Sensory Pathways
Sensory systems receive information from the environment via specialized receptors in the periphery and transmit this information through a series of neurons and synaptic relays to the CNS. The following steps are involved in transmitting sensory information (Fig. 3-3):
Figure 3–3 Schematic diagram of sensory pathways in the nervous system. Information is transmitted, via a series of neurons, from receptors in the periphery to the cerebral cortex. Synapses are made in relay nuclei between first- and second-order neurons, between second- and third-order neurons, and between third- and fourth-order neurons. Second-order neurons cross the midline either in the spinal cord (shown) or in the brain stem (not shown) so that information from one side of the body is transmitted to the contralateral thalamus and cerebral cortex.
1. Sensory receptors. Sensory receptors are activated by stimuli in the environment. The nature of the receptors varies from one sensory modality to the next. In the visual, taste, and auditory systems, the receptors are specialized epithelial cells. In the somatosensory and olfactory systems, the receptors are first-order, or primary afferent, neurons. Regardless of these differences, the basic function of the receptors is the same: to convert a stimulus (e.g., sound waves, electromagnetic waves, or pressure) into electrochemical energy. The conversion process, called sensory transduction, is mediated through opening or closing specific ion channels. Opening or closing ion channels leads to a change in membrane potential, either depolarization or hyperpolarization, of the sensory receptor. Such a change in membrane potential of the sensory receptor is called the receptor potential.
After transduction and generation of the receptor potential, the information is transmitted to the CNS along a series of sensory afferent neurons, which are designated as first-order, second-order, third-order, and fourth-order neurons (see Fig. 3-3). First-order refers to those neurons closest to the sensory receptor, and the higher-order neurons are those closer to the CNS.
2. First-order sensory afferent neurons. The first-order neuron is the primary sensory afferent neuron; in some cases (somatosensory, olfaction), it also is the receptor cell. When the sensory receptor is a specialized epithelial cell, it synapses on a first-order neuron. When the receptor is also the primary afferent neuron, there is no need for this synapse. The primary afferent neuron usually has its cell body in a dorsal root or spinal cord ganglion. (Exceptions are the auditory, olfactory, and visual systems.)
3. Second-order sensory afferent neurons. First-order neurons synapse on second-order neurons in relay nuclei, which are located in the spinal cord or in the brain stem. Usually, many first-order neurons synapse on a single second-order neuron within the relay nucleus. Interneurons, also located in the relay nuclei, may be excitatory or inhibitory. These interneurons process and modify the sensory information received from the first-order neurons. Axons of the second-order neurons leave the relay nucleus and ascend to the next relay, located in the thalamus, where they synapse on third-order neurons. En route to the thalamus, the axons of these second-order neurons cross at the midline. The decussation, or crossing, may occur in the spinal cord (illustrated in Fig. 3-3) or in the brain stem (not illustrated).
4. Third-order sensory afferent neurons. Third-order neurons typically reside in relay nuclei in the thalamus. Again, many second-order neurons synapse on a single third-order neuron. The relay nuclei process the information they receive via local interneurons, which may be excitatory or inhibitory.
5. Fourth-order sensory afferent neurons. Fourth-order neurons reside in the appropriate sensory area of the cerebral cortex. For example, in the auditory pathway, fourth-order neurons are found in the primary auditory cortex; in the visual pathway, they reside in the primary visual cortex; and so forth. As noted, there are secondary and tertiary areas, as well as association areas in the cortex, all of which integrate complex sensory information.
Sensory Receptors
Consider again the first step in the sensory pathway in which an environmental stimulus is transduced into an electrical signal in the sensory receptor. This section discusses the various types of sensory receptors, mechanisms of sensory transduction, receptive fields of sensory neurons, sensory coding, and adaptation of sensory receptors.
Types of Receptors
Receptors are classified by the type of stimulus that activates them. The five types of receptors are mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, and nociceptors. Table 3-2 summarizes the receptors and gives examples and locations of each type.
Table 3–2 Types and Examples of Sensory Receptors
CSF, Cerebrospinal fluid; PO2, partial pressure of oxygen.
Mechanoreceptors are activated by pressure or changes in pressure. Mechanoreceptors include, but are not limited to, the pacinian corpuscles in subcutaneous tissue, Meissner’s corpuscles in nonhairy skin (touch), baroreceptors in the carotid sinus (blood pressure), and hair cells on the organ of Corti (audition) and in the semicircular canals (vestibular system). Photoreceptors are activated by light and are involved in vision. Chemoreceptors are activated by chemicals and are involved in olfaction, taste, and detection of oxygen and carbon dioxide in the control of breathing. Thermoreceptors are activated by temperature or changes in temperature. Nociceptors are activated by extremes of pressure, temperature, or noxious chemicals.
Sensory Transduction and Receptor Potentials
Sensory transduction is the process by which an environmental stimulus (e.g., pressure, light, chemicals) activates a receptor and is converted into electrical energy. The conversion typically involves opening or closing of ion channels in the receptor membrane, which leads to a flow of ions (current flow) across the membrane. Current flow then leads to a change in membrane potential, called a receptor potential,which increases or decreases the likelihood that action potentials will occur. The following series of steps occurs when a stimulus activates a sensory receptor:
1. The environmental stimulus interacts with the sensory receptor and causes a change in its properties. A mechanical stimulus causes movement of the mechanoreceptor (e.g., sound waves move the hair cells in the organ of Corti). Photons of light are absorbed by pigments in photoreceptors on the retina, causing photoisomerization of rhodopsin (a chemical in the photoreceptor membrane). Chemical stimulants react with chemoreceptors, which activate Gs proteins and adenylyl cyclase. In each case, a change occurs in the sensory receptor.
2. These changes cause ion channels in the sensory receptor membrane to open or close, which results in a change in current flow. If ionic current flow is inward (i.e., positive charges move into the receptor cell), then depolarization occurs. If current flow is outward (i.e., positive charges move out of the cell), then hyperpolarization occurs. The resulting change in membrane potential, either depolarization or hyperpolarization, is called the receptor potential or generator potential. The receptor potential is not an action potential. Rather, the receptor potential increases or decreases the likelihood that an action potential will occur, depending on whether it is depolarizing or hyperpolarizing. Receptor potentials are graded electronic potentials, whose amplitude correlates with the size of the stimulus.
3. If the receptor potential is depolarizing, it moves the membrane potential toward threshold and increases the likelihood that an action potential will occur (Fig. 3-4). Because receptor potentials are graded in amplitude, a small depolarizing receptor potential still may be subthreshold and, therefore, insufficient to produce an action potential. However, a larger stimulus will produce a larger depolarizing receptor potential, and if it reaches or exceeds threshold, action potentials will occur. If the receptor potential is hyperpolarizing (not illustrated), it moves the membrane potential away from threshold, always decreasing the likelihood that action potentials will occur.
Figure 3–4 Receptor potentials in sensory receptor cells. Receptor potentials may be either depolarizing (shown) or hyperpolarizing (not shown). A, If a depolarizing receptor potential does not bring the membrane potential to threshold, no action potential occurs. B, If a depolarizing receptor potential brings the membrane potential to threshold, then an action potential occurs in the sensory receptor.
Receptive Fields
A receptive field defines an area of the body that when stimulated results in a change in firing rate of a sensory neuron. The change in firing rate can be an increase or a decrease; therefore, receptive fields are described as excitatory(producing an increase in the firing rate of a sensory neuron) or inhibitory (producing a decrease in the firing rate of a sensory neuron).
There are receptive fields for first-, second-, third-, and fourth-order sensory neurons. For example, the receptive field of a second-order neuron is the area of receptors in the periphery that causes a change in the firing rate of thatsecond-order neuron.
Receptive fields vary in size (Fig. 3-5). The smaller the receptive field, the more precisely the sensation can be localized or identified. Typically, the higher the order of the CNS neuron, the more complex the receptive field, since more neurons converge in relay nuclei at each level. Thus, first-order sensory neurons have the simplest receptive fields, and fourth-order sensory neurons have the most complex receptive fields.
Figure 3–5 Size of receptive fields of sensory neurons.
As noted, receptive fields can be excitatory or inhibitory, with the pattern of excitatory or inhibitory receptive fields conveying additional information to the CNS. Figure 3-6 illustrates one such pattern for a second-order neuron. The receptive field on the skin for this particular neuron has a central region of excitation, bounded on either side by regions of inhibition. All of the incoming information is processed in relay nuclei of the spinal cord or brain stem. The areas of inhibition contribute to a phenomenon called lateral inhibition and aid in the precise localization of the stimulus by defining its boundaries and providing a contrasting border.
Figure 3–6 Excitatory and inhibitory receptive fields of sensory neurons.
Sensory Coding
Sensory neurons are responsible for encoding stimuli in the environment. Coding begins when the stimulus is transduced by sensory receptors and continues as the information is transmitted to progressively higher levels of the CNS. One or more aspects of the stimulus are encoded and interpreted. For example, in seeing a red ball, its size, location, color, and depth all are encoded. The features that can be encoded include sensory modality, spatial location, frequency, intensity, threshold, and duration of stimulus.
Stimulus modality is often encoded by labeled lines, which consist of pathways of sensory neurons dedicated to that modality. Thus, the pathway of neurons dedicated to vision begins with photoreceptors in the retina. This pathway is not activated by somatosensory, auditory, or olfactory stimuli. Those modalities have their own labeled lines.
Stimulus location is encoded by the receptive field of sensory neurons and may be enhanced by lateral inhibition as previously described.
Threshold is the minimum stimulus that can be detected. Threshold is best appreciated in the context of the receptor potential. If a stimulus is large enough to produce a depolarizing receptor potential that reaches threshold, it will be detected. Smaller subthreshold stimuli will not be detected.
Stimulus intensity is encoded in three ways. (1) Intensity can be encoded by the number of receptors that are activated. Thus, large stimuli will activate more receptors and produce larger responses than will small stimuli. (2) Intensity can be encoded by differences in firing rates of sensory neurons in the pathway. (3) Intensity even may be encoded by activating different types of receptors. Thus, a light touch of the skin may activate only mechanoreceptors, whereas an intense damaging stimulus to the skin may activate mechanoreceptors and nociceptors. The intense stimulus would be detected not only as stronger, but also as a different modality.
Stimulus information also is encoded in neural maps formed by arrays of neurons receiving information from different locations on the body (i.e., somatotopic maps), from different locations on the retina (i.e., retinotopic maps), or from different sound frequencies (i.e., tonotopic maps).
Other stimulus information is encoded in the pattern of nerve impulses. Some of these codes are based on mean discharge frequency, others are based on the duration of firing, while others are based on a temporal firing pattern. The frequency of the stimulus may be encoded directly in the intervals between discharges of sensory neurons (called interspike intervals).
Stimulus duration is encoded by the duration of firing of sensory neurons. However, during a prolonged stimulus, receptors “adapt” to the stimulus and change their firing rates. Sensory neurons may be rapidly adapting or slowly adapting.
Adaptation of Sensory Receptors
Sensory receptors “adapt” to stimuli. Adaptation is observed when a constant stimulus is applied for a period of time. Initially, the frequency of action potentials is high, but as time passes, this frequency declines even though the stimulus continues (Fig. 3-7). The pattern of adaptation differs among different types of receptors. Some receptors are phasic, meaning they adapt rapidly to the stimulus (e.g., pacinian corpuscles), and others are tonic, meaning they adapt slowly to the stimulus (e.g., Merkel’s receptors).
Figure 3–7 Response of phasic and tonic mechanoreceptors.
The physiologic basis for adaptation also is illustrated in Figure 3-7. Two types of receptors are shown: a phasic receptor and a tonic receptor. A stimulus (e.g., pressure) is applied (on), and then the stimulus is removed (off). While the stimulus is on, the receptor potential and the frequency of action potentials are measured. (In the figure, action potentials appear as “spikes.”)
Phasic receptors are illustrated by the pacinian corpuscles, which detect rapid changes in the stimulus or vibrations. These receptors adapt rapidly to a constant stimulus and primarily detect onset and offset of a stimulus and a changing stimulus. The phasic receptor responds promptly at the onset of the stimulus with a depolarizing receptor potential that brings the membrane potential above threshold. A short burst of action potential follows. After this burst, the receptor potential decreases below the threshold level, and although the stimulus continues, there are no action potentials (i.e., there is silence). When the stimulus is turned off, the receptor is once again activated, as the receptor potential depolarizes to threshold, causing a second short burst of action potentials.
Tonic receptors are illustrated by mechanoreceptors (e.g., Merkel’s receptors) in the skin, which detect steady pressure. When compared with the pacinian corpuscles (which detect vibration with their fast on-off response), tonic mechanoreceptors are designed to encode duration and intensity of stimulus. The tonic receptor responds to the onset of the stimulus with a depolarizing receptor potential that brings the membrane to threshold, resulting in a long series of action potentials. Unlike the pacinian corpuscle, whose receptor potential returns quickly to baseline, here the receptor potential remains depolarized for a longer portion of the stimulus period, and the action potentials continue. Once the receptor potential begins to repolarize, the rate of action potentials declines and eventually there is silence. Tonic receptors encode stimulus intensity: The greater the intensity, the larger the depolarizing receptor potential, and the more likely action potentials are to occur. Thus, tonic receptors also encode stimulus duration: The longer the stimulus, the longer the period in which the receptor potential exceeds threshold.