Catad_tema Functional and laboratory methods diagnostics - articles
Catad_tema Parkinsonism - articles
Transcranial ultrasound scan of the brain in Parkinson's disease
A.O. Chechetkin
Chechetkin A.O.
Research Institute of Neurology, Russian Academy of Medical Sciences, Moscow
Research Institute of Neurology, Russian Academy of Medical Sciences, Moscow
The review evaluates the capabilities of transcranial ultrasound scanning (TCUS) in identifying structural changes in the brain in Parkinson's disease (PD). It has been shown that most patients with PD exhibit increased echogenicity of brain tissue in the area of the substantia nigra (SN), as well as dilation of the third ventricle. However, the presence of a hyperechoic ultrasound signal from the SN area and the size of its area are not specific signs of the disease, since similar changes were also found in individuals without clinical signs of PD. The role of TUS in the diagnosis of PD remains unclear. Carrying out further studies with verification of the obtained ultrasound data using positron emission tomography may help solve this problem.
The potentialities of transcranial ultrasound scanning (TCUS) in identification of structural changes of the brain in patients presenting with Parkinson's disease (PD) are reviewed. It is demonstrated that the majority of PD patients show an increase of brain tissue echogenicity in the area of substantia nigra (SN) and an enlargement of the third ventricle. However, the presence of the hyperechogenic ultrasound signal from the SN region and the size of its area are not specific signs of the disease because the similar changes were also revealed in persons without the clinical evidence of PD. The role of TCUS in the diagnosis of PD remains unclear. Further studies including verification of the ultrasound data by positron emission tomography are likely to be of help in solving this problem. (“Imaging in the Clinic.” 2000, 17.45-48)
Keywords: Parkinson's disease, transcranial ultrasound scanning, substantia nigra.Key words: Parkinson's disease, transcranial ultrasound scanning, substantia nigra.
Parkinson's disease (PD) is a chronic progressive disease manifested by akinetic-rigid syndrome and tremor, which most often manifests itself at the age of 55-60 years. Morphological studies have shown that in PD, degenerative changes occur in the nigrostriatal dopaminergic pathway, mainly in the area of the compact zone of the substantia nigra and the gray spot, consisting in a decrease in the number of pigment neurons and the proliferation of glial elements.
Diagnosing Parkinson's disease (PD) can present certain difficulties, especially at the onset of the disease. To recognize it from imaging research methods, magnetic resonance imaging (MRI) and positron emission tomography (PET) are used. MRI of the brain can detect only nonspecific and subtle changes. More accurate information about the nigrostriatal system allows us to obtain PET, which contributes to the early diagnosis of PD even in the preclinical stage. However, it is clear that such an expensive type of study cannot be used as a routine diagnostic method.
IN last years The role of transcranial ultrasound duplex scanning has increased in the diagnosis of cerebral diseases, which allows visualization vascular system and brain matter. Data on the use of transcranial ultrasound scanning (TCUS) of the brain in patients with PD are scarce and ambiguous.
In 1995, Becker G. et al. performed TUS of the brain for the first time in patients with PD. Since this method allows visualization of brain tissue depending on its echogenicity, the authors set themselves the task of finding out whether it can be used to detect structural changes in the brain in PD. To do this, they assessed the state of the area of the supposed anatomical location of the substantia nigra (SN) at the level of the midbrain peduncles, conducting a study through the temporal ultrasound window. The authors believed that normally the ultrasound signal from the SN is identical to the echogenicity of the adjacent brain tissue.
In a study of 30 patients with PD and 30 individuals without clinical manifestations of this disease, they obtained the following data: in 17 patients and 2 individuals in the control group, a homogeneous increase in the echogenicity of brain tissue in the region of the SN was detected. It should be noted that in 5 patients and two people in the control group, these changes were difficult to determine. Analysis of the data obtained showed that in patients with a clear hyperechoic zone in the area of the SN (12 patients, i.e. 40% of all patients), clinical symptoms were more pronounced, and the dose of antiparkinsonian drugs was higher than in patients with isoechoic SN ( 18 patients). The authors suggested that the increase in echogenicity in the region of the SN is likely due to a relative increase glial cells in combination with microstructural changes in cellular architecture, as reported by Bogerts B. et al. .
There is a hypothesis that with PD in the emergency, there is an accumulation of various microelements, in particular iron. Berg D. et al. suggested that increased iron concentration in the SN may underlie the effect of increased echogenicity of the ultrasound signal from this region. To confirm this hypothesis, experimental work was carried out in which salts of various metals - iron, zinc and ferritin - were injected directly into the SN by the stereotactic method. When ultrasound scanning the SN area, an increase in echogenicity was found only in those rats that were injected with iron salts.
In the works of V. Lelyuk et al. , published later, in a study of 39 and 111 patients with PD, an increase in echogenicity in the area of the SN was detected in absolutely all. The area of hyperechoic zones in the SN area in one study varied from 0.019 to 0.54 cm2 (average 0.26+/-0.13 cm2) on the right and from 0.066 to 0.585 cm2 (average 0.27+/-0.14 cm2) on the left, and in the other - from 0.011 to 0.62 cm2 (average 0.31+/-0.17 cm2) on the right and from 0.06 to 0.71 cm2 (average 0.32+/-0 .15 cm2) on the left. Thus, the mean values of hyperechoic zone area were approximately equal in the two studies and there were no significant interobserver differences. It should be noted that the authors were unable to detect changes in echogenicity in the area of the SN in any of the practically healthy individuals (51 people in two studies).
Comparison of the duration of the disease with the presence of an identified zone of increased echogenicity in the area of the emergency area gave conflicting results. Becker G. et al. found a hyperechoic zone in the area of the SN in approximately half of the patients with PD and only with a long course of the disease (on average 14.6+/-4.5 years), when the clinical diagnosis was no longer in doubt; at the same time, in patients with isoechoic ES, the duration of the disease averaged 6.5+/-4.2 years. V. Lelyuk et al. observed such changes in absolutely all patients, regardless of the duration of the disease from the moment the first symptoms appeared.
The results of measurements of the third ventricle indicate its expansion in patients with PD compared with the corresponding data in control groups. Thus, in the work of Becker G. et al. and V. Lelyuk et al. the dimensions of the third ventricle in patients with PD averaged 8.6+/-2.3 mm and 6.3+/-1.2 mm versus 7.4+/-2.2 mm and 2.6+/-1, 2 mm in control groups, respectively. The authors explain this by atrophic changes in the brain in patients with PD, described by Schneider E. et al. .
As stated above, Becker G. et al. found a hyperechoic signal in 2 individuals in the control group. These findings led Berg D. et al. to the idea of conducting a screening study of echogenicity in the area of the presumed anatomical location of the SN in individuals without clinical manifestations of PD. They studied 301 people (146 men and 155 women) under the age of 79 years (average age about 30 years). The study population included healthy volunteers (students and hospital staff), as well as patients suffering from herniated discs and non-inflammatory myopathy. Since the echogenicity (brightness) signal in B-mode is not a quantitative parameter, the outline of visible hyperechoic changes in the SN area was outlined, and then the resulting area was determined. The studies were carried out by two independent specialists. The data they obtained on the zone of increased echogenicity in the region of the SN was summarized, the average values were determined and used for further analysis. On average, the area of the hyperechoic signal on one or both sides in the studied individuals was 0.11 cm2, and a clear pattern of increasing values was observed with age. A group of 26 people (16 men and 10 women) was identified with a wider zone of hyperechoic signal, the area of which on one or both sides exceeded 0.25 cm2. The size of this group was 8.6% of the number of people included in the study. The area of the hyperechoic signal on the right and left was on average 0.32 cm2. For a more detailed study, 10 people were selected, comparable by gender and age, from those examined with an area of hyperechoic signal less than 0.2 cm2 (first group) and from patients with an area exceeding 0.25 cm2 (second group). They were tested on motor function (using a pinboard and a series of tests using a typewriter), cognitive function (a standardized psychometric test) and MRI. PET was performed only on individuals in the second group, and to compare the data obtained, 10 individuals were selected from among patients without PD previously examined in the clinic and administered -dopa. The following results were obtained: 1) tests assessing motor functions did not show statistically significant differences between the two groups; 2) when assessing cognitive functions, significant differences were revealed only in fluency of speech, which was worse in the second group; 3) the relative intensities of signals from the SN during MRI were increased in individuals of the second group; 4) the ratios of activity of intravenously administered -dopa in the basal ganglia in the second group found by PET were significantly lower than in the group of individuals taken for comparison. Despite the obtained correlation between ultrasound, MRI and PET data, the nature of the hyperechoic signal from the SN area remained unclear to the authors. They suggested that in individuals of the second group, the nigrostriatal system is more vulnerable to various pathogenetic factors (exo- and endotoxins), under the influence of which neuronal degeneration of the SN can occur, as reported by other authors. In our opinion, this statement is hypothetical, since only pathomorphological, histochemical and electron microscopic studies can confirm the changes occurring in the area of emergency situations.
Interesting results were obtained when comparing the area of the hyperechoic zone in the region of the SN in patients with PD and in individuals without clinical symptoms of this disease. Thus, in the work of V. Lelyuk et al. in patients with PD, it was almost equal to the area of the hyperechoic zone of the same area in individuals without clinical signs of PD, found by Berg D. et al. , and was approximately 0.32 cm2. These findings suggest that increased echogenicity and the size of its area are not specific signs of PD.
In the work of Berg D. et al. it was shown that a hyperechoic signal with an area of more than 0.25 cm2 in the SN area occurred in 8.6% of the total number of subjects studied. However, these results significantly exceed the data on the incidence of PD in the population, which, according to Golbe L., is 0.1%, and according to other authors, ranges from 60 to 140 cases per 100,000 population, which is 0.06 and 0.14%.
Based on the above, it can be noted that in patients with PD, the TUS method in most cases reveals an increase in the echogenicity of brain tissue in the area of the presumed anatomical location of the SN and an expansion of the third ventricle compared to the control group. However, data on the frequency of detected changes in the SN region in patients with PD, as well as on the relationship between the duration of the disease and their presence, are contradictory. In addition, in studies where PD patients underwent TCUS, PET was not performed to verify the data obtained, which, unlike MRI, is currently the most informative technique in diagnosing this disease.
The presence of a zone of increased echogenicity in the area of the SN during TUS is not a specific sign for PD, since it is not found in all patients with PD and the same changes are found in individuals without clinical manifestations of this disease. The size of the hyperechoic ultrasound signal area also cannot serve as a diagnostic criterion, since in patients with PD, as shown above, it was almost identical to the size of the studied area in people without PD.
It should be noted that visualization of brain structures during B-mode ultrasound largely depends on the ultrasound window, and the assessment of visible changes is very subjective (especially in relation to the measured area), since the echogenicity (brightness) signal is not a quantitative parameter.
Thus, the role of TUS in the diagnosis of PD remains unclear. Carrying out further studies with verification of the obtained ultrasound data using PET may help solve this problem.
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Ecology of life. Educational: Today we offer you a story about, although black, but essential substance(or substance) of our brain.
Today we offer you a story about the dark, but irreplaceable substance (or substance) of our brain.
Black substance(or Substantia nigra) does not take up as much space as the white matter. It is located in the midbrain, one of the oldest structures in the center of the brain. Namely, it is hidden under four of its hills. To be completely precise, each of us has two Substantia nigra - left and right.
Midbrain. Animation from Life Science Databases(LSDB).
Cross section of the midbrain at the quadrigeminal level. The substantia nigra is shown in guess what color.
Despite the fact that the Substantia nigra, like the gray matter, contains the bodies of neurons, it is much darker due to its “coloring” with neuromelanin (by the way, another form of this pigment - melanin - gives color to our eyes, skin and hair).
Neuromelanin monomer
In total, there are two layers in the substantia nigra: compact layer (pars compacta) and ventral (pars reticulata). Here we need to clarify the word “ventral”.
Doctors use two spatial antonyms: ventral and dorsal. "Ventral" means "abdominal". This does not mean at all that the ventral layer of the substantia nigra is located in the stomach. It is simply located more “in front” in the body. “Ventral” is anterior, “dorsal” is posterior (dorsal).
If we talk about the functionality of the layers, then the compact one is in some sense similar to a computer processor - it processes information and transmits it to the thalamus and quadrigeminal region of the midbrain, and the ventral one ensures the production of the neurotransmitter dopamine. The layers are arranged vertically, the pars compacta is located closer to the axis of the body than the pars reticulata.
Dopamine
Thanks to the substantia nigra, we can move our eyes, perform small and precise movements, in particular with our fingers, chew and swallow. And our body can carry out breathing, cardiac activity, and keep blood vessels in good shape.
Disturbances in the functioning of the substantia nigra lead to various diseases. There is a hypothesis that the secret of schizophrenia lies in it. And Parkinson’s disease, which we often write about on the portal, is caused precisely by a disruption in the production of dopamine in the substantia nigra: it causes the death of neurons there.
Histology of the corpus nigra in a patient with Parkinson's disease
Researchers have even found the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), which, just like Parkinson's disease, destroys dopamine neurons, and are now actively using it in mice to model the disease and searching for ways to treat it. published
Since 1922, after distribution epidemic encephalitis, similar changes in the neurons of the substantia nigra began to be detected in patients with postencephalitic and other forms of parkinsonism.
Cells, containing melanin, are usually damaged to a greater extent than cells free of pigment. Therefore, even upon macroscopic examination, the substantia nigra often appears discolored. The total number of dead neurons containing melanin can reach 90%.
The described changes in neurons substantia nigra are considered specific to all forms of parkinsonism. Repeated indications, especially in the works of older authors, of diffuse changes in the parenchyma and blood vessels of the brain, apparently, are nonspecific for parkinsonism and reflect senile and (or) atherosclerotic disorders in nerve tissue, which can also be observed in elderly people who do not suffer from parkinsonism.
Beginning with 20s this century began to pay attention to the natural involvement in the pathological process in parkinsonism of another pigmented structure of the brain stem - the so-called locus coeruleus, located in the covering of the oral parts of the pons. Still, it is believed that in parkinsonism, the decrease in the number of cells in the blue nucleus is less pronounced than in the compact zone of the substantia nigra.
Running a few forward, we emphasize the fact that in recent years new facts have been obtained that shed light on the physiological significance and neurochemical nature of the blue nucleus. According to modern concepts, the blue nucleus consists mainly of neurons with a high content of norepinephrine, its precursors, enzymes and metabolites.
Blue core has extensive connections with many brain structures, including the spinal cord, medulla oblongata and pons, cerebellum, mesencephalic structures of the brain stem, thalamus, hypothalamus, forebrain cortex, which ensures its modulating influence at different levels of the cerebral axis and participation in a variety of regulatory processes, requiring noradrenergic mediation.
Blue core takes part in the regulation of such complex psychophysiological reactions and functional states as arousal, maintaining the level of wakefulness, attention, in the mechanisms of desynchronized sleep phase (sleep with dreams) and stress. Apparently, the blue nucleus and the ascending reticular system of the brain stem are involved, along with the hypothalamic-pituitary-adrenal endocrine link, in the regulation different levels brain activation.
Substantia nigra and core blue in parkinsonism, they are affected most severely and with the greatest consistency, although most studies devoted to the pathomorphology of the brain in this disease describe pathological changes in other brain structures, among which the hypothalamus, reticular formation, and dorsal nucleus of the vagus nerve (nucleus dorsalis nervi vagi) are usually mentioned. , the innominate substance (substantia innominata), the nuclei of the borderline sympathetic trunk, the red nucleus, the globus pallidus, the putamen, as well as the subthalamic nucleus of Lewis, the inferior olive, the cerebral cortex and some other structures.
In recent years, the listed brain structures are subjected to intensive study using histochemical, fluorescent, electron microscopic, microelectrode and other techniques.
In these brain structures in addition to neuron death and depigmentation, other intracellular changes have been described. These include, first of all, peculiar concentric inclusions in the cytoplasm of these cells, called Lewy bodies by K. P. Tretyakov in honor of the author who first described them back in 1913. K. P. Tretyakov discovered them in the cells of the substantia nigra in 6 cases out of 9 observations .
Lewy bodies were later found in neurons of the substantia nigra, blue nucleus, dorsal nucleus of the vagus nerve, substantia innominata, neurons of the reticular formation, ganglia of the sympathetic chain and in other structures.
On its ventral surface there are two massive bundles of nerve fibers - the cerebral peduncles, through which signals are carried from the cortex to the underlying brain structures.
Rice. 1. The most important structural formations of the midbrain (cross section)
The midbrain contains various structural formations: quadrigeminal, red nucleus, substantia nigra and nuclei of the oculomotor and trochlear nerves. Each formation plays a specific role and contributes to the regulation of a number of adaptive reactions. Everything passes through the midbrain ascending paths, transmitting impulses to the thalamus, cerebral hemispheres and cerebellum, and descending pathways, conducting impulses to the medulla oblongata and spinal cord. The neurons of the midbrain receive impulses through the spinal cord and medulla oblongata from the muscles, visual and auditory receptors along the afferent nerves.
Anterior tubercles of the quadrigeminal are the primary visual centers, and they receive information from visual receptors. With the participation of the anterior tubercles, visual orientation and guard reflexes are carried out by moving the eyes and turning the head in the direction of the action of visual stimuli. The neurons of the posterior tubercles of the quadrigeminal form the primary auditory centers and, upon receiving excitation from auditory receptors, ensure the implementation of auditory orientation and guard reflexes (the animal's ears tense, it becomes alert and turns its head towards a new sound). The nuclei of the posterior colliculus provide a guard adaptive reaction to a new sound stimulus: redistribution of muscle tone, increased flexor tone, increased heart rate and respiration, increased blood pressure, i.e. the animal is preparing to defend, run, attack.
Black substance receives information from muscle receptors and tactile receptors. It is associated with the striatum and globus pallidus. Neurons of the substantia nigra are involved in the formation of an action program that ensures the coordination of complex acts of chewing, swallowing, as well as muscle tone and motor reactions.
Red core receives impulses from muscle receptors, from the cerebral cortex, subcortical nuclei and cerebellum. It has a regulatory effect on motor neurons of the spinal cord through the Deiters nucleus and the rubrospinal tract. The neurons of the red nucleus have numerous connections with the reticular formation of the brain stem and together with it regulate muscle tone. The red nucleus has an inhibitory effect on the extensor muscles and an activating effect on the flexor muscles.
Elimination of the connection between the red nucleus and the reticular formation of the upper part of the medulla oblongata causes a sharp increase in the tone of the extensor muscles. This phenomenon is called decerebrate rigidity.
|
Name |
Functions of the midbrain |
|
Nuclei of the roof of the superior and inferior colliculi |
Subcortical centers of vision and hearing, from which the tectospinal tract originates, through which indicative auditory and visual reflexes are carried out |
|
Nucleus of the longitudinal medial fasciculus |
Participates in ensuring a combined rotation of the head and eyes to the action of unexpected visual stimuli, as well as in case of irritation of the vestibular apparatus |
|
Nuclei of III and IV pairs of cranial nerves |
They participate in a combination of eye movements due to the innervation of the external muscles of the eye, and the fibers of the autonomic nuclei, after switching in the ciliary ganglion, innervate the muscle that constricts the pupil and the muscle of the ciliary body |
|
Red kernels |
They are the central link of the extrapyramidal system, since the paths from the cerebellum (tr. cerebellotegmenlalis) and basal nuclei (tr. pallidorubralis) end on them, and the rubrospinal path begins from these nuclei |
|
Black substance |
It has a connection with the striatum and cortex, is involved in complex coordination of movements, regulation of muscle tone and posture, as well as in coordinating the acts of chewing and swallowing, and is part of the extrapyramidal system |
|
Nuclei of the reticular formation |
Activating and inhibitory influences on the nuclei of the spinal cord and various areas of the cerebral cortex |
|
Gray central periaqueductal substance |
Part of the antinociceptive system |
The structures of the midbrain are directly involved in the integration of heterogeneous signals necessary for the coordination of movements. With the direct participation of the red nucleus, the substantia nigra of the midbrain, the neural network of the brain stem movement generator and, in particular, the eye movement generator is formed.
Based on the analysis of signals entering the stem structures from proprioceptors, vestibular, auditory, visual, tactile, pain and other sensory systems, a flow of efferent motor commands is formed in the stem movement generator, sent to the spinal cord along the descending pathways: rubrospinal, retculospinal, vestibulospinal, tectospinal. In accordance with the commands developed in the brain stem, it becomes possible to carry out not just contractions of individual muscles or muscle groups, but the formation of a certain body posture, maintaining body balance in various poses, making reflexive and adaptive movements when carrying out various types of body movement in space (Fig. 2 ).
Rice. 2. The location of some nuclei in the brain stem and hypothalamus (R. Schmidt, G. Thews, 1985): 1 - paraventricular; 2 - dorsomedial: 3 - preoptic; 4 - supraoptical; 5 - rear
The structures of the brainstem movement generator can be activated by voluntary commands that come from the motor areas of the cerebral cortex. Their activity can be enhanced or inhibited by signals from sensory systems and the cerebellum. These signals can modify already executed motor programs so that their execution changes in accordance with new requirements. For example, adapting posture to purposeful movements (as well as organizing such movements) is possible only with the participation of the motor centers of the cerebral cortex.
The red nucleus plays an important role in the integrative processes of the midbrain and its stem. Its neurons are directly involved in the regulation, distribution of skeletal muscle tone and movements, ensuring the maintenance of normal body position in space and the adoption of a posture that creates readiness to perform certain actions. These influences of the red nucleus on the spinal cord are realized through the rubrospinal tract, the fibers of which end on the interneurons of the spinal cord and have an excitatory effect on the a- and y-motoneurons of the flexors and inhibit the majority of the oto neurons of the extensor muscles.
The role of the red nucleus in the distribution of muscle tone and maintaining body posture is well demonstrated in experimental conditions on animals. When the brainstem is cut (decerebration) at the level of the midbrain below the red nucleus, a condition called decerebrate rigidity. The animal's limbs become straightened and tense, the head and tail are thrown back to the back. This body position occurs as a result of an imbalance between the tone of the antagonist muscles in the direction of a sharp predominance of the tone of the extensor muscles. After transection, the inhibitory effect of the red nucleus and cerebral cortex on the extensor muscles is eliminated, and the excitatory effect of the reticular and vestibular (Dagers) nuclei on them remains unchanged.
Decerebrate rigidity occurs immediately after transection of the brainstem below the level of the red nucleus. The y-loop is of utmost importance in the origin of rigidity. Rigidity disappears after cutting the dorsal roots and stopping the flow of afferent nerve impulses to the spinal cord neurons from the muscle spindles.
The vestibular system is related to the origin of rigidity. Destruction of the lateral vestibular nucleus eliminates or reduces the tone of the extensors.
In the implementation of integrative functions of brain stem structures important role plays the substantia nigra, which is involved in the regulation of muscle tone, posture and movements. It is involved in the integration of signals necessary to coordinate the work of many muscles involved in the acts of chewing and swallowing, and influences the formation of respiratory movements.
Through the substantia nigra, motor processes initiated by the brainstem movement generator are influenced by the basal ganglia. There are bilateral connections between the substantia nigra and the basal ganglia. There is a bundle of fibers that conducts nerve impulses from the striatum to the substantia nigra, and a path that conducts impulses in the opposite direction.
The substantia nigra also sends signals to the nuclei of the thalamus, and then these signal flows reach the cortex along the axons of the thalamic neurons. Thus, the substantia nigra is involved in closing one of the neural circuits through which signals circulate between the cortex and subcortical formations.
The functioning of the red nucleus, substantia nigra and other structures of the brain stem movement generator is controlled by the cerebral cortex. Its influence is carried out both through direct connections with many nuclei of the stem, and indirectly through the cerebellum, which sends bundles of efferent fibers to the red nucleus and other stem nuclei.
second higher education"Psychology" in MBA formatsubject: Anatomy and evolution of the human nervous system.
Manual "Anatomy of the central nervous system"
8.1. Roof of the midbrain
8.2. Brain stems
The midbrain is a short section of the brain stem, forming the cerebral peduncles on its ventral surface and the quadrigeminal on the dorsal surface. On a cross section, the following parts are distinguished: the roof of the midbrain and the cerebral peduncles, which are divided by a black substance into the roof and base (Fig. 8.1).
Rice. 8.1. Midbrain formations
8.1. Roof of the midbrain
The roof of the midbrain is located dorsal to the aqueduct, its plate is represented by the quadrigeminal. The hills are flat and have alternating white and gray matter. The superior colliculus is the center of vision. From it there are pathways to the lateral geniculate bodies. Due to the evolutionary transfer of vision centers to the forebrain, the centers of the superior colliculi perform only reflex functions. The inferior colliculi serve as subcortical hearing centers and are connected by the medial geniculate bodies. From the spinal cord to the quadrigeminal tract there is an ascending pathway, and downwards there are pathways that provide two-way communication between the visual and auditory subcortical centers with the motor centers of the medulla oblongata and spinal cord. The motor pathways are called the tegnospinal tract and the tegnobulbar tract. Thanks to these pathways, unconscious reflex movements are possible in response to sound and auditory stimuli. It is in the buffs of the quadrigeminal that the orienting reflexes are closed, which I. P. Pavlov called the “What is this?” reflexes. These reflexes play an important role in the implementation of the mechanisms of involuntary attention. In addition, two more important reflexes are closed in the upper tubercles. This is a pupillary reflex, which ensures optimal illumination of the retina, and a reflex associated with adjusting the lens for clear vision of objects located at different distances from a person (accommodation).
8.2. Brain stems
The cerebral peduncles look like two rollers, which, diverging upward from the pons, plunge into the thickness of the cerebral hemispheres.
The tegmentum of the midbrain is located between the substantia nigra and the aqueduct of Sylvius and is a continuation of the tegmentum of the pons. It is in it that there is a group of nuclei belonging to the extrapyramidal system. These nuclei serve as intermediate links between the cerebrum on the one hand, and on the other hand, with the cerebellum, medulla oblongata and spinal cord. Their main function is to ensure coordination and automaticity of movements (Fig. 8.2).
Rice. 8.2. Transverse section of the midbrain:
1 - roof of the midbrain; 2 - water supply; 3 - central gray matter; 5 - tegmentum; 6 - red nucleus; 7 - black substance
In the midbrain tegmentum, the largest are those with elongated shape red kernels. They stretch from the subthalamic region to the pons. The red nuclei reach their greatest development in higher mammals, in connection with the development of the cerebral cortex and cerebellum. The red nuclei receive impulses from the nuclei of the cerebellum and the globus pallidus, and the axons of the neurons of the red nuclei are sent to the motor centers of the spinal cord, forming the rubrospial tract.
In the gray matter surrounding the midbrain aqueduct, there are the nuclei of the III and IV cranial nerves, which innervate the oculomotor muscles. In addition, groups of vegetative nuclei are also distinguished: the accessory nucleus and the unpaired median nucleus. These nuclei belong to the parasympathetic division of the autonomic nervous system. The medial longitudinal fascicle unites the nuclei of the III, IV, VI, XI cranial nerves, which ensures combined eye movements when deviated in one direction or another and their combination with head movements caused by irritation of the vestibular apparatus.
Under the tegmentum of the midbrain is the locus coeruleus - the nucleus of the reticular formation and one of the sleep centers. Laterally from the locus coeruleus there is a group of neurons that influence the release of releasing factors (liberins and statins) of the hypothalamus.
At the border of the tegmentum with the basal part lies the substantia nigra; the cells of this substance are rich in the dark pigment melanin (where the name comes from). The substantia nigra has connections with the cortex of the frontal lobe of the cerebral hemispheres, with the nuclei of the subthalamus and reticular formation. Damage to the substantia nigra leads to disruption of fine coordinated movements associated with plastic muscle tone. The substantia nigra is a collection of neuron bodies that secrete the neurotransmitter dopamine. Among other things, dopamine appears to contribute to some pleasurable feelings. It is known to be involved in creating the euphoria for which drug addicts use cocaine or amphetamines. In patients suffering from Parkinsonism, neurons in the substantia nigra degenerate, which leads to a lack of dopamine.
The Sylvian aqueduct connects the III (diencephalon) and IV (pons and medulla oblongata) ventricles. The cerebrospinal fluid flow through it is carried out from the third to the fourth ventricle and is associated with the formation of cerebrospinal fluid in the ventricles of the hemispheres and diencephalon.
The basal part of the cerebral peduncle contains fibers of the descending pathways from the cerebral cortex to the underlying parts of the central nervous system.
