5 Firing Rate Adaptation in Sensory Neurons

In response to a constant stimulus, most sensory receptors do not maintain a constant receptor potential, but rather one that declines over time. This process is called adaptation and is mediated by several different mechanisms in different receptors including changes in the behavior of the accessory structures, intracellular signal cascades and even changes in the threshold for generating action potentials at the axon hillock of the sensory neuron. Some sensory receptors adapt slowly to a constant stimulus and others adapt more rapidly. Slowly-adapting receptors are sometimes called tonic receptors because their sensory neurons maintain a tonic or sustained level of discharge as long as the stimulus is present. Likewise, rapidly-adapting sensory receptors are referred to as phasic receptors because they only discharge during specific phases of the stimulus, normally at the onset, but sometimes also at the offset. Tonic receptors are designed to signal the continued presence of a stimulus, whereas phasic receptors primarily signal the start time and end time of the stimulus. Many tissues, like skin and sense organs, like muscle spindles are innervated by both tonic and phasic receptors.