Sleep

Normal sleep cycle

• NREM and REM sleep alternate throughout the night.
• A sleep cycle lasts about 90 minutes.
• REM episodes occur about every 90 minutes.
• Early night: predominantly deep NREM sleep.
• Later night: REM periods become progressively longer.
• In young adults, REM comprises about 25% of sleep; most remaining sleep is NREM.

NREM sleep

Features

• EEG waves are high-voltage and low-frequency, particularly in deeper sleep.
• It is the deep, restorative type of sleep, especially prominent in the first hour after a long period of wakefulness.
• Autonomic activity decreases:
  • Blood pressure decreases by about 10%-30%
  • Respiratory rate decreases
  • Basal metabolic rate decreases
  • Peripheral vascular tone decreases
• Dreams can occur, but are usually less vivid and commonly not remembered because they are not consolidated into memory.

NREM stages in current scoring

• Physical restoration and energy conservation
• Reduction in sympathetic and metabolic activity
• Growth hormone secretion is greatest during deep sleep
• Arousal threshold is high, especially in N3

REM sleep

• Paradoxical sleep
• Desynchronized sleep

Features

• Occurs in bouts lasting about 5-30 minutes.
• Strongly associated with vivid dreaming.
• Rapid eye movements occur.
• Brain activity and metabolism are high. Guyton describes an increase in overall brain metabolism of up to 20%.
• EEG resembles the waking pattern: low-voltage, fast, desynchronized activity.
• Skeletal muscle tone is markedly suppressed due to inhibition of spinal motor neurons, called REM atonia.
• Heart rate and respiration become irregular.
• Small irregular muscle twitches can occur despite generalized muscle atonia.
• A person is difficult to arouse during REM sleep, although spontaneous morning awakening often occurs from REM.

REM versus NREM: exam comparison

One-line memory aid

• NREM: body rests, brain slows.
• REM: brain is active, body is paralyzed.

Sleep cycle: simple diagram

Awake
  │
  ▼
N1  →  N2  →  N3
Light    Deep
sleep    slow-wave sleep
  │
  └──────────────► REM sleep
                   (dreaming, rapid eye movements)
                         │
                         ▼
               Cycle repeats every ~90 min

Across an 8-hour night

Beginning of night                           Morning
│--------------------------------------------------│

NREM deep sleep:   ██████████████  ██████   ██
REM sleep:               ██          ███     ██████
                         ↑            ↑        ↑
                   REM episodes become longer

How the pattern changes through the night

Cycle 1:  N1 → N2 → N3 (long) → REM (short)
Cycle 2:  N1 → N2 → N3         → REM (longer)
Cycle 3:  N1 → N2              → REM (longer)
Cycle 4:  N1 → N2              → REM (longest)

• Early night: more N3 deep NREM sleep, which is the most physically restorative sleep.
• Later night: less deep NREM sleep and progressively longer REM periods.
• REM sleep: brain activity resembles wakefulness, vivid dreams are common, but skeletal muscle tone is markedly reduced.
• An adult typically has 4-6 sleep cycles per night.

AWAKE, alert
Beta waves: fast, low amplitude
        ↓
AWAKE, relaxed, eyes closed
Alpha waves: 8-13 Hz
        ↓
N1: light sleep
Theta waves: 4-7 Hz
        ↓
N2: light sleep
Theta background + sleep spindles + K-complexes
        ↓
N3: deep slow-wave sleep
Delta waves: 0.5-2 Hz, high amplitude
        ↓
REM sleep
Low-amplitude, mixed-frequency waves
(similar to awake EEG) + rapid eye movements
        ↓
Back to N1/N2, then the cycle repeats

Simple sleep cycle with waves

                 First half of night                 Later night
              More deep N3 sleep                  More REM sleep

Awake     Alpha / Beta
            │
N1        Theta
            │
N2        Theta + sleep spindles + K-complexes
            │
N3        Delta  ███████  Deepest sleep
            │
REM       Low-voltage mixed-frequency (wake-like EEG)
            │
            └──────────── repeats every ~90 minutes ────────────┘

Memory sequence

1. Trace the taste pathway

Taste buds
   │
   ├─ Anterior 2/3 of tongue
   │      Lingual nerve → Chorda tympani → Facial nerve (CN VII)
   │
   ├─ Posterior 1/3 of tongue, circumvallate papillae, pharynx
   │      Glossopharyngeal nerve (CN IX)
   │
   └─ Base of tongue and pharyngeal region
          Vagus nerve (CN X)
                     │
                     ▼
      Nucleus of tractus solitarius, medulla
                     │
                     ▼
 Ventral posteromedial (VPM) nucleus of thalamus
                     │
                     ▼
 Gustatory cortex:
 lower end of postcentral gyrus + adjacent
 opercular-insular cortex, near Sylvian fissure

Important reflex pathway

2. Primary taste sensations

1. Sour: caused by acids, mainly hydrogen ions (H⁺).
2. Salty: mainly caused by sodium ions (Na⁺) from ionized salts.
3. Sweet: caused by many organic substances, especially sugars.
4. Bitter: often caused by alkaloids and long-chain nitrogen-containing organic substances, such as quinine, caffeine, nicotine, and strychnine.
5. Umami: a pleasant savory taste, especially associated with L-glutamate in foods such as meat extracts and aged cheese.

Sour  → H⁺
Salty → Na⁺
Sweet → sugars and many organic chemicals
Bitter → alkaloids
Umami → L-glutamate / protein-rich foods

3. Physiological anatomy and location of the olfactory membrane

Location

Superior nasal septum, medially
          +
Superior nasal turbinate, laterally
          +
Small upper part of middle turbinate

Structure

Nasal cavity
    │
    ▼
Mucus layer
    │
Olfactory cilia (olfactory hairs)
    │
Olfactory receptor cell
    │
Sustentacular supporting cells
    │
Axons of receptor cells
    │
Cribriform plate of ethmoid bone
    │
Olfactory bulb
    │
Glomeruli → Mitral and tufted cells
    │
Olfactory tract, CN I

Components and function

• Olfactory receptor cells are bipolar neurons, about 100 million in the olfactory epithelium.
• Their free end forms a knob bearing 4-25 olfactory cilia.
• The cilia project into the mucus and contain receptors for odorants.
• Bowman glands secrete mucus, which helps dissolve odorant molecules.
• Odorant binds the receptor on cilia → activates G protein → activates adenylyl cyclase → increases cAMP → opens sodium channels → receptor depolarization and action potentials.
• Axons pass through the cribriform plate to the olfactory bulb, where they synapse in glomeruli with mitral and tufted cells.