The field of sensory physiology is focused on understanding the relationship between stimulus location and activation of specific sensory neurons. A major concept in this field is the receptive field of a sensory neuron, which refers to the area that influences the activity of that neuron when stimulated. For instance, a sensory receptor may only be activated by pressure on a small portion of the skin, which is known as its excitatory receptive field. Additionally, a neuron in the central nervous system (CNS) can have a receptive field that is larger than that of a sensory receptor because it receives input from multiple sensory receptors, each with slightly different receptive fields. Therefore, the receptive field of a CNS neuron is the combination of the receptive fields of the sensory receptors that affect it. The location of the receptive field is determined by the location of the sensory transduction apparatus responsible for transmitting information about the stimulus to the sensory neuron.

In general, sensory receptive fields are excitatory. However, a central sensory neuron can have an excitatory or inhibitory receptive field, or even a complex receptive field that includes both stimulating and inhibiting areas. Examples of such complex receptive fields are discussed in the section on Coding of Information by Action Potentials.

The transmission of information between neurons is a fundamental aspect of CNS function, primarily achieved through action potentials. Action potentials are electrical signals that travel down the axon to the presynaptic terminals, where they cause the release of neurotransmitters, thereby signaling the postsynaptic cells. Unlike local signals, such as receptor or synaptic potentials, which decay with distance and are unsuitable for long-range transmission, action potentials are regenerative and can carry signals regardless of the length of the axon. However, the all-or-none nature of action potentials means that their shape and size do not convey information as effectively as the graded local potentials. Instead, variations in the rate or timing of action potentials are primarily utilized as "codes" for information transmission between neurons.

Rate coding refers to the encoding of information in the firing rate of a neuron, where firing rate represents the number of action potentials fired per unit time, typically expressed as spikes per second or hertz (Hz). For example, the force of a mechanical stimulus applied to the skin can be encoded in the firing rate of the primary afferent neuron that innervates the skin. The greater the force, the larger the resulting receptor potential in the primary afferent neuron, leading to a faster rate of action potentials triggered by the receptor potential. Research has shown that many neurons employ rate coding, where the firing rate of a neuron consistently corresponds to specific parameters of sensory stimuli, forthcoming movements, or other behavioral aspects.

The amount of information conveyed by rate codes is constrained by several factors. One factor is the range of firing rates exhibited by a neuron. The upper limit of this range is determined by the maximal frequency at which a neuron can fire action potentials, which is governed by the duration of absolute and relative refractory periods and typically does not exceed 1000 Hz. On the other hand, the lower limit of the firing range is 0 Hz, as neurons cannot fire at negative rates. To overcome this limitation, many neurons display spontaneous activity levels. These levels can be relatively high, such as Purkinje cells that exhibit spontaneous firing at 100 Hz. This allows a cell to increase or decrease its activity within a similar range in response to inputs. Another constraining factor is the variability in a neuron's firing rate, which determines the resolution of its information coding.

CHAPTER 5 Generation and Conduction of Action Potentials
The relationship between the location of a stimulus and activation of particular sensory neurons is a major theme in the field of sensory physiology. The receptive field of a sensory neuron is the region that, when stimulated, affects the activity of that neuron. For example, a sensory receptor might be activated by indentation of only a small area of skin. That area is the excitatory receptive field of the sensory receptor. Moreover, a neuron in the CNS might have a receptive field several times as large as that of a sensory receptor because it may receive information from many sensory receptors, each with a slightly different receptive field. The receptive field of that CNS neuron is thus the sum of the receptive fields of the sensory receptors that influence it. The location of the receptive field is determined by the location of the sensory transduction apparatus responsible for signaling information about the stimulus to the sensory neuron.
In general, sensory receptive fields are excitatory. However, a central sensory neuron can have either an excitatory or an inhibitory receptive field or, indeed, a complex receptive field that includes areas that excite it and areas that inhibit it. Examples of such complex receptive fields are discussed in
Coding of Information by Action Potentials
Central to CNS function is the transmission of information between neurons. This is accomplished primarily through action potentials, which propagate down the axon to the presynaptic terminals and cause neurotransmitter release, signaling the postsynaptic cells. As already explained, the regenerative nature of action potentials allows them to carry signals regardless of the length of the axon, whereas local signals, such as receptor or synaptic potentials (see ), decay with distance and are therefore not suitable for this purpose. The tradeoff, however, is that the all-or-none nature of action potentials means that their shape and size do not generally convey information in the way gradations of local potentials do. Instead, the variations in the rate or timing of action potentials appear to be used primarily as the “codes” for transmission of information between neurons.
Rate coding refers to information being coded in the firing rate of a neuron, where firing rate is defined as the number of spikes fired per unit time, usually expressed as spikes/second, also called hertz (Hz). For example, the force of a mechanical stimulus to the skin can be encoded in the firing rate of the primary afferent neuron that innervates the skin; the greater the force applied to the skin, the larger the resulting receptor potential in the primary afferent neuron will be and, as a consequence, the faster the rate of action potentials triggered by the receptor potential will be. Research has shown many neurons employ rate coding in the sense that the firing rate of a neuron shows a consistent relationship to particular parameters of sensory stimuli, upcoming movements, or other aspects of behavior.
The amount of information in such rate codes is constrained by several factors. One factor is a neuron’s range of firing rates. The upper limit of this range is set by the maximal frequency that a neuron can fire action potentials, which is determined by the duration of the absolute and relative refractory periods (see
Fig. 5.4 ) and rarely exceeds 1000 Hz. The lower limit of the firing range is, of course, 0 Hz, as neurons cannot fire at negative rates. To avoid this problem, many neurons have spontaneous activity levels. These can be quite high (e.g., some Purkinje cells fire spontaneously at 100 Hz) and let a cell either increase or decrease its activity over a similar range in response to inputs. A second constraining factor is the variability of neuron’s firing rate, which determines the resolution of the neuron’s information coding.
CHAPTER 5 Generation and Conduction of Action Potentials