Timothy C. Hain, MD Page last modified: March 3, 2014
The aim of this material is to orient the reader, who might not be familiar with neuroanatomy, to the main structures involved in dizziness and balance. This is by no means a comprehensive discussion, and we strongly recommend the interested reader to turn to one of the many excellent books on neuroanatomy.
This section is in progress, and is in no way complete.
|Midbrain -- contains the 3rd and 4th nerve nuclei|
|Pons -- contains 5th nerve, and has 6, 7 and 8th nerves exciting from bottom|
|Medulla. Contains the rest of the cranial nerves, as well as the cochlear and vestibular nuclei.|
|A view of the front of the brainstem is shown.|
The brainstem is divided into three regions. The uppermost portion is called the midbrain. It is small. It contains the 3rd and 4th nerve nuclei, the superior and inferior colliculi, and various fiber bundles. The middle portion is the pons. It contains cranial nerves 5-8, and largely is composed of fibers going to the cerebellum via the middle cerebellar peduncle. The lower portion is the medulla. It contains the lower cranial nerves and fiber pathways.
The cerebellopontine angle is at the junction between the pons, medulla and cerebellum. It contains the 8th nerve and AICA.
The cranial nerves are shown in yellow. The main function of the nerves is listed -- we have neglected to mention numerous 2ndary functions. The 1st (olfactory -- smell) and 2nd (optic, sight) cranial nerves are not shown. Cranial nerves 3,4 and 6 are the oculomotor nerves (eye movement). Cranial nerve 5 is the trigeminal nerve (facial movement). Cranial nerve 7 is the facial nerve (facial movement). Cranial nerve 8 is the cochleovestibular nerve (hearing and balance). Cranial nerve 9 is the vagus, 10 the glossopharyngeal (throat), 11 the accessory nerve (neck movement), and 12 the hypoglossal nerve (tongue movement).
The arteries to the brainstem are shown in on the diagram in red above and in real life, in white on the MRA picture below. More images of the blood supply in the neck are found here. The main blood supply comes from the vertebral arteries. The first branches off of the vertebrals are the posterior inferior cerebellar arteries (PICA). These supply blood to the vestibular nucleus and inferior cerebellum. The next named branches are the anterior inferior cerebellar arteries. These supply blood to the junction between the medulla and the pons, the cerebellum, and the inner ear. At the top of the pons, where it meets the midbrain, are the superior cerebellar arteries (SCA). These mainly supply the midbrain and upper cerebellum. The posterior cerebral arteries (PCA) mainly supply occipital cortex. Many variations are possible without compromise of function.
|Normal MRA scan of a larger region than the brainstem showing blood supply to the brain and brainstem from the heart.|
Vascular supply of the inner ear: the arterial circulation of the inner ear is completely supplied by the labyrinthine artery. The labyrinthine artery has a variable origin. Most often it is a branch of the anterior inferior cerebellar artery (AICA), but occasionally, it is a direct branch of the basilar artery. As it enters the inner ear, it divides into the anterior vestibular artery and the common cochlear artery. The anterior vestibular artery supplies the vestibular nerve, most of the utricle, and the ampullae of the lateral and anterior semicircular canals. The common cochlear artery divides into a main branch, the main cochlear artery, and the vestibulocochlear artery. The main cochlear artery supplies the cochlea. The vestibulocochlear artery supplies part of the cochlea, the ampulla of the posterior semicircular canal, and the inferior part of the saccule. The labyrinth has no collateral anastomotic network and is highly susceptible to ischemia. Only 15 seconds of selective blood flow cessation is needed to abolish temporarily auditory nerve excitability.
The vertebral-basilar arterial system provides the vascular supply for both the peripheral and central auditory vestibular system. The posterior inferior cerebellar arteries supply the surface of the inferior portions of the cerebellar hemispheres, as well as the dorsolateral medulla, which includes the inferior aspects of the vestibular nuclear complex. The basilar artery supplies central vestibular structures via perforator branches, which penetrate the medial pons, short circumferential branches, which supply the anterolateral aspect of the pons, and long circumferential branches which supply the dorsolateral pons. The anterior-inferior cerebellar artery or "AICA" supplies both the peripheral vestibular system, via the labyrinthine artery, as well as the ventrolateral cerebellum and the lateral tegmentum of the lower two-thirds of the pons.
There are two main targets for vestibular input from primary afferents, namely the vestibular nuclear complex and the cerebellum. The vestibular nuclear complex is the primary processor of vestibular input, and implements direct, fast connections between incoming afferent information and motor output neurons. The cerebellum monitors vestibular performance and keeps it calibrated. In both locations, vestibular sensory input is processed in association with somatosensory and visual sensory input.
The vestibular nuclear complex consists of 4 "major" nuclei (superior, medial, lateral, and descending) and at least 7 "minor" nuclei. It is a large structure, located primarily within the pons, but also extending caudally into the medulla. In the vestibular nuclear complex, processing of the vestibular sensory input occurs concurrently with the processing of extra‑vestibular sensory information (proprioceptive, visual, tactile, and auditory). Extensive connections between the vestibular nuclear complex, cerebellum, ocular motor nuclei, and brainstem reticular activating systems are required to formulate appropriate efferent signals to the vestibuloocular reflex (VOR) and vestibulo-spinal reflex (VSR) effector organs, the extraocular and skeletal muscles. The vestibular nucleus sends outflow to the oculomotor nuclei, the spinal cord, the cerebellum, and the vestibular cortex (mesial to primary acoustic cortex).
The cerebellum is a major recipient of outflow from the vestibular nucleus complex, and is also a major source of input to the vestibular nucleus. Most of the signal traffic is routed through the inferior cerebellar peduncle. The cerebellum is not required for vestibular reflexes, but when removed, vestibular reflexes become uncalibrated and ineffective. Most parts of the cerebellar vermis (midline) respond to vestibular stimulation. The cerebellar flocculus adjusts and maintains the gain of the VOR. Lesions of the flocculus reduce the ability of experimental animals to adapt to disorders which reduce or increase the gain of the VOR. Clinically, the Chiari-I malformation is the most commonly encountered example of a floccular lesion. The cerebellar nodulus adjusts the duration of VOR responses, and is also involved with processing of otolith input. Lesions are characterized by ataxia and central positional nystagmus. Clinically, the cerebellar medulloblastoma is the most common example of a nodular lesion.
Second Order Neuron: The cochlear nerve, a part of the cochleovestibular nerve (VIII) enters the brainstem at the ponto-medullary junction, bifurcates, and terminates in the two major subdivisions of the cochlear nucleus, the dorsal and ventral cochlear nuclei. The most important outflow of the cochlear nucleus is to the trapezoid body which contains fibers destined for the superior olivary nuclei on both sides of the brainstem. The superior olive is concerned with sound localization based in interaural differences in sound timing and intensity. The superior olive is also an essential part of the stapedius reflex, a protective reflex in the middle ear. Output from the superior olive joins crossed and uncrossed axons from the cochlear nucleus to form the lateral lemniscus, an ascending fiber pathway which proceeds to the inferior colliculus. Because the lateral lemniscus contains second order neurons from the cochlea, and third and fourth order neurons from the superior olive, it contributes to three waves of the auditory brainstem response (see later section on testing). Because of the extensive crossing which occurs early in the central auditory circuitry, central auditory lesions usually do not cause monaural hearing loss.
Third Order Neuron: The inferior colliculus is a major integrating center for the auditory system and serves as a center for feedback pathways to the lower auditory system. Output from the inferior colliculus ascends to the medial geniculate body of the thalamus. Bilateral lesions are required for persistent clinical symptoms.
Fourth Order Neuron: The medial geniculate body is the major auditory nucleus of the thalamus. Parts of the medial geniculate are hypothesized to function in directing auditory attention. It sends output to primary auditory cortex, also known as the transverse temporal gyri of Heschl (Brodman areas 41 and 42), and association auditory cortex (areas 22 and 52). The medial geniculate also sends output to auditory motor cortex which controls body responses in response to sound. As is the case elsewhere in the central auditory system, bilateral lesions are generally required for persistent clinical symptoms.
The central auditory cortex is mainly supplied by the middle cerebral artery.
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