In one breath
The sensory system detects a stimulus with a receptor, turns it into action potentials (transduction), carries them up afferent pathways, and lets the brain recognise them and act. Sensation is the basic recognition of a stimulus; perception is its interpretation. Sensations are somatic, visceral or special, and every sensation has four dimensions: modality, intensity, affect and acuity.
Builds on: Divisions of the nervous system · Leads to: Sensory receptors and their classification · Receptor potential and properties of receptors · Ascending tracts: dorsal column and anterolateral system · Somatosensory cortex
How the sensory system works
The sensory system keeps us aware of the world outside and of the body within. It all starts with a stimulus.
- The receptor, an afferent nerve ending, senses the stimulus and converts it into action potentials. The conversion is called transduction.
- Sensory nerves carry the impulses to the spinal cord, and ascending neurons carry them on to the brain.
- Centres in the CNS process the information, and the end result is a change in behaviour, carried out through efferent pathways.
| Component | Its job |
|---|---|
| Receptors | Receive the stimulus; transduce it into action potentials |
| Afferent pathways | Carry the action potentials to the CNS |
| CNS neurons | Modulate the sensory information |
| Brain areas | Recognise the stimulus |
Sensation
Sensation is the basic recognition of a stimulus. Clinicians call it esthesia, from the Greek for “feeling”, and name its disorders from that root.
| Term | Meaning |
|---|---|
| Anesthesia | No sensation |
| Paresthesia | Abnormal sensation |
| Kinesthesia | Sensation of movement |
Types of sensation
Somatic sensations
- Also called somesthesia.
- Receptors: on the body surface, and in the body wall, muscles, tendons, bones, joints and connective tissue.
- Examples: touch, pain, temperature, vibration, and joint movement (proprioception).
Visceral sensations
- Arise when receptors in the viscera are stimulated.
- The receptors lie in the wall of the organ, if it has one, or in its connective tissue.
- The viscera are found in the skull, thorax, abdomen and pelvis (see Visceral and referred pain).
Special sensations
- Arise in the special sense organs, the eye, ear, nose and tongue, from special sensory receptors.
- They are vision, audition (hearing), olfaction (smell), gustation (taste) and the vestibular senses.
Perception and modulation
- Perception is the appreciation and interpretation of a sensation. Recognition comes first; then the sensations are compared, discriminated and integrated.
- Sensory modulation is control of a sensation on its way in. The same stimulus can feel different in quality and intensity, depending on:
- which afferent pathway carries it;
- which CNS areas it activates;
- how those areas process it.
- Modulation can accentuate a sensation or inhibit it.
- Worth knowing, though not in your pages: the gate control of pain is the classic example of sensory modulation.
Sensation is not perception
Sensation is recognising that a stimulus is there. Perception goes further: comparing, discriminating and integrating, so that you know what the stimulus is. Keep the two definitions separate in a viva.
Dimensions of sensation
| Dimension | What it means | Example |
|---|---|---|
| Modality | The type or quality | Tactile sensation → touch |
| Intensity | How strongly it is perceived | Pain does not fade; touch fades fast |
| Affect | Its emotional component | Pain is negative |
| Acuity | How precisely it is localised | Depends on receptor density |
Modality
The quality, or type, of a sensation. The modality of tactile sensation, for example, is touch.
Intensity
The degree to which a stimulus is perceived. If the stimulus is kept on a little longer, the sensation may:
- fade quickly on its own: a rapidly adapting sensation, such as touch;
- fade slowly: a slowly adapting sensation;
- not change at all: a non-adapting sensation, such as pain.
The receptor side of this is adaptation (see Receptor potential and properties of receptors).
Affect
The emotional component of a sensation.
- Neutral: no emotional component.
- Positive: stirs a pleasant emotional response.
- Negative: stirs an unpleasant one. Pain has negative affect.
Acuity
The precision with which a stimulus is localised. It varies with:
- the concentration of receptors, or innervation density;
- the size of the receptive fields where the stimulus falls;
- the size of the cortical area that represents that part of the body (see Somatosensory cortex).
Worth knowing, though not in your pages: acuity is finest where receptors are dense, fields are small and the cortical area is large, as on the fingertips and lips. Two-point discrimination is the bedside test of it (see Sensory lesions and sensory function tests).
Exam-answer skeleton: "Types and dimensions of sensation" (short note)
- Define sensation (esthesia), with anesthesia, paresthesia and kinesthesia.
- Outline the sensory system: receptor → afferent pathway → CNS modulation → recognition in the brain.
- Types: somatic, visceral and special, with where their receptors lie and examples.
- Perception and sensory modulation, a line each.
- Modality, with an example.
- Intensity: rapidly adapting, slowly adapting and non-adapting sensations.
- Affect: neutral, positive or negative.
- Acuity and the three things it depends on.