The Brain and Its Functions
The cortex or cerebrum is the highest level of the Central Nervous System and is concerned with complex thought, perception and action. This cerebrum is divided into two halves called hemispheres. Between the hemispheres lies a thick bundle of nerve fibers, called the corpus callosum, which provides a communication link between the hemispheres. The left hemisphere controls the right half of the body, and vice-versa. Although the right and left hemispheres seem to be a mirror image of one another, there are important functional distinctions. In most people, for example, the areas that control speech are located in the left hemisphere, while areas that govern spatial perceptions reside in the right hemisphere.
Most people (approximately 87%) are right-handed and left hemisphere dominant. Cerebral dominance refers to the fact that one of the cerebral hemispheres is "leading" the other one in certain functions. This idea usually refers to the dominant control over speech and motor functions. This is why most people that have a stroke in the left side of their brain while have aphasia (speech/language disorder), apraxia (motor control disorder) and difficulty using the right side of their body. Broca's area (responsible for speech production) and Wernick's area (responsible for comprehension) are the two major language centers in the brain and lie in the left hemisphere.
The right cerebral hemisphere is often considered to be the non-dominant hemisphere in most people. Nevertheless, the right hemisphere specializes in certain abilities. For example, people with damage to their right hemisphere often show signs of visual and perceptual problems (can’t see and interpret visual images correctly). It is also believed that the right hemisphere may be dominant for certain musical abilities, different forms of creativity and various emotional states.
Lobes of the Brain – There are four basic lobes that divide up the cerebral hemispheres. These have been named for the bones of the skull that lie on top of them and include the Frontal, Temporal, Parietal and Occipital lobes.
The frontal lobes in the most forward portion of the brain, just underneath our foreheads. This part of the brain occupies the largest portion of our brain, approximately one-third of the entire brain. The frontal lobes help us to control our thoughts, emotions and actions; they are what make us unique human beings.
The frontal lobes are further subdivided into three regions:
While symptoms vary on an individual basis, observed problems following frontal lobe damage may include:
Frontal lobe damage most often occurs in cases of Traumatic Brain Injury, Stroke, and Tumors, but these symptoms may occur following any neurologic condition that leads to damage within the frontal lobes or their connections with other parts of the brain.
There are two temporal lobes, one on each side of the brain located at about the level of the ears. These lobes allow a person to tell one one sound from another and are believed to be involved in short-term memory. The left temporal lobe is mainly involved in verbal memory and specific language functions such as the comprehension of words. The right temporal lobe participates in visual memory, musical abilities and in the identification of visual objects. The temporal lobes rest on top of and are connected to the limbic system, which is considered the seat of raw emotions. While symptoms vary on an individual basis, observed problems following temporal lobe damage may include:
Temporal lobe damage most often occurs in cases of Traumatic Brain Injury, Stroke, and Encephalitis, but these symptoms may occur following any neurologic condition that leads to damage within the temporal lobes or their connections with other parts of the brain.
The parietal lobe is located just behind the frontal lobes and is primarily concerned with the control of bodily sensation, shape, texture, and position. The parietal lobe also enables us to put together touch sensations such as shape, size, texture, and weight, and merge these into a three-dimensional knowledge of the objects around us. The parietal lobe is believed to integrate many functions and the right side is involved in such things as directing attention, visual and spatial skills, while the left side often participates in overlearned motor routines, and linguistic skills such as reading, writing, and naming objects. While symptoms vary on an individual basis, observed problems following parietal lobe damage may include:
Parietal lobe damage most often occurs in cases of Stroke, but these symptoms may occur following any neurologic condition that leads to damage within the parietal lobes or their connections with other parts of the brain.
The occipital lobe is located in the rear of the head and is an important region controlling sight, reading, and visual images. If the visuosensory center of a person's occipital lobe were destroyed, that person would be blind even if their eyes and optic nerves were perfectly intact. Surrounding this visuosensory center is an area called the visual association area. If only the visual association area were destroyed, that person would not go blind, but instead would lose the power to recognize what is seen. The person would be able to see, but not to recognize what he/she was seeing. While symptoms vary on an individual basis, observed problems following occipital lobe damage may include:
Occipital lobe damage is not very common, but may occur in cases of Stroke and Tumors, but these symptoms may occur following any neurologic condition that leads to damage within the occipital lobes.
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The corpus callosum is a very thick bundle of nerve fibers located in the middle of the brain that connects the two cerebral hemispheres. When the corpus callosum is severed, there is perception but no messages travel back and forth between the brain's two hemispheres. Thus, although both eyes "see" an image, each hand drawing the figure does only what its half of the brain perceives. The left hemisphere, which handles language in most people, does not receive or send messages about what the two halves see. The brain is, indeed, divided.
The thalamus is a group of nuclei in the middle and on both sides of the brain that acts as a relay station between different parts of the brain. Signals from all of the sensory systems, with the exception of smell, pass through here on their way to the cerebral cortex, and the cortex itself uses it to relay signals to other parts of itself and down into the spinal cord. Multiple sensory inputs are processed by the thalamus, including those related to vision, touch, hearing etc. As such, the thalamus is also involved in directing the focus of our attention and screening out distracting stimuli.
The Hypothalamus is located deep within the brain at its’ base just underneath the thalamus. The main function of the hypothalamus is homeostasis, or maintaining the body's status quo. Factors such as blood pressure, body temperature, hunger, thirst, fluid/electrolyte balance, body weight, emotion, and circadian rhythms (sleep-wake cycles) are held to a precise value called the set-point. Although this set-point can migrate over time, from day to day it is remarkably fixed. To achieve this task, the hypothalamus must receive inputs about the state of the body, and must be able to initiate compensatory changes if anything drifts out of whack.
The Limbic System is a complex connection of brain regions deep within the brain underneath the temporal lobes. It includes the Hippocampus, Fornix, Thalamus, Hypothalamus, Amygdala, Olfactory cortex and Cingulate Gyrus. Generally speaking, the limbic system is considered our primitive brain and is responsible for things like emotions, instincts/basic drives (i.e., sex, hunger..), motivation, mood, pleasure/pain, smell and memory. Lesions in the Limbic System can result in voracious appetite, increased (often perverse) sexual activity, and docility (including loss of normal fear and anger responses).
The Hippocampus is located in the center of the limbic system and has a looping C shape. It is connected to the hypothalamus by the fornix. The hippocampus appears to be crucial for the transfer of memories into long-term storage. While it is debated as to exactly how this task is carried out, it is clear that the hippocampus is necessary to file away new memories as they occur.
The significance of the hippocampus is driven home by a famous patient named H.M. As part of an epilepsy surgery, doctors removed most of his medial temporal lobes including his hippocampus. Since that surgery, in 1953, he has formed no new memories. He can remember his childhood and everything before the surgery, and he still has working/short-term memory and the ability to form procedural (skill) memories. You can have a normal, lucid conversation with him, but if you leave the room for a moment, when you return he will not remember you or the conversation. He has completely lost the ability to lay down declarative (context-related) memory.
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The Basal Ganglia is a collection of cell bodies that are interconnected and lie next to and on the lateral (outer) side of the thalamus and the ventral (inner) side of the white matter of the cerebral cortex. The structures that make up the basal ganglia include: caudate, putamen, nucleus accumbens, globus pallidus, substantia nigra, and the subthalamic nucleus of the thalamus. The basal ganglia is generally concerned with the coordination of movement.
The overall function of the basal ganglia is considered inhibitory are as though it were putting on the brakes. To sit still, you must put the brakes on all movements except those reflexes that maintain an upright posture. To move, you must apply a brake to some postural reflexes, and release the brake on voluntary movement. In such a complicated system, it is apparent that small disturbances can throw the whole system out of whack, often in unpredictable ways. The deficits tend to fall into one of two categories: the presence of extraneous unwanted movements or an absence or difficulty with intended movements.
Lesions in specific nuclei tend to produce characteristic deficits. One well-known disorder is Parkinson's disease, which is the slow and steady loss of dopaminergic neurons in the Subsantia Nigra. An instant Parkinson-like syndrome will result if these neurons are damaged. This happened several years ago to an unfortunate group of people who took some home-brewed Demerol in search of a high. It was contaminated by a very nasty byproduct, MPTP ,which selectively zapped the Substantia Nigra neurons. The three symptoms usually associated with Parkinson's are tremor (shaking), rigidity (muscle tightness), and bradykinesia (slowed movements). Huntington's disease, or chorea, is a hereditary disease of unwanted movements. It results from degeneration of the caudate and putamen, and produces continuous dance-like movements of the face and limbs. A related disorder is hemiballismus, flailing movements of one arm and leg, which is caused by damage (i.e., stroke) of the subthalamic nucleus of the thalamus.
If the Basal Ganglia is the brake then the Cerebellum is the gas pedal. The cerebellum provides excitatory inputs that are involved in coordinating movement, balance and motor learning. The word "cerebellum" comes from the latin word for "little brain". The cerebellum is located behind the brain stem has a cortex that surrounds these hemispheres. There are connections between the cerebellum and brain stem, basal ganglia and cerebral hemispheres. These structures together control the smooth coordination of movement.
Damage to the cerebellum may result in the following symptoms:
The Brain Stem consists of the medulla oblongata, pons, and midbrain. These three structures connect the brain to the spinal cord. The medulla oblongata is continuous with the spinal cord. The medulla controls several reflexes such as heart rate, blood pressure, swallowing and breathing. There are structures within the medulla that have other responsibilities. Within the medulla, the nerve tracts cross and this is what is responsible for the halves of the brain controlling the opposite sides of the body.
Damage to the Brain Stem can lead to the following symptoms:
If fact, many believe that Attention Deficit Disorder involves dysfunction to the Reticular Activating System.
- Most of the cranial nerves originate in the brainstem. The brainstem is the pathway for all fiber tracts passing up and down from peripheral nerves and spinal cord to the highest parts of the brain. The cranial nerves are 12 pairs of nerves that can be seen on the bottom surface of the brain. Some of these nerves bring information from the sense organs to the brain; other cranial nerves control muscles; other cranial nerves are connected to glands or internal organs (for example, the heart and lung).
Below is a listing of the cranial nerves and their functions:
The entire surface of central nervous system is bathed by a clear, colorless fluid called cerebrospinal fluid (CSF). The CSF is contained within a system of fluid-filled cavities called ventricles. CSF is produced mainly by a structure called the choroid plexus located in the lateral, third and fourth ventricles. CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro). The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the Aqueduct of Sylvius). CSF then flows into the subarachnoid space and blood stream.
The CSF has several functions including:
Under some pathological conditions, CSF builds up within the ventricles. This condition is called hydrocephalus.
Hydrocephalus may result from:
The smells of a rose, perfume, freshly baked bread and cookies...these smells are all made possible because of your nose and brain. The sense of smell, called olfaction, involves the detection and perception of chemicals floating in the air. Chemical molecules enter the nose and dissolve in mucous within a membrane called the olfactory epithelium. In humans, the olfactory epithelium is located about 7 cm up and into the nose from the nostrils. Hair cells are the receptors in the olfactory epithelium that respond to particular chemicals. These cells have small hairs called cilia on one side and an axon on the other side. In humans, there are about 40 million olfactory receptors. The electrical activity produced in these hair cells is transmitted to the olfactory bulb. The olfactory tract (cranial nerve I) transmits the signals on to the brain to such areas as the olfactory cortex, hippocampus, amygdala, and hypothalamus.
Many of these brain areas are part of the limbic system. The limbic system is involved with emotional behavior and memory. That's why when you smell something, it often brings back memories associated with the object. As you probably know, when you have a cold and your nose is stuffed up, you cannot smell very well. This is because the molecules that carry smell cannot reach the olfactory receptors.
About 2 million people in the United States have NO sense of smell. This disorder is called anosmia. A serious head injury can cause anosmia. Most likely this results in damage to the olfactory nerves as they enter the olfactory bulb. It is also possible that damage of the frontal lobes caused by a tumor or surgery can cause anosmia.
The Optic Nerve connects the eye to the brain. Visual information enters the eye in the form of photons of light which are converted to electrical signals in the retina. These signals are carried via the optic nerves, chiasm, and tract to the lateral geniculate nucleus of each thalamus and then to the visual centers of the brain for interpretation.
The regions of the cortex are functionally differentiated. For example, in the frontal lobes along the central sulcus is located the primary motor cortex, important in voluntary movement. Adjacent to it are higher order motor areas (supplementary and premotor) involved in planning a movement. In the parietal lobe along the central sulcus is the primary somatic sensory cortex. Sensory signals from the body surface are mapped to it. The systematic relationship between position within the primary somatosensory cortex and the surface of the body is called somatotopy, and is often represented by a cartoon man.
We hear and interpret sound using areas of the brain called the auditory cortex. The primary auditory cortex registers and organizes the signals coming from the ears, and the auditory association cortex interprets them, as language, for example. It is this latter area only that is active in congenitally deaf people using sign language.
It is located in the medial side of the brain between the cingulate sulcus and the corpus callosum (principal fiber bundle connecting the two cerebral hemispheres). There is still much to be learned about this gyrus, but it is already known that its frontal part coordinates smells and sights with pleasant memories of previous emotions. This region also participates in the emotional reaction to pain and in the regulation of aggressive behaviour. Wild animals, submitted to the ablation of the cingulate gyrus (cingulectomy), become totally tamed. The cutting of a single bundle of this gyrus (cingulotomy) reduces pre-existent depression and anxiety levels, by interrupting neural communication across the Papez's circuit.
The pituitary gland, located at the base of the brain, is a small organ about the size of an acorn. It is surrounded by a bony saddle-like structure above the sinuses at the back of the nose, called the sella turcia. The pituitary gland is sometimes referred to as the "master gland" because it releases substances which control the basic functions of growth, metabolism, and reproduction. The pituitary gland is divided into two parts called lobes. These are referred to as the anterior and posterior lobes. Each lobe releases special substances, or hormones which control basic activities within the body. The specific hormones and their activities are shown in Figure 1.
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