This chapter describes the general organization of somatosensory pathways and the anatomy of the somatosensory pathways involved in processing discriminative touch and proprioceptive information, and those involved with sharp pain and cool/cold information. Discriminative touch and proprioceptive information allow for the recognition of objects by touch, provide for a sense of our body image and is used for maintaining balance and posture. Sharp, pricking pain and cool/cold information allows for the detection and localization of potential tissue-damaging stimuli in a timely manner.
General Organization of Somatosensory Pathways
Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations.
Common Anatomical Features
Somatosensory stimuli activate a chain of neurons starting with the peripheral first-order (1°) afferent and ending in the cerebral cortex (e.g., Figure 4.1).
Figure 4.1 |
Within each somatosensory pathway,
- The 1° afferent is a pseudounipolar neuron that has its cell body located in a peripheral (spinal or cranial) ganglion. It has a peripheral axon that forms or innervates somatosensory receptors and a central process that synapses with 2° afferent neuron(s) in a spinal cord or brain stem nucleus.
- The 2° afferent may synapse with 3° afferent neurons in the spinal cord or may ascend the neuraxis to synapse with 3° afferent neurons in the thalamus.
- There is a decussation (i.e., axons crossing the midline to the opposite side of the spinal cord or brain stem) in each somatosensory pathway below the level of the thalamus.
- All somatosensory pathways include a thalamic nucleus. The thalamic neurons send their axons in the posterior limb of the internal capsule to end in the cerebral cortex.
- Most somatosensory pathways terminate in the parietal lobe of the cerebral cortex.
Each somatosensory pathway is named after a major tract or nucleus in the pathway.
In general, conscious perception of sensory stimuli requires the involvement of neurons in the thalamus and cerebral cortex. For example, electrical stimulation of a structure in pathways connecting muscle and joint receptors to the cerebellum (e.g., electrical stimulation of the anterior spinocerebellar tract) will not produce a sensation of limb movement, as these pathways do not include the thalamus or cortex. In contrast, electrical stimulation of a structure in the posterior column-medial lemniscal pathway (e.g., electrical stimulation of the medial lemniscus) may result in a sensation of limb movement, as this pathway includes the thalamus and terminates in the cerebral cortex.
Peripheral Somatosensory Axons
The morphology of the peripheral somatosensory axon is related to the receptor it innervates or forms and to the sensory information it carries (Figure 4.2).
- The Group I and II 1° afferent axons, which form the muscle/tendon receptors and carry body proprioceptive information, have the largest diameter and the thickest myelin of all the somatosensory 1° afferent axons.
- The Type C 1° afferent axons, which form free nerve endings and carry dull pain, deep pain, crude touch or warm/hot information, are the smallest 1° afferent axons and are unmyelinated.
- The Type Aδ1° afferent axons, which form free nerve endings and carry sharp pain or cool/cold information, are thinly myelinated and larger than the Type C axons.
- The Type Aβ 1° afferent axons, which form encapsulated endings in skin and joints or hair follicle endings or Merkel disks in skin, are myelinated and have diameter less than Group I afferents and greater than the Type Aδ 1° afferent axons.
Figure 4.2 |
The morphology of the peripheral somatosensory axon is also related to the conduction velocity of the action potentials generated by the axon.
The conduction velocity of an axon is determined by electrically stimulating the axon and recording the time (latency) it takes the electrically elicited action potential to reach a recording electrode (Figure 4.3). The distance traveled from the electrical stimulating site to the recording site divided by the latency provides the conduction velocity of the axon.
As discussed in earlier chapters, the larger and more heavily myelinated the axon, the greater its conduction velocity (Figure 4.3). Consequently, the 1° afferent axons carrying information required for fine motor control and rapid reflex responses (i.e., those forming body proprioceptors) conduct action potentials rapidly, whereas those carrying information about body and object temperature conduct action potentials at a much slower rate.
The whole nerve potential or compound action potential (CAP) is recorded extracellularly from an electrically stimulated nerve and is the sum of the signals produced by each of the individual action potentials of the axons forming the nerve. (Figure 4.3) The mixed nerve (afferent and efferent axons) compound action potential has three prominent peaks that are called A, B and C. The conduction velocity of an axon determines the axon's contribution to the compound action potential peaks. Specifically, the faster the axon conduction velocity, the shorter the latency of axon response and the greater the axon's contribution to the shorter latency peaks (e.g., compare columns CAP Peak and Conduction Velocity in Table I). The axons contributing to a given compound action potential peak (e.g., peak A) are named according to the peak name (e.g., Type A axon). When the relative amplitudes of the peaks differ from those generated by "normal" nerves, the types of damaged axons can be assessed by determining which peaks are abnormal. Consequently, the compound action potential is used clinically to detect nerve damage and to monitor the progress of the regeneration of damaged nerves.
For historical reasons, the terminology based on axon conduction velocity (Group I, II, III and IV) is used for afferent and efferent axons innervating muscles and tendons. And the terminology based on the compound action potential (Type A, B or C) is used for afferent axons innervating the skin, joints and viscera.
Note that the fastest conducting somatosensory 1° afferents (Group Ia) innervate skeletal muscle and the slowest (C-fibers) form the receptors of the pain systems. While one might expect painful, tissue damaging stimuli to have priority over all other somatosensory stimuli, the afferent information required to control the reaction to the painful stimuli are conveyed by the faster conducting muscle and joint afferents. Even afferents providing more exact information about the location of a cutaneous stimulus, the Aβ axons, conduct at a faster rate than the Aδ and C axons carrying information about painful stimuli.
