Answer: The olivary nucleus is a part of the brain stem that is involved in auditory processing and cerebellum mediated learning.
The olivary nucleus is an oval shaped prominence in the medulla, a part of the brain stem. The olivary nucleus, or olive, is divided into two parts with two different functions. The superior olive is a part of the pons in the cerebellum, and functions in the auditory pathway. The inferior olive rests below that structure, and functions as a part of the cerebellar motor learning pathway.
Superior olivary complex (SOC)
When sound waves enter the ear canal, they vibrate the eardrum, in turn causing the ossicles (three tiny bones inside the ear) to vibrate at that same frequency. The vibration is passed into the cochlea, and that structure’s output is sent to the brain via the auditory nerve, a branch of the vestibulocochlear nerve (cranial nerve VIII). This auditory information enters into the superior olive for initial processing before arriving at the auditory cortex.
The superior olivary nucleus can be further divided into the medial and lateral superior olive, which serve slightly different functions in auditory processing. They are both paired structures, meaning on exists on each half of the spinal cord. Collectively, the medial and lateral superior olive serve to help the person identify which direction an auditory stimulus is coming from, the left or the right side of the head.
The medial superior olive functions to determine if a sound is coming from the left or right side of the head by calculating a difference in time from when the sound reaches one ear to the other. Using these data, the medial superior olive can calculate an angle for where a sound has originated. For example, sound waves travel at approximately 300 meters per second. The average width of a human head is about 6 inches. Using these numbers, it can be calculated that a sound wave that originates exactly from the left or right side of the head will reach one ear 700 microseconds before it reaches the other ear.
The lateral superior olive also functions to help identify the origin of a sound. Instead of using temporal differences to make determine the origin of the sound, it uses slight changes in volume. A sound originating on the left side will be louder in the left ear, and when that sound reaches the right ear, that sound wave will be slightly dampened because of the head. The structure receives excitatory inputs from the ipsilateral ear while receiving simultaneous inhibitory inputs from the contralateral ear (The right lateral superior olive gets excitation from the right ear and inhibition from the left ear.)
Inferior olivary complex (IOC)
The inferior olive is located close to the cerebellum and functions as a relay between the spinal cord and cerebellum. Injury to the cerebellum almost always results in inferior olive injury, and inferior olive injury likewise results in cerebellar injury. Such lesions lead to a difficulty with learning complex coordination tasks.
Some disorders affect the inferior olive and cerebellum along with other brain structures. For instance, progressive supranuclear palsy is a disease associated with a problem in the Tau protein that normally functions to maintain the microtubule structure of cells. In this progressive supranuclear palsy, a person experiences bradykinesia (slowed movements) and difficulty with gait and posture during locomotion. Sometimes, they may also exhibit psychological symptoms such as dementia or difficulty with speech. Because the symptoms may appear similar to other neurodegenerative disorders such as Parkinson’s disease or Alzheimer’s disease, progressive supranuclear palsy may be underdiagnosed.
It also is involved heavily in signaling of the spino-olivary tract, which carries information about proprioception from the muscles up the spinal cord. This pathway is sometimes also called Helwig’s tract.
Another important feature of the inferior olivary nucleus is related to the production of sex hormones in the body. For example, the inferior olive produces the enzyme aromatase, which is the primary enzyme that converts testosterone into estradiol.
On the level of neural circuitry, the inferior olive receives GABA-ergic projections from various other structures, including the deep cerebellar nuclei and the parasolitary nucleus (Adaptive Balance in Posterior Cerebellum).