OVERVIEW OF SENSORY RECEPTION

Objectives:

The aim of this section is to provide an overview of the structure and function of sensory receptors. Whereas there are differences among sensory receptors, they all work on a common plan, and it is this plan that we wish to emphasize. Structure and function specific to each particular sensory system will be discussed in subsequent sections. At the end of this section, you should be able to:

1. State the attributes of a physical stimulus that can be quantified and correlated with sensation, and which combine in various ways to form the sensory experience.

2. State the basic elements that make up the common plan for functional organization of all sensory systems.

3. State what is meant by sensory receptor, adequate stimulus, sensory transduction, neural coding, integrative action

4. State the mechanism that provides stimulus specificity.

5. State the basic receptor types and the stimulus and modality with which they are related.

6. Describe the four coding mechanisms used to transmit information about a stimulus to the central nervous systeM

INTRODUCTION

Sensory systems extract four basic attributes of a physical stimulus that can be quantified and correlated with sensation. These combine in various ways to form the sensory experience.

1. Stimulus Modality - Different forms of energy are transformed by the nervous system into different sensations or sensory modalities. We recognize six sensory modalities: vision, hearing, balance, touch, smell, and taste. Each modality may have many submodalities (e.g. vision may include color and movement, taste may include bitter, sweet, etc). A unique stimulus that activates a particular receptor is referred to as the adequate stimulus for that receptor. The combination of receptor specificity, the connections of that receptor type to the central nervous system, the patterns of neural activity evoked by the stimulus and the organization of the central pathways contribute to the specific sensation that arises in consciousness.

2. Stimulus Intensity - The intensity of a sensation depends on the strength of the physical stimulus. The lowest stimulus intensity that a subject can detect is referred to as the sensory threshold for that modality or submodality. While there is an absolute threshold, which is determined by the sensitivity of the receptors, the actual neuronal and perceptual thresholds may vary depending on a variety of factors, including the context of the stimulus, practice, fatigue etc. It can also be affected by damage to the receptor, peripheral nerve or central nervous pathways.

3. Stimulus Duration - Our perception of a stimulus changes over time even though the stimulus remains constant in intensity. For example, placing our hand in warm water is felt immediately as 'warm', but this sensation subsides and warmth may no longer be felt. Similarly, the smell we experience when entering a room soon subsides. We refer to this phenomenon as sensory adaptation, and it plays a major role in our abilities to function in complex sensory environments. Adaptation may take place at the level of the sensory receptor, as we shall see later.

4. Stimulus Location - With respect to an external stimulus, we need to know not only the quality of the stimulus (what the stimulus is) but where the stimulus is located. With respect to location, we need to know both the absolute location and be able to distinguish two (or more) closely spaced stimuli. These two abilities are closely linked. As examples, the location of a probe on the tip of the finger can be determined with and error of no more than 1-2 mm, and the position of a light in the center of the visual field is located to within 1o. Much of our spatial discrimination ability is based on the peripheral density of receptors. One reason why the fovea of the retina has greater visual accuity is because of relatively high receptor density. Fine tactile discrimination by fingertips is related to the fact that the density of receptors is far greater there than on the palm of the hand, for example.

Even though sensory systems have evolved specialized mechanisms to detect different kinds of physical energy, these systems have many structural and functional features in common. By first understanding these fundamental features of all sensory systems we can more easily understand the specific features of each of them. There are seven of them:

1. There is a physical stimulus (light, sound, touch, heat, cold, chemicals)

2. There are specialize structures, called receptors, that respond specifically to these physical stimuli. The terms 'sensory ending', 'sensory receptor', and sensory organ are often used loosely and interchangeably to describe receptors. More precisely, sensory endings are peripheral terminals of primary afferent nerve fibers, whereas sensory receptors and sensory organs are highly specialized structures which are either a part of, or come into contact with, peripheral afferent terminals.

3. Receptors perform a sensory transduction, changing physical energy into electrical energy of the nervous system in the form of all-or-none action potentials in peripheral afferent nerve fibers.

4. Receptors are activated over a limited spatial domain, called the receptive field. The receptive field is that spatial area over which a stimulus will evoke a neural discharge in a peripheral afferent nerve fiber.

5. Information about a sensory stimulus is transmitted to the brain by action potentials in a process called neural encoding. There are a limited number of codes available.

6. Neural circuits in sensory areas of the brain receive the encoded information and further transform it by complex interactions of excitation and inhibition, a process referred to as integrative action.

7. The end result of integrative action of sensory input is the perception or conscious experience of sensation and the appropriate motor response.

The sensory receptor is the first element in the sensory pathway. It is at the interface between the external environment and the nervous system. Although different sensory systems share a common organizational plan, the specific structure of the receptor and neural circuitry of the brain associated with it differ, reflecting the particular demands imposed by the special functions of that sensory system (Figure 1). The receptor may be a neuron with non-specialized (pain and temperature) or specialized (touch, taste, smell) peripheral structure or may be a modified epithelial cell in synaptic contact with a peripheral nerve fiber (vision, hearing, balance). In all cases, the afferent axon in contact with the receptor projects to the central nervous system. Receptors and their respective modalities are listed on the following chart:

Receptors perform sensory transduction

Sensory receptors transduce physical energy into nerve impulses in peripheral afferent nerve fibers. The key intermediate step in the transduction process is the production of the receptor potential (sometimes called a generator potential), a non-propagated electrical event produced by the opening of ion channels in the receptor membrane in response to an adequate stimulus. The receptor potential is usually, but not always, depolarizing. It is the result of the opening of cation channels selective for Na⁺, K⁺, Ca⁺⁺, similar to the mechanisms described earlier in the course involved in synaptic transmission. The trigger for channel opening may be mechanical deformation of the receptor (touch-pressure), receptor-ligand interactions (taste), second messengers systems activated by light (vision) or chemical substances (olfaction), or by displacement of cilia (hearing, balance). In all cases, inward flow of current associated with stimulation produces the receptor potential, which in turn triggers electrical or electro-chemical events which result in nerve impulses in peripheral afferent nerve fibers.

Sensory Coding

Information transmitted from sensory receptors to the central nervous system is encoded in trains of propagated action potentials in axons of peripheral afferent nerve fibers. The term "code" is simply a way of describing the manner in which information about a stimulus is represented in neural activity. In both the peripheral and central nervous systems, there is a small variety of neural activities and, hence, a limited number of candidate codes. In order to qualify as a neural code it must be shown first that the pattern of activity in question occurs under natural conditions or is evoked by natural stimuli, and second that there exists a sensitive receiver, that is a set of neurons whose activity changes in response to the candidate code activity it receives.

Four candidate codes have been studied extensively in the sensory-evoked discharges of peripheral sensory nerve fibers. Probably no one of them is capable of transmitting the vast array of information upon which a normal subject operates. Rather, present evidence indicates that these codes operate in various combination depending on the sensory environment and behavioral context and that for certain the coding strategy may change at different levels of the central sensory pathways.

Labeled line code

This means that information in a neural message depends on the fiber or set of fibers that is active. Once the identity of of an active line is known, the information stored in the system tells the "meaning" of the activity. Thus, for example, increased discharge in axons of the cochlear division of the eighth nerve gives rise to the sensation of sound only; stimulation of the optic nerve evokes a sensation only of light, and so on.

Rate code

Sensory information may be carried by the rate or frequency of the discharge of a neuron, averaged over some short period of time that is compatible with the integration time required by the postsynaptic elements and, ultimately, by the subject. The encoding of stimulus intensity is associated with this type of code: the greater the stimulus intensity the greater the discharge rate.

Temporal code

Nerve impulses are brief, usually on the order of 1 msec or less, and all-or-none in nature. Thus, a train of nerve impulses may be considered a string of instantaneous events in time. Information about a sensory stimulus may be carried in the temporal pattern of these events. Such a code is almost certainly used to convey information about the temporal sequence of events in a sensory stimulus. For example, the sensation of vibration on the skin is mediated by the timing of nerve impulses in afferent nerve fibers innervating skin receptors.

Ensemble code

It is unlikely that any natural stimulus engages but a single peripheral receptor and thereby excites but one or even a small number of sensory nerve fibers associated with that single receptor cell. Indeed, a single nerve fiber is probably not capable of encoding unambiguously. Rather, some finite number of receptor cells and their afferent fibers are brought to threshold level of activation. Thus, the full array of sensory information perceived by a subject is the result of activity in populations of neurons acting in some coordinated fashion.

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