Which part of the brain is responsible for relaying sensory information to the cerebrum?

The Cerebrum

The cerebrum is the largest part of the brain. It is responsible for memory, speech, the senses, and emotional response. It is divided into four sections called lobes: the frontal, temporal, parietal, and occipital. Each handles a specific segment of the cerebrum's jobs.

The diencephalon is inside the cerebrum above the brain stem. Its tasks include sensory function, food intake control, and the body's sleep cycle. As with the other parts of the brain, it is divided into sections. These include the thalamus, hypothalamus, and epitheliums.

The brain is protected from damage by several layers of defenses. Outermost are the bones of the skull. Beneath the skull are the meninges, a series of sturdy membranes that surround the brain and spinal cord. Inside the meninges, the brain is cushioned by fluid.8

In vivo [3H] TCP binding

In the cerebrum and the hippocampus, non-competitive NMDA antagonists, prevented [3H]TCP binding with no significant differences between ID50 values. Contrarily in the cerebellum, ID50 were significantly higher. BTCP prevented only slightly [3H]TCP binding in vivo in the cerebellum, with no effect in the cerebrum. Therefore, high affinity [3H]TCP binding sites in forebrain structures are related to the NMDA receptor-associated PCP receptor.

The time course of the 50% prevention of [3H]TCP binding by TCP [1 mg/kg] and PCP [3 mg/kg] showed a rapid preventing effect, similar between the cerebellum and the cerebrum. Contrarily, MK-801 [0.3 mg/kg] presented a stable prevention of [3H]TCP binding in the cerebrum and no significant effect in the cerebellum.

4.2.1 Brain modeling and boundary conditions

The cerebrum, cerebellum, brainstem, and the tumor are segmented from preoperative MRI. The cerebrum and tumor are meshed, with a higher density of elements in the tumor area in order to better capture its deformations. The tentorium cerebelli is identified as the border between the cerebrum and cerebellum. Since this membrane is quite rigid, the nodes of the model located on the tentorium cerebelli are assigned to fixed Dirichlet conditions.

The dura mater surface is generated as the external surface of the brain FE mesh at the beginning of the simulation. As this membrane is stuck to the skull, it is fixed throughout the simulation. Sliding constraints are used, allowing the brain to move along the dura mater without any friction. Displacements in the normal direction inside the cranial cavity are allowed.

During the simulation, loads are imposed through displacements to register the vascular tree embedded within the model onto the US extracted data. Both these vessels loads and contacts between the brain and dura mater are handled using Lagrangian Multipliers, with an ICP-inspired method proposed by Courtecuisse et al. [2014].

Lipid Extraction and Thin-Layer Chromatography

Cerebra from several rats are dissected and homogenized initially in a small volume of chloroform/methanol [1:1, v/v], with several brains pooled into one homogenate, and equal fractions are taken for each sample. This is carried out to reduce the variation in myo-[3H]inositol incorporation between the individual animals. The lipids are extracted essentially according to the method of Hauser and Eichberg [14] or Schacht [11]. Prior to use, it is essential to wash each test tube three times with chloroform/methanol [1:1, v/v] to remove possible contaminants. The initial neutral extraction removes primarily PI. In addition, during each step of the lipid extraction, the samples are kept under nitrogen using test tubes with Teflon-lined caps. After the acidified chloroform/methanol extraction step, the lipid phases are mixed with 0.2 volume of 1 M HCl, with each phase neutralized immediately with ammonia. The first acidified chloroform/methanol lipid extract is used for all lipid determinations, as this fraction contains most of the polyphosphoinositides.

Silica gel H preoxalated plates [0.25 mm, Analtech, Newark, DE] and silica gel 60 plates [0.5 mm, Merck], oxalated before use, are used to separate the inositol lipids. The plates are pre washed in chloroform/methanol [1:1, v/v], then activated at 110°C for 1 hr. The plates are placed in a sealed plexiglass spotting apparatus which allows nitrogen to flow continuously over the plate during spotting. An aliquot is spotted which gives PIP2 radioactive counts of 4000–5000 disintegrations/min [dpm]. Two primary solvent systems are used: [i] chloroform/methanol/ammonium hydroxide/water [90:90: 7:22, v/v] and [ii] chloroform/methanol/ammonium hydroxide/water [70:100:15:25, v/v] a solvent system developed by us. This latter solvent system is found to provide excellent separation of PIP3 and PIP2 in our experiments and has been tested independently by other groups [15, 16]. The Rf values for this system on Merck silica gel 60 [0.5 mm] plates are as follows: PIP3, 0.100; PIP2, 0.366; PIP, 0.500; PI, 0.626. The plates are developed using iodine vapors; the appropriate areas are scraped, and scintillation counting is performed [Packard Instrs., Meriden, CT Model 1900CA]. For fatty acid determinations, the plates are not developed in iodine and are kept under a nitrogen atmosphere continuously until ready for gas chromatography [GC] injection. PI, PIP, and PIP2 standards [Sigma, St. Louis, MO] are used in all runs. Flo-Scint IV [Packard] is a useful scintillation fluid with high concentrations of ammonium formate in samples. The radioactive counts are derived using computer programs for both quenching and chemiluminescence [Packard].

Cerebrum

The cerebrum consists of two cerebral hemispheres that are partially connected with each other by corpus callosum. Each hemisphere contains a cavity called the lateral ventricle. The cerebrum is arbitrarily divided into five lobes: frontal, parietal, temporal, occipital, and insula.2 On the lateral surface three sulci [central, lateral or Sylvian, and parietooccipital sulci] and two imaginary lines divide the cerebrum into four lobes [Fig. 1.1]. The first imaginary line [lateral parietotemporal line] is drawn from parietooccipital sulcus to preoccipital notch and second [temporo-occipital line] backward continuation of posterior ramus of lateral sulcus before it turns upward to meet first line. The central sulcus and posterior ramus of Sylvian fissure [SF] separate frontal lobe from parietal lobe and temporal lobe. Posteriorly parietooccipital sulcus and lateral parietotemporal line separate occipital lobe from parietal lobe and temporal lobe. Temporal and parietal lobes are separate by posterior ramus of SF and temporo-occipital line [Fig. 1.1].

The cerebral cortex is the outermost sheet of neural tissue of the cerebrum whereas white matter lies in the center. Cerebral cortex is folded into sulci and gyri, which actually increases the surface area of cortex. Sulci include the central lateral and parietooccipital.

The central sulcus begins by cutting the superomedial border of the hemisphere a little behind the midpoint between the frontal and parietal lobe. It runs on the superolateral surface obliquely downward and forward for about 8–10 cm and ends a slight above the posterior ramus of lateral sulcus. It separates precentral gyrus [motor area] from postcentral gyrus [sensory area] [Figs. 1.2 and 1.3]. It was originally called the fissure of Rolando or the Rolandic fissure.

The lateral sulcus or Sylvian fissure [SF] is one of the earliest-developing sulci of the human brain. It first appears around the 14th gestational week.3 It is the deepest and most prominent of the cortical sulci. The lateral sulcus [SF] separates frontal and parietal lobes from temporal lobe. It begins on the superomedial margin. The SF starts on basal and extends to the lateral surface of the brain. It has both a superficial part and a deep part. Superficial part has a stem and three rami [Figs. 1.2 and 1.3]. The anterior portion of the deep part [Sylvian cistern] is called the sphenoidal compartment and the posterior part is called the operculoinsular compartment. SF is an important corridor in neurosurgery as it connects the surface of anterior part of brain to its depth with all the neural and vascular components along the way. The structures within the reach through the Transylvian approach include middle cerebral artery; optic nerves; internal carotid artery; and its branched lamina terminalis, insula, basal ganglia, and interpeduncular fossa.

Parietooccipital sulcus begins on the medial surface of hemisphere nearly 5 cm in front of the occipital pole [Fig. 1.4]. The upper end of the sulcus reaches the superomedial border to meet the calcarine sulcus, and a small part of it is seen on the superolateral surface.

2.1.1 Cerebrum

The cerebrum represents one of the largest regions of the brain as seen in Fig. 3, and its functions are critical for survival. It is responsible for processing information associated with movement, smell, sensory perception, language, communication, memory, and learning. The left and right symmetrical hemispheres present in the cerebrum are responsible for a different set of tasks. This division of labor is where the terms “left brained,” meaning a person is more analytical and logical, and “right brained” where someone is more intuitive, arise—despite the lack of convincing scientific evidence to support such claims [7]. The cerebral cortex serves as the outer layer of the cerebrum and it consists of mostly of gray matter, which is a type of tissue labeled on the basis of its color [8]. Four lobes make up the cerebral cortex: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Each lobe has a distinct function. For example, the frontal lobe processes information associated with problem solving, speech, and emotions. The parietal lobe senses stimuli and movement, while the temporal lobe deals with processing auditory stimuli and speech. The occipital lobe processes visual information. Generally, this region controls voluntary action by working in coordination with the region of the brain known as the cerebellum, which is part of the brainstem.

The hippocampus, basal ganglia, and olfactory bulb are located in the deeper regions of the cerebrum, and these structures play unique roles in the brain function. The structure of the hippocampus resembles a seahorse, and accordingly it is named after the Greek word meaning seahorse. This region plays an important role in long-term memory [9]. It consists of two sections: the hippocampus proper region and the dentate gyrus. The dentate gyrus holds particular interest as it is one of the regions of the brain where adult neural stem cells are found, as well as a site of neurogenesis, which is the process of forming new neurons from stem cells [10]. This region of the brain becomes dysfunctional in patients suffering from Alzheimer's disease, and neuroscientists have been looking for connections between neurogenesis and Alzheimer's disease [11]. The basal ganglia consist of the nuclei [the command center of a cell] located laterally in a coronal section from a structure called the thalamus, which is found in the diencephalon region of the brain. These two structures work together to coordinate movement through signaling by the molecule glutamate. More on cell-to-cell signaling will be discussed in Section 4, which details the cells of the nervous system and their functions. The main functional cellular units of the nervous system are neurons. These cells rely on different types of signaling to transmit a variety of messages throughout the body. Multiple diseases and disorders are associated with improper basal ganglia function, including Parkinson's disease, attention deficit hyperactivity disorder [ADHD], and schizophrenia [12]. Some of the symptoms of these diseases manifest as disordered movement, which is consistent with the function of this region in healthy tissue.

As its name implies, the olfactory bulb plays a critical role in maintaining the sense of smell. This region contains several receptors that enable the body to sense and filter stimuli detected through olfaction [13]. This information is then transmitted to other regions of the brain where it is processed accordingly. The olfactory bulb also contains multipotent stem cells to replenish cells lost during the sensing process [14]. These neural stem cells migrate to the olfactory bulb from a region called the subventricular zone, which will be discussed later in this chapter. Interestingly, the loss of the ability to smell is observed in many neurodegenerative diseases, including dementia and Alzheimer's disease. This observation suggests a potential common link between these diseases caused by an inability of the brain to perform neurogenesis, the development of new neural tissue.

Cerebral white matter

The cerebrum mostly contains white matter, and its axons are either classified as association fibers, commissural fibers, or projection fibers. Association fibers, in one cerebral hemisphere, interconnect areas of the cerebral cortex. The shorter arcuate fibers curve similar to arcs, passing from one gyrus to another. Longer fibers form bundles known as fasciculi. Longitudinal fasciculi connect the frontal lobe to the other lobes within the same cerebral hemisphere.

Commissural fibers—interconnect between both hemispheres to allow communication. Their bands link the hemispheres and include the corpus callosum and anterior commissure. There are over 200 million axons in the corpus callosum [the largest commissure], which can carry about 4 billion impulses per second.

Projection fibers—link the cerebral cortex to the diencephalon, brain stem, cerebellum, and spinal cord. They pass through the diencephalon, where ascending axons linking sensory areas pass near descending axons from the motor areas which run horizontally. A dissected brain shows similarities of ascending and descending fibers. The internal capsule is the total collection of projection fibers.

Functional Neuroanatomy

The brain is conventionally divided into four distinct regions: cerebrum, diencephalon, cerebellum, and brainstem.

The cerebrum has the cortex [frontal, parietal, occipital, temporal lobes; eSlide 44.1], basal ganglia, and limbic system. The frontal lobe controls skeletal movement, executive function, and behavioral expression. The parietal lobe receives somatic sensation, spatial cognition [particularly the nondominant side], and houses some optic radiations. The temporal lobe involves hearing and olfactory function and also houses optic radiations. The occipital lobe receives optic radiations, and lesions of the occipital lobe cause vision dysfunction. Because the optic radiation travels through the parietal and temporal lobes, superior or inferior contralateral vision loss may occur from a stroke affecting the parietal or temporal lobe, respectively.

The diencephalon consists of the thalamus, hypothalamus, pituitary, and pineal glands. The thalamus is connected to all of the major areas of the brain. It receives sensory information from the face and body before it is relayed to the cerebrum, acting as a “switch board.” The thalamus also plays an important role in sleep and wakefulness. The hypothalamus controls the functions related to basic survival, including hunger, thirst, and autonomic and endocrine functions. The cerebellum is involved in the fine tuning of movement and balance.

The brainstem consists of the midbrain, pons, and medulla. The brainstem is responsible for vital functions, including respiration, circulation, wakefulness, and swallowing. This explains the high rate of early mortality in patients with brainstem strokes. Gates described the rule of four regarding the brainstem structures and corresponding functions [eSlide 44.2]. There are four longitudinal structures in the midline beginning with “M” and four longitudinal structures in the lateral beginning with “S” [Table 44.1]. The cranial nerve nuclei are horizontal structures spread out in the midbrain [III, IV], the pons [V, VI, VII, VIII], and the medulla [IX, X, XI, XII]. The midbrain involves coordination of eye movement. The medulla houses motor pathways [corticospinal track], which decussate at this location and are responsible for movement of the opposite side of the body. Combined examination of longitudinal and horizontal structures aids the diagnosis of brainstem stroke syndromes [eSlide 44.2].

Cerebral hemispheres

The cerebrum consists of two large masses, the cerebral hemispheres, which are nearly identical and side by side. A wide, flat, and heavily myelinated axon bundle called the corpus callosum connects the cerebral hemispheres. Their surfaces have gyri, which are many convolutions and ridges, separated by grooves. A groove that is shallow to slightly deep is called a sulcus, while an extremely deep groove is called a fissure [see Fig. 1.1]. These elevations and depressions are very complex. The longitudinal fissure separates the right and left cerebral hemispheres, while a transverse fissure separates the cerebrum and the cerebellum. Sulci divide each hemisphere into lobes. The lobes of the cerebral hemispheres are named from the skull bones that they underlie, and include the following:

Frontal lobe—located just behind the frontal bone, superior to the eyes. It extends from the forehead caudally to a curved vertical groove called the central sulcus. The frontal lobe is the center of abstract and conscious thought, as well as declarative or explicit memory. It is where emotional and cognitive processes occur: mood, foresight, motivation, decision-making, planning, emotional control, judgment of appropriate behaviors, speech production, and other voluntary motor controls. Frontal lobe lesions often cause a person to lose inhibitions, with impulsive sexual conversations and behaviors.

Parietal lobe—the upper part of the brain, underlying the parietal bone. It begins at the central sulcus, extending caudally to the parieto-occipital sulcus. The parietal lobe can be seen on each hemisphere’s medial surface. It handles visual processing [via optic radiations passing though the parietal lobe and temporal lobe, to reach their final destination at the occipital lobe]. The parietal lobe also controls spatial perception, often called “visuospatial perception.” Large lesions, such as a stroke in the right parietal lobe, cause the patient to neglect the left side of his or her world. Atrophy and shrinking of the parietal lobe is the reason why Alzheimer’s patients have difficulties with navigation and may get lost in their surroundings. This is because visuospatial tasks are disrupted due to atrophy of the parietal lobe.

Temporal lobe—deep to the temporal bone, lateral and horizontal, this lobe is separated from the frontal and parietal lobes above it via the deep lateral sulcus. The temporal lobe is used in hearing, emotion, smell, language comprehension, learning, memory related to grammar and vocabulary, formation of new long-term memories [memory consolidation], and storage of auditory, verbal, and visual memories. Temporal lobe lesions may cause memory problems, frequently seen in patients with Alzheimer disease. In Alzheimer’s disease, the temporal and parietal lobes are the main parts of the brain that are affected and atrophied.

Occipital lobe—situated at the back of the head, underlying the occipital bone, and caudal to the parieto-occipital sulcus. This lobe is the main visual center.

Insula—part of the temporal lobe, it is a small mass of cerebral cortex, deeply below the lateral sulcus. The insula can only be seen by retracting or removing some of the cerebrum above. It is used to process sensations of pain and taste, in visceral sensation, emotional responses and empathy, consciousness, and balancing heart rate and blood pressure when exercising, as well as other activities of cardiovascular homeostasis.

The various lobes of the brain are shown in Fig. 1.2.

Significant point

Magnetic resonance imaging studies of people with bipolar disorder indicate that there may be some structural abnormalities present, especially in the frontal and temporal lobes.

Localization and number

The cerebrum is the site of localization of 80–85% of all brain metastases, the cerebellum is the site of 10–15%, and the brain stem is the site of 3–5% [Delattre et al 1988; Haar & Patterson 1972; Takakura et al 1982]. There is a rough tendency for the overall distribution of brain metastases to correspond to the relative size and blood flow of regions in the brain; however, there are exceptions to this. Cerebral metastases have an increased tendency to occur at the temporo-parieto-occipital junction, where the terminal branches of the middle cerebral artery are located [Delattre et al 1988; Kindt 1964]. It is estimated that the weight of the cerebrum is nine times that of the cerebellum [Ask-Upmark 1956]; however, the relative frequency of metastasis to the cerebellum and brain stem is higher than is predicted by their relative weights. The reasons for this are unclear. The relative distribution of metastases may also vary with the tumor histology [Graf et al 1988], but this phenomenon is also poorly characterized. Some reports suggest that the rate of posterior fossa metastasis may be higher for pelvic and gastrointestinal tumors [Cascino et al 1983b; Delattre et al 1988; Takakura et al 1982], a phenomenon that may be caused by propagation via Batson's venous plexus [Batson 1942].

The relative frequency of single or multiple metastases varies with the type of primary tumor. Melanoma has the highest tendency to produce multiple lesions, followed by lung and breast primaries. Metastases from colon primaries present with multiple tumors 50% of the time [Cascino et al 1983b], whereas those from renal cancer usually present with only one lesion [Decker et al 1984; Gay et al 1987]. Table 45.3 shows the percentage of patients with multiple brain metastases in various studies. Overall, autopsy studies show that 60–85% of all patients dying of cancer harbor multiple brain metastases [Amer et al 1978; Chason et al 1963; Galluzzi & Payne 1956; Takakura et al 1982]. However, it is probable that patients diagnosed during life are less likely to present with multiple lesions. Indeed, CT studies show that 37–50% of patients present with a single metastasis [Delattre et al 1988; Takakura et al 1982]. Studies comparing contrast-enhanced CT with contrast-enhanced MR imaging, however, have indicated that patients demonstrating a single lesion by CT may demonstrate multiple lesions on MR imaging [Davis et al 1991; Sze et al 1990]. Thus, the percentage of patients with multiple lesions as demonstrated on MR imaging is likely to be higher than is indicated by CT scans.