The cerebrum is the largest part of the brain. It is divided into right and left hemispheres. The corpus callosum is the collection of white matter fibers that connect these hemispheres to each other.
Each of the cerebral hemispheres is divided into 4 lobes: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. The structures of the medial temporal lobe are considered by some to be part of what is known as the limbic lobe.
Briefly, the frontal lobe is distinguished from the parietal lobe posteriorly by the central sulcus (see figure below). The frontal lobe and parietal lobe are separated inferiorly from the temporal lobe by the lateral sulcus. The parietal lobe is distinguished from the occipital lobe by the parieto-occipital groove on the medial surface.
Lateral and medial surfaces of the brain with large sulci and gyri.
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The brain is further divided into the telencephalon and the diencephalon. The telencephalon consists of the cortex, subcortical fibers, and basal ganglia. The diencephalon consists mainly of the thalamus and hypothalamus. The telencephalon of the brain is disproportionately well developed in humans compared to other mammals.
Cortex and subcortical fibers
The outermost layer of the brain is the cortex, which appears slightly gray, hence the term "grey matter." The bark has a folded structure; Each fold is called a gyrus, while each groove between folds is called a groove. Cortical anatomy is discussed in more detail below.
Beneath the cerebral cortex are axons, which are long fibers that radiate from and connect neurons. Axons are insulated by myelin, which increases conduction velocity. Myelin is what gives these brain fibers their white appearance, hence the term "white matter."
The limbic system is a grouping of cortical and subcortical structures involved in memory formation and emotional responses. The limbic system enables complex interactions between the cortex, thalamus, hypothalamus, and brainstem. The limbic system is not defined by strict anatomical boundaries, but rather includes several important structures. Limbic structures conventionally include the amygdala, hippocampus, fornix, corpus mammary, cingulate gyrus, and parahippocampal gyrus.
The functional relationships within the limbic system can best be summarized by the Papez circuit. Signals are transmitted from the hippocampus through the fornix to the mammary bodies and through the mammillothalamic tract to the anterior nucleus of the thalamus. The thalamocingulate radiation is then projected to the cingulate gyrus and back to the hippocampus to complete the circuit. The hippocampus serves as the primary output structure of the limbic system.
Unlike the 6-layered neocortex, the hippocampus has only 3 layers and is called the archicortex. The hippocampus is believed to be a crucial structure for memory formation, more specifically, a type of memory called declarative or explicit memory. Declarative memory is essentially the ability to recall past life events, e.g. For example, what was eaten or where the car was parked.
However, over time, certain declarative memories from the distant past can be independently evoked without hippocampal structures. The hippocampus probably facilitates long-term memory encoding in the cortex and short-term memory retrieval. The hippocampus has also been shown to have a cellular memory called "long-term potentiation" in laboratory animal and human studies.
The tonsil is a collection of nuclei located in the uncus. It receives various types of sensory information as inputs. The outputs of the amygdala travel through the stria terminalis and the ventral amygdalofugal pathway. Output structures include the hypothalamus, as well as the thalamus, hippocampus, brain stem, and cortex. The amygdala appears to be involved in mediating the emotional aspects of memory, particularly the subjective aspects of fear responses.
The basal ganglia (formerly known as the basal ganglia) include the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. Pairs of these structures have different names. The putamen and globus pallidus unite to form the lentiform nuclei. The putamen and the caudate nucleus unite to form the striatum. The striatum derives its name from the striated appearance given by the junctions of gray matter that traverse the internal capsule. The basal ganglia are tightly integrated with the motor cortex, premotor cortex, and motor nuclei of the thalamus and play critical roles in modulating movement.
The main input to the basal ganglia is from the primary motor cortex and premotor cortex (Brodmann areas 4 and 6) and consists mainly of the pyramidal cells in cortical layer V. These excitatory projections give rise mainly to the striatum. The striatum also receives information from dopaminergic cells in the substantia nigra. The striatum, in turn, sends inhibitory projections to the external and internal globus pallidus. The external globus pallidus sends inhibitory projections to the subthalamic nucleus, which sends excitatory projections to the internal globus pallidus. The internal globus pallidus, in turn, projects to the ventral anterior and ventral lateral nuclei of the thalamus.
Certain movement disorders can be attributed to pathologies in the basal ganglia, the most notable of which isParkinson's disease, which is related to the lack of dopaminergic cells in the substantia nigra.Huntington's diseaseIt is an inherited disorder that involves degeneration of the striatum, resulting in progressive choreiform or jerky movements.
The diencephalon lies between the brainstem and the telencephalon and is made up of the thalamus, epithalamus, subthalamus, and hypothalamus. The thalamus serves as a relay station for ascending input to the cortex, receiving information from each of the cardinal senses (except smell). The thalamus is believed to have a blocking function in information filtering. The thalamus consists of several nuclei, which are briefly described here (see image below).
Principal nuclei of the thalamus.
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The 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, perhaps more than 100, are found in these regions.
The anterior nuclei of the thalamus are functionally connected to the limbic system and have reciprocal connections to the cingulate gyrus and mammary bodies. The medial nuclei project to the frontal association cortex and the premotor cortex with reciprocal connectivity.
The ventrolateral nuclei can be divided into ventral anterior (VA), ventral lateral (VL), ventral posterolateral (VPL), and ventral posteromedial (VPM) nuclei. The VA and VL nuclei share information from the globus pallidus and projections to the motor cortex. The VPL and VPM act as sensory relays on the body and face, respectively.
The lateral nuclei are divided into the lateral dorsal and lateral posterior nuclei, with projections to the cingulate gyrus and parietal cortex, respectively.
Other thalamic structures not included in the anatomical subdivisions above include the medial and lateral geniculate bodies, which process auditory and visual information, respectively. The pulvinar is interconnected with the parietal and occipital association cortex. Intralaminar nuclei within the internal medullary lamina receive input from the brainstem, cerebellum, and other thalamic nuclei and project to structures in the basal ganglia and other thalamic nuclei. Among the intralaminar nuclei, the centromedian nucleus is part of the reticular activation system that plays a role in maintaining cortical arousal.
The epithalamus is made up of the habenula, the habenular commissure, the posterior commissure, and the pineal gland.
The subthalamus lies between the midbrain and the thalamus and contains the subthalamic nucleus, the red nucleus, and the substantia nigra. The subthalamic structures are closely integrated with the basal ganglia and play a role in modulating movement.
Its hypothalamic nuclei are located in the walls of the third ventricle anteriorly. The hypothalamus is involved in mediating endocrine, autonomic, visceral, and homeostatic functions. It can be divided into anterior, posterior, and middle central groups.
The anterior nuclei include the preoptic, supraoptic, and paraventricular nuclei. The posterior nuclei include the supramammary nucleus, the mammillary nucleus, the intercalated 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 middle central groups, while sympathetic control can be attributed to the posterior and lateral central groups. Satiety can be located in the stimulation of the medial nuclei and hunger can be located in the stimulation of the lateral nuclei. Other functions of the hypothalamus include regulation of body temperature, heart rate, blood pressure, and fluid balance.
The hypothalamus has close connections to the cingulate gyrus, frontal lobe, hippocampus, thalamus, brain stem, spinal cord, basal ganglia, and pituitary gland.