How does vestibular information reach the cortex

When the stereocilia of hair cells are moved, the tip links pull associated ion channels open for a fraction of a millisecond.

This is long enough to allow ions to rush through the ion channels to cause depolarization of the hair cells. Depolarization of hair cells leads to a release of neurotransmitters and the stimulation of the vestibulocochlear nerve.

The hair cells associated with the semicircular canals extend out of the crista ampullaris into a gelatinous substance called the cupula , which separates hair cells from the endolymph. When the endolymph flows into the ampulla, however, it causes the distortion of the cupula, which leads to movement of hair cells. This prompts stimulation of the vestibulocochlear nerve, which transmits the information about head movement to the vestibular nuclei in the brainstem as well as to the cerebellum.

The vestibular system uses two other organs, known as the otolith organs, to detect linear acceleration, gravitational forces, and tilting movements.

There are two otolith organs in the vestibular labyrinth: the utricle and the saccule. The utricle is specialized to detect movement in the horizontal plane, while the saccule detects movement in the vertical plane. The process of sensation in the otolith organs bears some similarity to the process in the semicircular canals, but there are also some distinct differences.

Like the semicircular canals, the otolith organs also contain a sensory organ where hair cells can be found; in this case, however, it is called the macula. As in the semicircular canals, there is a gelatinous layer above the hair cells; in the otolith organs, however, there is another fibrous structure called the otolithic membrane above the gelatinous layer. The otolithic membrane has small crystals of calcium carbonate called otoconia embedded within it.

These crystals make the otolithic membrane heavier than the rest of the structure; when linear acceleration occurs, it causes the otolithic membrane to shift relative to the macula, which leads to the displacement of hair cells and thus the release of neurotransmitters from these cells.

The structure of the otolith organs makes them especially sensitive to movements like linear acceleration and head tilts. The ascending tracts are shown in blue. The lateral vestibulospinal tract is shown in green.

It descends ipsilaterally to the sacral cord. The medial vestibulospinal tract is shown in red. It descends bilaterally in the MLF to thoracic levels.

The cerebellar afferents are not shown in this summary, but these come from the medial and inferior vestibular nuclei. The 1st order vestibular afferents arise in Scarpa's ganglion, which is in the distal portion of the internal auditory meatus. The axons travel in the vestibular portion of the VIIIth cranial nerve and enter the brain stem at the pontomedullary junction. All four nuclei are found beneath the floor of the fourth ventricle in the medulla and pons, lateral to the sulcus limitans.

The main projections from these nuclei are to the spinal cord controlling head and body position , to the three, extraocular motor nuclei III, IV, VI, controlling eye movements , to the thalamus VPI, eventually reaching the cortex and conscious perception of movement and gravity , and to the cerebellum coordinating postural adjustments.

The main descending tracts are the lateral vestibulospinal tract from the lateral vestibular nucleus and the medial vestibulospinal tract from the medial vestibular nucleus. The lateral vestibular tract starts in the lateral vestibular nucleus and descends the length of the spinal cord on the same side.

This pathway helps us walk upright. The medial vestibular tract starts in the medial vestibular nucleus and extends bilaterally through mid-thoracic levels of the spinal cord in the MLF. This tract affects head movements and helps integrate head and eye movements. In summary, remember that the lateral vestibulo-spinal tract is ipsilateral and long; the medial vestibulo-spinal tract is bilateral but shorter.

The main ascending tracts are from the superior and medial vestibular nuclei to the extraocular muscles through the medial longitudinal fasciculus MLF.

Second-order afferents arise in the medial and superior vestibular nuclei. Connections from the MLF to the abducens, trochlear and occulomotor nuclei can be seen. They are involved in preparatory and movement-related activities, postural control, and modulation of some sensory and autonomic functions. The corticospinal tract is involved in voluntary movement.

The majority of fibres of the corticospinal tract cross over in the medulla oblongata, resulting in muscles being controlled by the opposite side of the brain. The pyramidal tracts are named because they pass through the pyramids of the medulla oblongata. The Ventromedial Pathway conveys information from diffuse areas of the cortex, midbrain, and cerebellum.

The medial lemniscus is formed in the medulla as fibers from the posterior column nuclei cross the midline. The primary function of the medial lemniscus is as a second-order neuron of the dorsal column-medial lemniscus pathway DCML is to transport the sensory spinothalamic information of conscious proprioception, vibration, fine touch, and 2-point discrimination of skin and joints of the body and head; from the caudal ….

Level of Decussation of the Medial Lemniscus Purple lines have been used to represent the internal arcuate fibres as they run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus.

The direct dorsal column pathway includes two large ascending pathways, the gracile and cuneate fasciculi. The gracile fasciculus tract of Goll is present throughout the length of the spinal cord and contains afferents from the lower trunk and extremities, below the T6 spinal cord segment. Ascending tracts are sensory pathways that begin at the spinal cord and stretch all the way up to the cerebral cortex.

There are three types of ascending tracts, dorsal column-medial lemniscus system, spinothalamic or anterolateral system, and spinocerebellar system. The ascending tracts refer to the neural pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex.

In some texts, ascending tracts are also known as somatosensory pathways or systems. The Spinal Cord Position of the Eight Major Ascending Tracts The largest ascending tracts are the gracile and cuneate fasciculi, the spinothalamic tracts, and the spinocerebellar tracts. Ascending pathway: A nerve pathway that goes upward from the spinal cord toward the brain carrying sensory information from the body to the brain. In contrast, descending pathways are nerve pathways that go down the spinal cord and allow the brain to control movement of the body below the head.

The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. The corticospinal tract controls primary motor activity for the somatic motor system from the neck to the feet.

The tract begins in the primary motor cortex where the soma of pyramidal neurons are located within cortical layer V. C Postural reflexes are NOT a function of the hypothalamus.

It is a function of the cerebellum. Functions of the hypothalamus include: Body temperature regulation. The corticospinal pathway is a one-neuron pathway from the cerebral cortex to the gray of the spinal cord. This pathway consists of all axons that: 1 originate from cells within the cerebral cortex, 2 pass through the pyramids of the medulla, and 3 terminate in the spinal cord.