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Section II: Sensory Systems
5. Somatosensory Processes
Part 3 of 10

Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).
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Clinical Examples

An excellent way to test your knowledge of the material presented thus far is by examining the effects of damage to structures within the somatosensory pathways. The observed sensory loss(s) provide clues to the pathway(s) affected; and the area(s) and side of the body/face affected provide clues to the level of the damage. The following section should help you determine how well you can utilize what you have learned thus far about the somatosensory system.

Clinical Examples: Example 1, the periperal nervous system Clinical Examples: Example 2, the periperal nervous system Clinical Examples: Example 3, the spinal cord Clinical Examples: Example 4, the spinal cord Clinical Examples: Example 5, the spinal cord Clinical Examples: Example 6, the spinal cord Clinical Examples: Example 7, the brain Clinical Examples: Example 8, the cortex

Peripheral Nervous System

Peripheral Nerve Damage: Damage to peripheral nerves often results in sensory and motor symptoms. The sensory losses would include all somatosensory sensations if the peripheral nerve contains all the afferent axons supplying the skin, muscles and joints of a given body part (e.g., the hand or jaw). The motor losses may be severe (i.e., total paralysis) if the peripheral nerve contains all of the motor axons controlling the muscles of the normally innervated body part.

example 1

The patient reports a loss of all sensation from his left hand.


Figure 5.7
 Symptoms: The patient complains of loss of sensation and weakness involving his left hand (Figure 5.7). The physical examination determines that he is insensitive to pain, touch, vibration and finger position in his left hand. However, touch, vibration, position and pain sensations are normal in the rest of his body and face.

You conclude that the somatosensory losses in his left hand include

  • Loss of discriminative touch and proprioception (touch, vibration & position).
  • Severe decrement in pain sensation (analgesia).
Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.8) may have been affected
  • the medial lemniscal (touch, vibration & position) pathway
  • the spinothalamic (pain and temperature) pathway

The medial lemniscal pathway (MLP) and neospinothalamic pathway (NSTP) carry somatosensory information from the left hand to the right cortex. Press press the Play button to view the MLP and NSTP.

Figure 5.8

Side & Level of Damage: The sensory losses (Figure 5.9)
  • are limited to the left hand
  • involve all somatosensory pathways
  • are associated with motor dysfunction
Figure 5.9

The results of testing somatosensory sensation for Example 1.

A pin prick to the left hand produces no perceived pain sensations; and application of a vibrating tuning fork on the left hand or manipulating the fingers of the left hand produce no vibration or proprioceptive sensations. Press thumb to view the course of action potentials generated in response to application of a vibrating tuning fork or a pin prick to the left hand. Vibration and pain sensations are normal in the rest of the body and face.

Press foot to view the course of action potentials generated in response to application of a vibrating tuning fork or a pin prick to the left foot.

So, you conclude that

  • damage is to the peripheral nerve(s) innervating the left hand (Figure 5.10)
  • symptoms are ipsilesional (i.e., on the same (left) side as the damaged nerve)

Figure 5.10
The ulnar and meridian nerves provide sensory innervation to the hand. When these nerves are severed, the area normally innervated loses all sensations and motor functions.

Damage to peripheral nerves results in a loss of all somatosensory modalities and motor function in a restricted area of the body defined by the nerve distribution. Electrophysiological methods can be used to determine the nerves involved and the degree of nerve damage (Refer to the section "Peripheral Somatosensory Axons" in the chapter on Somatosensory Pathways).

Posterior or Cranial Nerve Root Damage: The central processes of the 1° somatosensory afferents collect to form a posterior root prior to entering the spinal cord. Consequently, the area of the body supplied by a single posterior root is represented by the sum of receptive fields of the 1° afferents in the root. The area of the body innervated by a posterior root is called a dermatome (Figure 5.11). Posterior root damage would result in somatosensory losses in the dermatome supplied by the root. All sensations would be lost in the central area of the dermatome. The more peripheral areas of the dermatome will have some sensation, albeit less than normal, as consecutive roots have partially overlapping dermatomes.


Figure 5.11

The dermatome of each posterior root is illustrated and represented by a root number (e.g. T4 for the fourth thoracic root). A given dermatome (e.g. T4) represents the collective receptive fields of all the 1° afferents making up that (e.g. T4) posterior root.

The symptoms produced by cranial nerve root damage depend upon the cranial nerve involved. For example, the trigeminal nerve root contains somatosensory (major) and chemosensory (minor) 1° afferent axons innervating the face, as well as efferent (motor) axons controlling the jaw muscles (Refer to Table 2 in the chapter on Somatosensory Pathways for the cranial nerves providing somatosensory innervation of the face and dura).

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