Each fiber class in turn forms a heterogeneous group with regard to sensitivity to stimuli. Thus afferents are classified according to both size and their sensitivity to mechanical, thermal, and chemical stimuli. Fibers may have a low or high threshold to mechanical stimulation or be completely insensitive to it. Thermal sensitivity has been classified as responsiveness to warmth, noxious heat, cool, and noxious cold. Note that 43°C and 15°C are the approximate limits above and below which, respectively, thermal stimuli are sensed as painful. Chemical sensitivity to a variety of irritating compounds has been tested, including capsaicin (found in chili peppers), mustard oil, and acids.
Afferent fibers may be sensitive to one or more types of stimuli and have been named accordingly. For example, C fibers sensitive only to high-intensity (damaging) mechanical stimuli are called C mechanosensitive fibers, whereas those sensitive to heat and mechanical stimuli are labeled C mechanoheat-sensitive fibers (also called polymodal fibers). Other identified fiber types include Aδ and C cold-sensitive, Aδ mechanosensitive, and mechanoheat-sensitive fibers. Thus there is quite a variety of afferent types; however, the most common afferent type is the C polymodal fiber, which accounts for nearly half of the cutaneous C fibers. Surprisingly the second most common type is the mechanoheatinsensitive afferent (i.e., an afferent that is not sensitive to noxious stimuli until sensitized—see later).
The central portion of the Aδ and C axons carrying pain and temperature information from the body terminates in the dorsal horn of the spinal cord. The Aδ fibers target lamina I (dorsalmost part of dorsal horn), V (base of dorsal horn), and X (area surrounding central canal) of the gray matter, whereas the C fibers terminate in lamina I and II. The distinct termination patterns of the Aδ and C fibers in the spinal cord suggest that the messages they are carrying to the CNS are kept separated, and this is consistent with our ability to feel two distinct types of pain.
The primary afferent termination patterns in the spinal cord are also important because they may help determine the possible interactions pain fibers can have with other afferents and with descending control systems (see later). Indeed, the gate control theory of pain refers to the phenomenon that
CHAPTER 7 The Somatosensory System innocuous stimuli, such as rubbing a hurt area, can block or reduce painful sensations. Such stimulation activates the large-diameter (Aα and Aβ) fibers, and their activity leads to release of GABA and other neurotransmitters by inter-neurons within the dorsal horn. GABA then acts by both presynaptic and postsynaptic mechanisms to shut down the activity of spinothalamic tract cells. Presynaptically, GABA activates both GABAA and GABAB receptors, which leads to partial depolarization of the presynaptic terminal and blocking of Ca++ channels, respectively. Both actions will decrease release of transmitter by the afferent terminal and thereby lessen excitation of the tract cell (see section on presynaptic inhibition).
Nociceptive and thermoreceptive information that originates from regions of the head is processed in a fashion similar to that for the trunk and limbs. The primary afferent fibers of nociceptors and thermoreceptors in the head enter the brainstem through the trigeminal nerve (some also enter through the facial, glossopharyngeal, and vagus nerves). Of note, the trigeminal distribution includes both tooth and headache pain. These fibers then descend through the brainstem as far as the upper cervical spinal cord via the descending tract of the trigeminal nerve. Some mechanoreceptive afferent fibers also join the descending tract of the trigeminal nerve. Axons in the descending tract synapse on second-order neurons in the descending nucleus of the trigeminal nerve.