Coordination and Movement
2. Sensory Receptors
Learning outcomes
- I can define sensory receptors and explain their function.
- I can identify different types of sensory receptors.
- I can explain how receptors detect environmental changes.
- I can describe how sensory information is processed.
- I can explain how sensory systems contribute to survival.
What Are Sensory Receptors?
Animals are constantly exposed to changes in both their external environment and their internal environment.
These changes are called stimuli.
Examples include:
- Light becoming brighter.
- A sudden sound.
- An increase in temperature.
- Pressure against the skin.
- The smell of food.
- Changes in blood carbon dioxide concentration.
- Changes in body position.
To respond to these changes, an animal must first detect them.
A sensory receptor is a specialised cell, group of cells, or nerve ending that detects a particular type of stimulus and converts information about that stimulus into signals that can be transmitted through the nervous system.
This conversion of stimulus energy into an electrical signal is called sensory transduction.
A general pathway is:
Stimulus → sensory receptor → sensory neuron → central nervous system → processing → response
Receptors Detect Specific Types of Stimuli
Sensory receptors are specialised.
Different receptors respond particularly well to different forms of energy or chemical change.
Major categories include:
- Photoreceptors – detect light.
- Mechanoreceptors – detect mechanical forces.
- Chemoreceptors – detect chemicals.
- Thermoreceptors – detect temperature.
- Nociceptors – detect potentially damaging stimuli.
- Proprioceptors – provide information about body position and movement.
This specialisation allows the nervous system to distinguish between different types of information.
Photoreceptors
Photoreceptors detect light.
In vertebrates, photoreceptors are found in the retina at the back of the eye.
Humans have two major types:
- Rod cells.
- Cone cells.
Rod Cells
Rods are highly sensitive to light.
They are especially useful in:
- Dim conditions.
- Night vision.
- Detecting changes in brightness.
- Peripheral vision.
Rods do not provide detailed colour information.
This is one reason colours become difficult to distinguish under very low light.
Cone Cells
Cones function best in brighter light.
They are important for:
- Colour vision.
- Fine visual detail.
- High visual acuity.
Humans normally have three types of cone cells with different sensitivities to wavelengths of visible light.
The brain compares information from these receptors to produce our perception of colour.
From Light to Vision
Seeing an object requires much more than light simply entering the eye.
A simplified sequence is:
Light from object
↓
Light enters eye
↓
Image forms on retina
↓
Photoreceptors detect light
↓
Electrical signals are produced
↓
Signals travel through the optic nerve
↓
Brain processes the information
↓
Visual perception
The eye detects light, but much of what we experience as vision results from processing within the brain.
Mechanoreceptors
Mechanoreceptors respond to mechanical forces.
These can include:
- Pressure.
- Touch.
- Stretch.
- Vibration.
- Sound waves.
- Movement.
Mechanoreceptors occur in many parts of the body.
Touch and Pressure Receptors
The skin contains several types of sensory nerve endings that respond to mechanical changes.
These allow us to detect:
- Light touch.
- Pressure.
- Vibration.
- Stretching of the skin.
Different regions of the body have different densities of receptors.
For example, the fingertips contain many touch receptors and can distinguish fine details much better than many areas of the back.
Mechanoreceptors and Hearing
Sound consists of vibrations travelling through a medium.
In the ear, sound vibrations eventually cause movement of specialised sensory structures in the cochlea.
Hair cells within the cochlea act as mechanoreceptors.
A simplified sequence is:
Sound wave → vibration of ear structures → movement within cochlea → hair cells stimulated → nerve signals → brain
The brain interprets these signals as sound.
Balance and Movement
Mechanoreceptors are also important for maintaining balance.
The inner ear contains structures that detect:
- Rotation of the head.
- Linear acceleration.
- Head position relative to gravity.
These signals are sent to the brain, where they are combined with information from:
- Eyes.
- Muscles.
- Joints.
This allows the nervous system to coordinate posture and balance.
Chemoreceptors
Chemoreceptors detect chemicals or changes in chemical concentration.
They are involved in senses such as:
- Taste.
- Smell.
Chemoreceptors also monitor internal conditions.
For example, some receptors detect changes related to:
- Carbon dioxide concentration.
- Oxygen concentration.
- Blood pH.
Smell
The sense of smell depends on olfactory receptors.
Chemical molecules in the air enter the nasal cavity and interact with receptors in specialised tissue.
Different molecules activate different combinations of receptors.
Signals travel to the brain, where the pattern is interpreted as a particular smell.
This allows animals to detect chemical information in their environment.
Taste
Taste receptors are found mainly in structures called taste buds.
They respond to dissolved substances.
Human taste perception includes sensations associated with:
- Sweet.
- Sour.
- Salty.
- Bitter.
- Umami.
Taste and smell work together closely.
This is why food can appear to have much less flavour when the nose is blocked.
Internal Chemoreceptors
Chemoreceptors are not limited to the traditional senses.
Some monitor the chemical composition of the body's internal fluids.
For example, changes in blood carbon dioxide and pH can be detected by chemoreceptors associated with the brainstem and major blood vessels.
If carbon dioxide increases significantly:
Chemoreceptors detect change → CNS receives information → breathing is adjusted
This contributes to homeostasis.
Thermoreceptors
Thermoreceptors detect changes in temperature.
They are found in places including:
- Skin.
- Hypothalamus in the brain.
Skin thermoreceptors provide information about external temperature.
Internal thermoreceptors help monitor the temperature of the body itself.
Thermoreceptors and Homeostasis
Suppose body temperature rises.
Temperature-sensitive receptors detect the change.
Information is processed by the nervous system.
Responses can include:
- Increased sweating.
- Increased blood flow near the skin.
If body temperature falls:
- Shivering may begin.
- Blood flow near the skin may decrease.
This creates a regulatory pathway:
Temperature change → receptors → CNS → effectors → corrective response
Sensory receptors therefore contribute directly to homeostasis.
Nociceptors
Nociceptors are sensory receptors that respond to stimuli associated with actual or potential tissue damage.
These can include:
- Extreme heat.
- Extreme cold.
- Strong mechanical pressure.
- Chemicals released by damaged tissue.
Their activity contributes to the sensation of pain.
Pain is unpleasant, but it has an important protective function.
It can cause an animal to:
- Withdraw from danger.
- Protect an injured body part.
- Avoid repeating harmful behaviour.
Proprioceptors
Close your eyes and raise one arm.
Even though you cannot see your arm, you still have a good idea where it is.
This ability depends partly on proprioception.
Proprioceptors are sensory receptors associated with structures such as:
- Muscles.
- Tendons.
- Joints.
They provide information about:
- Muscle length.
- Muscle tension.
- Joint position.
- Movement.
This information allows the nervous system to continuously monitor the position of different body parts.
Proprioception and Movement
Imagine walking down a staircase.
Your nervous system must constantly determine:
- Where your feet are.
- Whether your knees are bent.
- How much your muscles are stretched.
- Whether your body is balanced.
Much of this information is provided automatically by proprioceptors.
Without proprioception, smooth coordinated movement would be extremely difficult.
Receptor Specialisation
Why have different receptors?
Different stimuli involve different forms of energy or chemical change.
For example:
- Light consists of electromagnetic radiation.
- Sound involves mechanical vibrations.
- Odours involve chemical molecules.
- Temperature involves thermal conditions.
A single receptor type would not detect all of these equally effectively.
Specialised receptors allow animals to collect many different types of information simultaneously.
Sensory Transduction
A receptor must convert a stimulus into a form the nervous system can transmit.
This process is called sensory transduction.
For example:
Light → photoreceptor → electrical change
Pressure → mechanoreceptor → electrical change
Chemical molecule → chemoreceptor → electrical change
Temperature change → thermoreceptor → electrical change
The nervous system can then transmit information using patterns of action potentials.
Stimulus Strength
Sensory systems must also provide information about the strength of a stimulus.
A stronger stimulus does not normally produce a "bigger" individual action potential.
Instead, stimulus intensity can be represented partly by:
- Increased frequency of action potentials.
- Activation of additional sensory receptors.
Therefore, the nervous system can distinguish between:
- Dim and bright light.
- Gentle and strong pressure.
- Quiet and loud sounds.
Sensory Neurons
After a receptor detects a stimulus, information is commonly transmitted toward the CNS by sensory neurons.
A simplified pathway is:
Receptor → sensory neuron → spinal cord and/or brain
Sensory neurons therefore connect detection with processing.
The receptor detects the change.
The sensory neuron carries information about it.
Sensory Information Is Processed
Detecting a stimulus is only the beginning.
The central nervous system must interpret the incoming information.
For example, imagine touching an object.
Your brain may need to determine:
- Where you were touched.
- How strong the pressure was.
- Whether the object is moving.
- Whether it is hot or cold.
- Whether it may cause injury.
Information from several receptor types can be combined to produce a useful interpretation.
The Brain Constructs Perception
Sensory receptors provide information, but our conscious experience is produced through processing by the nervous system.
Consider vision.
The retina detects patterns of light.
The brain processes information about:
- Colour.
- Shape.
- Movement.
- Depth.
- Position.
- Previous experience.
Together, this produces visual perception.
This explains why sensory perception is more complex than simply detecting a stimulus.
Sensory Adaptation
Have you ever entered a room and noticed a strong smell, only to notice it much less a few minutes later?
This is an example of sensory adaptation.
Sensory adaptation is a reduction in sensitivity to a constant stimulus over time.
Examples can include:
- Becoming less aware of clothing touching your skin.
- Becoming less aware of a constant smell.
- Adjusting to changes in light level.
Adaptation allows the nervous system to focus attention on new or changing information.
Sensory Systems and Survival
Sensory systems provide animals with information needed for survival.
They help animals:
- Locate food.
- Detect predators.
- Find mates.
- Navigate.
- Communicate.
- Avoid harmful environments.
- Maintain homeostasis.
Different species have evolved sensory systems suited to their environments and lifestyles.
Predator Detection
Imagine a grazing animal detects movement nearby.
Photoreceptors detect the visual change.
The information travels to the brain.
The brain integrates this with other information, such as:
- Sound.
- Smell.
- Previous experience.
If the movement is identified as a predator, the nervous system can rapidly coordinate escape.
The sequence might be:
Movement → photoreceptors → sensory neurons → brain → motor neurons → muscles → escape
A rapid sensory response can make the difference between survival and capture.
Finding Food
Sensory systems also help animals locate food.
Different animals may rely heavily on different senses.
Examples include:
- Vision for detecting prey.
- Smell for locating food at a distance.
- Taste for evaluating potential food.
- Touch for locating food in dark environments.
- Hearing for detecting moving prey.
The sensory system most useful to an animal depends strongly on its habitat and feeding strategy.
Sensory Adaptations in Nocturnal Animals
Animals active at night often have sensory adaptations that allow them to function under low-light conditions.
These may include:
- Eyes adapted for greater light sensitivity.
- Large numbers of rod photoreceptors.
- Highly sensitive hearing.
- Strong sense of smell.
- Specialised touch receptors.
An animal does not necessarily rely on only one sensory system.
Combining several types of information can provide a more accurate picture of the environment.
Echolocation
Bats and some other animals can use echolocation.
The animal produces sound.
The sound travels through the environment and reflects from objects.
Returning echoes are detected by auditory receptors.
The nervous system analyses characteristics of the returning echoes.
This can provide information about:
- Distance.
- Direction.
- Size.
- Movement.
Echolocation demonstrates how sensory systems can become highly specialised for particular lifestyles.
Sensory Systems in Aquatic Animals
Water transmits many stimuli differently from air.
Aquatic animals therefore possess specialised sensory adaptations.
Fish, for example, possess a lateral line system.
The lateral line contains mechanoreceptors that detect water movement and vibrations.
This can help fish:
- Detect nearby movement.
- Avoid obstacles.
- Detect predators.
- Locate prey.
- Coordinate schooling behaviour.
Sensory Systems and Communication
Sensory receptors also allow animals to receive signals from other members of their species.
Communication signals can include:
- Visual displays.
- Sounds.
- Chemical signals.
- Touch.
- Vibrations.
For communication to work, the receiver must have receptors capable of detecting the signal.
For example:
Sound-producing animal → sound waves → auditory receptors of another animal → nervous processing
Communication therefore depends on both signal production and sensory detection.
Sensory Systems and Homeostasis
Sensory receptors do not only monitor the outside world.
Internal receptors continually monitor conditions within the body.
These can detect changes associated with:
- Temperature.
- Blood pressure.
- Carbon dioxide concentration.
- Oxygen concentration.
- Muscle stretch.
- Water balance.
This information allows the nervous system to coordinate corrective responses.
Therefore, sensory receptors contribute to both:
survival in the external environment + regulation of the internal environment
Worked Example: Touching a Hot Surface
Imagine accidentally touching a hot metal surface.
Detection
Thermoreceptors and nociceptors detect potentially damaging conditions.
Transmission
Sensory neurons carry signals toward the spinal cord.
Processing
A reflex pathway can rapidly activate motor neurons.
Response
Arm muscles contract and withdraw the hand.
Further Processing
Information also reaches the brain, producing conscious awareness of heat and pain.
The complete pathway can be simplified as:
Hot surface → receptors → sensory neuron → CNS → motor neuron → muscle → withdrawal
Worked Example: Smelling Food
Suppose you walk past a bakery.
Chemical molecules from food enter your nose.
Step 1
The molecules interact with olfactory receptors.
Step 2
Receptor cells generate electrical signals.
Step 3
Signals travel toward the brain.
Step 4
The brain processes the pattern of signals.
Step 5
The smell is recognised, potentially using stored memories.
The response might include:
- Increased attention.
- Salivation.
- Movement toward the food.
This demonstrates how sensory information can influence both behaviour and physiological responses.
Worked Example: Maintaining Balance
Imagine standing on one leg.
The nervous system receives information from:
- Eyes.
- Vestibular receptors in the inner ear.
- Proprioceptors in muscles and joints.
- Pressure receptors associated with contact between the foot and ground.
The brain combines these signals.
Motor commands are continually adjusted.
Muscles make small corrections to maintain posture.
Balance is therefore produced by the integration of several sensory systems, rather than by one receptor alone.
Comparing Receptor Types
| Receptor | Main stimulus detected | Example |
|---|---|---|
| Photoreceptor | Light | Rods and cones in retina |
| Mechanoreceptor | Pressure, vibration or movement | Touch receptors, inner-ear hair cells |
| Chemoreceptor | Chemicals | Taste and smell receptors |
| Thermoreceptor | Temperature | Receptors in skin |
| Nociceptor | Potential tissue damage | Pain-sensitive nerve endings |
| Proprioceptor | Body position and movement | Muscle and tendon receptors |
This classification is useful, but biological sensory systems can be complex and some receptors respond to more than one aspect of a stimulus.
Sensory Information Is Integrated
The brain rarely relies on one piece of sensory information alone.
Imagine crossing a busy street.
You might use:
- Vision to detect vehicles.
- Hearing to detect engines or horns.
- Proprioception to coordinate walking.
- Balance receptors to maintain posture.
- Touch receptors to detect the ground beneath your feet.
The nervous system combines this information to create an appropriate response.
This process is called sensory integration.
What Happens If Receptors Are Damaged?
Damage to sensory receptors can reduce or eliminate particular types of sensory information.
For example:
- Damage to retinal photoreceptors can affect vision.
- Damage to cochlear hair cells can cause hearing loss.
- Damage to sensory nerves can reduce touch or pain sensation.
The consequences demonstrate how important sensory information is for safe interaction with the environment.
Loss of pain sensation, for example, can be dangerous because injuries may occur without being detected.
Receptor Density and Sensitivity
Sensory receptors are not distributed equally throughout the body.
Some areas contain many more receptors than others.
The fingertips and lips, for example, have high densities of touch receptors.
This allows fine discrimination between nearby stimuli.
This can be investigated using a two-point discrimination test.
Two nearby points are touched against the skin.
If the points stimulate sufficiently separate sensory fields, they are perceived as two individual contacts.
Areas with high receptor density can usually distinguish points that are closer together.
Common Mistakes
Saying Sensory Receptors Produce the Response
Receptors detect stimuli. Effectors such as muscles and glands produce responses.
Saying All Receptors Detect the Same Stimulus
Different receptors are specialised for different forms of information.
Confusing Receptors With Sensory Neurons
A receptor detects a stimulus. Sensory neurons carry information toward the central nervous system. In some sensory systems, the receptor itself is a specialised neuron; in others, a separate receptor cell communicates with a neuron.
Saying Eyes "See" Everything Directly
Photoreceptors detect light. The brain performs extensive processing that produces visual perception.
Saying Pain Has No Useful Function
Pain can provide important information about actual or potential tissue damage and can trigger protective behaviour.
Thinking Sensory Adaptation Means the Stimulus Has Disappeared
The stimulus may still be present. The sensory system has simply become less responsive to a constant stimulus.
Thinking All Animals Sense the World in the Same Way
Different species have sensory systems adapted to different environments and lifestyles.
Check Your Understanding
1. Define a sensory receptor.
2. What is a stimulus?
3. Explain what is meant by sensory transduction.
4. Name five major types of sensory receptors and identify the stimulus detected by each.
5. Compare rods and cones.
6. Explain how mechanoreceptors contribute to hearing.
7. Give two examples of chemoreceptors.
8. Explain how thermoreceptors contribute to homeostasis.
9. What is the function of nociceptors?
10. Explain the importance of proprioceptors.
11. Describe the pathway of sensory information from a receptor to the CNS.
12. What is sensory adaptation, and why might it be useful?
13. Explain how several sensory systems work together to maintain balance.
14. Give two examples of specialised sensory adaptations in animals.
15. An animal is active almost entirely at night and has highly sensitive hearing, many rod photoreceptors, and a strong sense of smell. Explain how these features could increase its chances of survival.
Key Terms
- Sensory receptor – specialised cell, group of cells, or nerve ending that detects a stimulus.
- Stimulus – detectable change in the internal or external environment.
- Sensory transduction – conversion of stimulus information into electrical signals used by the nervous system.
- Photoreceptor – receptor specialised for detecting light.
- Mechanoreceptor – receptor responding to mechanical forces such as pressure, vibration, or movement.
- Chemoreceptor – receptor responding to chemicals or changes in chemical concentration.
- Thermoreceptor – receptor responding to temperature.
- Nociceptor – receptor responding to potentially damaging stimuli.
- Proprioceptor – receptor providing information about body position and movement.
- Rod – retinal photoreceptor specialised for high sensitivity in low light.
- Cone – retinal photoreceptor important for colour vision and fine detail.
- Retina – light-sensitive tissue at the back of the eye.
- Olfactory receptor – chemoreceptor involved in smell.
- Sensory neuron – neuron carrying sensory information toward the CNS.
- Sensory adaptation – reduction in responsiveness to a constant stimulus.
- Sensory integration – combination and processing of information from different sensory sources.
- Proprioception – awareness of body position and movement.
- Echolocation – use of emitted sounds and returning echoes to obtain information about surroundings.
- Lateral line – mechanosensory system in fish and some aquatic vertebrates that detects water movement.
Key Takeaways
- Sensory receptors allow animals to detect changes in their internal and external environments.
- A stimulus is a detectable environmental or internal change.
- Receptors convert stimulus information into signals through sensory transduction.
- Photoreceptors detect light.
- Mechanoreceptors detect mechanical forces such as touch, vibration, and movement.
- Chemoreceptors detect chemical substances or changes in chemical conditions.
- Thermoreceptors detect temperature.
- Nociceptors detect potentially damaging stimuli.
- Proprioceptors provide information about body position and movement.
- Sensory information is transmitted toward the central nervous system for processing.
- The brain combines information from multiple receptors to produce useful perceptions and responses.
- Sensory adaptation reduces responsiveness to some constant stimuli.
- Internal sensory receptors contribute to homeostasis.
- Sensory systems help animals find food, avoid predators, navigate, communicate, and reproduce.
- Different species possess sensory adaptations suited to their particular habitats and lifestyles.
- Sensory systems are most effective when detection, nervous-system processing, and coordinated responses work together.