Physiology 5th Ed.

SOMATOSENSORY SYSTEM AND PAIN

The somatosensory system processes information about touch, position, pain, and temperature. The receptors involved in transducing these sensations are mechanoreceptors, thermoreceptors, and nociceptors. There are two pathways for transmission of somatosensory information to the CNS: the dorsal column system and the anterolateral system. The dorsal column system processes the sensations of fine touch, pressure, two-point discrimination, vibration, and proprioception (limb position). The anterolateral system processes the sensations of pain, temperature, and light touch.

Types of Somatosensory Receptors

Somatosensory receptors are categorized according to the specific sensation they encode. The major groups of receptors are mechanoreceptors (for touch and proprioception), thermoreceptors (for temperature), and nociceptors(for pain or noxious stimuli).

Mechanoreceptors

Mechanoreceptors are subdivided into different types of receptors, depending on which kind of pressure or proprioceptive quality they encode. Some types of mechanoreceptors are found in nonhairy skin and other types in hairy skin. Mechanoreceptors are described in Table 3-3 according to their location in the skin or muscle, the type of adaptation they exhibit, and the sensation they encode, and they are illustrated in Figure 3-8.

Table 3–3 Types of Mechanoreceptors

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Figure 3–8 Types of mechanoreceptors found in nonhairy skin and hairy skin. (Modified from Schmidt RF: Fundamentals of Sensory Physiology, 3rd ed. Berlin, Springer-Verlag, 1986.)

An important characteristic of each receptor is the type of adaptation that it exhibits. Among the various mechanoreceptors, adaptation varies from “very rapidly adapting” (e.g., pacinian corpuscle), to “rapidly adapting” (e.g., Meissner’s corpuscle and hair follicles), to “slowly adapting” (e.g., Ruffini’s corpuscle, Merkel’s receptors, and tactile discs). Very rapidly and rapidly adapting receptors detect changes in the stimulus and, therefore, detect changes in velocity. Slowly adapting receptors respond to intensity and duration of the stimulus.

image Pacinian corpuscle. Pacinian corpuscles are encapsulated receptors found in the subcutaneous layers of nonhairy and hairy skin and in muscle. They are the most rapidly adapting of all mechanoreceptors. Because of their very rapid on-off response, they can detect changes in stimulus velocity and encode the sensation of vibration.

image Meissner’s corpuscle. Meissner’s corpuscles are also encapsulated receptors found in the dermis of nonhairy skin, most prominently on the fingertips, lips, and other locations where tactile discrimination is especially good. They have small receptive fields and can be used for two-point discrimination. Meissner’s corpuscles are rapidly adapting receptors that encode point discrimination, precise location, tapping, and flutter.

image Hair follicle. Hair-follicle receptors are arrays of nerve fibers surrounding hair follicles in hairy skin. When the hair is displaced, it excites the hair-follicle receptors. These receptors are also rapidly adapting and detect velocityand direction of movement across the skin.

image Ruffini’s corpuscle. Ruffini’s corpuscles are located in the dermis of nonhairy and hairy skin and in joint capsules. These receptors have large receptive fields and are stimulated when the skin is stretched. The stimulus may be located some distance from the receptors it activates. Ruffini’s corpuscles are slowly adapting receptors. When the skin is stretched, the receptors fire rapidly, then slowly adapt to a new level of firing that corresponds to stimulus intensity. Ruffini’s corpuscles detect stretch and joint rotation.

image Merkel’s receptors and tactile discs. Merkel’s receptors are slowly adapting receptors found in nonhairy skin and have very small receptive fields. These receptors detect vertical indentations of the skin, and their response is proportional to stimulus intensity. Tactile discs are similar to Merkel’s receptors but are found in hairy, rather than nonhairy, skin.

Thermoreceptors

Thermoreceptors are slowly adapting receptors that detect changes in skin temperature. The two classes of thermoreceptors are cold receptors and warm receptors (Fig. 3-9). Each type of receptor functions over a broad range of temperatures, with some overlap in the moderate temperature range (e.g., at 36°C, both receptors are active). When the skin is warmed above 36°C, the cold receptors become quiescent, and when the skin is cooled below 36°C, the warm receptors become quiescent.

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Figure 3–9 The response profiles of skin temperature receptors.

If skin temperature rises to damaging levels (above 45°C), warm receptors become inactive; thus, warm receptors do not signal pain from extreme heat. At temperatures above 45°C, polymodal nociceptors will be activated. Likewise, extremely cold (freezing) temperatures also activate nociceptors.

Transduction of warm temperatures involves transient receptor potential (TRP) channels in the family of vanilloid receptors (i.e., TRPV). These channels are activated by compounds in the vanilloid class, which includes capsaicin, an ingredient in spicy foods.

Transduction of cold temperatures involves a different TRP channel, TRPM8, which is also opened by compounds like menthol (which gives a cold sensation).

Nociceptors

Nociceptors respond to noxious stimuli that can produce tissue damage. There are two major classes of nociceptors: thermal or mechanical nociceptors and polymodal nociceptors. Thermal or mechanical nociceptors (TRPV or TRPM8 channels) are supplied by finely myelinated A-delta afferent nerve fibers and respond to mechanical stimuli such as sharp, pricking pain. Polymodal nociceptors are supplied by unmyelinated C fibers and respond to high-intensity mechanical or chemical stimuli and hot and cold stimuli.

Damaged skin releases a variety of chemicals including bradykinin, prostaglandins, substance P, K+, and H+, which initiate the inflammatory response. The blood vessels become permeable, and, as a result, there is local edema and redness of the skin. Mast cells near the site of injury release histamine, which directly activates nociceptors. In addition, axons of the nociceptors release substances that sensitize the nociceptors to stimuli that were not previously noxious or painful. This sensitization process, called hyperalgesia, is the basis for various phenomena including reduced threshold for pain.

Somatosensory Pathways

There are two pathways for transmission of somatosensory information to the CNS: the dorsal column system and the anterolateral or spinothalamic system (Fig. 3-10). Each pathway follows the general pattern already described for sensory systems.

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Figure 3–10 Comparison of the dorsal column (A) and the anterolateral (B) somatosensory systems. The dorsal column system crosses the midline in the brain stem. The anterolateral system crosses the midline in the spinal cord.

1.     The first-order neuron in the somatosensory pathway is the primary afferent neuron. Primary afferent neurons have their cell bodies in dorsal root or cranial ganglia, and their axons synapse on somatosensory receptor cells (i.e., mechanoreceptors). The signal is transduced by the receptor and transmitted to the CNS by the primary afferent neuron.

2.     The second-order neuron is located in the spinal cord (anterolateral system) or in the brain stem (dorsal column system). The second-order neurons receive information from first-order neurons and transmit that information to the thalamus. Axons of the second-order neurons cross the midline, either in the spinal cord or in the brain stem, and ascend to the thalamus. This decussation means that somatosensory information from one side of the body is received in the contralateral thalamus.

3.     The third-order neuron is located in one of the somatosensory nuclei of the thalamus. The thalamus has a somatotopic arrangement of somatosensory information.

4.     The fourth-order neuron is located in the somatosensory cortex, called S1 and S2. Higher-order neurons in the somatosensory cortex and other associative cortical areas integrate complex information. The S1 somatosensory cortex has a somatotopic representation, or “map,” similar to that in the thalamus. This map of the body is called the somatosensory homunculus (Fig. 3-11). The largest areas of representation of the body are the face, hands, and fingers, which are densely innervated by somatosensory nerves and where sensitivity is greatest. The sensory homunculus illustrates the “place” coding of somatosensory information.

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Figure 3–11 The somatosensory homunculus. (Modified from Wilder P, Rasmussen T: The Cerebral Cortex of Man. New York, Macmillan, 1950. Reprinted by permission of The Gale Group.)

Dorsal Column System

The dorsal column system is used for transmitting somatosensory information about discriminative touch, pressure, vibration, two-point discrimination, and proprioception. The dorsal column system consists mainly of group I and II nerve fibers. The first-order neurons have their cell bodies in the dorsal root ganglion cells or in cranial nerve ganglion cells and ascend ipsilaterally to the nucleus gracilis(lower body) or nucleus cuneatus (upper body) in the medulla of the brain stem. In the medulla, first-order neurons synapse on second-order neurons, which cross the midline. The second-order neurons ascend to the contralateral thalamus, where they synapse on third-order neurons, which ascend to the somatosensory cortex and synapse on fourth-order neurons.

Anterolateral System

The anterolateral (spinothalamic) system transmits somatosensory information about pain, temperature, and light touch. The anterolateral system consists mainly of group III and group IV fibers. (Recall that group IV fibers have the slowest conduction velocities of all the sensory nerves.) In the anterolateral system, first-order neurons have their cell bodies in the dorsal horn and synapse on thermoreceptors and nociceptors in the skin. The first-order neurons synapse on second-order neurons in the spinal cord. In the spinal cord, the second-order neurons cross the midline and ascend to the contralateral thalamus. In the thalamus, second-order neurons synapse on third-order neurons, which ascend to the somatosensory cortex and synapse on fourth-order neurons.

Fast pain (e.g., pin prick) is carried on A delta, group II, and group III fibers, has a rapid onset and offset, and is precisely localized. Slow pain (e.g., burn) is carried on C fibers and is characterized as aching, burning, or throbbing pain that is poorly localized.

Referred pain is of visceral origin. The pain is “referred” according to the dermatomal rule, which states that sites on the skin are innervated by nerves arising from the same spinal cord segments as those innervating the visceral organs. Thus, according to the dermatomal rule, ischemic heart pain is referred to the chest and shoulder, gallbladder pain is referred to the abdomen, kidney pain is referred to the lower back, and so forth.