Law of Specific Nerve Energies

Stimulation of a sensory nerve fiber produces only one sensation— touch, cold, pain, and so on. According to the law of specific nerve energies, the sensation characteristic of each sensory neuron is that produced by its normal stimulus, or adequate stimulus (table 10.1).

Also, although a variety of different stimuli may activate a receptor, the adequate stimulus requires the least amount of energy to do so. The adequate stimulus for the photoreceptors of the eye, for example, is light, where a single photon can have a measurable effect. If these receptors are stimulated by some other means—such as by the high pressure produced by a punch to the eye—a flash of light (the adequate stimulus) may be perceived.

The effect of paradoxical cold provides another example of the law of specific nerve energies. When the tip of a cold metal rod is touched to the skin, the perception of cold gradually disappears as the rod warms to body temperature. Then, when the tip of a rod heated to 45° C is applied to the same spot, the sensation of cold is perceived once again. This paradoxical cold is produced because the heat slightly damages receptor endings, and by this means produces an "injury current" that stimulates the receptor.

Regardless of how a sensory neuron is stimulated, therefore, only one sensory modality will be perceived. This specificity is due to the synaptic pathways within the brain that are activated by the sensory neuron. The ability of receptors to function as sensory filters so that they are stimulated by only one

Resting membrane potential c

Action potentials

Tonic receptor slow-adapting

Resting membrane potential

Stimulus

Stimulus

(a)

applied

withdrawn

Phasic receptor -

fast-adapting

Stimulus

Stimulus

(b)

applied

withdrawn

■ Figure 10.1 A comparison of tonic and phasic receptors. Tonic receptors (a) continue to fire at a relatively constant rate as long as the stimulus is maintained. These produce slow-adapting sensations. Phasic receptors (b) respond with a burst of action potentials when the stimulus is first applied, but then quickly reduce their rate of firing if the stimulus is maintained. This produces fast-adapting sensations.

type of stimulus (the adequate stimulus) allows the brain to perceive the stimulus accurately under normal conditions.

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    What is the law of specific nerve energies?
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