Structure and Function of the Cerebrum

The cerebrum comprises the largest portion of the human brain. It is the site of higher-order thinking and memory, and it centralizes the information received by the senses. It also controls voluntary movements made by the muscles. The cerebrum consists of two hemispheres, which are actually separate pieces connected by a small tissue on the underside called the corpus callosum. The surface of the cerebrum is not smooth, but instead consists of bumps and ridges. Each ridge that protrudes from the surface is called a gyrus (plural, gyri). Each of the grooves that separates the gyri is called a sulcus (plural, sulci). There are also grooves that run deeper into the surface of the cerebrum, called fissures. Anatomists use the gyri and sulci as landmarks to determine the positions of the various sense receptor centers. Scientists believe that senses, such as vision and hearing, are isolated to specific areas of the cerebrum in terms of processing and interpreting of stimuli. However, they also feel that the ability to create and use language and form memories is the result of many different areas spread across the cerebrum.

The cranium, the bones of the skull, tightly fit over the cerebrum, protecting it from damage. There are several openings within the skull that allow for the passage of nerves and blood vessels from the body to the brain. The thin outer layer is called the cerebral cortex, the area within the brain where thinking and voluntary actions originate. When a person decides to start running or pick up a pencil to write something down, the cerebral cortex is the location where the nervous signals initiate. The cerebral cortex is composed of gray matter; that is, neurons that lack myelin, a fatty sheath that surrounds the axons of other neurons and increases the speed of impulse transmission. The cortex surrounds the white matter of the cerebrum. The axons of these nerve cells are covered with myelin, which is why this area appears light in color. White matter functions as a relay center between the gray matter and the other areas of the central nervous system (such as the spinal cord). The myelinated cells of white matter form into long fibers that travel and penetrate into other locations. These fibers are used to transmit signals.

The basal nucleus is a group of neuron clusters found deep within the white matter of the cerebrum that control motor function and voluntary movement. The basal nuclei work in conjunction with the structures of the diencephalon (the thalamus, hypothalamus, and epithalamus) to receive signals from the brain and then initiate motor responses. Should the nuclei malfunction, a person is more prone to developing Parkinson's disease or Huntington's disease.

The cerebrum is the part of the brain responsible for higher thinking and voluntary motor skills. It also receives and processes information from the five senses. As such, the cerebrum is divided into four main lobes: the frontal, parietal, occipital, and temporal lobes, each having a particular function.

The frontal lobe, located near the front of the cerebrum, is sensitive to dopamine and is associated with motivation, short-term memory, and planning. It is here that higher-level thinking takes place, along with reasoning and the performing of motor skills, such as writing with a pencil or playing tennis. The frontal lobe has an area called the primary motor cortex that receives information from the other lobes of the brain and uses it to create a motor response. The primary motor cortex is responsible for controlling most movements assisted by the premotor cortex, which offers sensory and spatial guidance. The frontal lobe is also responsible for the production and interpretation of language. A specific site on the frontal lobe, called Broca's area, is in charge of muscles that produce sound for speech. People with damage to this area are able to understand what they hear but are unable to talk. However, damage to Wernicke's area, an area located between the temporal and parietal lobes, interferes with speech comprehension—but not speech production.

The temporal lobe is found at the bottom of cerebrum, located inferior to the parietal lobe, beneath the lateral fissure on both cerebral hemispheres. It is responsible for processing sensory input and interpreting meaning for memory and emotional associations. The temporal lobe also functions as the main site for deciphering auditory signals and producing language. The part of the brain that is believed to form and store memories, the hippocampus, is also found in the temporal lobe.

The occipital lobe is located posterior to the parietal lobe toward the back part of the cerebrum. The occipital lobe is the part of the brain that receives and processes visual stimuli. The visual cortex receives the nervous signals from the eyes through the optic nerve and then processes the information accordingly. The occipital lobe (along with the other lobes) is connected via white matter fibers to the areas of the brain that contain memories. This way when a person sees, hears, or tastes something familiar, they are able to isolate the memory where they first experienced the stimuli and identify what it is.

The parietal lobe, found toward the middle areas of the cerebrum between the temporal lobe and the frontal lobe, is important in processing language. This part of the brain is also responsible for determining and processing information that comes from touch (such as pain, pressure, heat, and cold).

The limbic system is the part of the brain that creates and controls emotions. It consists of a collection of structures that are located in the center of the cerebrum, above the diencephalon, and surrounding the corpus callosum and the upper parts of the brain stem. It includes the amygdala, which sits close to the hippocampus, and the hypothalamus. The limbic system, specifically the amygdala, controls a person's emotions. This part of the brain is essential when a person comes into contact with a possible threat. The amygdala responds by producing the reactions of fear or aggression. This reaction is commonly known as the "fight-or-flight response," which produces an increase in the hormones norepinephrine and epinephrine. This increases heart rate and blood flow, allowing for quicker thoughts and movements needed for "escape."

The limbic system also correlates emotions and helps organize thoughts when choices need to be made. For example, odors have long been shown to produce extreme reactions in people. Rotting garbage causes a person to screw up their face, indicating a strong displeasure with the smell. This reaction comes from previous experience with the smell. On the other hand, walking into a house that smells of freshly baked bread might trigger a memory of a person's childhood at a relative's house. This reaction would be positive because the odor brings back good memories.

Most of the information processed by the limbic system goes through the hypothalamus. The hypothalamus is found below the thalamus in the diencephalon of the brain. It serves several different functions including, but not limited to, controlling the autonomic nervous system as well as various hormones of the endocrine system, regulating sensations of hunger and thirst, and controlling sleep cycles. It also acts as a relay station for the sense of smell.

The limbic system is associated with the higher-thinking areas of the brain that cause people to react with strong emotions. This means that when a person is exposed to a familiar stimulus or familiar situation, such as the death of a loved one, they are able to respond with appropriate feelings.