67 The Cerebrum

Overview of the Cerebrum

With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.

The cerebrum, which lies in front or on top of the brainstem, comprises a large portion of the brain. In humans, it is the largest and best-developed of the brain’s five major divisions. The cerebrum is the newest structure in the phylogenetic sense, with mammals having the largest and most developed among all species.

The cerebrum contains the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In larger mammals, the cerebral cortex is folded into many gyri and sulci, which allows it to expand in surface area without taking up much greater volume. With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.

Cerebral Cortex

The cortex is composed of two hemispheres, right and left, separated by a large sulcus. A thick fiber bundle, the corpus callosum, connects the two hemispheres, allowing information to be passed from one side to the other. The right hemisphere controls and processes signals from the left side of the body, while the left hemisphere controls and processes signals from the right side of the body.

The Four Brain Lobes

Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital.

The frontal lobe is located at the front of the brain, over the eyes, and contains the olfactory bulb. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement.

Two of the parietal lobe’s main functions are processing somatosensation (touch sensations such as pressure, pain, heat, cold) and proprioception (the sense of how parts of the body are oriented in space).

The temporal lobe is located at the base of the brain by the ears. It is primarily involved in processing and interpreting sounds. It also contains the hippocampus, which processes memory formation.

The occipital lobe is located at the back of the brain. It is primarily involved in vision: seeing, recognizing, and identifying the visual world.

Cerebrum Function

The cerebrum directs the conscious or volitional motor functions of the body. These functions originate within the primary motor cortex and other frontal lobe motor areas where actions are planned. Upper motor neurons in the primary motor cortex send their axons to the brainstem and spinal cord to synapse on the lower motor neurons, which innervate the muscles. Damage to motor areas of cortex can lead to certain types of motor neuron disease. This kind of damage results in loss of muscular power and precision rather than total paralysis.

The olfactory sensory system is unique in that neurons in the olfactory bulb send their axons directly to the olfactory cortex, rather than to the thalamus first. Damage to the olfactory bulb results in a loss of the sense of smell. The olfactory bulb also receives “top-down” information from such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra. Its potential functions can be placed into four non-exclusive categories: discriminating among odors, enhancing sensitivity of odor detection, filtering out background odors, and permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors.

Speech and language are mainly attributed to parts of the cerebral cortex. Motor portions of language are attributed to Broca’s area within the frontal lobe. Speech comprehension is attributed to Wernicke’s area, at the temporal-parietal lobe junction. Damage to the Broca’s area results in expressive aphasia (non-fluent aphasia) while damage to Wernicke’s area results in receptive aphasia.

Cerebral Lobes

The cortex is divided into four main lobes: frontal, parietal, occipital, temporal.

Brain lobes were originally a purely anatomical classification, but we now know they are also associated with specific brain functions. The telencephalon (cerebrum), the largest portion of the human brain, is divided into lobes like the cerebellum. If not specified, the expression “lobes of the brain” refers to the telencephalon. There are four uncontested lobes of the telencephalon:

The Frontal Lobe

The frontal lobe is an area in the mammalian brain located at the front of each cerebral hemisphere and positioned anterior to (in front of) the parietal lobe and superior and anterior to the temporal lobes. It is separated from the parietal lobe by a space between tissues called the central sulcus and from the temporal lobe by a deep fold called the lateral (Sylvian) sulcus. The precentral gyrus, forming the posterior border of the frontal lobe, contains the primary motor cortex, which controls voluntary movements of specific body parts.

The frontal lobe contains most of the dopamine-sensitive neurons in the cerebral cortex. The dopamine system is associated with reward, attention, short-term memory tasks, planning, and motivation. Dopamine tends to limit and select sensory information that the thalamus sends to the forebrain. A report from the National Institute of Mental Health indicates that a gene variant that reduces dopamine activity in the prefrontal cortex is related to poorer performance in that region during memory tasks; this gene variant is also related to slightly increased risk for schizophrenia.

The frontal lobe is considered to contribute to our most human qualities. Damage to the frontal lobe can result in changes in personality and difficulty planning. The frontal lobes are the most uniquely human of all the brain structures.

The Parietal Lobe

The parietal lobe is a part of the brain positioned above (superior to) the occipital lobe and behind (posterior to) the frontal lobe. The parietal lobe integrates sensory information from different modalities, particularly spatial sense and navigation. For example, it comprises the somatosensory cortex and the dorsal stream of the visual system. This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions.

Several portions of the parietal lobe are also important in language processing. Also, this lobe integrates information from various senses and assists in the manipulation of objects. Portions of the parietal lobe are involved with visuospatial processing.

The Occipital Lobe

The two occipital lobes are the smallest of the four paired lobes in the human cerebral cortex. Located in the rearmost portion of the skull, the occipital lobes are part of the forebrain. At the front edge of the occipital there are several lateral occipital gyri separated by lateral occipital sulci. The occipital lobe is involved in the sense of sight; lesions in this area can produce hallucinations.

The Temporal Lobe

The temporal lobe is a region of the cerebral cortex located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobes are involved in many functions, such as retaining visual memories, processing sensory input, comprehending language, storing new memories, feeling and expressing emotion, and deriving meaning. The temporal lobe contains the hippocampus and plays a key role in the formation of explicit long-term memory, modulated by the amygdala. It is involved in the senses of smell and sound as well as in processing of complex stimuli.

Adjacent areas in the superior, posterior, and lateral parts of the temporal lobes are involved in high-level auditory processing. The temporal lobe is involved in primary auditory perception such as hearing and holds the primary auditory cortex. The superior temporal gyrus includes an area where auditory signals from the ear first reach the cerebral cortex and are processed by the primary auditory cortex in the left temporal lobe.

The areas associated with vision in the temporal lobe interpret the meaning of visual stimuli and establish object recognition. The ventral part of the temporal cortices appear to be involved in high-level visual processing of complex stimuli such as faces (fusiform gyrus) and scenes (parahippocampal gyrus). Anterior parts of this ventral stream for visual processing are involved in object perception and recognition.

White Matter of the Cerebrum

White matter is composed of myelinated axons and glia and connects distinct areas of the cortex.

White matter is one of the two components of the central nervous system (CNS). It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the cerebrum and the superficial parts of the spinal cord. In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue containing capillaries. The axons of white matter transmit signals from various grey matter areas (the locations of nerve cell bodies) of the cerebrum to each other and carry nerve impulses between neurons. While grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.

Tracts

There are three different kinds of tracts (bundles of axons) that connect one part of the brain to another within the white matter:

• Projection tracts extend vertically between higher and lower brain areas and spinal cord centers, and carry information between the cerebrum and the rest of the body. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a broad, dense sheet called the internal capsule between the thalamus and basal nuclei, then radiate in a diverging, fanlike array to specific areas of the cortex.
• Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. The great majority of commissural tracts pass through the large corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
• Association tracts connect different regions within the same hemisphere of the brain. Long association fibers connect different lobes of a hemisphere to each other, whereas short association fibers connect different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain.

Corpus Callosum

The corpus callosum (Latin: “tough body”), also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200 to 250 million contralateral axonal projections.

The posterior portion of the corpus callosum is called the splenium, the anterior is called the genu (or “knee”), and the area between the two is the truncus or body of the corpus callosum. The part between the body and the splenium is often markedly thin and thus called the isthmus. The rostrum is the part of the corpus callosum that projects posteriorly and inferiorly from the anteriormost genu. The rostrum is so named for its resemblance to a bird’s beak.

Agenesis of the corpus callosum (ACC) is a rare congenital disorder in which the corpus callosum is partially or completely absent. It is usually diagnosed within the first two years of life and may manifest as a severe syndrome in infancy or childhood, as a milder condition in young adults, or as an asymptomatic incidental finding. Initial symptoms of ACC usually include seizures that may be followed by feeding problems and delays in holding the head erect, sitting, standing, and walking. Hydrocephaly may also occur.

Other possible symptoms include impairments in mental and physical development, hand-eye coordination, and visual and auditory memory. In mild cases, symptoms such as seizures, repetitive speech, or headaches may not appear for years.

Basal Ganglia

The basal ganglia is important for the control of movement and forming habits, and each of its components has a complex internal anatomical and neurochemical organization.

The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus, and other brain areas. The components of the basal ganglia include the striatum, pallidum, substantia nigra, and subthalamic nucleus. Each of these components has a complex internal anatomical and neurochemical organization.

Structure

The main components of the basal ganglia are:

• Striatum, or neostriatum: This component consists of 3 divisions: the caudate, putamen, and ventral striatum (includes the nucleus accumbens). The striatum receives input from many brain areas, but sends output only to other components of the basal ganglia.
• Globus pallidus, or pallidum: This component is composed of the globus pallidus externa (GPe) and globus pallidus interna (GPi). The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex.
• Substantia nigra: This component consists of the substantia nigra pars compacta (SNc) and substantia nigra pars reticulata (SNr). The SNr functions similarly to the pallidum, and the SNc cells contain neuromelanin and produce dopamine (a neurotransmitter) for input to the striatum.
• Subthalamic nucleus (STN): The STN receives input mainly from the striatum and cortex, and projects to a portion of the pallidum (interna portion or GPi). It is the only portion of the ganglia that produces an excitatory neurotransmitter, glutamate. The role of the subthalamic nucleus is to stimulate the SNr-GPi complex, and it receives inhibitory input from the GPe and sends excitatory signal to the GPi.

Function

The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or habits such as bruxism, eye movements, and cognitive, emotional functions. Currently, popular theories implicate the basal ganglia primarily in action selection, that is, the decision of which several possible behaviors to execute at a given time. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems and that a release of this inhibition permits a motor system to become active. The behavior switching that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.

The basal ganglia play a central role in a number of neurological conditions, including several movement disorders. The most notable are Parkinson’s disease, which involves degeneration of the melanin-pigmented dopamine-producing cells in the substantia nigra pars compacta (SNc), and Huntington’s disease, which primarily involves damage to the striatum. Basal ganglia dysfunction is also implicated in some other disorders of behavior control such as Tourette’s syndrome, ballismus (particularly hemibalismus), obsessive-compulsive disorder, and Wilson’s disease (hepatolenticular degeneration). With the exception of Wilson’s disease and hemiballismus, the neuropathological mechanisms underlying diseases of ganglia such as Parkinsons’ and Huntington’s are not very well understood or are at best still developing theories.

The basal ganglia have a limbic sector whose components are the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). This limbic sector is thought to play a central role in reward learning, particularly a pathway from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine. A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal.

Limbic System

The limbic system makes up the inner border of the cortex and is vital for emotion, motivation, and memory.

The limbic system, or paleomammalian brain, is a set of brain structures in the precortex and subcortex of the brain. It includes the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex, and fornix, and supports a variety of functions including emotion, behavior, motivation, long-term memory, and olfaction. The term “limbic” comes from the Latin limbus, for “border” or “edge,” because the limbic system forms the inner border of the cortex.

Limbic System Anatomy

The limbic system consists of various structures that each support distinctive brain functions.

Hippocampus and Associated Structures

• Hippocampus: Required for the formation of long-term memories and implicated in maintenance of cognitive maps for navigation.
• Amygdala: Involved in signaling the cortex of motivationally-significant stimuli, such as those related to reward and fear, and in social functions, such as mating.
• Fornix: A white matter structure that carries signals from the hippocampus to the mammillary bodies and septal nuclei.
• Mammillary body: Important for the formation of memory.

Septal Nuclei

• These lie below the rostrum of the corpus callosum and anterior to the lamina terminalis. The septal nuclei receive reciprocal connections from the olfactory bulb, hippocampus, amygdala, hypothalamus, midbrain, habenula, cingulate gyrus, and thalamus.

Limbic Lobe

A phylogenetically old structure consisting of the following structures:

• Parahippocampal gyrus: Plays a role in the formation of spatial memory
• Cingulate gyrus: Conducts autonomic functions regulating heart rate, blood pressure, and cognitive and attentional processing
• Dentate gyrus: Thought to contribute to the formation of new memories

Additional Structures

• Entorhinal cortex: Important memory and associative components
• Piriform cortex: Processes olfactory information
• Fornicate gyrus: Region encompassing the cingulate and parahippocampal gyrus
• Nucleus accumbens: Involved in reward, pleasure, and addiction
• Orbitofrontal cortex: Involved in cognitive processing during decision-making

Limbic System Function

The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain’s pleasure center, which plays a role in sexual arousal and the “high” derived from certain recreational drugs.

The structures of the limbic system are involved in motivation, emotion, learning, and memory.

The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery called a prefrontal lobotomy. Patients who underwent this procedure often became passive and lacked motivation.