Human Anatomy - Lecture Notes - Savchuk O. I. 2017

Anatomy of the Brain

Lecture Outline

1. Structure OF THE Medulla Oblongata.

2. Anatomy of the Hindbrain

3. The Fourth ventricle.

4. Structure of the Midbrain.

5. Diencephalon.

6. Structure of the Telencephalon.

7. Lateral ventricles.

The Brain (encephalon) is located in the cranial cavity. It consists of two cerebral hemispheres (the evolutionarily newest part) and the Brainstem (medulla oblongata, Pons, cerebral peduncles) with the Cerebellum. The Superolateral surface of the brain is convex, and the inferior surface (base) is uneven, where 12 pairs of Cranial Nerves emerge from the brain.

The brain mass of an adult is 1245–1375 g, and that of a newborn is 330–340 g. During the Embryonic period and the first 6 years of life, the brain grows quite intensively, but it only reaches its stable size by the age of 20.

The BRAIN AND SPINAL cord develop from the outer germ layer (ectoderm). The neural tube forms with an expansion at its cranial end, consisting of three brain vesicles: the Forebrain, midbrain, and hindbrain. The forebrain and hindbrain divide to form 5 vesicles: 1) myelencephalon, metencephalon, mesencephalon, diencephalon, and telencephalon. The various Regions of the brain develop from each of these vesicles.

The medulla oblongata (myelencephalon) develops from the myelencephalic vesicle. The boundary between the medulla oblongata and the Spinal Cord is defined by the exit of the rootlets of the first cervical Spinal Nerves or the level of the foramen magnum. Superiorly, it borders the pons. On its ventral surface, the medulla oblongata has two elevations: the pyramids (located medially) and the olives (located laterally). On the dorsal surface of the medulla oblongata, there are the gracile and cuneate fasciculi, which extend from the spinal cord and terminate in the corresponding tubercles containing the respective nuclei. On the dorsal surface of the medulla oblongata, which forms part of the floor of the IV ventricle (Rhomboid fossa), are the nuclei of cranial nerves IX–XII. The fibers of these nuclei emerge posterior to the olive, as well as between the olive and the pyramid.

The medulla oblongata performs reflex and conduction Functions. It houses vital centers, namely the respiratory and cardiovascular centers. The slightest damage to this area of the brain leads to severe functional impairment and even death.

The medulla oblongata contains the nervous centers for digestive Reflexes: sucking, salivation, pancreatic and gastric secretion, chewing, and swallowing, as well as metabolic centers.

The centers for vomiting, sneezing, coughing, and lacrimation are also localized in the medulla oblongata.

The 12 pairs of cranial nerves, which form part of their reflex arcs, participate in the execution of these reflexes.

The reflex arcs of motor reflexes close in the medulla oblongata: postural reflexes, and Changes in the tone of the neck and trunk Muscles. Maintaining posture requires the redistribution of extensor Muscle tone.

Pons

Posteriorly, the pons borders the medulla oblongata, and anteriorly, the cerebral peduncles. The middle cerebellar peduncles emerge from its lateral parts. On the ventral surface of the pons, there is the basilar sulcus, which contains the Basilar artery.

The Gray matter of the ventral surface forms the pontine nuclei. The dorsal surface of the pons forms the upper (anterior) part of the floor of the IV ventricle (rhomboid fossa). In the dorsal part of the pons, the superior olivary nuclei, the nuclei of cranial nerves V–VIII, and the reticular formation are located. The axons of the cranial nerve nuclei emerge at the Base of the brain lateral to the pons and posterior to it, along the line bordering the cerebellum and medulla. The White matter OF the pons consists of ascending and descending fiber systems.

Cerebellum

The cerebellum lies dorsal to the pons and medulla oblongata. It consists of two hemispheres and a middle part, the vermis. The surface of the cerebellum is covered by a layer of gray matter (cerebellar cortex), which forms narrow folds (folia) separated by sulci. These sulci divide the cerebellar surface into lobules. The central part of the cerebellum consists of white matter containing clusters of gray matter, the cerebellar nuclei. The largest of these is the dentate Nucleus. The cerebellum has three pairs of peduncles: superior (connecting it to the midbrain), middle (to the pons), and inferior (to the medulla oblongata). Fiber tracts pass through these peduncles, connecting the cerebellum with various Divisions of the brain and spinal cord.

The spinal cord and brainstem regulate muscle tone, maintain posture, and enable locomotor acts; however, the cerebellum is essential for performing complex, coordinated, rapid, and precise movements.

Four groups of motor activity alterations can be distinguished in cerebellar lesions:

1) changes in muscle tone: muscle tone drops sharply (atonia)

2) rapid muscle fatigue (asthenia)

3) tremors of the limbs and HEAD (astasia)

4) impaired coordination of movements (ataxia); movements are imprecise, clumsy, and inappropriate in force.

ISTHMUS OF THE rhombencephalon.

The isthmus of the rhombencephalon is the boundary between the hindbrain and the midbrain. It includes the superior cerebellar peduncles, the superior medullary velum lying between the peduncles, and the lemniscal triangle, which lies lateral to the superior cerebellar peduncles.

Fourth ventricle

The fourth ventricle (ventriculus quartus) is the cavity of the rhombencephalon (hindbrain vesicle). It has a roof and a floor. The floor is formed by the dorsal surfaces of the medulla oblongata and the pons, while the roof is formed by the superior and inferior medullary vela. The ventricle is filled with CEREBROSPINAL FLUID and communicates with the Third ventricle via the cerebral aqueduct (aqueduct of Sylvius), and with the central canal and the subarachnoid space via the paired lateral apertures (foramina of Luschka) and the median aperture (foramen of Magendie). The floor of the IV ventricle forms the so-called rhomboid fossa, onto which the nuclei of the cranial nerves (pairs V to XII) project. The fossa is bounded superiorly by the superior cerebellar peduncles and inferiorly by the inferior cerebellar peduncles.

Midbrain.

The midbrain develops from the third brain vesicle (mesencephalon). It includes the cerebral peduncles and the tectum of the midbrain.

Its cavity is the cerebral aqueduct (aqueduct of Sylvius), which connects the III and IV ventricles. The tectum consists of two superior and two inferior colliculi, which serve as subcortical centers for Vision and Hearing, respectively.

In a cross-section of the midbrain, the tectum and the cerebral peduncles can be observed. The cerebral peduncles consist of the tegmentum and the base, separated by the substantia nigra (pigmented interneurons). The Pyramidal Tracts and the tracts connecting the Cerebral Cortex with the nuclei of the pons and cerebellum pass through the base of the peduncles. The tegmentum contains both gray and white matter. The gray matter comprises the red nucleus, the reticular formation, and the nuclei of cranial nerves III and IV. The white matter is represented by the fibers of the ascending (sensory) pathways, which form the medial and lateral (auditory) lemnisci here.

The midbrain performs a reflex function. Visual and auditory orienting reflexes are mediated at the level of the colliculi of the midbrain tectum.

Diencephalon.

The diencephalon is located beneath the corpus callosum. It consists of parts such as the thalamus and Hypothalamus, and its cavity is the III ventricle.

The thalamus is a paired accumulation of gray matter, which is divided into the thalamus proper, metathalamus, and epithalamus. Within the thalamus proper, anterior, lateral, and medial groups of nuclei are distinguished. In the lateral nuclei, all sensory pathways heading to the cerebral cortex are relayed. In addition, the anterior part of the thalamus is called the anterior tubercle, and the posterior part is the pulvinar, which contains subcortical visual centers. The medial surfaces of the thalamus proper form the lateral walls of the III ventricle.

The metathalamus includes the medial and lateral geniculate bodies, which serve as subcortical centers for hearing and vision, respectively. The medial geniculate body is connected to the inferior colliculi of the midbrain tectum via the inferior brachia, and the lateral geniculate body is connected to the superior colliculi via the superior brachia.

The epithalamus consists of the Pineal Gland (pineal body), which is located between the superior colliculi of the midbrain tectum. Habenulae extend from the pineal gland, holding it in this position.

Hypothalamus

It is located ventral to the thalamus proper and consists of the subthalamic region itself and A number of structures located at the base of the brain. These include the lamina terminalis, optic chiasm, tuber cinereum, infundibulum (on which the Pituitary Gland is located), and mammillary bodies. The subthalamic region contains nuclei with Nerve Cells that have The ability to secrete substances (neurosecretion).

The latter travels along the axons of nerve cells to the posterior pituitary gland and then into the bloodstream.

Third ventricle.

It is a narrow slit whose lateral walls are formed by the medial surfaces of the thalamus proper. The anterior wall of the III ventricle is formed by the columns of the fornix and the anterior commissure, the posterior wall by the epithalamus, the inferior wall by the structures of the hypothalamus, and the superior wall by the choroid plexus, which produces cerebrospinal fluid. The III ventricle communicates with the lateral ventricles via the interventricular foramen and with the IV ventricle via the cerebral aqueduct.

Reticular formation

In the brainstem, there are clusters of Neurons with highly branched processes that form a dense network. This system of neurons is called the 'reticular' formation. Non-specific pathways originate from the neurons of the reticular formation. They ascend to the CEREBRAL CORTEX AND subcortical nuclei, and descend to the neurons of the spinal cord.

The reticular formation regulates the functional state of the spinal cord and brain, as well as muscle tone. It has connections with all regions of the brain: cerebral cortex cells, autonomic subcortical centers, the cerebellum, motor nerve nuclei, and receptors.

Thus, two systems pass through the brainstem to the cerebral cortex: one is specific (sensory pathways carrying impulses from all receptors—interoceptors, exteroceptors, and proprioceptors) and terminates in the 4th layer of the cortex. The second is non-specific, formed by the reticular formation, and terminates on the dendrites of all cortical layers.

The telencephalon.

The telencephalon develops from the anterior brain vesicle and consists of the right and left cerebral hemispheres. The hemispheres are separated by the longitudinal fissure, deep within which lies the corpus callosum. It is composed of fibers that connect the hemispheres with each other. Beneath the corpus callosum lies the fornix, represented by two fibrous bands. In the middle portion, these bands join, while anteriorly and posteriorly they diverge, forming columns and crura. Anterior to the columns is the anterior commissure. A thin plate of brain tissue, the septum pellucidum, is stretched between the anterior part of the corpus callosum and the fornix. It belongs to the rhinencephalon (olfactory brain).

Each hemisphere is composed of white and gray matter. The gray matter forms the cerebral cortex (Pallium), which features sulci and gyri. Accumulations of gray matter within the hemispheres constitute the basal nuclei.

Each hemisphere has three surfaces: superolateral, inferior, and medial. The surface of the hemispheres features numerous sulci and elevations between them called gyri. The size and shape of the sulci and gyri vary individually, but several constant sulci exist. The hemispheres are divided into the frontal, parietal, temporal, and occipital lobes, as well as the insula, which lies at the bottom of the lateral sulcus. The temporal lobe is separated from the others by the lateral sulcus (Sylvian fissure), the frontal lobe by the central sulcus, and the occipital lobe by the parieto-occipital sulcus.

In the frontal lobe, the precentral sulcus runs parallel to the central sulcus. The superior and inferior frontal sulci extend perpendicularly from it, dividing the lobe into the superior, middle, and inferior frontal gyri. The precentral gyrus is located between the central and precentral sulci.

In the parietal lobe, the postcentral sulcus runs parallel to the central sulcus. The intraparietal sulcus extends perpendicularly from it, dividing the lobe into the superior and inferior parietal lobules.

In the temporal lobe, the superior and inferior temporal sulci run parallel to the lateral sulcus, separating the superior, middle, and inferior temporal gyri.

Cortex of the cerebral hemispheres.

The gray matter On the surface of the cerebral hemispheres ranges from 3 to 5 mm in thickness and is composed of more than 50 billion nerve cells. Based on morphological characteristics, the neurons of the cerebral cortex are divided into pyramidal and non-pyramidal cells. Pyramidal cells are pyramid-shaped, while non-pyramidal cells include basket, stellate, and spindle-shaped (fusiform) cells.

The cerebral cortex consists of six layers: molecular, external granular, external pyramidal, internal granular, ganglionic, and polymorphic Cell layers. Schematically, the cerebral cortex can be conceptualized as a collection of nuclei of various analyzers, interspersed with scattered elements of these analyzers:

1) the postcentral gyrus of the cortex contains the cortical center (nucleus) for general sensitivity (pain, Temperature, tactile) and Proprioception;

2) the motor center (analyzer) is located in the precentral gyrus;

3) the center for conjugate head and eye deviation to the opposite side lies in the posterior PARTS OF THE middle frontal gyrus;

4) the motor center for writing (graphic motor center) is located in the posterior parts of the middle frontal gyrus ("writing center");

5) the motor speech center (Broca's area) is located in the posterior parts of the left inferior frontal gyrus.

Lateral ventricles

The lateral ventricles (right and left) are the cavities of the telencephalon. They consist of the anterior, posterior, and inferior horns, and a central part.

The anterior horn is located in the frontal lobe, the posterior horn in the occipital lobe, the inferior horn in the temporal lobe, and the central part in the parietal lobe. The walls of the horns are formed by the white matter of the hemispheres and the caudate nucleus. The ventricles are filled with cerebrospinal fluid and communicate with the third ventricle through the interventricular foramen.

White matter of the hemispheres.

The white matter of the hemispheres occupies the space between the cortex and the basal nuclei. It consists of numerous nerve fibers running in various directions. Three systems of hemispheric fibers are distinguished: association fibers (connecting different areas within the same hemisphere), commissural fibers (connecting symmetrical areas of the left and right hemispheres), and projection fibers (connecting the cerebral cortex with other regions of the brain and spinal cord).

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