Neurophysiology II PHS 415 by Adedayo Lawrence Department
Neurophysiology II (PHS 415) by Adedayo, Lawrence Department of Physiology Faculty of Basic Medical and Health Sciences Bowen University Iwo First Semester 2018/19 session 12/27/2021 1
SLEEP, ALERT BEHAVIOUR AND ELECTRICAL ACTIVITY OF THE BRAIN Lecture 2 12/27/2021 2
What is sleep? v Sleep is defined as unconsciousness from which the person can be aroused by sensory or other stimuli. v Or sleep is a readily reversible state of reduced responsiveness to, and interaction with, the environment. v It is to be distinguished from coma, which is unconsciousness from which the person cannot be aroused. v Its duration varies inversely with age (average 18 hrs in infants, 8 hrs in adults and 6 hrs in old persons). v The sleep/wakefulness 24 hrs rhythm is determined mainly by synchronization with the 24 -hours light/dark cycle. v It is also affected by various factors such physical, psychological etc 12/27/2021 3
PHYSIOLOGICAL CHANGES DURING SLEEP v Circulatory system : the heart rate, cardiac output, vasomotor tone and arterial blood pressure all decreased and the circulation time is prolonged v Respiratory system : the rate and depth of respiration are decreased (pulmonary ventilation is decreased with tendency to acidosis. v Nervous system: most reflexes disappear, the voluntary activity and sensory perception are abolished, and a positive Babinski’s sign is obtained, the activity of different parts of the CNS is also restored by sleep. 12/27/2021 4
Cont’d v. The skeletal muscle tone, body temperature and metabolic rate are decreased (later by 10 -30%). v. Most endocrine secretions are decreased (but the secretion of the growth hormone increases during sleep). On the other hand, the secretions of the GIT tend to increase during sleep. 12/27/2021 5
TYPES OF SLEEP • There are two types of sleep that normally alternate with each other namely: v. Slow wave sleep (non-rapid eye movement, NREM) v. Rapid Eye Movement (REM, desynchronized or paradoxical) sleep 12/27/2021 6
Slow wave sleep (non-rapid eye movement, NREM) v This type is the first to occur when the person falls asleep. It is the characterized by slow EEG waves and absence of rapid eye movements REM. EEG recording during this type shows 4 stages that occur respectively as follows: v Stage 1: sleep is very light and EEG shows a slow theta wave rhythm. Marked by a presence of some theta activity (3. 5 -7. 5 Hz). This stage is actually a transition between sleep and wakefulness. 12/27/2021 7
Cont’d v Stage 2: sleep is light and EEG shows characteristics sleep spindles (bouts of large alpha-like waves with frequency of 1215 Hz that superimpose on theta rhythm, and each bout last for 1 -2 seconds). K complexes are sudden, sharp wave forms, which, unlike sleep spindles, are usually found only during stage 2 sleep. The subject is sleeping soundly now; but if awakened, she might report that she has not been asleep. v Stage 3: sleep is moderately-deep, and the EEG shows delta waves at a frequency of about 3 -4 Hz 12/27/2021 8
Cont’d • Stage 4: sleep is deepest and the EEG shows delta waves with maximal slowing (frequency 0. 5 -1. 5 Hz), they are sometimes called delta-max waves. • The distinction between stage 3 and stage 4 is not clear-cut; stage 3 contains 20 -50 percent of delta activity, and stage 4 contains more than 50 percent delta activity. • During NREM sleep, sleep talking and walking often occur, and dreams do also occur, but they are not consolidated in memory (so they are usually not remembered). 12/27/2021 9
Rapid Eye movement (REM, desynchronized or paradoxical) sleep v For the rapid eye movements that characterize it. REM sleep has also been called paradoxical sleep, because of the presence of beta activity, which is usually seen during wakefulness. v The EEG shows desynchronized beta rhythm as that encountered in the arousal response. This indicates brain activity but the person is still asleep and threshold for arousal is high. Dreams can be remembered, and penile erection frequently occur during REM. 12/27/2021 10
EEG recording during the various stages of sleep 12/27/2021 11
Distribution of sleep stages (sleep cycle) v Sleep starts normally by the slow wave type for about 70 -110 (average 90) minutes, then REM sleep follows for about 20 mins, and this is repeated cyclically with gradual prolongation of the REM sleep periods. Thus, there 46 sleep cycles during a single night, and the REM sleep constitutes about 20 -25% of the total sleep time. 12/27/2021 12
Mechanisms (Theories of sleep) Sleep primarily results from depression of the cerebral cortex secondary to inhibition of the ARAS activity, which can occur by one of the following mechanisms: Passive mechanism (Theory): this assumes that sleep occurs as a result of passive inhibition of the ARAS, which can be induced by either its fatigue (after a period of wakefulness) or by decreasing its activity (by elimination of its exciting stimuli e. g visual, auditory, painful and other stimuli). However, such factors alone cannot lead to sleep, although they may help its onset. 12/27/2021 13
Cont’d Active mechanism (theory): It was found that excitation of certain centres in the brainstem inhibits the ARAS and it is believed that sleep occurs as a result of active inhibition of ARAS by signals discharged from such centres. The centre are: v The raphe nuclei in the medulla and lower pons: stimulation of these nuclei depresses the ARAS and produces slow wave (NREM) sleep. The neurons of these nuclei release serotonin. 12/27/2021 14
Cont’d v The hypothalamic suprachiasmal area also promotes sleep by inhibiting the ARAS activity, and is probably the centre that synchronizes the sleep/wakefulness rhythm with 24 hrs lightdark cycle (acting as a waking/sleeping regulator centre) v The reticular formation in the upper brainstem: stimulation of this area produces REM sleep by discharging cholinergic signals directly from the cerebral cortex. It was believed that REM sleep is produced by noradrenergic signals from the nucleus ceruleus. 12/27/2021 15
SLEEP DISORDERS OR DISTURBANCES v Insomnia: which may be defined as the subjective problem of insufficient or non-restorative sleep despite an adequate opportunity for sleep, occurs at one time or another in almost all adults. Persistent insomnia can be due to many different mental and medical conditions. It can be relieved temporarily by "sleeping pills, " especially benzodiazepines. but prolonged use of any of these pills is unwise because they compromise daytime performance and can be habit -forming. v Fatal familial insomnia: is a progressive prion disease that occurs in inherited and sporadic forms. It is characterized by worsening insomnia, impaired autonomic and motor functions, dementia, and death. Patients with the disease have severe neuronal loss and gliosis in the ventral and mediodorsal nuclei of the thalamus and the olives in the medulla oblongata. 12/27/2021 16
Cont’d v Sleepwalking (somnambulism), bed-wetting (nocturnal enuresis), and night terrors occur during slow-wave sleep or, more specifically, during arousal from slow-wave sleep. v They are not associated with REM sleep. Episodes of sleepwalking are more common in children than in adults and occur predominantly in males. v Narcolepsy is a disease characterized by episodic sudden loss of muscle tone and an eventual irresistible urge to sleep during daytime activities. In some cases it has been shown to start with the sudden onset of REM sleep almost never occurs without previous slow-wave sleep in normal individuals. v On the hand, dreaming, penile erection, excessive swallowing and bruxism (teeth grinding) occur during REM sleep. 12/27/2021 17
ELECTRICAL ACTIVITY OF THE BRAIN v The cerebral cortex consists of 6 layers, and its neurons are mostly pyramidals cells having extensively branching dendrites that may reach the surface. v Their axons usually give recurrent collaterals that synapse with the dendrites. Fibres from the specific thalamic nuclei terminate mostly in the 4 th layer, while those from the non-specific nuclei terminate at layers 14. 12/27/2021 18
Electroencephalogram 12/27/2021 19
Cont’d v The brain has a marked electric activity, and two types of potentials can be recorded, evoked and spontaneous: v EVOKED CORTICAL POTENTIAL: These are the potential changes that occur in the cerebral cortex after stimulation of a receptor, and they are recorded by exploring electrode applied on the surface of the brain over the excited cortical area. They superimpose on the spontaneous electric activity of the brain, so they are best studied while the subject is under barbiturate anaesthesia. 2 types of these potentials are recorded. Primary evoked potential: this consists of a positive wave followed by a negative wave, which are produced as a result of depolarization followed by hyperpolarization of the nerve cells in the deep layers of the cortex. They are localized to the brain area at which the sensory pathway terminates (recording of this potential has been used for mapping the specific cortical sensory areas). 12/27/2021 20
Cont’d v Secondary evoked potential (Diffuse secondary response): this follows the primary potential, and consists of a large and more prolonged positive wave. It is not localized (appearing in most areas of the cortex at the same time). It is produced by afferent signals discharge to the cortex most probably from the nonspecific thalamic nuclei 12/27/2021 21
The Electroencephalogram (ECG) Or Brain Waves v The EEG is a record of the spontaneous brain electric activity in conscious subjects. If the electrodes are placed directly on the cortical surface, the record is called the ELECTROCORTICOGRAM v Recording of the human EEG may be achieved through applying unipolar or bipolar leads, and 4 main types of waves are usually recorded namely: • Alpha: most dominant waves in conscious adult , voltage is 50 m. V and produced by non-specific thalamic nuclei frequency of 8 -12 Hz marked at parietal and occipital region 12/27/2021 22
Cont’d • Beta: have the lowest voltage, recorded in infants and during brain activity and increased tension in adults. Rhythm is 18 -30 Hz and is marked in the frontal lobe region. • Theta: have higher voltage than alpha wave, recorded in children, emotional stress in some adult, and during light sleep rhythm is 47 Hz and marked in the temporal and parietal region • Delta: have the highest voltage and lowest frequency (0. 5 -4 Hz), and are recorded during deep sleep. They originate from cortical neurons themselves. 12/27/2021 23
The arousal or alert response v This is an EEG response that occurs when the subject becomes alert e. g on opening of his eyes or when solving a problem. v The synchronized alpha rhythm is replaced by rapid irregular low -voltage beta waves. It represents breaking up of the synchronized neuronal alpha activity, so it is also called alpha block or desynchronization. • Such response is reversible (so if the eyes are closed again, the alpha rhythm is resumed). It is due to stimulation of the ascending reticular activating system (ARAS). Such system is also stimulated by corticofugal fibres. 12/27/2021 24
Clinical significance of EEG recording • It helps in determining the sites of focal pathological processes in the brain e. g sites of tumors (in which the EEG waves are distorted) or the sites of fluid collection e. g a subdural haematoma in which the EEG waves are damped. • Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body • EEGs can be used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions • A flat EEG (no electrical activity) is clinical evidence of death • A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure • Epilepsy is not associated with, nor does it cause, intellectual impairments • Epilepsy occurs in 1% of the population 12/27/2021 25
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