Study sheet: Sensory Physiology

📋 Course Outline

1. Sensory Classification
2. Somatic Sensation Types
3. Mechanoreceptors and Receptors
4. Touch and Tactile Discrimination
5. Pressure Vibration and Kinesthesia
6. Tickle Itch and Their Pathway
7. Thermal Receptors and Coding
8. Sensory Pathways Overview

📖 1. Sensory Classification

🔑 Key Concepts & Definitions

• Sensation : a feeling caused by a stimulus

📝 Essential Points

• Sensations are classified as: somatic sensations from skin, muscles, and joints, visceral sensations from painful viscera, micturition, and defecation, organic sensations including hunger, thirst, and sex, special sensations including hearing, smell, taste, vision, and equilibrium

💡 Memory Hook

SVOS: Somatic, Visceral, Organic, Special

📖 2. Somatic Sensation Types

📝 Essential Points

📌 Somatic sensations are felt from somatic structures, conducted by somatic nerves, and perceived in the cerebral cortex.

• Somatic sensations include:
  • mechanoreceptive sensations
  • thermoreceptive sensations
  • pain sensation

📌 Exteroreceptive sensations arise from cutaneous receptors and include pain, touch, and temperature, whereas proprioceptive sensations arise from deep receptors and include position and movement sensations.

💡 Memory Hook

Exteroception reads the skin; proprioception reads position and movement.

📖 3. Mechanoreceptors and Receptors

📝 Essential Points

📌 Mechanoreceptive sensations are detected by mechanoreceptors and are divided into cutaneous and deep mechanoreceptive sensations.

• Cutaneous mechanoreceptors include: free nerve endings and basket endings around hair follicles, expanded endings such as Merkel discs and Ruffini corpuscles, encapsulated endings such as Pacinian corpuscles and Meissner corpuscles

• The muscle spindle is a stretch receptor in the fleshy part of skeletal muscle, whereas the Golgi tendon organ is a tension receptor in skeletal muscle tendons.

💡 Memory Hook

A skin-to-muscle map: free, expanded, and encapsulated endings above; spindles and Golgi organs below.

📖 4. Touch and Tactile Discrimination

🔑 Key Concepts & Definitions

• Tactile discrimination : the ability to perceive two stimuli applied simultaneously as two separate points while the eyes are closed

★ Must-know

📌 Crude touch is poorly localized and requires strong stimuli, whereas fine touch is highly localized and detects the number and characteristics of stimuli.

• Threshold distance is the shortest distance at which two stimuli are perceived as two separate points; it is 1 mm at the tip of the tongue, 3 mm at the fingertips, and 70 mm on the back, shoulders, and thighs.

• Stereognosis is the ability to identify an object without vision and requires previous learning and experience.

Further detail

• Crude touch is examined by touching the skin with a piece of cotton while the subject keeps the eyes closed.

💡 Memory Hook

Touch → localize → discriminate → recognize.

📖 5. Pressure Vibration and Kinesthesia

🔑 Key Concepts & Definitions

• Pressure sensation : a mechanoreceptive sensation produced by several mechanoreceptors in subcutaneous tissues and requires more energy than touch sensations

★ Must-know

• Vibration sensation results from repetitive stimulation of mechanoreceptors; Meissner corpuscles respond up to 80 cycles per second, whereas Pacinian corpuscles respond up to 500–800 cycles per second.

📌 Kinesthetic sensations include sense of position and sense of joint movement.

Further detail

• Pacinian corpuscles rapidly adapt to pressure, whereas Ruffini spray-type endings slowly adapt to pressure.

• Position sense uses Ruffini endings, muscle spindles, and Golgi tendon organs in joint capsules and ligaments, whereas joint movement sense uses Pacinian corpuscles and Golgi tendon organs.

💡 Memory Hook

Pressure measures force; vibration measures repeated stimulation; kinesthesia measures position and movement.

📖 6. Tickle Itch and Their Pathway

🔑 Key Concepts & Definitions

• Itch : a sensation of skin irritation that produces a desire to scratch and may result from moving tactile stimulation or substances such as histamine

📝 Essential Points

• Itch is transmitted by specialized, highly sensitive, rapidly adapting free nerve endings through slowly conducting type C fibers and may travel in the ventral spinothalamic tract, initiating a scratch reflex that removes the irritant.

💡 Memory Hook

Moving stimulation or histamine → C-fiber activity → scratch reflex.

📖 7. Thermal Receptors and Coding

🔑 Key Concepts & Definitions

• Thermoreceptive sensation : the perception of different temperatures ranging from freezing to burning

★ Must-know

• Thermal receptors are located in the epidermis in cold spots, warm spots, and insensitive areas, and cold spots are 3–10 times more numerous than warm spots.

• Cold receptors include Krause end bulbs and some free nerve endings, discharge between 10 and 35°C, reach maximal discharge at 20–25°C, and stop discharging at 35°C.

• Warm receptors are specialized free nerve endings that discharge at 25°C, reach maximal discharge at 37°C, stop discharging at 45°C, and conduct impulses through C fibers.

Further detail

• Paradoxical cold sensation is a false cold sensation caused by stimulating cold receptors when the temperature exceeds 45°C.

• In Weber's illusion, a cold metal object feels heavier than a wooden object of the same weight because metal conducts heat away from the skin more rapidly and stimulates pressure-sensitive Ruffini endings.

💡 Memory Hook

Cold receptors peak at 20–25°C; warm receptors peak at 37°C.

📖 8. Sensory Pathways Overview

📝 Essential Points

• Sensory pathways generally contain three orders of neurons, and sensations are perceived in the cerebral cortex on the side opposite the stimulated body.

📌 The gracile tract carries sensations from the lower body, whereas the cuneate tract carries sensations from the upper body.

• The gracile and cuneate tracts carry:
  • tactile localization
  • tactile discrimination
  • stereognosis
  • vibration sensation
  • weight discrimination
  • kinesthetic sensations

• The spinothalamic tracts carry:
  • crude touch
  • pressure sensations
  • pain sensation
  • temperature sensation
  • tickle and itch sensations
  • sexual sensations

💡 Memory Hook

First-order neuron → second-order neuron → third-order neuron → opposite cortex.

📊 Synthesis Tables

Mechanoreceptor Categories

Category	Receptors	Main role
Cutaneous	Free, expanded, and encapsulated endings	Touch and other skin sensations
Deep	Muscle spindle and Golgi tendon organ	Stretch and tension
Kinesthetic	Ruffini, muscle spindle, Golgi tendon, and Pacinian receptors	Position and joint movement

Major Sensory Pathways

Pathway	Body region or tract	Sensations
Gracile tract	Lower body	Discriminative touch, vibration, weight, and kinesthesia
Cuneate tract	Upper body	Discriminative touch, vibration, weight, and kinesthesia
Spinothalamic tracts	Lateral and ventral spinothalamic tracts	Crude touch, pressure, pain, temperature, tickle, itch, and sexual sensations