Saccule

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Saccule
Bigotolith.jpg
illustration of otolith organs showing detail of utricle, otoconia, endolymph, cupula, Macula of saccule, hair cell filaments, and saccular nerve
Latinsacculus
Gray'ssubject #232 1052
MeSHSaccule+and+Utricle
 
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Saccule
Bigotolith.jpg
illustration of otolith organs showing detail of utricle, otoconia, endolymph, cupula, Macula of saccule, hair cell filaments, and saccular nerve
Latinsacculus
Gray'ssubject #232 1052
MeSHSaccule+and+Utricle

The saccule is a bed of sensory cells situated in the inner ear. The saccule translates head movements into neural impulses which the brain can interpret. The saccule is sensitive to linear translations of the head, specifically movements up and down (for example, moving on an elevator). When the head moves vertically, the sensory cells of the saccule are disturbed and the neurons connected to them begin transmitting impulses to the brain. These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem.

The vestibular system is important in maintaining balance, or equilibrium. The vestibular system includes the saccule, utricle, and the three semicircular canals. The vestibule is the name of the fluid-filled, membranous duct than contains these organs of balance. The vestibule is encased in the temporal bone of the skull.

Contents

Anatomy

The saccule, or sacculus, is the smaller of the two vestibular sacs. It is globular in form and lies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea. Its cavity does not directly communicate with that of the utricle. The anterior part of the saccule exhibits an oval thickening, the macula acustica sacculi, or macula, to which are distributed the saccular filaments of the vestibular branch of the vestibulocochlear nerve, also known as the acoustic nerve or cranial nerve VIII.

Within the macula are hair cells, each having a hair bundle on the apical aspect. The hair bundle is composed of a single kinocilium and many (at least 70) stereocilia. Stereocilia are connected to mechanically-gated ion channels in the hair cell plasma membrane via tip links. Supporting cells are interdigitate between hair cells and secrete the otolithic membrane, a thick, gelatinous layer of glycoprotein. Covering the surface of the otolithic membrane are otoliths, which are crystals of calcium carbonate. For this reason, the saccule is sometimes called an "otolithic organ."

From the posterior wall of the saccule is given off a canal, the ductus endolymphaticus. This duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater.

From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity.

Physiology

The saccule gathers sensory information to orientate the body in space. It primarily gathers information about linear movement in the vertical plane. The structures that enable the saccule to gather this vestibular information are the hair cells.

Not much is known of how this organ is used in other species. Research has shown, like songbirds, females in some species of fish show seasonal variation in auditory processing and the sensitivity of the saccule of females peaks during the breeding season. This is due to an increase in the density of saccular hair cells, partly resulting from reduced apoptosis.[1] The increase the hair cells make also increase the sensitivity to male mating calls. An example of this is seen in Porichthys notatus,or plainfin midshipman fish.

Evolution of the Ear from Saccule

Research suggests in the vertebrate lineage, sensory cells became specialized as gravistatic sensors after they became assembled to form the ear. After this aggregation, growth, including duplication and segregation of existing neurosensory epithelia, gave rise to new epithelia and can be appreciated by comparing sensory epithelia from the inner ears of different vertebrates and their innervation by different neuronal populations. Novel directions of differentiation were apparently further expanded by incorporating unique molecular modules in newly developed sensory epithelia. For example, the saccule gave rise to the auditory epithelium and corresponding neuronal population of tetrapods, starting possibly in an aquatic environment.[2]

Assessing Saccular Function

It is possible to assess saccular function through the use of vestibular evoked myogenic potential (VEMP). The VEMP response is a brief latency muscle relaxation potential that is generated by synchronous discharges of motor units in the sternocleidomastoid (SCM) muscle when a loud sound, usually a click, is detected. The myogenic potential is felt to assess saccular function.[3]

References

  1. ^ Coflfin B. Allison Saccular-Specific Hair Cell Addition Correlates with Reproductive State-Dependent Changes in the Auditory Saccular Sensitivity of a Vocal Fish Journal of Neuroscience, January 25, 2012
  2. ^ Duncan S. Jeremy Cochlear neurosensory specification and competence University of Iowa, 2012
  3. ^ Cushing,& Lynn, S. (2008). "Relationship between sensorineural hearing loss and vestibular and balance function in children." (Master's thesis, University of Toronto, Canada)Retrieved from url: [1]