8. Body in Balance Flashcards
Homeostasis
Tendency of body to maintain a condition of balance. Depends on active regulation which often involves the hypothalamus
Example of body’s internal clock
Faster pulses of peristaltic waves in gut during day and dip in blood pressure during night
How are daily rhythms coordinated?
Clocks cannot detect daylight, so they can’t tell time on their own. Coordinated by suprachiasmatic nucleus (SCN), group of neurons in hypothalamus. Emits a steady stream of action potentials during day and does nothing at night.
How is the shift between active and silent states of suprachiasmatic nucleus controlled?
Controlled by cyclic interactions between two sets of proteins encoded by clock genes
How does the suprachiasmatic nucleus track time?
Via signals received from photoreceptors in retina
Why is the nudge from the suprachiasmatic nucleus important for our internal clocks?
Because clock proteins take slightly more than 24 hours to complete a cycle. Studies of animals deprived of light have discovered that they go to sleep and wake up a bit later each day
How is activity of SCN tied to other clocks in body?
Autonomic neural pathway ties daily activity of SCN to other clocks in body. Neurons in the SCN stimulate paraventricular nucleus (PVN) which sends signals down the spinal cord to the peripheral organs of the body
Melatonin
Hormone that influences sleep behaviours. Electrical activity originating in SCN enters PVN and sends signals to pineal gland, which secretes melatonin into bloodstream at night. Melatonin binds to many different types of cells, has no direct effect on clock gene expression in SCN but reduces alertness and increases sleepiness. Light exposure stops melatonin secretion.
What happens when your body prepares to wake?
Levels of cortisol peak in blood, releasing sugars from storage and increasing appetite. Core body temperature rises, raising metabolic rate
Circadian rhythm disruptions
Jet lag, late-shift jobs, blindness
Outline of neuroendocrine system
Hypothalamus oversees production and release of hormones through ties to pituitary gland. Paraventricular and supraoptic nuclei of hypothalamus send axons into posterior part of pituitary gland. Activation of specific neurons either releases vasopressin or oxytocin into capillaries. Two substances serve as both neurotransmitters and hormones.
Vasopressin
Antidiuretic hormone, increases water retention in kidneys, constricts blood vessels.
Oxytocin
Promotes uterine contractions during labour and milk release during nursing
Activation of anterior pituitary
Other hypothalamic regions send axons to the median eminence (capillary-rich area above pituitary). When these neurons are activated, they release their hormones into the blood to anterior pituitary, where they trigger or inhibit the secretion of another hormone.
Anterior pituitary hormones (7)
5 are trophic hormones (travel in bloodstream to stimulate activity in specific glands). Other 2 are non-endocrine tissues.
Growth hormone: stimulates growth of bone and soft tissues
Prolactin: stimulates milk production
How are hormone levels regulated?
Negative feedback loops. Many hormones produced by the pituitary affect the amount of hormone released by the hypothalamus
First three steps in hormone regulation of reproduction in mammals
Gonadotropin releasing hormone (GnRH) from hypothalamus makes anterior pituitary release luteinizing hormone (LH) and follicle stimulating hormone (FSH), which make the gonads secrete sex hormones and start development of egg/sperm. Sex hormones attach to receptors in hypothalamus and anterior pituitary and modify release of hypothalamic and pituitary hormones. How sex hormones regulate these feedback loops differs between males and females
Regulation of reproduction in males
Simple negative feedback loops that reduce secretion of GnRH, LH, and FSH. interplay among hormones. Creates a repetitive pulse of GnRH that peaks every 90 minutes. Waxing and waning of GnRH keeps testosterone levels steady, maintains libido, and keeps testes making sperm each day
Regulation of reproduction in females
More complex, over course of menstrual cycle, hormones exert both positive and negative feedback on 3 hormones. When circulating levels of estrogen and progesterone are low, rising FSH levels trigger egg maturation and estrogen production. Rising estrogen levels make LH levels rise. As levels of sex hormones rise, they exert negative feedback on FSH secretion, limiting number of eggs that mature each month, but positive feedback on LH, eventually producing th LH surge that triggers ovulation. After ovulation, high levels of sex hormones again exert negative feedback on 3 hormones, reducing ovarian activity. Levels of sex hormones therefore decrease, allowing cycle to restart
Examples of hormones not regulated by pituitary gland
Released by specific tissues, brain contains receptors but doesn’t regulate secretion.
Leptin: helps maintain body weight within set range. Produced by fat cells, released when fat stores are large. Suppresses activity of hunger circuits
Ghrelin: keeps body fed. Released by wall of gastrointestinal tract when stomach is empty. Activates hunger circuits in hypothalamus
Heat-shock proteins
In single-celled organisms/cells, heat-shock proteins guide damaged proteins to where they can be repaired/degraded, protecting the cell from toxicity
Somatic nervous system in stress response
Voluntary nervous system, prime boy to fight or flee
Autonomic nervous system in stress response
Involuntary nervous system, redirect nutrients and oxygen to muscle
Sympathetic branch in stress response
Tells adrenal medulla to release epinephrine (adrenaline), making heart pump faster and relaxing arterial walls
Parasympathetic branch in stress response
Restricts blood flow to other organs
Glucocorticoid hormones
Adrenal cortex releases it. Bind to many tissues and produce widespread effects that prepare body for threat. Stimulate production and release of sugar from storage. Bind to areas that ramp up attention and learning. Inhibit nonessential functions like growth and immune responses until crisis ends
Inhibition of immune system in stress response
Activation of hypothalamic-pituitary-adrenal axis. Increased production of corticotropin releasing factor (CRF) in hypothalamus, CRF travels to pituitary gland to release adrenocorticotropin releasing factor (ACTH) which travels to adrenal gland to release cortisol. Cortisol suppresses immune function
Chronic stress
Adrenal glands keep secreting epinephrine and glucocorticoids. Causes muscles to atrophy, pushes body to store energy as fat, keeps blood sugar abnormally high.
Mothers with high glucocorticoid levels during pregnancies often have babies with…
Lower birth weights, developmental delays, more sensitive stress responses. Stressful environments push fetuses to develop stress-sensitive metabolisms that store fat easily to deal with environment. However, this can backfire, especially if the child eventually grows up in a healthy environment with a lot of food
Negative effects of chronic stress
Contributes to development of hypertension and atherosclerosis (hardening of arteries). Reduces resistance to infection and inflammation, sometimes pushes immune system to attack body. Inhibit neuron growth in hippocampus. Can speed deterioration of brain function caused by aging. Can lead to sleep disorders (cortisol is an important wakeful signal)
Responses to disease regulated primarily by…
Hypothalamus
Neural signals for immune system sent via…
C-fibres and vagus nerve from liver
Cytokines
Important immune signals. Group of over 100 proteins. Normally produced at very low levels, but are switched on in response to disease/injury. Cause most of the responses to disease and infection, stimulating immune system and key parts of inflammation (swelling, changes to blood flow). Shown to also be active contributors to brain diseases (Alzheimer’s, stroke, multiple sclerosis)
Examples of cytokines
Interferons, interleukins, tumour necrosis factors, and chemokines
