Anatomy of Brain

Before we precede, know this information (you can skip): The lateral surface of the brain The medial surface of the brain The inferior surface of the brain This is coronal section This is sagittal section This is horizontal (cross) section 2|Page Let’s start with the lateral surface anatomy: the cerebrum is the largest part of the brain and consists of two hemispheres, each hemisphere is composed of four anatomical lobes and the most prominent part of the lobe is called pole (they’re three in number), they are marked on the following drawing; (poles in dark arrows & lobes in thine arrows) parietal lobe Frontal lobe occipital lobe Frontal pole Temporal pole occipital pole Temporal lobe The lobes and poles are named after the bones of the cranium under which they lie. Note that there are two poles directed anteriorly (frontal and temporal) and one pole directed posteriorly (occipital); ( according to this in the previous drawing the anterior direction is to the left while the posterior direction is to the right). Hint: if the cerebellum is present then it’s easy to determine the direction as its always directed posteriorly. The question now is: How to know the borders for each lobe? The surface layer of each hemisphere is called the cortex and is composed of grey matter. The cerebral cortex is thrown into elevated folds called (gyri), separated by depressed fissures called (sulci). Several large sulci subdivide the surface of each hemisphere into lobes. 3|Page Note: The gyrus is an elevation while the sulcus is a depressed area. The gyri are composed of the functional areas that control the body. For example: there is a gyrus to control the motor, a gyrus to control the sensory, a gyrus for the visual center, a gyrus for the auditory center and so on. Your intelligence is directly proportional to the number of gyri on the surface membrane :o Note: the right cerebral hemisphere is controlling the left side of the body, while the left cerebral hemisphere is controlling the right side of the body. There is an important sulcus located on the lateral surface of the brain called the central sulcus (in the center of the brain), it divides the hemisphere into anterior region and posterior region. It meets another sulcus called the lateral fissure (fissures are deeper than sulci). They are marked in Blue in the next figure; 4|Page These two sulci divide the brain hemisphere into three parts (anterior posterior and inferior) , but we knew that the number of lobes is four so how does this happens? There is another sulcus called parietooccipital sulcus, it separates the parietal lobe (anteriorly) from the occipital lobe (posteriorly). Look to the following figure and examine the arrowed sulcus, this is the parietooccipital sulcus from the lateral view, its mainly appeared on the medial surface and its only one inch one the lateral surface. On the inferior border there is a notch called preoccipital notch (its one inch far from the occipital pole). There is an imaginary line between the parietooccipital sulcus and the preoccipital notch and another imaginary line between the first imaginary line and the lateral fissure, these two lines complete the division of the lateral surface of brain hemisphere into four anatomical lobes. parietooccipital sulcus preoccipital notch Quick summary: The frontal lobe is situated in front of the central sulcus and above the lateral sulcus. The parietal lobe is situated behind the central sulcus and above the lateral sulcus. The occipital lobe lies below the parietooccipital sulcus. The temporal lobe lies below the lateral sulcus. During our previous discussion I emphasized on the word anatomical when we said that the lobes are four, in reality there are two another functional lobes buried inside the brain. These lobes are: - The insula (the 5th lobe; as it does part of the brain functions and it has gyri and sulci) which is deep to the lateral fissure. 5|Page - The limbic lobe (the 6th lobe) on the medial surface of the brain hemisphere, it controls the emotions, the smell function and the sexual activity. Later in this sheet we will dig deeper in them 😊 The following figure is a coronal section and it shows the content of the cerebral hemispheres: Outer grey matter (cerebral cortex) Inner white matter (neural fibers) Basal nuclei (inner grey matter) Lateral ventricle lined with ependymal cells 1 2 3 4 The cerebral cortex is only 2-3 mm in thickness, but it consists 40% of the brain mass, as its folded into gyri and sulci. The cerebral cortex is the most important part because it has the functional areas that control your body (allows for sensation, voluntary movement, self-awareness, communication, recognition and more). The cerebral cortex is composed of six layers as shown in the table below: 6|Page Notes about the table: - There are two type of cells; stellate cells (represented by stars) & pyramidal cells (represented by triangles). The pyramidal cells are more important than the stellate cells. - The molecular layer (1st layer) has NO cells, it only has axons and dendrites that extends from the 2nd layer. - The internal granular layer (4th layer) isn’t important; as it has only stellate cells. - The internal pyramidal layer (5th layer) is the most important; as it has large pyramidal cells (Betz cells) only. There is a tract called pyramidal tract extent from this layer to the brain stem then to the internal capsule and then the spinal cord where it synapses with the anterior horn cells (found inside the spinal cord) and give the motor nerves which supply the muscles. - So, the higher center of all your body movement is the internal pyramidal layer. Each hemisphere has three surfaces: Superomedial border Medial surface Inferolateral border Inferior surface Inferomedial border Note: The hemispheres are separated by a deep cleft, the longitudinal fissure, into which projects the falx cerebri. 7|Page There are also three borders separate the surfaces from each other (they are located between them and named accordingly); look to the red names in the previous figure (will be understood better in the lab). Now we will study the gyri and sulci on the lateral surface: Important sulci & gyri: 1) Central sulcus (of Rolando): Extends from the superomedial border at a point a little behind the midpoint between the frontal & occipital poles. It ends slightly above the middle of the posterior ramus of lateral fissure. Begins on medial surface 2) Lateral fissure (of Sylvius): It begins on the inferior surface (stem) lateral to the anterior perforated substance & curves laterally to reach the lateral surface where it divides into 3 branches: - Anterior ramus: Runs forwards in the inferior frontal gyrus (more horizontal). - Ascending ramus: Ascends in the inferior frontal gyrus (more vertical). - Posterior ramus: Runs backwards & ends by turning upwards in the parietal lobe. So, the lateral fissure is composed of four parts; stem on the inferior surface, ascending ramus, anterior ramus and posterior ramus 3) Parieto – occipital fissure: Between Parietal & occipital lobes. Central sulcus Parietooccipital Ascending ramus fissure Anterior ramus Lateral fissure 8|Page Posterior ramus Now well zoom in the frontal lobe; ** Sulci of the frontal lobe: - The frontal lobe is situated in front of the central sulcus - Precentral sulcus: Parallel to & one finger in front of 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: - Precentral gyrus: Between central & precentral sulci. - Superior & inferior frontal sulci divide the remaining part equally into superior, middle & inferior frontal gyri. Remember when we talked about the parts of the lateral fissure, we said that it gives ascending and anterior rami in the frontal lobe, around these rami and as parts of the inferior frontal gyrus there is three portions called: 1- Orbital: anterior to the anterior ramus 2- Triangular: between the anterior ramus and the ascending ramus 3- Opercular: posterior to the ascending ramus The following drawing summarizes all these sulci & Gyri: Precentral sulcus Superior frontal sulcus Inferior frontal sulcus Lateral fissure 9|Page Central sulcus The Temporal lobe: ** Sulci & Gyri of the Temporal lobe - The temporal lobe lies below the posterior ramus of the lateral fissure. - Parallel and inferior to the posterior ramus there are two sulci: Superior & inferior temporal sulci. - The 2 sulci divide the temporal lobe into 3 gyri: superior, middle & inferior temporal gyri. The following drawing summarizes all these sulci & Gyri: Superior temporal sulcus Inferior temporal sulcus Now we will talk about the two functional lobes mentioned previously; 1- The insula (Island of Reil); the 5th lobe To be able to see it you must open between the two lips of the posterior ramus of the lateral fissure (shown in the following figure). • It lies at the bottom of the lateral fissure. It is conical in shape having a base (surrounded by circular sulcus) & an apex directed inferiorly towards the anterior perforated substance. • The apex is directed anteriorly & inferiorly, while the base is directed posteriorly 10 | P a g e • In its center, there is sulcus centralis insulae divide the insula into two parts: - Anterior part divided into 3-4 short gyri. - Posterior part with one long gyrus which is usually divided near its upper part. The function of insula is related to taste (gustatory area), it’s the higher gustatory center (there are certain nuclei in the brain stem receive the taste sensation and transmit it to insula; that’s way it’s called the higher gustatory center). Also, it plays a role in: fast and slow pains (will be discussed in physiology) & several autonomic functions. Sulcus centralis insulae short gyri Long gyrus The 6th lobe will be discussed after we talk about the medial surface The Parietal lobe It extends between the central sulcus anteriorly and the parietooccipital sulcus posteriorly (remember the two imaginary lines) **Sulci of the Parietal lobe; - Postcentral sulcus: parallel to & one finger behind the central sulcus, sometimes its divided into anterior sulcus and posterior sulcus (not a one continuous sulcus). - Intraparietal fissure (deep sulcus): Begins at the middle of the postcentral sulcus 11 | P a g e These two sulci divide the parietal lobe into three gyri: - Postcentral gyrus: Between the central & postcentral sulci. - Superior parietal lobule (the lobule is smaller than the gyrus): above the intraparietal fissure - Inferior parietal lobule: below the intraparietal fissure and its further divided into:  Supramarginal gyrus: Above the upturned end of the post ramus of lateral fissure (area 40).  Angular gurus: Above the upturned end of superior temporal sulcus (area 39), (in the next lecture Inshallah we will study the brain map & the numbers). These two gyri have an important function in language area, so its important to know their precise locations.  Posterior part: Above the upturned end of the inferior temporal sulcus central sulcus Postcentral sulcus Intraparietal sulcus 12 | P a g e The Occipital Lobe It extends between the parietooccipital sulcus anteriorly and the occipital pole posteriorly. ** Sulci & Gyri of the occipital lobe - Transverse occipital sulcus (lunate) - Lateral occipital sulcus (horizontal): divides the lateral surface of the occipital lobe into a superior and an inferior gyrus. Keep in mind that these areas are present only on the lateral surface of the occipital lobe and they are not present on the medial surface. Now we will discuss the medial surface of the brain hemisphere It can be seen through a sagittal section in the brain - In the center of the brain there is a commissural fiber attach the two hemispheres to each other, this structure is called Corpus Callosum, it’s a marker for the medial surface. - Below the corpus callosum there is another commissural fiber called Fornix. - Between the corpus callosum and the fornix there is a septum cover the lateral ventricle and called Septum Lucidum. - below the fornix you can find the Thalamus & the Hypothalamus 13 | P a g e So, these structures are arranged in sagittal section as following: Corpus Callosum Septum Lucidum Fornix Thalamus Hypothalamus **Sulci of the medial surface - Callosal sulcus surrounds (above) Corpus Callosum (CC), in this sulcus there is an important artery called anterior cerebral artery, this artery supplies all the medial surface and a lesion in it will affect the areas controlled by the affected centers on the medial surface. Lesion in the artery damage in the control center supplied by the artery loss of the function in the organs controlled by this center - Cingulate sulcus runs parallel to CC & terminates by turning upwards to meet the superomedial border and ends above as marginal sulcus. It gives ascending branch above the middle of the body of CC which divides the area above cingulate sulcus into anterior part: medial frontal gyrus & paracentral lobule. - The beginning of the central sulcus (small part of it), the area around it is very important as it has the motor and sensory of the lower limb and the perineum (damage in this area will lead to incontinence, motor & sensory loss for the lower limb). - Parieto-occipital fissure between the parietal & occipital lobes. 14 | P a g e Note: the central sulcus and the parietooccipital fissure are opposite to each other; most of the central sulcus is on the lateral surface and only small part of its beginning is on the medial surface, while for the parietooccipital fissure its mainly found on the medial surface and only one inch of it is present on the lateral surface. - Calcarine sulcus begins near the occipital pole, this sulcus has two legs fused at one pole (V shape) near the posterior end of the corpus callosum; one is the parietooccipital fissure and the other is the calcarine (similar to the foot of the calcar bird). - Subparietal (suprasplenial) sulcus appears as a continuation of cingulate sulcus and lies in the middle of the parietal lobe. **Gyri of medial surface; - Cingulate gyrus lies between CC & cingulate sulcus. - Paracentral lobule: around the beginning of the central sulcus (in front of the precuneus) and has the motor and sensory center of the lower limb and perineum (discussed earlier in page 14). Keep in mind: lesion in the anterior cerebral artery may lead to loss in sensation in the small toe for example by affecting certain areas in the paracentral lobule (will be discussed more in the coming lectures). - Medial frontal gyrus (in the frontal lobe medially), in front of the paracentral lobule. - Cuneus is the wedge area between the parieto-occipital fissure & the calcarine sulcus. - Precuneus lies between the marginal sulcus anteriorly and the parieto-occipital fissure posteriorly. - Lingual gyrus below calcarine sulcus (similar to tongue shape). The following figure illustrate all these structures: 15 | P a g e Ascending Callosal sulcus Central Marginal Parieto - occipital fissure Subparietal sulcus Corpus Calcrine sulcus Cuneus Cingulate sulcus This area is responsible of your personality and behaviour, it’s called prefrontal cortex Finally, we will talk about the limbic loop (6th lobe); It can be seen from the medial surface only Its function -as mentioned previously-: - To controls the emotions, the smell function and the sexual activity. Its named loop based on its shape The limbic loop starts from the subcallosal area and ascend with the cingulate gyrus then to the isthmus, parahippocampal gyrus and finally ends in the uncus 2 1 Follow the numbers 16 | P a g e 3 4 5 17 | P a g e