Sensory physiology

Sensation is the process by which sensory receptors detect and transduce physical or chemical energy from the environment into neural signals. A sensory modality, like vision or hearing, is a distinct channel defined by its specific stimulus, the specialized receptors that transduce it, and the dedicated neural pathway that carries the signal to the brain. This process involves reception (detecting the stimulus) and transduction (creating an electrical receptor potential), which, upon reaching a threshold, triggers the transmission of action potentials to the central nervous system.

The field of psychophysics quantifies the relationship between physical stimuli and our perception of them, using concepts like the absolute threshold (the minimum stimulus detected 50% of the time) and the difference threshold (the smallest detectable change, as described by Weber's Law). Furthermore, the signal detection theory clarifies that our ability to detect a faint stimulus depends not only on its intensity but also on our psychological state and the surrounding background "noise."

Overview of sensory modalities

The classic sensory modalities include vision, hearing, smell, taste, and touch, the latter encompassing sensations of pressure, temperature, and pain. Within each modality, multiple sensory qualities can be differentiated (e.g., sweet, sour, and bitter within taste).

Sensory cells

Sensory cells are specialized cells that use sensory receptors to detect physical or chemical stimuli and transduce them into electrical signals. There are two principal types of sensory cells, classified by whether they can generate their own action potentials (see also "Stimulus reception and transmission" below).

• Primary sensory cell: a specialized neuron that transduces a stimulus into a receptor potential, which in turn generates an action potential that is transmitted along its own axon to the CNS (e.g., olfactory receptor cells)
• Secondary sensory cell: a specialized epithelial cell that transduces a stimulus into a receptor potential, which in turn causes the release of neurotransmitters onto an adjacent sensory neuron that transmits the signal to the CNS (e.g., taste cells, hair cells in the inner ear)

Sensory receptors

Types of sensory receptors

By stimulus

• Mechanoreceptors
• Thermoreceptors
• Nociceptors
• Photoreceptors
• Chemoreceptors

By location

• Exteroceptors
• Interoceptors
• Proprioceptors

By adaptation behavior

Stimulus reception and transmission

Stimulus reception and transmission involve two main steps. First, physical or chemical stimuli are detected by sensory receptors and converted into an electrical signal called a receptor potential (transduction). If the receptor potential reaches a threshold, transformation occurs: action potentials are generated and transmitted to the CNS. Conscious perception requires further processing within the CNS.

Overview

1. The stimulus acts on the sensory receptor.
2. Transduction: The sensory cell generates the receptor potential.
3. Transformation: A sequence of action potentials is generated by either a primary sensory cell or a sensory neuron that receives synaptic input from a secondary sensory cell.

Transduction (sensory physiology)

• Sequence
  1. Stimulus reception by the receptor
  2. The stimulus is converted into a receptor potential
• Characteristics of the receptor potential
  • Graded potential: amplitude and duration are proportional to the stimulus intensity and duration
  • Spreads passively from its site of origin within the receptor cell (= passive electrotonic conduction) .
• Examples
  • Sensory modality: hearing
    • Receptors: hair cells
    • Transduction process in hair cells: The mechanical deflection of sensory hairs is converted into a receptor potential.
  • Sensory modality: vision
    • Receptors: photoreceptors
    • Transduction process: Light, i.e., electromagnetic waves, is converted into a receptor potential.

Transformation (sensory physiology)

During transformation, the receptor potential is converted into action potentials, which are then transmitted via afferent nerve fibers (usually via saltatory conduction) to the CNS.

• Sequence
  1. The receptor potential spreads electrotonically to a trigger zone (rich in voltage-gated channels).
  2. If the threshold of neighboring voltage-gated Na⁺ and K⁺ channels is exceeded, an action potential is generated
  3. Propagation of the action potential along the afferent neuron to the CNS
• Characteristics of the transformation
  • Amplitude of the receptor potential is encoded by the frequency of action potentials
  • Action potentials follow the all-or-none law

The stronger the stimulus, the larger the receptor potential and the higher the frequency of action potentials!

The receptor potential spreads electrotonically, meaning that transmission occurs passively and weakens with distance. In contrast, action potentials follow the “all-or-none law” and regenerate along the axon.

Definitions

Weber's law

• States that the perceptible difference between two weights of different heaviness is in a constant ratio to the initial weight
• Formula: k = ΔI / I
  • k = Weber's constant, ΔI = just-noticeable difference, I = initial stimulus intensity
• Calculation example: In a series of experiments on the sense of force, a participant reports that a weight must reach 105 g to be perceived as heavier than a 100 g reference weight.
  According to Weber’s law, by how many grams must a weight be lighter than 500 g for the difference to be perceptible to the same person?
  • Find: ΔI₂
  • Given: I₁ = 100 g, I₂ = 500 g
  • Formula: k = ΔI₁ / I₁ = ΔI₂ / I₂
  • Steps:
    • 1. Calculate ΔI₁: ΔI₁ = 105g - 100 g = 5 g
    • 2. Calculate Weber fraction (k): k = ΔI₁ / I₁ = 5 g/ 100 g = 0.05
    • 3. Calculate ΔI₂: k = ΔI₂ / I₂ ⇔ ΔI₂ = k × I₂ = 0.05 x 500 g = 25 g
  • Interpretation: In this series of experiments, a weight would have to be at least 25 g less than 500 g (i.e., 475 g or lighter) to be perceived by the person as lighter than a weight of 500 g.

Signal detection theory (SDT)

The SDT states that the ability to detect a stimulus (a signal) depends not only on the signal's intensity but also on a person's sensory sensitivity and their psychological decision-making criterion used to distinguish it from background "noise". SDT analyzes decision-making by looking at four possible outcomes.

• Sensitivity: represents how well a signal can be distinguished from background noise
  • High sensitivity: occurs when the signal is very strong compared to the background noise → results in many correct identifications (hits) and correct rejections
  • Low sensitivity: occurs when the signal is weak/hard to detect → results in many errors due to a higher likelihood of false alarms (incorrectly identifying background noise as a signal) and misses (failing to identify a signal present)
• Signal detection strategies: two common strategies can influence outcomes in signal detection
  • Liberal strategy: You say "Yes" at the slightest hint of a potential signal.
    • Outcome: many hits, but many false alarms ; beneficial, when catching every possible signal is prioritized over accuracy
  • Conservative strategy: You say "Yes" only if the signal is unmistakable.
    • Outcome: many correct rejections and fewer false alarms, but many misses; effective in scenarios where incorrect identification could have serious consequences

• Application example: a radiologist is looking at an X-ray for a tumor
  • Hit: tumor is there; the doctor finds it
  • False alarm: no tumor; doctor says there is one (leads to unnecessary biopsy)
  • Miss: tumor is there; doctor misses it (dangerous for the patient)
  • Correct rejection: no tumor; doctor says the X-ray is clear

Sensory pathways

See "Overview of sensory pathways" in "The somatosensory system" article.

Receptive fields

A receptive field is the specific physical area where a stimulus alters a sensory neuron's activity. Afferent nerve fibers carry this information to the CNS, where convergence on a single neuron may occur. High convergence leads to larger central receptive fields, reducing location accuracy, while minimal convergence results in smaller fields and greater spatial precision. Thus, receptive field size directly affects sensory acuity.

• Large receptive fields
  • Resolution: low (blurry)
  • Convergence: high (many-to-one)
    • Principle: many afferent fibers converge on one central neuron → the neuron cannot precisely localize where within the receptive field the stimulus occurred → poor spatial discrimination
  • Two-point discrimination: poor (greater distance needed)
  • Example: skin on the back or thigh, where large receptive fields make two nearby touches feel like a single point
• Small receptive fields
  • Resolution: high (sharp)
  • Convergence: low (one-to-one)
    • Principle: only one or a few afferent fibers converge on each central neuron → the neuron can accurately localize the stimulus → high spatial discrimination
  • Two-point discrimination: excellent (minimal distance needed)
  • Example: fingertips or lips, where small, densely packed receptive fields enable fine tactile acuity