The Ear: Hearing

The ear is a sense organ that is specialized for two distinct functions: hearing and equilibrium. It can be divided into external, middle, and inner sections, with the neurological elements housed in and protected by structures in the inner ear. The vestibular complex of the inner ear is the primary sensor for equilibrium. The remainder of the ear is used for hearing.

The external ear consists of the outer ear, or pinna, and the ear canal (Fig. 10.15). The pinna is another example of an important accessory structure to a sensory system, and it varies in shape and location from species to species, depending on the animals’ survival needs. The ear canal is sealed at its internal end by a thin membranous sheet of tissue called the tympanic membrane, or eardrum.

The tympanic membrane separates the external ear from the middle ear, an air-filled cavity that connects with the pharynx through the Eustachian tube. The Eustachian tube is normally collapsed, sealing off the middle ear, but it opens transiently to allow middle ear pressure to equilibrate with atmospheric pressure during chewing, swallowing, and yawning. Colds or other infections that cause swelling can block the Eustachian tube and result in fluid buildup in the middle ear. If bacteria are trapped in the middle ear fluid, the ear infection known as otitis media {oto-, ear + -itis, inflammation + media, middle} results.

Three small bones of the middle ear conduct sound from the external environment to the inner ear: the malleus {hammer}, the incus {anvil}, and the stapes {stirrup}. The three bones are connected to one another with the biological equivalent of hinges. One end of the malleus is attached to the tympanic membrane, and the stirrup end of the stapes is attached to a thin membrane that separates the middle ear from the inner ear.

The inner ear consists of two major sensory structures. The vestibular apparatus with its semicircular canals is the sensory transducer for our sense of equilibrium, described in the following section. The cochlea of the inner ear contains sensory receptors for hearing. On external view the cochlea is a membranous tube that lies coiled like a snail shell within a bony cavity. Two membranous disks, the oval window (to which the stapes is attached) and the round window, separate the liquid-filled cochlea from the air-filled middle ear. Branches of cranial nerve VIII, the vestibulocochlear nerve, lead from the inner ear to the brain.

Hearing Is Our Perception of Sound

Hearing is our perception of the energy carried by sound waves, which are pressure waves with alternating peaks of compressed air and valleys in which the air molecules are farther apart (Fig. 10.16a). The classic question about hearing is, “If a tree falls in the forest with no one to hear, does it make a noise?” The physiological answer is no, because noise, like pain, is a perception that results from the brain’s processing of sensory information. A falling tree emits sound waves, but there is no noise unless someone or something is present to process and perceive the wave energy as sound.

FIG. 10.16 Sound waves

Figure Question

  1. What are the frequencies of the sound waves in graphs (1) and (2) in Hz (waves/second)?

  2. Which set of sound waves would be interpreted as having lower pitch?

Sound is the brain’s interpretation of the frequency, amplitude, and duration of sound waves that reach our ears. Our brains translate frequency of sound waves (the number of wave peaks that pass a given point each second) into the pitch of a sound. Low-frequency waves are perceived as low-pitched sounds, such as the rumble of distant thunder. High-frequency waves create high-pitched sounds, such as the screech of fingernails on a blackboard.

Sound wave frequency (Fig. 10.16b) is measured in waves per second, or hertz (Hz). The average human ear can hear sounds over the frequency range of 20–20,000 Hz, with the most acute hearing between 1000–3000 Hz. Our hearing is not as acute as that of many other animals, just as our sense of smell is less acute. Bats listen for ultra-high-frequency sound waves (in the kilohertz range) that bounce off objects in the dark. Elephants and some birds can hear sounds in the infrasound (very low frequency) range.

Loudness is our interpretation of sound intensity and is influenced by the sensitivity of an individual’s ear. The intensity of a sound wave is a function of the wave height, or amplitude (Fig. 10.16b). Intensity is measured on a logarithmic scale in units called decibels (dB). Each 10 dB increase represents a 10-fold increase in intensity.

Normal conversation has a typical noise level of about 60 dB. Sounds of 80 dB or more can damage the sensitive hearing receptors of the ear, resulting in hearing loss. A typical heavy metal rock concert has noise levels around 120 dB, an intensity that puts listeners in immediate danger of damage to their hearing. The amount of damage depends on the duration and frequency of the noise as well as its intensity.

Concept Check

  1. What is a kilohertz?

Sound Transduction Is a Multistep Process

Hearing is a complex sense that involves multiple transductions. Energy from sound waves in the air becomes mechanical vibrations, then fluid waves in the cochlea. The fluid waves open ion channels in hair cells, the sensory receptors for hearing. Ion flow into hair cells creates electrical signals that release neurotransmitter (chemical signal), which in turn triggers action potentials in the primary auditory neurons.

These transduction steps are shown in Figure 10.17. Sound waves striking the outer ear are directed down the ear canal until they hit the tympanic membrane and cause it to vibrate (first transduction). The tympanic membrane vibrations are transferred to the malleus, the incus, and the stapes, in that order. The arrangement of the three connected middle ear bones creates a “lever” that multiplies the force of the vibration (amplification) so that very little sound energy is lost due to friction. If noise levels are so high that there is danger of damage to the inner ear, small muscles in the middle ear can pull on the bones to decrease their movement and thereby dampen sound transmission to some degree.

FIG. 10.17 Sound transmission through the ear

As the stapes vibrates, it pulls and pushes on the thin tissue of the oval window, to which it is attached. Vibrations at the oval window create waves in the fluid-filled channels of the cochlea (second transduction). As waves move through the cochlea, they push on the flexible membranes of the cochlear duct and bend sensory hair cells inside the duct. The wave energy dissipates back into the air of the middle ear at the round window.

Movement of the cochlear duct opens or closes ion channels on hair cell membranes, creating electrical signals (third transduction). These electrical signals alter neurotransmitter release (fourth transduction). Neurotransmitter binding to the primary auditory neurons initiates action potentials (fifth transduction) that send coded information about sound through the cochlear branch of the vestibulocochlear nerve (cranial nerve VIII) and the brain.

The Cochlea Is Filled with Fluid

As we have just seen, the transduction of wave energy into action potentials takes place in the cochlea of the inner ear. Uncoiled, the cochlea can be seen to be composed of three parallel, fluid-filled channels: (1) the vestibular duct, or scala vestibuli {scala, stairway; vestibulum, entrance}; (2) the central cochlear duct, or scala media {media, middle}; and (3) the tympanic duct, or scala tympani {tympanon, drum} (Fig. 10.18). The vestibular and tympanic ducts are continuous with each other, and they connect at the tip of the cochlea through a small opening known as the helicotrema {helix, a spiral + trema, hole}. The cochlear duct is a dead-end tube, but it connects to the vestibular apparatus through a small opening.

The fluid in the vestibular and tympanic ducts is similar in ion composition to plasma and is known as perilymph. The cochlear duct is filled with endolymph secreted by epithelial cells in the duct. Endolymph is unusual because it is more like intracellular fluid than extracellular fluid in composition, with high concentrations of K+ and low concentrations of Na+.

The cochlear duct contains the organ of Corti, composed of hair cell receptors and support cells. The organ of Corti sits on the basilar membrane and is partially covered by the tectorial membrane {tectorium, a cover}, both flexible tissues that move in response to fluid waves passing through the vestibular duct (Fig. 10.18). As the waves travel through the cochlea, they displace basilar and tectorial membranes, creating up-and-down oscillations that bend the hair cells.

Hair cells, like taste receptor cells, are non-neural receptor cells. The apical surface of each hair cell is modified into 50–100 stiffened cilia known as stereocilia, arranged in ascending height (Fig. 10.19a). The stereocilia of the hair cells are embedded in the overlying tectorial membrane. If the tectorial membrane moves, the underlying cilia do as well.

FIG. 10.19 Signal transduction in hair cells

When hair cells move in response to sound waves, their stereocilia flex, first one way, then the other. The stereocilia are attached to each other by protein bridges called tip links. The tip links act like little springs and are connected to gates that open and close ion channels in the cilia membrane. When the hair cells and cilia are in a neutral position, about 10% of the ion channels are open, and there is a low tonic level of neurotransmitter released onto the primary sensory neuron.

When waves deflect the tectorial membrane so that cilia bend toward the tallest members of a bundle, the tip links pop more channels open, so cations (primarily K+ and Ca2+) enter the cell, which then depolarizes (Fig. 10.19b). Voltage-gated Ca2+ channels open, neurotransmitter release increases, and the sensory neuron increases its firing rate. When the tectorial membrane pushes the cilia away from the tallest members, the springy tip links relax and all the ion channels close. Cation influx slows, the membrane hyperpolarizes, less transmitter is released, and sensory neuron firing decreases (Fig. 10.19c).

The vibration pattern of waves reaching the inner ear is thus converted into a pattern of action potentials going to the CNS. Because tectorial membrane vibrations reflect the frequency of the incoming sound wave, the hair cells and sensory neurons must be able to respond to sounds of nearly 20,000 waves per second, the highest frequency audible by a human ear.

Concept Check

  1. Normally when cation channels on a cell open, either Na+ or Ca2+ enters the cell. Why does K+ rather than Na+ enter hair cells when their cation channels open?

Sounds Are Processed First in the Cochlea

The auditory system processes sound waves so that they can be discriminated by location, pitch, and loudness. Localization of sound is a complex process that requires sensory input from both ears coupled with sophisticated computation by the brain (see Fig. 10.4). In contrast, the initial processing for pitch and loudness takes place in the cochlea of each ear.

Coding sound for pitch is primarily a function of the basilar membrane. This membrane is stiff and narrow near its attachment between the round and oval windows but widens and becomes more flexible near its distal end (Fig. 10.20a). High-frequency waves entering the vestibular duct create maximum displacement of the basilar membrane close to the oval window and consequently are not transmitted very far along the cochlea. Low-frequency waves travel along the length of the basilar membrane and create their maximum displacement near the flexible distal end.

FIG. 10.20 Sensory coding for pitch

This differential response to frequency transforms the temporal aspect of frequency (number of sound waves per second) into spatial coding for pitch by location along the basilar membrane (Fig. 10.20b). A good analogy is a piano keyboard, where the location of a key tells you its pitch. The spatial coding of the basilar membrane is preserved in the auditory cortex as neurons project from hair cells to corresponding regions in the brain. Loudness is coded by the ear in the same way that signal strength is coded in somatic receptors. The louder the noise, the more rapidly action potentials fire in the sensory neuron.

Auditory Pathways Project to the Auditory Cortex

Once the cochlea transforms sound waves into electrical signals, sensory neurons transfer this information to the brain. The cochlear (auditory) nerve is a branch of cranial nerve VIII, the vestibulocochlear nerve [here]. Primary auditory neurons project from the cochlea to cochlear nuclei in the medulla oblongata (Fig. 10.21). Some of these neurons carry information that is processed into the timing of sound, and others carry information that is processed into the sound quality.

FIG. 10.21 The auditory pathways

From the medulla, secondary sensory neurons project to two higher nuclei, one ipsilateral (on the same side of the body) and one contralateral (on the opposite side). Splitting sound signals between two ascending tracts means that each side of the brain gets information from both ears. These ascending tracts then synapse in nuclei in the midbrain and thalamus before projecting to the auditory cortex (see Fig. 10.3). Collateral pathways take information to the reticular formation and the cerebellum.

The localization of a sound source is an integrative task that requires simultaneous input from both ears. Unless sound is coming from directly in front of a person, it will not reach both ears at the same time (see Fig. 10.4). The brain records the time differential for sound arriving at the ears and uses complex computation to create a three-dimensional representation of the sound source.

Hearing Loss May Result from Mechanical or Neural Damage

There are three forms of hearing loss: conductive, central, and sensorineural. In conductive hearing loss, sound cannot be transmitted either through the external ear or the middle ear. The causes of conductive hearing loss range from an ear canal plugged with earwax (cerumen), to fluid in the middle ear from an infection, to diseases or trauma that impede vibration of the malleus, incus, or stapes. Correction of conductive hearing loss includes microsurgical techniques in which the bones of the middle ear can be reconstructed.

Central hearing loss results either from damage to the neural pathways between the ear and cerebral cortex or from damage to the cortex itself, as might occur from a stroke. This form of hearing loss is relatively uncommon.

Sensorineural hearing loss arises from damage to the structures of the inner ear, including death of hair cells as a result of loud noises. The loss of hair cells in mammals is currently irreversible. Birds and lower vertebrates, however, are able to regenerate hair cells to replace those that die. This discovery has researchers exploring strategies to duplicate the process in mammals, including transplantation of neural stem cells and gene therapy to induce nonsensory cells to differentiate into hair cells.

Therapy that replaces hair cells would be an important advance. The incidence of hearing loss in younger people is increasing because of prolonged exposure to rock music and environmental noises. Ninety percent of hearing loss in the elderly—called presbycusis {presbys, old man + akoustikos, able to be heard}—is sensorineural. Currently, the primary treatment for sensorineural hearing loss is the use of hearing aids, but amazing results have been obtained with cochlear implants attached to tiny computers (see Biotechnology box).

Hearing is probably our most important social sense. Suicide rates are higher among deaf people than among those who have lost their sight. More than any other sense, hearing connects us to other people and to the world around us.

Concept Check

  1. Map or diagram the pathways followed by a sound wave entering the ear, starting in the air at the outer ear and ending on the auditory cortex.

  2. Why is somatosensory information projected to only one hemisphere of the brain but auditory information is projected to both hemispheres? (Hint: See Figs. 10.4 and 10.8.)

  3. Would a cochlear implant help a person who suffers from nerve deafness? From conductive hearing loss?