Chapter 1: Outline of the Nervous System

Nachum Dafny, Ph.D., Department of Neurobiology and also Anatomy, The UT Medical School at Houston Reregarded and revised 07 Oct 2020

The human nervous device is separated into the main nervous mechanism (CNS) and also the peripheral nervous mechanism (PNS). The CNS, subsequently, is separated right into the brain and the spinal cord, which lie in the cranial cavity of the skull and the vertebral canal, respectively. The CNS and the PNS, acting in concert, integrate sensory indevelopment and also control motor and cognitive features.

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1.1 The Central Nervous System (CNS)

Figure 1.1 Lateral see of humale embryo at the start of the 3rd (A) and also 5th (B) week of gestation.

The adult humale brain weighs in between 1,200 to 1,500 g and also contains around one trillion cells. It occupies a volume of about 1400 cc - roughly 2% of the total body weight, and also receives 20% of the blood, oxygen, and also calories provided to the body. The adult spinal cord is about 40 to 50 cm long and also occupies around 150 cc. The brain and the spinal cord aclimb in at an early stage development from the neural tube, which increases in the front of the embryo to form the three major brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain) (Figure 1.1A). These 3 vesicles additionally identify right into 5 subdivisions: telencephalon, diencephalon, mesencephalon, metencephalon, and also the myelencephalon (Figure 1.1B). The mesencephalon, metencephalon, and also the myelencephalon consist of the brain stem.

The telencephalon contains the cerebral cortex (cortex is the external layer of the brain), which represents the highest level of neuronal organization and also attribute (Figures 1.2A and also 1.2B). The cerebral cortex is composed of miscellaneous types of cortices (such as the olfactory bulbs, Figure 1.2B) as well as carefully associated subcortical frameworks such as the caudate nucleus, putaguys, globus, amygdala and also the hippocampal development (Figure 1.2C).

Figure 1.2 Lateral (A) and also ventral (B) views of the cerebral cortex. Coronal view (C) of subcortical structures.

The diencephalon consists of a complicated repertoire of nuclei lying symmetrically on either side of the midline. The diencephalon includes the thalamus, hypothalamus, epithalamus and subthalamus (Figure 1.3).

Figure 1.3 The major diencephalon nuclei.

The mesencephalon (or midbrain) consists of numerous structures roughly the cerebral aqueduct such as the periaqueductal gray (or main gray), the mesencephalic reticular development, the substantia nigra, the red nucleus (Figure 1.4), the remarkable and inferior colliculi, the cerebral peduncles, some cranial nerve nuclei, and the estimate of sensory and also motor pathmethods.

Figure 1.4 Subcortical diencephalic and mesencephalic frameworks.

Figure 1.5 Lateral watch of the metencephalon and also a spinal cord section through ventral and also dorsal root fibers, and also dorsal root ganglia.

The metencephalon consists of the pons and the cerebellum. The myelencephalon (spinal cord-like) has the open and closed medulla, sensory and also motor nuclei, forecast of sensory and also motor pathmethods, and also some cranial nerve nuclei.

The caudal finish of the myelencephalon establishes into the spinal cord. The spinal cord is an elongated cylindrical framework lying within the vertebral canal, which includes the main canal and also the bordering gray matter. The gray matter is composed of neurons and also their sustaining cells and is enclosed by the white matter that is written of a dense layer of ascending and also descending nerve fibers. The spinal cord is an important link between the peripheral nervous device and also the brain; it conveys sensory information originating from various outside and also inner sites through 31 pairs of spinal nerves (Figure 1.5). These nerves make synaptic relationships in the spinal cord or in the medulla oblongata and ascfinish to subcortical nuclei.

Figure 1.6 The central nervous device, which has the spinal cord and also the brain, is the most safeguarded organ in the human body. It is protected from the exterior environment by 3 barriers: skull, meninges, and cerebral spinal fluid (CSF).

The meninges are composed by three fibrous connective tworries (Figure 1.6). The many external is a thick collagenous connective tissue envelope known as the dura mater (Latin for “hard mother”). The second, or the intermediated membrane, is a delicate non-vascular membrane of fine collagenous layer of reticular fibers developing a web-like membrane, recognized as the arachnoid (Greek for “spider”). It is separated from the inner pia layer by subarachnoid room, which is filled with cerebrospinal fluid. The inner the majority of vulnerable connective tproblem membrane of collagenous is the pia mater, a thin translucent elastic membrane adherent to the surface of the brain and the spinal cord. Blood vessels located on the surface of the brain and the spinal cord are discovered on top of the pia issue. The meninges are topic to viral and bacterial infection recognized as meningitis, a life-threatening problem that calls for instant medical treatment.

The room in between the skull and also the dura is well-known as the epidural room. The room between the dura and also the arachnoid is well-known as the subdural room. The space between the arachnoid and the pia is recognized as the subarachnoid space. In this room, tbelow is a clear liquid well-known as the CSF. The CSF serves to assistance the CNS, and also to cushion and also safeguard it from physical shock and also trauma. The CSF is produced by the choroid plexus, which is created of a specialized secretory ependymal layer located in the ventricular mechanism.

The ventricular mechanism is a derivative of the primitive embryonic neural canal. This device is an interconnected series of spaces within the brain, which has the CSF (Figure 1.7).

Figure 1.7 The ventricular device in 4 different angles.

In basic, the CNS have the right to be split right into 3 main sensible components: the sensory system, the motor system, and homeostatic and greater brain functions. The sensory system is composed of the somatosensory, viscerosensory, auditory, vestibular, olmanufacturing facility, gustatory, and also visual units. The motor device consists of motor units, and also the somatic (skeletal muscle) device, the spinal reflexes, the visceral (autonomic) system, the cerebellum, several subcortical and cortical sites, and the brain stem ocular motor control device. The homeostatic and also higher functional device contains the hypothalamus, cortical areas associated in inspiration, insight, personality, language, memory, creativity, creativity, thinking, judgment, mental processing, and subcortical locations involved in finding out, believed, consciousness, memory, attention, emotional state, sleep and arousal cycles.

1.2 The Cerebral Hemispheres

The biggest and the majority of apparent components of the humale brain are the cerebral hemispheres. The cerebrum has an external layer - the cortex, which is created of neurons and their supporting cells, and also in living brain, has actually a gray shade for this reason called the gray matter. Under the gray issue is the white matter, which is composed of myelinated ascending and also descfinishing nerve fibers, and also in living brain have actually a white color. Embedded deep within the white issue are aggregates of neurons exhibiting gray shade and also recognized as subcortical nuclei. The cerebral hemispheres are partly separate from each various other alengthy the midline by the interhemispheric fiscertain (deep groove) the falx cerebri (Figure 1.8A); Posteriorly, there is a transverse fiscertain that separates the cerebral hemisphere from the cerebellum, and consists of the tentorium cerebellum. The hemispheres are linked by a large C-shaped fiber bundle, the corpus calloamount, which carries information in between the two hemispheres.

Figure 1.8 Six cortical lobes: Dorsal watch (A), Lateral see (B), Mid-sagittal area showing the limbic lobe (in green) (C), and Horizontal section mirroring the insular cortex (D).e

For descriptive functions each cerebral hemispright here deserve to be divided into 6 lobes. Four of these lobes are named according to the overlying bones of the skull as follows: frontal, parietal, occipital and temporal (Figures 1.8A and 1.8B), the fifth one is situated internally to the lateral sulcus – the insular lobes (Figure 1.8B and also 1.8D), and the 6th lobe is the limbic lobe (Figure 1.8C) which includes the limbic device nuclei. Neither the insular lobe nor the limbic lobe is a true lobe. Although the borders of the various lobes are somewhat arbitrary, the cortical areas in each lobe are histologically distinctive.

The surconfront of the cerebral cortex is highly convoluted via folds (gyri), separate from each various other by elongated grooves (sulci). These convolutions permit for the development of the cortical surchallenge location without boosting the size of the brain. On the lateral surface of the cerebral hemispright here there are 2 major deep grooves-sulci (or fissure), the lateral fiscertain (of Sylvian) and also the central sulci (of Rolando), these sulci provide landmarks for topographical orientation (Figure 1.9A). The central sulcus sepaprices the frontal lobe from the parietal lobe and also runs from the remarkable margin of the hemisphere near its midallude obliquely downward and forward until it almost meets the lateral fiscertain (Figures 1.8A and 1.8B). The lateral fiscertain, separating the frontal and parietal lobes from the temporal lobe, starts inferiorly in the basal surface of the brain and extends laterally posteriorly and also upward, separating the frontal and parietal lobes from the temporal lobe (Figure 1.9A). The frontal lobe is the percent which is rostral to the central sulcus and also over the lateral fissure, and it occupies the anterior one third of the hemispheres (Figures 1.8 and 1.9). The boundaries of the parietal lobe are not specific, other than for its rostral border – the central sulcus. The occipital lobe is the portion which is caudal to the parietal lobe (Figures 1.8 and 1.9). Alengthy the lateral surconfront of the hemispright here, an imaginary line connecting the pointer of the parietal-occipital sulcus and the preoccipital notch (Figure 1.9A), sepaprice the parietal lobe from the occipital lobe. On the medial surconfront of the hemispbelow (Figure 1.9B), parieto-occipital sulcus creates the rostral boundary of the parietal lobe. The tempdental lobe lies ventral to the lateral sulcus, and also on its lateral surface, it display screens three diagonal oriented convolutions-the superior, middle, and also inferior tempdental gyri (Figure 1.9A). The insula lies in the depths of the lateral sulcus. It has actually a triangular cortical area with gyri and also sulci (Figures 1.8B and 1.8D, and Figure 1.9A). The limbic lobe is composed of a number of cortical and subcortical locations (Figure 1.9B).



Figure 1.9 Drawing of the various cortices, sulci and also gyri (A) and mid-sagittal illustration emphasizes the limbic lobe (in green) (B).

The cerebral cortex is a functionally arranged body organ. A functionally organized system is a collection of neurons connected together to convey a details type(s) of information to accomplish a certain job. It is possible to determine on the cerebral cortex primary sensory locations, additional sensory locations, main motor area, premotor location, supplementary motor location and association areas, which are devoted to the integration of motor and also sensory indevelopment, intellectual task, reasoning and comprehension, execution of language, memory storage and also recontact.

The frontal lobe is the largest of the brain lobes and also is consisted of of four gyri, precentral gyrus that parallels the main sulcus, and 3 horizontal gyri: the superior, middle, and inferior frontal gyri. The inferior frontal gyrus is comprised of three parts: the orbital, the triangular and opercular. The term opercular refers to the “lips of the lateral fissure. Finally, the directly gyrus (gyrus rectus) and the orbital gyri create the base of the frontal lobe (Figure 1.9B). Four basic practical areas are in the frontal lobe. They are the main motor cortex, where all components of the body are represented, the premotor and also supplementary motor areas. A area concerned with the motor mechanisms of speech formulation comprised of the opercular and also triangular parts of the inferior frontal gyrus are well-known as Broca’s speech location, and also the remainder of the prefrontal cortex is associated in mental activity, personality insight, foresight, and reward. The orbital portion of the prefrontal cortex is essential in the proper switching in between mental sets and the regulation of eactivity.

The parietal lobe is consisted of of 3 gyri: postmain gyrus, exceptional and also inferior parietal gyri (Figure 1.9A). The postmain gyrus is immediately behind the main sulcus which forms its anterior boundary. The postcentral gyrus comprises the main somatosensory cortex which is concerned via somatosensory reception, integration and handling sensory indevelopment from the surchallenge of the body and also from the viscera, and is necessary for the formulation of perception. Caudal to the postcentral gyrus is the inferior parietal gyrus. The intraparietal sulcus separates the posterior parietal gyrus from the inferior parietal gyrus. The inferior parietal gyrus represents the cortical association location which integrates and also processes sensory information from multiple modalities such as auditory and also visual information. The inferior parietal gyrus, which is well-known as Wernicke"s location, is also vital for language and also reading abilities, whereas the premium parietal gyrus is involved with body photo and spatial orientations.

The temporal lobe is formed by three obliquely oriented gyri: the remarkable, middle, and inferior tempdental gyri (Figure 1.9A). Inferomedial to the inferior temporal gyrus are the occipitotemporal and the parahippocampal gyri, which are separated by the collateral sulcus. The top surface of the exceptional temporal gyrus, which exhas a tendency right into the lateral fiscertain, is dubbed the transverse temporal gyrus (of Heschl) and also is the main auditory cortex. The caudal part of the superior tempdental gyrus, which exhas a tendency up to the parietal cortex, develops part of Wernicke’s area. Wernicke’s area is involved, in part, via handling the auditory information and is crucial in the understanding of language. The inferior part of the temporal lobe (i.e., the occipitotempdental gyri) is affiliated in visual and also cognitive processing. More medially is the parahippocampal gyrus, which is involved in discovering and also memory. Portions of the frontal, parietal, and temporal lobes, which are adjacent to the lateral sulcus and overlie the insular cortex, are known as the operculum. The inferomedial surconfront of the tempdental lobe is made up of the uncus and the parahippocampal gyrus medially. The inferior surconfront of the temporal lobe rests on the tentorium cerebelli.

The occipital lobe is the most caudal component of the brain, lies on the tentorium cerebelli (Figure 1.9A) and is comprised of a number of irconstant lateral gyri. On its medial surchallenge, tright here is a prominent fissure – the calcarine fissure and parieto-occipital sulcus. The calcarine fissure (sulcus) and also the parieto-occipital sulcus also define a cortical region recognized as the cuneus. The cuneus sulcus divides the occipital lobe into the cuneus dorsally and also ventrally right into the lingual gyrus. The occipital lobe includes the major and also higher-order visual cortex.

The insula lobe is located deep inside the lateral fissure and also can be checked out only once the tempdental and also the frontal lobes are separated (Figures 1.8B and also 1.8D). The insula is identified by a number of lengthy gyri and sulci, the gyri breves and gyri longi. Tright here is some evidence that the insular cortical areas are associated in nociception and regulation of autonomic function.

The limbic lobe is not a true lobe and is comprised of a number of cortical areas such as the cingulate and also parahippocampal gyri, some subcortical locations favor the hippocampus, amygdala, septum, and various other locations via their particular ascfinishing and descfinishing connections (Figures 1.8C and also 1.9B). The limbic lobe is associated in memory and discovering, drive associated behavior, and emotional feature.

There are subcortical locations in the telencephalon choose the basal ganglia and the amygdaloid nucleus facility. The corpus calloamount is a arsenal of nerve fibers that connect the 2 hemispheres. The corpus callosum is separated right into rostrum (head), body, the a lot of rostrally component is the genu (knee) via connecting the rostrum and also the body, and also the splenium at the caudal extremity (Figure 1.10). The corpus calloamount plays a critical role in delivering indevelopment from one hemispright here to the other.

1.3 The Diencephalon

The second major derivative of the prosencephalon is the diencephalon. The diencephalon is the most rostral structure of the brain stem; it is embedded in the inferior element of the cerebrum. The posterior commissure is the junctional landnote between the diencephalon and also the mesencephalon. Caudally, the diencephalon is constant with the tegmentum of the midbrain. During advance the diencephalon differentiates into 4 regions: thalamus, hypothalamus, subthalamus and also epithalamus (Figure 1.11) The epithalamus comprises the stria medullaris habenular trigone, pineal gland and also the posterior commiscertain

Figure 1.11 Mid-sagittal illustration mirroring the main structures of the diencephalon and also rhombencephalon.

1.4 The Brain Stem

The brain stem consists of mesencephalon (midbrain), metencephalon, and also myelencephalon. The metencephalon and myelencephalon together compose the rhombencephalon (hindbrain), which divides right into pons, and medulla oblongata (Figures 1.11 and also 1.12).

Mesencephalon (midbrain) is constant with the diencephalon rostrally and with the pons caudally. The midbrain is the smallest component of the brain stem, being about 2 cm in size. It consists of a tectum posteriorly, a tegmentum inferiorly, and also a base anteriorly. The tectum develops the roof of the cerebral aqueduct, which connects the 3rd ventricle through the fourth ventricle and also the tegmentum its floor. The base of the midbrain consists of the cerebral peduncle, which contain nerve fibers descending from the cerebral cortex. The nuclei of the 3rd (oculomotor), the fourth (trochlear) and also component of the fifth (trigeminal) are situated in the midbrain tegmentum. The red nucleus and also the substantia nigra, two significant nuclei, are also discovered in the midbrain tegmentum. The midbrain tectum is created by two pairs of rounded structures: the superior and also inferior colliculi. The superior and also inferior colliculi (Figure 1.12) are associated in visual and auditory attributes respectively.

Pons is continuous via the midbrain and also is composed of 2 components, the pontine tegmentum (located internally) and the basilar pons. At the level of the pons, the cerebral aqueduct has expanded to develop the fourth ventricle (Figure 1.12). The cerebellum is situated posterior to the pons and forms part of the roof (tectum) of the forth ventricle. The pons contains nuclei that obtain axons from various cortical locations. Projections from the axons of these pontine neurons form huge transverse fiber bundles that traverse the pons and ascend to the contralateral cerebellum using the middle cerebellar peduncles. Also, within the pons base and tegmentum are longitudinally ascending and descending fibers. The nuclei of the fifth (trigeminal), 6th (abducens), 7th (facial) and also the 8th (vestibulocochlear) nerves are situated in the pons tegmentum.

Medulla Oblongata (myelencephalon is also known as the medulla). The medulla lies in between the pons rostrally and the spinal cord caudally. It is constant via the spinal cord just over to foramales magnum and the initially spinal nerve. The posterior surface of the medulla forms the caudal half of the fourth ventricle floor and also the cerebellum, its roof (Figure 1.12). The base of the medulla is created by the pyramidal-descfinishing fibers from the cerebral cortex. The medulla tegmentum contains ascfinishing and also descfinishing fibers and also nuclei from the nine (glossopharyngeal), 10th (vagus), 11th (accessory) and the 12th (hypoglossal) nerves. The corticospinal fibers (pyramid) are alongside the anterior median fissure, and also decussate (cross the midline) to the contralateral side on their way to the spinal cord. Other influential structures in the medulla are the inferior olive, and the inferior cerebellar peduncle. The medulla consists of nuclei which control respiration, swpermitting, sweating, gastric secretion, cardiac, and vasomotor activity.

The arterial blood supply to the brain is acquired from two arterial systems: the carotid system and also the vertebrobasilar mechanism. A series of an anastomotic networks lying at the base of the brain, recognized as the circle of Willis, permits communication between these two systems (Figure 1.13).

The arterial blood supply to the spinal cord is acquired from 2 branches of vertebral artery, the anterior and also 2 posterior spinal arteries which run the size of the spinal cord and develop an ircontinuous plexus around it (Figure 1.14).

The PNS consists of 31 pairs of spinal nerves, 12 pairs of cranial nerves, the autonomic nervous device and also the ganglia (groups of nerve cells outside the CNS) connected with them. Also had in the PNS are the sensory receptor organs. The receptor organs are scattered in all components of the body, sense and perceive changes from external and internal organs, then transform this indevelopment to electric signals, which are brought through a substantial nervous netoccupational to the CNS (Figure 1.15). The cranial and also spinal nerves contain nerve fibers that conduct indevelopment to-afferent-(Latin for bring toward) and from-efferent (Latin for carry away) the CNS. Afferent fibers convey sensory information from sensory receptors in the skin, mucous membranes, and also inner organs and also from the eye, ear, nose and mojonathanlewisforcongress.com to the CNS; the efferent fibers convey signals from cortical and subcortical centers to the spinal cord and also from tright here to the muscle or autonomic ganglia that innervate the visceral organs. The afferent (sensory) fibers enter the spinal cord through the dorsal (posterior) root, and the efferent (motor) fibers leave the spinal cord via the ventral (anterior) root. The spinal nerve is developed by the joining of the dorsal and also the ventral roots. The cranial nerves leave the skull and also the spinal cord nerves leave the vertebrae through openings in the bone referred to as foramina (Latin for opening).

The PNS is divided right into two systems: the visceral mechanism and the somatic system. The visceral device is likewise recognized as the autonomic device. The autonomic nervous system (ANS) is frequently taken into consideration a separate entity; although created partly in the PNS and partly in the CNS, it interdeals with in between the PNS and the CNS. The major feature of the ANS is to manage and regulate unconsciousness attributes consisting of visceral, smooth muscle, cardiac muscle, vessels, and also glandular function (Figure 1.16). The ANS can be split right into 3 subdivisions:

The sympathetic (or the thoracolumbar) subdivision connected through neurons located in the spinal gray between the thoracic and the top lumbar levels; The parasympathetic (or craniosacral) subdivision is linked with the 3rd, 7th, ninth and the 10th cranial nerves and through the 2nd, third, and also 4th sacral nerves; The enteric subdivision is a complicated neuronal netjob-related within the walls of the gastrointestinal system and consists of even more neurons than the spinal cord. The visceral (autonomic) system regulates the internal organs external the realm of conscious control. The PNS component of the somatic device has the sensory receptors and also the neurons innervating them and also their nerve fibers entering the spinal cord. The visceral and the somatic nervous mechanism are mainly involved via their own features, yet likewise work in harmony via various other aspects of the nervous mechanism.

1.6 Orientation to the Central Nervous System

This section illustprices representative sections through the CNS in order to acquaint the reader through significant structures and assist in the recognition of the level and orientation of the section. It likewise provides landmarks for locating nuclei and tracts involved in sensory and also motor features. Directional terms are offered in describing the locations of frameworks in the CNS.

Figure 1.17 Orientation of the central nervous system of the spinal cord and various brain sections.

Keep in mind that certain terms were emerged to define the nervous mechanism of quadrupeds and may have a slightly different meaning once used to bipeds. For example, the ventral surchallenge of the quadruped spinal cord is similar to the anterior surconfront of the biped (Figure 1.18). In the following descriptions, the terms are used to a standing humale. The terms rostral and also anterior describe a direction in the direction of the face/nose. The terms caudal and also posterior describe a direction towards the buttocks/tail. The terms inferior and also exceptional mainly describe spatial relationships in a vertical direction (Figure 1.18). A coronal section is parallel to the vertical plane and also a midcoronal section would certainly divide the head into anterior and also posterior halves (Figure 1.19). The sagittal area is also parallel to the vertical aircraft, but a midsagittal area would certainly divide the head into ideal and left halves. The horizontal (axial) section is parallel to the horizontal plane and also a mid-horizontal area would certainly divide the head into premium and also inferior halves. Transverse or cross sections of the spinal cord of humans are taken in a plane perpendicular to the vertical (i.e., in the horizontal aircraft of the head). Many electromagnetic imaging techniques produce imperiods of the brain in the coronal, horizontal (axial) and also sagittal planes. The representative sections are transverse sections with the spinal cord and brain stem and coronal sections via the telencephalon and also diencephalon (Figure 1.17).

Transverse Section via the Spinal Cord. Figure 1.17A illustrates a area taken at the level of the thoracic spinal cord. The spinal cord neuron (gray matter) form a central core taking a butterfly configuration that is surrounded by nerve fibers (white matter). In the left and also appropriate halves of the spinal cord, the gray issue is arranged into a dorsal horn and also ventral horn with the intermediate gray situated in between them. In the thoracic spinal cord, which is shown in this figure, a lateral horn extends laterally from the intermediate gray (Figure 1.17A). The spinal cord white issue is subseparated into the posterior white column, the anterior white column and also the lateral white column. The anterior white commiscertain joins the two halves of the spinal cord and is located ventral to the intermediate gray. The dorsal root fibers enter the spinal cord at the dorsolateral sulcus and also the fibers of the ventral root fibers departure the spinal cord in countless fine bundles with the ventral funiculus (view Figure 1.5).

Transverse Section through the Medulla. Figure 1.17B is a area taken at the level of the upper medulla. Landnote structures encompass the fourth ventricle, hypoglossal nucleus, inferior cerebellar peduncle, inferior olivary complicated and the pyramids. As in the spinal cord area, the fiber tracts, the inferior cerebellar peduncle and also pyramids, appear light in this section whereas the nuclei in the inferior olivary complicated show up dark.

Transverse Section via the Pons. Figure 1.17C is a area taken at the level of the mid pons. Landnote frameworks include the fourth ventricle, the pons tegmentum, which has the abducens nuclei; the pons base, which has the corticofugal fibers and also pontine nuclei; and the middle cerebellar peduncles.

Coronal Section with the Rostral Telencephalon. Figure 1.17D is a section taken at the level of the decussation of the anterior commiscertain. Landmark frameworks incorporate the head of the cauday nucleus, the anterior limb of the internal capsule, the globus pallidus and putaguys (vital for controlling motor functions). The anterior commiscertain, a fiber bundle connecting the appropriate and also left frontal lobes, have the right to be checked out decussating (crossing the midline). The corpus calloamount forms a thick band of decussating nerve fibers located above the lateral ventricles. Below the telencephalon afferent nerve fibers from each eye decussate in the optic chiasm and join uncrossed fibers to develop the optic tract.

Coronal Section through the Midbrain-Diencephalon Junction. Figure 1.17E is a section taken at the level of the junction of the midbrain via the diencephalon. Notice that the airplane of area differs from those of the previous sections. At this level, a landmark structure of the diencephalon is the thalamus, which surrounds the 3rd ventricle. The posterior limb of the inner capsule separates the thalamus from the neighboring telencephalic structures (i.e., the globus pallidus and putamen). Lateral to the putaguys is the insula while more dorsomedially the corpus callosum overlies the cavities of the lateral ventricles. Below the 3rd ventricle are the red nucleus, substantia nigra and also crus cerebri of the midbrain, which are the extension of the internal capsule.

Section through the Midbrain. Figure 1.17F is a area that mirrors the main midbrain nuclei which include the tectum (superior colliculi) the periaqueductal gray, the red nuclei, substantia nigra and also the cerebral peduncles.

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