Introduction
Sensory experience has four fundamental attributes that are encoded by specialized subcategories of neurons within the nervous system. A modality is a general class of stimulus defined by the type of energy that the stimulus transmits and by the specialized receptors that sense this energy. The location of a stimulus is given by the set of sensory receptors within the sensory system that are activated by that stimulus. Sense organs generally have a topographic distribution of receptors so as to provide information about the size and position in space of the stimulus. The amplitude of responses of each receptor gives the intensity of a stimulus, which is a function of the total amount of energy conferred by the stimulus to the receptor. The timing of stimulation is given by the onset and end of the receptor response, i.e. the speed of energy gain and loss in the receptor. Thus the firing pattern of active sensory neurons encodes both the intensity and time course of stimulus presentation.
Sensory Modality
There are five principle sensory modalities that were first identified in ancient times: hearing, vision, touch, taste and olfaction. In modern times, the somatic senses of pain, temperature, itch and proprioception, as well as the vestibular sense of balance, are also included.
Sensory receptors, their central pathways, and corresponding target areas in the brain constitute a sensory system. The sensory receptor converts stimulus energy into electrical energy, a processing mechanism found in all sensory systems. This receptor generated electrical signal is called the receptor potential, and the transformation of stimulus energy into electrical energy is called stimulus transduction. The morphology of a sensory receptor is usually specific and selective for transduction of a particular form of energy, a phenomenon referred to as receptor specificity. This property of receptor specificity is the basis of the labeled line code, the key coding mechanism for the modality of the stimulus. Receptor selectivity for a particular type of stimulating energy indicates that the corresponding axon is a line of communication specific for a given modality and carries information only about that type of stimulus. Sensory receptors make connections with and activate particular structures within the central nervous sytem. Thus a given modality is represented by a set of neurons that are connected to a specific class of receptors and constitute a sensory system.
Humans have four classes of receptors, each of which responds selectively to one form of energy: mechanical, chemical, thermal or electromagnetic. In the somatosensory system, mechanoreceptors permit touch, proprioception and joint position, while hearing and balance occur through mechanoreceptors in the inner ear. Mechanoreceptors sense physical deformation of and pressure on tissue, which is transduced into electrical energy by the physical impact of the stimulus on cytoskeleton-linked cation channels in the membrane. The resulting deformation of stretch-sensitive channels increases ion conductances that depolarize the receptor. Pain, itch, taste and olfaction occur through chemoreceptors, which generally respond to the appropriate ligand with a depolarization. Dermal thermoreceptors sense temperature in the body and surrounding environment. Retinal photoreceptors respond with hyperpolarization to electromagnetic energy.
The major modalities each have several submodalities, since each of the four receptor classes described above is not homogeneous, but rather consists of various different receptor subtypes that are specialized to a limited stimulus energy range. Thus receptors effectively act as filters for a narrow bandwidth of energy, and exhibit tuning to a unique stimulus that activates the receptor at low energy, also known as the receptor's adequate stimulus. Data from physiological experiments can be used to generate a tuning curve that displays the receptor's sensitivity range and preferred stimulus energy band, at which it is activated by the lowest possible stimulus amplitude.
It should be noted that while sensory neurons are generally sensitive to only one type of stimulus, the specificity is not absolute: for example, retinal photoreceptors may be activated by very strong mechanical stimulation.
Encoding Stimulus Location
Information regarding the site of stimulation on the body or the location of a stimulus in space, the size and shape of objects, and the fine detail of a stimulus or of the environment are crucially represented by the spatial arrangement of stimulated receptors in a sense organ. The spatial area within which stimulation excites a sensory neuron is referred to as its receptive field.
In vision and somatic sensation, this receptive field confers a specific topographic location to the sensory output of the corresponding sensory neuron. A receptor will only respond to stimulation within its receptive field, and a stimulus larger than a single receptive field will activate neighboring receptors. Thus stimulus size affects the number of receptors that are stimulated. The resolution of a sensory system can be a function of its receptor density. Finer resolution of spatial detail is possible with denser receptor populations since each receptor will have a more restricted receptive field. Receptor density, however, is not uniform throughout a sensory sheet. The resulting differences in the desnity of afferent inputs to the central nervous system leads to discrepancies in the topographic representation of various body parts in central nervous system maps: densely innervated body parts occupy the largest areas, while sparsely innervated parts occupy the smallest areas.
The receptors for hearing , taste and smell are spatially distributed according to the sensitivity, or energy spectrum, of these modalities. Auditory receptors are organized according to the sound frequency to which they respond, while chemoreceptors on receptive surfaces in the nose and on the tongue are distributed based on their particular chemical sensitivity.
Encoding Stimulus Intensity
The intensity or amount of sensation experienced is a function of stimulus strength.
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