellar lesions more closely approximated the impairment in the amplitude of the conditioned response when the force-generating properties of the conditioned and unconditioned stimuli were equated. It was hypothesized, therefore, that one reason why conditioned responses are so easily disrupted by cerebellar lesions is because they are of low force and not simply because they are learned. It was proposed that the two peaks of acceleration that characterize the conditioned response represent the function of two distinct anatomical systems. The first, a short-latency system, initiates the response and is most likely mediated by circuits that traverse the pontomedullary reticular formation. The second, a longer-latency system, amplifies response amplitude and its neural basis remains to be elucidated. The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles. It was concluded that the vulnerability of the second component of the conditioned response to cerebellar damage reflects an important role for the cerebellum in modulating the degree to which long-latency neural systems contribute to the ongoing performance of learned and unlearned behaviors.

Activity of spinal trigeminal pars oralis and adjacent reticular formation units during differential conditioning of the rabbit nictitating membrane response

Spinal trigeminal nucleus pars oralis (SpoV) is anatomically linked to brain circuitry thought to subserve unconditioned and conditioned nictitating membrane responses in rabbit. Single-unit recording from SpoV and adjacent reticular formation obtained during conditioning from awake, behaving animals revealed modulation of unit firing related to CS, US, and CR occurrence. SpoV participates directly in the unconditioned response and probably relays US information to other brain areas subserving conditioning. The presence of CR-related activity suggests that SpoV may participate in the CR motor output pathway, and may also provide CR-related information to cerebellum. Sensory convergence and CR-related activity in reticular formation mark this structure as a candidate locus of primary neuronal plasticity in this example of conditioning.

Anatomical observations on the afferent projections to the retractor bulbi motoneuronal cell group and other pathways possibly related to the blink reflex in the cat

In the cat retractor bulbi (RB) muscle reflexively retracts the eye ball into the orbit. This reflex action is called the nictitating membrane response which, together with the reflex contraction of the orbicularis oculi muscle, constitutes the blink reflex. The retractor bulbi (RB) motoneuronal nucleus is a small cell group located in the lateral tegmentum of the caudal pons, just dorsal to the superior olivary complex. The nucleus is identical to the accessory abducens nucleus and sends its fibers through the abducens nerve. Autoradiographical tracing results indicate that the RB nucleus receives some fibers from the principal and rostral spinal trigeminal nuclei and from the dorsal red nucleus and dorsally adjoining tegmentum. The same areas project to the intermediate facial subnucleus, containing motoneurons innervating the orbicularis oculi muscle. It is suggested that the trigeminal projections take part in the anatomical framework for the R1 component of the blink reflex. Two other brainstem areas i.e.: a portion of the caudal pontine ventrolateral tegmental field and the medullary medial tegmentum at the level of the hypoglossal nucleus were also found to project to the RB motoneuronal cell group and to the intermediate facial subnucleus. These projections were much stronger than those derived from the trigeminal nuclei and red nucleus. Moreover, the medullary premotor area projects not only to the blink motoneuronal cell groups but also to the pontine premotor area. It is suggested that both areas are involved in the R2 blink reflex component. The medullary blink premotor area receives afferents especially from oculomotor control structures in the reticular formation of the brainstem while the pontine blink premotor area receives afferents from the olivary pretectal nucleus and/or the nucleus of the optic tract and from the dorsal red nucleus and its dorsally adjoining area. Because the oculomotor control structures in the reticular formation (by way of the superior colliculus) and the red nucleus receive afferents from trigeminal nuclei, they may play an important role in tactually induced reflex blinking, while the pretectum could take part in the neuronal framework of the visually induced blink reflex.

Anatomical study of the rabbit's corneal-VIth nerve reflex: connections between cornea, trigeminal sensory complex, and the abducens and accessory abducens nuclei

The corneal-VIth nerve reflex of the rabbit, involving retraction of the eyeball by the retractor bulbi muscle and the correlated extension of the nictitating membrane, has been suggested to be mediated by retractor bulbi motoneurons in the accessory abducens-(ACC) nucleus but not by those in the abducens (ABD) nucleus, and to consist of both a fast, disynaptic, component and a slower component mediated by the reticular formation (RF). We, therefore, employed the anterograde and retrograde transport of horseradish peroxidase (HRP) to examine the neural connections between anatomical structures proposed to be involved in the afferent limb of the corneal VIth nerve reflex. The transganglionic transport of HRP from cornea indicated a primary projection to the ventral half of pars oralis of the trigeminal sensory complex. The retrograde transport of HRP infused into ACC resulted in a bilateral labeling of cells in ventral pars oralis with 75% of the labeled cells being ipsilateral to the side of infusion. In contrast, there was no retrograde labeling of cells in the trigeminal sensory complex after HRP infusions into ABD. Infusion of HRP into ACC and ABD also revealed retrogradely labeled cells in the RF caudal to these two nuclei and infusion of HRP into this area of the RF resulted in both the retrograde labeling of cells in ventral pars oralis and anterograde-like labeling in both ACC and ABD. These data provide anatomical support for a direct relationship of the ACC, but not ABD, to the trigeminal sensory system and for the suggested existence of two components of the corneal-VIth nerve reflex: a disynaptic component from cornea to ventral pars oralis which in turn projects only to the ACC nucleus; and a multisynaptic component consisting of projections from the ventral pars oralis to RF cells which, in turn, are premotor to the ACC and ABD nuclei.

Eyeball retraction latency in the conscious rabbit measured with a new photodiode technique

A new technique is described for accurate, reliable measurement of eyeball retraction in the rabbit. A narrow film strip, on which a linear light intensity grating has been exposed, is attached to a contact lens which is placed on the animal's cornea. The other end of the intensity grating slides between a photodiode and an LED. The contact lens-film grating assembly moves freely with eyeball retraction and relaxation, causing changes in photodiode output. This device appears to be well tolerated by the animal. Large amplitude eyeball retractions occur in response to air puff stimulation directed at the upper or lower eyelid and to periorbital shock. Average eye retraction latency to stimulation of the abducens (VI) nerve with a chronically implanted stimulating electrode was 5.3 ms (S.D. = 0.75 ms) in the conscious rabbit as measured with our device. Latency to periorbital electrical shock was 9.3 ms (S.D. = 2.1 ms). Eye retraction latency decreased with increasing shock amplitudes. Rabbits readily acquired classically conditioned eyeball retractions, monitored with this device, when a white noise auditory stimulus was paired with an air puff directed at the eyelid.

Convergence of trigeminal afferents on retractor bulbi motoneurones in the anaesthetized cat

Retractor bulbi motoneurones were identified by intracellular recording of their antidromic invasion following stimulation of the motor axons. Characteristics of excitatory post-synaptic potentials (e.p.s.p.s) evoked by electrical stimulation of long ciliary nerves (corneal afferents), the supraorbital nerve and the ipsilateral or contralateral vibrissae were analysed. Comparison of the orthodromic responses induced by supra-threshold stimulation of the four trigeminal inputs showed that the most powerful excitatory effect was due to corneal afferent stimulation. Excitatory synaptic potentials were followed in some cases by a period of hyperpolarization lasting 15-20 msec. It is suggested that this is an inhibitory potential of post-synaptic origin. Interaction between condition and test e.p.s.p.s evoked by long ciliary nerve and supraorbital nerve stimulation revealed a partial blocking of test e.p.s.p.s over a longer period (more than 30 msec), and it is suggested that inhibitory mechanisms within the trigeminal nucleus may be in part responsible for the absence of facilitation at the level of the motoneurone.

Comparison of antidromic and orthodromic action potentials of identified motor axons in the cat's brain stem

Recordings were made from identified central axons at a known distance from their somata, to compare the action potentials resulting from antidromic and synaptic excitation. By taking advantage of the anatomical configuration within the brain stem of the motoneurones innervating the retractor bulbi muscle in the orbit, their axons were penetrated in the VIth nucleus and labelled by electrophoretic injection of horseradish peroxidase. Excitatory post-synaptic potentials recorded in the retractor bulbi axons at about 3 mm from the soma were six times smaller than in the soma. The space constant of the axonal segment between the retractor bulbi and the abducens nucleus was estimated to be 1.7 mm. When the axons propagated action potentials the attenuation was increased to eighteen times due to the nodes of Ranvier intercalated between the soma and the site of recording. Antidromic action potentials displayed stepwise changes in amplitude and shape when stimuli were applied at intervals decreasing from 5 ms to 0.7 ms. The changes were related to the different lengths of refractoriness of the soma, initial segment and axon. Orthodromic action potentials evoked by synaptic excitation displayed similar changes in amplitude and shape. These observations lead to the conclusion that the soma, initial segment and neighbouring nodes of Ranvier contribute significantly to the shape of the action potential. Contrary to the generally accepted view, it appears that the efferent discharge along motor axons can be initiated without a simultaneous activation of the somato-dendritic or even the initial segment membrane, as revealed by the lack of somato-dendritic and/or initial segment contribution to the shape of the synaptically evoked action potentials.

The nictitating membrane response: an electrophysiological study of the abducens nerve and nucleus and the accessory abducens nucleus in rabbit

The nictitating membrane response to periocular electrostimulation was investigated in anesthetized and paralyzed rabbits. Recordings from the abducens nerve, which carries the fibers innervating retractor bulbi muscles that are primarily involved in this reflex, showed two distinct volleys to effective stimulation: a short duration volley with a minimum latency of approximately 4 ms and a longer duration volley beginning approximately 10 ms after stimulus onset and lasting up to 25 ms. Recordings of antidromically evoked field potentials via microelectrodes indicated large responses to abducens nerve stimulation in the vicinity of the accessory abducens nucleus. Single unit recordings from the accessory abducens nucleus produced spike trains with minimal latencies of 3.7-5 ms to eyeshock. The latency of spike discharge was inversely related to stimulus current. Units in the abducens nucleus did not show stimulus-elicited spiking, suggesting that the accessory abducens, but not the abducens, is primarily involved in the reflex pathway. Transverse knife cuts which separated caudal areas of the sensory trigeminal complex from the accessory abducens nucleus did not attenuate the efferent volley to suprathreshold stimulation, suggesting that more rostral components of the trigeminal complex are primarily involved in the reflex pathway.

Latency of the nictitating membrane response to periocular electrostimulation in unanesthetized rabbits

The latency of the nictitating membrane response (NMR) in rabbit (Oryctolagus cuniculus) to periocular electro-stimulation is a negative exponential function of stimulus current with an asymptote of approximately 17 msec. The NMR was recorded by means of a precision low-torque potentiometer like that employed in previous studies of NMR latency, and the criterion of response initiation employed here was similar to that employed in studies of classical conditioning in this preparation. Using estimates from physiological studies on surgically acute preparations, the minimum latency of the NMR of 17 msec can be decomposed as follows: 4 msec to fire motoneurons of the accessory abducens nucleus; 9 msec for conduction, synaptic transmission, and recruitment of retractor bulbi muscle fibers; 4 msec for the nictitating membrane to initiate its sweep after eyeball retraction. The implications of these estimates for chronic unit-recording studies of the conditioned NMR are discussed.