How the nervous system converts the physical world — touch, pain, temperature, light — into the language of electrical impulses.
Our perceptions of signals within our bodies and of the world around us are mediated by a complex system of sensory receptors that detect such stimuli as touch, sound, light, pain, cold, and warmth.
The purpose of this chapter is to discuss the basic mechanisms by which these receptors change sensory stimuli into nerve signals that are then conveyed to and processed in the central nervous system.
In this chapter, we discuss the function of a few specific types of receptors, primarily peripheral mechanoreceptors, to illustrate some of the principles by which receptors operate.
Each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli.
Highly responsive to light, almost completely nonresponsive to heat, cold, pressure, or chemical changes in the blood.
Detect minute changes in osmolality of body fluids in the supraoptic nuclei of the hypothalamus, but have never been known to respond to sound.
Almost never stimulated by usual touch or pressure, but become highly active the moment tactile stimuli become severe enough to damage tissues.
"Each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads."
Five basic types of sensory receptors are classified by the type of stimulus they detect:
Detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor.
Detect changes in temperature, with some receptors detecting cold and others warmth.
Pain receptors that detect physical or chemical damage occurring in the tissues.
Detect light on the retina of the eye.
Detect chemical stimuli including taste, smell, and blood chemistry.
When a sensory receptor is stimulated, the membrane permeability changes, allowing ions to flow and creating a local electrical signal called the receptor potential.
Unlike action potentials, receptor potentials are graded — their amplitude is proportional to the strength of the stimulus. When the receptor potential reaches threshold (~−55 mV), it triggers action potentials in the attached nerve fiber.
Mechanical, thermal, or chemical stimulus deforms the receptor membrane
Membrane permeability increases; Na⁺ flows in, causing local depolarization
Graded, decremental potential spreads to the first Node of Ranvier
If threshold is reached, all-or-none action potentials propagate along the nerve fiber
The Pacinian corpuscle is the best-studied mechanoreceptor. Its onion-like lamellar structure filters out slow stimuli, making it respond only to rapid vibration.
Sigmoidal relationship: amplitude increases rapidly at first, then progressively less rapidly at high stimulus strength. (Data from Loewenstein, 1961)
All sensory receptors adapt — they decrease their response to sustained stimuli. The rate of adaptation varies dramatically between receptor types.
Signal change in stimulus. Fire at onset (and offset) but not during sustained stimulation.
Signal sustained stimulus. Continue firing throughout stimulation, encoding intensity.
Nerve fibers come in all sizes between 0.5 and 20 μm in diameter — the larger the diameter, the greater the conducting velocity. The range of conducting velocities is between 0.5 and 120 m/second.
Click a fiber type to see details
| Group | General Type | Diameter (μm) | Velocity (m/s) | Sensory Function |
|---|---|---|---|---|
| Ia | Aα | 12–20 | 70–120 | Muscle spindle primary endings (proprioception) |
| Ib | Aα | 12–20 | 70–120 | Golgi tendon organs (muscle tension) |
| II | Aβ | 5–12 | 30–70 | Muscle spindle secondary endings; touch, vibration, pressure |
| III | Aδ | 1–5 | 5–30 | Temperature (cold); pricking pain; crude touch |
| IV | C | 0.2–1.5 | 0.5–2 | Aching pain; warmth; itch; crude touch |
The area of skin (or other tissue) from which a single sensory neuron can be activated. Receptive fields can be as large as 5 cm in diameter for some pain fibers.
A stronger stimulus activates more nerve fibers simultaneously — the signal is encoded by the number of active fibers. A pinprick activates many pain fibers in the surrounding area, with the periphery of the field having more nerve endings than the center.
Signal strength is also encoded by increasing the frequency of nerve impulses in each fiber. As stimulus strength increases, action potentials fire more rapidly.
Translation of signal strength into a frequency-modulated series of nerve impulses
Each receptor type responds maximally to one specific stimulus modality
Ordered projection to the cortex — each fiber leads to a specific brain region
Receptor potentials are proportional to stimulus intensity
Receptors decrease their response to sustained stimuli over time
Test your understanding of sensory receptors. Select the best answer for each question.