A. Sensory receptors—general
- are specialized epithelial cells or neurons that transduce environmental signals into neural signals.
- The environmental signals that can be detected include mechanical force, light, sound, chemicals, and temperature.
1. Types of sensory transducers a. Mechanoreceptors
- Pacinian corpuscles
- Joint receptors
- Stretch receptors in muscle
- Hair cells in auditory and vestibular systems b. Photoreceptors
- Rods and cones of the retina c. Chemoreceptors
- Olfactory receptors
- Taste receptors
- Carotid body 02 receptors d. Extremes of temperature and pain
2. Fiber types and conduction velocity (Table 2-4)
- is an area of the body that, when stimulated, changes the firing rate of a sensory neuron. If the firing rate of the sensory neuron is increased, the receptive field is excitatory. If the firing rate of the sensory neuron is decreased, the receptive field is inhibitory.
4. Steps in sensory transduction a. Stimulus arrives at the sensory receptor. The stimulus may be a photon of light on the retina, a molecule of NaCl on the tongue, a depression of the skin, and so forth.
b. Ion channels are opened in the sensory receptor, allowing current to flow.
- Usually the current is inward, which produces depolarization of the receptor.
- The exception is in the photoreceptor, where light causes hyperpo-larization.
Table 2-4. Characteristics of Nerve Fiber Types
General Fiber Type and Example
7-Motoneurons to muscle spindles (intrafusal fibers)
Touch, pressure, temperature, and pain
Preganglionic autonomic fibers
Slow pain; postganglionic autonomic fibers
Sensory Fiber Type and Example la
Muscle spindle afferents lb
Golgi tendon organs II
Secondary afferents of muscle spindles; touch and pressure
Touch, pressure, fast pain, and temperature
Pain and temperature (unmyelinated)
Conduction Diameter Velocity
Largest Fastest Largest Fastest
Slowest c. The change in membrane potential produced by the stimulus is the receptor potential, or generator potential (Figure 2-2).
- If the receptor potential is depolarizing, it brings the membrane potential closer to threshold. If the receptor potential is large enough, the membrane potential will exceed threshold and an action potential will be fired by the sensory neuron.
- Receptor potentials are graded in size depending on the size of the stimulus.
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