Basic Ideas

Two fields have emerged to investigate sensory systems.
- The first is psychophysics, which studies the relationship between the physics of a stimulus and our perception of it. This field asks questions like: “What is the minimum light intensity that we can perceive?” (Turns out the answer is a few photons.)
- The second is sensory physiology. The name is narrower than the field, as this line of study also encompasses genetics and molecular biology, &etc. The field attempts to understand the neural basis for stimulus transduction. It asks questions like: “How do we transform incoming photons into neural signals?”(Answer at the end of these notes.)
Any stimulus has four central attributes:
- Modality – the nature and quality of the stimulus … is it a visual image? A smell?
- Intensity / amplitude
- Location
- Duration

Features common to all sensory systems

In all sensory systems, the signaling must begin with a primary sensory neuron, sometimes called a sensory receptor (even though we are referring to a cell, not a protein). These receptors are specialized for the particular sensory modality, and they are responsible for the transformation of the physical stimulus into an electrical signal.
- The receptor cells can be either epithelial or neuronal in origin. The hair cells in the cochlea are epithelial; whereas the receptors in the nose are true neurons.
- Despite these developmental differences, all primary sensory receptors are spatially polarized: they have front and back ends. There is a detection end, and then a synaptic signaling end.
  - An olfactory neuron, for example, has sensory cilia protruding into the nasal cavity, which connect to a soma, which then projects an axon to an olfactory glomerulus. At the glomerulus, the axon synapses onto the next cell in line.
  - A rod, found in the retina, has an outer segment packed with photon transduction proteins. This outer segment is connected to the cell soma, which immediately synapses on the next cell in line (a bipolar cell). Note that a rod does not have an axon and so does not fire action potentials –– the passive signal is enough to cause synaptic transmission since the distance from the outer segment to the synapse is so small. See your book for a diagram, which may be helpful.
  - Auditory neurons are flask-shaped cells with stereocilia on their sensory end. (Despite the misnomer, the “cilia” are actually microvilli.) These stereocilia connect to a soma that makes a synapse onto the next cell, again without an axon.
- In all of these cells, one often sees surface area maximized at the sensory end in order to maximize the likelihood of picking up a stimulus.
Once these sensory neurons have captured a signal, what is the result?
- A detected stimulus gives rise to a change in the membrane potential of the receptor cell. This change is called the receptor potential. These voltage changes are always graded, meaning that the strength of the depolarization directly correlates with the strength of the stimulus. If the depolarization is sufficiently large, an action potential will be generated (either in that cell or in the first in line that is able, for reasons discussed above). The farther past threshold the cell is depolarized, the higher the frequency of action potentials that will fire as a result.
- If the signal falls short of threshold, then no further signaling will occur, and you (the brain) will never know about the stimulus.

Features of stimuli

We should revisit the four qualities of a stimulus in more detail.

o Modality

§ The law of specific energies states that specialized receptors are attuned to a certain kind of energy. This is generally true: photoreceptors detect electromagnetic energy, whereas olfactory receptors are sensitive to chemicals (smells). Remember, however, that under extreme conditions, a certain stimulus can cause any receptor to respond. A punch in the eye results in a flash of light, even though it is a mechanical, and not electromagnetic, energy.

§ There are four classes of receptors in humans: mechanical, chemical, electromagnetic, and thermal.

· Mechanical receptors are triggered by sheer physical activity. For example, touch receptors in your skin respond to physical (somatosensory) input because that input is sufficient to cause protein conformation change and, thus, depolarization. (By pushing on your skin, you are literally opening channels.) The auditory system has a complex mechanical arrangement for processing sound stimuli. See book for details on the cochlea.

· Chemical receptors respond to molecular ligands. The olfactory and gustatory systems use chemical receptors.

· Electromagnetic receptors respond to light energy. Rods and cones are examples, since they respond to photons.

· Thermal receptors respond to changes in temperature.

§ Receptors are attuned to a particular modality, but they are also more finely attuned to particular qualities within that modality. For example, all cones respond to light, but “blue” cones give maximal responses to wavelengths of 420 nm, whereas “red” cones are tuned to wavelengths of 560 nm. A tuning curve for a particular receptor plots the average response against a varied parameter; for example, one could plot how much response a particular photoreceptor gives when subjected to photons with different wavelengths. These plots allow one to determine the stimulus parameters to which that receptor gives the greatest response (and to which it is, hence, maximally tuned.)

§ Once the signal has passed beyond the initial receptor, how is its modality coded? In other words, we know that by occurring in a photoreceptor cell, an action potential must be encoding a visual stimulus –– but how do you specify the modality once the signal has moved beyond the initial receptor cell?

· Labeled line networks.

o An electromagnetic receptor projects to a unique network of neurons, so if those particular neurons are firing, it must be in response to an electromagnetic input. Hence the labeled line –– a designated network.

o Amplitude

§ Amplitude is typically described with dose-response curves. Along the x-axis is the intensity of the stimulus, often on a logarithmic scale. On the y-axis is the percentage of trials in which a subject can accurately detect the stimulus at that intensity. The plot usually looks sigmoidal, and the intensity for which one has a 50% accuracy rate is deemed the threshold intensity –– the minimum intensity the stimulus must have in order to be detected.

§ One problem with self-reported perception is that current experiences are modulated by previous ones. For example, if you closed your eyes and were given a one-pound weight, you would reliably report when you had received the weight. If you were first saddled with a fifty-pound weight, it would be hard to tell when the one pound weight had been added, even though its magnitude had not changed.

· The Weber-Fechner law gives the just noticeable difference (JND), the minimum possible change in stimulus intensity that can be detected.

§ On the neural scale, how is amplitude encoded? Action potential frequency and population frequency. The former corresponds to how often a single neuron fires, the latter to how many neurons related to that stimulus are firing.

o Location

§ Determining the location of a stimulus is very important to an organism vying to survive. It is thus a fast and robust mechanism in several of the sensory systems. There are a few methods for encoding location.

§ (1) In many systems, each neuron has a receptive field, a region of the environment to which it is sensitive. If a stimulus appears in a neuron’s receptive field, that neuron will fire. One can thus know where a particular object is based on which neurons are firing.

· Note that the resolution of this system varies from location to location, based on the density of receptor cells: one can reliably report feeling two adjacent but distinct somatosensory inputs when those inputs are on the finger, but not when they are on the back. On your back, they just feel like a single input. The somatosensory receptors are thus denser in the fingers than on the back.

· Also, as networks converge, resolution goes down. If ten rods, each with a discrete receptive field, all synapse onto the same bipolar cell (the next neuron in the visual network), the receptive field of that bipolar cell is equal to the sum of the fields for the rods. Resolution has thus decreased, as the field input has been reduced to a single averaged unit, instead of ten unique ones.

§ (2) Another mechanism for locating stimuli is based on comparing inputs to the two main sensors (the two eyes, two ears, etc.).

· In the visual system, we can calculate depth by comparing the differences between the images on the right and left eyes.

· In the auditory system, we can calculate location based on differences in signal arrival times for each ear, as well as differences in signal intensities. For example, if a sound comes from the left field, it will reach the left ear before the right ear. Also, the signal will be louder at the left ear than at the right, because of interference from the skull. These differences can be analyzed to determine location.

§ Finally, with smell, we (3) determine location based on gradients. We smell something and then move in some direction. If the smell has gotten more intense, we’re going toward its source. This is a warmer/colder location mechanism.

o Duration

§ Some cells fire as long as the stimulus is present. Some cells briefly fire as soon as the stimulus goes on, then stop, and then briefly fire when the stimulus goes off. Still other cells fire continuously in response to the stimulus, though the magnitude of the response decreases over time (within the single stimulus presentation).

§ Let’s look closer at the last phenomenon: this is adaptation. In some sensory systems, like smell, your perception of a stimulus weakens over time. Why? There are biochemical reasons, but the effect can happen on the channel, neuronal, or cortical level. There are different terms for each.

· Desensitization is what channels do.

· Adaptation is what cells do.

· Habituation is what cortex does.

Adaptation

What is the real purpose of adaptation? It can be a long-term (i.e. seconds to hours) effect. Above, we discussed a decrease in firing within a single stimulus trial, but adaptation can also affect one’s processing of all trials within an environmental context. So, whereas before we talked about neurons adapting (i.e. showing a decreased response) to the single signal, here we are talking about neurons adapting to the environmental context, and adjusting their responses appropriately.
This sort of adaptation allows us to remain sensitive over a wide range of stimulus intensities. Consider the photoreceptors, for example. These are cells sensitive to over six orders of magnitude of stimulus intensity. You can distinguish between objects in incredible bright rooms, and you can do the same in very dark rooms. How can photoreceptors, despite this incredible scope, remain highly sensitive to small changes in light? In other words, if you plotted a response vs. intensity (dose response) curve, how can you keep the slope extremely steep at any given point?
- If you consider your plot, instead of having a single, monotonic increase in response based on intensity –– a line that would have a shallow slope, given the broad domain –– you have a shifting sigmoidal curve that allows for high changes in response for little changes in stimulus intensity.
- What in the world am I talking about? Consider the following graph:
- - First consider only the blue line. This line indicates what the stimulus response curve would look like without any adaptation –– a linear relationship between stimulus intensity and response. But consider that the slope of this line at any point is 10,000/350=28.6. So increasing the stimulus intensity from 125 to 175 gives a response change of 28.6*50=1,430. This is okay, but we can do better.
  - Consider, now, the green line. This would be the dose-response curve for a receptor narrowly tuned to stimuli with intensities between 75 and 200. If we increase the stimulus from 125 to 175 on this line, we get a response change of about 4900. This is about three and a half times greater than the change in response to the same change in stimulus for the blue line. So the cell is much more sensitive to changes in stimulus intensity, which is a good thing, because it means it can easily distinguish between two stimuli presenting at similar intensities. This is contrast, on which vision is based.
  - But what about stimuli outside the range of 75 and 200? The sensitivity of the receptor, if we consider only the green line, is extremely poor in these regions. The response is the same no matter the stimulus! This is where adaptation comes in. Adaptation changes the sensitivity of the neuron in question from the green line to the red line. The neuron adapts to the new range of stimuli intensities and changes its domain of sensitivity.
  - What is a real life example? The above graph is made-up and with arbitrary units. But imagine that it corresponds to light intensity on the x-axis and visual response on the y-axis. If you are in a dark room, the photoreceptors have the green response curve. You can clearly distinguish between stimuli as long as their visual intensity is between 75 and 200 AU (arbitrary units), since an increase of 1 AU in stimulus intensity produces a significant change in receptor response. When you go outside into the daylight, however, all of the stimuli are >200 AU. Since the response is flat in this region for the green curve, the stimuli all generate the same responses, and hence look the same. So you cannot see anything. Through biochemical processes, though, the neuron shifts itself to the red response curve. You are then sensitive to differences in intensity of brighter lights and can see again. But if you go back into the dark room, you will see nothing, of course, until the neurons shift back to the green curve.

Transduction

The mechanisms vary for each sensory system, but let us consider how the visual system transforms visual signals into neural/electrical signals.
Within the outer segment of the photoreceptors, there is a protein called rhodopsin. This protein was the first GPCR ever discovered, and its G-protein (called transducin) was also the first G-protein to be discovered.
Rhodopsin has a molecule called retinal bound to one of its residues. (Remember that because it is a GPCR, rhodopsin is a seven-pass transmembrane protein.)
- Retinal starts in a form called 11-cis, aptly named since the double bond at the eleventh carbon is cis. Here is a picture:
- 11-cis retinal just sits there and does nothing. When a photon arrives at the photoreceptor, however, it gives retinal just enough energy to isomerize its cis double bond, thereby resulting in all-trans retinal, pictured here:
- Rhodopsin can hold 11-cis retinal, but it cannot hold all-trans retinal without changing conformation. It has no choice, then, but to adapt a different three-dimensional structure, which initiates the G-protein mediated signaling cascade that results in the electrical signal encoding vision.
- Note that only photons of certain wavelengths have the correct energy to isomerize the double bond –– less energy is insufficient, and greater energy is dangerous. Perhaps not surprisingly, but rather amazingly, the photons that can isomerize 11-cis to all-trans are those within the visual spectrum –– 400 to 700 nanometers. (If you are interested in details on how the three different kinds of cones have different sensitivities to different wavelengths, see your textbook.)