Each of the cerebral hemispheres is further divided into 4 lobes: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. The medial temporal lobe structures are considered by some to be part of the so-called limbic lobe.
Briefly, the frontal lobe is distinguished from the parietal lobe posteriorly by the central sulcus (see the image below). The frontal lobe and parietal lobes are divided inferiorly from the temporal lobe by the lateral sulcus. The parietal lobe is distinguished from the occipital lobe by the parieto-occipital sulcus on the medial surface.
Lateral and medial surfaces of cerebrum, showing mLateral and medial surfaces of cerebrum, showing major sulci and gyri.
The cerebrum is further divided into the telencephalon and diencephalon. The telencephalon consists of the cortex, the subcortical fibers, and the basal nuclei. The diencephalon mainly consists of the thalamus and hypothalamus. The telencephalon of the cerebrum is disproportionately well-developed in humans as compared with other mammals.
Cortex and subcortical fibers
The outermost layer of the cerebrum is the cortex, which has a slightly gray appearance--hence the term "gray matter." The cortex has a folded structure; each fold is termed a gyrus, while each groove between the folds is termed a sulcus. Cortical anatomy is discussed in greater detail below.
Below the cortex are axons, which are long fibers that emanate from and connect neurons. Axons are insulated by myelin, which increases the speed of conduction. Myelin is what gives the white appearance to these fibers of the brain--hence the term "white matter."
Limbic system
The limbic system is a grouping of cortical and subcortical structures involved in memory formation and emotional responses. The limbic system allows for complex interactions between the cortex, the thalamus, the hypothalamus, and the brainstem. The limbic system is not defined by strict anatomic boundaries but incorporates several important structures. The limbic structures conventionally include the amygdala, the hippocampus, the fornix, the mammillary bodies, the cingulate gyrus, and the parahippocampal gyrus.
The functional connections within the limbic system are best summarized by the Papez circuit. From the hippocampus, signals are relayed via the fornix to the mammillary bodies and via the mammillothalamic tract to the anterior nucleus of the thalamus. The thalamocingulate radiation then projects to the cingulate gyrus and back to the hippocampus to complete the circuit. The hippocampus serves as a primary output structure of the limbic system.
Unlike the 6-layered neocortex, the hippocampus only has 3 layers and is termed the archicortex. The hippocampus is felt to be a structure that is crucial to formation of memory--more specifically, a type of memory called declarative or explicit memory. Declarative memory is essentially the ability to recall life events of the past such as what meal was eaten for breakfast or where the car is parked.
Over time, however, certain declarative memories from the distant past can be independently recalled without the hippocampal structures. The hippocampus likely allows long-term memory encoding in the cortex and allows short-term memory retrieval. In laboratory studies of animals and humans, the hippocampus has been shown to also have a cellular memory termed "long-term potentiation."
The amygdala is a collection of nuclei that lies within the uncus. It receives multiple modes of sensory information as inputs. The outputs from the amygdala travel through the stria terminalis and the ventral amygdalofugal pathway. Output structures include the hypothalamus, as well as the thalamus, hippocampus, brainstem, and cortex. The amygdala appears to be involved in mediating the emotional aspects of memory, especially the subjective aspects of fear responses.
Basal nuclei (ganglia)
The basal nuclei (formerly referred to as the basal ganglia) comprise the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. Pairs of these structures bear different names. The putamen and globus pallidus combined form the lentiform nuclei. The putamen and caudate nucleus combined form the striatum. The striatum derives its name from the striped appearance given by the gray matter connections bridging across the internal capsule. The basal nuclei are closely integrated with the motor cortex, premotor cortex, and motor nuclei of the thalamus and plays a crucial role in modulation of movements.
The primary input to the basal nuclei is from the primary motor cortex and premotor cortex (Brodmann areas 4 and 6) and consists primarily of the pyramidal cells in cortical layer V. These excitatory projections lead primarily to the striatum. The striatum also receives input from the dopaminergic cells of the substantia nigra. In turn, the striatum sends inhibitory projections to the globus pallidus externa and interna. The globus pallidus externa sends inhibitory projections to the subthalamic nucleus, which sends excitatory projections to the globus pallidus interna. The globus pallidus interna in turn projects to the ventral anterior and ventral lateral nuclei of the thalamus.
Certain movement disorders can be traced to pathologies in the basal nuclei, the most notable being Parkinson disease, which is related to deficiencies of dopaminergic cells of the substantia nigra. Huntington disease is a heritable disorder that involves degeneration of the striatum and leads to progressive jerky, or choreiform, movement.
Thalamus
Positioned between the brainstem and the telencephalon, the diencephalon is composed of the thalamus, the epithalamus, the subthalamus, and the hypothalamus. The thalamus serves as a relay station for ascending input to the cortex and receives information from each of the cardinal senses (except smell). It is hypothesized that the thalamus serves a gating function in filtering information. The thalamus consists of multiple nuclei that are briefly described here (see the image below).
Major nuclei of thalamus. Major nuclei of thalamus.
Left and right sides of the thalamus are divided by the third ventricle. Each side is then divided by the internal medullary lamina into a series of anterior nuclei, ventrolateral nuclei, and medial nuclei. Smaller nuclei are found within these regions, numbering perhaps in excess of 100.
The anterior thalamic nuclei are functionally associated with the limbic system and share reciprocal connections with the cingulate gyrus and the mammillary bodies. The medial nuclei project to the frontal association cortex and premotor cortex, with reciprocal connectivity.
The ventrolateral nuclei can be further divided into the ventral anterior (VA), ventral lateral (VL), ventral posterolateral (VPL), and ventral posteromedial (VPM) nuclei. The VA and VL nuclei share input from the globus pallidus and projections to the motor cortex. The VPL and VPM serve as sensory relays in the body and face, respectively.
The lateral nuclei are divided into lateral dorsal and lateral posterior nuclei, with projections to the cingulate gyrus and parietal cortex, respectively.
Other thalamic structures not included in the anatomic divisions above include the medial and lateral geniculate bodies, which process auditory and visual information, respectively. The pulvinar connects reciprocally with the parietal and occipital association cortex. Intralaminar nuclei within the internal medullary lamina obtain input from the brainstem, cerebellum, and other thalamic nuclei and project to basal nuclei structures and other thalamic nuclei. Amongst the intralaminar nuclei, the centromedian nucleus is a part of the reticular activating system, which plays a role in maintaining cortical arousal.
Epithalamus
The epithalamus is made up of the habenula, the habenular commissure, the posterior commissure, and the pineal gland.
Subthalamus
Located between the midbrain and the thalamus, the subthalamus contains the subthalamic nucleus, the red nucleus, and the substantia nigra. Subthalamic structures are closely integrated with the basal ganglia and play a role in modulation of movement.
Hypothalamus
Thy hypothalamic nuclei lie in the walls of the third ventricle anteriorly. The hypothalamus is involved in mediating endocrine, autonomic, visceral, and homeostatic functions. It can roughly be divided into anterior, posterior, and middle groups of nuclei.
The anterior nuclei include the preoptic, the supraoptic, and paraventricular nuclei. The posterior nuclei include the supramammillary nucleus, the mammillary nucleus, the intercalate nucleus, and the posterior nucleus. The middle nuclei include the infundibular, tuberal, dorsomedial, ventromedial, and lateral nuclei.
Parasympathetic control can be attributed to the anterior and medial nuclear groups, whereas sympathetic control can be attributed to the posterior and lateral nuclear groups. Satiety can be localized to stimulation of medial nuclei, and hunger can be localized to stimulation of lateral nuclei. Other functions of the hypothalamus include regulation of body temperature, heart rate, blood pressure, and water balance.
The hypothalamus has close connections with the cingulate gyrus, frontal lobe, hippocampus, thalamus, brainstem, spinal cord, basal ganglia, and pituitary gland.
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