Diagram depicting the main subdivisions of the embryonic vertebrate brain.
|anterior cerebral, middle cerebral, posterior cerebral|
The cerebrum (Latin for brain) refers to the parts of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In humans, the cerebrum is the superior-most region of the central nervous system (CNS). The telencephalon refers to the embryonic structure from which the mature cerebrum develops. In mammals, the dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into approximately symmetric left and right cerebral hemispheres.
With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.
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The cerebrum is composed of the following regions:
- Cerebral cortex, or cortices of the cerebral hemispheres
- Basal ganglia, or basal nuclei
- Limbic System
The cerebrum comprises the largest part of the brain. It lies in front or on top of the brainstem and in humans is the largest and best-developed of the five major divisions of the brain. The cerebrum is the newest structure in the phylogenetic sense, with mammals having the largest and best-developed structures among all species. In larger mammals, the cerebral cortex is folded into many gyri (ridges) and sulci (furrows), which has allowed the cortex to expand in surface area without taking up much greater volume. The cerebral cortex is present only in mammals.
In humans, the cerebrum surrounds the older parts of the brain. The limbic, olfactory, and motor systems project fibers from the cerebrum to the brainstem and spinal cord. Cognitive and volitive systems project fibers from the cerebrum to the thalamus and to specific regions of the midbrain. The neural networks of the cerebrum facilitate complex behaviors such as social interactions, thought, judgement, learning, working memory, and in humans, speech and language.
In the developing vertebrate embryo, the neural tube subdivides into four unseparated sections which then develop further into distinct regions of the CNS; these are the prosencephalon, the mesencephalon the rhombencephalon and the spinal cord. The prosencephalon develops further into the telencephalon (forebrain or cerebrum), and the much smaller diencephalon which develops into the optic vessels and the hypothalamus. The telencephalon then forms two lateral telencephalic vesicles which develop into the left and right cerebral hemispheres.
Note: As the cerebrum is a gross division with many subdivisions and sub-regions, it is important to state that this section lists the functions that the cerebrum as a whole serves. See main articles on cerebral cortex and basal ganglia for more information. The cerebrum is a major part of the brain, controlling emotions, hearing, vision, personality and much more. It controls all voluntary actions.
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.
It functions as the center of sensory perception, memory, thoughts and judgement; the cerebrum also functions as the center of voluntary motor activity.
The primary sensory areas of the cerebral cortex receive and process visual, auditory, somatosensory, gustatory, and olfactory information. Together with association cortical areas, these brain regions synthesize sensory information into our perceptions of the world around us.
The olfactory bulb, responsible for the sense of smell, takes up a large area of the cerebrum in most vertebrates. However, in humans, this part of the brain is much smaller and lies underneath the frontal lobe. The olfactory sensory system is unique since the 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 olfaction (the sense of smell).
Language and communication
Speech and language are mainly attributed to the 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. These two regions are interconnected by a large white matter tract, the arcuate fasciculus. Damage to the Broca's area results in expressive aphasia (non-fluent aphasia) while damage to Wernicke's area results in receptive aphasia (also called fluent aphasia).
Learning and memory
Explicit or declarative (factual) memory formation is attributed to the hippocampus and associated regions of the medial temporal lobe. This association was originally described after a patient known as HM had both his left and right hippocampus surgically removed to treat chronic temporal lobe epilepsy. After surgery, HM had anterograde amnesia, or the inability to form new memories.
Implicit or procedural memory, such as complex motor behaviors, involves the basal ganglia.
Short-term or working memory involves association areas of the cortex, especially the dorsolateral prefrontal cortex, as well as the hippocampus.
In the most primitive vertebrates, the hagfishes and lampreys, the cerebrum is a relatively simple structure receiving nerve impulses from the olfactory bulb. In cartilaginous and lobe-finned fishes and also in amphibians, a more complex structure is present, with the cerebrum being divided into three distinct regions. The lowermost (or ventral) region forms the basal nuclei, and contains fibres connecting the rest of the cerebrum to the thalamus. Above this, and forming the lateral part of the cerebrum, is the paleopallium, while the uppermost (or dorsal) part is referred to as the archipallium. The cerebrum remains largely devoted to olfactory sensation in these animals, in contrast to its much wider range of functions in amniotes.
In ray-finned fishes the structure is somewhat different. The inner surfaces of the lateral and ventral regions of the cerebrum bulge up into the ventricles; these include both the basal nuclei and the various parts of the pallium and may be complex in structure, especially in teleosts. The dorsal surface of the cerebrum is membranous, and does not contain any nervous tissue.
In the amniotes, the cerebrum becomes increasingly large and complex. In reptiles, the paleopallium is much larger than in amphibians and its growth has pushed the basal nuclei into the central regions of the cerebrum. As in the lower vertebrates, the grey matter is generally located beneath the white matter, but in some reptiles, it spreads out to the surface to form a primitive cortex, especially in the anterior part of the brain.
In mammals, this development proceeds further, so that the cortex covers almost the whole of the cerebral hemispheres, especially in more developed species, such as the primates. The paleopallium is pushed to the ventral surface of the brain, where it becomes the olfactory lobes, while the archipallium becomes rolled over at the medial dorsal edge to form the hippocampus. In placental mammals, a corpus callosum also develops, further connecting the two hemispheres. The complex convolutions of the cerebral surface are also found only in higher mammals. Although some large mammals (such as elephants) have particularly large cerebra, dolphins are the only species (other than humans) to have cerebra accounting for as much as 2 percent of their body weight.
The cerebrum of birds are similarly enlarged to those of mammals, by comparison with reptiles. The increased size of bird brains was classically attributed to enlarged basal ganglia, with the other areas remaining primitive, but this view has been largely abandoned . Birds appear to have undergone an alternate process of encephalization, as they diverged from the other archosaurs, with few clear parallels to that experienced by mammals and their therapsid ancestors.
- Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 536–543. ISBN 0-03-910284-X.
- T.L. Brink. (2008) Psychology: A Student Friendly Approach. "Unit 4: The Nervous System." pp 62 
- Jarvis ED, Güntürkün O, Bruce L, et al. Avian brains and a new understanding of vertebrate brain evolution. Nat. Rev. Neurosci. 2005;6(2):151–9. Available at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2507884&tool=pmcentrez&rendertype=abstract.
- This article uses anatomical terminology; for an overview, see anatomical terminology.
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