Higher brain functions

Sleep

Dr. Rajaa Ahmad





Sleep is defined as unconsciousness from which the person can be aroused by sensory or other stimuli. It is to be distinguished from coma, which is unconsciousness from which the person cannot be aroused. There are multiple stages of sleep, from very light sleep to very deep sleep;

Two Types of Sleep. During each night, a person goes through stages of two types of sleep that alternate with each other. They are called
(1) slow-wave sleep,
because in this type of sleep the brain waves are very strong and very low frequency,
(2) rapid eye movement sleep (REM sleep),
because in this type of sleep the eyes undergo rapid movements despite the fact that the person is still asleep.

Most sleep during each night is of the slow-wave variety; this is the deep , restful sleep that the person experiences during the first hour of sleep after having been awake for many hours.
This sleep is associated with 10 -30 % decrease in
-blood pressure,
-respiratory rate,
-and basal metabolic rate.
Although slow-wave sleep is frequently called “dreamless sleep,” dreams and sometimes even nightmares do occur during slow-wave sleep. The difference between the dreams that occur in slow-wave sleep and those that occur in REM sleep is that the dreams of slow-wave sleep usually are not remembered.


REM sleep, occurs in episodes that occupy about 25 per cent of the sleep time in young adults; each episode normally recurs about every 90 minutes. This type of sleep is not so restful, and it is usually associated with vivid dreaming.

In a normal night of sleep, bouts of REM sleep lasting5 to 30 minutes usually appear on the average every90 minutes.
There are several important characteristics of REM sleep:
1. This type of sleep is not so restful, and it is usually associated with dreaming and active bodily muscle movements.
2. The person is even more difficult to arouse by sensory stimuli than during deep slow-wave sleep and yet people usually awaken spontaneously in the morning during an episode of REM sleep.
3. Muscle tone throughout the body is depressed.
4. Heart rate and respiratory rate usually become irregular, which is characteristic of the dream state.

5. The rapid movements of the eyes.

6. The brain is highly active in REM sleep.
The electroencephalogram(EEG) shows a pattern of brain waves similar to those that occur during wakefulness. This type of sleep is also called paradoxical sleep because it is a paradox that a person can still be asleep despite marked activity in the brain.


REM sleep is a type of sleep in which the brain is quite active. However, the brain activity is not channeled in the proper direction for the person to be fully aware of his or her surroundings, and therefore the person is truly asleep.

Basic Theories of Sleep

Sleep Is Believed to Be Caused by an Active Inhibitory Process.
An earlier theory of sleep was that the excitatory areas of the upper brain stem, the reticular activating system, simply fatigued during the waking day and became inactive as a result.
This was called the passive theory of sleep


An important experiment changed this view
to the current belief that sleep is caused by an active
inhibitory process: it was discovered that transecting
the brain stem at the level of the mid pons creates a
brain whose cortex never goes to sleep. In other words, there seems to be some center located below the mid pontile level of the brain stem that is required to cause sleep by inhibiting other parts of the brain

Brain Waves

Electrical recordings from the surface of the brain or
even from the outer surface of the head demonstrate
that there is continuous electrical activity in the brain.
Both the intensity and the patterns of this electrical
activity are determined by the level of excitation of different parts of the brain resulting from sleep, wakefulness, or brain diseases such as epilepsy or even psychoses.

The undulations in the recorded electrical

Potentials are called brain waves, and the entire record is called an EEG ((electroencephalogram

The intensities of brain waves recorded from the

surface of the scalp range from 0 to 200 microvolts, and their frequencies range from once every few seconds to 50 or more per second. The character of the waves is dependent on the degree of activity in respective parts of the cerebral cortex, and the waves change markedly between the states of wakefulness and sleep and coma.

In normal healthy people, most waves in the EEG can be classified as alpha, beta, theta, and delta waves




Alpha waves are rhythmical waves that occur at frequencies between 8 and 13 cycles per second and are found in the EEGs of almost all normal adult people when they are awake and in a quiet, resting state.
Beta waves occur at frequencies greater than 14 cycles per second up to 80 cycles per second. Occur when the awake person’s attention is directed to some specific type of mental activity

Theta waves have frequencies between 4 and 7 cycles per second. They occur in children and in adult during emotional stress and in many brain disorders, often in degenerative brain states.
Delta waves EEG with frequencies less than 3.5 cycles per second. They occur in very deep sleep, in infancy, and in serious organic brain disease.
Alert wakefulness (beta waves)
Quiet wakefulness (alpha waves)

Origin of Brain Waves

The discharge of a single neuron or single nerve fiber
in the brain can never be recorded from the surface of
the head. Instead, many thousands or even millions of
neurons or fibers must fire synchronously; only then will
the potentials from the individual neurons or fibers
summate enough to be recorded all the way through the skull.

Epilepsy

Epilepsy (also called “seizures”) is characterized by
uncontrolled excessive activity of either part or all of the central nervous system. A person who is predisposed to epilepsy has attacks when the basal level of excitability of the nervous system (or of the part that is susceptible to the epileptic state) rises above a certain critical
threshold. As long as the degree of excitability is heldbelow this threshold, no attack occurs.


Epilepsy can be classified into three major types:
grand mal epilepsy, petit mal epilepsy, and focal epilepsy.