The vestibular nerve is one of the two branches of the vestibulocochlear nerve. In humans the vestibular nerve transmits sensory information transmitted by vestibular hair cells located in the two otolith organs and the three semicircular canals via the vestibular ganglion. Information from the otolith organs reflects gravity and linear accelerations of the head. Information from the semicircular canals reflects rotational movement of the head. Both are necessary for the sensation of body position and gaze stability in relation to a moving environment. Axons of the vestibular nerve synapse in the vestibular nucleus are found on the lateral floor and wall of the fourth ventricle in the pons and medulla, it arises from bipolar cells in the vestibular ganglion, ganglion of Scarpa, situated in the upper part of the outer end of the internal auditory meatus. Structure The peripheral fibers divide into three branches: the superior branch passes through the foramina in the area vestibularis superior and ends in the utricle and in the osseous ampullae of the superior and lateral semicircular ducts.
The primary role of the vestibular nerve is to transform vestibular information into an egocentric frame of reference based on the position of the head in relation to the body. The vestibular nerve dynamically updates the frame of reference of motor movement based on the orientation of the head in relation to the body; as an example, when standing upright and facing forward, if you wished to tilt your head to the right you would need to perform a slight leftward motor movement to maintain balance. While the head is still in motion, the response magnitude of alteration to motor coordinates is reduced when compared to when the head is fixated in one position. Due to its role in transforming motor coordinates, the vestibular nerve implicitly plays a role in maintaining stable blood pressure during movement, maintaining balance control, spatial memory and spatial navigation; the most common causes of damage to the vestibular nerve are exposure to ototoxic antibiotics, Ménière's disease and some rare autoimmune disorders.
Patients with a damaged nerve suffer from acute attacks of vertigo accompanied by nausea/vomiting, inability to maintain posture and horizontal nystagmus. Rapid compensation to damage of the vestibular nerve occurs within seven to ten days of receiving the damage. A small percentage of patients with damage to the vestibular nerve experience recurrent symptoms; these patients have not been able to undergo vestibular compensation and are left with long-term attacks of vertigo. By administering betahistine to the damaged nerve over a long period of time, the process of vestibular compensation can be accelerated to alleviate symptoms. Patients can learn strategies to recover their balance through physical therapy. Vestibular system This article incorporates text in the public domain from page 906 of the 20th edition of Gray's Anatomy Illustration at dizziness-and-balance.com
Posterior meningeal artery
The posterior meningeal artery is the largest vessel supplying the dura region of the posterior fossa. It arises from the ascending pharyngeal artery although other origins have been seen, such as the occipital artery; the artery or its branches enter the cranium through jugular foramen, foramen magnum or hypoglossal canal. MeningesThis article incorporates text in the public domain from page 558 of the 20th edition of Gray's Anatomy Diagnostic Cerebral Angiography, 2nd edition, Anne G. Osborn
The medulla oblongata is a long stem-like structure located in the brainstem. It is anterior and inferior to the cerebellum, it is a cone-shaped neuronal mass responsible for autonomic functions ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory and vasomotor centers and therefore deals with the autonomic functions of breathing, heart rate and blood pressure. During embryonic development the medulla oblongata develops from the myelencephalon; the myelencephalon is a secondary vesicle which forms during the maturation of the rhombencephalon referred to as the hindbrain. The bulb is an archaic term for the medulla oblongata and in modern clinical usage the word bulbar is retained for terms that relate to the medulla oblongata in reference to medical conditions; the word bulbar can refer to the nerves and tracts connected to the medulla, by association to those muscles innervated, such as those of the tongue and larynx. The medulla can be thought of as being in two parts: an upper open part or superior part where the dorsal surface of the medulla is formed by the fourth ventricle.
A lower closed part or inferior part where the fourth ventricle has narrowed at the obex in the caudal medulla, surrounds part of the central canal. The anterior median fissure contains a fold of pia mater, extends along the length of the medulla oblongata, it ends at the lower border of the pons in a small triangular area, termed the foramen cecum. On either side of this fissure are raised areas termed the medullary pyramids; the pyramids house the pyramidal tracts–the corticospinal and the corticobulbar tracts of the nervous system. At the caudal part of the medulla these tracts cross over in the decussation of the pyramids obscuring the fissure at this point; some other fibers that originate from the anterior median fissure above the decussation of the pyramids and run laterally across the surface of the pons are known as the anterior external arcuate fibers. The region between the anterolateral and posterolateral sulcus in the upper part of the medulla is marked by a pair of swellings known as olivary bodies.
They are caused by the largest nuclei of the inferior olivary nuclei. The posterior part of the medulla between the posterior median sulcus and the posterolateral sulcus contains tracts that enter it from the posterior funiculus of the spinal cord; these are the gracile fasciculus, lying medially next to the midline, the cuneate fasciculus, lying laterally. These fasciculi end in rounded elevations known as the cuneate tubercles, they are caused by masses of gray matter known as the cuneate nucleus. The soma in these nuclei are the second-order neurons of the posterior column-medial lemniscus pathway, their axons, called the internal arcuate fibers or fasciculi, decussate from one side of the medulla to the other to form the medial lemniscus. Just above the tubercles, the posterior aspect of the medulla is occupied by a triangular fossa, which forms the lower part of the floor of the fourth ventricle; the fossa is bounded on either side by the inferior cerebellar peduncle, which connects the medulla to the cerebellum.
The lower part of the medulla lateral to the cuneate fasciculus, is marked by another longitudinal elevation known as the tuberculum cinereum. It is caused by an underlying collection of gray matter known as the spinal trigeminal nucleus; the gray matter of this nucleus is covered by a layer of nerve fibers that form the spinal tract of the trigeminal nerve. The base of the medulla is defined by the commissural fibers, crossing over from the ipsilateral side in the spinal cord to the contralateral side in the brain stem. Blood to the medulla is supplied by a number of arteries. Anterior spinal artery: This supplies the whole medial part of the medulla oblongata. Posterior inferior cerebellar artery: This is a major branch of the vertebral artery, supplies the posterolateral part of the medulla, where the main sensory tracts run and synapse, it supplies part of the cerebellum. Direct branches of the vertebral artery: The vertebral artery supplies an area between the other two main arteries, including the solitary nucleus and other sensory nuclei and fibers.
The medulla oblongata forms in fetal development from the myelencephalon. The final differentiation of the medulla is seen at week 20 gestation. Neuroblasts from the alar plate of the neural tube at this level will produce the sensory nuclei of the medulla; the basal plate neuroblasts will give rise to the motor nuclei. Alar plate neuroblasts give rise to: The solitary nucleus, which contains the general visceral afferent fibers for taste, as well as the special visceral afferent column; the spinal trigeminal nerve nuclei. The cochlear and vestibular nuclei, which contain the special somatic afferent column; the inferior olivary nucleus, which relays to the cerebellum. The dorsal column nuclei, which contain the gracile and cuneate nuclei. Basal plate neuroblasts give rise to: The hypoglossal nucleus, which contains general somatic efferent fibers; the nucleus ambiguus. The dorsal nucleus of vagus nerve and the inferior salivatory nucleus, both of which form the general visceral efferent fibers; the medulla oblongata connects the higher levels of the brain to the spinal cord, is responsible for several functions of the autonomous nervous system which include: The control of ventilation via signals from the carotid and aortic bodies.
Respiration is regulated by groups of chemoreceptors. These sensors detect changes in the acidity of the blood.
Inferior tympanic artery
The inferior tympanic artery is a small branch of the ascending pharyngeal artery. It is a small branch which passes through a minute foramen in the petrous portion of the temporal bone, called tympanic canaliculus or inferior tympanic canaliculus, in company with the tympanic branch of the glossopharyngeal nerve, to supply the medial wall of the tympanic cavity and anastomose with the other tympanic arteries. In the case of a missing or underdeveloped cervical ICA, the Inferior tympanic artery can provide collateral ICA circulation by reversing flow of the caroticotympanic artery; this can result in pulsatile tinnitus. The resulting Aberrant Carotid artery can mimic neoplasm on CT; this article incorporates text in the public domain from page 557 of the 20th edition of Gray's Anatomy
The brainstem is the posterior part of the brain, continuous with the spinal cord. In the human brain the brainstem includes the midbrain, the pons and medulla oblongata of the hindbrain. Sometimes the diencephalon, the caudal part of the forebrain, is included; the brainstem provides the main motor and sensory nerve supply to the face and neck via the cranial nerves. Of the thirteen pairs of cranial nerves, ten pairs come from the brainstem; the brainstem is an important part of the brain as the nerve connections of the motor and sensory systems from the main part of the brain to the rest of the body pass through the brainstem. This includes the corticospinal tract, the dorsal column-medial lemniscus pathway, the spinothalamic tract; the brainstem plays an important role in the regulation of cardiac and respiratory function. It regulates the central nervous system, is pivotal in maintaining consciousness and regulating the sleep cycle; the brainstem has many basic functions including heart rate, breathing and eating.
The midbrain is divided into three parts: tectum and the ventral tegmentum. The tectum, which forms the ceiling; the tectum comprises the paired structure of the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct. The inferior colliculus, is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei, as well as inputs from the auditory cortex, its inferior brachium reaches to the medial geniculate nucleus of the diencephalon. Superior to the inferior colliculus, the superior colliculus marks the rostral midbrain, it is involved in the special sense of vision and sends its superior brachium to the lateral geniculate body of the diencephalon. The tegmentum which forms the floor of the midbrain, is ventral to the cerebral aqueduct. Several nuclei and the reticular formation are contained here; the ventral tegmental area is composed of paired cerebral peduncles. These transmit axons of upper motor neurons; the midbrain consists of: Periaqueductal gray: The area of gray matter around the cerebral aqueduct, which contains various neurons involved in the pain desensitization pathway.
Neurons synapse here and, when stimulated, cause activation of neurons in the nucleus raphe magnus, which project down into the posterior grey column of the spinal cord and prevent pain sensation transmission. Oculomotor nerve nucleus: This is the third cranial nerve nucleus. Trochlear nerve nucleus: This is the fourth cranial nerve. Red nucleus: This is a motor nucleus that sends a descending tract to the lower motor neurons. Substantia nigra pars compacta: This is a concentration of neurons in the ventral portion of the midbrain that uses dopamine as its neurotransmitter and is involved in both motor function and emotion, its dysfunction is implicated in Parkinson's disease. Reticular formation: This is a large area in the midbrain, involved in various important functions of the midbrain. In particular, it contains lower motor neurons, is involved in the pain desensitization pathway, is involved in the arousal and consciousness systems, contains the locus coeruleus, involved in intensive alertness modulation and in autonomic reflexes.
Central tegmental tract: Directly anterior to the floor of the fourth ventricle, this is a pathway by which many tracts project up to the cortex and down to the spinal cord. Ventral tegmental area: A dopaminergic nucleus located close to the midline on the floor of the midbrain. Rostromedial tegmental nucleus: A GABAergic nucleus located adjacent to the ventral tegmental area; the pons lies between the midbrain. It contains tracts that carry signals from the cerebrum to the medulla and to the cerebellum and tracts that carry sensory signals to the thalamus; the pons is connected to the cerebellum by the cerebellar peduncles. The pons houses the respiratory pneumotaxic center and apneustic center that make up the pontine respiratory group in the respiratory center; the pons co-ordinates activities of the cerebellar hemispheres. The pons and medulla oblongata are parts of the hindbrain; the medulla oblongata just referred to as the medulla, is the lower half of the brainstem continuous with the spinal cord.
Its upper part is continuous with the pons. The medulla contains the cardiac and ventral respiratory groups, vasomotor centres, dealing with heart rate and blood pressure. Another important medullary structure is the area postrema whose functions include the control of vomiting. From the front In the medial part of the medulla is the anterior median fissure. Moving laterally on each side are the medullary pyramids; the pyramids contain the fibers of the corticospinal tract, or the upper motor neuronal axons as they head inferiorly to synapse on lower motor neuronal cell bodies within the anterior grey column of the spinal cord. The anterolateral sulcus is lateral to the pyramids. Emerging from the anterolateral sulci are the CN XII rootlets. Lateral to these rootlets and the anterolateral sulci are the olives; the olives are swellings in the medulla containing underlying inferior nucleary nuclei. Lateral to the olives are the rootlets for CN IX, CN X and CN XI; the pyramids end at the pontine medulla junction, noted most by the large basal pons.
From this junction, CN VI, CN VII