The Nervous System is Divided into 2 Major Divisions

The Nervous system is divided into 2 major divisions: 1) Central Nervous System (CNS): found within bones & consists of: - The Brain: within the skull, composed of cerebrum, cerebellum and brain stem. -The spinal cord: found within the vertebral canal, composed of 31 spinal segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) 2) Peripheral Nervous System (PNS): Consists of: -Autonomic nervous system: divided into: * Sympathetic nervous system * Parasympathetic nervous system. -Somatic nerves: * Cranial nerves (12 pairs): Connected to the brain. * Spinal nerves (31 pairs): Connected to the spinal cord. -The brain consists of: 1) The Cerebrum 2) Cerebral hemispheres separated by a median fissure) ‐ The Diencephalon is a structure located deep within the brain underneath the cerebrum and can only be viewed through a midsagittal section of the brain. The midsagittal aspect of the cerebrum reveals structures spanning the fissure, the first and most prominent of them being the Corpus Callosum; which is the largest group of commissural fibers connecting the right and left cerebral hemispheres. 1|Page *Commissural fibers are one of the many types of fibers in the brain, which function in connecting the two hemispheres together in order to achieve coordination and integration in the function of the central nervous system. In contrast, Association fibers connect regions within the same hemisphere of the brain, and Projection fibers connect each region to other parts of the brain or to the spinal cord. Ascending and descending tracts are projection fibers. 2) The Brainstem which consists of midbrain, pons and medulla oblongata. 3) The Cerebellum which consists of 2 cerebellar hemispheres connected by a tree-like structure, “The Vermis” which resembles the corpus callosum of the cerebrum in function. Note: Before making a midsagittal section, the midbrain cannot be seen, whereas the pons and medulla oblongata can be seen anteriorly but not posteriorly as they are covered by the cerebellum from the back. 2|Page Inner Structures of the brain: 1) Basal Ganglia (The more anatomically correct term is Basal nuclei since its located within the CNS): This structure is buried deep within the basal surface of the brain; it contains the Caudate, Lentiform and amygdaloid nuclei. It can be seen lateral to the thalamus. 2) Thalamus: Is an egg-shaped structure seen medially in the midsagittal section. It is the biggest and most important part of the Diencephalon. 3) Hypothalamus: A group of nuclei situated at the basal surface of the brain in front of the brainstem and antero-inferior to the thalamus 4) Subthalamus: A group of nuclei at the upper surface of the midbrain (separating the thalamus from the midbrain) 5) Epithalamus: A small structure posterior to the thalamus. It consists of the pineal gland (which has a role in secreting melatonin that regulates sleep cycles). This gland is found between 2 commissural fibers, the Habenular commissure superiorly and the posterior commissure inferiorly. (These 3 structures together make up the epithalamus) 6) Metathalamus: consists of the medial and lateral geniculate bodies (nuclei) situated at the Pulvinar (posterior part of the thalamus). The medial geniculate nucleus is a part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex, whereas the lateral geniculate nucleus is a part of the visual thalamus and is connected to the superior colliculus. 3|Page The Embryology of the brain: The neural tube is a sheet, derived from the surface ectoderm, that gets differentiated in the embryo into a proximal and distal end representing an open tube, divided into 3 major vesicles (proximal, middle and distal): 1) Forebrain (Prosencephalon): consists of two cerebral hemispheres and the diencephalon 2) Midbrain (Mesencephalon) 3) Hindbrain (Rhombencephalon): consists of the pons, medulla and cerebellum The Cerebral Hemisphere: -The cerebrum consists of 3 poles: 1) Frontal pole: Supero-anteriorly (rounded) 2) Temporal pole: Infero-anteriorly 3) Occipital pole: Posteriorly (pointed) *In order to differentiate between the anterior and posterior aspects of the cerebrum (In the absence of the cerebellum which is easily located posteriorly), one must identify two poles anteriorly and 1 pole posteriorly. -The brain has 3 surfaces: 1) Supero-lateral surface 2) Medial surface (in the midsagittal section) 3) Inferior surface 4|Page There are also 3 borders: 1) Supero-medial border: Between Supero-lateral and medial surfaces 2) Infero-lateral border: Between Supero-lateral and Inferior surfaces 3) Infero-medial border: Between Inferior and Medial surfaces The Superolateral Surface: -Fissures divide the brain into lobes; the most important one is the Lateral Sylvian fissure *Sulcus: Depression in the brain, Gyrus: Elevation in the brain and Fissure: Deep sulcus -The Lateral fissure is divided into 4 parts anatomically: -Stem of the lateral fissure: It begins on the inferior surface (dividing it into ventral “orbital” and dorsal “Tentorial” part), lateral to the anterior perforated substance & extends laterally to reach the lateral surface where it divides into the remaining 3 branches. -Posterior ramus of the lateral fissure: When the stem curves on the lateral surface, it heads towards the posterior end of the brain and ends right before reaching the supero-medial border by turning upwards in the parietal lobe. -Anterior ramus: Runs forwards in the inferior frontal gyrus -Ascending ramus: Ascends in the inferior frontal gyrus 5|Page Central Sulcus of Ronaldo: It separates the anterior and posterior areas of the brain, with the frontal lobe directly anterior to it and the parietal lobe directly posterior to it. It begins 1 cm into the medial surface and extends from the supero-medial border at a point a little behind the midpoint between the frontal & occipital poles and ends right before reaching the posterior ramus of the lateral fissure without communicating with it. There is a notch in front of the occipital lobe known as the Pre-occipital notch. In contrast to the central sulcus, the Parieto-occipital fissure begins its course in the medial surface and extends 1 cm into the lateral surface. 2 Imaginary lines are drawn to divide the brain into 4 lobes: 1) Frontal lobe: Anterior to central sulcus 2) Parietal lobe: Posterior to central sulcus 3) Occipital lobe: Posterior to the Parieto-occipital fissure 4) Temporal lobe: Most inferior lobe -The first line is drawn between the Parieto-occipital fissure and preoccipital notch -The second line is drawn as a backward continuation of the posterior ramus of the lateral fissure to meet the first imaginary line. -There are 6 lobes in the brain; 4 major anatomical lobes and 2 minor lobes, known as the insula and Limbic lobe. *The limbic lobe can only be viewed in the medial surface 6|Page Supero-medial border Corpus Callosum Septum pelluidum Third Ventricle -If we take a coronal section of the brain, we can see the following structures: 1) Grey matter. 2) White matter. 3) Basal nuclei. 4) Lateral ventricle: Triangular cavity in the center -The doctor mentioned the following facts regarding the coronal section of the brain which will be described more thoroughly in the lab lecture: -The Septum Pellucidum makes up the medial wall of the lateral ventricle and is found between the fornix (Commissural fibers) and the corpus callosum. -The Third ventricle is situated between the two thalami -The Thalamus (Medial) is separated from the basal nuclei (Lateral) by the internal capsule -The anterior limb of the internal capsule passes between the caudate and the lentiform nucleus whereas the posterior limb passes between the thalamus posteriorly and the lentiform. 7|Page The Frontal Lobe: The anatomical boundaries of the Frontal lobe: -Anteriorly: Frontal pole -Posteriorly: Central sulcus -Inferiorly: Posterior ramus of the lateral fissure -Superiorly: Supero-medial border The Sulci of the frontal lobe: -Precentral sulcus: Parallel to & one finger breadth in front of the central sulcus. It begins on the lateral surface unlike the central sulcus. -Superior frontal sulcus. ‐ Inferior frontal sulcus Gyri of the Frontal lobe: It is divided by the sulci of the frontal lobe into: A) Precentral gyrus: Between central & precentral sulci, it is also known as the Primary motor area. B) Superior & Inferior sulci divide the remaining part equally into superior, middle & inferior frontal gyri The inferior frontal gyrus is subdivided into 3 gyri known as the orbital, triangular, and opercular gyri by the ascending and anterior rami of the lateral fissure. This region contains a very important area called the broca’s area of speech (Brodmann area 44, 45). The middle frontal gyrus also contains an important area called frontal eye field (Brodmann area 8). The area in front of the precentral gyrus is called premotor area (Brodmann area 6) extending over the medial surface as supplementary motor area (SMA). 8|Page -At the primary motor cortex, motor representation is orderly arranged in an inverted fashion where the head is represented by a large area below, followed by the trunk and upper limb then the thigh, all of which are on the lateral surface. Meanwhile the foot, leg and perineum’s representation extends into the medial surface on an area around the central sulcus called paracentral lobule. An injury in the anterior cerebral artery which supplies the paracentral lobule would cause incontinence in urine and stool as a result of not being able to control the muscles in the perineum area. Sulci & Gyri of the Temporal lobe -It contains 2 sulci: Superior & inferior temporal sulci, which divide it into 3 gyri: Superior, middle and inferior temporal gyri. -The superior temporal gyrus contains the auditory association area (which gives meaning to the auditory stimulus). The Insula (Island of Reil) The insula is a buried part of the cerebral cortex inside the lateral fissure, it lies at the bottom of the lateral Fissure and is not considered an anatomical lobe. It is conical (pyramidal) in shape having a base directed superiorly (surrounded by circular sulcus) and an apex directed antero-inferiorly towards the anterior perforated substance. The insula collects all taste sensations thus named the gustatory area. It is connected to nerves involved in carrying the taste sensations such as the facial, glossopharyngeal and vagus nerves, which synapse in the solitary nucleus of the medulla oblongata (tractus solitarii). The Insula is divided by sulcus centralis insulae into:An anterior part which is divided into 3‐4 short gyri and a posterior part with one long gyrus which is usually divided near its upper part. 9|Page Inferior surface of the brain RECALL: The stem of the lateral fissure begins on the inferior surface and divides it into a smaller anterior part (orbital surface) and posterior part (tentorial surface). -On the orbital surface, the olfactory sulcus lies near and parallel to the median fissure and lateral to the Gyrus Rectus, it is overlapped by the olfactory tract which ends as the olfactory bulb. -The limbs of the olfactory tract are called medial and lateral striae. -In between the two striae, the anterior perforated substance is found. This perforation is due to the presence of the anterior and middle cerebral arteries which supply the central structures of the brain (Basal ganglia, internal capsule...) - The H‐shaped orbital sulcus divides the remaining part into anterior, posterior, lateral & medial orbital gyri. - The posterior perforated substance (named so due to the presence of branches of the posterior cerebral artery in it) is found in the interpeduncular fossa (also known as the base of the brain or ventral surface of the midbrain) - The interpeduncular fossa is bounded anteriorly by the optic chiasm, antero-laterally by the optic tract, postero-lateral by the cerebral peduncles. - The cerebral peduncles are a part of the brain tissue that connects between the midbrain and the brain itself. 10 | P a g e - The mammillary bodies are a pair of small round nuclei, located on the inferior surface of the brain and forms part of the hypothalamus. - The pituitary gland is found between the optic chiasm and mammillary bodies. It is connected to the hypothalamus by the infundibulum. - Contents of the interpeduncular fossa: 1) Mammillary bodies 2) Posterior perforated substance 3) Pituitary gland and infundibulum The Parietal lobe Anatomical boundaries: - Anteriorly: Central sulcus - Posterior: Parieto-occipital fissure - Superiorly: Supero-medial border - Inferiorly: The imaginary line mentioned previously Sulci and gyri of the parietal lobe: -The first sulcus found behind the central sulcus is the post-central sulcus and in between these two structures, a very important area known as the primary somatosensory area of the body or the Post-central gyrus is found. This region represents the sensations of the body in an inverted fashion (upside down) just like the primary motor cortex - In the back, The Intra-parietal sulcus is found (It is so deep that it can be called a fissure). It divides the parietal lobe into 2 parts that are bigger than a gyrus and thus called lobule: 1) Superior parietal lobule 2) Inferior parietal lobule: contain 2 hook-shaped gyri; 11 | P a g e a) Supra-marginal gyrus: found above the posterior ramus of the lateral fissure b) Angular gyrus: found above the posterior end of superior temporal sulcus - Wernicke’s area includes the Supra-marginal gyrus (Area 40), angular gyrus (Area 39) and the superior part of the auditory association area (area 22) found in the superior temporal gyrus. A lesion in Wernicke’s area leads to Receptive Aphasia (Inability to understand spoken or written words). The Occipital lobe: Contains one upper sulcus called the Transverse occipital sulcus (lunate) and one lateral sulcus called the lateral occipital sulcus (horizontal). The lateral occipital sulcus divides the lateral surface of the occipital lobe into superior and inferior occipital gyri. Sulci and Gyri of the medial surface: The most important area found on the medial surface is the limbic lobe, which is considered the 6th lobe of the brain. It is a collection of cortical structures (represented by the rounded area) and subcortical structures (buried deep within the brain). The limbic system is in control of emotions and behavior and has a role in smell and autonomic function through their connection with the hypothalamus. 12 | P a g e The most important landmark of the medial surface is the corpus callosum (largest commissural fiber which connects between the right and left cerebral hemispheres). It is made up of four parts 1) Rostrum 2) Genu 3) Body 4) Splenium Inferior to the corpus callosum is the fornix which is arching white matter. It is separated from the corpus callosum by a septum called septum pellucidum. If we removed the septum pellucidum, the cavity of the lateral ventricle is seen. The Callosal sulcus is found above the corpus callosum and it houses the artery supplying the medial surface of the brain; Anterior cerebral artery. The Cingulate sulcus is found inferior to the corpus callosum and begins at the rostrum and curves upwards then ends approximately above the splenium of the corpus callosum to meet the supero-medial border. The end of this sulcus is known as marginal sulcus. It also gives an ascending branch (Ascending ramus) above the middle of the body of Corpus Callosum 13 | P a g e which divides the area above cingulate sulcus into anterior part: medial frontal gyrus & posterior part: paracentral lobule The paracentral lobule is found around the central sulcus. If this area was stimulated (motor stimulation) will cause a movement in the leg and foot Anatomical boundaries of the Para-central lobule: - Anteriorly: Ascending ramus of the cingulate sulcus - Posteriorly: Marginal sulcus - Inferiorly: Cingulate sulcus - Superiorly: Supero-medial border -Subparietal (suprasplenial) sulcus appears as a continuation of cingulate sulcus inside the parietal lobe -Parieto‐occipital fissure is represented on the medial surface and extends 1cm into the lateral surface between the parietal (anterior) & occipital lobes (posterior). -Calcrine sulcus begins near the occipital pole. The visual area is represented on the limb of the calcrine sulcus (Area 17) Gyri: -Cingulate gyrus lies between the corpus callosum and cingulate sulcus. It is the most important component of the limbic system (loop). The limbic loop consists of the cingulate gyrus and its continuation in the inferior temporal lobe. This continuation passes through the parahippocampal gyrus (Found on the inferior surface surrounding the hippocampal sulcus) If a deep dissectionin the parahippocampal gyrus was done, the hippocampus is found. The hippocampus is in charge of the short term memory 14 | P a g e The gyri from posterior to anterior: 1) Lingual gyrus : which is found below the Calcrine sulcus 2) Cuneus: This is the wedge area between the parieto‐occipital fissure & the calcrine sulcus. 3) Precuneus: lies in front the parieto‐occipital fissure 4) Paracentral lobule 5) Medial Frontal Gyrus (The rest of the frontal cortex) -Green box (In the picture above): The Subcallosal gyrus is found below the rostrum of the corpus callosum -Yellow Oval (In the picture above): Para terminal gyrus is found Infront of lamina terminalis -The hypothalamus contains Mamillary bodies -Infront of hypothalamus: Lamina terminalis -Para terminal gyrus is continuous with the cingulate gyrus so it’s considered part of the limbic loop 15 | P a g e