Neuronal encoding of sound

The neuronal encoding of sound is the representation of auditory sensation and perception in the nervous system.

This article explores the basic physiological principles of sound perception, and traces hearing mechanisms from sound as pressure waves in air to the transduction of these waves into electrical impulses (action potentials) along auditory nerve fibers, and further processing in the brain.

Introduction

The complexities of contemporary neuroscience are continually redefined. Thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change.

This article is structured in a format that starts with a small exploration of what sound is followed by the general anatomy of the ear which in turn will finally give way to explaining the encoding mechanism of the engineering marvel that is the ear. This article traces the route that sound waves first take from generation at an unknown source to their integration and perception by the auditory cortex.

Basic physics of sound

Sound waves are what physicists call longitudinal waves, which consist of propagating regions of high pressure (compression) and corresponding regions of low pressure (rarefaction).

Waveform

Waveform is a description of the general shape of the sound wave. Waveforms are sometimes described by the sum of sinusoids, via Fourier analysis.

Amplitude

Graph of a simple sine wave

Amplitude is the size of the pressure variations in a sound wave, and primarily determines the loudness with which the sound is perceived. In a sinusoidal function such as Csin(2πft), C represents the amplitude of the sound wave.

Frequency and wavelength

The frequency of a sound is defined as the number of repetitions of its waveform per second, and is measured in hertz; it is inversely proportional to the wavelength. The wavelength of a sound is the distance between any two consecutive matching points on the waveform. The audible frequency range for humans is about 20 Hz to 20 000 Hz at infants. Hearing of higher frequencies decreases with age limiting to about 16000 Hz for adults and even down to 3000 Hz for elders.^{[citation needed]}

Anatomy of the ear

Flowchart of sound passage - outer ear

Given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail.

External ear

The external ear consists of the pinna or auricle (visible parts including ear lobes and concha), and the auditory meatus (the passage way for sound). The fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum. Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz.^[1]

The pinna as a result of its asymmetrical structure is able to provide further cues about the elevation from which the sound originated. The vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of it mechanical design.^[1][2]

Middle ear

Flowchart of sound passage - middle ear

The middle ear plays a crucial role in the auditory process, as it essentially converts pressure variations in air to perturbations in the fluids of the inner ear. In other words, it is the mechanical transfer function that allows for efficient transfer of collected sound energy between two different media.^[1] The three small bones that are responsible for this complex process are the malleus, the incus, and the stapes, collectively known as the ear ossicles.^[3][4] The impedance matching is done through via lever ratios and the ratio of areas of the tympanic membrane (ear drum) and the footplate of the stapes, creating a transformer-like mechanism.^[3] Furthermore the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy.^[4] A certain degree of top-down control is present at the middle ear level primarily through two muscles present in this anatomical region: the tensor tympani and the stapedius. These two muscles can restrain the ossicles so as to reduce the amount of energy that is transmitted into the inner ear in loud surroundings.^[2][3]

Inner ear

Flowchart of sound passage - inner ear

The cochlea of the inner ear, a marvel of physiological engineering, acts as both a frequency analyzer and nonlinear acoustic amplifier.^[1]

The cochlea has over 32,000 hair cells. Outer hair cells primarily provide amplification of traveling waves that are induced by sound energy, while inner hair cells detect the motion of those waves and excite the (Type I) neurons of the auditory nerve. The basal end of the cochlea, where sounds enter from the middle ear, encodes the higher end of the audible frequency range while the apical end of the cochlea encodes the lower end of the frequency range. This tonotopy plays a crucial role in hearing, as it allows for spectral separation of sounds. A cross section of the cochlea will reveal an anatomical structure with three main chambers (scala vestibuli, scala media, and scala tympani).^[4] At the apical end of the cochlea, at an opening known as the helicotrema, the scala vestibuli merges with the scala tympani. The fluid found in these two cochlear chambers is perilymph, while scala media, or the cochlear duct, is filled with endolymph.^[2]

Transduction

Auditory hair cells

The auditory hair cells in the cochlea are at the core of the auditory system's special functionality (similar hair cells are located in the semicircular canals). Their primary function is mechano-transduction, or conversion between mechanical and neural signals. The relatively small number of the auditory hair cells is surprising when compared to other sensory cells such as the rods and cones of the visual system. Thus the loss of low number (in the order of thousands) of auditory hair cells can be devastating while the loss of a larger number of retinal cells (in the order to hundreds of thousands) will not be as bad from a sensory standpoint.^[5]

Cochlear hair cells are organized as inner hair cells and outer hair cells; inner and outer refer to relative position from the axis of the cochlear spiral. The inner hair cells are the primary sensory receptors and a significant amount of the sensory input to the auditory cortex occurs from these hair cells. Outer hair cells on the other hand boost the mechanical signal by using electromechanical feedback.^[5]

Mechano-transduction

A hair bundle is found on the apical surface of each hair cell. Each hair bundle has about 300 projections of actin cytoskeleton known as stereocilia.^[6] These stereocilia are anatomically arranged in order of progressive height. The actin filaments present in these stereocilia are highly interlinked and even cross linked with fibrin that makes these pseudo ciliary projections quite stiff. In addition to stereocilia, a true ciliary structure known as the kinocilium exists and is believed to play a role in hair cell degeneration that is caused by exposure to high frequencies.^[1][6]

The stereocilia are hinged where they attach to the apical membrane. When displaced along a plane parallel to the tallest stereocilium, the tallest stereocilium depolarizes, which in turn causes subsequent depolarizations in the smaller stereocilia in that specific bundle.^[6] This serial depolarization is due to interconnected MET (mechano-electrical transduction) channels which open when mechanical perturbations occur in the endolymph that bathes the apical ends of the hair cells. These MET channels are interconnected with filaments known as tip links and fall under the category of cation selective transduction channels. Potassium is the ion that initiates the depolarization cascade by entering the cell through an open MET channel. This depolarization event induces calcium vesicles to fuse with the basal end of the hair cell, which in turn causes the generation of an action potential in the auditory neuron.^[6] Hyperpolarization of the hair cell, which occurs when potassium leaves the cell, is equally important as it is what prevents the merging of calcium vesicles with the basal end of the hair cell. Thus, as elsewhere in the body, the transduction is dependent on the concentration/distribution of ions.^[6] The perilymph which is found in the scala tympani has a low potassium concentration while the endolymph found in the scala media has a high potassium concentration with an electrical potential of 80mV in comparison to the perilymph.^[1] The stereocilia are highly sensitive with the ability to measure perturbations as small as fluid fluctuations of 0.3 nm, and can convert this depolarizing potential into a nerve impulse in about 10 microseconds.^{[citation needed]}

Nerve fibers from the cochlea

There are two types of afferent neurons found in the cochlear nerve: Type I and Type II. Each type of neuron has specific cell selectivity within the cochlea.^[7] The Mechanism that determines the selectivity of each type of neuron for a specific hair cell has been proposed by two diametrically opposed theories in neuroscience known as the peripheral instruction hypothesis and the cell autonomous instruction hypothesis. The peripheral instruction hypothesis states that phenotypic differentiation between the two neurons are not made until after these undifferentiated neurons attach to hair cells which in turn will dictate the differentiation pathway. The cell autonomous instruction hypothesis states that differentiation into Type I and Type II neurons occur following the last phase of mitotic division but preceding innervations.^[7] Both types of neuron participate in the encoding of sound for transmission to the brain.

Type I neurons

Type I neurons innervate inner hair cells. There is significantly greater convergence of this type of neuron towards the basal end in comparison with the apical end.^[7] A radial fiber bundle acts as an intermediary between Type I neurons and inner hair cells. The ratio of innervation that is seen between Type I neurons and inner hair cells is 1:1 which results in high signal transmission fidelity and resolution.^[7]

Type II neurons

Type II neurons on the other hand innervate outer hair cells. However, there is significantly greater convergence of this type of neuron towards the apex end in comparison with the basal end. A 1:30-60 ratio of innervation is seen between Type II neurons and outer hair cells which in turn make these neurons ideal for electromechanical feedback.^[7] Type II neurons can be physiologically manipulated to innervate inner hair cells provided outer hair cells have been destroyed either through mechanical damage or by chemical damage induced by drugs such as gentamicin.^[7]

Brainstem and midbrain

Levels of transmission of neuronal auditory signals

Main article: Central auditory system

The auditory nervous system includes many stages of information processing between the ear and cortex.

Auditory cortex

Primary auditory neurons carry action potentials from the cochlea into the transmission pathway shown in the image to the right. Multiple relay stations act as integration and processing centers. The signals reach the first level of cortical processing at the primary auditory cortex (A1), in the superior temporal gyrus of the temporal lobe.^[5] Most areas up to and including A1 are tonotopically mapped (that is, frequencies are kept in an ordered arrangement). Like lower regions, this region of the brain has combination-sensitive neurons that have nonlinear responses to stimuli.^[5]

Recent studies conducted in bats and other mammals have revealed that the ability to process and interpret modulation in frequencies primarily occurs in the superior and middle temporal gyri of the temporal lobe.^[5] Lateralization of brain function exists in the cortex, with the processing of speech in the left cerebral hemisphere and environmental sounds in the right hemisphere of the auditory cortex. Music, with its influence on emotions, is also processed in the right hemisphere of the auditory cortex. While the reason for such localization is not quite understood, lateralization in this instance does not imply exclusivity as both hemispheres do participate in the processing, but one hemisphere tends to play a more significant role than the other.^[5]

Recent ideas

• Alternation in encoding mechanisms have been noticed as one progresses through the auditory cortex. Encoding shifts from synchronous responses in the cochlear nucleus and later becomes dependent on rate encoding in the inferior colliculus.^[8]
• Despite advances in gene therapy that allows for the alteration of the expression of genes that affect audition, such as ATOH1, and the use of viral vectors for such end, the micro-mechanical and neuronal complexities that surrounds the inner ear hair cells, artificial regeneration in vitro remains a distant reality.^[9]
• Recent studies suggest that the auditory cortex may not be as involved in top down processing as was previous thought. In studies conducted on primates for tasks that required the discrimination of acoustic flutter, Lemus found that the auditory cortex played only a sensory role and had nothing to do with the cognition of the task at hand.^[10]
• Due to the presence of the tonotopic maps in the auditory cortex at an early age, it has been assumed that cortical reorganization had little to do with the establishment of these maps. However, recent work by Kandler et al. has shown that these maps are formed as a result of plastic reorganization on a sub-cellular and circuit level.^[11]

References

1. ^ ^{a b c d e f} Hudspeth, A. J, (1989) "How the Ear Works". Nature, 341, 397-404.
2. ^ ^{a b c} Hudspeth, A. J, (2001) "How the Ear works work: Mechanoelectrical transduction and amplification by hair cells of the internal ear". Harvey Lect, 97, 41-54.
3. ^ ^{a b c} Hudde, Herbert and Weistenhöfer Christian , (2006) "Key Features of the Human Middle Ear". Journal for Otorhinolaryngology, 324-329.
4. ^ ^{a b c} Hudspeth, A. J, (2000) "Auditory Neuroscience: Development, Transduction and Integration". Proc. National Academy of Sciences, 11690-11691.
5. ^ ^{a b c d e f} Kass, Jon H. et al, (1999) Auditory processing in primate cerebral cortex. Current Opinions in Neurobiology, 9, 500.
6. ^ ^{a b c d e} Fettiplace, Robert et al (2006) The Sensory and Motor roles of auditory hair cells. Nature Reivews, 7, 19-29.
7. ^ ^{a b c d e f} Rubel, Edwin W., (2002) "Auditory System Development:Primary Auditory Neurons and their Targets". Annual Reviews in Neuroscience, 25, 51-101.
8. ^ Frisina, Robert D., (2001) "Subcortical neural coding mechanisms for auditory temporal processing". Hearing Research, 158, 1-27.
9. ^ Brigande, John V., (2009) "Quo vadis, hair cell regeneration?". Nature Neuroscience, 12, 679-685.
10. ^ Lemus, Luis,(2009) "Neural codes for perceptual discrimination of acoustic flutter in the primate auditory cortex". Proceeding of the National Academy of Sciences, 106, 9471–9476.
11. ^ Kandler, Karl et al,(2009) "Tonotopic reorganization of developing auditory brainstem circuits". Nature Neuroscience, 12, 711-717.