2 point discrimination and acuity of touch
The acuity of touch sensation differs at different areas of the body. This is mainly due to two factors:
- the concentration of sensory receptors in that region
- the more that are present, the more likely more than one will be innervated by the same stimulus
- the size of the receptive fields of the receptors
- the smaller the receptive fields, the more accurate a representation of the stimulus is signalled to the brain- the body cannot differentiate between two stimuli acting on the same receptive field
The acuity of single and two-point discrimintation is improved by lateral inhibition. This is the process by which neurons inhibit signals from nearby neurons via an intermediate neuron in the CNS. This enhances the difference between strong signals (at the point of the stimulus) and weaker signals (generated nearby the stimulus), thus the body can pinpoint more exactly where the stimulus is coming from.
Different parts of the body are better at two-point discrimination:
- Your fingertip can detect a gap of around 2mm
- Your palm- 10mm
- back of the arm- 40mm
This acuity can be represented as the sensory homunculus. I.e. there are more receptors with finer receptive fields in the lips and fingertips than anywhere alse in the body.
Vibration sense is detected by the rapidly adapting mechanoreceptors (see fibre types), each of which has a different threshold sensitivity to vibration (in Hz). These thresholds overlap somewhat, so for middle frequency vibrations, they will be firing simultaneously. The receptors generate an AP for every ‘cycle’ of vibration.
- Pacinian corpuscles
- 60-400Hz; peak at 250 Hz
- Meissner’s corpuscles
- 5-300Hz; most sensitive at 20-50Hz
This means the body is most sensitive to vibrations of around 250Hz.
Cutaneous temperature sensation is most sensitive at the face and chest (most receptors) and least in the periphery. (NB Central body temperature is detected in the hypothalamus- not described here). In general, thermoreceptors can be divided into two groups:
- Warmth receptors
- Cold receptors
At constant temperatures,
- Warmth receptors are active in temperatures from 29-45° (most active at 44-45°)
- Cold receptors are active between 5-40° (most active at 25°)
- At body temperature, cold receptors are more active than warmth receptors:
During a change in temperature
- Warmth receptors show a dramatic rise in firing rate when temperature rises and a dramatic fall during cooling
- Cold receptors show a dramatic rise in firing rate during cooling and a dramatic fall when temperature rises (i.e. opposite)
This change in firing is more marked the greater the change in temperature. If temperature remains constant, the firing gradually comes to a steady state.
Specific temperature gated channels
Recent research has shown that there are different ion channels that open/close to specific pockets of temperature. These are TRP receptors. TRPM(menthol)8 is thought to be associated with cold receptors, whereas the TRPV group are thought to be associated with warmth receptors. Different TRPV channels open to different degrees of heat.