Neuroanatomy III: The Auditory System

For the most point, sound is something most of us take for granted, it has been suggested to be the second most important part of the human sensory system (Carlson 2013), for those of us with sight impairments, it is arguably the most important sense for it helps us detect the location of objects, people, threats and sounds. Of course, we know that hearing is done by our ears – an organ which tells us much more about our human selves than we have ever truly thought into – but how does it work? How can those two flaps of skin on the side of our head help us to hear?

It is the structure internal to the ear – an area one cannot see – which is responsible for our hearing. The anatomy of the ear is relatively simple, there is the outer ear (that which we can see) which helps us hear by picking up the sound waves and directing them into the middle ear via the ear drum. It is in this middle ear where the sound vibrations bounce off the ear drum and make the structure vibrate, thus sending emphasised waves onto the bone structure which meets the end of the middle ear. These are known as the hammer, the anvil and the stirrup, these bone like structures emphasise the sound waves within the ear so that when the vibrations are passed onto the oval window (the first structure of the inner ear, or last structure of the middle ear) the information can be passed correctly onto the cochlea. It is here, in the main area of the middle ear that the process of neuraltransmission (and so the important part when it comes to the processing of sound identification and so the understanding and identification of sound properties.)

Inside the cochlea there are two membranes, the basilar membrane and the tectorial membrane, these are essential tissue barriers between fluid filled crevices within the cochlea named the scala tympani and the scala vestibuli. These fluid filled areas vibrate when alerted to sound waves (the levels of such vibrations fully depend on the sounds pitch, timbre and intensity) these then bend the membranes (specifically the basilar membrane) and move the little hairs which are located in the scala media (these are known an cilia) these cilia play the role of neurons – refer to neuroanatomy I if you are unsure of this meaning.

When the cilia vibrate as a result of the sound waves, dependent on the level of vibration this causes activity resulting from the potassium based fluid surrounding them. Consequently, this leads to information being passed to the bipolar neurons whose axons make up the cochlear nerve (VIII cranial nerve) and thus pass messages for processing in the brain. It is in the brain where this information is sent via the primary auditory cortex. It is important to register that in the auditory system, the neurotransmitter comes from the hair cells and is passed to the bipolar neurons which then – unlike most neurons – can sustain action potentials on both ends of their soma (cell body of the neuron) and so both ends of the neuron act as dendrites in the synapse of the neighbouring/adjacent neuron, following this process the synapse of the cochlear nerve results in synapse with the auditory nerve which axons travel and synapse with the neurons of the medulla. (this process will be talked of further later in the article.)

Right now, readers should note that each cochlear nerve is house to approximately 50,000 afferent (neuroanatomy I, moving toward the brain – think afferent 'attract', efferent 'escape') axons. These axons are each mylinated for faster neurotransmission. The hair cells (sensory receptors see neuroanatomy II) make up around 30% of all the neural transmission cells in the auditory system (Carlson 2013) though do seem to be of primary importance with regard to communication with the central nervous system (neuroanatomy I).

It is when the sound reaches the medulla – via the auditory nerve, which leads to the talk of efferent communication. These fibres are found in the olivary complex. (Singularly known as 'olives'.). The pathways are complex from here, as they clearly follow a complex route from the auditory nerve to the cortex of the brain. Thus the image below should assist your understanding of the system.


One will notice, as explained above that the sound information travels up the auditory nerve to the medulla, from there they pass up the lateral lemniscus to the inferior collicullus of the midbrain and finally into areas of the cerebrum after interacting with the thalamus (the 'recall' area of the human brain) and so then to the auditory cortex in the temporal lobe of the brain.(Some may now wish to consult a diagram of the brain to find the location of the lobes on the cortex of the brain, one should be aware that the temporal lobe is located posterior to the frontal lobe - associated with memory and personality, and anterior to the occipital lobe mentioned in neuroanatomy II.)The auditory system, unike the visual system, is contralateral, simply meaning it recieves information from both sides equally and does not switch from left to right (thus allows for better and more accurate location information of sound.

With regard to the basilar membrane, it responds via different ends to different levels of sound. (tonotopic representation) Again, with reference to neuroanatomy II, we see that there are similarities between the layouts of the auditory and visual systems, in as much of a hierachy of order. Essentially, both systems communicate via different locations in the neural conduction process and pass information to different areas of the brain to identify different features (where and what?) If one is interested in this, they should then be aware that there are three main levels of the auditory cortex in the human brain and should research the following areas; the core region, the belt region and parabelt region. Again one will then notice that as with the visual system there is presence of dorsal - ventral communication(s) between the belt and parabelt regions of the primary auditory cortex (overlapping into the medial geniculate nucleus). It is in these anterior and posterior systems we will find that the auditory cortex overlaps from the parietal lobe into the temporal lobe.

Perceptions

Most of us will know that there are three very important dimensions of sound, these being pitch (hertz) intensity/loudness (amplitude) and timbre/complexity. It is dependent on all of these factors that we pick up vibrations of sounds and so the hearing process begins.

Pitch

When measuring pitch waves, one should be aware that the more waves passing a particular point per second (or per any time frame) represents a higher pitch than the fewer waves passing the point. These pitches then cause a different level of activiation in different areas of the basilar membrane within the cochlea. The higher the frequency of waves (and so higher the pitch of the sound) the closer to the middle ear the basilar membrane is activated (see page 216, figure 7.11, Carlson 2013).

This process then shows us that different neurons are activiated by different sound pitches they are exposed to. This is then known as place coding - the process which suggests that certain locations are activated by specific stimulus. This then is something interesting in regard to loss of hearing, for it is interesting to know that with the death of hair cells in the basil end of the cochlea (close to the basilar membrane) leads to loss of higher frequencies and these are always the first frequencies to go. - It is then probably depressing news to most of you when I inform you that by the time one reaches twenty, around 50% of these have died, though certain drugs/pharmaceutricals have been reported as responsible for higher rates of death in these cells - specifically certain antibiotics taken for an extended period of time.) This is often then where cochlear implants are used - to replace the hair cell loss which causes loss of hearing. Different cells will respond to different hertz of pitch, this has been mentioned however, a point to mention also is varience in reception to these hertz can be seen according to the intensity of the sound. (basically, the louder the sound the further a receptor will respond)

With regard to low frequency sounds, this is done by the apical end of the cochlea and thus means it is not place coding processes used to register such pitches but a process by name of rate coding, although the process seems to be similar.

Intensity/Loudness

The loudness of sound is detected by the movement at the end of the hair cells in the cochlea, depedent on the number on nanometers these move (between 1 and 100) depends then on the level of volume (strength of vibrations). The higher the number of nanometers suggests the louder the sound. It is with these movements of the tiny hair cells that neurons fire at different rates and so with this firing send different communications of the auditory cortex. The louder the noise results in higher levels of firing of neurons, the pitch then determines which neurons fire.

Timbre

For this, one must understand the concept of sine waves (waves within waves), for most sounds we hear are complex sound waves made up of frequency, amplitude and sine waves - these are what gives complex waves their ragged formation, these are then the very things that allow us to identify what it is we hear. This is done by the way the cochlear nerve will differentiate between the beginning, middle and end of the wave from we percieve and will carefully seperate these sounds - and changes within the wave - to identify the object making the sound.

The Localisation of Sound - Where is it? Based on the arrival time..

When we hear a sound, we are relatively good at detecting where the sound comes from (we are much better at left or right - think due to the location of our ears). This then means that when a sound happens on our left side, the left ear will recieve the information faster and more intensely than the right ear. Although this is an unnoticable time difference to us, and we appear to hear the sound in both ears at the same time, it is still the case that mostly, we can identify from which side the sound came. Most neurons of the auditory system will respond to sound information from both ears, however, they react dependent on time of signal arriving - so the noise from the left side results in faster perception of sound from left ear and cochlea causing neurons to fire from the left side faster than the right side, consequently this activity leads to us assuming - often quite accurately - that the sound came from our left side.
This is then further backed by the higher intensity recieved by the closer ear when a sound is percieved, again this is minimal and rather hard to notice, but is part of the reason why we can detect from which side a sound came.

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