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13.04.2014 admin
The midbrain is the front part of the brainstem, and controls visual, auditory, and motor system functions. The midbrain, also called the mesencephalon, is a small region of the brain that serves as a relay center for visual, auditory, and motor system information.
The tectum is the dorsal, or roof, part of the midbrain and controls visual and auditory reflexes.
These control centers relay the information they receive to the thalamus, which in turn sends it to the cerebral cortex.
Within this region is the red nucleus, which is involved in motor coordination, and the substantia nigra, the brain’s largest dopamine-producing center. A stroke in the midbrain, also called a posterior cerebral artery stroke, is less common than those which affect the anterior or middle cerebral arteries. We're really lucky to be living in a time when there are so many treatments available for midbrain problems.
FIGURE 12-1 The loci of the major cell groups and fiber tracts at the level of the midbrain. FIGURE 12-2 Midsagittal section of the brainstem depicting the midbrain and the levels at which the cross-sectional diagrams were taken, as indicated in Figures 12-3 and 12-4. FIGURE 12-3 Cross-sectional diagram in which the principal structures of the midbrain at the level of the inferior colliculus are depicted. Because of the arrangement of the principal cell groups and fiber bundles of the midbrain, it is convenient to examine the organization of these structures at two levels. In the caudal half of the midbrain, the tectal region contains the inferior colliculus (Figs.
FIGURE 12-4 Photograph of a cross section taken at the level of the inferior colliculus (Weigert stain). FIGURE 12-5 Cross-sectional diagram in which the principal structures of the midbrain at the level of the superior colliculus are depicted. FIGURE 12-6 Photograph of a cross section taken at the level of the superior colliculus (Weigert stain).
These groups of neurons supply the inferior colliculus via a pathway called the lateral lemniscus, which ascends in the lateral aspect of the pons and caudal midbrain (Fig.
Just beneath the PAG lies the nucleus of the trochlear nerve (a general somatic efferent [GSE] nucleus). The lateral aspect of the tegmentum contains a number of important ascending sensory pathways. Embedded within the tegmentum at this level of the mid-brain is the decussation of the superior cerebellar peduncle (Fig. Situated in a transitional position between the crus cer-ebri and the tegmentum is the substantia nigra. It is front part of the brainstem, and any disruption to this area can cause irreversible damage and impairment.
It's divided into the corpora quadrigemina, which consists of two superior and two inferior colliculi. The superior ones are located below the thalamus and receive visual sensory input from the oculomotor nucleus, a bundle of nerves that connect to the eyes.


There, the neural connections are made that enable the brain to decide how to act in response to the sensory information it receives. It regulates autonomic functions, those that the body carries out without conscious thought, such as digestion, heart rate, and breathing rate. This progressive illness develops when dopamine-producing nerve cells in the pars compacta die off in large numbers.
Midbrain strokes typically affect an individual's motor and sensory functions including speech, vision, body movement, and sensation.
The dopamine hypothesis of psychosis developed as researchers noted that dopamine production is often abnormally high in people with certain mental illnesses, like schizophrenia. The trochlear nerve (cranial nerve [CN] IV) emerges on the dorsal aspect of the caudal midbrain just caudal to the inferior colliculus (Figs. The caudal half includes the level of the inferior collicu-lus, and the rostral half includes the level of the superior colliculus.
The PAG, a transitional region between the tectum and tegmentum, is composed mainly of tightly packed cells that surround the cerebral aqueduct (Figs. Its axons pass dorsally and caudally until they exit the brain on the contralateral side at the caudal aspect of the inferior col-liculus.These axons then innervate the superior oblique muscle, which moves the eye downward when it is displaced medially. These pathways include the lateral lemniscus, which provides auditory inputs to the inferior colliculus from other auditory relay nuclei of the upper medulla, and the spinotha-lamic, trigeminothalamic, and medial lemniscal pathways (Figs. The central tegmental area contains the reticular formation, which extends ros-trally from the medulla as described previously (Fig.
It contains two groups of cells; one group is located medially and is highly compacted (called the pars compacta), and one is located laterally, reticulated in appearance, and called the pars reticulata (Fig. The inferior colliculi are involved with processing auditory stimuli that come from the ears through the trochlear nerve bundle. The tectum and the four colliculi within it are, therefore, the first step of the neural pathway that determines how people react to what they see and hear. These nerve cells are important in regulating motor function and emotion, and cell death leads to symptoms such as tremors, physical instability, and emotional changes. There are several pieces of evidence to support this hypothesis, including the fact that some the most effective medications for the treatment of psychosis are those that reduce dopamine activity. When viewed in cross section at either rostral or caudal levels of the midbrain, it is customary to divide this region of the brainstem into three anatomically distinct components (Fig.
The inferior colliculus consists of a large nuclear structure that represents an important relay component of the auditory pathway. The inferior colliculus, in turn, projects its axons through a fiber bundle called the brachium of the inferior colliculus (to the medial geniculate nucleus of the thalamus) (see Fig.
Damage to this cranial nerve is manifest, in particular, in the form of diplopia when the patient tries to look downward such as when attempting to walk down a flight of stairs. The superior cerebellar peduncle originates from the dentate and interposed nuclei of the cerebellum and passes into the brainstem at the level of the upper pons.
This disease is typically treated with medications that provide the brain with additional dopamine, but the drugs involved have side effects and are not always effective. Another strong piece of evidence is that the substantia nigra, where most dopamine is produced, has been seen to undergo structural and cellular changes in a person with schizophrenia.


No one entertains the idea that the ringing is due to this growth, and it started not long after the tumor. The oculomotor nerve (CN III) emerges on the ventromedial aspect of the midbrain at a position just medial to the crus cerebri (Fig.
12-6, which does not include the medial geniculate nucleus because it lies at a more rostral position). Although not typically visible through normal stained material, axons arising from both the PAG as well as from regions of the forebrain descend through different levels of the PAG to terminate either within the PAG or at lower regions of the brainstem.
Beneath the nucleus of the trochlear nerve lies the medial longitudinal fasciculus; this serves as an afferent source to the tro-chlear nucleus (Fig. At this level of the brainstem, the reticular formation contains a variety of nuclei and fiber tracts. At the level of the inferior colliculus, these fibers decussate as they continue to ascend to the red nucleus and ventrolateral nucleus of the thalamus, where they terminate (Fig. Each of these regions is important because of the neurotransmitters that they synthesize as well as their projection targets. In some cases, a device can be placed in the brain to stimulate the areas related to movement and help control symptoms, but this is usually only recommended for patients who don't respond to medications. I also am out of work due to cognitive issues, and frankly, I have no idea how to find a really good brain surgeon or diagnostician who would look at tying all these things together. The PAG contains high concentrations of the neurotransmitter peptide, enkephalin.The PAG plays important roles in the regulation of autonomic functions and affective and emotional processes and in the modulation of pain impulses. Along its medial edge can be found raphe nuclei, which include serotonin-containing neurons that project to the forebrain and to lower regions of the brainstem. Fibers in the lateral fifth arise from the parietal, occipital, and temporal neo-cortices and terminate upon deep pontine nuclei. Dopamine, for example, is associated with the pars compacta and is released onto neurons of the neostriatum.
Nuclei of this region of the tegmentum also contribute to the regulation of somatomo-tor, autonomic, and other visceral processes. Fibers contained in the medial fifth also terminate upon deep pontine nuclei, but they arise from the frontal lobe. In contrast, neurons of the pars reticulata project to the thalamus, using gamma-aminobutyric acid (GABA) as a neurotransmitter.
Fibers contained within the medial three fifths constitute the corticobulbar and corticospinal tracts (Figs. Clinically, it is known that loss of dopaminergic neurons in the substantia nigra results in a motor disorder called Parkinson’s disease. It includes massive bundles of fibers that pass from the cerebral cortex to the brainstem and spinal cord.
The organization of these fibers is such that fibers associated with the head region are located medially (cor-ticobulbar fibers); whereas fibers associated with the upper limb, trunk, and lower limbs are located laterally.



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