The upper olive or olive complex is a set of oval-shaped nuclei, similar to that of an olive, which are found in the lower area of the brain stem. These structures are very important for maintaining auditory function, forming part of the ascending and descending pathways of the auditory system..
In addition, the superior olive complex is related to the decussing trapezoidal body, the junction between the axons of the contralateral neurons of the two cochlear subnuclei. The neurons of the superior olivary complex, for the most part, are placed dorsal to the axons of the neurons of the trapezoidal body. In addition, some neurons of the olive are embedded in this structure.
The superior olive complex is located in the brainstem and extends from the rostral medulla to the middle of the brain. This set of nuclei receives its projections especially from the anterior ventral cochlear nucleus through the trapezoidal body and from the posterior ventral nucleus through the intermediate acoustic stria. Thus, the upper olive complex is thus connected to the cochlea, a fundamental piece for the hearing of humans and animals that is found in the inner ear and that appears as a spiral-shaped tube, in charge of sending the vibrations of sounds to the rest of the brain thanks to the glial cells that it has inside, specifically in the organ of Corti.
The upper olive complex is divided into the following main nuclei:
As we have mentioned, the upper olive complex has different functions in relation to hearing. These functions are very demarcated in each nucleus of the structure:
The superior medial olive, one of the nuclei found in the superior olive complex and containing approximately 15,000 neurons, is responsible for measuring the different intensities of the sounds that arrive between the ears, and its function is very important to classify sounds. high frequency. Its main function is to detect the signals of interaural time differences.
In addition, the upper lateral olive appears to be related to the measurement of the difference in sound intensities between the two ears. This is also essential for evaluating high-frequency sounds..
Likewise, the medial superior olive has an important function when classifying the angle from which the sound comes, whose source can be located to the right or to the left. This horizontal information of the sound is therefore processed by the upper olive grove, although not the vertical information, in which it is decided whether a sound comes from a higher or lower localized source of the listener. This process is carried out instead by the inferior colliculus..
On the other hand, the superior lateral olive has functions very similar to the medial superior olive. However, it receives excitatory glutamatergic inputs from the ipsilateral cochlear nucleus and inhibitory inputs from the medial nucleus of the trapezoidal body that manage to sharpen the intensity for localization of the sound source. In addition, this structure projects to the central nucleus of the inferior follicle and to the lateral lemniscus..
The medial nucleus of the trapezoidal body is made up of neurons that transmit the neurotransmitter glycine and whose function takes place in the ascending auditory pathway. Its functions are essential for locating sound and encoding the temporal characteristics of complex sounds. This nucleus has been found in humans in very recent times, since before it was only contemplated in other animals, however, recent studies have confirmed the existence of this essential nucleus for these functions
In addition to the superior olive complex, there is a lower olive complex or inferior olive nucleus that is related to the cerebellum and is placed between the lateral spinal cord and the pyramid of the medulla oblongata. The lower olive nucleus, in turn, is made up of the primary olive nucleus, which configures its greater structure; the accessory medial olivaceous nucleus that rests between the first nucleus and the bulbar pyramid and the dorsal accessory olivary nucleus, which appears as a curved lamina behind the primary nucleus. The main function of this complex is involved in motor activity and learning of the cerebellar structure.