| Saccule | |
|---|---|
 Inner ear, showing saccule near center. | |
 illustration of otolith organs showing detail ofutricle,otoconia,endolymph, cupula,macula of saccule,hair cell filaments, andsaccular nerve | |
| Details | |
| Part of | Inner ear |
| System | Balance |
| Identifiers | |
| Latin | sacculus |
| TA98 | A15.3.03.065 |
| TA2 | 7001 |
| FMA | 61116 |
| Anatomical terminology | |
Thesaccule (Latin: sacculus) is a bed of sensory cells in theinner ear that detectslinearacceleration and head tilting in thevertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the head moves vertically, the sensory cells of the saccule are moved due to a combination of inertia and gravity. In response, the neurons connected to the saccule transmit electrical impulses that represent this movement to the brain. These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in thebrainstem.
The vestibular system is important forbalance, orequilibrium. It includes the saccule,utricle, and the threesemicircular canals. Thevestibule is the name of the fluid-filled, membranous duct that contains these organs of balance and is in turn encased in thetemporal bone of the skull as a part of the inner ear.
The saccule, or sacculus, is the smaller of the two vestibular sacs. It is globular in form and lies in therecessus sphæricus near the opening of thevestibular duct of thecochlea. Its cavity does not directly communicate with that of theutricle. The anterior part of the saccule exhibits an oval thickening, themacula acustica sacculi, or macula, to which are distributed the saccular filaments of the vestibular branch of thevestibulocochlear nerve, also known as thestatoacoustic nerve orcranial nerve VIII.
Within the macula arehair cells, each having a hair bundle on the apical aspect. The hair bundle is composed of a singlekinocilium and many (at least 70)stereocilia. Stereocilia are connected to mechanically gated ion channels in the hair cell plasma membrane viatip links. Supporting cells interdigitate between hair cells and secrete theotolithic membrane, a thick, gelatinous layer of glycoprotein. Covering the surface of the otolithic membrane areotoliths, 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, theductus endolymphaticus (endolymphatic duct). This duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouchsaccus endolymphaticus (endolymphatic sac) on the posterior surface of thepetrous portion of the temporal bone, where it is in contact with thedura mater.
From the lower part of the saccule a short tube, thecanalis reuniens of Hensen, passes downward and opens into theductus cochlearis near its vestibular extremity.
Both the utricle and the saccule provide information about acceleration. The difference between them is that the utricle is more sensitive to horizontal acceleration, whereas the saccule is more sensitive to vertical acceleration.
| Saccule |
| Components of theinner ear |
|---|
The saccule gathers sensory information to orient the body in space. It primarily gathers information about linear movement in the vertical plane, including the force due to gravity. The saccule, like the utricle, provides information to the brain about head position when it is not moving.[1] The structures that enable the saccule to gather this vestibular information are thehair cells. The 2 by 3 mm patch of hair cells and supporting cells are called a macula. Each hair cell of a macula has 40 to 70 stereocilia and one true cilium called a kinocilium. The stereocilia are oriented by the striola, a curved ridge that runs through the middle of the macula; in the saccule they are oriented away from the striola[2] The tips of the stereocilia and kinocilium are embedded in a gelatinous otolithic membrane. This membrane is weighted with protein-calcium carbonate granules called otoliths, which add to the weight and inertia of the membrane and enhance the sense of gravity and motion.[3]
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 inauditory processing and the sensitivity of the saccule of females peaks during the breeding season. This is due to an increase in thedensity of saccular hair cells, partly resulting from reducedapoptosis.[4] The increase the hair cells make also increase the sensitivity to male mating calls. An example of this is seen inPorichthys notatus, or plainfin midshipman fish.
Saccular function can be assessed by the cervical vestibular evoked myogenic potential (cVEMP). This is a middle latency (P1 between 12 and 20 ms) waveform denoting inhibition of the sternocleidomastoid (SCM) muscle ipsilateral to the stimulus. While not truly a unilateral reflex (response waveforms can be detected in the SCM contralateral to the stimulus in approximately 40% of cases), cVEMPs are more unilateral than the closely related ocular vestibular evoked myogenic potential (oVEMP). The most reliable points on the cVEMP waveform are known as P1 and N1. Of all waveform characteristics, P1-N1 amplitude is the most reliable and clinically relevant. cVEMP amplitude is linearly dependent upon stimulus intensity and is most reliably elicited with a loud (generally at or above 95 dB nHL) click or tone burst. The cVEMP can also be said to be low-frequency tuned, with largest amplitudes in response to 500–750 Hz tonebursts. This myogenic potential is felt to assess saccular function, because the response is present in completely deafened ears and because it is routed through the inferior vestibular nerve, which is known to dominantly innervate the saccule. .[5]
Research suggests that invertebrate evolution,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 newepithelia 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.[6]
