Brain Divisions and Cerebral Hemispheres

Brain Divisions

• The brain can be divided into the forebrain, midbrain, and hindbrain
• The forebrain consists of the telencephalon (cerebrum) and diencephalon
• The hindbrain includes the pons, medulla oblongata, and cerebellum

Regions of the Brain

• The brain consists of the cerebral hemispheres, diencephalon, brainstem, and cerebellum

Cerebrum

• The cerebrum includes the cerebral hemispheres and basal ganglia
• The cerebrum is the largest division of the forebrain
• It is divided into two hemispheres, each with four lobes

Cerebral Hemispheres

• The cerebral hemispheres are two halves of the cerebrum separated by a deep median longitudinal fissure
• The median longitudinal fissure lodges a dural fold called the falx cerebri
• The hemispheres are connected in the depth of the fissure by a bundle of fibers: the corpus callosum

Cerebral Hemisphere Structure

• Cerebral hemispheres include a superficial layer of gray matter called the cerebral cortex
• Deeper to the cortex, axons running to and from the cells of the cortex form the white matter
• Within the white matter lie nuclear masses (caudate, putamen, globus pallidus), collectively known as the basal ganglia
• The cavity of the hemisphere is called the lateral ventricle

Cerebral Cortex

• The cerebral cortex is the outermost layer of gray matter

Gyri, Sulci, and Fissures

• The surface of the cerebral hemispheres are marked by elevated ridges (gyri) separated by grooves (sulci)
• Gyri increase the surface area of the cerebrum, thus increasing the number of neurons in the cortex
• Deeper grooves are called fissures, and they separate larger regions of the brain

Fissures

• The longitudinal fissure divides the two cerebral hemispheres
• The transverse fissure separates the cerebrum from the cerebellum
• The Sylvian/lateral fissure divides the temporal lobe from the frontal and parietal lobes

Lobes of the Cerebrum

• The central sulcus, lateral fissure, and parieto-occipital sulcus divide each hemisphere into lobes
• The cerebrum includes the frontal, parietal, temporal, and occipital lobes, plus the insula
• The insula is sometimes considered the fifth lobe and is deep to the temporal lobe

Frontal Lobe

• The frontal lobe extends from the anterior tip of the brain or frontal pole to the central sulcus
• The frontal lobe plays an integral role in motor functions, memory formation, emotions, decision-making/reasoning, and personality
• Anatomically, the frontal lobe is divided by three sulci to form four gyri
• The precentral gyrus lies between the central and precentral sulci
• The superior, middle, and inferior frontal gyri lie anterior to the precentral sulcus and are divided by the superior and inferior frontal sulci

Functional Areas of Frontal Lobe

• The frontal lobe contains primary motor areas, premotor areas, frontal eye fields, Broca's area, and prefrontal cortex

Primary Motor Area

• Neurons generate motor orders to the voluntary muscles
• Damage leads to muscle paralysis
• Large neurons (pyramidal cells) in these gyri consciously control voluntary skeletal muscle movements
• The body's representation in the motor area is called the motor homunculus, and it is contralateral, inverted, and disproportional
• Muscles on each side of the body represented in the opposite motor cortex
• The body is represented upside down with the head in the inferior part, on the lateral surface
• The hands are small but occupy a large area in the motor cortex
• The chest, abdomen etc are large but are represented in a small area
• The body is represented spatially in the primary motor cortex of each hemisphere in motor somatotopy
• Most neurons in these gyri control muscles in body areas with precise motor control
• Areas with the most control are the face, tongue, and hands
• Damage to localized areas of the primary motor cortex paralyzes the muscles controlled by this area
• If the lesion is in the right hemisphere, the left side is paralyzed and vice versa

Premotor Area

• The premotor area (Brodmann's area 6) stores programs for previous motor activity/experience; related to voluntary movements
• Controls repetitive/patterned motor skills e.g., typing or piano
• Sends activating impulses to the primary motor cortex
• This area is a memory bank of skilled motor activities, and it is involved with motor planning

Frontal Eye Field (FEF)

• Controls conjugate eye movements in the horizontal plane, e.g., tracking a moving target
• Each FEF moves both eyes to the contralateral side
• Unilateral lesions in the FEF impair contralateral saccadic eye movements, and the affected eye may deviate ipsilaterally toward the lesion

Broca’s Area

• Located in the inferior frontal gyrus (IFG) of the dominant, usually left, hemisphere
• Production of written and spoken language, directs muscles of the tongue, throat, and lips during articulation
• Damage causes motor (expressive) aphasia
• The patient knows what they want to say but is unable to express those thought in normal speech
• Has two areas: the triangular and opercular gyri
• The triangular gyrus is located between the anterior and ascending rami of the lateral fissure
• Opercular gyrus is located behind the triangular gyrus
• The orbital gyrus is also a part of the IFG

Prefrontal Cortex

• An extensive region of the frontal lobe anterior to the premotor area
• Concerned with personality, intellect, complex learning abilities, initiative, judgment, foresight, mood, and reasoning
• The inferior frontal gyrus is visibly divided into three parts: the orbital, triangular, and opercular parts
• The orbital part is most anterior and continuous with the inferior surface of the frontal lobe
• The medial surface of the frontal lobe contains extensions of the precentral gyrus, superior frontal gyrus, and rectus gyrus
• The rectus gyrus (Greek for "straight gyrus") is the most medial and extends onto the medial surface of the hemisphere
• Small cortical areas near the corpus callosum's rostrum are part of the limbic lobe
• The olfactory sulcus lies between the rectus gyrus and the orbital gyri, containing the olfactory bulb and tract

Parietal Lobe

• Extends from the central sulcus to an imaginary line connecting the top of the parietooccipital sulcus and the preoccipital notch
• Occupies the area posterior to the central sulcus and superior to the lateral fissure
• Functions to sense and integrate sensations and spatial awareness and perception
• Awareness of body/body parts in space and in relation to each other (proprioception)
• Lateral surface divided by two sulci into three gyri: postcentral gyrus, superior and inferior parietal lobules
• The postcentral gyrus lies between the central sulcus and postcentral sulcus
• The intraparietal sulcus runs posteriorly from the middle of the postcentral sulcus
• The superior parietal lobule (gyrus) is superior to the intraparietal sulcus and the inferior parietal lobule (gyrus) is inferior to it
• Consists of the supramarginal gyrus and angular gyrus
• The supramarginal gyrus caps the upturned end of the lateral fissure
• The angular gyrus caps the superior temporal sulcus

Angular Gyrus

• The supramarginal gyrus is part of the somatosensory association cortex
• Involved in in language processing, visuospatial awareness, social interaction, proprioception, and sensorimotor integration
• The dominant (typically left) angular gyrus divide the pathway between the visual association area and the anterior part of the Wernicke area
• Destructive lesions to it (often considered a part of the Wernicke area)
• Being unable to read (alexia) or write (agraphia) results
• Its also involved in word reading and comprehension, number processing, retrieval, attention, spatial cognition, reasoning, and social cognition
• Supramarginal and angular gyri lesions may cause Gerstmann's syndrome in the dominant hemisphere
• Lesions may manifest as symptoms of acalculia, finger agnosia, right-left disorientation, and agraphia
• Lesions may also manifest as Hemineglect, or unilateral neglect

Functional Areas of the Parietal Lobe

• Includes the primary somatosensory area (postcentral gyrus), somatosensory association area and primary gustatory area
• The primary somatosensory area (postcentral gyrus) for processing tactile and proprioceptive information (Areas 1, 2, & 3)
• Assist with the integration and interpretation of sensations relative to body position and orientation in space
• Assist with visuomotor coordination in the somatosensory association area
• Involved in gustatory perception and taste discrimination in the primary gustatory area (43)
• The main structural components on its medial surface : the medial extension of the postcentral gyrus and the precuneus
• The extensions of the precentral and postcentral gyri are sometimes referred to together as the paracentral lobule, partly in both lobe

Occipital Lobe

• The occipital lobe occupies the small area behind the parieto-occipital sulcus
• It is located deep to the occipital bone of the skull
• The medial surface is divided by the calcarine sulcus into the cuneus and lingual gyrus
• Key functional area in the occipital lobe is the primary visual cortex, which is responsible for visual perception (size, color, light, motion, dimensions, etc.)
• Located on the medial surface of the hemisphere, in the gyri surrounding the calcarine sulcus (Brodmann's area 17)
• Involved in visual association area, which Interprets information acquired through the primary visual cortex
• It relates to visual cognition

Temporal Lobe

• Occupies the area inferior to the lateral fissure
• The lateral surface is divided by superior and middle temporal sulci into three gyri ○ Superior, middle, and inferior temporal gyri
• The inferior temporal gyrus continues onto the lobe's inferior surface
• The inferior surface consists of the broad occipitotemporal (fusiform) gyrus
• The collateral sulcus separates the occipitotemporal (fusiform) gyrus from the limbic lobe
• The occipitotemporal gyrus is partly in the occipital lobe and in the temporal lobe
• The temporal lobes are located on the sides of the brain, deep to the temporal bones of the skull
• It play an important function in Visual information processing, learning, memory, and comprehension of language
• The inferior surface is involved in higher-order processing
• The most medial part of the temporal lobe is involved in complex aspects of learning and memory
• Key function is its key areas. primary auditory cortex and Wernicke's area

Functional Areas of the Temporal Lobe

• The primary auditory cortex (Brodmann's area 41, 42) is responsible for hearing
• Wernicke's (sensory speech) area, partially located in the posterior portion of the superior temporal gyrus (usually the left), is to comprehend language
• The auditory association cortex is located immediately around the primary auditory cortex and includes Wernicke's area
• Wernicke’s area, which is a sensory speech area, is important for language comprehension
• Connected to Broca's area by a bundle of nerve fibers called the arcuate fasciculus
• It receives fibers from the visual cortex (occipital lobe)
• Also receives input from the auditory cortex of the superior temporal gyrus
• It permits one to understand written and spoken language and enables one to read a sentence, understand it, and say it out loud
• Localized in the dominant hemisphere (mainly left), mainly in the superior temporal gyrus, with extensions around the posterior end of the lateral fissure

Parahippocampal Gyrus

• Located in the inferomedial part of the temporal lobe
• Is part of the limbic system
• Plays a role in memory encoding and retrieval
• Is part of the hippocampus and amygdala
• The anterior end of the parahippocampal gyrus hooks backward on itself
• Forms the uncus
• The superior border of the gyrus is the hippocampal sulcus
• Is part of the hippocampus, a differently structured area of cortex
• A compound structure found in the brain

Hippocampal Formation

• A compound structure in the medial temporal lobe of the brain
• Consists of the dentate gyrus, hippocampus proper, and the subiculum
• Helps in memories, spatial navigation and control of attention

Insula

• The gyri in the depth of the lateral fissure are covered by parts of frontal, parietal, & temporal lobes called the opercula
• Its involved in the processing of olfactory and taste stimuli
• Additionally, plays a role in emotional regulation and interoception, empathy, decision-making, speech and articulation, and cognitive functions

Language Areas

• Organized around the lateral fissure
• Includes Broca's area, Wernicke's area, and angular gyrus

Hemispheric Dominance

• This is defined by the localization of speech and mathematical ability in the cerebral hemisphere
• About 90% are right-handed, and the control resides in the left hemisphere
• The remainder are left-handed, and a few are ambidextrous
• In 96% of individuals, speech and understanding of spoken and written language are controlled by the left hemisphere
• Cerebral dominance becomes established in the first few years after birth

White Matter

• Underlies the cortex and contains nerve fibers, neuroglia cells, and blood vessels
• Nerve fibers run in different directions and originate, terminate, or both, within the cortex
• Categorized as association, commissural, and projection fibers, based on origin and termination
• Association fibers unite different parts of the same hemisphere
• Commissural fibers connect corresponding regions of the two hemispheres
• Projection fibers include afferent fibers conveying impulses to the cerebral cortex and efferent fibers conveying impulses away from the cortex

Association Fibers

• Association fibers connect cortical regions of same hemisphere & are short /long fibers
• Short ones connect adjacent gyri
• Uncinate fasciculus helps to connect to motor speech area and inferior surface of frontal lobe to the temporal lobe
• Cingulum within cingulate gyrus connects frontal and parietal lobes to parahippocampal and temporal cortical regionals
• Superior longitudinal fasciculus connects to portions of temporal lobes with frontal part
• Inferior longitudinal fasciculus occipital lobe with temporal lobe connects
• Frontal lobe and temporal lobe connects to occipital with fronto-occipital fasciculus

Commissure Fibers

• Connect corresponding regions of the two hemispheres
• These include the Corpus callosum, which is Rostrum, Genu, Body, Splenium
• Anterior commissure
• Posterior commissure
• Fornix
• Habenular commissure

Projection Fibers

• Are afferent and efferent nerve fibers brainstem of spinal cord/entire cerebral córtex
• Main pass between: caudate nucleus and thalamus lentiform nucleus
• Form a compact band at the brainstem:internal capsule
• Anterior limb internal capsule
• Genu of internal capsule
• Posterior limb: internal capsule
• Fibers radiate superiorly to cortex areas corona radiata