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Chapter 18. Sensory System

Recommended Article: 【Biology】 Biology Table of Contents


1. Overview

2. Types of Sensory Receptors


a. Photoreceptors: Eyes

b. Auditory Receptors: Ears

c. Equilibrium Receptors: Ears

d. Olfactory Receptors: Nose

e. Gustatory Receptors: Tongue

f. Cutaneous Receptors



1. Overview

⑴ Stimulus

① Definition: Changes in the surrounding environment

② Characteristics: Modality, intensity, location, timing

⑵ Sensation: Subjective experience triggered when sensory receptors are activated by a stimulus

① Mechanism: Sensory transformation → Amplification → Transmission → Integration → Consciousness

○ Sensory receptors: Generate action potential when physical stimulus exceeds the threshold

○ Sensory transduction: Conversion of light, chemical, mechanical, thermal energy to electrical energy directly or through GPCR mediation

② Sensory Encoding: Mechanisms to distinguish stimuli

○ Modality of stimulus: Labeled-line sensory channel by specific sensory receptors

○ Law of specific nerve energy: Nature of sensation is determined by the type of sensory receptor, not the type of stimulus

○ Intensity of stimulus: Magnitude of sensory receptor voltage, frequency of action potential, repetition

○ Location of stimulus: Topographical organization

○ Examples: Somatosensory homunculus, Retinotopic map, Tonotopic map

③ Types of Sensory Receptors

○ Types of sensory receptors based on stimuli

○ Electromagnetic receptors

○ Chemical receptors

○ Mechanical receptors

○ Nociceptors

○ Thermoreceptors

○ Types of sensory receptors based on structure

○ Simple receptors: Neurons with free nerve endings

○ Complex neural receptors: Nerve endings encased in connective tissue sheath

○ Specialized sensory receptors: Release neurotransmitters to initiate action potentials in sensory neurons

○ Types of sensory receptors based on principles

○ Tonic receptors: Slowly adapting receptors

○ Maintain depolarization even during sustained stimulus

○ Convey stimulus magnitude (e.g., proprioceptors, nocioceptors)

○ Phasic receptors: Rapidly adapting receptors

○ Cease firing if stimulus intensity remains constant (e.g., olfactory receptors, touch receptors)

○ Detect onset, offset, and changes of stimulus primarily

④ When Neurons are Sensory Cells

○ Olfactory receptors

○ Cutaneous receptors

○ Pacinian receptors

○ Thermoreceptors

○ Stretch receptor (e.g., muscle spindle [cerebellum, unconscious sensation])

⑵ Characteristics of the Sensory System

Decussation (chiasm): Sensory and motor tracts in the brain cross over, governing opposite side (contralateral) regions of the body

○ Reasons remain unclear

② Synaptic Integration: Sensory information interaction due to connection structure

○ Convergence

○ Divergence

○ Facilitation

○ Occlusion

③ Lateral Inhibition: Sensory receptor excitation inhibits nearby receptor activity

○ Two-point discrimination: Ability to distinguish two adjacent stimuli

○ Example: Recognizing spikes in food


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Figure. 1. Effect of lateral inhibition on two-point discrimination


④ Adaptation: Gradual decrease in sensory receptor voltage due to sustained stimulus

○ SA (slowly adapting receptor)

○ RA (rapidly adapting receptor)

○ TA (transiently adapting receptor)

⑤ Parallel Processing

⑥ Descending Control: Sensory information feedback regulation downwards as well

○ Example: Temporarily forgetting pain through other senses


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Figure. 2. Example of descending control


⑦ Except for olfactory information, all sensory information passes through the thalamus

⑧ Plasticity

○ Advantage: Increased efficiency in brain’s information processing

○ Disadvantage: May lead to conditions like hallucinogenic pain


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Figure. 3. Effects of finger amputation and training on brain cortex


⑷ Psychophysics

① Principles: Physical reality ≠ Psychological reality, active, evolutionary

② Psychophysical Measurement Functions

○ Different reactions from different individuals

○ Measuring reactions when stimulus is suddenly introduced in absence of prior stimuli

○ Concepts: Detection rate, theoretical threshold curve, threshold-below stimulus, absolute threshold, obtained threshold curve

③ Just Noticeable Difference (JND)

○ Weber’s Law: ΔI/Ii = constant

○ Light: ΔIlight / Ii, light × 100 = 8 (%)

○ Weight: ΔIweight / Ii, weight = 2 (%)

○ Sound: ΔIsound / Ii, sound = 0.3 (%)


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Figure. 4. Intensity-response curve of stimuli


④ Signal Detection Theory (SDT)

○ Detecting faint signals in noise

○ Assumption: No single absolute threshold

○ Concepts: Liberal criterion, conservative criterion, noise, signal

⑤ Psychophysical Adaptation

⑸ McGurk Effect

① Definition: Integrating different sensory information to perceive when perceiving sensory information

② Example: Seeing ‘ga’ but hearing ‘ba’ results in perception as ‘da’



2. Types of Sensory Receptors

Photoreceptors

Auditory Receptors

Equilibrium Receptors

Olfactory Receptors

Gustatory Receptors

Cutaneous Receptors

Nociceptors

⑻ Chemical Receptors

① Chemical Sensation

○ Oldest and most widespread sensory organs

○ Detect chemical signals in our internal and external environment

○ All species and cells detect various chemicals

② Olfactory Receptors in Vertebrates: Nose

③ Gustatory Receptors in Vertebrates: Tongue

④ Moth’s Chemical Receptors: Antennae

○ In adult males, the receptors can respond even to a single molecule of bombykol secreted by the female.

○ Bombykol → male antenna → sensillum (sensory hair) → olfactory receptor → dissolves in fluid → acts on the receptor membrane → dissolution.

⑼ Electromagnetic Receptors

① Lorenzini Organs: Found in sharks, salamanders, and can sense electrical signals

⑽ Interoception

① Example: Knowing the position of your arm even with closed eyes

⑾ Thermal Receptors

① Example: Snakes have pit organs, thermal sensors



Input: 2015.07.26 22:50

Modified: 2018.07.15 08:09

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