Point 13 Intro Receptors Pathway Deficits Eyes Eye Defic. Overview Problems Contents Anatomy

Once vestibular input from the semicircular canals and otolith organs has reached the vestibular nuclei, the information is used to maintain balance and to stabilize the visual image on the retina during head movements. First we will consider only the projections of the vestibular nuclei that reach the spinal cord in order to help us maintain our BALANCE. For example, let's say that you are walking to lecture this morning and slip on the icy sidewalk. Your feet fly to the LEFT and your upper body and head fly to the RIGHT (right ear down). Information coming out of your semicircular canals will be related to the accelerating head. This particular angular acceleration affects several different canals (beyond what you need to know for this course), but what I want you to know is that the RIGHT vestibular nuclei are turned on. Once your head is not moving the information will come from the utricles. Again, you do not have to know the specific pattern from each side, but only that the RIGHT vestibular nuclei are turned on. The increased activity in the RIGHT vestibular nuclei can affect the body musculature via the RIGHT LATERAL VESTIBULOSPINAL TRACT. This will result in increased activity in the RIGHT arm and leg in order to right ourselves after slipping. The cells of origin of the lateral vestibulospinal tract lie in the lateral vestibular nucleus. Axons arising from this nucleus descend through the caudal brain stem (you don't see these fibers on the cross sections) and upon reaching the spinal cord course within the ventral funiculus and innervate neurons for the ENTIRE length of the cord. This projection is UNCROSSED. Through this tract, the vestibular apparatus--which detects whether the body is on an even keel--exerts its influence on those muscles that restore and maintain upright posture. Such muscles are proximal rather than distal. REMEMBER--LATERAL VESTIBULAR NUC.--LATERAL VESTIBULOSPINAL TRACT--UNCROSSED--ENTIRE LENGTH OF CORD--VENTRAL FUNICULUS--PROXIMAL MUSCLES--MAINTAINS BALANCE BY ACTING MAINLY ON LIMBS. The increased activity in the RIGHT vestibular nuclei can also affect body musculature via a second, smaller, descending pathway to the spinal cord. This smaller pathway is called the MEDIAL VESTIBULOSPINAL TRACT (or descending medial longitudinal fasciculus [MLF]). Cells within the medial vestibular nucleus possess axons that descend bilaterally (the ipsilateral projection is denser) in a position just off the midline near the dorsal surface of the pons and medulla. These descending axons course caudally and enter the spinal cord, where they lie within the medial part of the ventral funiculus. This pathway makes connections with cervical motor neurons that play a role in maintaining the normal position of the head via innervation of spinal cord neurons that innervate neck musculature. Thus when your head flies to the RIGHT, it will reflexively be brought to an upright position via information flowing out of the RIGHT medial vestibular nucleus. REMEMBER--MEDIAL VESTIBULAR NUC.--MEDIAL VESTIBULOSPINAL TRACT--BILATERAL--CERVICAL CORD ONLY--MAINTAINS HEAD ERECT.