A Tour of the Brain
A Tour of the Brain
This tour of the brain is going to start big (the brain as a whole) and wind up small (the brain at the cellular level), and hopefully by the time we’re done you’ll have a foundation of knowledge on which to build. Here are two illustrations showing various views of the brain, to help you understand the information which follows.
Brain Anatomy 1
Brain Anatomy 2
The surface of the brain consists of rounded ridges (technically, one is a gyrus, and several are gyri), which are separated from one another by grooves (technically, fissures or sulci [singular, sulcus]). See Part A of Brain Anatomy 1.
The brain, which is shaped like a boxing glove, is divided into three major parts:
- The cerebrum: The largest part of the brain. Its nerve centers control movement, sensation (touch, feel, position, heat, cold, vision, hearing, etc.), as well as cognition, memory, reasoning and higher mental functioning.
- The cerebellum: Primarily but not exclusively involved in coordination of muscle movement and balance.
- The brain stem (also called the primitive brain): Involved in relaying information from the cerebrum and cerebellum to the rest of the body through the spinal cord, as well as being involved in regulation of basic bodily functions like breathing, control of heart rate, control over blood pressure, and other functions. See Part A of Brain Anatomy 1.
The two parts of the cerebrum are called cerebral hemispheres. Connecting the hemispheres is an arched bridge of nerve tissue called the “corpus callosum,” which allows the right and left sides of the brain to communicate. See Part B of Brain Anatomy 1.
The Brain’s Four Lobes
Each hemisphere consists of four lobes that are located at the front, middle, back and side. Their technical names are based on the part of the skull bone covering that particular area. The frontal lobe is located just where it sounds like, up front, just behind the forehead. The middle (parietal) lobe is in the central part of the brain. The back (occipital) lobe is also just where it sounds—at the back of the head. The temporal lobe is on the side, near the ear. The frontal and middle (parietal) lobes are separated by the central sulcus (groove). The lateral (side) sulcus separates the frontal and temporal lobes. See Part C of Brain Anatomy 2.
The four lobes of the cerebrum are the frontal lobe, just behind the forehead and front part of the skull, the temporal lobe near the ear, the parietal lobe which lies behind the frontal lobe, and the occipital lobe which is at the back of the head. The central sulcus is a deep grove that separates the frontal lobe from the parietal lobe, the lateral sulcus is a deep groove separating the frontal lobe from the temporal lobe. It was, of course, too easy to just call the separation between the center and back lobes, the “back groove,” and thus the correct term: the parieto-occipital sulcus. See Part C of Brain Anatomy 2.
While the functioning of the brain is very complex and specific functions are affected by input from other areas of the brain, the four lobes do have specific functions associated with each of them. (These lists are not intended to be all-inclusive.)
Frontal lobe: abstract thought; problem solving; creative thought; emotion; judgment; inhibition (impulse control); initiative; smell, sexual urges, control of voluntary and coordinated movements.
Parietal lobe: Appreciation of form, sensory combination and comprehension (pain, pressure, heat, cold, touch); some language, reading and visual functions.
Occipital lobe: Reading and vision, including an important visual center that is critical for the complex processing of vision and for relating vision to other sensory experiences.
Temporal lobe: Hearing, memories (auditory, visual, other), fear, music, and some speech, hearing, language and behavior.
The cerebellum is located below the occipital lobe at the back and towards the base of the brain, just behind the brain stem. It also has two hemispheres which are connected by tissue called the vermis. The cerebellum has a series of folds on its surface. The cerebellum has three sets of paired bundles of nerves that carry impulses from one area of the brain to another, and connect the cerebellum with the brain stem. The superior (top) pair connects the cerebellum with the midbrain; the middle connects it with the pons, and the inferior (bottom) connects it with the medulla oblongata. See Part D of Brain Anatomy 2.
The impulses are modulated in the cerebellum, which acts a reflex center that coordinates the movement of muscles. It is also the primary center for proprioception, which is another expensive word for position sense. It just means the ordinary, innate ability to know where your arms and legs are in space without having to see them. To understand better, just try this: Scoot forward toward the edge of your computer chair (after all, you’re probably reading this on your screen), close your eyes, put one hand behind your back and open and close your fist. You obviously knew where your hand was and how it was moving, even without looking at it. The cerebellum is critical for the coordinated movements required for fine motor skill, normal gait in walking or running, tone of muscles and balance.
Other Parts of the Brain
The brain stem is located at the base of the brain beneath the cerebrum and in front of the cerebellum. It consists of many fiber tracts carrying nerve impulses from the brain to the spinal cord, plus areas called nuclei. The nuclei are clusters of brain cells having specific functions that are primarily related to basic bodily functions and the senses. The brain stem consists of several distinct areas called the diencephalon, pons, midbrain, and medulla oblongata. See Part D of Brain Anatomy 2.
The diencephalon includes the thalamus, which is a relay station for sensory impulses that come from other parts of the brain and which are forwarded to other regions of the brain for interpretation. Pain, touch and temperature sensation are functions that are affected both by the cortex and the thalamus. The diencephalon also includes the hypothalamus, which plays a key role in regulation of heart rate, blood pressure, body temperature, control of fluids, hunger glandular secretions, sleep, wakefulness and endocrine functions. Optic nerve fibers also cross over in the thalamus, and damage to this area can adversely affect vision.
The midbrain is a small area in the brain stem between the diencephalon and the pons. See Part B of Brain Anatomy 1, and Part D of Brain Anatomy 2. It connects the brain stem and spinal cord with the cerebral cortex. It contains several important reflex centers, that help control posture and balance , hearing, movement of the head and eyes in a coordinated way, visual reflexes and responses to auditory stimuli.
The pons is a round protuberance on the brain stem that lies between the midbrain and the medulla oblongata. See Part B of Brain Anatomy 1, and Part D of Brain Anatomy 2. It relays information between the medulla oblongata, cerebrum and cerebellum. It contains centers that also impact the rate and depth of respirations. Damage to the pons can result in an inability to close the mouth, chewing difficulty, atrophy of the muscles involved in chewing, visual problems, a serious loss of coordination in motor functions of the head, neck and face, and/or hearing problems. One mother reported trouble with the tongue and swallowing.
The medulla oblongata (or just “the medulla”) is the connection between the brain stem and the rest of the brain. See Part B of Brain Anatomy 1, and Part D of Brain Anatomy 2. All nerve impulses running up and down the body run through the medulla, which contains critical clusters of nerve cells called nuclei that control basic body functions. The medulla has a respiratory center to regulate breathing, a cardiac center that can increase or decrease heart rate and a vasomotor center that regulates how blood vessels dilate and constrict, thereby affecting blood pressure. The possible consequences to damage to the medulla are obvious.
The motor cortex is a ribbon of tissue located next to the central sulcus (groove); it controls motor movement
The corticospinal tracts connect the motor cortex to the body, running through the brain stem into the spinal cord. The major function of this pathway is fine voluntary motor control of the limbs. The pathway also controls voluntary body posture adjustments.
Turning to the microscopic level, the brain is made up of many different kinds of cells. The brain functions discussed above are carried out at the cellular level by nerve cells called neurons, which use electro-chemical impulses to both receive information from other cells (via dendrites) and send information (via axons). The dendrites are the short branches lying close to the neuron body in the illustration below. The axons are the longer nerve fibers. The axons are lined with myelin that serves to “insulate” the nerve fibers. The area where electro-chemical impulses are communicated from one cell to another is called the synapse. The brain has many other cells that serve support functions in the brain.
There are multiple short dendrites attached to a neuron (nerve cell) body, which receive information from other neurons through an electro-chemical impulse. The information is then “processed” by the neuron body and another electro-chemical impulse is used to send it on to another cell via the axon. Generally there is only one axon per neuron. Think of Federal Express. Sometimes several trucks will arrive at a FedEx distribution center with packages for a particular destination (information received via multiple dendrites); the center (neuron body) “bundles” the information and sends it on in a single shipment via the axon. Other times, the single “package” (nerve impulse) arrives and is then forwarded to its destination. Whether there is bundling or a straight shot depends on the location, function, “strength” or intensity of the impulse, the size and shape of the neuron and other factors.
The cell bodies of neurons are generally located in an area of the brain that is called “grey matter” while the axons with a myelin sheath, connecting cell bodies of neurons to other areas, are referred to as “white matter,” which forms most of the deeper parts of the brain. Groups of cell bodies with discrete functions are called nuclei. These generally appear as grey matter and are mixed within the white matter in the brain stem.
If a neuron is damaged to the extent that the cell dies it will not regenerate. Damage to an axon may or may not be repaired depending upon the degree of damage.
The other major type of cells in the nervous system are the glial cells, which provide support and nutrition, form myelin to sheath neurons, and are part of the process of transmitting signals within the nervous system.
Although the underlying Greek word for this type of cell means “glue” and many people consider these cells as the “glue” which holds the nervous system together, this is actually not true. Glial cells are kind of a “support group” for neurons so that neurons can function most effectively. Glial cells supply nutrients and oxygen to the neurons; they insulate neurons from one another; they surround and hold the neurons in place, and destroy pathogens (infectious agents) and remove dead neurons. As noted above, they also help with signal transmission.
Back to Location
The “brain tour” above hopefully provided some basic understanding about the types of problems that are associated with injuries to various parts of the brain. Please remember, though, that while knowing the location of an injury gives valuable information about the types of impairment that can occur, due to the complexity of the brain, prognosis cannot not be determined based solely upon an assessment of injury location. The other component of prognosis is the simple passage of time, how quickly recovery occurs, or the progression of the recovery of function.