The lateral sulcus is a deep groove on the inferior flank of each cerebral hemisphere of the brain. The depth of this cerebral cortex cleft is vast and partitions the parietal and frontal lobes entirely from and temporal lobe. This sulcus is referred to as the Sylvian fissure, named after 17th-century neuroanatomist Franciscus Sylvius.
The lateral sulcus is a significant cerebral feature. It’s on each side of the brain’s hemispheres. This immensely deep groove has characteristic bulges that fold over and obscure it- called opercula. The frontal, temporal, and parietal operculum. The insular lobe resides deep in the lateral sulcus.
Studying the lateral sulcus is crucial. Reasons include understanding the biological progression and cranial architecture of the human brain. In addition, this predominant fissure is a major landmark of the brain's development in utero. The cortical surface of the brain (sulci and gyri) has distinct characteristics that other mammalian brains don’t. This indicates higher levels of cognitive capacity.
The Lateral Sulcus in Utero
The biological development of the lateral sulcus and cerebral cortex surface in utero reveals the brain's exceptional cytoarchitecture and the human evolution of the neocortex. The lateral sulcus is intricate, and its developmental sequence allows neurological psychologists insight into its form, function, and cranial influence.
Examination of fetal sulcus maturation is extensive. Generally, fetal cortical development is complex, highly involved, and incredibly organized. Ultrasounds are used to observe the in utero growth of the lateral sulcus. In utero investigations disclose the gene expression profile of the human brain.
Initially, from conception to about 14 weeks, the brain’s surface is smooth. The cortical progression in utero is a vigorous and energetic process. The Sylvian fissure is realized at 22 weeks of gestation and is a significant milestone in fetus cortical development. The width of the Sylvian fissure steadily grows in accordance with the fetus's maturation.
The relevance of examining gyri/sulci development lies in discerning cranial patterns for diagnoses, assessment, and insight. Generally, the cerebral cortex convolutions develop predictably. However, variations among individuals do occur. Therefore, the estimations of scheduled sulci/gyri appearances have a wide margin of error.
Brain surface anatomy reveals the immense migration of glial and neurons from the germinal matrices to the cortical surface during in utero maturation. This neuronal network expansion is unique to humans. In addition, the realization of surface cortical convolutions reveals a greater surface area per unit in contrast to the smooth agyric cortex that other mammalian brains exhibit.
The Lateral Sulcus- Sulcus Versus Gyrus
A cerebral sulcus (plural sulci) is a groove or indent on the brain’s cortical surface. Every sulcus encloses a gyrus (gyri) that appears folded and wrinkled. The larger sulci are referred to as fissures. However, generally, the term sulcus and fissure are used interchangeably. The sulci and gyri increase the brain’s surface area.
The cerebral cortex (the brain’s surface) features countless folds of sulci and gyri. These convolutions are the basic units that determine our cerebral cortex's sophisticated and complex capacity. The indentations and ridges increase the brain’s surface, allowing neurons to grow and accumulate. Therefore, more information can be processed and stored.
Any deficiencies or alternations cause abnormal cognitive, emotional, or behavioral functioning. Malformation results in disorders realizing symptoms like delayed cognitive responses, cerebral palsy, and seizures. Sonography is used to recognize and diagnose cerebral surface deformity.
The ridges form boundaries and divide the brain into distinct cranial sections. Specifically, the brain is partitioned into two cerebral hemispheres- the right and left. These are divided by the longitudinal fissure. Cranial divisions reveal the increased sophistication and intricacy of the human brain and its adaption via strategic organization.
The Lateral Sulcus Location
The lateral sulcus is a comparatively salient cortical sulcus. It's the deepest fissure and the most discernable cranial feature. It divides the frontal and parietal lobes (superiorly) from the temporal lobe (inferiorly) in each cranial hemisphere. It resides within proximity to the anterior perforated substance. The insular lobe is generally unknown, as the lateral sulcus and opercula cover it.
In the brain’s left hemisphere, the lateral fissure is longer and lies amid the Broca area and Wernicke area. The difference in length is due to the Yakovlevian torque. This refers to the brain’s natural inclination to bulge slightly forward in the right hemisphere and, to some extent, backward in the left. Therefore, the lateral sulcus is longer and not as warped on the left.
The Sylvian sulcus begins close to the basal forebrain and continues to the lateral surface. The insular cortex is situated deep within the lateral sulcus. However, this ‘fifth’ lobe cortex isn’t readily observable, as the lateral sulcus bulges into the opercula, which envelops and obscures this inner cranial lobe.
The Structure of the Lateral Sulcus
The Sylvian fissure is found within proximity to the Sylvian point. The arachnoid mater overlays the fissure. The arachnoid membrane coating creates the Sylvian cistern (insular cistern). This subarachnoid space belongs to the deep layer of the lateral sulcus and consists of vital vascular structures immersed in cerebrospinal fluid (CSF).
The Sylvain cistern or insular cistern consists of several arteries, including the clinically significant middle cerebral artery, frontal-orbital veins, and tributaries of the basal vein of Rosenthal. It relays information with the carotid cistern in a medial direction.
It’s vital to examine the Sylvian fissure, arachnoid mater, and Sylvian cisterns to diagnose various pathophysiological connections like schizophrenia. In addition, the relevance of the lateral sulcus microneurosurgery presents the structure of neurological diseases and improved patient prognosis.
The lateral sulcus is about 1.5 inches long and consists of two components- superficial and deep. It’s noticeable within these spheres- between the frontal, parietal, and temporal lobes. However, the posterior parietal and temporal lobes continue without a discernable border.
The Superficial Layer of the Lateral Sulcus
The superficial aspect of the lateral sulcus is discernable on the encephala’s lateral and basal hemispheres. The superficial aspect is complex and apparent on the brain’s surface. It’s partitioned into a stem and three rami: the anterior horizontal ramus, the anterior ascending ramus, and the posterior ramus.
The stem is 1.53 inches long and aligns laterally along the anterior perforated surface. It then travels anteriorly, between the temporal pole and the orbital gyri, surfacing laterally. The stem’s structural relationship with the sphenoid bone’s lesser wing is noteworthy. It’s partitioned into the three rami (mentioned above).
The anterior (also referred to as the horizontal) and the anterior ascending ramus partition the inferior frontal gyri (on the frontal lobe’s lateral surface) into three distinct areas: pars orbitalis, pars triangularis, and pars opercularis.
The ascending ramus (also referred to as anterior ascending ramus or vertical ramus) divides the pars triangularis from the pars opercularis. The anterior (horizontal) ramus divides the pars orbitalis from the pars triangularis.
The posterior ramus is 2.95 inches and travels posteriorly as an extension of the lateral fissure. The posterior ramus partitions the frontoparietal operculum (upper) from the temporal operculum (lower). This ramus terminates in the inferior parietal lobule, shrouded by the supramarginal gyrus- at its endpoint. It’s the longest ramus.
The Deep Layer of the Lateral Sulcus
The deep aspect of the lateral sulcus is found beneath the brain’s surface and is also referred to as cisternal. It’s known as the Sylvian cistern. Two facets comprise the deeper part- the sphenoidal (anterior) and operculoinsular (posterior). The anterior perforated substance envelops the sphenoidal compartment.
The Lateral fissure’s sphenoid compartment is a thin layer between the temporal and frontal lobes. The sphenoid compartment shares a common border with the carotid cistern. It stretches along the middle cerebral artery (M1 segment) and terminates at the limen insulae.
The sphenoidal compartment is covered by the anterior perforated substance and the posterior aspect of the frontal lobe’s basal surface. It resides above the anterior area of the planum polare. The sphenoidal compartment connects medially through the Sylvian vallecula (tubular opening) to the subarachnoid spaces around the carotid artery and the optic nerve.
The operculoinsular compartment is realized from the opercular and insular clefts. This lateral sulcus feature aligns posterior to the limen insulae- the opercular and insular clefts are subdivisions of the operculoinsular.
The Lateral Sulcus- Opercula
There are cranial features that bulge around the lateral sulcus. Areas include parts of the frontal, temporal, and parietal lobes- they swell and envelop the lateral sulcus. These protrusions are referred to as opercula. The operculum (Latin for ‘little lid’) wraps around the insular cortex. Therefore, the opercula obscure the insular cortex from an external view.
The opercula are partitioned into three cortical areas: the frontal, parietal, and temporal operculum. The frontal operculum involves articulation, cueing phonological stimuli for movement, value coding of social behavior (expectancy and beliefs), and the taste cortex composition.
The parietal operculum resides among the inferior aspect of the postcentral gyrus and the posterior rami of the lateral sulcus. The frontal and parietal areas are collectively referred to as the frontoparietal operculum. The temporal operculum is directly inferior to the lateral sulcus. It’s structured by the superior temporal and traverse temporal gyri (Heschl’s gyri).
The Function of the Opercula
The opercula operate according to the place along the brain they occupy. Therefore, the location of each portion determines its function. Generally, each operculum's role is revealed from lesions to each section observed and documented by individual case studies.
The Lateral Sulcus- The Frontal Operculum
The frontal operculum is directly related to the prefrontal association cortex. Its alignment across the lateral fissure includes the pars triangularis and opercularis of the inferior frontal gyrus. The frontal operculum is a significant feature in a neuronal system that administers cognitive control and strategies for socially cued behavior.
The frontal operculum has critical speech and language neuronal connections. This important lateral sulcus feature includes Broca’s area. Broca’s area resides immediately above the lateral sulcus, along the third frontal convolution, and anterior to the motor cortex. This cortical region is responsible for speech- production, articulation, comprehension, and interpretation.
Case studies investigating lesions to the frontal operculum reported an evident connection between speech aphasia and the lesioned region. The lesions reveal the relevant role of the frontal operculum in speech production and language systems.
The frontal operculum also comprises the taste cortex. The frontoparietal operculum (explained below) administers the gustatory cortex. This cortical region is tasked with regulating taste discrimination and assessing taste quality. Stimuli, registered by both the taste and gustatory cortex, are consolidated by other features like olfactory and somatosensory properties.
The frontal operculum originates at the Sylvian fissure’s anterior ramus and continues to the inferior aspect of the precentral gyrus. The precentral gyrus is a major feature of voluntary bodily motion. This particular site relays facial movement.
The frontal and parietal operculum together form the frontoparietal operculum. The frontoparietal operculum consists of inferior aspects of the precentral (pars opercularis) and postcentral gyri. This region enables motor function and processing of somatosensory sensations.
Seizures characteristic of the frontoparietal operculum are evident in the rapid tightening and slackening facial movements of the mouth and tongue (clonic). Other symptoms include struggling to articulate, swallow, chew, and hyper-salivation. Frequently, gustatory hallucinations are reported.
The Lateral Sulcus- The Parietal Operculum
The parietal operculum contains the secondary somatosensory cortex- situated on the upper verge of the lateral fissure. The location of the secondary somatosensory cortex is intrinsically linked to the function of its relative anatomical cranial structures. However, the extent of its role remains mostly undiscovered.
Investigating the parietal cortex of the Sylvian fissure with electrical stimulation revealed the secondary somatosensory cortex homologous connection with varied sensations. The different sensations include visceral, tactile, and pain perception.
Observing the macaque monkey’s parietal operculum- it's noted that the anatomical structure can be partitioned into two distinct features. These two aspects are differentiated by neuronal network connectivity and their architecture. However, there are four cytoarchitectonic areas identified in the human parietal operculum.
The primary somatosensory cortex also referred to as the parietal ventral area, resides adjacent to the secondary somatosensory cortex. This area lies within the posterior of the parietal operculum. Deeper within the lateral sulcus resides the ventral somatosensory area. The parietal operculum exhibits Brodmann areas 40 and 43.
The parietal ventral area and the second somatosensory cortex’s specialized neurons receive input from an extensive range of the body’s surface. These distinct neurons have large receptive fields. They react immediately to stimuli that will elicit and influence neuron firing. A good example is applying cream over a broad area of the skin's surface.
The parietal operculum occupies an interesting role in history. Albert Einstein’s brain, preserved after his death, was reported not to possess a parietal operculum in either hemisphere. It was believed that this vacancy facilitated increased communication and development of specific cranial features- that relay logic processes. However, the absent operculum remains disputed.
The Lateral Sulcus- The Temporal Operculum
The temporal operculum is an extension of the superior temporal and transverse temporal gyri (Heschl’s gyri). It aligns inferior to the lateral sulcus. Afferent auditory projections terminate in the temporal operculum, which also receives neuronal connections from the medial geniculate nucleus of the thalamus.
Heschl’s gyrus features the primary auditory complex. This is a challenging area for neurologists and neuropsychologists to examine, as measuring the direct neuronal connections has inherent restrictions. Therefore, the operational makeup and degree of performance remain unclear.
The transverse temporal gyrus is where a person’s inner dialogue or inner speech exists. Specifically, spontaneous, unprompted inner speech elicits neuronal activation from Heschl’s gyri- shown by functional Magnetic Resonance Imaging (fMRI).
Heschl’s gyri are different from other gyri as they travel in the direction of the brain’s center (mediolaterally), while other temporal lobe gyri travel front to back (anteroposteriorly). The transverse temporal gyrus lies superior to the planum temporale- a cortical area responsible for synthesizing language.
The Insular Cortex/Lobe
The insular cortex, or the ‘Island of Reil’, is partitioned into specialized physiological divisions. The insular lobe is hidden deep within the lateral fissure, obstructed from surface view. The subdivisions of the insular cortex are characterized by individualized sulcal landmarks, cytoarchitectonics, and connectivity.
The function of the insular remains largely unknown due to its location. Access is challenged by the lateral sulcus. Essentially, the insular cortex enables visceral, motor responses and somatic sensory reactions. In addition, it operates to serve cognitive capabilities like consciousness, decision-making, and emotions.
The insular lobe is surrounded by the peri-insular sulcus and various insular gyri. Essentially, it’s been observed to support the paralimbic processes. It was traditionally described as an aspect of the paralimbic cortex.
Conclusion
The lateral sulcus is a deep groove on each flank of the cerebral hemisphere. It completely divides the temporal lobe from the frontal and parietal lobes. This major fissure is a predictable and significant milestone of a fetus’s cranial development in utero. The lateral sulcus has two components- The superficial and deep.
The lateral sulcus covers the insular cortex. Some neurological scholars acknowledge it as the fifth lobe. It plays a significant role in our paralimbic processes. The insular cortex is enveloped and obscured by the Lateral fissure’s opercula: the frontal, parietal, and temporal.
