19.3 States of Consciousness

19.3 States of Consciousness

Quick Answer: Consciousness is awareness of self and environment, regulated by the reticular activating system (RAS) in the brainstem. It fluctuates along a continuum from alert to drowsy to comatose; destruction of the RAS or bilateral thalamic damage causes coma.

Sleep Stages and EEG Patterns

Sleep is an active, cyclic process, not a passive state. The cortex shows characteristic electrical patterns measured by electroencephalography (EEG):

A normal night cycles through ~90-minute ultradian cycles, with N3 dominating early cycles and REM lengthening toward morning. REM sleep is characterized by muscle atonia (prevents acting out dreams), vivid dreaming, and high cortical activity — hence "paradoxical sleep." Most but not all dreaming occurs in REM; NREM dreams are less vivid and emotional.

Sleep architecture shifts with age: N3 declines and sleep fragmentation rises in older adults. Sleep deprivation impairs attention, memory consolidation, immune function, and glucose regulation; chronic short sleep correlates with metabolic and cardiovascular risk.

Functions of Sleep and Sleep Disorders

Theories of sleep function include restorative (cellular repair, glycogen replenishment, glymphatic clearance of beta-amyloid), consolidation (memory reactivation and synaptic pruning during N3 and REM), and adaptive/energy conservation. Deprivation of REM specifically produces REM rebound — increased REM pressure and longer REM episodes on recovery nights.

Key sleep disorders:

• Insomnia — difficulty falling or staying sleep; treated with CBT-I and short-acting hypnotics.
• Obstructive sleep apnea (OSA) — repeated upper-airway collapse causing arousal and desaturation; risk factors include obesity and male sex; treated with CPAP.
• Narcolepsy — excessive daytime sleepiness with cataplexy (sudden loss of muscle tone with emotion), hypnagogic (falling asleep) and hypnopompic (waking) hallucinations, and sleep paralysis. Associated with loss of orexin/hypocretin neurons in the lateral hypothalamus (type 1).
• Restless legs syndrome — urge to move legs at night, often with iron deficiency.

Circadian rhythm disorders (delayed sleep phase, advanced sleep phase, shift work disorder) reflect misalignment between the internal clock and the external environment.

Circadian Rhythms

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master clock. Light strikes melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) → retinohypothalamic tract → SCN. The SCN suppresses melatonin from the pineal gland during light; melatonin rises in darkness, signaling night. The SCN drives peripheral clocks via neural and humoral signals. Shift work and jet lag desynchronize these rhythms, producing fatigue, cognitive impairment, and metabolic disturbance. Morning light exposure and scheduled melatonin are common chronobiologic interventions.

Altered States and Anesthesia

Consciousness can be altered by trauma, metabolic derangement, drugs, or induced anesthesia. Levels of impairment progress from confusion → delirium → obtundation → stupor → coma. The Glasgow Coma Scale (eye, verbal, motor responses; total 3–15) quantifies depth — lower scores indicate deeper depression.

General anesthetics produce reversible unconsciousness, analgesia, amnesia, immobility, and attenuation of autonomic responses. Mechanisms are diverse:

• Propofol and barbiturates enhance GABA-A receptors.
• Ketamine blocks NMDA receptors, producing dissociative anesthesia.
• Volatile agents (isoflurane, sevoflurane) act on multiple targets including GABA-A and two-pore K+ channels.
• Opioids (mu-receptor agonists: morphine, fentanyl) provide analgesia but not amnesia or immobility.

The MAC (minimum alveolar concentration) quantifies volatile anesthetic potency — the concentration preventing movement in 50% of subjects to a standardized stimulus. MAC is reduced by age, opioids, benzodiazepines, and hypothermia, and increased by chronic alcohol use and hyperthyroidism. Balanced anesthesia combines induction agents (propofol, etomidate, ketamine), neuromuscular blockers (succinylcholine, rocuronium — facilitate intubation), opioids for analgesia, and volatile agents for maintenance. Local anesthetics (lidocaine, bupivacaine) block voltage-gated sodium channels to prevent action potential conduction in peripheral nerves.

Psychoactive Drugs and Consciousness

Psychoactive substances alter mood, perception, and cognition by modifying neurotransmission:

• Depressants (alcohol, benzodiazepines, barbiturates) enhance GABA; reduce anxiety, cause sedation, and at high doses coma.
• Stimulants (cocaine, amphetamine) increase dopamine and norepinephrine; euphoria and vigilance, then crash.
• Opioids (morphine, heroin, fentanyl) mimic endogenous endorphins; analgesia, euphoria, and respiratory depression.
• Hallucinogens (LSD, psilocybin) act partly at 5-HT2A receptors; perceptual distortions.
• Cannabinoids (THC) act at CB1 receptors; altered perception of time, appetite stimulation.
• Dissociatives (ketamine, PCP) block NMDA; depersonalization.

Tolerance is reduced response requiring higher doses; dependence is neuroadaptation producing withdrawal on cessation; addiction is compulsive use despite harm. The mesolimbic dopamine pathway (ventral tegmental area → nucleus accumbens) is the common reward circuit across addictive substances.