Saccule

Key takeaways

Saccule belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Saccule to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Saccule from memory before moving on to harder problems.

Reference excerpt

The saccule (Latin: sacculus) is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the head accelerates 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 vestibulocochlear nerve (CNVIII) to the vestibular nuclei in the brainstem. The vestibular system is important for balance, or equilibrium. It includes the saccule, utricle, and the three semicircular canals. The vestibule is the name of the fluid-filled, membranous duct that contains these organs of balance and is in turn encased in the temporal bone of the skull as a part of the inner ear.

Structure The saccule, or sacculus, is the smaller of the two vestibular sacs. It is globular in form and lies in the spherical recess (one of the three recesses in the vestibule) near the opening of the vestibular duct 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 of saccule, 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 at least 70 stereocilia. Stereocilia are connected to mechanically gated ion channels in the hair cell plasma membrane via tip links. 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 endolymphatic duct. This duct is joined by the ductus utriculosaccularis, and then passes along the vestibular aqueduct and ends in a blind pouch the endolymphatic sac 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 ductus reuniens, passes downward and opens into the cochlear duct near its vestibular extremity. Both the utricle and the saccule provide information about acceleration. The utricle is more sensitive to horizontal acceleration, and 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. In fish, such as the midshipman fish, and reptiles, such as geckos, the saccule, plays dual roles in both balance and low frequency sound or vibration hearing.

Clinical significance

Assessment 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. .

Role in evolution of the ear Research suggests that in vertebrate 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 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.

Illustrations

Worked examples

Example 1 — a first encounter with Saccule

Start with the simplest possible case. Write down what Saccule claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Saccule before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Saccule ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Saccule

In research: Saccule appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Saccule in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Saccule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ear, Vestibular system, so understanding it makes those chapters shorter.
In everyday life: Look for Saccule outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Saccule in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Saccule means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Saccule out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Saccule in simple terms?

The saccule (Latin: sacculus) is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the head accelerates vertically, the sensory cells of the sac…

Why does Saccule matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Saccule?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Saccule.