Sensory receptors may be (1) modified nerve endings (dendrites) of first order sensory nerve cells (PRIMARY RECEPTORS), e.g. tactile receptors & retinal photoreceptors, or (2) modified epithelial cells, innervated by first order sensory neurones (SECONDARY RECEPTORS) e.g. taste cells & acoustico-vestibular hair cells. Different types of receptors are specialized to respond to different forms of stimulation, and produce different qualities of sensation. The main categories of sensations are referred to as different SENSORY MODALITIES: smell, vision, taste, hearing and touch. Receptors respond most readily to one form of stimulation, and less readily or not at all, to others. The preferred stimulus for a receptor is termed its ADEQUATE STIMULUS.

Photoreceptors of the retina will respond best to light (electro-magnetic energy), but intense pressure will abnormally stimulate them. It is important to note that both stimuli will produce action potentials in the axons leading from the receptors, and the sensation produced by both will be the same. Thus, a punch in the eye, will produce the sensation of light (stars), and if a cold sensitive thermoreceptor is stimulated by a hot probe (it can be) the sensation produced is COLD not WARMTH.

Receptors can be classified based upon their ADEQUATE STIMULUS as photoreceptors, mechanoreceptors, thermoreceptors & chemoreceptors.

These broad categories of stimulus energies (electromagnetic, mechanical, thermal, chemical) may produce different categories of sensory perception or SENSORY MODALITIES (see above), which can be broken down into sub-modalities:

PHOTOSENSITIVITY - Vision: Red sensitivity, blue sensitivity, green sensitivity.

MECHANORECEPTION - Tactile, pressure, vibration, sound (20 Hz- 2 kHz), proprioception.

THERMORECEPTION - Warmth (heating up of body); Cold (cooling down of body).

CHEMORECEPTION - Smell; Taste: sweet, sour, salty.

H⁺ ions and O₂ molecules act on the chemoreceptors in the carotid & aortic bodies, to produce various reflex responses, but elicit no known conscious sensation.

When a sensory stimulus falls on a receptor, the energy of the stimulus is converted into electrochemical energy, in the form of de- or hyperpolarisation of nerve cells, and the generation or inhibition of action potentials. The process of energy conversion into a form usable in the CNS is referred to as TRANSDUCTION.

The mechanism of transduction varies with the nature of the adequate stimulus, but some common features do exist. In olfactory, visual and some taste receptors, the transducer molecules are integral membrane proteins, which are homologous with the neurotransmitter receptor molecules. These are usually linked to G-proteins which, when activated, in turn activate second messenger systems, which amplify the primary response, causing either opening or closing of specific ion channels, and subsequent depolarisation or hyperpolarisation of the receptor membrane (a receptor potential). The process of transduction must preserve information concerning the quality (modality), duration, intensity and location of the stimulus.

Quality is determined by which receptor type is stimulated, the pathway along which its responses are routed, and the destination to which it projects in the brain. All receptors generate action potentials which are indistinguishable from each other. The different sensations are produced in the brain usually at the level of the cerebral cortical sites to which the inputs project. This is referred to as the principle of LABELLED LINES.

The duration of a stimulus may be coded for in terms of the duration of the response of the receptor. Receptors which continue to respond throughout the duration of a prolonged stimulus are termed SLOWLY ADAPTING or STATIC RECEPTORS (e.g. some pressure receptors). Continuous monitoring of an unchanging, static situation however, is redundant and wasteful. It is more important to note changes, and accordingly update the status quo. Thus, many receptors respond best to change and soon stop responding if an unchanging stimulus is maintained. These are termed RAPIDLY ADAPTING or DYNAMIC receptors (e.g. olfactory & warmth receptors). Many receptors have in their response profile, a dynamic component, proportional to the rate of change, and a static component, proportional to the degree of maintained change.

Intensity may be coded for in two ways: (1) During transduction, a stimulus applied to a receptor causes a change in membrane potential (a receptor potential) the magnitude of which depends on the intensity of the stimulus. This graded, RECEPTOR POTENTIAL causes proportional change in the frequency of discharge of action potentials, such that the change in frequency is a logarithmic function of the ratio of the degree of change to the baseline stimulus strength (c.f. the WEBER-FECHNER RULE). The principle whereby the stimulus strength is related to the action potential frequency and thence to the perceived intensity, is referred to as the principle of FREQUENCY CODING. (2) Most sensory receptor organs are made up of many sensory receptors grouped together in overlapping fields. These receptors may all respond to the same type of stimulus but with differing sensitivity. Low intensity stimuli excite only the most sensitive receptors, with the number responding being proportional to the stimulus strength. This is referred to as POPULATION CODING.

Every receptor has a defined area in the periphery, over which it can receive input. Stimuli falling outside this area will be unable to affect the receptor. This area is known as the RECEPTIVE FIELD of the receptor. When a receptor, or its pathway (labeled line) is stimulated, the resulting sensation is normally referred to the receptive field of the receptor. In this way, a stimulus exciting a receptor on the forearm can be localized. Receptors with adjacent receptive fields tend to send their inputs along adjacent pathways to adjacent destinations in the brain. Thus, topographically recognizable maps of the receptive surfaces are represented in the destinations in the brain, to which sensory inputs project. Receptors which receive inputs from remote sources have complex receptive fields, and the mechanisms involved in creating sensation of a source located at a distance from the actual receptor, may be quite complex. This is probably why the three initial expansions of the brain developed in the first place: in order to carry out the complex information processing associated with the three main distance receptors: the nose, the eyes and the ears (or the related lateral line system in fish).

EXTEROCEPTORS receive inputs from outside of the body; INTEROCEPTORS monitor the internal conditions in the body; PROPRIOCEPTORS are mechanoreceptors which sense movement and relative position of the body parts, and orientation of the body. NOCICEPTORS are a special category of receptors which are stimulated by conditions which

actually or incipiently, cause damage to tissues of the body. They respond to intense heat, cold and/or mechanical stimuli. Many are chemoreceptive, responding to chemicals produced by damaged tissues. The sensation produced by stimulation of nociceptors is PAIN. Intense stimulation of non-nociceptive neurones will not normally cause pain.

All sensory receptors are associated with first order sensory neurones whose cell bodies lie outside of the CNS, in the dorsal root ganglia of the spinal nerves, or in the corresponding sensory ganglia of the cranial nerves. (The one exception is the cells of the mesencephalic nucleus of the trigeminal nerve, where e.g. cell bodies of stretch receptors in the jaw muscles are found.) These first order cells synapse ipsilaterally in the CNS on second order cells, whose axons decussate (cross over) to the opposite side, and project on to third order sensory cells, located in specific nuclei in the thalamus (wall of the diencephalon). Axons of the thalamic cells project in a topographically organized fashion to specific target areas of the cerebral cortex (occipital cortex - visual system; Superior temporal cortex - auditory; Post-central gyrus - somatic sensory). The one exception to this pattern is the olfactory system, whose inputs project directly to primitive areas of the cerebral cortex (pyriform cortex), without first relaying in the thalamus.

All sensory pathways give off many branches (collaterals) before reaching the thalamus. Some of these branches create excitation in diffusely connected cells in the mid- and hind-brain (the reticular activating system), which may bypass the thalamus, or relay in non-specific thalamic nuclei, to project to wide-spread areas of the cerebral cortex, in a non-topographical manner, causing back-ground arousal.

Sensory receptors of many different types throughout the body, transduce myriad stimuli into bursts of action potentials, racing along numerous pathways in the CNS, to cause complex, interacting, and integrated patterns of neuronal activity. The final patterns of activity caused by any given stimulus configuration will depend upon genetic programming in conjunction with the cumulative environmental influences through development and growth. PERCEPTS are created in the brain depending on the complex and varying pattern of excitation in the sensory systems. Complex percepts (form, movement, melody) may be built by integration of simpler percepts (light, shade, distance, pitch, loudness, timing). The process by which complex, discriminative precepts are produced is termed COGNITION. The brain activity involved in producing these percepts is termed NEURAL PROCESSING, and involves the passage of action potentials along converging and diverging pathways, which may be excitatory or inhibitory, to produce highly complex patterns of activity in neuronal ensembles distributed widely throughout the brain. How these patterns of activity give rise to conscious perceptions, remains one of the profound mysteries of neuroscience. It is widely accepted however, that our CONSCIOUS PERCEPTIONS, with the exception of pain, are generated mostly on the basis of activity in the cerebral cortex.

Most if not all sensory receptors do not serve as passive transducers, but receive efferent output from the brain, to modulate the sensitivity and response properties of the sense organ, depending on the immediate circumstances, and past experiences of the organism. This modulation can occur at relays along the central pathway, at the first synapse in the CNS (e.g. dorsal horn or dorsal column nuclei) or at the sensory cell in the receptor itself (e.g. cochlea, retina). It usually represents feedback from centres to which the sensory input projects.

Read Somjen, NEUROPHYSIOLOGY - the essentials, Chapter 6

Ganong, Review of Medical Physiology, Chapter 5 [Return to Start] [Return to Lecture Schedule]

Berne & Levy, PHYSIOLOGY, Chapters 6 & 7