Sensory Transduction
The Adequate Stimulus. The adequate somatosensory stimulus (i.e., the stimulus to which a somatosensory neuron is most sensitive) is either a mechanical force, a temperature change, tissue damage, or a chemical action. The discriminative touch and proprioceptive systems are most sensitive to mechanical force. Consequently, their sensory receptors are of the mechanoreceptor category.
Sensory Transduction. The non-neural tissue surrounding the peripheral ending of the somatosensory 1° afferent helps concentrate and deliver the stimulus (e.g., mechanical force) onto the 1° afferent terminal membrane. Somatosensory mechanoreceptors function to transduce the applied mechanical force into an electrical potential change in the 1° afferent neuron.
The mechanoreceptor 1° afferent terminal membrane contains ion channels that respond to mechanical distortion by increasing sodium and potassium conductance (i.e., the channels are stress gated). Generator potentials are produced as sodium and potassium flow down their electrochemical gradients to depolarize the terminal ending (see Figure 2.3B). In most cases, the magnitude and duration of the generator potentials are related to the applied mechanical force: the greater the mechanical force, the greater is the depolarization, and the longer the mechanical force is applied, the longer the terminal remains depolarized (Figure 2.7). Terminals that do not sustain the depolarization for the duration of the mechanical distortion are called rapidly adapting. Terminals that sustain the depolarization with minimal decrease in amplitude for the duration of a stimulus are called slowly adapting.
The generator potential spreads passively along the 1° terminal fiber to the axon trigger zone - that part of the 1° afferent axon containing voltage-sensitive sodium and potassium channels (see Figure 2.3B). If the depolarization reaches threshold at these voltage-sensitive sites, action potentials are generated by the 1° afferent peripheral axon. When the action potentials reach the central terminals of the 1° afferent, they initiate the release neurotransmitters on 2° afferents within spinal cord or brain stem nuclei. If, as in the example in Figure 2.8, the generator potential is slowly adapting, the 1° afferent produces a sustained discharge of action potentials that continue for the duration of the stimulus.
If the generator potential is rapidly adapting (Figure 2.9), the 1° afferent produces a transient, short burst of action potentials and falls silent even in the continued presence of the stimulus.
The rapidly adapting receptors produce generator potentials and action potential discharges that follow the time-varying waveform of pressure changes produced by a vibrating stimulus (Figure 2.10, left panel). In contrast, the slowing adapting receptors produce generator potentials and action potential discharges that are sustained and unable to mimic the time-varying pattern of the stimulus (Figure 2.10, right panel). Consequently, the responses of rapidly adapting 1° afferents are best suited for representing time varying (e.g., vibrating or moving) stimuli, whereas slowly adapting 1° afferents better represent static stimuli (e.g., sustained pressure).
