Stimuli of different intensities may also activate different sets of sensory receptors. The limit of a neuron’s firing rate of action potentials can also limit its range of response to a stimulus. However, mechanoreceptors with different thresholds can overcome this problem: Those with low thresholds can signal over a range of low input intensities, whereas others with higher thresholds can signal higher input intensities. Together they allow fine resolution over an extended range of intensities. In addition, still higher intensities might recruit nociceptors, and that will also change the perceived quality of the stimulus.
Stimulus frequency can sometimes be encoded by action potentials whose interspike intervals correspond exactly to the intervals between stimuli (e.g., at intervals corresponding •Fig. 5.15 Responses of Slowly and Rapidly Adapting Mechanoreceptors to Displacement of the Skin. A toC arethedischargesofprimaryafferentfibersduringaramp-and-holdstimulusshowninD. A, Theresponseofaslowlyadaptingreceptorthatsignalsthemagnitudeanddurationofdisplacement.B, Theresponseofarapidlyadaptingreceptorwhoseoutputsignalsthevelocityofdisplacement.C, Theresponseofadifferentrapidlyadaptingreceptorthatrespondstoacceleration.p,displacement;R,response;t,time.
to that of a low-frequency vibration). However, this mechanism is limited by the firing rate limits of neurons as discussed earlier. When higher frequencies need to be encoded (e.g., the auditory system, which in humans is capable of detecting frequencies up to 20,000 Hz; see ), other strategies are needed. Other candidate codes depend on the spatiotemporal patterns of firing across populations of neurons.
The duration and the onset and offset of events are encoded by different populations of sensory neurons. For example, slowly adapting receptors in the skin produce a repetitive discharge throughout a prolonged stimulus. However, rapidly adapting receptors produce spikes at the onset (or offset) of the same stimulus.
Fig. 5.15 shows the responses of three types of receptors to the slow deflection of the skin, which is depicted in the graph at the bottom of the figure. The functional implication is that different temporal features of a stimulus can be signaled by receptors with different adaptation rates.
1.
Ion channels are integral membrane proteins that have ion-selective pores. An ion channel typically has two states: high conductance (open) and zero conductance (closed). Different regions of an ion channel protein act as gates to open and close the channel. The channel flips spontaneously between the open and closed states.
2.
For a voltage-dependent channel, the fraction of time that the channel spends in the open state is a function of the transmembrane potential difference.
3.
The action potential is generated by the rapid opening and subsequent voltage inactivation of voltage-dependent Na+ channels and by the delayed opening and closing of voltage-dependent K+ channels.
4.
The absolute and relative refractory periods result from voltage inactivation of Na+ channels and the delayed closure of K+ channels in response to membrane repolarization. These refractory periods limit the firing rate of action potentials.
5.
Subthreshold signals and action potentials are conducted along the length of a cell by local circuit currents. Subthreshold signals are conducted only electrotonically, and thus decrease with distance.
6.
The action potential is propagated rather than merely conducted; it is regenerated as it moves along the axon. In this way, an action potential retains the same size and shape as it travels along the axon.
7.
A large-diameter axon has greater propagation velocity because increased axon diameter lowers axial resistance and allows greater amounts of current to flow farther down the axon.
8.
Myelination dramatically increases the conduction velocity of a nerve axon because myelin increases membrane resistance and lowers membrane capacitance. Myelination allows an action potential to
Fain GL. Molecular and Cellular Physiology of Neurons. 2nd ed. Cambridge, MA: Harvard University Press; 2014.