The saccule is a bed of sensory cells in the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. 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
Vestibule or Vestibulum can have the following meanings, each primarily based upon a common origin, from early 17th century French, derived from Latin ''vestibulum, -i n.'' "entrance court".
Anatomy
In general, vestibule is a small space or cavity ...
is the name of the fluid-filled, membranous duct that contains these organs of balance. The vestibule is encased in the
temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex.
The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears. ...
of the skull.
Structure
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
vestibular duct of the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory o ...
. 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
statoacoustic 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 link
Tip links are extracellular filaments that connect stereocilia to each other or to the kinocilium in the hair cells of the inner ear.Pickles JO, Comis SD, Osborne MP. 1984.Cross-links between stereocilia in the guinea pig organ of Corti, and thei ...
s. Supporting cells 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 (
endolymphatic duct). This duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch saccus endolymphaticus (
endolymphatic sac) on the posterior surface of the
petrous portion of the temporal bone, where it is in contact with the
dura mater
In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. ...
.
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.
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.
Function
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. The structures that enable the saccule to gather this vestibular information are the
hair 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 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.
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
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
. 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.
Clinical significance
Assessment
Saccular function can be assessed by the cervical vestibular evoked myogenic potential (cVEMP). This is a middle latency (P1 between 12-20 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. .
Evolution of the Ear from Saccule
Research suggests that in
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
evolution,
sensory cells
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...
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.
[Duncan S. Jerem]
Cochlear neurosensory specification and competence
University of Iowa, 2012
See also
*
Saccular acoustic sensitivity
References
{{Authority control
Ear
Vestibular system