Horizontal Sections of the Brain
Previous articles have highlighted the gross structure of the brain and some important grey matter structures. This article intends to show the relationship of these previously described structures in axial (horizontal) planes, similar to those encountered in radiological imaging of the brain. Please note that this article will not describe structures observed at every possible axial plane. The three planes chosen – at the level of the corpus callosum, insula and corpora quadrigemina – demonstrate a variety of unique and overlapping structures that will be briefly discussed.
Level of the Corpus Callosum
An axial section passing through the brain at the level of the corpus callosum reveals mainly the superior lobes of the cerebrum. Additionally, some grey matter structures and parts of the ventricular system can be appreciated.
At this level, the anterior portion of the section is occupied by the frontal lobes. The frontal lobes are located anterior to the central sulcus and superior to the lateral fissure. They occupy the anterior cranial fossa and are divided into four gyri by three sulci.
These lobes are home to the following:
- primary motor cortex (in the precentral gyrus; Brodmann’s area 4)
- premotor cortex (anterior to the primary motor cortex, Brodmann’s area 6)
- frontal eye field (Brodmann’s area 8)
- Broca’s area (Brodmann’s areas 44 and 45)
- part of the cingulate gyrus (inferior to the cingulate sulcus).
Input to the primary motor area is from the somatosensory cortex and indirectly from the cerebellum via the ventral posterolateral nucleus of the thalamus. The major motor pathways project fibers via the corticospinal and corticobulbar tracts.
Segments of the cingulate gyrus can be appreciated anteromedial to the genu of the corpus callosum and posteromedial to the posterior part of the body of the corpus callosum. This gyrus extends from the ventral surface of the genu of the corpus callosum, curves superiorly then continues posteriorly along the dorsal surface of the corpus callosum, back to the splenium of the corpus. The cingulate gyrus is a part of the limbic system, which is involved in regulating emotions, memory and learning.
Parts of the corpus callosum – the genu and the posterior body – can be appreciated in the midline of the section. The genu is located anteriorly and medial to the frontal lobes, while the body is seen posteriorly and medial to the parietal lobes. The corpus callosum is made up of commissural fibers that permit communication between the two cerebral hemispheres. A notable band of commissural fibers that passes through the genu of the corpus is the forceps minor. It permits communication between the medial and lateral parts of the frontal lobes.
The superior portions of the lateral ventricle can be seen in the midline. The frontal horn (occupying the frontal lobe) and body (extending into the parietal lobe) of the right and left lateral ventricles can be seen separated anteroposteriorly by the thin band of septum pellucidum. The ventricles are home to the choroid plexus, which produces cerebrospinal fluid (CSF).
Just lateral to the lateral ventricles, is the head and tail of the caudate nucleus. This grey matter structure is an intricate part of a system of grey matter nuclei responsible for the regulation and production of movement. This system is known as the basal ganglia. The caudate nucleus is a ‘C’ shaped collection of nuclei that lies in close proximity to the lateral ventricle and is located lateral to the internal capsule and the thalamus.
The posterior half of this section is occupied by the parietal lobe. This lobe is limited anteriorly by the central sulcus, inferiorly by the lateral fissure and posteriorly by the parieto-occipital sulcus. It is divided into three gyri by two sulci. The parietal lobe houses the primary somatosensory cortex (Brodmann’s areas 3,1,2) and the sensory association cortex.
Its major afferent fibers arise indirectly from the:
- dorsal column medial lemniscus pathway (relaying fine touch and proprioception)
- the spinothalamic pathway (carrying pain, temperature, coarse touch and pressure perception)
- the trigeminothalamic tract (containing neurones which enter the brainstem in the trigeminal nerve).
Level of the Insula
At the level of the insula (island of Reil) most of the structures encountered previously are still visible. The recurring structures include the frontal lobe, the genu of the corpus callosum, parts of the parietal lobe and a segment of the frontal horn of the lateral ventricle. The following are additional structures that will be encountered in axial sections at the level of the insula:
The head of the caudate nucleus can be observed anteromedially with respect to the anterior limb of the internal capsule and posteromedial to the frontal lobe. The caudate nucleus is part of the basal ganglia, together with other grey matter nuclei, namely the putamen and globus pallidus. The putamen is the largest and most lateral of the three structures. Medial to the putamen is the globus pallidus externa (lateral segment of the globus pallidus), which is larger and darker than the medial segment or globus pallidus interna. The latter also has additional connections to the substantia nigra of the midbrain (not visible at this level).
The internal capsule is an extensive band of white matter fibers that link the cerebral cortex to subcortical structures such as the corpus striatum, thalamus and brain stem. It is made up of an anterior limb, a genu, a posterior limb, a retrolenticular (behind the lentiform nucleus – globus pallidus and putamen) part and a sublenticular part (ventral to the lentiform nucleus; not visible in this section). The internal capsule is essentially a conduit for both ascending and descending fibers. Frontopontine and thalamocortical fibers traverse the anterior limb. The genu contains corticospinal tracts, while the posterior limb has corticobulbar, corticospinal and thalamocortical fibers. The retrolenticular segment carries fibers of the auditory and visual pathways.
The insular lobe, or island of Reil, is typically concealed within the Sylvian fissure (lateral fissure). The lobe has two gyri and two sulci. In the horizontal section, it is located lateral to the extreme capsule – a band of white matter separating the insula from the claustrum. The claustrum is a band of grey matter that permits reciprocal communication between the frontal, parietal and occipital lobes. It also separates the extreme capsule from the external capsule. The external capsule is another band of white matter separating the claustrum from the putamen.
The fornix is formed as superior continuations of the hippocampal fimbriae. Posteriorly, there are left and right crura of the fornix, which fuse and continue anteriorly as the body of the fornix. In the axial section, the left and right fornicial (of or relating to the fornix) crura are located lateral to the quadrigeminal cistern (potential space filled with CSF posterior to the corpora quadrigemina). In this particular section, the post-commissural part of the fornix (just posterior to the anterior commissure and anterior to the interventricular foramen of Monro) can be seen. The fornix is classified as a commissural fibre that connects the hippocampus to the hypothalamus, and is consequently considered as part of the limbic system.
Posteromedial to the trigone of the lateral ventricle is the splenium of corpus callosum. This posterior portion of the corpus callosum houses the forceps major, which connects the left and right occipital lobes. The trigone of the lateral ventricle is the point of trifurcation that gives rise to the frontal, temporal and occipital horns of this particular ventricular system. Each horn occupies the cerebral lobe after which it is named.
Level of the Corpora Quadrigemina
The frontal lobe, trigone of the lateral ventricle, insula and Sylvian fissure, that were described in the last axial section are also visible in the section at the corpora quadrigemina. The corpora quadrigemina refers to two pairs (four) rounded bodies – the superior and inferior colliculi – that are found inferior to the pineal gland. Here are the other structures that can be appreciated at this level:
Cisterns are potential spaces within the cranial vault that are occupied by CSF. In this section, the suprasellar cistern (just superior to the sella turcica and pituitary gland), interpeduncular cistern (between the cerebral peduncles) can be seen in the anterior field, while the quadrigeminal cistern and the superior cerebellar cistern (just superior to the superior surface of the cerebellum and inferior to the tentorium cerebelli) are observed in the posterior field.
Hypothalamus & Mammillary Bodies
The hypothalamus and mammillary bodies are midline diencephalic structures. The former is a slit-like space inferior to the hypothalamic sulcus on the walls of the third ventricle. The latter is a pair of rounded bodies at the posterior extent of the tuber cinereum (floor of hypothalamus) and anterior to the posterior perforated substance in the interpeduncular fossa (just superior to the pons). Both the hypothalamus and mammillary bodies are a part of the limbic system.
In the lateral field, adjacent to the midbrain and the trigone of lateral ventricle, is part of the temporal lobe. The lobe is limited superiorly by the lateral fissure and extends posteriorly to an imaginary line stretching from the apex of the lateral fissure to the margin of the occipital lobe. Two sulci separate the lobe into three gyri. It houses the primary auditory cortex (in superior temporal gyrus) that receives its inputs from the medial geniculate body of the thalamus.
The cerebral peduncles of the midbrain can be seen in the midline of the field with the red nucleus and cerebral aqueduct of Sylvius. The peduncles carry corticospinal and corticobulbar fibers to the rest of the body, while the red nucleus participates in the activities of the basal ganglia and projects both descending and ascending fibers. The cerebral aqueduct is a communicating pathway between the third and fourth ventricles.
The superior colliculus can also be seen extending from the tectum of the midbrain into the quadrigeminal cistern. The superior colliculus acts as a relay station through which fibers of the optic tract pass via the superior brachium to the lateral geniculate body of the pulvinar of the thalamus.
Occipital Lobes & Lateral Ventricles
Finally, the occipital lobe with the occipital horn of lateral ventricle is observed at the posterior aspect of the field. The lobe is situated in the posterior cranial fossa and is limited anteriorly and superiorly by the parieto-occipital sulcus. On the medial surface of the brain, the occipital lobe is also bordered inferiorly by the calcarine sulcus. In addition to housing the occipital horn of the lateral ventricle, it is also home to the visual cortex, superior and inferior to the calcarine sulcus on the medial side of the brain. Three parts can be identified as the primary visual cortex (Brodmann 17), secondary visual association cortex (Brodmann 18) and tertiary visual association cortex (Brodmann 19).