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SDL TOPICS - Physiology Answers

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1. Circadian Rhythm & Sleep Physiology

• The word "circadian" comes from Latin circa dies meaning "about a day"
• The master circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus
• The intrinsic period is approximately 24.2 hours; light entrains it to exactly 24 hours
• Light is the primary zeitgeber (time-giver); it resets the pacemaker via retinal inputs
• Melatonin is secreted by the pineal gland in response to darkness; dim-light melatonin onset (DLMO) occurs ~2 hours before habitual sleep time (~8-9 PM in normal adults)

• Sleep consists of NREM (stages N1, N2, N3) and REM cycles, each cycle ~90 minutes, 4-6 cycles per night
• N3 (slow-wave sleep) is deepest, most restorative; predominates in first half of night
• REM sleep predominates in second half; associated with dreaming and memory consolidation
• Homeostatic drive (process S) - sleep pressure builds with prolonged wakefulness via adenosine accumulation
• Circadian drive (process C) - promotes wakefulness during the day via SCN

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2. Stress and Its Effects on the Human Body

• Alarm stage: Hypothalamus activates the HPA axis and sympathoadrenal system
• Resistance stage: Body adapts using sustained cortisol and catecholamine secretion
• Exhaustion stage: Prolonged stress depletes adaptive reserves

1. Sympatho-Adrenal System (SAM): Hypothalamus → sympathetic nerves → adrenal medulla → Adrenaline (Epinephrine) & Noradrenaline
   • Causes: tachycardia, increased BP, bronchodilation, hyperglycemia (fight-or-flight)
2. HPA Axis: Hypothalamus → CRH → Anterior Pituitary → ACTH → Adrenal cortex → Cortisol
   • Cortisol effects: gluconeogenesis, anti-inflammatory, immunosuppression, protein catabolism

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3. Physiological Adaptations to Aerobic & Anaerobic Exercise

Aerobic (Endurance) Exercise

• ↑ Heart rate, ↑ stroke volume, ↑ cardiac output (up to 4-5x resting)
• Redistribution of blood to working muscles (vasodilation via NO, CO2, lactate)
• ↑ Ventilation to maintain blood O2
• Increased use of fat as fuel at moderate intensity

• Cardiac: Eccentric hypertrophy ("athlete's heart"), ↑ stroke volume at rest, lower resting HR (bradycardia)
• Respiratory: ↑ Respiratory muscle efficiency, ↑ VO2 max
• Skeletal Muscle: ↑ Mitochondrial density, ↑ oxidative enzymes, ↑ myoglobin, ↑ capillary density
• Metabolic: ↑ fat oxidation, sparing glycogen (glycogen-sparing effect)
• Blood: ↑ Blood volume, ↑ plasma volume, ↑ total Hb

Anaerobic Exercise (High Intensity/Sprinting)

1. Phosphagen system (ATP-PCr): Immediate, lasts ~10 seconds
2. Anaerobic glycolysis: Lasts 30-90 seconds, produces lactic acid → lactate

• During intense exercise, O2 demand exceeds supply → oxygen deficit
• After exercise: elevated O2 consumption persists = Excess Post-exercise Oxygen Consumption (EPOC)

• ↑ Muscle fiber size (hypertrophy), especially Type II fast-twitch fibers
• ↑ Phosphocreatine stores
• ↑ Glycolytic enzyme activity (phosphofructokinase, lactate dehydrogenase)
• ↑ Buffering capacity for lactate

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4. Autonomic Nervous System (ANS)

• α1: Vasoconstriction, pupil dilation (mydriasis), bladder sphincter contraction
• α2: Presynaptic inhibition; ↓ insulin secretion
• β1: ↑ Heart rate (chronotropy) and contractility (inotropy)
• β2: Bronchodilation, vasodilation in skeletal muscle, ↑ glucagon
• Sympathetic fibers innervate all blood vessels except capillaries (Guyton & Hall)

• Bradycardia (via vagus - CN X on SA node)
• ↑ GI motility and secretion
• Bronchoconstriction
• Miosis (pupillary constriction)
• Promotes urination and erection

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5. Physiology of Meditation & Yoga

• Meditation activates the parasympathetic system - termed the "relaxation response" (Benson)
• ↓ Heart rate, ↓ blood pressure, ↓ respiratory rate
• ↑ Heart Rate Variability (HRV) - a marker of vagal tone

• ↓ Cortisol levels (HPA axis downregulation)
• ↓ Catecholamines (adrenaline, noradrenaline)
• ↑ GABA and serotonin activity
• ↑ DHEA levels
• ↑ Melatonin and oxytocin

• ↑ Alpha wave activity on EEG (calm alertness)
• ↑ Theta wave activity in experienced meditators
• Neuroplasticity: ↑ cortical thickness in prefrontal cortex and insula
• ↓ Amygdala reactivity (reduced fear response)

• Pranayama (yogic breathing): stimulates vagus nerve, ↑ parasympathetic tone
• Improves flexibility via stretch reflex modulation
• Core muscle activation → improved posture and spinal mechanics
• ↓ Inflammatory markers (IL-6, TNF-alpha, CRP)

• ↓ Hypertension, improved glucose control
• Enhanced immune function
• ↓ Chronic pain perception (altered central pain processing)
• Improved sleep quality

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6. Physiological Basis & Prevention of Obesity

• Adipose is not merely a storage depot - it is an active endocrine organ secreting adipokines (leptin, adiponectin, resistin, TNF-alpha)

• Body weight = Energy intake - Energy expenditure
• BMI ≥ 30 kg/m² = Obesity

• POMC/CART neurons: Produce alpha-MSH → reduce food intake
• NPY/AgRP neurons: Increase food intake; activated by fasting

• Insulin Resistance: Free fatty acids impair insulin signaling (IRS-1 serine phosphorylation)
• Hypertension: Hyperinsulinemia → Na+ retention; leptin → sympathetic activation
• Obstructive Sleep Apnea: Fat deposition in pharyngeal tissues
• Non-alcoholic Fatty Liver Disease (NAFLD): Excess lipid delivery to liver

• Caloric restriction + aerobic exercise → ↑ insulin sensitivity, ↑ lipolysis
• High-fiber diet → slower gastric emptying, sustained GLP-1 release
• Sleep optimization → normalizes ghrelin/leptin balance
• Behavioral modification → reset hypothalamic set-point

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7. Regulation of Blood Pressure (BP)

1. Baroreceptor Reflex (Most rapid - seconds):
   • Baroreceptors in carotid sinus and aortic arch
   • ↑ BP → ↑ firing → NTS in medulla → ↑ vagal output + ↓ sympathetic → ↓ HR, ↓ TPR
   • ↓ BP → opposite response
   • Acts as a buffer - normalizes BP within seconds
2. Chemoreceptors:
   • Peripheral (carotid bodies): respond to ↓ PO2, ↑ PCO2, ↓ pH
   • ↑ BP as compensatory response
3. CNS Ischemic Response (Cushing Reflex):
   • Extreme ↓ BP → massive sympathetic discharge → ↑ BP (last ditch mechanism)

• Stress relaxation of vessel walls
• Capillary fluid shift
• Renin release from kidney

• ↓ BP → juxtaglomerular cells release Renin
• Renin → Angiotensinogen → Angiotensin I → (ACE) → Angiotensin II
• Angiotensin II: vasoconstriction + aldosterone release → Na+ & water retention → ↑ BP

• Vasoconstrictors: Angiotensin II, catecholamines, endothelin, vasopressin (ADH)
• Vasodilators: Nitric Oxide (NO), kinins, prostaglandins, ANP

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8. Acclimatization to High Altitude, Deep Sea Diving, Heat & Cold

A. High Altitude Acclimatization

• AMS (Acute Mountain Sickness): Headache, nausea, fatigue (>2500m); treat with descent, O2, acetazolamide
• HACE (High-Altitude Cerebral Edema): Ataxia, altered consciousness - EMERGENCY
• HAPE (High-Altitude Pulmonary Edema): Most lethal; due to hypoxic pulmonary vasoconstriction

B. Deep Sea Diving

• Nitrogen narcosis: Increased N2 partial pressure at depth → anesthetic effect
• Oxygen toxicity: High PO2 → CNS and pulmonary toxicity
• Decompression sickness ("the bends"): Rapid ascent → N2 bubbles form in blood/tissues → joint pain, neurological symptoms; treated with hyperbaric O2

C. Heat Acclimatization

• ↑ Sweat rate and earlier onset of sweating
• ↓ Sweat Na+ concentration (aldosterone effect)
• ↑ Plasma volume
• ↓ Core temperature at same workload after acclimatization
• Heat disorders: Heat cramps → Heat exhaustion → Heat stroke (core temp >40°C, CNS dysfunction = emergency)

D. Cold Acclimatization

• Thermogenesis: Shivering (rapid) + Non-shivering thermogenesis via BAT (brown adipose tissue)
• ↑ Basal metabolic rate
• Vasoconstriction in periphery to preserve core temperature
• ↓ Cold-induced vasoconstriction over time (hunting reaction in acclimatized individuals)
• Cold injuries: Chilblains → Frostbite → Hypothermia (core <35°C)

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9. Changes in CVS, Respiratory, Nervous, Endocrine & Musculoskeletal Systems with Ageing

Cardiovascular System

• ↓ Maximum heart rate (HR max ≈ 220 - age)
• ↓ Cardiac output, ↓ stroke volume
• ↑ Systolic BP (stiffening of aorta - arteriosclerosis)
• ↑ Left ventricular wall thickness (concentric hypertrophy)
• ↓ Baroreceptor sensitivity → orthostatic hypotension
• ↑ Risk of AF due to fibrosis of conduction system

Respiratory System

• ↑ Residual Volume (RV); ↓ FVC, FEV1
• ↓ Elastic recoil of lungs (loss of elastin)
• ↑ Functional Residual Capacity
• ↓ Mucociliary clearance
• ↓ Respiratory muscle strength
• ↓ Ventilatory response to hypoxia and hypercapnia
• ↓ PaO2 (but PaCO2 remains normal)

Nervous System

• ↓ Brain volume (especially frontal lobes and hippocampus)
• ↓ Neuronal density and synaptic connections
• ↓ Nerve conduction velocity (demyelination)
• ↓ Dopamine activity → increased risk of Parkinson's
• ↓ Acetylcholine → memory decline
• ↓ Sleep efficiency (less N3 slow-wave sleep, more wakefulness)
• Slowed reaction time and psychomotor speed

Endocrine System

Musculoskeletal System

• Sarcopenia: Progressive loss of muscle mass and strength (↓ Type II fiber size and number)
• Osteoporosis: ↓ Bone mineral density (↓ osteoblast activity; ↑ osteoclast activity)
• ↓ Joint cartilage → osteoarthritis
• ↓ Tendon elasticity
• ↓ Proprioception → fall risk ↑

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10. Effects of Prolonged Screen Time on Vision, Sleep, Posture, Mental Health & Circadian Rhythm

Vision

• Computer Vision Syndrome (CVS): Eye strain, headache, blurred vision, dry eyes
• ↓ Blink rate during screen use (normal ~15-20/min → ↓ 3-5/min) → tear film disruption
• Digital Myopia Progression: Prolonged near work → ↑ axial length of eye
• Blue light exposure → potential retinal photoreceptor damage (mitochondrial stress in RPE cells)

Sleep

• Blue light (~480 nm) from screens suppresses melatonin secretion by up to 50%
• ↑ Alertness via ↑ cortisol and ↑ noradrenaline → delays sleep onset
• Emotional arousal from social media → ↑ amygdala activity → ↑ arousal

Posture

• "Tech neck"/"Forward Head Posture": For every inch of forward head displacement, cervical spine load increases by ~10 lbs
• ↑ Tension in trapezius, levator scapulae, posterior cervical muscles
• ↓ Thoracic mobility → kyphotic posture
• ↑ Risk of chronic neck pain, shoulder pain, and cervicogenic headaches

Mental Health

• Dopamine dysregulation: Social media provides unpredictable reward (variable ratio reinforcement - strongest behavior schedule) → addictive behavior
• ↑ Cortisol due to passive scrolling and social comparison
• ↑ Rates of anxiety and depression (especially in adolescents)
• ↓ Social bonding (oxytocin deficiency from reduced face-to-face interaction)
• Sleep deprivation from screens → ↑ emotional reactivity (amygdala hyperresponsiveness)

Circadian Rhythm

• Evening blue light shifts DLMO (dim-light melatonin onset) to a later time
• Causes social jet lag - misalignment between biological clock and social schedule
• Disrupts the "forbidden zone for sleep" (4-6 PM high alertness window controlled by SCN)
• Chronic circadian disruption → ↑ risk of metabolic syndrome, obesity, cardiovascular disease, and certain cancers

• 20-20-20 rule (every 20 min, look 20 feet away for 20 sec)
• Blue light-filtering glasses or Night Mode (warm display) after 6 PM
• Screen-free 60-90 minutes before bedtime
• Ergonomic setup (screen at eye level, arm's length distance)
• Regular breaks with stretching; correct posture habits

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