The Brainstem

Glonidine. Clonidine decreases blood pressure, heart rate, cardiac output, stroke volume, and total peripheral resistance. It activates central a2 adrenoceptors ia the brainstem vasomotor center and produces a prolonged hypotensive response. Clonidine, most efficaciously used concomitantly with a diuretic in long-term treatment, decreases renin and aldosterone secretion. 

The raphe nuclei are a cluster of nuclei found in the brainstem, where they are located in the medial portion of the formatio reticularis, the raphe. (The raphe is the junction of the left and right brainstem hemisphere, hence the name raphe=seam). Serotonergic nerve cells in the CNS originate from the raphe nuclei, i.e., their rostral portion, and because of their wide-ranging projections appear to supply serotonin (5HT) to the rest of the brain. 

The sensitivity and specificity of DWI depend to some extent on the technique being used and the amount of imaging time that can be dedicated to the DWI sequence. DWI pulse sequences typically require between approximately 30 seconds and 4 minutes of imaging time to image the entire brain and achieve sensitivity and specificity approaching 100% (Fig. 2.2). The rare infarcts that are not apparent on DWI are usually very small and are often located in the brainstem. 

Figure 8.2 The distribution of noradrenergic neurons in the brain. The cell bodies are clustered in nuclei (A1 A7) in the pons/medulla regions of the brainstem and their axons project both rostrally and caudally to most regions of the neuraxis. The major nucleus is the locus coeruleus...

HT nuclei, as well as connections with other monoaminergic nuclei in the brainstem which are also implicated in these physiological functions. 

These receptors are quite different from any other monoamine reeeptor in that they are not coupled to G proteins. Instead, they eomprise a pentamerie eomplex of subunits that ineorporates an ion ehannel. This is seleetive for the eations Na+ and K+ whieh, when opened, leads to depolarisation of the host eell. 5-HT3 reeeptors are found at high eoneentrations in the brainstem and area postrema. However, they are also found elsewhere in lower eoneentrations, notably in the eortex, amygdala and hippoeampus, where they are thought to be assoeiated mainly with GABAergie neurons. 

This peptide itself has no selectivity for the two CCK receptors, CCK-A and B, which have so far been established to stimulate IP3/DAG while, like substance P, can close potassium channels to increase neuronal activity. The CCK-B receptor is thought to predominate in the CNS but species differences may make this interpretation difficult. It has a wide distribution in the CNS but is also found in the gut whereas the CCK-A receptor is more restricted but is found in the hypothalamus, hippocampus and in the brainstem. There are high levels of the natural peptide, CCK-8 in cortex, hippocampus, hypothalamus, ventral tegmentum, substantia nigra, brainstem and spinal cord. CCK is one of the most abundant peptides in the brain and CCK co-exists with dopamine, substance P, 5-HT and vasopressin. Interestingly, in the dopamine areas, CCK co-exists in the mesolimbic pathways but in the nigrostriatal projections, the peptide and... 

There are other important sites of opiate actions located in the 5-HT and noradrenergic nuclei of the brainstem and midbrain including the raphe nuclei, the periaquaductal... 

The large numbers of opioid receptors in areas of the brainstem such as the solitary tract and adjacent areas are probably related to respiratory effects of opiates, cough suppression and nausea and vomiting. Opiates acting in the brainstem reduce the sensitivity of the respiratory centres to pC02 and this is the most common cause of death from overdose with street use of opiates. 

Opiates activate the chemoreceptor trigger zone in the medulla (by disinhibition) to cause nausea and vomiting, and cough suppression also occurs because of the inhibitory effects of opiates on the brainstem nuclei in the cough reflex pathway. Dextromethorphan is the non-opiate isomer of the opiate levorphanol and is an effective cough suppressant. 

Ascending inputs from the brainstem ascending reticular activating system (ARAS). As described below, these seem to be particularly important and probably disrupt the thalamo-cortical synchrony. 

Boakes, RJ. Bradely, P.B. Candy, J.M. and Wolstencraft, J.H. Actions of noradrenalin, other sympatomimetic amines and antagonists on neurones in the brainstem of the cat. Brit J Pharmacol 41 462-479, 1971. 

Warenycia MW, Goodwin LR, Benishin CG, et al. 1989a. Acute hydrogen sulfide poisoning Demonstration of selective uptake of sulfide by the brainstem by measurement of brain sulfide levels. Biochem Pharmacol 38 973-981. 

Robinson GS, Keith RW, Bomschein RL, et al. 1987. Effects of environmental lead exposure on the developing auditory system as indexed by the brainstem auditory evoked potential and pure tone hearing evaluations in young children. In Lindberg SE, Hutchinson TC. eds. International Conference on Heavy Metals in the Environment, Vol. 1, New Orleans, LA. September. Edinburgh, UK CEP Consultants, Ltd., 223-225. 

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of 12 pairs of cranial nerves that arise from the brainstem and 31 pairs of spinal nerves arising from the spinal cord. These peripheral nerves carry information between the CNS and the tissues of the body. The PNS consists of two divisions ... 

The brain is the integrative portion of the nervous system that serves to receive, process, and store sensory information and then plan and orchestrate the appropriate motor response. It is divided into several anatomically and functionally distinct regions (see Table 6.2). The forebrain consists of the cerebrum, basal ganglia, thalamus, and hypothalamus. The midbrain, along with the pons and the medulla of the hindbrain, composes the functional region referred to as the brainstem. The cerebellum is also considered a component of the hindbrain but is functionally distinct from the brainstem. 

The functional region known as the brainstem consists of the midbrain, and the pons and medulla of the hindbrain. It is continuous with the spinal cord and serves as an important connection between the brain and spinal cord because all sensory and motor pathways pass through it. The brainstem consists of numerous neuronal clusters or centers, each of which controls vital, life-supporting processes. 

In addition, the brainstem contains a diffuse network of neurons known as the reticular formation. This network is best known for its role in cortical alertness, ability to direct attention, and sleep. It is also involved with coordination of orofacial motor activities, in particular those involved with eating and the generation of emotional facial expressions. Other functions include coordination of eating and breathing, blood pressure regulation, and response to pain. 

The cerebellum (Latin, little brain) is part of the hindbrain and is attached to the dorsal surface of the upper region of the brainstem. Although it constitutes only 10% of the total volume of the brain, it contains more than half of all its neurons. Its surface consists of a thin cortex of gray matter with extensive folding, a core of white matter, and three pairs of nuclei embedded within it. 

As discussed, the first-order neuron is the afferent neuron that transmits impulses from a peripheral receptor toward the CNS. Its cell body is located in the dorsal root ganglion. This neuron synapses with the second-order neuron whose cell body is located in the dorsal horn of the spinal cord or in the medulla of the brainstem. The second-order neuron travels upward and synapses with the third-order neuron, whose cell body is located in the thalamus. Limited processing of sensory information takes place in the thalamus. Finally, the third-order neuron travels upward and terminates in the somatosensory cortex where more complex, cortical processing begins. 

All ascending tracts cross to the opposite side of the CNS. For example, sensory input entering the left side of the spinal cord ultimately terminates on the right side of the cerebral cortex. These tracts may cross — at the level of entry into the spinal cord a few segments above the level of entry or within the medulla of the brainstem. The locations of specific ascending tracts are illustrated in Figure 7.2 and a summary of their functions is found in Table 7.1. 

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