Sensory adaptation · Lobeworks/17

Sensory adaptation is the process by which a sensory receptor answers less and less to a stimulus that stays the same, a fall in the firing rate of its nerve fibre, and receptors are sorted by how fast it happens: phasic (rapidly adapting, RA) receptors fire at the start and the end of a stimulus and fall silent in bet

Sensory adaptation. Sensory adaptation is the process by which a sensory receptor answers less and less to a stimulus that stays the same, a fall in the firing rate of its nerve fibre, and receptors are sorted by how fast it happens: phasic (rapidly adapting, RA) receptors fire at the start and the end of a stimulus and fall silent in between, tonic (slowly adapting, SA) receptors keep firing for as long as it lasts.

Every receptor turns a physical event into voltage (for touch, mechanotransduction: the skin is deformed, stretch-gated channels such as Piezo2 open, the ending depolarises) and then into a train of action potentials whose rate codes the stimulus. Adaptation can enter at each of those steps. The tissue around the ending may filter the stimulus before the channels see it (the onion-like capsule of the Pacinian corpuscle passes quick movements and absorbs steady pressure), the channels themselves may close while the push goes on, and the ending's membrane may slow down through the same potassium currents that give spike-frequency adaptation in central neurons. The result is two kinds of message. A phasic receptor gives a dynamic response: it codes change and its speed (a slip, a vibration, the start of a stretch). A tonic receptor gives a static response: it codes what persists (a pressure held, an edge under the finger, a muscle's length).

Two classifications that are easy to mix up. Phasic and tonic describe the pattern of response; Aβ, Aδ, C, Ia, Ib and II classify the afferent fibres by diameter, myelin and conduction speed. They cross: the Pacinian and the Merkel endings both travel on Aβ fibres, one phasic and one tonic.

The skin has two of each. Meissner corpuscles and Pacinian corpuscles adapt rapidly; Merkel discs and Ruffini endings adapt slowly. In the muscle the muscle spindle carries both kinds of answer at once.

A shirt goes quiet. Pulling it on, the rapidly adapting endings fire as the cloth slides over the skin; once it rests they fall nearly silent though it still touches you, and only the slowly adapting ones keep a faint report of the pressure.

The flotation tank is the extreme. In flotation REST the body floats in Epsom-salt water held at skin temperature (35 °C in Feinstein's 2018 study), still, in the dark: nothing changes for the phasic receptors to report, the tonic ones adapt with no edges or stretch to measure, and with no muscle holding the body up the spindles and Golgi tendon organs are unloaded. One hour lowered anxiety sharply in an open-label study of 50 patients; the authors suggest less somatomotor and exteroceptive input may contribute, without pinning the effect on it.

Most of the senses report change, and what does not change fades from the message.

Phasic receptors say that something is happening, tonic ones that it is still there, and a stimulus that never moves is soon left to the tonic ones alone.

Questions: You stop hearing a fan while you study: is that adaptation or habituation? Mostly habituation, and attention. In sensory adaptation the receptor itself sends less, so the signal entering the nervous system changes; in habituation the receptor goes on signalling and a central circuit learns to answer less to a repeated stimulus that has no consequence. The test is that the fan comes back at once when someone mentions it: the ear never stopped sending it. Are phasic and tonic the same thing as the fibre classes Aβ, Ia or C? No, they are two classifications that cross. Phasic and tonic describe how a receptor answers in time; Aβ, Aδ, C, Ia, Ib and II sort the fibres that carry the answer by diameter, myelin and conduction speed. The four touch receptors of the skin all ride Aβ fibres, two of them phasic and two tonic, and a single muscle spindle sends a phasic and tonic message on Ia and a mostly tonic one on II. Is a receptor adapting the same as a neuron's spike-frequency adaptation? Partly. Sensory adaptation can arise before any spike, in the tissue that filters the stimulus (the Pacinian capsule) or in the channels that close while the push goes on; spike-frequency adaptation is a brake in the membrane that fires, the potassium currents that pile up with each spike. A sensory ending has both: whatever reaches its channels, its own spikes still slow it down. What makes the Pacinian corpuscle stop answering a pressure that is held? Mostly its capsule. The layers pass a quick deformation through to the ending in the middle but take up a steady one as they slide and redistribute it, so a held push stops reaching the ending; removed from its capsule, the same ending adapts slowly. The capsule's cells also release GABA during a sustained push, which damps the ending further, so the filter is partly chemical as well as mechanical. Why does the body nearly fall silent in a flotation tank? Because nothing changes and nothing has to be held. The water is at skin temperature and still, so the phasic receptors have no movement to report and the tonic ones adapt with no edges or stretch to measure; floating, no muscle carries the body, so the spindles and tendon organs are unloaded. Whether that quiet is why people come out calmer is not settled: one hour lowered anxiety sharply in an open-label study of 50 patients, and the authors name reduced sensory and motor input as one plausible contributor among several. Why do you stop feeling a shirt a minute after putting it on? Because most of what reports it only reports change. While the cloth slides over the skin, the rapidly adapting Meissner endings near the surface fire hard; once it rests they fall nearly silent although it still touches you. The slowly adapting endings keep a faint report of the pressure, and attention filters out most of what is left, so the shirt returns only when it moves again.