Coronal sections of the brain

Level of the genu of the corpus callosum

A coronal section of the head is viewed and interpreted from the point of view that the clinician is facing the patient. Therefore, the patient’s left side will be on the physician’s right side. The following are important structures seen on a coronal section at the level of the genu of the corpus callosum:

• The frontal lobe occupies majority of the field in this section. Recall that the frontal lobe is the largest of the cerebral lobes and is located in the anterior cranial fossa. Its posterior border is formed by the central sulcus (Rolandic fissure) and it is limited posterolaterally by the lateral sulcus (Sylvian fissure). In the midline of the upper half of this section, the longitudinal fissure (interhemispheric fissure) can be seen dividing the frontal lobes into left and right counterparts. The longitudinal fissure, along with the falx cerebri that runs within the fissure, divides the cerebrum into left and right hemispheres.

• The genu (Latin for knee) of the corpus callosum is observed in the center of the section, medial to the frontal lobes and the frontal (anterior) horns of the lateral ventricles . The genu is the second part of the corpus callosum (preceded by the rostrum) and it is just posteroinferior to the cingulate gyrus (or cingulum). Immediately superior to the genu are the left and right cingulate gyri, which extend anteroposteriorly along the corpus callosum. Superior to the cingulate gyri is the cingulate sulcus, which in this particular section separates the superior frontal gyri (right and left) from the cingulum.

• The temporal lobes are inferior to the Sylvian fissure in this section. Each temporal lobe is divided by a superior and an inferior temporal sulcus into three gyri. Medial to the temporal lobes is an osseous space relevant in neurosurgery. It is the sphenoid air sinus and it is a common surgical route for approaching the pituitary gland.

Level of the lamina terminalis

At the level of the lamina terminalis, the frontal lobe occupies the upper half of the section. The temporal lobes appear larger in this section, and are still inferior to the Sylvian fissure and lateral to the sphenoid sinus. Important structures that we can observe at this level include:

• The body of the corpus callosum appears as a horizontal band of white matter just superior to the frontal (anterior) horns of the lateral ventricles. The left and right frontal horns are separated from each other by the septum pellucidum. The body of the corpus callosum is just inferior to the cingulate gyri.

• The head of the caudate nucleus is located inferolateral to the frontal horns of the lateral ventricle. The caudate nucleus is an important part of the basal ganglia that extends anteroposteriorly along the lateral ventricle forming the lateral wall and floor of the frontal horn.

• The anterior limb of internal capsule separates the head of the caudate nucleus from the putamen. Classically, the caudate nucleus and the putamen are collectively referred to as the striatum, due to bundles of thinly myelinated and un-myelinated axons (Wilson’s pencils) that connect both structures and give them a striped appearance. Both these structures are important components of the basal ganglia, and are involved in the regulation of movement. At this coronal section, the putamen and caudate nucleus coalesce inferomedially to form the ventral striatum, an important component of the limbic system. Most of the ventral striatum consists of the nucleus accumbens which is involved in pleasure, reward, motivation and addiction. The internal capsule is a band of white matter in which important ascending and descending fibers, such as the corticospinal (pyramidal ) tract.
• The optic chiasm corresponds to the decussation of the optic nerves (CN II) and is located in the suprasellar cistern, superior to the diaphragma sellae of the pituitary gland. The suprasellar cistern is a potential space superior to the sella turcica and pituitary gland, while the diaphragma sellae is a sheet of dura mater covering the cranial aspect of the pituitary gland.
• The insula (island) of Reil is located lateral to the basal ganglia and is located deep in the Sylvian fissure, covered by the opercular parts of the frontal and temporal lobes.

Level of the interventricular foramen of Monro

The frontal and temporal lobes are observed in their previously described locations. The body of the corpus callosum forms the roof of the body of the left and right lateral ventricles, which are separated from each other by the septum pellucidum. The insula of Reil and the Sylvian fissure maintain their lateral relationship to the basal ganglia. The novel structures not present in previous sections are the following:

• The fornix (arch in Latin) is responsible for connecting the hippocampus to the other parts of the limbic system . It begins as left and right crura, arising from the respective hippocampal fimbriae. The crura subsequently fuse to form the body of the fornix, which takes a C-shaped (like an arch) form as it courses superior to the dorsum of the thalamus.
• The third ventricle is the continuation of the ventricular system in the brain and is responsible for producing (by way of the choroid plexuses) and conveying cerebrospinal fluid (CSF). In this section, the third ventricle is in the midline, inferior to the body of the fornix and medial to the left and right pallidum (globus pallidus internus and externus). The third ventricle communicates anteriorly with the lateral ventricles (located superolaterally) via the interventricular foramen of Monro and posteriorly with the fourth ventricle via the cerebral aqueduct of Sylvius. The hypothalamus forms the inferolateral aspect of the third ventricle.
• The tail of the caudate nucleus is a posterior continuation of the head, and is located lateral to the body of the lateral ventricles. Like the head of the caudate nucleus, the tail of the caudate is also separated from the putamen and two additional structures – globus pallidus externus and internus – by the internal capsule. The only difference in this case is that the separation is done by the genu of the internal capsule and not its anterior limb. The pallidum and putamen together are referred to as the lentiform nucleus. The globus pallidus internus is found medial to the globus pallidus externus; both are a part of the basal ganglia.

Level of the midbrain (mesencephalon) and pons

The frontal lobe begins to taper off at this level and consequently occupies less of the section. The temporal lobes and the temporal horns of the lateral ventricles are located inferolaterally. The cingulate gyrus and body of the corpus callosum is superior to the body of the lateral ventricle. The body of the fornix is just inferior to the septum pellucidum, as it separates the left and right lateral ventricles. Other notable structures encountered in this section include:

• The thalami are a pair of ovoid diencephalic structures located bilaterally, with respect to the third ventricle. In this section, the thalami are inferior to the body of the lateral ventricle and lateral to the body of the fornix. The thalami act as a relay station for afferent and efferent fibers communicating between the cortex and subcortical and peripheral structures. It is separated from the putamen by the posterior limb of the internal capsule.

• The red nucleus, subthalamic nucleus and substantia nigra are three pairs of nuclei found in the midbrain. The red nucleus and substantia nigra are named for their gross appearance in dissected specimen. The latter is a black (nigra) substance (owing to its high concentration of neuromelanin) that is considered part of the basal ganglia and produces dopamine as neurotransmitter, while the former is a highly vascular, pink structure that has various afferent and efferent communications within the brain. The subthalamic nuclei communicate with the pallidum and substantia nigra via glutaminergic neurons (uses glutamate as neurotransmitter). The midbrain is the most rostral aspect of the brain stem that permits communication between the prosencephalon (forebrain) and the rhombencephalon (hindbrain) via the cerebral peduncles (white matter columns of afferent and efferent fibers). The cerebral peduncles are surrounded by two communicating cisterns, namely the interpeduncular cistern – an intracranial space just inferior to the posterior perforated substance – and the perimesencephalic cistern – just lateral to the cerebral peduncles.
• The pons is caudal to the midbrain and medial to the temporal lobes. It is home to the pontine nuclei, which serve as a relay station between the cerebrum and the cerebellum.

Level of the splenium of the corpus callosum

At the level of the splenium, three cerebral lobes – frontal, parietal and temporal – can be visualized from superomedial to inferolateral. The quadrigeminal cisterns (inferior to the splenium, superior to the cerebellum and posterior to the corpora quadrigemini) and the cisterna magna can be seen inferior to the cerebellum. In addition to these structures:

• The trigone of the lateral ventricle can be seen in open communication with the temporal horn of the lateral ventricle. The trigone is lateral to the splenium of the corpus callosum.
• The fourth ventricle is the most caudal of the cerebral ventricles. Its roof is formed by the cerebellum and its floor is comprised by the pons and the medulla.). In the roof of the fourth ventricle is the superior medullary velum, superior cerebellar peduncles and the anterior cerebellar lobe. Posteroinferiorly to the fourth ventricle is the vermis and the posteroinferior cerebellar lobes. The medial foramen Magendie and the lateral foramina of Luschka provide a route of communication between the fourth ventricle and the cisterns.