Higher Brain Functions

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Chapter: Anatomy and Physiology for Health Professionals: Central Nervous System

Although ongoing studies continue, the human brain’s higher functions are extremely difficult to truly understand.


Higher Brain Functions

Although ongoing studies continue, the human brain’s higher functions are extremely difficult to truly understand. Brain waves are based on electrical activity, and normal brain functions involve con-tinuous electrical activity of the neurons. Certain aspects of electrical brain activity can be recorded on an electroencephalogram­ (EEG), which involves placing electrodes on the patient’s scalp. The EEG measures voltage differences between the areas of the cerebral cortex. Brain waves are the patterns of neuronal electrical activity that is recorded (FIGURES­ 12-11A and B). They are generated by the ­activity of synapses at the surface of the cortex. Every individual’s brain wave patterns are unique but are grouped into four primary types:

Alpha waves: Relatively regular, rhythmic, synchronous­ waves of low amplitude (8–13 Hz), they usually indicate calm and relaxed ­wakefulness.

Beta waves: Rhythmic but less regular waves that have a higher frequency than alpha waves (14–30 Hz), they occur during mental alertness such as when concentrating or looking at visual stimuli.

Theta waves: Irregular waves that are more common in children and have a low frequency (4–7 Hz), they may occur in adults when concentrating.

Delta waves: High amplitude (4 Hz or less) waves occurring in deep sleep or when something (such as anesthesia) dampens the reticular activating system; if these waves exist in a conscious adult, they indicate brain damage.

Brain waves change with brain disease, aging, ­sensory stimuli, and the chemical balance of the body.



Consciousness

Consciousness is defined as conscious perception of sensation, capabilities related to higher mental pro-cessing, voluntary initiation, and control of move-ment. Consciousness levels are graded based on alertness, drowsiness or lethargy, stupor, and coma. It involves simultaneous activity of large portions of the cerebral cortex and is superimposed on other types of neural activity (both motor control and cognition). It is holistic and completely interconnected, for example, a memory triggered by a location, an odor, a person, or other stimuli.


Sleep and Sleep Patterns

Sleep is a state of partial unconsciousness from which we may be aroused by stimulation. It is dif-ferent from coma, from which a person cannot be aroused by stimulation. During sleep, most cortical activity is depressed, but brain stem functions con-tinue. These functions include control of heart rate, blood pressure, and respiration. The two major types of sleep are non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Each of these has different patterns on an EEG. TABLE 12-2 lists the stages of sleep.


Sleep patterns are normally based on a natural 24-hour circadian rhythm. The sleep cycle is con-trolled by the hypothalamus. Sleep occurs because of the inhibition of the brain stem’s reticular activat-ing system. The preoptic nucleus is the actual com-ponent that turns off arousal and puts the cerebral cortex to sleep.

Sleep patterns alternate through most of the sleep cycle. There are four stages of NREM sleep and also REM sleep. As NREM and REM sleep alternate, partial arousals occur occasionally. These are defined by their EEG patterns. In the first 30–45 minutes of sleep, the first two stages of NREM pass, followed by stages 3 and 4, which are known as slow -wave sleep. This type of sleep is thought to have restor-ative properties for the mind and body. As a result, sleep -deprived people­ will spend more time in slow-wave sleep the next time they do fall asleep. After these stages, as the sleep patterns deepen, the fre-quency of EEG waves declines while their amplitude increases. There are progressive decreases in blood pressure and heart rate.

After about 90 minutes, once NREM stage 4 has been reached, there is an abrupt change in EEG patterns,­ which become irregular. There is a quick backtracking through the sleep stages until alpha waves reappear. The alpha waves are more typical of brain activity when we are awake. This return of alpha waves indicates the onset of REM sleep. There are increases in heart and respiratory rates, along with blood ­pressure. There is a decrease in motility inside the gastrointestinal tract. During REM sleep, the brain uses more oxygen than during waking hours. The skeletal muscles are limp due to active inhibition, but the eyes move quickly underneath the eyelids. The majority of dreaming occurs during REM sleep. The body’s temporary paralysis keeps us from acting out what we are dreaming.

Sleep Regulation

Sleep and wakefulness cycles occur in a natural 24-hour or circadian rhythm. The hypothalamus regulates the timing of sleep cycles. Its suprachias-matic nucleus acts like the body’s clock to regulate the sleep-inducing area called the preoptic nucleus. As the reticular activating system inside the brain stem is inhibited, the preoptic nucleus causes the cerebral cortex to enter sleep. The arousal system is switched off, and the RAS centers help maintain the awake state as well as dreaming and other sleep stages. Just prior to awakening, hypothalamic neu-rons release orexins, which are peptides that ­function as “waking up” chemicals. Therefore, some brain stem reticular formation neurons fire at heightened rates, arousing the cerebral cortex. Many chemicals in the body are linked to sleepiness; the importance of their functions is not fully understood.

Sleep Deprivation

Deprivation of REM sleep causes depression and moodiness, resulting in various personality dis-orders. Dreaming may help an individual to focus thoughts while awake. REM sleep helps the brain analyze life events and manage emotional problems via dream images. It also eliminates synaptic con-nections that are not needed. Dreaming actually helps us to forget problematic occurrences. The need for sleep declines from infancy (from approx-imately 16 hours per day) to reach a plateau of 7.5–8.5 hours (in early adulthood),­ and then declines again in old age. Sleep patterns may change differently ­throughout life for every individual. Stage 4 sleep declines steadily over time and may not even occur in elderly individuals. REM sleep occupies about 50% of the sleep of infants but declines to about 25% in adults.

1. What are the four primary types of brain wave patterns?

2. What are the two major types of sleep?

3. What are the results of REM sleep deprivation?

4. What are the functions of Broca’s area and Wernicke’s area?

Language Functions

In the brain, language involves nearly all the left asso-ciation cortex, especially Broca’s area and ­Wernicke’s area. Lesions of Broca’s area may cause difficulty speak-ing, writing, typing, or using sign language. Lesions of Wernicke’s area may cause lack of understanding of language or the use of excessive nonsense words while speaking. Language, as controlled by the brain, also involves the basal nuclei and surrounding portions of the cerebral cortex. The right association cortex is involved in nonverbal ­language or body language.

Memory Functions

Memory involves storage and retrieval of informa-tion and is required for learning, establishing behav-iors, and normal conscious activity. ­Short -term memory (working memory) focuses on small pieces of information needed for a few moments and is based on approximately seven to eight groups of information. Long -term memory may be unlim-ited, but is affected by changes to the body over time and declines with aging. The transfer of information from short-term to long-term memory is influenced by your emotional state, rehearsing or repeating material, associating new information to stored infor-mation, and automatic memory (which is the uncon-scious memorizing of something that occurred such as what a person was wearing).

Memories that are transferred to long-term ­memory become permanent over time. Memory consolidation appears to involve “inserting” new facts into areas of knowledge previously stored in the cerebral cortex. This process is primarily han-dled by the hippocampus and surrounding temporal cortical areas. They communicate with the prefrontal cortex and thalamus during these functions. How-ever, widespread amnesia occurs if there is bilateral destruction. Consolidated memories are retained, yet new sensory input is not associated with older sensory input. The individual lives in the present time with little ability to connect with the past, a con-dition known as anterograde amnesia. If a physi-cian consulted a patient with anterograde amnesia, then left the room and returned a short while later, the patient would not remember the physician. The loss of memories from the distant past is called retrograde­ amnesia.

It is believed that certain portions of every memory are stored close to areas of the brain that need to utilize them. In this manner, new sensory input can be quickly related to older sensory input of a similar type. For example, musical memory is stored in the temporal cortex, while visual memories are stored in the occipital cortex.

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