Question

3. We discussed parallel pathways emerging at the level of regular and irregular afferents. What features of vestibular motion are represented by these two pathways?

Answer 1

Afferents can be functionally grouped on the basis of the regularity of their resting discharge measured by a normalized coefficient of variation. It is grouped as regular and irregular units.  

To maintain postural and perceptual stability and accurately guide behavior, the nervous system must differentiate between vestibular signals imposed by the external world and those that result from our own actions. semicircular canal afferents similarly encode self-generated (i.e., active) and externally applied (i.e., passive) movements , the differentiation between active and passive movements first occurs at the next level of signal processing, that is the vestibular nuclei.

The vestibular nuclei comprise a large set of neural elements in the brainstem that receive motion and other multisensory signals, then regulate movement responses and sensory experience. Many vestibular nuclei neurons have reciprocal connections with the cerebellum that form important regulatory mechanisms for the control of eye movements, head movements, and posture. There are four major vestibular nuclei that lie in the rostral medulla and caudal pons of the brainstem; all receive direct input from vestibular afferents. Many of these nuclei neurons receive convergent motion information from the opposite ear through an inhibitory commissural pathway that uses gamma-aminobutyric acid (GABA) as a neurotransmitter. The commissural pathway is highly organized such that cells receiving horizontal excitatory canal signals from the ipsilateral ear will also receive contralateral inhibitory horizontal canal signals from the opposite ear This fact gives rise to a “push-pull” vestibular function, whereby directional sensitivity to head movement is coded by opposing receptor signals. Because vestibular nuclei neurons receive information from bilateral inner ear receptors and because they maintain a high spontaneous firing rate (nearly 100 impulses/sec), they are thought to act to “compare” the relative discharge rates of left vs. right canal afferent firing activity.