The ectodermally derived membranous labyrinth consists of four fluid-filled compartments, all of which communicate. These compartments are contained within the components of the bony labyrinth and include the saccule and utriculus within the bony vestibule connected by the utriculosaccular duct, the three semicircular ducts within the bony semicircular canals that connect to the utriculus and a cochlear duct within the bony cochlea that is connected to the saccule by the ductus reuniens. The endolymphatic duct is an extension from the utriculosaccular duct through the bony vestibular aqueduct to the intracranial dura where the duct expands into a endolymphatic sac. The endolymph contained within the membranous labyrinth is thought to be derived from the blood vessels and epithelium of the stria vascularis along the peripheral wall of the cochlear duct and is absorbed back into the blood through the blood vessels surrounding the endolymphatic sac. The three semicircular ducts are the anterior (vertical), posterior (vertical), and lateral (horizontal). Each semicircular duct is oriented at right angles to the others. Thus rotation of the head around any plane causes endolymph to flow within one or more of the ducts. Each semicircular duct connects at both ends with the utriculus.
The crista ampullaris is the receptor organ associated with each semicircular duct. At one end of each membranous semicircular duct is a dilation called the ampulla. On one side of the membranous ampulla, a proliferation of connective tissue forms a transverse ridge called the crista ampullaris. It is lined on its internal surface by columnar neuroepithelial cells. On the surface of the crista is a gelatinous structure that is composed of a protein-polysaccharide material called the cupula, which extends across the lumen of the ampulla. This neuroepithelium is composed of two basic cell types: hair cells and supporting cells. The dendritic zones of the vestibular neurons are in synaptic contact with the base of the hair cells. These hair cells have on their luminal surface 40 to 80 hairs, or modified microvilli (stereocilia), and a single modified cilium (kinocilium). These structures project into the overlying cupula. Movement of fluid in the semicircular ducts causes deflection of the cupula, which is oriented transversely to the direction of flow of the endolymph. This deflection bends the stereocilia, which is the source of the stimulus by way of the hair cells to the dendritic zone of the vestibular neuron that is in synaptic relationship with the plasmalemma of the hair cell. In one end of each semicircular duct is one membranous ampulla with its crista ampullaris. Because the three semicircular ducts are all at right angles to each other, movement of the head in any plane or angular rotation affects a crista ampullaris and stimulates vestibular neurons. These cristae function in dynamic equilibrium.
The macula is the receptor organ found in the utriculus and saccule, which are located in the bony vestibule. These maculae are on one surface of each of these saclike structures. Each macula is an oval-shaped plaque in which the membranous labyrinth has proliferated. The surface of the macula consists of columnar epithelial cells. This neuroepithelium is composed of hair cells and supporting cells. Covering the neuroepithelium is a gelatinous material, the statoconiorum (otolithic) membrane. On the surface of this membrane are calcareous crystalline bodies known as statoconia (otoliths). Similar to the hair cells of the cristae, the macular hair cells have projections of their luminal cell membranes — stereocilia and kinocilia — into the overlying statoconiorum membrane. Movement of the statoconia away from these cells is the initiating factor in bending the stereocilia to stimulate an impulse in the dendritic zones of the vestibular neurons that are in synaptic relationship with the base of the hair cells. The macula of the saccule is oriented in a vertical direction (sagittal plane), whereas the macula of the utriculus is in a horizontal direction (dorsal plane). Thus gravitational forces continually affect the position of the statoconia relative to the hair cells. These structures are responsible for the sensation of the static position of the head and linear acceleration or deceleration. They function in static equilibrium. The macula of the utriculus may be more important as a receptor for sensing changes in head posture, whereas the macula of the saccule may be more sensitive to vibrational stimuli and loud sounds.
The dendritic zone of the vestibular portion of cranial nerve VIII is in a synaptic relationship with the hair cells of each crista ampullaris and the macula utriculi and macula sacculi. The axons course through the internal acoustic meatus with those of the cochlear nerve. The cell bodies of these bipolartype sensory neurons are inserted along the course of the axons within the petrous portion of the temporal bone, where they form the vestibular ganglion. The vestibular nerve axons pass to the lateral surface of the rostral medulla where they enter the medulla and terminate in telodendria at one of two sites. The majority terminate in the vestibular nuclei in the medulla and pons. A few course directly into the cerebellum by way of the caudal cerebellar peduncle.
Cochlear Duct-Spiral Organ
The most highly developed and differentiated portion of the membranous labyrinth is the cochlear duct. This duct has a triangular shape with its base, the stria vascularis adjacent to the peripheral wall of the cochlea. A thin vestibular membrane forms the roof of the cochlear duct and a thicker basilar membrane forms the floor of the duct. The spiral organ (organum spirale), formerly organ of Corti, is a collection of hair cells and supporting cells that rests on the basilar membrane. These structures are involved in the transduction and transmission of sound impulses via the cochlear nerve to the brain. Special somatic afferent axons in the cochlear nerve make synaptic contact with these hair cells. Their neuronal cell bodies are centrally located in the attachment of the spiral lamina to the modiolus. The axons leave the inner ear through the internal acoustic meatus accompanied by the vestibular nerve and synapse in the cochlear nuclei on the lateral side of the medulla oblongata. The basilar membrane separates the endolymph of this duct from the perilymph of the scala tympani, which is a part of the cochlea. The thin vestibular membrane separates the endolymph of the cochlear duct from the perilymph in the scala vestibuli of the cochlea. Thus the membranous cochlear duct filled with endolymph is enclosed within the bony cochlea (scala tympani-scala vestibuli), which is filled with perilymph. These fluids have different chemical compositions and are not in open communication with each other. Likewise the membranous semicircular ducts containing endolymph are enclosed within the semicircular canals that contain perilymph. The distinction between the osseous and the membranous labyrinth is sometimes blurred in textbooks by the carefree use of canal for duct and cochlea for cochlear duct. To understand the structural and functional relationships of the labyrinths to each other, the terminology must be kept clear and consistent.
Hearing depends on the ability of sound waves in the external gaseous environment reaching the tympanic membrane via a patent external acoustic meatus. Here the oscillations in the air are converted to oscillations in the three auditory ossicles in the tympanic cavity. At the vestibular window the bony oscillations are converted to oscillations of the perilymphatic fluid, which in turn affects the spiral organ by oscillations of the basilar membrane in the cochlear duct. Any interruption of this pathway can result in loss of hearing. Most deafness is related to disorders of the spiral organ. Foss and Flottorp (1974) found that hearing in puppies first occurred as a functional modality at 14 days after birth (on average), which coincides with the opening of the eyelids. Congenital inherited sensorineural deafness is very common in many breeds of dogs and is usually present shortly after birth. Johnsson et al (1973) studied deafness and pathology of the cochlear in Dalmatian dogs. Rouse et al (1984) considered abnormal otoconia and calcification of the labyrinth in deaf Dalmatian dogs. Albinotic and abiotrophic forms are recognized and can be diagnosed in puppies using brainstem auditory response testing
Shambaugh (1923) injected the vessels of the dog labyrinth and found that the arterial supply was from a single labyrinthine artery that entered through the internal acoustic meatus. In some specimens a small artery came along the cochlear canaliculus and was distributed to a small area of periosteum along the scala tympani. The first branch to be given off from the labyrinthine artery was the anterior vestibular artery, a vessel that supplied the macula of the utriculus and the cristae of the lateral and anterior semicircular ducts. The second branch of the labyrinthine artery supplied the crista of the posterior duct, the posterior crura of the posterior and horizontal canals and ducts and the crus commune. The venous drainage from the labyrinth was collected by two trunks, the larger one leaving along the cochlear canaliculus collected all of the blood from the cochlea as well as most of the blood from the capillaries supplied by the anterior and posterior vestibular arteries. The lesser vein leaving along the vestibular aqueduct drained the remainder of the labyrinth.