The Human Brain

This month we will take a break from technology and do a little anatomy and physiology instead.

The human brain weighs about 3 lbs, is pink-gray in color (the source of the old saying “using your gray matter”), soft, spongy, and loaded with nerves. It has surprisingly little blood for such a large tissue mass. The skull surrounds the brain on five of the six sides with hard bone, but, on the side toward the mouth, the bone is less rigid. It also has large internal voids called ventricles, which change in size as we age.

The brain is further protected by three layers of tissue. The outer layer, closest to the bone, is the dura mater, which is thick, fibrous, and inelastic and adheres to the skull bones.

The middle layer, the arachnoid membrane, is thin and delicate, and is sometimes compared to a spider’s web. It is separated from the dura by the subdural space. Subdural bleeding, often caused by head trauma, can lead to a subdural hematoma or clot. It can be fatal if not found and treated. The subarachnoid space, between the arachnoid membrane and the innermost membrane, the pia mater, is filled with cerebrospinal fluid (CSF).

The pia mater is vascular. Together the three membranes are called the meninges. Meningitis is the inflammation of these membranes and is not a specific disease but a pathologic condition. The problem is to find the cause and treat the source of the inflammation, which can be bacterial, viral, or, occasionally, fungal in origin.

The cerebrum is located in the anterior portion of the forebrain and is split into left and right cerebral hemispheres. Each hemisphere has four lobes: the frontal lobe, located at the front of the cerebrum; the parietal lobe, which is in front of the occipital lobe; the temporal lobe, which is beneath the parietal lobe; and the occipital lobe.

Intelligence, motor skills, short-term memory, and behavior are the responsibility of the frontal lobes, which are the largest of the lobes.

The parietal lobes are responsible for pain, touch sensation, cognition, information processing, speech, and visual perception. Damage to these lobes can produce defects in vision and aphasia (the loss of the power of expression by speech, writing, or signs).

The temporal lobes are the centers for hearing, long-term storage of sensory input, and visual object recognition and categorization.

The occipital lobes are responsible for visual processing, color discrimination, and discrimination of movement.

The deep creases between the lobes are called fissures or sulci. Between the frontal, temporal, and parietal lobes is the lateral sulcus (fissure of Sylvius). The central sulcus, (fissure of Rolando) is between the frontal and parietal lobes. The parieto-occipital fissure is between the parietal and occipital lobes. The transverse fissure runs between the cerebellum and the occipital and temporal lobes. Between the hemispheres of the cerebrum is the longitudinal fissure. Some researchers have tried to correlate the deep creases in the brain tissue with intelligence and memory, but more research is needed.

At the base of the cerebrum is the cerebellum, which comprises two lateral spheres connected by a narrow middle portion called the vermis. This section of the brain is important for a number of motor and cognitive functions, including learning. Some early researchers thought that this was the original brain and that the cerebrum grew out of it as man got smarter over the centuries.

At the very base of the cerebrum and anterior to the cerebellum is the medulla oblongata, which in the terms of a BMET is the interface or port between the brain and the spinal cord. At the base of the brain stem is the pons, consisting of an anterior and posterior part. It is part of the central nervous system and relays sensory information between the cerebellum and cerebrum.

This total complex in the cranial cavity and the spinal column contains CSF, which aids in the protection of the brain, spinal cord, and membranes by acting as a shock absorber. It is produced in the choroid plexus of the four ventricles. The normal pressure is about 5 mm Hg or 130 mm H2O. Increased fluid pressure can cause a multitude of problems, and shunts may have to be installed to drain off excess fluids.

Increases in cranial pressures can indicate many problems, from tumors to injuries to just producing more CSF than can be absorbed.

The electroencephalogram (EEG) is a common testing method used to diagnose problems by measuring the electrical activity of the brain. The EEG amplifier requires a gain of at least 10,000 and a frequency response between DC (0 Hz) and 50 Hz. The chart has a normal speed of 30 mm/sec, but most have up to 16 speeds. The channel width is 28 mm with as many as 21 channels per machine. The rate, height, and length of the waves vary between parts of the brain; some researchers say that each person has a unique and characteristic EEG. The alpha waves (8–30 Hz) are obtained from the occipital lobe at the back of the brain; the patient opening their eyes or excitement can block these waves. Beta waves come from the central part of the brain. They have the same frequency as the alpha waves and can be blocked the same way. During sleep, the voltage of the wave will increase, but the frequency generally decreases to 2–3 Hz. The delta waves (2–3 Hz) are normally found when a patient is in deep sleep. They are common in young people but could signal a problem in an awake adult. The slow rhythmic waves of 4–7 Hz are called theta waves. Electrical silence is the absence of waves and is an indicator of brain death.

One of the challenges in doing these columns is finding the right information. For this one I used seven different references; some did not agree with others. This can lead to problems with the column, but of more concern is that there may be questions on the exam that are not covered in any of the texts used for reference. Some months back I made a mistake on the 3 db point on an amplifier: I used a definition that had been in place for many years and it was wrong. So please be careful when reading references, as they may not always be current or correct.

 

Review Questions

1)     How many lobes are there in the cerebrum?
        a.    4
        b.    6
        c.    7
        d.    8

2)    The speed of an EEG chart recorder is normally _____________.
        a.    25 mm/sec
        b.    30 cm/sec
        c.    30 mm/sec
        d.    15 mm/sec

3)    How many layers of tissue make up the meninges?
        a.    5
        b.    3
        c.    2
        d.    1

4)    Which lobe of the brain is associated with hearing?
        a.    frontal
        b.    temporal
        c.    occipital
        d.    parietal

Answers: 1-d; 2-c; 3-b; 4-b

David Harrington, PhD, director of staff development and training at Technology in Medicine, Holliston, Mass, is a member of 24×7’s editorial advisory board.