Basic Cerebrum Structures

shocks the brain might receive and helps to make the brain more buoyant within the skull itself. Without the CSF, the brain would weigh heavily on the blood vessels that supply it, which could affect the circulation to the brain. It also helps to balance the chemistry of the CNS by transferring oxygen, ions, proteins, and nutrients into and outside the CNS to the periphery.

The sense organs are also important parts of the nervous system. The special senses like sight, smell, balance, taste, and hearing all require special organs or receptors in order to pick up mechanical or chemical signals from the body surfaces or within the body itself, sending these signals to the brain or spinal cord, where they can be acted on or interpreted. Reflexes are reactions that happen when the sense organs pick up something that gets acted on without conscious choice over whether or not to have the specific response involved.

BASIC CEREBRUM STRUCTURES

As mentioned, the three broad categories of brain structures are the cerebrum, the cerebellum, and the brainstem. These are subdivided in different ways, which we will soon talk about. The cerebrum is the biggest part of the brain. This is the five-lobed, two-hemispheric structure involved in higher-level interpretation of data, thought processes, and initiation of voluntary activities. Reasoning, learning, and judgment are held within and integrated inside the cerebrum.

The cerebellum or “little brain” is in the back and below the cerebrum. It helps to control balance, normal posture, and muscle movement, among other things. The brainstem is the deeper and less conscious part of the brain. It helps to control emotions, sleep cycles, all of the automatic functions of the body, swallowing, sneezing, cough, vomiting, and digestion. Figure 4 shows the main structures of the brain from a midline viewpoint:

Figure 4.

The cerebrum has two hemispheres and is partly lateralized, which means that each side of the brain is unique in what it can do. If a person has a stroke or tumor on one side of the brain, the symptoms will be different than if the same dysfunction would have happened on the other hemisphere. Many motor and sensory functions in the brain are linked to the opposite side of the body. A stroke in the left hemisphere, for example, will cause paralysis on the right side of the body. Figure 5 shows what lateralization of the hemispheres look like. About 92 percent of people are left-hemisphere dominant:

Figure 5.

The deeper gyri between each of the four lobes of the brain are called fissures. They help you identify the different lobes. The five lobes of the brain are called the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. There are subdivisions in many of the lobes. For example, the frontal lobe has Broca’s area, which is one of the two major speech centers of the brain and the temporal lobe has Wernicke’s area, which is responsible for the understanding of speech. There is a motor strip near the back of the frontal lobe that abuts against the sensory strip on the most anterior part of the parietal lobe. Figure 6 shows what the different lobes of the brain look like and are named:

Figure 6.

There are different broad categories of function of each of the lobes of the brain. You should try to remember these as they help you keep track of where the important aspects of brain function come from or are integrated by. This is a summary of the functions of the four lobes:

• Frontal lobe—this is the lobe responsible for judgment, problem-solving, planning, emotional interpretation, behavior, personality, speech and writing production, movement, self-awareness, concentration, and intelligence.

• Parietal lobe—this is the lobe responsible for word and language interpretation, sensory functions of pain, touch, and temperature, interpretation of the special senses, memory integration, and visuospatial perception.

• Occipital lobe—the function of this lobe is almost exclusively for the way the brain interprets all aspects of vision, including movement, light, and color.

• Temporal lobe—the function of this lobe is the understanding of language in

Wernicke’s area, organization and sequencing, hearing, and memory consolidation.

You will hear a lot about gray matter and white matter when it comes to brain tissue. These are named for their respective gray and white appearances. Gray matter tends to be on the outer surface of the cerebrum and consists of the cell bodies of the neurons, which is where the nuclei and other internal cell structures are located. The white matter tends to be seen in the deeper brain regions. The reason it is white is that it contains myelin, which is the fatty sheath around nerve axons. These are where brain pathways are located and where signals are sent from one place to another.

The longitudinal fissure is what separates the two hemispheres. This is the deepest brain sulcus. The falx cerebri is a deep fold of dura mater that separates the two halves of the brain in this fissure. The other main sulci you should recognize are the central sulcus, which separates the frontal and parietal lobes, the lunate sulcus, which is in the occipital lobe, and the lateral sulcus, which separates the temporal lobe from both the parietal and frontal lobes. Figure 7 shows the major gyri and sulci in the brain:

Figure 7.

The three main gyri to know about are the precentral gyrus, just in front of the central sulcus, which is where the motor strip is located, the postcentral gyrus, which is just behind the central gyrus and is where the somatosensory strip is located, and the superior temporal gyrus, which is near the top of the temporal lobe and is where sound is received and processed.

The blood supply to the cerebrum involves three major arteries. These are the anterior cerebral arteries, the middle cerebral arteries, and the posterior cerebral arteries. They supply the front, middle, and back parts of the brain, respectively. The venous drainage involves small cerebral veins that together drain into the large dural venous sinuses on the outer surface of the brain.

The ventricles are the fluid-filled central cavities inside the brain. Each of these has a choroid plexus lining it that makes cerebrospinal fluid or CSF. The lateral ventricles are large and most superior in the brain. There is a small third ventricle in the deeper brain tissue as well as several channels that connect the different ventricles. The fourth

ventricle is just in front of the cerebellum and beneath the cerebellum is the cisterna magna. The outflow downward from the cisterna magna is the central canal of the spinal cord, which is also filled with CSF. There is normally a balance between the production of CSF and the reuptake of CSF in the brain. If this does not happen, fluid can build up in the ventricles, leading to hydrocephalus, or in the spinal cord, leading to syringomyelia. Figure 8 shows what the ventricular system of the brain looks like:

Figure 8.

The brain is surrounded by the skull and is protected by it. There are eight bones of the skull that are fused together. The points where the bones are fused are called “suture lines”. The main skull bones are the frontal bone, the paired parietal bones, the paired

temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone. Inside the floor of the skull are three levels, called the anterior fossa, the middle fossa, and the posterior fossa. These fit the brain snuggly in the brain case or cranium.

Figure 9 shows what the skull bones look like:

Figure 9.

The blood supply to the entire brain comes from the internal carotid artery anteriorly and the vertebral artery posteriorly. The internal carotid artery is what separates into the anterior, middle, and posterior cerebral arteries plus the small ophthalmic artery that goes to the eyes. It is connected to the vertebral arterial system through the basilar artery. The vertebral artery supplies mainly the cerebellum, the underside of the

cerebrum, and the brainstem. The whole thing together comes together in what is called the Circle of Willis, shown in figure 10:

Figure 10.

The Circle of Willis is the way in which blood supply can come to one area of the brain to make up for narrowed or blocked arteries in other parts of the brain. It is not a perfect system but does allow for some redundancy in the circulation of the brain.