Homeostasis, Sleep & Wakefulness

Homeostasis

Maintenance of a relatively stable internal environment despite constantly changing conditions.

CNS control of vital functions

The ANS maintains functions such as heart pumping, blood pressure, and digestion; respiration is regulated primarily by reticular formation nuclei in the pons.

Hypothalamus in homeostasis

The hypothalamus is the “boss” of the ANS; it receives information from the viscera, limbic system, and cerebral cortex and adjusts ANS output to maintain homeostasis.

Hypothalamus-to-brainstem homeostatic pathway

Hypothalamus preserves homeostasis by relaying instructions to reticular formation nuclei in the pons and medulla.

Vasopressor center

Center in the anterolateral pons and medulla that increases heart rate, increases force of heart contraction, and constricts blood vessels, raising blood pressure.

Vasodepressor center

Center located inferior and medial to the vasopressor center whose effects oppose the vasopressor center and decrease blood pressure.

Other homeostatic centers

Reticular formation nuclei also participate in regulation of digestive processes and control of urination.

Hypothalamus and body temperature

Hypothalamus acts as the body’s thermostat and normally maintains temperature around 36.1–37.2°C (97–99°F).

Response to increased body temperature

Hypothalamic nuclei initiate a negative feedback response, including sweating, to cool the body.

Response to decreased body temperature

Hypothalamic nuclei initiate negative feedback mechanisms that conserve body heat.

Fever

Elevation of normal body temperature caused when pyrogens interact with the hypothalamus and raise its normal temperature range.

Pyrogens

Substances secreted by immune cells and certain bacteria that interact with the hypothalamus and raise the body’s temperature set range.

Effects of fever

The elevated temperature setting produces shivering, increased muscle tone and muscle aches, and constriction of skin blood vessels, causing a feeling of coldness.

Antipyretics

Drugs such as acetaminophen and aspirin that block formation of pyrogens, allowing the hypothalamus to reset to its normal temperature range.

Hypothalamus and feeding

Stimulation of hypothalamic nuclei induces hunger and feeding behaviors that help preserve blood glucose concentration.

Orexins

Neurotransmitters secreted in high amounts by neurons in the lateral hypothalamic area during fasting; contribute to hunger and also help maintain wakefulness.

Sleep

Reversible and normal suspension of consciousness.

Functions of sleep

Allows the brain to replenish glycogen and provides time for metabolic waste products to be cleared through the brain’s lymphatic vessels.

Effects of sleep deprivation

May cause impaired temperature homeostasis, weight loss, decreased cognitive abilities, hallucinations, and even death.

Typical sleep requirements

Adults require about 7–8 hours per night; infants require about 17 hours per day, and sleep requirements generally decrease with age.

Circadian rhythm

Pattern in which part of the 24-hour cycle is spent awake and another part asleep.

Suprachiasmatic nucleus (SCN)

Hypothalamic nucleus that functions as the body’s “biological clock” and controls circadian rhythms.

SCN activity

SCN generates action potentials cyclically and fires more rapidly during the day than at night; its activity is influenced especially by light but also by food intake, movement, and hormones.

Light and the SCN

As light levels decrease, neurons from the eye signal the SCN through the optic nerve.

SCN and melatonin

When decreasing light is detected, the SCN triggers melatonin production from the epithalamus.

Melatonin in sleep

Melatonin from the epithalamus decreases activity of the SCN and cerebral cortex and directly suppresses cortical activity, decreasing consciousness.

Preoptic nuclei and sleep

The median preoptic and ventrolateral preoptic nuclei suppress wakefulness-promoting areas of the reticular formation, leading to sleep.

Lateral hypothalamic area and wakefulness

Functions as an “on switch” for arousal from sleep and secretes orexin to help maintain wakefulness during the day.

Orexin deficiency

May be responsible for narcolepsy.

Histamine and wakefulness

Histamine is a neurotransmitter in the reticular formation involved in wakefulness, helping explain why antihistamines can cause drowsiness.

Sleep induction sequence

Decreasing light → eye signals SCN → SCN triggers melatonin production → melatonin decreases SCN and cortical activity → preoptic nuclei suppress wakefulness-promoting reticular formation areas → sleep.

Electroencephalogram (EEG)

Test that measures electrical activity of the brain using electrodes attached to the skin; its tracings are called brain waves.

Brain waves

EEG tracings that differ in amplitude (height) and frequency (speed of generation).

Beta waves

Low-amplitude, high-frequency brain waves occurring when a person is awake and engaged in mental activity.

Stages I–III sleep

Progress from drowsiness to moderately deep sleep; waking beta waves slow into theta waves.

Theta waves

Brain waves of stages I–III sleep whose amplitude increases and frequency decreases as sleep deepens.

Stage IV sleep

Deepest stage of sleep; characterized by delta waves and takes less than one hour to reach.

Delta waves

Low-frequency, high-amplitude waves characteristic of stage IV sleep; also seen during general anesthesia and in brain-damaged adults.

REM sleep

Rapid eye movement sleep; characterized by rapid back-and-forth eye movements, dreams, brain waves resembling awake beta waves, and muscle paralysis.

Dreams

Hallucination-like episodes that often have highly emotional content and usually occur during REM sleep.

REM sleep duration

Generally lasts about 10–15 minutes initially and becomes longer with each sleep cycle.

Movement during sleep

The body can move during stages I–IV, but muscle paralysis occurs during REM sleep.

Physiological changes during stages I–IV

Blood pressure, heart rate, respiratory rate, body temperature, and metabolic rate decrease.

Physiological changes during REM

Most physiological variables that decrease during stages I–IV increase during REM sleep.

Possible functions of REM sleep

May consolidate memories and recently learned tasks or help dispose of unwanted accumulated memories; the exact function is unknown.

Brain activity during dreams

Limbic system is highly active, while the prefrontal cortex involved in logic and reason is barely active.

Stupor

Diminished cortical activity in which a person can still be aroused by strong or painful stimuli.

Coma

Unarousable unconsciousness with no purposeful responses even to pain; brainstem reflexes and some involuntary movements may remain intact.

Coma and the reticular formation

Damage to the reticular formation or a related component prevents normal arousal of the cerebral cortex; EEG shows a slow fixed pattern without normal sleep/wake cycles.

Persistent vegetative state

State caused by cerebral cortex damage in which the person is awake but unaware, lacks voluntary movement, and retains sleep/wake cycles and brainstem reflexes.

Brain death

State in which the EEG shows no brain activity, brainstem reflexes are absent, cerebral metabolism and blood flow are essentially zero, and consciousness cannot be regained.

Stupor vs. coma vs. persistent vegetative state vs. brain death

Stupor = arousable with strong stimuli; coma = unarousable but brainstem reflexes remain; persistent vegetative state = awake but unaware with brainstem reflexes and sleep/wake cycles; brain death = no EEG activity or brainstem reflexes.