Sensory receptors: how the body notices the world

Introduction

Perception begins when the nervous system detects a change. Sensory receptors are specialised cells or nerve endings that receive information from the environment around us or from conditions inside the body.

Vision, hearing, smell, taste, touch, temperature, pain, balance, and awareness of body position all depend on sensory information. Receptors collect these signals and convert them into electrical changes that can travel through sensory neurons to the central nervous system.

The brain combines incoming signals with memory and context. This allows a person to identify a sound, recognise a surface by touch, notice a change in temperature, or adjust movement when balance changes.

Structure of sensory receptors

Some receptors are the specialised endings of sensory neurons. Others are separate receptor cells that communicate with a sensory neuron using chemical signals. Their shapes and supporting tissues vary according to the stimulus they detect.

For example, free nerve endings can detect some forms of touch, temperature, and potentially damaging stimuli. Encapsulated endings are enclosed by supporting tissue that can help them respond to particular mechanical changes. In the eye, photoreceptor cells in the retina respond to light and pass information through a chain of retinal cells towards the optic nerve.

How sensory receptors are classified

There are two useful ways to organise sensory receptors: by where they receive information and by the type of stimulus that activates them.

Classification by location

1. Exteroceptors respond to events outside the body. Receptors in the skin and photoreceptors in the eyes are examples.
2. Interoceptors monitor the internal environment. They can detect changes such as pressure, stretch, acidity, oxygen, carbon dioxide, or chemical signals within organs and blood vessels.
3. Proprioceptors provide information about body position and movement. They are found in muscles, tendons, and joints and help the brain coordinate posture and action.

Function and physiology

A stimulus must be strong enough to influence a receptor. When a receptor responds, it produces a graded electrical change. If the change reaches the necessary threshold, the connected sensory neuron can generate action potentials that carry information towards the spinal cord and brain.

Stimulus and sensory modality

A sensory modality describes the kind of information being detected, such as light, sound, pressure, temperature, or a chemical. The nervous system preserves important features of a stimulus, including its strength, location, timing, and duration.

From receptor to response

The signal does not represent a complete perception on its own. The brain compares activity from many receptors and pathways, then uses that information to guide an appropriate response. This may include moving away from a painful stimulus, adjusting grip, changing posture, or focusing attention on a new sound.

Functional receptor types

1. Chemoreceptors detect chemicals. They contribute to smell and taste and help monitor chemical conditions such as oxygen and carbon dioxide levels.
2. Mechanoreceptors respond to physical forces, including touch, pressure, vibration, stretch, and sound-related movement. Skin receptors and receptors in muscles and joints are examples.
3. Nociceptors respond to potentially damaging mechanical, thermal, or chemical stimuli. Their activity contributes to protective pain responses.
4. Thermoreceptors detect changes in temperature. Information from these receptors helps the nervous system regulate behaviour and body processes.
5. Photoreceptors respond to light in the retina. Rods and cones support different aspects of vision, including sensitivity in dim light and colour information.

Other specialised receptors contribute to the detection of pressure in blood vessels, electrical changes in some animals, humidity, and additional features of the internal or external environment.

References

• Saladin, K. S. Anatomy & Physiology: The Unity of Form and Function. McGraw-Hill, 2011.
• Boron, W. F. and Boulpaep, E. L. Medical Physiology: A Cellular and Molecular Approach. Elsevier, 2005.