Coding of stimulus intensity: firing frequency and recruitment
In the somatosensory system (the body's system for touch, pain, temperature and position sense), a stronger stimulus makes sensory neurones fire more often and activates more neurones.
Remember
Stimulus strength is coded mainly by how often afferent neurones (neurones carrying signals towards the central nervous system) fire, with more neurones also being activated as the stimulus gets stronger, not by the size of each action potential.
The somatosensory system has to tell the brain how strong a stimulus is as well as what it is. The receptor first produces an analogue signal, meaning a graded signal that rises smoothly as the stimulus gets stronger. The first order sensory neurone (the neurone that connects the receptor to the spinal cord) then converts this into a digital code, which is trains of action potentials (the electrical impulses of nerve cells).
Action potentials are all or none, so each one has the same size. Their size cannot carry the message, but their rate can. In rate coding, strong receptor activation gives a high frequency of action potentials and weak activation gives a lower frequency. A light bump and a blow from a cricket ball both stimulate touch receptors, but the bump gives few action potentials and the cricket ball gives a rapid burst.
Intensity is thought to be coded in two ways at once, mainly by firing rate. A stronger stimulus also activates more afferent neurones, including those with higher thresholds, which is called recruitment. Receptive fields (the patches of skin supplied by one neurone) overlap, so a firmer pressure brings more neurones to threshold. This is population coding, and perceived intensity is thought to depend on the size of the active population as well as the firing rates, which makes perceived pressure rise smoothly rather than in steps.
For a light touch, the kind of sensation stays the same as the force rises. Receptor type tells the brain the kind of sensation, while the firing rate and the number of active neurones tell it the intensity.
Key facts
- Receptor signal
- The receptor produces an analogue signal, a graded signal that rises smoothly as the stimulus gets stronger.
- Where conversion happens
- The conversion to a digital code of action potentials happens in the first order sensory neurone, not before it.
- Action potential size
- Each action potential has the same size, so the size cannot carry the message.
- Rate coding
- A stronger stimulus gives a higher frequency of action potentials and a weaker one gives a lower frequency.
- Recruitment
- A stronger stimulus also activates more afferent neurones, including those with higher thresholds.
- Receptor type
- The type of receptor tells the brain the kind of sensation, not how strong it is.
Exam traps
- Larger action potentials do not signal a stronger stimulus, because action potentials are all or none and the neurone fires more often instead.
- A stronger stimulus does not bring in a different type of receptor on the same neurone, because the receptors feeding one neurone are of the same type.
- Lateral inhibition (the suppression of neighbouring neurones) sharpens contrast between neighbouring receptive fields, so it does not represent how strong a stimulus is.
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