Table of Contents
Sense organs form the interface between an organism and its environment. They transform physical and chemical events “out there” into nerve signals that the nervous system can process “inside.” In this chapter, the focus is on this mediating role itself, not on the detailed structure of individual senses (which are treated in later chapters).
From Environmental Stimulus to Neural Signal
The basic task of all sense organs is transduction: converting one form of energy or information into another.
- In the environment: light, mechanical pressure, sound waves, temperature changes, chemical substances, electrical or magnetic fields.
- In the organism: changes in membrane potential of sensory cells, generation of action potentials, patterned nerve impulses.
The steps are:
- Reception (Detection)
A receptor cell or a receptor molecule in its membrane interacts with a specific environmental stimulus (for example, a photon, a sound-induced vibration, or an odor molecule). - Transduction
The stimulus is converted into a change in the receptor cell’s electrical state (a receptor potential). This may directly or indirectly open ion channels and alter the membrane potential. - Encoding
If the receptor potential reaches threshold (directly in the receptor cell or in an associated neuron), action potentials are generated. - Stimulus intensity is typically encoded in the frequency of action potentials (stronger stimulus → higher firing rate).
- Stimulus duration is encoded in the time course of firing.
- Stimulus location is often encoded by which specific receptors or receptor fields are activated.
- Transmission
The generated nerve impulses travel via sensory (afferent) neurons to the central nervous system (CNS), where they are processed and interpreted.
At no point is the environment itself “inside” the organism; only coded signals produced by sense organs reach the CNS. Sense organs therefore determine what and how an organism can perceive.
Types of Stimuli and Corresponding Modalities
Sense organs mediate between environment and organism by being specialized for particular modalities:
- Photoreception: light intensity and wavelength (visual sense).
- Mechanoreception: pressure, vibration, tension, sound waves (touch, hearing, some balance functions).
- Thermoreception: temperature changes (heat and cold).
- Chemoreception: dissolved or airborne molecules (taste and smell).
- Nociception: potentially damaging stimuli (pain; mechanical, thermal, or chemical).
- Electroreception and Magnetoreception: electrical and magnetic fields (in some fish, birds, and other animals).
Each modality corresponds to a particular receptor type and a particular sensory pathway. The physical event in the environment is different for each modality, but the result in the nervous system is always the same sort of “language”: sequences of action potentials.
Sensory Filters: Species-Specific Access to the World
Different organisms live in different environments and have evolved different sense organs. Each species therefore experiences only a limited excerpt of all possible environmental information, sometimes called its sensory world or Umwelt.
Examples:
- Bees perceive ultraviolet patterns on flowers that humans cannot see.
- Many snakes can detect infrared radiation from warm prey.
- Bats and dolphins use high-frequency sound and echolocation to build an acoustic map of their surroundings.
- Some fish detect weak electric fields produced by other animals.
Sense organs thus filter the environment: they pass some kinds of information to the nervous system and ignore others. This filtering is:
- Biologically useful: only information relevant to survival and reproduction is processed.
- Species-specific: different animals live in different “worlds” even in the same physical place.
This mediation is therefore selective, not neutral; sense organs shape what is even available for behavior and perception.
Functional Characteristics of Sense Organs
Although sense organs differ in structure and modality, they share several functional properties that determine how they mediate between organism and environment.
Sensitivity and Threshold
Each receptor type has a threshold: the minimal stimulus intensity necessary to produce a detectable response.
- High sensitivity → low threshold (even weak stimuli trigger receptor potentials).
- Low sensitivity → high threshold.
Sense organs often operate close to the minimal physical limits of detectability (for example, very low light levels, minimal sound intensity), but sensitivity is tuned to ecological needs.
Range and Dynamic Adjustment
Sense organs have a limited working range of intensities within which differences can be distinguished.
- If stimuli are too weak → below threshold, not perceived.
- If stimuli are too strong → receptors saturate, further increases are not encoded.
Many sense organs adjust dynamically (adaptation, gain control) so that they remain useful in changing environments (e.g., bright midday vs. dim twilight for vision).
Specificity and Adequate Stimulus
Every receptor type has an adequate stimulus: the form of energy to which it is most sensitive under natural conditions.
- Photoreceptors: light (photons).
- Hair cells in the inner ear: mechanical deflection by sound-induced fluid movement.
- Olfactory receptors: binding of specific molecule types.
Receptors can sometimes be activated by other, stronger stimuli (for example, mechanical pressure on the eye causing “flashes of light”), but their normal function is tied to their adequate stimulus.
Receptive Fields and Spatial Resolution
Many receptors are organized so that each one is responsible for a limited region of the environment, its receptive field.
- On the skin: each touch receptor covers a small skin area.
- In the retina: each photoreceptor responds to a small portion of the visual field.
Small, densely packed receptive fields → high spatial resolution (fine detail, precise localization). Large or sparse fields → low spatial resolution but sometimes higher sensitivity.
By combining signals from many receptive fields, the nervous system constructs a spatial map of the environment (for example, a visual image, a spatial pattern of touch).
Adaptation and Change Detection
Sense organs often emphasize changes rather than constant conditions.
- Many receptors show adaptation: their response declines when a stimulus remains constant.
- Rapid changes in stimulus intensity or pattern produce stronger, more sustained responses.
This adaptation:
- Prevents flooding the nervous system with unchanging information.
- Makes organisms particularly sensitive to new or moving stimuli (for example, a moving predator against a constant background).
Parallel Processing and Integration
Different aspects of the same environmental object are often encoded by different receptor types in parallel.
Example (using vision conceptually, without going into structural details):
- Some receptors are more sensitive to brightness differences.
- Others to color (wavelength).
- Others to motion or contrast.
The CNS integrates these parallel inputs into a unified percept. Thus, the mediating role of sense organs is not merely detection; it includes pre-sorting and pre-processing information before it reaches higher centers.
Internal vs. External Environment
Sense organs do not only mediate between the organism and the external environment; some receptors monitor the internal environment and provide feedback necessary for regulation and control.
- Exteroceptors: respond to external stimuli (light, sound, external temperature, external touch).
- Interoceptors: respond to conditions inside the body (blood pressure, blood gas levels, stretch in internal organs, nutrient levels).
- Proprioceptors: provide information on body position and movement (muscles, tendons, joints, parts of the balance organ).
Together, these receptors deliver the information needed for:
- Orientation in space and in relation to other objects.
- Maintenance of internal balance (homeostasis).
- Coordination of movement and posture.
Thus, sense organs continuously mediate not only between organism and environment, but also between different parts and states of the organism itself.
Evolutionary and Ecological Aspects of Mediation
As interfaces with the environment, sense organs are strongly shaped by natural selection:
- Species in dark habitats often rely more on chemical or mechanical senses.
- Predators and prey frequently have highly developed visual or auditory systems suited to their lifestyle.
- Migratory animals may possess particularly refined magnetoreception or celestial orientation mechanisms.
In each case, sense organs mediate exactly those environmental features that are most relevant for the organism’s survival, such as:
- Finding food and mates.
- Avoiding predators and dangers.
- Navigating and selecting suitable habitats.
The form and sensitivity of sense organs therefore tell us a great deal about how an organism “experiences” and uses its environment.
Limits of Perception and Constructed Reality
Because sense organs filter, transform, and encode environmental stimuli:
- Perception is always indirect: we do not experience the physical stimulus itself, but the brain’s interpretation of nerve signals.
- There is no “complete” picture of the environment; every organism has a partial, species-specific representation.
For humans as well:
- We cannot hear ultrasound like bats, nor perceive polarized light like many insects.
- Our experience of color, tone, warmth, or pain is the result of processing in the nervous system based on inputs from sense organs.
Sense organs are therefore not just passive windows, but active mediators that shape what the environment means for each organism.