Figure 4‑1: The lateral line system of bass. Note the extensive branching of the canals across the face.
Human beings detect sound strictly through our ears. Bass, on the other hand, have two sensors for acoustical disturbances: the lateral line system and the inner ears. Together, along with the bass's sensory control for balance and equilibrium, these senses make up what is known as the octavo‑ system. While the inner ears of bass bear some resemblance to that of humans, their lateral line is wholly unlike any sensory system we own.
The Lateral Line System
A bass's lateral line system is comprised of two major subdivisions: one on the head (the cephalic system) and one along the body trunk (the lateralis system). These two subdivisions differ not only in structure and in the nerve branches that supply them but also in how the bass uses them. The cephalic system is a network of highly branched canals spread over the head and face of the bass, especially around the eyes, across the top of the head, beneath the jaw, and along the front portion of the gill cover. The lateralis system consists of a single long canal on either side of the trunk, starting just behind the eyes and extending to the tail. The head canals are partially or completely housed in the underlying bone, whereas the main trunk canal is constructed from specialized scales overlying the skin.
Along the canals are regularly spaced openings, or pores. The pores allow water to flow freely in either direction along the canal's length. Housed within the canal between each pair of pores is a flexible gelatin‑like mass called a cupula. The cupula juts into the canal cavity, partially obstructing the flow of water. Beneath each cupula is a sensory patch, the neuromast, that contains an array of unique sensory cells called hair cells, so named because of their hair‑like extensions on top. The hair cells reach into the overlying cupula and are extremely sensitive to its every move. There is always only one neuromast between any two pores, but the total number of pores and neuromasts continues to increase throughout the bass's life. The lateral line system covering the face is serviced by a branch of its own cranial nerve called the anterior lateral line nerve; the posterior lateral line nerve, which stems from the same cranial nerve, services the main trunk canals. These two nerve branches eventually come together and enter the rear of the bass's brain, slightly ahead of the spinal cord. Each lateral line neuromast receives about ninety nerve fibers. Each nerve fiber connects to about seven hair cells.
The Ear
Despite their similar functions, bass ears differ from our own in many ways. Human ears, for example, consist of three relatively distinct divisions: the outer, middle, and inner ear. All of our actual hearing is accomplished in the inner ear; the outer and middle sections merely aid in gathering and transmitting sound waves to the inner chamber. Bass, in contrast, have only an inner ear. They do not have anything remotely similar to our external ear for gathering sound, and they lack the three small bones in our middle ear‑the
Figure 4‑2 The lateral line is made up of a series of pores (formed by modified scales) opening into a canal system just beneath the surface (A). Between each pair of pores lies a gelatinous mass, the cupula, which rests on the canal floor and partially obstructs flow along the canal (B). As the fluid pushes past the cupula it causes the cupula to bend with the flow (Q. This distortion, in turn, bends the underlying hair cells contained within the sensory patch beneath the cupula, causing the hair cells to fire off a series of nervous impulses to the brain (D). Within each sensory patch, roughly half of the hair cells face in one direction while the other half face the opposite direction.
Figure 4‑3 Each inner ear is held within a bony capsule of the skull just behind the eye (A). The upper portion of the inner ear has three semicircular canals needed for maintaining balance and a lower portion that houses three otolith organs: the saccule, lagena, and utricule. Each otolith organ consists of a hair cell patch, or macula, overlaid by a calcified ear stone called an otolith (B and Q. When an otolith slides across its macula it distorts the underlying hair cells, triggering them to fire. Hair cell distribution within each macula is arranged into discrete sections. All cells within each section face the same direction. The composite neural signal sent to the brain varies according to the pathway taken by the drifting otolith, that is, different pathways yield different neural results.
malleus (hammer), incus (anvil), and stapes (stirrup)‑that are key to our internal sound amplification. The inner ear of a bass has no outside connections whatsoever. Instead, it lies completely out of sight, buried beneath the skin and housed within the skull.
The general structure of a bass's inner ear is much like our own. It consists of three arches, or semicircular canals, that are responsible as they are in humans for controlling equilibrium. These three semicircular canals are connected to an open sac called the labyrinth. Inside the labyrinth are located three sensory patches, or maculae, called the saccule, lagena, and utricule. Each of these subdivisions contains a patch of hair cells, just like those found in the lateral line system.
Overlying each macula is a calcified ear stone, called an otolith. Together the combination of a macula with its overlying ear stone is called an otolith organ. The shape of an otolith is unique for its particular macula. Moreover, the shapes differ across various species of bass enough that they can often be used in species identification. And, because their cross sections display annual growth rings, otoliths are used to help age bass as well. The otolith is free to slip and slide over the underlying macula as the bass moves about. Each otolith organ is estimated to contain twenty thousand or more sensory hair cells. All of these cells are connected to nerve fibers from the eighth cranial nerve, known as the octaval nerve. The octaval nerve enters the rear of the bass's brain just below the area that receives the lateral line nerve fibers. We know that the total number of the hair cells in the otolith increases with age.
An important difference between bass and humans is that bass ears lack a cochlea, our own seat of hearing. Later we'll examine how the lack of this structure has major implications for how bass and humans differ in hearing capabilities.
Figure 4‑4: Nerves for the lateral line and inner ear enter the hindbrain just beneath the cerebellum (see side view). Nerve fibers from the inner ear and the lateral line follow separate but parallel pathways as they project for‑ward to several nerve sites in the mid‑ and forebrain. As seen in the nerve projection coming from the left ear (top view), most of the nerve fibers cross over the midline to be processed by the opposite side ofthe brain.
Converting Sound to Brain Signals
The hair cell is the root of sound perception in all vertebrates. Hair cells specialize in translating mechanical movements into nervous impulses. On its top end, a hair cell sports a bundleof hair‑like extensions, or cilia, that extend above the skin surface. One of these, the kinocillum, is extra long and lies to one side of the cell's upper surface. The remaining cilia, the stereocilia, are all shorter and slope upwards towards the kinocilium. This arrangement appears to give the hair cell a lengthwise axis rising along the cilia.
Excerpted from Knowing Bass copyright 2002 by Keith A. Jones, PhD with permission from The Lyons Press, an imprint of the Globe Pequot Press, Guilford, CT www.globepequot.com
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