Lobes of the brain
The brain is a complex organ with many layers and components that play their roles, in one way or another, in almost every function performed by the body. To complicate matters, an otherwise uniform-looking region can contain sub-regions responsible for performing vastly different functions. To simplify the endeavor of learning about the brain as a whole, its structures, and functions, it can be beneficial to take a piece-by-piece approach based on separating and grouping parts of the brain by their embryological origins.
Early on in development, the neural tube forms three outpouchings called the primary brain vesicles. The prosencephalon, mesencephalon, and rhombencephalon will form the forebrain, midbrain, and hindbrain, respectively. Shortly after these form, in the fifth week of gestation, the prosencephalon and rhombencephalon undergo further divisions, such that the prosencephalon gives rise to the telencephalon and diencephalon, and the rhombencephalon gives rise to the metencephalon and myelencephalon. The telencephalon eventually forms the cerebral hemispheres. The diencephalon develops into the thalamus, epithalamus, and hypothalamus and mammillary bodies. The mesencephalon forms the midbrain; the metencephalon forms the pons; and the myelencephalon gives rise to the medulla oblongata.
Approaching the brain by separating it into parts based on embryological origins in those five secondary brain vesicles can simplify the process of learning about neuroanatomy and neuroscience. Focusing on a specific region allows one to understand the more general significance of that region as well as the important details which separate it into its own unique constituents. This article will focus in particular on the derivatives of the telencephalon, which are the cerebral hemispheres, endeavoring to describe their components, the four lobes contained within each hemisphere: the frontal lobes, parietal lobes, temporal lobes, and occipital lobes. The anatomical features and boundaries of the lobes as well as their functions will be discussed.
The frontal lobe
Boundaries & landmarks
In an examination of the brain, it can be enormously helpful to begin by identifying clear anatomical landmarks. One of the most important landmarks of the brain lies on the lateral surface as an infolding known as the central sulcus.
The central sulcus is a longitudinal infolding that begins on the medial surface of the brain. It extends laterally over the superior surface of the brain and continues almost all the way down the lateral surface of the brain to the Sylvian fissure, the lateral cerebral sulcus. This central sulcus identifies the posterior border of the frontal lobe, and the Sylvian fissure demarcates the inferior border of the frontal lobe.
The frontal lobe is considered as all cortex situated anterior to the central sulcus and superior to the Sylvian fissure, encompassing the frontal pole of brain. The frontal lobe is the largest lobe of the brain comprising almost one-third of the hemispheric surface. The most rostral part of the frontal cortex is known as the prefrontal cortex. This region comprises the lateral and medial aspects of frontal lobe, as well as the inferior aspects of the frontal lobe, including the orbital gyri. The prefrontal cortex plays a crucial role in the processing of intellectual and emotional information, including aggression, and facilitates judgement and decision-making.
On the medial surface, the frontal lobe extends down to the cingulate sulcus and consists of the paracentral lobule, an extension of precentral and postcentral gyri. This region plays an important role in bladder control.
The four principal convolutions in the convexity of frontal lobe are i) the precentral gyrus, ii) the superior gyrus, iii) the middle gyrus, and iv) the inferior gyrus.
The precentral gyrus is situated between and parallel to the central and precentral sulci, and is the most posterior structure considered part of the frontal lobe. The precentral gyrus contains the primary motor cortex (Brodmann’s area 4), which is responsible for integrating signals from different brain regions to modulate motor function: each primary motor cortex sends instructions for voluntary movement to the body and limbs on the contralateral (opposite) side.
The primary motor cortex is where the corticospinal tract originates, and neurons within it are arranged somatotopically. This means that, depending on where in the precentral gyrus they originate, they will supply different regions of the body. The foot and leg, for example, receive input from neurons originating in the inner, medial part of the gyrus; whereas the arms, hands, face, tongue, trunk, etc., receive input from neurons originating from the part of the gyrus running along the outer (lateral) surface of the cerebral cortex.
The body parts which are controlled by each part of the motor cortex are typically represented by the homunculus, a cartoon human whose body parts are drawn at sizes proportional to the input they receive from their corresponding areas and neuronal populations within the motor cortex (and sensory information received from the body by the somatosensory cortex is often depicted similarly). This arrangement is continuous from the primary motor cortex, through the internal capsule, to the brainstem, and down the corticospinal tract of the spinal cord.
Rostral (anterior) to the primary motor cortex in the precentral gyrus precentral gyrus is the premotor area, or premotor cortex (Brodmann’s area 6), and the supplemental motor cortex. These regions of cortex occupy the anterior part of the precentral gyrus and posterior part of the three other previously noted convolutions present in the convexity of frontal lobe: the superior, middle, and inferior gyri. These regions receive input from sensory cortex, thalamus, and basal ganglia, and assist in the facilitation of movement in the contralateral side of body. As such, they play an important role in the initiation and sequencing of movements.
Superior frontal gyrus
The superior frontal sulcus is the lateral boundary of the superior frontal gyrus. This sulcus is a deep horizontal sulcus which can be recognized in coronal and horizontal sections. Many shallow sulci are present in the superior frontal gyrus.
Middle frontal gyrus
The middle frontal gyrus is a wide gyrus that lies between the superior and the inferior frontal sulci. The anterior part of the middle frontal gyrus is occupied by a deep sulcus known as the middle frontal sulcus or the intermediate frontal sulcus which divides this part into dorsal and ventral middle frontal tiers.
Inferior frontal gyrus
The inferior frontal gyrus constitutes a large part of the anterolateral prefrontal cortex. It can be divided into three parts: i) the pars opercularis, ii) the pars triangularis, and iii) the pars orbitalis. The pars opercularis and triangularis in the dominant hemisphere (usually left in right handed people) are referred to as Broca’s speech area and is vital for producing the motor component of speech.
The inferior surface of the frontal lobe lies on the superior surface of the orbital part of frontal bone. Near the medial margin of the hemisphere, the olfactory sulcus lies along with the olfactory bulb and tract in it. Medial to this sulcus is the gyrus rectus and lateral to it are present two longitudinal sulci called medial & lateral orbital sulci ,joining together to form an impression of H or a K pattern. Medial orbital gyrus is present between olfactory and medial orbital sulcus.
Encompassing part of the middle and inferior frontal gyri, just rostral to the premotor region, is an area called the frontal eye fields (Brodmann's area 6,8,9), which is responsible for voluntary control of conjugate (horizontal) eye movements.
The medial surface of the frontal cortex is supplied primarily by branches of the anterior cerebral artery; whereas parts of the lateral surface receive blood supply from branches of the medial cerebral artery.
The insular lobe
Boundaries & landmarks
The insula is the part of the cerebral cortex where the temporal, parietal, and frontal lobes meet. Located at the base of Sylvian fissure and buried beneath the temporal lobe, the insula can be viewed when temporal lobe is retracted from cortex. The insula has been linked with the processing and integration of various types of information, including taste sensation, visceral sensation, pain sensation, and vestibular function.
The parietal lobe
Boundaries & landmarks
The anterior border of the parietal lobe is demarcated by the central sulcus, and the inferior border is demarcated by the Sylvian fissure. The parietal lobe extends posteriorly until it meets the occipital lobe, and is involved in the perception and processing of various types of sensory information.
The postcentral gyrus–which is bordered on its anterior side by the central sulcus and on its posterior side by the postcentral sulcus–is the most anterior component of the parietal lobe. Otherwise known as the somatosensory cortex, this region receives and integrates both tactile (related to touch sensation) and kinesthetic (related to sensing body position and movement) information from the body.
Each primary somatosensory cortex occupying the post central gyrus receives sensory input from the ventral posterior lateral and ventral posterior medial thalamic nuclei. This area is comprised of Brodmann area 3,1,2 in the form of narrow strips. Area 3 lies just posterior to the central sulcus and responds to tactile stimuli. The homotypical structure of area 1 and 2, lying posterior to area 3, react to deep stimuli and joint movement. As in the primary motor cortex, the somatosensory cortex is organized somatotopically; and, as such, can also be represented by homunculus. The body image is inverted: the pharynx, tongue and jaws are inferolateral, followed by the face, hand, arm and trunk while the lower limb is superomedial. Most somatic inputs are contralateral, but some are ipsilateral and a few even project bilaterally.
The remainder of parietal lobe can be divided into two main regions: the superior and inferior parietal lobules, which are separated anatomically by the intraparietal sulcus.
Inferior parietal lobule
The inferior parietal lobule (area 39,40) is composed of two gyri, the supramarginal gyrus and the angular gyrus. The supramarginal gyrus lies superior to the posterior aspect of the Sylvian fissure, whereas the angular gyrus lies immediately posterior to the supramarginal gyrus and is associated with posterior part of superior temporal sulcus.
The supramarginal and angular gyri receive auditory and visual input from the auditory and visual cortices respectively, and as such are deeply involved in complex perceptual processes. A region called Wernicke’s area, which is comprised of the ventral regions of supramarginal and angular gyri as well as part of superior temporal gyrus of the left temporal lobe, is vital for the comprehension of spoken language.
Superior parietal lobule
The superior parietal lobule (area 5,7), also known as the somesthetic association area, integrates sensory and motor functions, providing input to the premotor cortex. As such, neurons in this lobule assist with the organization and planning of complex motor functions. This area is also concerned with discriminative aspects of sensation such as qualities of shape, roughness, size, and texture.
Another part of the parietal lobe, the parietal operculum, is not visible on the free surface of the hemisphere: it lies deep to the posterior part of the lateral sulcus, connecting the postcentral gyrus and the anterior part of supramarginal gyrus with the insula.
The medial surface of the parietal lobe is supplied by branches of the anterior and posterior cerebral arteries; whereas the lateral surface of parietal lobe is supplied primarily by branches of the middle cerebral artery.
The temporal lobe
Boundaries & landmarks
The superior border of temporal lobe is demarcated by the Sylvian fissure. The temporal lobe extends ventrally from this fissure to the inferior surface of the cerebral cortex, and also extends posteriorly to temporo-occipital junction.
Superior, middle & inferior temporal gyri
The temporal lobe consists of three main gyri, the superior, middle and inferior temporal gyri, which are visible on the lateral surface and separated by superior and inferior temporal sulci. The temporal lobes receive both auditory signals and visual signals, and as such are responsible for aspects of auditory and visual perception.
The transverse gyri of Heschl, also known as the primary auditory area (area 41, 42), is located on the internal, superior part of the superior temporal gyrus, is a specialized region of cortex primarily responsible for the reception of auditory information. The secondary auditory area (area 22) lies posterior to the primary auditory area in the superior temporal gyrus and receives impulses from the primary auditory area and thalamus. The posterior part of area 22 belongs to the Wernicke’s region in the dominant hemisphere, known as the sensory speech area of Wernicke. Unlike the superior temporal gyrus, the middle and inferior temporal gyri are responsible for aspects of visual perception. The middle temporal gyrus is associated with the perception of movement within the visual field; whereas the inferior temporal gyrus contains the fusiform face area (FFA), which is necessary for face recognition.
The lateral surface of the temporal lobe is supplied primarily by branches of the middle cerebral artery; and the inferior surface of temporal lobe is supplied primarily by branches of the posterior cerebral artery.
The occipital lobe
Boundaries & landmarks
The occipital lobe refers to a region of cortex encompassing the posterior pole of the brain. The occipital lobe lies over the tentorium cerebelli while its medial surface faces the falx cerebri. There is no clear defined sulcus separating the occipital lobe from parietal and temporal lobes; however, it is separated from the other lobes by a theoretical line starting from parieto-occipital fissure and extending to temporo-occipital incisure.
There is significant anatomic variability in the sulci and gyri of this lobe. The two sulci, superior and inferior, on its lateral surface form three gyri: the superior, middle and inferior gyri. While the superior sulcus, also known as the intra-occipital sulcus, is a continuation of intra-parietal sulcus, the inferior occipital sulcus is inconstant and usually difficult to identify.
Superior occipital gyrus
The superior gyrus is the only gyrus in occipital lobe which is clearly defined. Because the middle gyrus stretches between the superior and inferior sulci and covers the major part of the lateral surface, it is sometimes termed the lateral occipital gyrus. There is sometimes an intermediate sulcus, the lateral occipital sulcus, which divides the middle occipital lobe into superior and inferior parts.
Inferior occipital gyrus
The inferior occipital gyrus is indistinct and sometimes forms part of the middle gyrus. At the occipital pole the three gyri merge and are no longer identified as separate. A fissure called the calcarine sulcus begins slightly above the occipital pole just behind the parieto-occipital sulcus. The inferior end of the parieto-occipital sulcus merges with the anterior end of calcarine sulcus to form the anterior calcarine sulcus.
The primary visual cortex covers the banks of the calcarine sulcus. The portion of the medial occipital lobe superior to calcarine sulcus is known as cuneus and the portion inferior to it is known as lingual gyrus. The primary visual cortex (Brodmann area 17, recent nomenclature V1) in the occipital lobes receives special sensory input from the eyes via the optic radiations, and is therefore responsible for integration and perception of visual information.
The primary visual cortex possesses column of cells preferentially responding to specific visual stimuli, such as line orientation. The cortex is retinotopically organized with the upper half of the visual field represented in the cortex inferior to the calcarine sulcus and the lower half of the visual field represented in the cortex superior to the calcarine sulcus. The information from the primary visual cortex is sent to the secondary visual, or prestriate cortex (Brodmann areas 18 & 19), and then to the inferior temporal cortex (Brodmann areas 20 & 21). These secondary visual areas are important for color, motion, and depth perception.
The occipital lobe receives its blood supply primarily from the branches of the posterior cerebral artery.
The limbic lobe
The limbic lobe refers to a region of the cerebral cortex that borders the corpus callosum in the medial aspect of each hemisphere. Structures in this region play influential roles in the modulation of emotions, visceral functions, autonomic functions, hormonal functions, and learning and memory. The structures which comprise the region of cortex considered the limbic lobe are the subcallosal, cingulate, and parahippocampal gyri. The hippocampal formation is also included, the constituents of which are the hippocampus proper, the dentate gyrus, the subicular complex and the entorhinal cortex.
The subcallosal gyrus
The subcallosal gyrus is a narrow lamina in front of the lamina terminalis, behind the para-olfactory area and below the rostrum of the corpus callosum on the medial surface of the hemisphere. It is continuous around the genu of corpus callosum with the supracallosal gyrus.
The subcallosal area is a small triangular region of cortex in front of the subcallosal gyrus separated by the posterior para-olfactory sulcus. It is continuous with the olfactory trigone below it and with the cingulate gyrus above and in front of it. The anterior limit of this area is the anterior para-olfactory sulcus.
The cingulate gyrus
The cingulate gyrus lies directly above the corpus callosum. It continues anteriorly around the genu, blending with subcallosal gyrus. Posteriorly, it wraps around the splenium of the corpus callosum. It has the callosal sulcus as its anterior boundary and the cingulate sulcus as its posterior boundary. Posteriorly, the cingulate gyrus tapers to a narrow isthmus. The band of white matter in the fold of gyrus is known as the cingulum
The cingulate cortex refers to the cortex of the cingulate gyrus, lying deep within the cerebral longitudinal fissure and spanning the corpus callosum like an arc. It commences caudal to the medial prefrontal cortex, and extends inferior to the precentral and postcentral cortices to terminate just rostrally to the occipital lobe. The cingulate cortex plays a vital role in modulating emotion, as well as in visceral motor processes.
The parahippocampal gyrus is a cortical region in the medial temporal lobe that surrounds the hippocampus and lies between the hippocampal fissure and collateral sulcus. Part of the anterior end of the parahippocampal gyrus projects medially, forming a structure called the uncus. The amygdala and hippocampal formation lie deep to parahippocampal gyrus and uncus within the temporal lobe, with the amygdala located anterior to the hippocampus. The parahippocampal gyrus includes areas 27, 28 (the entorhinal cortex), 35, 36, 48, and 49, in addition to the temporal cortical fields.
Anterior cerebral artery syndrome
Anterior cerebral artery syndrome occurs when there is an interruption of the blood flow in one of the anterior cerebral arteries. These arteries supply various regions of the brain–including large parts of the medial surfaces of the frontal and parietal lobes–and as such anterior cerebral artery syndrome can present with with markedly different functional deficits depending on the exact region damaged.
The part of the precentral gyrus–the primary motor cortex–that lies on the medial surface of the frontal lobe provides motor input to the contralateral lower limb. As such, if occlusion or interruption of blood flow results in ischemic damage to this region, an affected patient will typically present with contralateral hemiparesis (weakness or paralysis) of the lower limb. Accordingly, occlusion of both anterior cerebral arteries can result in bilateral hemiparesis of the lower limbs.
The part of the postcentral gyrus–the somatosensory cortex–that lies on the medial surface of the parietal lobe interprets sensory information from the contralateral lower limb. If this region is subjected to ischemic damage, an affected individual is more likely to experience contralateral loss of sensation, including light touch, position, vibration, and proprioception, mainly in the lower limb.
Middle cerebral artery syndrome
Middle cerebral artery syndrome occurs when blood flow within the main trunk of one of the middle cerebral arteries is interrupted. These arteries supply large parts of the lateral surfaces of the temporal and parietal lobes.
Because the lateral surface of the postcentral gyrus in the parietal lobe provides motor input to the contralateral upper extremities and face, ischemic damage to this structure frequently results in contralateral hemiplegia of these body parts.
The lateral surface of the precentral gyrus in the frontal lobe interprets sensory information from the contralateral upper extremities and face. If this part of the cortex is damaged, an affected individual will typically present with contralateral loss of sensation (light touch, position, vibration, and proprioception) in those same regions.
Because the speech centers are also located on the lateral surface of the left hemisphere, they can be damaged in middle cerebral artery syndrome as well. Their involvement can result in aphasia, which can present as varying difficulties with reading, writing, and speech.
Posterior cerebral artery syndrome
Posterior cerebral artery syndrome occurs when there is obstruction of the blood flow through the calcarine branch of the posterior cerebral artery. The calcarine branch supplies the visual cortex in the occipital lobe, the part of the brain necessary for sight.
Ischemic damage to this region can lead to a condition called contralateral homonymous hemianopia (or hemianopsia), which is the loss of vision in the contralateral visual field. For example, if calcarine branch of the left posterior cerebral artery is occluded, an affected individual will typically experience vision loss in the right visual field of each eye. In cases of temporal lobe ischemia, amnesia may result.
The presence of dual blood supply to parts of the occipital lobe means that certain regions of the visual field may be spared from damage when the posterior cerebral artery is occluded: this typically presents as macular sparing, or sparing of the central visual field.
A watershed area is a region of the body where the territories of two or more major arteries overlap. In the brain, watershed areas are wedge-shaped regions supplied by a combination of the distal branches of the anterior, middle, and posterior cerebral arteries. Although these areas have dual blood supply, because they are supplied by distal branches of the main arteries, they are especially prone to ischemia if there is a decrease in systemic blood pressure that reduces flow to these regions. This can lead to “watershed stroke.”
Watershed strokes accounts for approximately 10% of ischemic strokes. Symptoms depend on affected area, and can result in:
- weakness or hemiplegia
- loss of sensation
- confusion or loss of consciousness
- vision loss
Broca’s aphasia, otherwise known as motor aphasia, is associated with damage to Broca’s area in the inferior frontal gyrus of the left hemisphere. It is called “motor” aphasia because affected persons can comprehend language, but they have difficulty with language output, or expression: they struggle with speech production, particularly word repetition and object naming.
Wernicke’s aphasia, also known as sensory aphasia, is associated with damage to Wernicke’s area in the left parietal lobe. It is called “sensory” aphasia because affected persons cannot make sense of language input: they cannot comprehend spoken language, and cannot repeat what is spoken to them. Although their speech remains fluent, it tends to be irrelevant and nonsensical.
Superior parietal lobule damage and Gerstmann syndrome
Damage within the superior lobule of the parietal lobe can result in disturbances including apraxia (difficulties with motor planning) and sensory neglect.
Damage to the angular gyrus region of the parietal lobe of the dominant (left in most) cerebral hemisphere, either as a result of ischemia or trauma, can result in Gerstmann syndrome. This syndrome is characterized by:
- agraphia/dysgraphia, the inability to write
- acalculia, the inability to perform arithmetic functions
- finger agnosia, the inability to recognize one’s own (or someone else’s) fingers
- the inability to distinguish between the right and left sides of the body
The term prosopagnosia refers to as “face blindness", or the inability to recognise faces (despite being able to describe their characteristics). It is a form of visual agnosia. Acquired prosopagnosia is most commonly caused by lesion in the medial occipitotemporal gyrus due to the occlusion of posterior cerebral artery, although damage to the FFA in the inferotemporal cortex may also have this outcome.
Herpes simplex virus (HSV), part of the human herpesvirus (HHV) family, is an enveloped, double-stranded DNA virus. It is most famously recognized for causing sores, with HSV-1 typically associated with the development of sores in the mouth and HSV-2 typically associated with the development of sores in the genital region. HSV-1, however, is also associated with another, dangerous sequela: HSV encephalitis.
Although encephalitis can occur in an infected individual of any age, it more commonly occurs in children and young adults. An affected person may present with mood and behavioral changes, as well as alteration in memory. The infection initiates in the inferior and medial regions of the temporal lobes and in the orbital region of the frontal lobes, and spreads from there, causing extensive destruction of inferior frontal and anterior temporal lobes with what can be severe necrosis and hemorrhage. This temporofrontal encephalitis tends to be apparent on imaging, and is highly suggestive of the diagnosis. Definitive identification of the etiological agent can be made using molecular diagnostics, including nucleic acid amplification tests like polymerase chain reaction (PCR) which has an 80% sensitivity for HSV in cerebrospinal fluid (CSF). Histological examination typically reveals a predominantly lymphocytic perivascular infiltration of immune cells; and eosinophilic intranuclear inclusion bodies called Cowdry type A bodies may also be seen in neurons and glia.
In 1848, a young railroad worker in Vermont by the name of Phineas Gage experienced a horrific accident: premature detonation of explosive powder sent a tamping iron upward into his cheek, through his brain, and right out the top of his skull. Shockingly, for the most part he recovered physically (although he was left blind in one eye); but his personality changed dramatically after the accident. Once a very capable foreman, after the accident he became disorganized, irritable, and even hostile at times.
In its upward trajectory through his skull, the tamping iron damaged the prefrontal cortex of Phineas Gage’s frontal lobes. The changes observed in Phineas Gage after his accident provided the first evidence of the role of the prefrontal cortex in modulating emotion, aggression, judgment and decision-making, linking the prefrontal cortex with personality.