Auditory Nerve Responses
Neurons produce all-or-none electrical discharges called action potentials which are recorded as spikes. The number of spikes per second is called the firing or discharge rate. Auditory neurons have a certain ongoing firing rate even when they are not being stimulated. called the spontaneous firing rate. Stimulation causes the firing rate to increase and raising the level causes the neuron to fire even faster, at least within certain limits. The faintest sound level that induces a response from a neuron is called its threshold.
An auditory neuron has a dynamic range over which its firing rate increases with stimulus level, after which the firing rate saturates or stops growing as the stimulus level increases further. Auditory neurons vary in threshold over a range of ~60dB or more, and their dynamic ranges are ~25dB wide for some groups of auditory neurons and ~40dB or more for others. These gradations of thresholds and dynamic ranges contribute to the ability of the auditory nerve to represent a wide range of sound intensities.
Auditory neurons represent frequency information by both place and temporal coding. The place coding mechanism is represented by tuning curves. Each tuning curve shows the range of frequencies to which a given nerve fiber responds at different levels of stimulation. A given curve was obtained here by finding the threshold for a particular neuron for many different frequencies. The peak of the curve shows the lowest threshold of the neuron, and the frequency where this occurs is its characteristic frequency. The orderly arrangement of CFs reflects the places along the length of the cochlea where the neurons are connected. The neuron responds to a slightly wider range of frequencies around its CF as the stimulus level is raised high enough, then the neuron will respond to a wide range of frequencies extended well below its characteristic frequency. This phenomenon appears as the 'tail" on the low-frequency side of the curve. However, the neuron is not responsive to much higher frequencies even when they are presented at very intense stimulus levels.
An auditory neuron has a dynamic range over which its firing rate increases with stimulus level, after which the firing rate saturates or stops growing as the stimulus level increases further. Auditory neurons vary in threshold over a range of ~60dB or more, and their dynamic ranges are ~25dB wide for some groups of auditory neurons and ~40dB or more for others. These gradations of thresholds and dynamic ranges contribute to the ability of the auditory nerve to represent a wide range of sound intensities.
Auditory neurons represent frequency information by both place and temporal coding. The place coding mechanism is represented by tuning curves. Each tuning curve shows the range of frequencies to which a given nerve fiber responds at different levels of stimulation. A given curve was obtained here by finding the threshold for a particular neuron for many different frequencies. The peak of the curve shows the lowest threshold of the neuron, and the frequency where this occurs is its characteristic frequency. The orderly arrangement of CFs reflects the places along the length of the cochlea where the neurons are connected. The neuron responds to a slightly wider range of frequencies around its CF as the stimulus level is raised high enough, then the neuron will respond to a wide range of frequencies extended well below its characteristic frequency. This phenomenon appears as the 'tail" on the low-frequency side of the curve. However, the neuron is not responsive to much higher frequencies even when they are presented at very intense stimulus levels.
The Central Auditory Pathways
The major aspects of the ascending central auditory nervous system, as well as briefly overview the olivocochlear pathways and the central connections of the vestibular system.
Afferent Auditory Pathways
The auditory pathway is quite redundant. Information originating from each ear is carried by pathways on both sides of the brain. The neural connections have what might be called a "series-parallel wiring diagram. The term "series-parallel" is used because the two arrangements exist together in the central auditory pathways. For example, points A and C are connected by many neural fibers ("wires"); some of them go from A to B to C and others bypass B on their way from A to C. As a result of the redundant pathways, a given lesion (analogous to a cut wire or a blown bulb) that occurs within the central auditory pathway rarely causes a loss of hearing sensitivity because the signal is usually also represented along an alternative route. However, this does not mean that a central lesion does not cause auditory impairments. On the contrary, significant disturbances are caused by central lesions because they affect the processing of auditory information that allows you to determine sound locations, differentiate among sounds and background noises, and interpret speech.
What are the major pathways of the ascending auditory system? The auditory nerve exits the temporal bone through the internal auditory meatus and enters the brainstem at a location called the cerebellopontine angle, which is a term that describes the relationship of the pons and cerebellum in this area. The auditory nerve fibers, constituting the first order neurons of the auditory pathway, terminate at either the ventral cochlear nucleus or the dorsal cochlear nucleus, where they synapse with the next level of nerve cells, called second-order neurons. Some second-order neurons go to the superior olivary complex on the same (ipsilateral) side of the brainstem, but a majority will decussate (cross to the opposite side) via the trapezoid body and proceed along the contralateral pathway. The fibers that cross over will either synapse with the opposite superior olivary complex or ascend in the contralateral lateral lemniscus. As a result, each superior olivary complex receives information from both ears, so that bilateral representation exists as low as this low level in the auditory nervous system.
Third order neurons arise from the superior olivary complex and rise via the lateral lemniscus. Fibers also arise from the superior olivary complex and rise via the lateral lemniscus. Fibers also originate from the nuclei of the lateral lemniscus. Notice here that the pathway rising out of the lateral lemniscus contains neurons that originated from several different levels. Neurons ascending from this point may synapse at the inferior colliculus or may bypass the inferior colliculus on their way to the next level. Neurons originating from the inferior colliculus and those that bypass it ascend via the brachium of the inferior colliculus to terminate at the medial geniculate body of the thalamus. This is the last subcortical way station in the auditory pathway, and all ascending neurons that reach the medial geniculate body will synapse here. Neurons from the medial geniculate then ascend along a pathway called the auditory radiations (thalamocortical radiations) to the auditory cortex located in the transverse temporal (Heschl's) gyrus.
Recall that decussations between the two sides of the brain occur beginning with second order neurons from the cochlear nuclei, so that bilateral representations exists as low as the superior olivary complex. Commissural tracts also connect auditory nuclei on the two sides at the levels of the lateral lemniscus, the inferior colliculus and the auditory cortex via the corpus callosum. However, communication does not occur between the medial geniculate bodies on the two sides.
The systemic organization of frequency by location is called tonotopic organization and is seen at every level of the auditory system from the cochlea up to and including the cortex. Tonotopic relationships are determined by measuring the characteristic frequencies of many individual cells for each nucleus, and have been reported for the auditory nerve, cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate bodies and the cortex.
What are the major pathways of the ascending auditory system? The auditory nerve exits the temporal bone through the internal auditory meatus and enters the brainstem at a location called the cerebellopontine angle, which is a term that describes the relationship of the pons and cerebellum in this area. The auditory nerve fibers, constituting the first order neurons of the auditory pathway, terminate at either the ventral cochlear nucleus or the dorsal cochlear nucleus, where they synapse with the next level of nerve cells, called second-order neurons. Some second-order neurons go to the superior olivary complex on the same (ipsilateral) side of the brainstem, but a majority will decussate (cross to the opposite side) via the trapezoid body and proceed along the contralateral pathway. The fibers that cross over will either synapse with the opposite superior olivary complex or ascend in the contralateral lateral lemniscus. As a result, each superior olivary complex receives information from both ears, so that bilateral representation exists as low as this low level in the auditory nervous system.
Third order neurons arise from the superior olivary complex and rise via the lateral lemniscus. Fibers also arise from the superior olivary complex and rise via the lateral lemniscus. Fibers also originate from the nuclei of the lateral lemniscus. Notice here that the pathway rising out of the lateral lemniscus contains neurons that originated from several different levels. Neurons ascending from this point may synapse at the inferior colliculus or may bypass the inferior colliculus on their way to the next level. Neurons originating from the inferior colliculus and those that bypass it ascend via the brachium of the inferior colliculus to terminate at the medial geniculate body of the thalamus. This is the last subcortical way station in the auditory pathway, and all ascending neurons that reach the medial geniculate body will synapse here. Neurons from the medial geniculate then ascend along a pathway called the auditory radiations (thalamocortical radiations) to the auditory cortex located in the transverse temporal (Heschl's) gyrus.
Recall that decussations between the two sides of the brain occur beginning with second order neurons from the cochlear nuclei, so that bilateral representations exists as low as the superior olivary complex. Commissural tracts also connect auditory nuclei on the two sides at the levels of the lateral lemniscus, the inferior colliculus and the auditory cortex via the corpus callosum. However, communication does not occur between the medial geniculate bodies on the two sides.
The systemic organization of frequency by location is called tonotopic organization and is seen at every level of the auditory system from the cochlea up to and including the cortex. Tonotopic relationships are determined by measuring the characteristic frequencies of many individual cells for each nucleus, and have been reported for the auditory nerve, cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate bodies and the cortex.
Efferent Auditory Pathways
The efferent neurons that communicate with the organ of Corti are derived from the superior olivary complexes on both sides of the brainstem, constituting the olivocochlar bundle also known as Rasmussen's bundle. The major aspects of the OCB are depicted in two systems rather than one. The uncrossed olivocochlear bundle is the olivocochlear pathway derived from the same side of the head as the cochlea in question. Most of its fibers are from the area of the lateral superior olive (LSO) and terminate at the afferent fibers of the inner hair cells and some of its fibers are from the vicinity of the medial superior olive and go to the outer hair cells. The opposite arrangement exists for the crossed olivocochlear bundle whose neurons originate on the opposite side of the brainstem and cross along the floor of the fourth ventricle to the side of the cochlea in question. Here, most of the fibers are from the MSO and go to the outer hair cells, while a much smaller number come from the LSO and terminate at the inner hair cells. In other words, the uncrossed olivocochlear bundle extends mainly from the LSO to the afferent neurons of the IHCs and the crossed olivocochlear bundle extends mainly from the MSO to the OHCs.
The olivocochlear bundle is by no means the only descending neural pathway in the auditory system. We have already seen that efferent signals influence the transmission system of the middle ear by innervating the stapedius and tensor tympani muslces. In addition, higher centers exert influences over lower ones at many levels of the auditory system, with some efferents going to the ear and others going to lower centers within the nervous system itself. For example, some of these efferent connections descend to lower levels from the cortex, lateral lemniscus, and inferior colliculus, and are received by various central auditory centers such as the medial geniculates and the inferior colliculi.
The olivocochlear bundle is by no means the only descending neural pathway in the auditory system. We have already seen that efferent signals influence the transmission system of the middle ear by innervating the stapedius and tensor tympani muslces. In addition, higher centers exert influences over lower ones at many levels of the auditory system, with some efferents going to the ear and others going to lower centers within the nervous system itself. For example, some of these efferent connections descend to lower levels from the cortex, lateral lemniscus, and inferior colliculus, and are received by various central auditory centers such as the medial geniculates and the inferior colliculi.