The cochlear nuclei of snakes

Central projections of auditory nerve fibers in the western rat snake (Pantherophis obsoletus)

Despite the absence of tympanic middle ears, snakes can hear. They are thought to primarily detect substrate vibration via connections between the lower jaw and the inner ear. We used the western rat snake (Pantherophis obsoletus) to determine how vibration is processed in the brain. We measured vibration-evoked potential recordings to reveal sensitivity to low-frequency vibrations. We then used tract tracing combined with immunohistochemistry and Nissl staining to describe the central projections of the papillar branch of the VIIIth nerve. Applications of biotinylated dextran amine to the basilar papilla (homologous to the organ of Corti of mammals) labeled bouton-like terminals in two first-order cochlear nuclei, a rostrolateral nucleus angularis (NA) and a caudomedial nucleus magnocellularis (NM). NA formed a distinct dorsal eminence, consisted of heterogenous cell types, and was parvalbumin positive. NM was smaller and poorly separated from the surrounding vestibular nuclei. NM was distinguished by positive calbindin label and included fusiform and round cells. Thus, the atympanate western rat snake shares similar first-order projections to tympanate reptiles. Auditory pathways may be used for detecting vibration, not only in snakes but also potentially in atympanate early tetrapods.

Organization of the auditory brainstem in a lizard, Gekko gecko. I. Auditory nerve, cochlear nuclei, and superior olivary nuclei

We used tract tracing to reveal the connections of the auditory brainstem in the Tokay gecko (Gekko gecko). The auditory nerve has two divisions, a rostroventrally directed projection of mid- to high best-frequency fibers to the nucleus angularis (NA) and a more dorsal and caudal projection of low to middle best-frequency fibers that bifurcate to project to both the NA and the nucleus magnocellularis (NM). The projection to NM formed large somatic terminals and bouton terminals. NM projected bilaterally to the second-order nucleus laminaris (NL), such that the ipsilateral projection innervated the dorsal NL neuropil, whereas the contralateral projection crossed the midline and innervated the ventral dendrites of NL neurons. Neurons in NL were generally bitufted, with dorsoventrally oriented dendrites. NL projected to the contralateral torus semicircularis and to the contralateral ventral superior olive (SOv). NA projected to ipsilateral dorsal superior olive (SOd), sent a major projection to the contralateral SOv, and projected to torus semicircularis. The SOd projected to the contralateral SOv, which projected back to the ipsilateral NM, NL, and NA. These results suggest homologous patterns of auditory connections in lizards and archosaurs but also different processing of low- and high-frequency information in the brainstem.

Calcium-binding protein immunoreactivity characterizes the auditory system of Gekko gecko

Geckos use vocalizations for intraspecific communication, but little is known about the organization of their central auditory system. We therefore used antibodies against the calcium-binding proteins calretinin (CR), parvalbumin (PV), and calbindin-D28k (CB) to characterize the gecko auditory system. We also examined expression of both glutamic acid decarboxlase (GAD) and synaptic vesicle protein (SV2). Western blots showed that these antibodies are specific to gecko brain. All three calcium-binding proteins were expressed in the auditory nerve, and CR immunoreactivity labeled the first-order nuclei and delineated the terminal fields associated with the ascending projections from the first-order auditory nuclei. PV expression characterized the superior olivary nuclei, whereas GAD immunoreactivity characterized many neurons in the nucleus of the lateral lemniscus and some neurons in the torus semicircularis. In the auditory midbrain, the distribution of CR, PV, and CB characterized divisions within the central nucleus of the torus semicircularis. All three calcium-binding proteins were expressed in nucleus medialis of the thalamus. These expression patterns are similar to those described for other vertebrates.

Snake bioacoustics: toward a richer understanding of the behavioral ecology of snakes

Snakes are frequently described in both popular and technical literature as either deaf or able to perceive only groundborne vibrations. Physiological studies have shown that snakes are actually most sensitive to airborne vibrations. Snakes are able to detect both airborne and groundborne vibrations using their body surface (termed somatic hearing) as well as from their inner ears. The central auditory pathways for these two modes of "hearing" remain unknown. Recent experimental evidence has shown that snakes can respond behaviorally to both airborne and groundborne vibrations. The ability of snakes to contextualize the sounds and respond with consistent predatory or defensive behaviors suggests that auditory stimuli may play a larger role in the behavioral ecology of snakes than was previously realized. Snakes produce sounds in a variety of ways, and there appear to be multiple acoustic Batesian mimicry complexes among snakes. Analyses of the proclivity for sound production and the acoustics of the sounds produced within a habitat or phylogeny specific context may provide insights into the behavioral ecology of snakes. The relatively low information content in the sounds produced by snakes suggests that these sounds are not suitable for intraspecific communication. Nevertheless, given the diversity of habitats in which snakes are found, and their dual auditory pathways, some form of intraspecific acoustic communication may exist in some species.

Neuronal architecture of the dorsal nucleus (cochlear nucleus) of the frog, Rana pipiens pipiens

The neuronal architecture of the dorsal nucleus of the Northern leopard frog (Rana pipiens pipiens), which is a homolog of the cochlear nucleus of mammals and birds, was investigated. Our study showed that the frog dorsal nucleus contains a number of morphologically distinct cell types that are discernible in terms of the cellular architecture as derived from Nissl-stained material and in terms of the dendritic profile as revealed by horseradish peroxidase-filled single neurons. These cell types are bushy cells, bipolar (or fusiform) cells, octopus cells, stellate cells, giant cells, radiate (or round) cells, and a variety of small cells. The different cell types occupy different regions of the nucleus. Therefore, our results suggest that the dorsal nucleus should no longer be considered to be a uniform nucleus containing a homogeneous population of neurons. Homologies of these cell types with those described in other vertebrate species, including mammals, are proposed.

Further serial transmission electron microscopy studies of auditory hair cell innervation in lizards and in a snake

Auditory hair cells of three lizard and one snake species were studied by serial transmission electron microscopy (TEM) sections of two unidirectional hair cells (UHC) and two bidirectional hair cells (BHC) and by nonserial section montages of each entire papilla cut at 2-microns intervals across the papillar width. The unidirectional hair cell region of the agamid lizard, Acanthosaura crucigera, lacked efferent innervation. Another agamid lizard, Agama agama, studied by nonserial section only, also lacked efferent innervation to the UHC. Afferent innervation to both the UHC and BHC of Acanthosaura was primarily exclusive (each nerve fiber innervates only one hair cell), although an occasional nerve fiber innervated two hair cells. Both the UHC and the BHC of the anguid, Celestus costatus, were exclusively innervated. Both hair cell types of the varanid, Varanus exanthematicus, were nonexclusively innervated (all afferent nerve fibers innervate two or more hair cells). The auditory papilla of the colubrid snake, Elaphe obsoleta, has only one type of hair cell and each is nonexclusively innervated. The numbers of afferent and efferent nerve fibers and of afferent synapses are presented in tabular form.

Primary projections and efferent cells of the VIIIth cranial nerve in the monitor lizard, Varanus exanthematicus

In order to describe the central relations of both the afferent and efferent components of the VIIIth cranial nerve in one reptile, the methods of anterograde and retrograde axonal transport and anterograde degeneration were used to study the vestibular and cochlear projections and the efferent system of this nerve in Varanus exanthematicus. On the basis of cresyl violet and Klüver-Barrera staining, five vestibular nuclei, four cochlear nuclei, and two clusters of small cells which could not be designated as strictly auditory or vestibular are distinguished. The vestibular nuclei include the nucleus dorsolateralis, nucleus ventrolateralis, nucleus tangentialis, nucleus ventromedialis, and nucleus descendens. The well-developed cochlear nuclear complex includes the nucleus angularis, nuclei magnocellulares medialis and lateralis, and nucleus laminaris. The two cell clusters are located dorsolaterally in the brainstem just ventrolateral to the acoustic tubercle. The primary afferent vestibular fibers coursing in the anterior VIIIth nerve root distribute to the ventral portions of all vestibular nuclei except nucleus ventromedialis, whereas the fibers coursing in the posterior root project to the dorsal portions of these nuclei. In nucleus ventromedialis fibers of both roots do not segregate into ventral and dorsal portions. Other targets of the vestibular fibers are the two cell clusters, the granular layer of the ipsilateral cerebellum, the reticular formation, and the descending trigeminal tract and its nucleus. The primary cochlear fibers coursing in the posterior root terminate in nucleus angularis, nuclei magnocellulares medialis and lateralis, and the inner cell strand of nucleus laminaris. The efferent system is, ipsi- and contralaterally in the brainstem, composed of ventral and dorsal cell groups that extend from the level of the principal abducens nucleus caudally where they overlap with the facial motor nucleus. The fibers, which originate from the contralaterally located efferent cells, course beneath the IVth ventricle to exit the brainstem on the ipsilateral side.

The nucleus magnocellularis in the red-eared turtle, Chrysemys scripta elegans: eighth nerve endings and neuronal types

Eighth nerve endings and neuronal types in the nucleus magnocellularis were analyzed in the red-eared turtle (Chrysemys scripta elegans). One group of turtles had horseradish peroxidase (HRP) injected into the surgically exposed inner ear. Following injection the animals survived for 3-5 days and then their 8th nerve fibers and endings were analyzed. A second group of turtles were impregnated by the Golgi-Kopsch technique. This also demonstrated 8th nerve endings and neurons in the nucleus magnocellularis. A third group had brains stained with cresyl violet to demonstrate normal morphology. Two types of neurons were present in the nucleus magnocellularis; bushy and stellate. Bushy neurons had a single primary dendrite with little branching and spines. Stellate neurons had 3-5 primary dendrites with secondary order branching and dendritic spines. Thick 8th nerve axons gave rise to endbulbs of Held and their axons formed bouton-type ending. Endbulbs of Held were presently only on bushy neurons while bouton-type terminals were only on stellate neurons. Endbulbs of Held had two patterns; one group was cup-shaped and surrounded approximately 1/2 of the soma without appendages. A second type had a smaller cup with 3-5 extensions to other parts of the cell. Stellate neurons had terminal boutons adjacent to the cell membrane. They appeared in some situations as a necklace surrounding the neuron.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways

Extracellular, iontophoretic injections of horseradish peroxidase were used to anterogradely fill axons efferent from the optic tectum in garter snakes. The tectal efferent pathways consist of six axon types with distinct projections and terminal morphologies. Tectogeniculate axons pass into the diencephalon via the optic tract, bearing collaterals that form spatially restricted, rodlike arbors in the pretectum, the ventral lateral geniculate nucleus, and the ventrolateral nucleus. Tectoisthmi axons exit the tectum as a thin-caliber component of the ventral tectobulbar tract. They form spatially restricted, spherical arbors within nucleus isthmi. Tectoisthmobulbar axons also give rise to small, spherical arbors within nucleus isthmi, but the parent axons continue caudally into the pontine and medullary reticular formation issuing many short collateral branches. Tectorotundal axons reach the diencephalon via the tectothalamic tract and give rise to fine terminal collaterals in the nucleus of the tectothalamic tract ipsilaterally and in nucleus rotundus bilaterally. Single axons form sheetlike terminal fields that span the rostrocaudal extent of nucleus rotundus. Ipsilateral tectobulbar axons descend into the midbrain tegmentum where they issue several thick collaterals that terminate widely throughout the nucleus lateralis profundus mesencephali. The parent axon continues caudally giving off several widely spreading collaterals within the pontine and medullary reticular formation. Crossed tectobulbar axons enter the dorsal tectobulbar tract and cross the midline to form the predorsal bundle. Single axons give rise to terminal collaterals in the nucleus lateralis profundus mesencephali bilaterally, the contralateral pontine and medullary reticular formation, and the intermediate gray of the cervical spinal cord.