Sensory Systems

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

- Osmoreceptors

- Carotid body 02 receptors d. Extremes of temperature and pain

- Nociceptors

2. Fiber types and conduction velocity (Table 2-4)

3. Receptive field

- 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


Large a-motoneurons


Touch, pressure


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

Medium Medium








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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