Lateral View of the Brain
IntroductionThe lateral view of the brain shows the three major parts of the brain which are the cerebrum, cerebellum and brainstem. These brain parts are marked with visible gross features like the gyri and sulci of the cerebrum. They are each also divided into subparts or regions for simplified localization of structures, for example the brainstem is composed of the midbrain, pons and medulla oblongata, while the cerebrum is divisible into lobes. Sulci are small grooves; but there are also large grooves called fissures. Fissures divide the cerebral cortex into lobes and also divide the cerebrum into the right and left cerebral hemispheres along the sagittal plane. The fissure involved in this division is called the medial longitudinal fissure.
Also worthy of note regarding the lateral view of the brain is that the structures seen from are specifically referred to as “cortex”, except for the brainstem part. Thus we can see the cerebellar and cerebral cortices. The cortex is the most superficial layer of these brain parts, and make up the largest portion of the brain’s grey matter.
The brainstem is made up of (from above downwards) the midbrain, the pons and the medulla. It is continuous above with the cerebral hemispheres, below with the spinal cord and posteriorly with the cerebellum. The surface of the brainstem is intimately related to the meninges, arteries, veins, and a number of cranial nerves are attached to it.
The medulla is about 3 cm long, 2 cm wide, and is marked on the lateral surface by the 9th to 12th cranial nerves, as well a series of fissures or sulci that divide it into a number of regions. These fissures and regions include the anterior median fissure, anterolateral sulcus and the region between these sulci which is an elevation called the pyramid. There is also a posterolateral sulcus or fissure which defines an area between it and the anterolateral sulcus. This area is an elongated, oval swelling called the olive. Some features of the posterior surface of the medulla can also be seen on the lateral view; for example, the fasciculus gracilis and the fasciculus cuneatus. The 6th to 8th cranial nerves emerge at the junction of the medulla and pons.
The 5th cranial nerve (Trigeminal nerve) emerge from the anterior surface of the pons. The pons shows a convex anterior surface, marked by prominent transversely running fibres. Laterally these fibres collect to form a bundle, the middle cerebellar peduncle.
The 3rd and 4th cranial nerves emerge from the surface of the midbrain. However, one striking feature of the lateral view of the midbrain is the trochlear nerve (CN IV) which emerges from the velum, and then winds round the side of the midbrain to reach its ventral aspect.
The cerebellum (or small brain) weighs about 10% of the cerebral hemispheres and it is about 150 g in the adult. It has a superficial layer of grey matter, the cerebellar cortex, and like other parts of the brain, it is marked by numerous fissures. The cerebellum lies behind the brainstem, and it is separated from the cerebrum by a fold of dura mater called the tentorium cerebelli.
The cerebellum consists of a part lying near the midline called the vermis, and of two lateral hemispheres. It has two surfaces, superior and inferior. The surface of the cerebellum is marked by a series of fissures that run more or less parallel to one another. The fissures sub-divide the surface of the cerebellum into narrow leaf-like bands or folia. The long axis of the majority of folia is more or less transverse. Sections of the cerebellum cut at right angles to the axis have a characteristic tree-like appearance to which the term arbor-vitae (tree of life) is applied. Some of the features of the surface of the cerebellum are deeper than others, and they divide the cerebellum into lobes within which smaller lobule may be recognised. The deepest fissures in the cerebellum are the primary fissure (fissure prima) which runs transversely across the superior surface, and the posterolateral fissure seen on the inferior aspect.
These features divide the cerebellum into three lobes; a part anterior to the primary fissure, the anterior lobe, a part between the two fissures, the posterior lobe (sometimes called the middle lobe). The remaining part is flocculonodular lobe. The anterior and posterior lobes together form the corpus cerebelli.
The cerebral cortex, which is the area of the cerebrum seen at a lateral view of the brain, is about 2-5 mm thick and accounts for about 80% of the brain’s totalling mass. Its total area has been estimated to be about 2000 cm. It is thrown into a complicated series of tortuous folds, the gyri (singular: gyrus) and between these gyri are grooves or indentations called sulci (singular: sulcus).
The highest level of cognitive functions and information processing takes place in the cerebral cortex. Some functions of the cerebral cortex include thinking and reasoning, memory, consciousness, attention, perceptual awareness and language. The cerebral cortex is made up of three large surfaces, namely:
- The superolateral surface
- The medial surface
- The inferior surface
However, only the superolateral surface is seen at the lateral view. These surfaces are clearly defined by three edges called borders of the cerebral hemisphere. They are the:
- superomedial border
- inferomedial border
- inferolateral border
Although the cerebral cortex has surfaces, they are not smooth owing to their embryonic development. These surfaces are characterized by elevations or folds called gyri and depressions or grooves called sulci. These sulci define each gyrus, while the large sulci called fissures (as described above) define and demarcate the cerebral cortex into four major subdivisions called lobes, which are:
- the frontal lobe
- the parietal lobe
- the temporal lobe
- the occipital lobe
These lobes are named according to their relation to bones of the skull. Hence the frontal lobe is the part just right under the frontal bone, right next to the parietal bone is the parietal lobe, the temporal lobe is by the temporal bone and occipital lobe is located in relation to the occipital bone of the cranium.
Additional striking external features of the cerebral cortex are the poles. When the cerebrum is viewed from the lateral aspect, in which three somewhat pointed ends can be recognised. These pointed ends are the poles of the cerebral cortex. These poles, which are also named in relation to the cranial bones, are the:
- frontal pole – anteriorly
- occipital pole – posteriorly
- temporal pole – lying between the frontal and occipital poles, and points forwards and somewhat downwards
The superolateral surface of the cerebral cortex is marked with striking features which divide the cortex into the frontal lobe, temporal lobe, parietal lobe, occipital lobe and the insula, and define these lobes as follows:
The frontal lobe is defined on this surface by a fissure called central sulcus (fissure of Rolando) which runs downwards and forwards towards a second fissure called the lateral sulcus (fissure of Sylvius), which also defines the this lobe. It is further subdivided as follows: a sulcus called the precentral sulcus runs downwards and forwards parallel to and a little anterior to the central sulcus (central fissure). The area between it and the central sulcus is the precentral gyrus. In the region anterior to the precentral gyrus, there are two sulci that run in an anteroposterior direction. These are the superior and inferior frontal sulci. These sulci divide this region into superior, middle, and inferior frontal gyri.
The inferior boundary of the frontal lobe, which is defined by the lateral fissure, has a stem – stem of the lateral sulcus from which an anterior ramus (anterior branch) and an ascending ramus extend into the inferior frontal gyrus to divide it into three parts:
- Pars orbitalis
- Pars triangularis
- Pars opercularis
The part below the anterior ramus is the pars orbitalis, that between the anterior and ascending rami (pleural for ramus) is the pars triangularis (shaped like a triangle), and the part posterior to the ascending ramus is the pars opercularis.
This lobe is defined on the superolateral surface by the lateral sulcus and the inferior temporal sulcus (inferior temporal fissure). It has two sulci that run parallel to the posterior ramus of the lateral sulcus (which is the third branch originating from the stem of the lateral sulcus). These two sulci are termed the superior and inferior temporal sulci, and they divide the superolateral surface of the temporal lobe into the superior, middle and an inferior temporal gyri.
This lobe is defined on the superolateral surface by the central fissure anteriorly, and the parieto-occipital sulcus, posteriorly. The parieto-occipital sulcus is joined to the inferior temporal sulcus by an imaginary line. Inferiorly the lobe is defined by the posterior ramus of the lateral sulcus and an imaginary line that runs posteriorly from this ramus of the lateral sulcus to join the first imaginary line and the inferior temporal sulcus at the point where they meet (1st imaginary line and the inferior temporal sulcus).
Within the parietal lobe, the postcentral sulcus runs downwards and forwards parallel to and a little behind the central fissure; thus defining a gyrus. This gyrus is the postcentral gyrus and is located between the postcentral sulcus and central fissure. The remaining part of the parietal lobe is divided into a superior parietal lobule and an inferior parietal lobule by the intraparietal sulcus – which runs anteroposteriorly.
The inferior parietal lobule is further divided into three parts by the posterior ends of three sulci: the upturned posterior end of the posterior ramus of lateral sulcus, the upturned posterior ends of the superior and inferior temporal sulci. These three parts are the:
- Supramarginal gyrus – which is the part that arches over the upturned posterior end of the posterior ramus of the lateral sulcus
- Angular gyrus – this part arches over the superior temporal sulcus
- Arcus temporooccipitalis – which arches over the posterior end of the inferior temporal sulcus.
This lobe is defined anteriorly by the parieto-occipital sulcus, the first imaginary line and the upturned posterior end of the inferior temporal sulcus. It shows three short sulci. One of these, the lateral occipital sulcus lies horizontally and divides the lobe into superior and inferior occipital gyri. The second sulcus is the lunate sulcus which runs downwards and slightly forwards just in front of the occipital pole and defines a vertical strip just in front of it. This strip is called the gyrus descendens. The third is the transverse occipital sulcus located in the uppermost part of the occipital lobe. The transverse occipital sulcus defines an area just superior to itself. This area is called the arcus parieto-occipitalis (as its name suggests, it belongs partly to the parietal lobe and partly to the occipital lobe).
The arcus parieto-occipitalis arches over the parieto-occipital sulcus – which is actually a fissure of the medial surface but just reaches the superolateral surface.
Insula is a Latin word meaning hidden. As its name implies, it is a part of the cerebral cortex that does not belong to any of the four lobes described above. It is located deep in the stem and posterior ramus of the lateral sulcus.
The surface of the insula is divided into a number of gyri. During development of the cerebral cortex, the insula grows less than surrounding areas which, therefore, come to overlap it and occlude it from surface view. These surrounding areas are called opercula (meaning lids; singular of opercula is operculum), and are three in number:
- The frontal operculum: lying between the anterior and ascending rami of lateral sulcus
- The frontoparietal operculum: which lies above the posterior ramus of lateral sulcus
- The temporal operculum: which lies below the posterior ramus of lateral sulcus. This operculum has a superior surface which is hidden in the depth of the lateral sulcus.
Important functional Areas of the Cerebrum
Certain areas of the cerebral cortex have long been identified with specific functions. These areas can be defined in terms of the gyri and sulci described above. However, various workers who have studied the microscopic structure of the cerebral cortex have found that there is a considerable variation from region to region and that definition of these functional areas is not confined to the boundaries of gyri and sulci, but often cross them. Most of these authors have also worked out “maps” of the cerebral cortex indicating areas of differing structure. The best known scheme is that by Brodmann, who represented different areas of the cortex by numbers.
Motor Area (Primary Motor Cortex)
This is the area corresponding to area 4 of Brodmann and possibly to the part of area 6 which lies in the precentral gyrus. The motor area is located in the precentral gyrus on the superolateral surface, and in the anterior part of the paracentral lobule on the medial surface. This area of the cortex is responsible for initiation of voluntary movement. However, specific regions within the area are responsible for movements in specific parts of the body. Stimulation of the paracentral lobule produces movement in the lower limbs. The trunk and upper limbs are represented in the upper part of the precentral gyrus, while the face and head are represented in the lower part of the gyrus. This concept is referred to as “Homunculus” and basically, it is about how the brain sees the body.
Another feature of interest is that the area of cortex representing a part of the body is not proportional to the size of the part, but rather to the intricacy of movements in the region. Thus relatively large areas of cortex are responsible for movements in the hands or in the lips.
Pre-motor Area (Motor Association Cortex)
This is the area corresponding to areas 6,8,44 and 45 of Brodmann. It is located just anterior to the motor area, occupying the posterior parts of the superior, middle and inferior frontal gyri. The part of the premotor area located in the superior and middle frontal gyri corresponds to area 6 and 8 of Brodmann. The part in the inferior frontal gyrus corresponds to areas 44 and 45, and constitutes the motor speech area (of Broca) or what is also called the anterior speech area (of Broca).
This Broca's area is usually situated in the inferior frontal gyrus on the left side (in right handed and in most left handed people) below and in front of the face area, and centred on the pars triangularis between the anterior and ascending rami of the lateral fissure (see description of frontal lobe). Damage to this area produce motor aphasia – difficulty in finding the right words, but not paralysis of laryngeal musculature. This is because the Broca's area is responsible for speech production and articulation, while the entire premotor area functions to coordinate complex movement.
Closely related to the motor area is two specific areas, one is the Broca's area described above, and the other is the frontal eye field.
The frontal eye field lies in the middle frontal gyrus just anterior to the precentral gyrus. It corresponds to parts of areas 6, 8 and 9 of Brodmann. Stimulation of this area causes both eyes to move to the opposite side. These are called conjugate movements. Movements of the head and dilation of the pupil may also occur. This area is connected to the visual cortex (areas 17, 18, 19 of Brodmann) and also to the thalamus (particularly to the medial dorsal nucleus). The frontal eye field is involved in voluntary eye movements and the accommodation pathway.
Apart from the anterior speech area, there is also a posterior speech area (of Wernicke). This area is in the posterior parts of the superior and middle temporal gyri and extends into the lower part of the parietal lobe. Its integrity is necessary for understanding of speech (language comprehension). It corresponds to areas 22, 39 and 40 of Brodmann.
Sensory Area (Primary Somatosensory Cortex)
This functional area is located in the postcentral gyrus. It corresponds to area 1, 3 and 2 of Brodmann. It extends into the medial surface, from the lateral surface, where it lies in the posterior part of the paracentral lobule.
Just like the motor area, the concept of Homunculus applies on the sensory area. Responses can be recorded from the sensory area when individual parts of the body are stimulated. Mapping of the representation of various parts of the body in the sensory area shows the body is represented upside down. The area of the cortex that receives sensations from a particular part of the body is not proportional to the size of that part, but rather to the complexity of sensations received from it. Thus the digits, lips and the tongue have a disproportionately large representation. Lying in the inferior part of the postcentral gyrus (also called frontoparietal operculum) is another important functional area responsible for the conscious appreciation of taste. This area is called the gustatory area, and it corresponds to area 43 of Brodmann.
Immediately posterior to the somatosensory area is the Sensory Association Cortex. This cortex is responsible for processing of multisensory information.
Auditory (Acoustic) Area
This is the cortex for hearing, situated in the temporal lobe. It is mostly hidden in the lateral sulcus, in the anterior transverse temporal gyrus. It corresponds to areas 41 and 42 of Brodmann. It extends into the superior temporal gyrus below the sulcus, and is here surrounded by the auditory association area (area 22). These regions receive fibres from the medial geniculate body through the auditory radiation. The cochleae are bilaterally represented, so a lesion of one cortex does not cause deafness. The acoustic areas are responsible for processing auditory information and detection of sound quality like loudness and tone.
The areas concerned with vision are located in the occipital lobe, mainly on the medial surface, both above and below the calcarine sulcus (area 17). Area 17 extends into the cuneus, and into the lingual gyrus. Posteriorly, it may extend onto the superolateral surface where it is limited anteriorly by the lunate sulcus. Area 17 is continuous, both above and below with area 18 and beyond this area with area 19. Areas 18 and 19 are described as psychovisual areas (or visual association areas) and are responsible for the interpretation of visual impulses reaching area 17.
Occipital eye field
As stated above under frontal eye field, fibres from the visual areas reach the frontal eye field which is concerned with eye movements. The visual areas are therefore regarded as partly motor in function. This view is substantiated by the fact that movements of the eyeballs and head can be produced by stimulation of areas 17 and 18 which constitutes an occipital eye field. Efferents from the visual areas also reach the superior colliculus, the pretectal region, and the nuclei of cranial nerves supplying muscles that move the eyeballs.
Therefore, as a way of summary, the visual areas can be categorized as follows:
- visual area – area 17 – sensory
- occipital eye field – area 17 & 18 – motor
- psychovisual area – area 18 & 19 – sensory
Prefrontal Areas (Prefrontal Cortex)
The part of the frontal lobe excluding the motor, premotor and motor association cortex is referred to as the prefrontal area. It includes the parts of the frontal gyri anterior to the motor association area, most of the anterior parts of the orbital gyri, most of the medial frontal gyrus, and the anterior part of the gyrus cinguli. This area is concerned with normal expression of emotions, the ability to predict consequences of actions, and complex thoughts. The medial part of the prefrontal area is associated with auditory and visual functions.