Neuroanatomy - Special Senses: Position Sense (PDF)

Pag. 1 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense Neuroanatomy Special senses: position sense Prof. Dellavia – 05/11/21 – Author: Davide Pizzi – Reviser: Neil C...

Pag. 1 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense Neuroanatomy Special senses: position sense Prof. Dellavia – 05/11/21 – Author: Davide Pizzi – Reviser: Neil Connolly 1. The vestibule The vestibule is the central part of the bony labyrinth in the inner ear. Anteriorly it is continuous with the cochlea, and posteriorly with three semicirclar canals. The vestibule and semicircular canals together are often referred to as the vestibular system. On the medial wall of the vestibule, it is possible to identify some circular depression: At the forepart, is a small circular depression, the recessus sphæricus, which is perforated, at its anterior and inferior part, by several minute holes for the passage of nerves to the saccule; and behind this depression is an oblique ridge, the crista vestibuli. Inferior to the recessus sphaericus there is a small depression, the fossa cochlearis, which is perforated by a number of holes for the passage of filaments of the cochlear nerve. On the upper wall or roof, there is a transversely oval depression, the recessus ellipticus, separated from the recessus sphæricus by the crista vestibuli already mentioned. Here are located an opening for vestibular aqueduct and openings for nerves to the semicircular canals  Pag. 2 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense The vestibular apparatus contains specific membranous structure: The vestibule contains the saccule and utricle. The first one sits in the recessus sphæricus and is continuous anteriorly with the cochlear duct, the second one is located in the recessus ellipticus and is continuous with the semicircular ducts The semicircular canals contain the semicircular ducts. These ducts follow the shape of the bony labyrinth, but their size corresponds to ¼ of the one of samicircular canals. Each of this membranous structure is filled by endolymph and surrounded by the perilymph of the bony labyrinth The membranous portion inside semcircular canals, saccula and utricle consists of three layers: 1. An outer layer made of dense connective tissue. 2. An intermediate portion that is made of loose connective tissue and hosts vessels. 3. An inner layer that faces the fluid (endolymph) and consist of simple squamous epithelium. This simple epithelium becomes specialized in specific regions where receptors are located.  Pag. 3 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 2. Receptors The receptors in the vestibular apparatus are organized in two different ways: 1. In the semicircular canals they are found in crests (formed by extroflexions of the outer layer of the membranous portion) located at the level of the ampulla (therefor called ampullary crests). Ampullae are enlargements formed by the semicircular ducts at the level of one of the two connections formed with the vestibule. 2. In the saccule and utricle, receptors are grouped in enlarged laminae or plaques that are called maculae. Ampullary crests and Maculae have a different organization. In the ampullary crest receptors are covered by a gelatinous cupula. It is attached through connective tissue to the opposite wall thus maintaining its position. Differently, in the maculae receptors are covered by an otolithic membrane. An otolithic membrane is a gelatinous membrane covered by otoliths. For this reason, the maculae of utricle and saccule are often referred to as otolithic organs. The orientation of maculae in the saccule and in the utricle differ from each other: maculae in the utricle are oriented in a horizontal plane while in the saccule they are oriented in a vertical plane. This is due to the fact that the utricle is designed to detect an anteroposterior movement of the head while the saccule is designed to detect a movement along the vertical axis. In both ampullary crests and maculae there are not only receptors but also supportive cells that allow for nourishment and maintenance of a specific level of ions inside the endolymph.  Pag. 4 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 3. Hair cells The receptors of the vestibular apparatus are hair cells similar to the ones located in the cochlear duct: they are covered by stereocilia on their apical surface and present a kinocilium. There are two different types of receptors in both maculae and ampullary crests: 1. Type 1 sensory cells are more discriminative since they have larger afferent fibers at the synaptic button that surround them. Moreover, all type 1 fibers in a given macula or crest have the same polarization. This means that all of them have the kinocilium oriented towards the same side of the cell. 2. Type II sensory cells have ordinary boutons and can receive both afferents and efferents. They show different polarities in a given macula or crest. 4. Macula The macula can detect a linear acceleration. As explained before, in the utricle they perceive a horizontal acceleration while in the saccule they perceive a vertical acceleration. These accelerations correspond to an inclination of the head along a frontal or sagittal plane respectively.  Pag. 5 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense The maculae have a continuous frequency of discharging modified by the bending of the cilia entrapped into the otolithic membrane. When the head is inclined gravity leads to movement of the endolymph that in turn moves the otolithic membrane thus allowing the displacement of stereocilia. Tilting of stereocilia in the direction of the kinocilium leads to an excitatory polarisation (depolarisation). Tilting of stereocilia in the opposite direction leads to a hyperpolarisation. Through this mechanism the macula doesn’t perceive a continuum of the movement but it’s only able to sense the inclination of the head in a static way. In other words, it can just sense the beginning and the end of the inclination. The kinocilium can be oriented towards or away from the midline of the macula. The midline is recognizable through the striola formed by supporting cells and by the corresponding accumulation of otoliths that create the snow-bank line. The kinocilia of type 1 sensory receptors are oriented away from the striola in the saccule and towards it in the utricle.  Pag. 6 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 5. Ampullary crest The ampullary crest allows to perceive angular acceleration. Therefor it is activated when moving the head in a circular way. Differently from the utricle and saccule, the information from the semicircular ducts can give the idea of a continuum of the movement and not just the beginning and the end. The semicircular ducts can provide an idea of the position of the head and this is why they are three and they are oriented in the three axes of the three-dimensional space. The cells are immersed in a gelatinous cupula. Forces that bend the cilia of hair cells in the ampullary crest are generated by movements of the endolymph. Indeed, since the cupula is attached to its extremities, the movement of the endolymph bends the central core of the cupula, and this leads to a deflection of the cilia. As well as in the maculae the movement of the stereocilia towards the kinocilium leads to a depolarization and the movement of the stereocilia away from it leads to a hyperpolarization. 6. Functional coupling In order to have a reconstruction of the position of the head information are collected from both ears. The labyrinth is located in the temporal bone in such a way that the axis running from the cochlear canal to the semicircular canals is parallel to the major axis of the pyramids. Therefor the anterior semicircular canal on one side is parallel to the posterior semicircular canal on the other side. For this reason, functional couples formed by the vestibular pathway when collecting information from both sides are: 1. Right posterior semicircular canal – left anterior semicircular canal 2. Right anterior semicircular canal – left posterior semicircular canal 3. Right horizontal semicircula canal – left horizontal semicircular canal  Pag. 7 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 7. Vestibular nerve The vestibular nerve forms, together with the cochlear nerve, cranial nerve VIII. They follow the same pathway from the pyramids to the brainstem. The vestibular nerve is the result of many bundles collected in two main branches: The superior branch collects information from the utricle plus the superior and lateral semicircular canals. The inferior branch is made by fibers from saccule and posterior semicircular canal. These two branches are combined at the level of the vestibular ganglion. and from here the proper vestibular nerve starts and moves in an anterior direction to reach the cochlear nerve. The vestibular and cochlear nerve then exit the temporal bone from the internal acoustic meatus in the posterosuperior surface of pyramids. Together with vestibular and cochlear component there is also the passage of CN VII in the acoustic meatus.  Pag. 8 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense Internal auditory meatus is partitioned in separate entrance for different nerves and each nerve has its own dimension. The vestibular nerve measures ¼ of the cochlear. 8. Vestibular nuclei Information coming from ampullary crests and maculae reach different vestibular nuclei: Fibers arising from semicircular canals go to superior vestibular nucleus and rostral portion of medial vestibular nucleus Fibers arising from utricle and saccule arrive mainly at the lateral vestibular nucleus and a minor component to the inferior vestibular nucleus At the same time fibers from all organs (ampullary crest and macula) can give a collateral that directly enter the cerebellum. These fibers form the so-called juxtarestiform body. Collaterals forming the juxtarestiform body are mainly ipsilateral but some of them can be contralateral. The vestibular nuclei can give rise to both ascending and descending tracts.  Pag. 9 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 9. Ascending tract The majority of the fibers that project to the cortex is from the superior and lateral vestibular nuclei. These two nuclei give fibers mainly to the thalamus. More specifically they enter the ventral-posterior lateral nucleus of the thalamus (some of them in the superior aspect and some in the inferior aspect). These fibers ascend to the thalamus using the medial longitudinal fasciculus. For this reason, along their course they form connections with the motor ganglia of different cranial nerves. The most important among these ones are the connections with CN VI, CN IV, and CN III since they allow the vestibulo-ocular reflexes. The fibers can ascend in two sides (ipsilaterally and contralaterally). Then from the thalamus the information reaches the primary somatosensory cortex. The vestibular cortex is not well defined but fibers reach mainly area 2v and area 3a. 10. Descending tracts The information of position of the head are important for maintenance of posture. For this reason, vestibular nuclei can also give rise to descending pathways: 1. Medial vestibulospinal tract The medial vestibulospinal tract originates from the medial vestibular nucleus and descends together with the medial longitudinal fasciculus. This descending tract enters the spinal cord but stops at the level of the mid-thoracic neuromeres. The medial vestibulospinal tract synapses at the level of rexed laminae VIII and IX with spinal nerves and with the nucleus of CN XI (that controls movements of trapezius and sternocleidomastoid). This tract excites the extensor muscles of the neck and the extensor muscles superior limbs.  Pag. 10 a 10 International Medical School – NEUROAN #9 – prof. Dellavia – Special senses: position sense 2. Lateral vestibulospinal tract The lateral vestibulospinal tract is the main one. This tract originates from the lateral vestibular nucleus and descends ipsilaterally independently from the medial longitudinal fasciculus. It extends to the entire length of spinal cord. It excites inferior limb flexors and excites neck and limbs extensors. The vestibulospinal tract is one of the extrapyramidal tracts and is independent from the cerebral cortex. Therefor even if there is awareness of the movements of the head (thanks to fibers that project to the cortex via the medial longitudinal fasciculus) the corrections of the posture in response to these same movements is not controlled consciously.