To some degree, the role of the pituitary hormones adrenocorticotropin , thyrotropin , and growth hormone in supporting lactation in women is inferred from the results of studies done on animals and from clinical observations that are in agreement with the results of animal studies. Adrenal corticoids also appear to play an essential role in maintaining lactation. The stimulus of nursing or suckling supports continued lactation. It acts in two ways: it promotes the secretion of prolactin and possibly other pituitary hormones of value in milk formation , and it triggers the release of yet another hormone from the pituitary gland— oxytocin , which causes the contraction of special muscle cells around the alveoli in the breast and ensures the expulsion of milk.
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About 30 seconds elapse between the beginning of active suckling and the initiation of milk flow. The nerve supply to the mammary glands is not of great significance in lactation, for milk production is normal after the experimental severing of nerves to the normal mammary glands in animals or in an udder transplanted to the neck of a goat. Conversely, embarrassment or fright can inhibit milk ejection by interfering with the release of oxytocin; alcohol, also, is known to block milk ejection in women, again by an action on the brain.
Beyond its action on the mammary glands, oxytocin affects uterine muscle, so that suckling can cause contractions of the uterus and may sometimes result in cramp. Since oxytocin release occurs during sexual intercourse , milk ejection in lactating women has been observed on such occasions. Disturbance of oxytocin secretion, or of the milk-ejection reflex, stops lactation just as readily as a lack of the hormones necessary for milk production, for the milk in the breast is then not extractable by the infant.
Many instances of nursing failure are due to a lack of milk ejection in stressful circumstances; fortunately, treatment with oxytocin, coupled with the reassurance gained from a successful nursing, is ordinarily successful in overcoming the difficulty. Suckling can initiate lactation in nonpregnant women. This has been seen most often in women of childbearing age but also has been observed in older persons. A baby who had lost his mother was suckled by his year-old grandmother, who had borne her last child 18 years before.
The grandmother produced milk after a few days and continued to nurse the baby until he was a year old and could walk. Rarely, lactation has been reported to set in after operations on the chest; in such instances it is attributed to injury or irritation of the nerves in this region. Such observations argue against the possibility that lactation continues simply as a consequence of emptying the breasts.
Milk can be regarded as an emulsion of fat globules in a colloidal solution of protein together with other substances in true solution.
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Two constituents of milk—the protein casein and milk sugar, or lactose —are not found elsewhere in the body. Breastfeeding is particularly advantageous because of the nutritional, immunologic, and psychological benefits. The bonding that is established through breast-feeding is advantageous to building the parent-child relationship. The nutritional status of the mother is important throughout this period. The use of drugs or smoking by the mother can adversely affect the infant; many drugs are secreted in breast milk, and smoking reduces breast milk volume and decreases infant growth rates.
The milk released from the breast when lactation starts differs in composition from the mature milk produced when lactation is well established. The early milk, or colostrum , is rich in essential amino acids , the protein building blocks essential for growth; it also contains the proteins that convey immunity to some infections from mother to young, although not in such quantity as among domestic animals. The human infant gains this type of immunity largely within the uterus by the transfer of these antibody proteins through the placenta; the young baby seldom falls victim to mumps , measles , diphtheria , or scarlet fever.
For a short time after birth, proteins can be absorbed from the intestine without digestion, so that the acquisition of further immunity is facilitated. The growth of harmful viruses and bacteria in the intestines is probably inhibited by immune factors in human milk.
After childbirth the composition of milk gradually changes; within four or five days the colostrum has become transitional milk, and mature milk is secreted some 14 days after delivery.
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There is no typical age at which human infants are weaned, for this varies from country to country and among the social classes of a nation. In India women in the higher socioeconomic groups tend to use artificial feeding, while the reverse relationship holds in Britain and the United States. Most commonly, weaning is a gradual process, with a gradual increase in the proportion of solid food supplied to the infant together with breast milk.
With the reduced demand of the baby, lactation slowly declines and stops. In fact, your brain and body stay remarkably active while you sleep. Recent findings suggest that sleep plays a housekeeping role that removes toxins in your brain that build up while you are awake. Everyone needs sleep, but its biological purpose remains a mystery. Sleep affects almost every type of tissue and system in the body — from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance.
Research shows that a chronic lack of sleep, or getting poor quality sleep, increases the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression, and obesity. Sleep is a complex and dynamic process that affects how you function in ways scientists are now beginning to understand. This booklet describes how your need for sleep is regulated and what happens in the brain during sleep.
The hypothalamus , a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal. Within the hypothalamus is the suprachiasmatic nucleus SCN — clusters of thousands of cells that receive information about light exposure directly from the eyes and control your behavioral rhythm. Some people with damage to the SCN sleep erratically throughout the day because they are not able to match their circadian rhythms with the light-dark cycle. The brain stem , at the base of the brain, communicates with the hypothalamus to control the transitions between wake and sleep.
The brain stem includes structures called the pons, medulla, and midbrain. Sleep-promoting cells within the hypothalamus and the brain stem produce a brain chemical called GABA , which acts to reduce the activity of arousal centers in the hypothalamus and the brain stem. The thalamus acts as a relay for information from the senses to the cerebral cortex the covering of the brain that interprets and processes information from short- to long-term memory. During most stages of sleep, the thalamus becomes quiet, letting you tune out the external world.
But during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams. People who have lost their sight and cannot coordinate their natural wake-sleep cycle using natural light can stabilize their sleep patterns by taking small amounts of melatonin at the same time each day. The basal forebrain , near the front and bottom of the brain, also promotes sleep and wakefulness, while part of the midbrain acts as an arousal system.
Release of adenosine a chemical by-product of cellular energy consumption from cells in the basal forebrain and probably other regions supports your sleep drive. Caffeine counteracts sleepiness by blocking the actions of adenosine. The amygdala , an almond-shaped structure involved in processing emotions, becomes increasingly active during REM sleep.
Each is linked to specific brain waves and neuronal activity. Stage 1 non-REM sleep is the changeover from wakefulness to sleep. During this short period lasting several minutes of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brain waves begin to slow from their daytime wakefulness patterns. Stage 2 non-REM sleep is a period of light sleep before you enter deeper sleep.
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Your heartbeat and breathing slow, and muscles relax even further. Your body temperature drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity. You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.
Stage 3 non-REM sleep is the period of deep sleep that you need to feel refreshed in the morning.
It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to their lowest levels during sleep. Your muscles are relaxed and it may be difficult to awaken you. Brain waves become even slower. REM sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels.
Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. As you age, you sleep less of your time in REM sleep.
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Two internal biological mechanisms —circadian rhythm and homeostasis—work together to regulate when you are awake and sleep. Circadian rhythms direct a wide variety of functions from daily fluctuations in wakefulness to body temperature, metabolism, and the release of hormones.
They control your timing of sleep and cause you to be sleepy at night and your tendency to wake in the morning without an alarm. Circadian rhythms synchronize with environmental cues light, temperature about the actual time of day, but they continue even in the absence of cues. Sleep-wake homeostasis keeps track of your need for sleep.
The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity. This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation. Factors that influence your sleep-wake needs include medical conditions, medications, stress, sleep environment, and what you eat and drink.
Perhaps the greatest influence is the exposure to light. Specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle. Exposure to light can make it difficult to fall asleep and return to sleep when awakened. Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted.
In the case of jet lag, circadian rhythms become out of sync with the time of day when people fly to a different time zone, creating a mismatch between their internal clock and the actual clock. Your need for sleep and your sleep patterns change as you age, but this varies significantly across individuals of the same age.