HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 14 AUDITORY AND VESTIBULAR ORGANS

A SITUATION AND EXTERNAL RELATIONS

l Organs - cochlea and vestibular apparatus - sensitive respectively to air vibrations (sound), and movement of the head and its position relative to the gravitational field (balance), are combined in the inner ear within communicating spaces - the bony labyrinth - of the temporal bone. . Ear PowerPoint
2 Actually the receptors are enclosed in membranous tubes forming a membranous labyrinth that lies within, but does not fill the bony labyrinth.
3 The two separate systems contain different fluids. The membranous labyrinth is filled with endolymph and is a closed system, although it extends a ductus endolymphaticus through the bone to end blindly by the brain as an intradural sac involved in metabolic functions. This sac can be drained surgically to relieve damaging excess endolymphatic pressure - endolymphatic hydrops.
4 The space between the tubules of the membranous labyrinth and the bone is occupied by perilymph, which is in communication via the aqueductus cochleae and aqueductus vestibularis with the meninges and with the CSF of the brain's subarachnoid space.
5 The fluid in the bony labyrinth can interact with the middle ear (and indirectly with the external environment) by means of two soft areas in its bony walls: 6 The stimulus in the environment that causes movement of the oval window and pressure changes in the fluids is movement of air/sound, allowed a little way into the head via the external ear.
7 The fluids of the labyrinth are also subject to the gravitational force and that accompanying movement of the head.

B DIVISIONS OF THE EAR

l External ear
l Auricle: core of elastic cartilage; lobule of adipose tissue; skin-covered.
2 External auditory meatus: lined with skin and stratified squamous epithelium; has ceruminous (modified apocrine sweat) and sebaceous glands; supported by cartilage and, further in, by bone.
3 Tympanic membrane/eardrum: inner limit of the external ear, core of atypical collagen with thin epidermis externally, and a mostly simple squamous epithelium internally; the manubrium of the malleus bone inserts into the collagen. Elastin is present in the flaccid region.

2 Middle ear
l Epithelium-lined, air-filled, bony spaces of the tympanic cavity.
2 Communicates with the nasopharynx via the Eustachian/auditory/ pharyngotympanic tube, allowing equalization of air pressures on either side of the tympanic membrane. The mucosa of the tube and middle ear has several kinds of cell, and defensive systems.
3 Auditory ossicles articulate with one another - malleus, incus and stapes. The malleus is vibrated by air moving the tympanic membrane. This movement is then transmitted via the incus to the stapes with its foot held in the oval window by the annular ligament.
4 Elastic membrane of the round window transforms the fluid pressures generated in the inner ear into other forms of energy, thus acting as a pressure-release.
5 Fine skeletal muscles, (a) stapedius and (b) tensor tympani, inserting into the stapes and malleus are protective, and influence sound discrimination. Fine nerves pass to them.

3 Inner ear
The outer and middle ear thus have an exteroceptor function, transmitting air vibrations (20-20000 cycles per second is the perceptible range) to the perilymph fluid in the bone of the inner ear. Although the resulting pressure changes involve all perilymph, the receptors sensitive to the changes are localized in only one part of the labyrinth, the cochlea, and lie in the inner endolymph-filled system. Elsewhere in this inner system lie the intero- or proprioceptors for balance and movement, located in the vestibular apparatus.

C VESTIBULAR APPARATUS

l Bony vestibule houses the membranous utricle and saccule.
2 The vestibule extends into three semicircular tubes or canals distributed in three planes perpendicular to one another and containing the membranous semicircular ducts/canals, each swelling out at one end into an ampulla.
3 Movement of endolymph within the connecting membranous chambers stimulates receptors in maculae and cristae - modified, small, neuroepithelial areas of the lining membrane.

4 Utricle (with a macula) is oriented in the plane of the base of the skull, and the saccule (macula) in the sagittal plane. Both are responsive to gravity and linear acceleration, thus giving information on how the head is positioned.

5 Ampulla (with a crista) oriented in each of the horizontal, sagittal and transverse planes; responsive to movement of the head in the plane of that canal, thus furnishing information on the rate of angular acceleration.

7 The insensitive remainder of the vestibular membranous labyrinth is lined by a simple squamous epithelium on CT, which is supported by collagen and fibroblasts passing to the periosteum of the bony labyrinth, except on the side it fastens to the bony wall.

D COCHLEA

l Structures and elements
The tube - the cochlear duct - containing the cochlear endolymph is not surrounded by perilymph, but has it on two of its triangular sides. Thus, three chambers are contained within the bony cochlea which spirals for 2 1/2 turns around an axis of spongy bone, the modiolus. The spiralling unit comprises:
  1. Scala vestibuli with perilymph and mesothelium-lined.
  2. Reissner's membrane (membrana vestibularis), thin;
  3. Cochlear duct/scala media with K+-rich endolymph made in the stria vascularis; epithelium-lined, and containing the
  4. Organ of Corti (the actual receptor) on the
  5. basilar membrane, stretched from the tympanic lip of the bony spiral lamina to the spiral ligament.
  6. Scala tympani with perilymph and mesothelium-lined, separated by bone from the scala vestibuli adjacent in the spiral.
  7. Scalae vestibuli and tympani connect by a helicotrema at the apex of the cochlea, whilst the cochlear duct ends blindly as the caecum cupulare.
    The cochlear duct at its base communicates with the saccule via the ductus reuniens.
 Fig. 5 Turn of the cochlea
                      # # # # # bone
                    #  (  (  #
                  # (        #
               #  (          #
             #  (    SCALA   #
           #  (* ~ VESTIBULI # b
          # (*     ~r        # o      
         # (*  SCALA ~m      # n
        # (*   MEDIA   ~   # # e   
        # (___________!! ~_# Modiolus
        # (Basilar membrane  #      r~m Reissner's membrane
         # (                #        
          # (   SCALA     #         !! Organ of Corti
           # ( TYMPANI  #
            # (       #             *** Stria vascularis
             # (    #
              # # #   SCALA
                # (   VESTIBULI of the next turn
2 Organ of Corti
l Rests on the tympanic lip and basilar membrane.
2 Internal border cells and internal hair cells (receptive).
3 Internal pillar cells lean outwards towards inwardly leaning
4 external pillar cells, thereby enclosing an inner tunnel.
5 Phalangeal cells/Dieter's supporting cells support
6 external hair cells, (50-l00 hairs per cell); contractile to amplify the response of the mechano-sensory system; in three rows; damaged by loud sounds, streptomycin, cisplatin, etc.
7 Hairs (stereocilia of graded lengths) of outer cells go through a reticular plate to attach to the overlying
8 tectorial membrane - a gelatinous body attached at the vestibular lip to the CT limbus spiralis.
9 Nerve fibres derived from bipolar neurons of the spiral ganglion/ganglion of Corti in the bony spiral lamina, passing through the bone, serve the inner and outer hair cells.
(Centrifugal fibres also run from the brain stem to the outer hair cells, to enhance the response.)
l0 The centripetal fibres of Scarpa's vestibular and Corti's cochlear ganglia join to form the auditory/VIIth cranial nerve.

3 Organ of Corti's transducer function
Inner hair cells convert into neuronal discharges fluid pressure changes, transmitted through the basilar membrane to the cochlear endolymph, from the perilymph of the scala tympani. These changes originated at the oval window in response to vibration of the auditory ossicles caused by air moving the tympanic drum.
Discrimination of pitch (sound frequency) is based on different cochlear regions responding preferentially to particular tones, with high frequency received at the basal cochlea and lower ones apically where the basilar membrane is broader.

4 Fluids and gelatinous bodies
Although these are lost or grossly distorted in the histological processing, they are very important. The fluids transmit forces, and provide a metabolic pathway and favourable ionic environment for the receptor and other cells of the membranous labyrinth. In life, the gelatinous cupola and tectorial and otolithic membranes are large, filling or almost filling their respective membranous chambers.


William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159