Somatosensation

Pain

Pain is the conscious perception of noxious stimuli. A noxious stimulus is one that is capable of producing tissue damage; it can be thermal, chemical, or mechanical. The receptor for noxious stimuli is the nociceptor, a naked (not encapsulated) nerve ending.

As a rule, axons transmitting noxious infor­mation are smaller and less myelinated than those carrying tactile or body position informa­tion. Activation of pain fibers of medium diam­eter and myelination (so-called Aδ fibers) is associated with a sharp, pricking quality of pain as reported by human beings. Activation of the smallest diameter C fibers, which are unmy­elinated, produces a dull, burning type of pain. The preponderance of C fibers in visceral sensory fibers explains the burning, aching quality of visceral pain.

As indicated in Chapter 9, a number of ascending spinal cord tracts transmit informa­tion about noxious stimuli to brain structures. in addition to projecting to the cerebral cortex for conscious perception, pain pathways typi­cally have strong connections to autonomic centers in the brainstem and parts of the brain that produce increased mental alertness and behavioral and emotional responses to painful stimuli. These connections are responsible for producing signs of sympathetic stimulation (e.g., increased heart and respiratory rates, dila­tion of pupils), emotional responses, and escape behaviors (Fig. 11-3).

The ability of a given noxious stimulus to produce a perception of pain is a highly mutable property that can be modified in the periphery, in the spinal cord, and in the brainstem.

The threshold of nociceptors in the periphery is not a constant. importantly, many substances released by injured tissues and inflammatory cells stimulate or lower the threshold of noci­ceptors. Thus, in damaged or inflamed tissue, stimuli that would normally be below thresh­old for detection may produce activity in noci­ceptive afferents.

Figure 11-3. Divergence in nociceptive pathways. The primary afferent neuron brings information about painful stimuli into the CNS. From there the informa­tion produces reflex movements and goes to the brain­stem to produce autonomic and other involuntary responses and to the cerebral cortex for conscious perception.

Events in the dorsal horn of the spinal cord can also affect transmission of nociceptive information. Rapid, prolonged firing of action potentials in the primary afferent neuron can produce changes in the neuron on which it synapses, making it respond more vigorously to subsequent stimulation. This is called wind­up or spinal facilitation of pain.

Activity in spinal nociceptive pathways is also strongly influenced by descending antino­ciceptive systems that originate in the brain­stem. The midbrain and medulla both possess a series of midline nuclei that inhibit nocicep­tion via their connections with nociceptive pathways. These nuclei use multiple neurotrans­mitters, most notably endorphins, transmitters with powerful antinociceptive properties.

Stoicism, an apparent indifference to pain, is largely determined by personality and training among both humans and animals. High-strung individuals often exhibit exaggerated reactions to stimuli that scarcely merit attention in more “laid-back” individuals. Interestingly, the effectiveness of applying a twitch to a horse’s upper lip as a method of restraint during mildly painful procedures has long been attributed to redi­recting the horse’s attention from the proce­dure to the moderately noxious stimulus of the twitch.

Proprioception

Proprioception is the nonvisual perception of body position. It is a complex sensory modality that is created through the input of a variety of specialized receptors called proprioceptors. These include joint receptors (providing infor­mation on tension and pressure within joints), muscle spindles (signaling changes in muscle length), Golgi tendon organs (signaling tension in tendons), and skin mechanoreceptors (which report contact with the environment).

Ascending proprioceptive pathways project both to the cerebral cortex and the cerebellum. The cerebral cortex uses proprioception to help formulate voluntary motor plans; the cerebel­lum uses it to adjust ongoing motor movements so that they are smooth and accurate. The information carried in the separate tracts to these targets arise from the same peripheral receptors and primary afferent neurons; it is only the ultimate destination (and therefore use) that is different.

For the cerebral cortex and cerebellum to make effective use of feedback on body posi­tion to guide movements, the proprioceptive information must be delivered very rapidly to these brain regions. Consequently, propriocep­tive tracts typically have few synapses and are composed of very large diameter, highly myelin­ated axons (called Aafibers). In fact, the very fastest (up to 120 m/second) axons of the entire nervous system transmit proprioceptive information.

We, and presumably animals, are generally not aware of proprioception, but it is critically important in the execution of accurate, well- coordinated movements. Injury to the proprio­ceptive pathways results in awkward, inaccurate, uncoordinated gait and movement. The inco­ordination typical of proprioceptive deficits is referred to as ataxia.

Touch

Touch is the modality associated with non- noxious mechanical contact with the body. Touch receptors are encapsulated, and the axons that transmit touch information to the brain are typically medium in diameter and degree of myelination. Spinal cord tracts associ­ated with touch are found in all the funiculi of the cord.