Our senses link our mind to the environment, and are the core driver of behavior both in humans and in other animals. These days, we count many more senses than the classical five (sight, hearing, touch, smell, and taste). Most of them, however, share some common principles. Each sensory system is obviously quite different in the type of stimulation that it responds to and the manner in which environmental stimuli are converted to neuronal signaling. Further, there are senses that humans just don’t have, such as the ability to see in the ultraviolet or infrared frequencies, or to hear ultrasound. The world we see is thus different from that seen by other animals, with other or differently tuned senses.

However, regardless of the species or the sensory modality, there are many principles that can be generalized across sensory systems.

Sensory transduction

First, changes in the environment must be received. Our sensory systems work by converting different types of stimuli in the environment (i.e. visible light, sound waves, chemical molecules) into action potentials in the nervous system. This conversion is called sensory transduction and occurs in all sensory systems. Transduction is usually mediated by specialized molecules that exhibit a conformational change upon the arrival of the sensory stimulus.

Sensory receptor cells

Transduction usually occurs in cells which may be neurons, or may be specialized receptor cells of non-neural origin. Confusingly, these cells are called ‘receptors’, a term also used for the molecules that mediate transduction. In this course, we will try to be careful avoid ambiguity by using the terms ‘receptor cell’ when we mean the sensory cell, and ‘receptor’ when we mean the transducing molecule or structure. Each sensory system has specialized cells that are able to detect the environmental stimuli. Photoreceptors detect light, chemical receptors in the tongue and nose detect odors and taste, mechanoreceptors detect touch, and hair cells detect sound.

Receptor Potentials

In Lesson 4, we learned about graded potentials, and briefly discussed receptor potentials. A receptor potential is a graded potential seen in receptor cells upon the arrival of a stimulus. Receptor potentials, like post-synaptic potentials (PSP), can increase or decrease membrane potential. When the receptor cell is a neuron, receptor potentials work much like PSPs, by diffusing to the axon hillock and eliciting neuronal action potentials. In other senses, the receptor cell is not a neuron – for example, in taste or vision. In these cases, the receptor cell itself is not capable of firing action potentials, and instead, the receptor potential causes a release of neurotransmitters. The post-synaptic cell in such cases is a neuron, that serves as the afferent signal carrier.

Receptive fields

Receptive fields are easiest to understand in the visual and somatosensory systems. The receptive field for a neuron is the region of the retina or skin where a stimulus (light or touch) will evoke a change in the activity of a neuron. Receptive fields in the auditory system can consist of a certain frequency of sound and/or the location of sound in space.

Receptive fields can vary in size and shape depending on the characteristics of neuron (i.e. type, location in body, location in pathway). Receptive fields become more complex as information travels to the brain.

Lateral Inhibition

Lateral inhibition is a process used by sensory systems to enhance the perception of signals, particularly at edges, points, or other changes in the stimulus. It occurs because overlapping receptive fields can inhibit each other. This inhibition enhances the perceived differences between the stimulus and the area not stimulated.

Neural Coding

As we discussed earlier, sensory reception involves the conversion of sensory stimuli into action potentials that are sent to the CNS by sensory afferents. Properties of the stimuli must, therefore, be transformed into a form that the CNS can decode – this process is called sensory encoding. There are a number of different ways in which the nervous system encodes information such as intensity, stimulus location, or even frequency or pitch of sounds.

Rate Coding

This is the simplest form of coding: an increase in stimulus intensity is encoded by an increase in the rate of activity of the sensory neurons. It is easy to imagine how a more intense stimulus may activate more sensory receptors, and thus produce a larger receptor potential in sensory cells. A larger receptor potential results in more depolarization at the axon hillock, and in turn, a higher rate of action potentials that are generated and propagated to the CNS. Increases in sound intensity, for example, lead to more activity in neurons of the auditory nerve.

Temporal Coding

Sometimes, information can be carried not just by the number of action potentials, but by their pattern or time of arrival. In the auditory system, the receptor cells are hair cells, which respond only during one phase of the sound. Thus, at low sound frequencies, the CNS can identify the pitch of the sound by how often it receives bursts of activity from the ear.

Labeled Line Coding

In the labeled line coding of information, one cell encodes for one type of sensory quality. Pain is a good example of this. If a pain receptor is activated, the resulting sensation will be pain, regardless of the manner in which the receptor is stimulated. In other words, the sensory neurons are specifically tuned to one sensory stimulus. If that receptor-cell type was dysfunctional, the sensation will not be perceived. For example, there is a mutation that prevents sodium channels in pain receptors (but not other cell types) from working. When this mutation occurs, the subject cannot feel pain. Labeled line codes most often carry information about sensory sub-modalities (color of a visual stimulus, or pitch of a sound) or about the location of the stimulus.

It’s important to understand that rate, temporal and labeled-line codes above are not mutually exclusive: sensory neurons may carry information using all 3 of the above codes at the same time. However, CNS neurons that receive sensory information are usually tuned to only one of the sensory codes. For example, neurons in the auditory nerve carry information using both a temporal and a rate code. The axons of such neurons bifurcate and project to two different midbrain nuclei in the barn owl. One branch travels to a midbrain nucleus that judges intensity, and only cares about the rate code. The other nucleus, however, cares about the time of arrival of the sound at each ear, and doesn’t respond much to changes in intensity.

Population Coding

Sometimes, a stimulus cannot be encoded by a single sensory afferent, and is represented by the joint activity of a group of sensory neurons. A population code may involve information from the same sensory modality or sub-modality, or may combine information across sensory modalities. For, each color photoreceptor is most sensitive to a specific color (blue, green, or red), but a range of wavelengths can elicit changes in firing rates in the neuron. Therefore, the responses from a population of color photoreceptors must be combined to perceive the full spectrum of color.

Population activity can also be used to decode sensory information, or to extract higher order information by combining information from multiple neurons or multiple sensory codes. For example, higher level processing of taste and olfaction also uses population coding – sometimes the sense of smell is needed in addition to the sense of taste to fully perceive a flavor. Have you ever been congested from a cold and food just doesn’t taste the same? That’s due to this combining of the senses for a full perception.

Similarly, movement is also encoded by the action of groups of neurons in motor cortex, not by the activity of single motor neurons. Population coding, in combining the output from multiple neurons, has the advantage of reducing noise and averaging out errors that individual neurons may make.

Pathways

In general, the route sensory information takes from the periphery to the central nervous system is similar among most of the systems. Environmental stimuli become encoded by a specialized receptor in the periphery. Information then enters the central nervous system via the spinal cord or brainstem and relays through the thalamus, a structure that sits deep in the forebrain. The only sensory system that does not relay through the thalamus is the olfactory system. The thalamus then sends projections out to the primary cortical regions for each sensory system.

Role of the Thalamus

It’s common to hear that sensory information “relays” through the thalamus on the way to the cortex (for example, in the paragraph above). This language can give the impression that the thalamus is only responsible for making sure the sensory signal gets from periphery to the cortex. This greatly underestimates the thalamic role. The thalamus is known to contribute to the processing and modification of the sensory signal.

Attributions

This chapter was adapted from “General Principles of Sensory Systems” in Foundations of Neuroscience by Casey Henley which is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.