Study guidelines
- 1.
The most important objective is that you become able to recite all the central nervous system items identified in the MRI pictures without looking at the labels.
- 2.
Try to get the nomenclature of the component parts of the basal ganglia into long-term memory. Not easily done!
- 3.
Because of its clinical importance, you must be able to pop up a mental image of the position and named parts of the internal capsule and to appreciate the continuity of the corona radiata, internal capsule, and crus cerebri.
Surface features
Lobes
The surfaces of the two cerebral hemispheres are furrowed by sulci , and the intervening ridges are called gyri . Most of the cerebral cortex is concealed from view in the walls of the sulci. Although the patterns of the various sulci vary from brain to brain, some are sufficiently constant to serve as descriptive landmarks. The deepest sulci are the lateral sulcus (Sylvian fissure) and the central sulcus (Rolandic fissure) ( Figure 2.1A ). These two serve to divide the hemisphere (side view) into four lobes , with the aid of two imaginary lines, one extending back from the lateral sulcus, the other reaching from the upper end of the parietooccipital sulcus ( Figure 2.1B ) to a blunt preoccipital notch at the lower border of the hemisphere (the sulcus and notch are labelled in Figure 2.2 ). The lobes are called frontal , parietal , occipital , and temporal .
The blunt tips of the frontal, occipital, and temporal lobes are the respective poles of the hemispheres.
The opercula (lips) of the lateral sulcus can be pulled apart to expose the insula ( Figure 2.3 ). The insula was mentioned in Chapter 1 as being relatively quiescent during prenatal expansion of the telencephalon.
The medial surface of the hemisphere is exposed by cutting the corpus callosum , a massive band of white matter connecting matching areas of the cortex of the two hemispheres. The corpus callosum consists of a main part or body , a posterior end or splenium , an anterior end or genu (‘knee’), and a narrow rostrum reaching from the genu to the anterior commissure ( Figure 2.2B ). The frontal lobe lies anterior to a line drawn from the upper end of the central sulcus to the trunk or body of the corpus callosum ( Figure 2.2B ). The parietal lobe lies behind this line, and it is separated from the occipital lobe by the parietooccipital sulcus. The temporal lobe lies in front of a line drawn from the preoccipital notch to the splenium.
Figures 2.2 and 2.4 to 2.6 should be consulted along with the following description of surface features of the lobes of the brain.
Frontal lobe
The lateral surface of the frontal lobe contains the precentral gyrus bounded in front by the precentral sulcus . Further forward, superior , middle , and inferior frontal gyri are separated by superior and inferior frontal sulci . On the medial surface, the superior frontal gyrus is separated from the cingulate gyrus by the cingulate sulcus . The inferior or orbital surface is marked by several orbital gyri . In contact with this surface are the olfactory bulb and olfactory tract .
Parietal lobe
The anterior part of the parietal lobe contains the postcentral gyrus bounded behind by the postcentral sulcus . The posterior parietal lobe is divided into superior and inferior parietal lobules by an intraparietal sulcus . The inferior parietal lobule shows a supramarginal gyrus , capping the upturned end of the lateral sulcus, and an angular gyrus capping the superior temporal sulcus.
The medial surface contains the posterior part of the paracentral lobule and, behind this, the precuneus . The paracentral lobule (partly contained in the frontal lobe) is so called because of its relationship to the central sulcus.
Occipital lobe
The lateral surface of the occipital lobe is marked by several lateral occipital gyri . The medial surface contains the cuneus (‘wedge’) between the parietooccipital sulcus and the important calcarine sulcus . The inferior surface shows three gyri and three sulci. The lateral and medial occipitotemporal gyri are separated by the occipitotemporal sulcus . The lingual gyrus lies between the collateral sulcus and the anterior end of the calcarine sulcus.
Temporal lobe
The lateral surface of the temporal lobe displays superior , middle , and inferior temporal gyri separated by superior and inferior temporal sulci . The inferior surface shows the anterior parts of the occipitotemporal gyri . The lingual gyrus continues forward as the parahippocampal gyrus , which ends in a blunt medial projection, the uncus . As will be seen later in views of the sectioned brain, the parahippocampal gyrus underlies a rolled-in part of the cortex, the hippocampus .
Limbic lobe
A fifth, limbic lobe of the brain surrounds the medial margin of the hemisphere. Surface contributors to the limbic lobe include the cingulate and parahippocampal gyri. It is more usual to speak of the limbic system , which includes the hippocampus, fornix, amygdala, and other elements ( Chapter 34 ).
Diencephalon
The largest components of the diencephalon are the thalamus and the hypothalamus ( Figures 2.6 and 2.7 ). These nuclear groups form the side walls of the third ventricle. Between them is a shallow hypothalamic sulcus , which represents the rostral limit of the embryonic sulcus limitans.
Midline sagittal view of the brain
Figure 2.8 is taken from a midline sagittal section of the head of a cadaver, displaying the brain in relation to its surroundings.
Internal anatomy of the cerebrum
The arrangement of the following structures will now be described: thalamus, caudate and lentiform nuclei, internal capsule; hippocampus and fornix; association and commissural fibres; lateral and third ventricles.
Thalamus, caudate and lentiform nuclei, internal capsule
The two thalami face one another across the slot-like third ventricle. More often than not, they kiss, creating an interthalamic adhesion ( Figure 2.9 ). In Figure 2.10 , the thalamus and related structures are assembled in a mediolateral sequence. In contact with the upper surface of the thalamus are the head and body of the caudate nucleus . The tail of the caudate nucleus passes forward below the thalamus but not in contact with it.
The thalamus is separated from the lentiform nucleus by the internal capsule , which is a common site for a stroke resulting from local arterial embolism (blockage) or haemorrhage. The internal capsule contains fibres running from thalamus to cortex and from cortex to thalamus, brainstem, and spinal cord. In the interval between cortex and internal capsule, these ascending and descending fibres form the corona radiata . Below the internal capsule, the crus of the midbrain (cerebral peduncle) receives descending fibres continuing down to the brainstem.
The lens-shaped lentiform nucleus is composed of two parts: putamen and globus pallidus . The putamen and caudate nucleus are of similar structure, and their anterior ends are fused. Behind this they are linked by strands of grey matter that traverse the internal capsule, hence the term corpus striatum (or, simply, striatum ) used to include the putamen and caudate nucleus. The term pallidum refers to the globus pallidus .
The caudate and lentiform nuclei belong to the basal ganglia , a term originally applied to a half-dozen masses of grey matter located near the base of the hemisphere. In current usage, the term designates four nuclei known to be involved in motor control: the caudate and lentiform nuclei, the subthalamic nucleus in the diencephalon, and the substantia nigra in the midbrain ( Figure 2.11 ).
In horizontal section, the internal capsule has a dog-leg shape (see photograph of a fixed-brain section in Figure 2.12 , and the living-brain magnetic resonance image [MRI] ‘slice’ in Figure 2.13 ). The internal capsule has five named parts in horizontal sections:
- 1.
anterior limb , between the lentiform nucleus and the head of the caudate nucleus;
- 2.
genu ;
- 3.
posterior limb , between the lentiform nucleus and the thalamus;
- 4.
retrolentiform part , behind the lentiform nucleus and lateral to the thalamus;
- 5.
sublentiform part (auditory radiation).
