A Decreased Sensitivity to a Continued Stimulus Defines the Term

Chapter 5: Somatosensory Processes

Previous chapters described the ways in which the different somatosensory receptors respond to specific types of somatosensory stimuli and that the receptors, by virtue of their selective sensitivities, extract specific information about the somatosensory stimulus. The specificity of the receptors forms the basis for a parsing (i.e., a sorting) of somatosensory experience into separate "information channels" or pathways. For example, sharp-pricking pain is mediated in the neospinothalamic (information channel) pathway, whereas proprioception is mediated in the medial lemniscus pathway. Recall that the receptor's extraction of somatosensory information is very specific (e.g., during limb movement, muscle spindles respond to muscle stretch, whereas Golgi tendon organs respond to muscle contraction) and the processing of this extracted information is kept separate along most of the ascending pathway. In addition to this parsing of stimulus information, the somatosensory system is also organized to provide a somatotopic representation of the body surface and parts. The resulting spatial maps provide the anatomical basis for our ability to localize somatosensory stimuli and for our sense of a 'body image".

As described above, the nervous system reduces somatosensory experience into parallel streams of neural activity - a decomposition of the experience into stimulus fragments spread over body pieces. So how does one have a sense of "oneness" of the body and how does one identify an object by handling it? One can do so because somatosensory information converges in the parietal lobe of the cerebral cortex to provide a cohesive perception of the body and of somatosensory stimuli.

The first part of this chapter will present additional details about the general organization of the somatosensory system and how somatosensory information is represented and processed in the parietal cortex. This understanding of the general organization of the somatosensory pathways will be used in the clinical assessments of somatosensory function.

5.1 Sensory Pathways Decussate before Reaching the Thalamus

In each of the somatosensory pathways covered thus far, afferent axons decussate (cross the midline) once on their course to the thalamus (Figure 5.1).

Figure 5.1
Decussation within the somatosensory pathways. The second-order (2°) axons of the neospinothalamic pathway (NSTP) decussate in the spinal cord. The 2° axons of the medial lemniscal pathway (MLP), main sensory trigeminal pathway (MSTP) and spinal trigeminal pathway (STP) decussate at different levels of the brain stem.

Above the level of decussation, the neurons in a somatosensory pathway represent the contralateral (i.e., opposite) side of the body or face. It is important to learn the decussation site, as it will aid in clinical diagnosis. When an afferent pathway is damaged somewhere below the site of decussation, the sensory loss will be on the side ipsilateral to the lesion (i.e., the loss is on the same side as the lesion or ipsilesional). When an afferent pathway is damaged somewhere above the site of decussation, the sensory loss will be on the side contralateral to the lesion (i.e., the loss is on the side opposite the lesion or contralesional).

In the medial lemniscal pathway, the axons of the gracile and cuneate nuclei decussate in the medulla. The decussation in the neospinothalamic pathway is in the spinal cord and involves the axons of the posterior marginal nucleus. The spinal trigeminal nucleus axons decussate upon leaving the nucleus in the medulla and lower pons, whereas the main sensory trigeminal nucleus axons decussate at mid pons levels immediately upon leaving the nucleus.

5.2 Modality Specificity is Maintained up to the Cortex

The sensory information necessary for discriminative touch, proprioception, pain and thermal sensations are kept separate within the somatosensory pathways (Figure 5.2).

Figure 5.2
Modality specificity of the somatosensory pathways. The sensory information used in discriminative touch and proprioception are processed in separate channels within the medial lemniscal pathway (MLP) for the body and the main sensory trigeminal pathway (MSTP) for the face. The sensory information necessary for the perception of sharp pain and cool/cold sensations are processed in separate channels within the neospinothalamic pathway (NSTP) for the body and the spinal trigeminal pathway (STP) for the face.

To ensure the fidelity of stimulus representation in the discriminative touch-proprioceptive pathways, there is minimal convergence of modality specific information along the pathways. That is, within the medial lemniscal and main sensory trigeminal pathways, there is little mixing of information from different receptor types and from afferents with different adaptive properties up to the level of the cerebral cortex. For example, a cuneate nucleus neuron (2° medial lemniscal afferent) will synapse only with one type of posterior root neuron (e.g., 1° afferents with small "touch" receptive fields and rapidly adapting discharges). Within VPL and VPM, the medial lemniscal and ventral trigeminal lemniscal fibers terminate in different regions based on the sensory information they are carrying. Fibers carrying cutaneous information terminate within the core of the nucleus, whereas those carrying proprioceptive information terminate in the surrounding, peripheral shell of the nucleus.

Within the cerebral cortex, there is a convergence of modality specific information and the response properties of cortical neurons become more complex. The primary somatosensory cortex is responsible for the first stage of cortical processing. Within the primary sensory cortex, discriminative touch and proprioceptive information from overlapping areas may be combined such that a cortical neuron may respond to both cutaneous and proprioceptive stimulation of a digit.

Figure 5.3
Receptive fields of somatosensory afferents. The receptive fields of somatosensory 1° afferents are illustrated by the terminal branches of each afferent (bottom) and by the colored patches of skin where the terminals form receptors. The receptive fields are smallest in the digits of the hand (A) and largest in the torso (C).

5.3 Somatosensory Neurons have Receptive Fields

Each subcortical somatosensory neuron responds to modality-specific stimuli applied to a specific region of the body or face.

For example, an axon in the medial lemniscus (i.e., the fiber tract) that responds to tactile stimulation of the right index finger pad will not respond to tactile stimulation of any other area in the hand, body or face. The stimulated area producing the response is called the neuron's receptive field (Figure 5.3). The neuron's receptive field can also be defined anatomically as that area of the sense organ (i.e., skin, muscles or joints) innervated directly or indirectly by the neuron. Consequently, a somatosensory neuron can be described to channel information about stimulus location - as well as stimulus modality. Furthermore, the size of a neuron's receptive field is related to the body area innervated/represented. The receptive fields of neurons innervating/representing the finger pads, lips, and tongue are the smallest, whereas those of neurons innervating/representing the shoulders, back and legs are the largest. For greater accuracy in locating the point of stimulus contact or movement, smaller cutaneous receptive fields are required. For fine motor control, as in playing the piano or speaking, small proprioceptive receptive fields are required.

5.4 Spatial Information is Topographically Mapped in Sensory Pathways

Within each somatosensory structure, neurons are organized to provide a spatial representation of the body and face called the somatotopic map (Figure 5.4). That is, within the spinal cord, brain stem, thalamus and postcentral gyrus, the location of a neuron is related to its receptive field. Consequently, body and face (i.e., the receptive fields) are represented spatially (topographically) within nuclei and cortex such that, neurons with contiguous receptive fields are located adjacent to one another within a given structure. For example, adjoining areas of the body are represented in adjoining areas of the cortex (Figure 5.4). The resulting neural maps of the body and face are not isomorphic representations and appear distorted because of the disproportionate representation of the hand and face areas (Figure 5.4). That is, as the neurons representing the hand and face have small receptive fields (Figure 5.3), a greater number of neurons are required to represent the hand and face. Because somatosensory neurons represent specific stimulus features and specific areas of the body or face, electrical stimulation of a restricted area of the postcentral gyrus (e.g., the area representing the tongue) will produce a somatic (and not gustatory) sensation that is perceived as arising from the specific region of the body (i.e., the tongue).

Figure 5.4
The somatotopic representation of the body and face in the postcentral gyrus and posterior paracentral lobule. The somatotopic map was developed from reports of the sensation and the location of the sensation from conscious patients whose cortices were electrically stimulated during neurosurgery.

5.5 The Somatosensory Cortex

Somatosensory information converges in the parietal lobe of the cerebral cortex where it is processed to provide a cohesive perception of your body and your physical environment.

5.6 Primary Cortical Receiving Area

The primary somatosensory cortex, SI, includes the postcentral gyrus and the posterior paracentral lobule of the parietal lobe (Figure 5.4 & 5.5).

Figure 5.5
Somatosensory cortical areas. The primary somatosensory cortex (SI) consists of the postcentral gyrus and posterior paracentral lobule. The secondary cortex (SII) resides in the operculum of the parietal cortex. The posterior (superior) parietal cortex and part of the superior temporal gyrus contains the somatosensory association area.

SI extends from the depths of the central sulcus up superiorly to form the posterior lip of the central sulcus. SI is considered the primary somatosensory cortex because it is the major site of termination of VPL and VPM axons:

its neurons are responsive to somatosensory stimuli almost exclusively;
lesions of SI produce severe somatosensory deficits; and
electrical stimulation of SI produces somatosensory perceptions.

SI is somatotopically organized. The body and face are mapped in the contralateral cortex with the foot and leg represented in the posterior paracentral lobule and the trunk, chest, arm and hand in the upper half of the postcentral gyrus. The face is represented in the lower half of the postcentral gyrus (Figure 5.4).

Differential projections to the SI areas arise from the central core and shell of VPM and VPL. However, there is also convergence of somatotopic and modality specific information in SI. To appreciate the shape, texture, size, weight, and movement of a given object, the somatosensory cortex must integrate the parallel streams of information carried by the medial lemniscal pathway. To achieve this integration, the parallel streams converge at cortical levels, starting in SI. As a result of this convergence, receptive fields become larger, modality specificity diminishes, and the cortical neural responses become more complex.

SI neurons send their axons to the secondary somatosensory cortex, adjacent areas of the parietal lobe, and to cortical motor areas (Figure 5.6) as well as to subcortical nuclei, brain stem and spinal cord. Unilateral destruction of SI produces severe deficits in all aspects of discriminative touch and proprioception on the contralesional side of the body. In addition to deficits in the abilities to accurately localize and to recognize objects by shape, texture and size and to appreciate vibrating/moving stimuli, there are deficits in fine motor coordination.

Figure 5.6
Diagram of the flow of information from mechanoreceptors in the body and face to various cortical areas. Information flows predominantly from the thalamus to the primary somatosensory cortex (SI). From there the information is forwarded to the secondary somatosensory cortex (SII), the primary and supplementary motor cortex (in the frontal lobe), and the posterior parietal cortex. The SII sends information to the same areas and also to the insula, which connects with cortical regions involved with learning and memory of somatosensory information. The superior temporal polysensory area integrates somatosensory information from the posterior parietal cortex with information from various other sensory systems.

5.7 Secondary Cortical Receiving Area

The secondary somatosensory cortex, SII, is located inferiorly - in the pars opercularis of the parietal lobe, which forms part of upper lip of the lateral sulcus (Figure 5.4 & Figure 5.5). SII neurons send their axons to SI, association cortex, motor cortex, and insula (Figure 5.6). The latter projection, to the insula, influences structures such as the amygdala and hippocampus. These structures are important in tactile learning and memory. The projection to the somatosensory association cortex is involved in higher order processing required for recognizing hand-held objects by texture and size. Consequently, lesions in SII produce deficits in learning by object manipulation and in recognizing the texture and size of hand-held objects.

5.8 Association Cortical Area

The somatosensory association cortex is located in the superior parietal lobe (a.k.a. posterior parietal cortex), which is posterior to SI. The highest degree of convergence of somatosensory information occurs in the posterior parietal cortex. The posterior parietal cortex receives the axons of SI and SII neurons and also receives input from the visual system and other systems involved in attention and motivation.

Neurons in the posterior parietal cortex are responsive to somatosensory and visual stimuli, have large somatic receptive fields in which responsiveness is based on stimulus context, and are often more responsive to stimulus movement.

Large lesions involving the posterior parietal cortex and the adjoining superior temporal gyrus may result in an attentional deficit called "neglect", wherein there is a partial neglect (inattention) to tactile, proprioceptive and/or visual stimuli delivered contralateral to the lesion site. The patient is described as ignoring the contralesional half of her/his body and space. The perception of a "whole" body is lost and the body parts affected may be considered to belong to someone else. Visual stimuli on the contralesional side may also be ignored.

5.9 Cortical Areas for Pain Sensation

Pain information is processed in multiple pathways (see Table 1 in the chapter on Somatosensory Systems) involving multiple thalamic nuclei that project to multiple cortical areas. In addition to the somatosensory cortex, painful stimuli activate neurons in the rostral cingulate gyrus and the insula. Consequently, all pain sensation is not lost when the primary somatosensory cortex is damaged. Primary somatosensory cortex neurons that have small receptive fields and are selectively responsive to sharp, cutting painful stimuli are considered to provide the ability to accurately localize the exact point of contact with the painful stimulus. Lesions of the primary somatosensory cortex will affect the quality of pain sensations and the ability to localize the exact location of the painful stimulus.

5.10 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.

Example 1

Example 2

Example 3

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Example 6

Example 7

Example 8

5.11 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.

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

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

Figure 5.8
The medial lemniscal pathway (MLP) and neospinothalamic pathway (NSTP) carry somatosensory information from the left hand to the right cortex. Press to view the MLP and NSTP.

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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Example 8

5.12 Clinical Examples:
Peripheral Nervous System (continued)

The patient reports a loss of sensation along the lateral aspect of his left arm that extends down to include the thumb of his left hand.

Symptoms. The patient complains of a loss of sensation along the side of his left arm that extends down to include the thumb of his left hand (Figure 5.12). Physical examination determines that there are decreases in the abilities to detect vibration and position involving the left elbow and thumb and loss of touch and pain sensations along the lateral edge of the left arm down to the thumb. Touch, vibration, position, and pain sensations are normal for the rest of the body and face.

You conclude that the somatosensory losses in his left arm involve

discriminative touch and proprioception
pain sensation

Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.8) have been affected

the medial lemniscal pathway
the spinothalamic pathway

Figure 5.13
The results of testing somatosensory sensation for Example 2. A pin prick to the left thumb produces no perceived pain sensations; and a vibrating tuning fork in contact with the left arm or manipulating the left arm and thumb produce no vibration or proprioceptive sensations. Press HAND to view the course of action potentials generated in response to application of a vibrating tuning fork or a pin prick applied to the left thumb. Vibration and pain sensations are normal for the rest of the body. Press FOOT to view the course of action potentials generated in response to application of a vibrating tuning fork or a pin prick applied the left foot.

Side & Level of Damage: As the sensory losses (Figure 5.13)

are limited to the lateral aspect of the left arm and extended down to include the left thumb
involve both somatosensory pathways
involve a dermatomal pattern
do not involve motor function

You conclude that

damage may involve the fifth, sixth and seventh cervical posterior roots (Figure 5.11)
symptoms are ipsilesional (i.e., involve the left posterior roots, Figure 5.14)

Figure 5.14
The fifth, sixth and seventh cervical posterior roots provide sensory innervation to the lateral edge of the arm. Compression of the posterior roots will prevent action potentials generated by somatic stimulation from reaching the spinal cord

Section of a Posterior Root results in the loss of all somatosensory modalities in a restricted area of the body defined by the root dermatome (Figure 5.11). Consequently, the damaged posterior root can be identified by the dermatomal pattern of sensory loss. Radiographic methods can be used to determine if the roots are being compressed by abnormalities in the vertebra.

Example 1

Example 2

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5.13 Clinical Examples:
Central Nervous System: The Spinal Cord

Spinal Cord Damage: Although there are numerous tracts in the spinal cord, the tracts considered to be of major clinical importance are limited. There are three major ascending tracts in the spinal cord, the posterior funiculus (which includes the gracilis and cuneatus fasciculi, aka posterior columns); the spinothalamic tract (in the anterior and lateral funiculi); and the posterior spinocerebellar tract (in the lateral funiculus).

Clinical symptoms of spinal cord damage are obvious when the posterior columns and/or spinothalamic tract are affected. The posterior spinocerebellar tract carries unconscious proprioceptive information and damage to it does not result in sensory losses.
When a somatosensory tract is sectioned in the spinal cord, the sensory losses start at the level of the lesion and extend down to encompass lower parts of the body. Such a loss occurs because the flow of ascending afferent information from the lesion site and lower body is interrupted and cannot reach the thalamus and cerebral cortex.
The spinothalamic tracts are crossed in the spinal cord. The decussation in the medial lemniscal pathway occurs in the lower medulla.
The cranial nerves, which are associated with the brain stem, supply the motor and sensory innervation of the face. Consequently, spinal cord lesions do not produce sensory or motor losses involving the face. When somatosensory losses are isolated to the body, spinal cord lesions should be suspected.
Unilateral lesion of one descending tract, the corticospinal tract in the lateral funiculus, produces the most obvious motor deficits (i.e., deficits in voluntary motor control below the level of the lesion).

The patient suffers from loss of discriminative touch and proprioception (i.e., vibration and position sensations) from the right half of the body starting just below the right nipple and extending down to and including his right foot.

Symptoms: The patient complains of problems with walking, especially at night when there is little light. He also reports a loss of sensation in his right foot. Physical examination determines that there are decreases in vibration and position sensations and poor localization of tactile stimuli involving the right half of his body starting just below the right nipple and extending down to include his right foot (Figure 5.15). Pain sensation is normal in the right torso, leg and foot. Touch, vibration, position and pain sensations are normal for the rest of the body and face. The Romberg test is positive. (i.e., The patient has difficulty standing upright with his feet together and his eyes closed.)

You conclude that the somatosensory losses in his right lower body involve

discriminative touch and proprioception

Pathway(s) Affected: You conclude that structures in the following somatosensory pathway may have been affected (Figure 5.16)

the medial lemniscal pathway

Figure 5.16
Neurons in the medial lemniscal pathway (MLP) process discriminative touch and proprioceptive information from the body. The MLP 1° afferents ascend uncrossed in the spinal cord within the posterior funiculus. In contrast, the 2° afferents of the neospinothalamic pathway (NSTP), which carry pain and temperature information, decussate in the spinal cord and ascend the cord in the lateral funiculus. Consequently, within the spinal cord, discriminative touch and proprioception of the right side of the body is represented in the ipsilateral (right) posterior funiculus and pain and temperature from the right side of the body is represented in the contralateral (left) lateral funiculi.

Side & Level of Damage: The sensory losses (Figure 5.17)

do not involve the face
involve only the medial lemniscal pathway
start at the nipple and extend to the foot
are limited to the right side of the body

Figure 5.17
The results for testing somatosensory sensations for example 3. Applying a vibrating tuning fork on the right foot and manipulating right foot produce no vibration or proprioceptive sensations. However, a pin prick to the right foot produces a well-localized sensation of sharp pain. Press FOOT to view the course of action potentials generated in response to the tuning fork on the right foot and pin prick to the right foot. Vibration and pain sensations are normal in the rest of the body. Press HAND o view the course of action potentials generated in response to application of a vibrating tuning fork to the right and left hands.

So, you conclude that

damage is limited to the posterior column of the spinal cord as pain sensation is not affected (Figure 5.18)
the fifth thoracic segment of the spinal cord may be involved (Figure 5.11)
symptoms are ipsilesional (i.e., involve damage to the right half of the spinal cord, Figure 5.18)

Figure 5.18
The posterior column has been damaged at upper thoracic level (T5) on the right side. The afferents pain and temperature sensations from the right and left side of the body were spared as the lateral and anterior columns were not damaged.

When only the posterior column of the spinal cord is damaged, there are losses involving discriminative touch and proprioception, but no loss of pain, temperature or crude touch sensitivity. The deficit is ipsilesional and extends down the body from the level of the lesion. There is an inability to appreciate vibrating stimuli and the position and movement in the ipsilesional lower body. The remaining tactile sense in the ipsilesional lower body is poorly localized as the spinothalamic tracts are undamaged. The Romberg test is positive as the patient has lost proprioception in a leg and cannot maintain normal posture with eyes closed.

Example 1

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5.14 Clinical Examples:
Central Nervous System: The Spinal Cord (continued)

The patient suffers from loss of pain and temperature sensations from the left half of the body starting just below the left nipple and extending down to and including his left foot.

Symptoms: The patient presents with a complaint of repeatedly injuring his left foot. Physical examination determines that there are losses of pain and temperature sensations involving the left half of his body starting just below the left nipple and extending down to include his left foot (Figure 5.19). However, discriminative touch, and position sensations are normal in the left torso, leg and foot. Touch, vibration, position, pain, and temperature sensations are normal for the rest of the body and face. The result of the Romberg test is negative.

You conclude that the somatosensory losses in his left side of his body involve

pain and temperature sensations

Pathway(s) Affected: You conclude that structures in the following somatosensory pathway (Figure 5.20) have been affected

the spinothalamic pathways

Figure 5.20
Neurons of the neospinothalamic pathway (NSTP) process sharp, cutting pain, and cool/cold information from the body. The 2° afferents of the neospinothalamic pathway decussate in the spinal cord and ascend the cord in the lateral funiculus. In contrast, the 1° afferents of the medial lemniscal pathway (MPL), which carry discriminative touch and proprioceptive information, ascend uncrossed in the spinal cord within the posterior funiculus. Consequently, within the spinal cord, sharp pain and cool/cold from the left side of the body is represented in the contralateral (right) lateral funiculus, and discriminative touch and proprioception of the left side of the body is represented in the ipsilateral (left) posterior funiculus.

Side & Level of Damage: The sensory losses (Figure 5.21)

do not involve the face
involve the spinothalamic pathways
start at the nipple and extend to the foot
are limited to the left side of the body

Figure 5.21
The results of testing somatosensory sensation for Example 4. A pin prick to the left foot does not produce a well localized sensation of sharp pain. However, a vibrating tuning fork on the left foot or manipulating the foot produces vibration or proprioceptive sensations, respectively. Press FOOT to view the course of action potentials generated in response to the tuning fork on, and a pin prick to, the left foot. Pin pricks to the upper body produce well localized sensations of sharp pain. Press HAND to view the course of action potentials generated in response to pin pricks to the left and right hands.

So, you conclude that

damage is in the lateral funiculus (lateral spinothalamic tract) of the spinal cord (Figure 5.22)
the fifth thoracic segment of the spinal cord may be involved (Figure 5.11)
symptoms are contralesional (i.e., the damage is to the right side of the spinal cord)

Figure 5.22
Part of the anterior and lateral funiculi, which contain the spinothalamic tracts, has been damaged at an upper thoracic level (T5) on the right side. The discriminative touch and proprioceptive afferents from the left and right side of the body were spared as the 1° afferents of the medial lemniscal pathway, which are in the posterior funiculus, were not damaged.

Anterolateral cordotomy has been used to relieve intractable pain. When the cut is limited to section of the spinothalamic tract, there is a decrease in pain and temperature sensitivity. As the posterior funiculus is not involved in the section, discriminative touch and proprioception remain intact. The deficit in pain and temperature sensitivity is contralesional and extends down the length of the body from the site of the lesion. However, pain sensation often returns, albeit in a different form, following the surgical section of the spinothalamic tract.

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Example 8

5.15 Clinical Examples:
Central Nervous System: The Spinal Cord (continued)

The patient suffers from loss of pain and temperature sensations from the left half of the body starting just below the left nipple and extending down to and including his left foot. He also exhibits loss of discriminative touch and proprioception in a corresponding area on the right side of his body.

Symptoms: The patient exhibits a loss in voluntary control of the right leg. He also reports loss of sensation in both feet (Figure 5.23). Physical examination determines that there are losses of pain and temperature sensations involving the left half of his body starting just below the left nipple and extending down to include his left foot. There are also loss of vibration and position sensations and poor localization of tactile stimuli on the right side of his body starting just below the right nipple and extending down to include his right foot. Touch, vibration, position and pain sensations are normal for the rest of the body and face. The Romberg test is positive (i.e., The patient has difficulty standing upright with his feet together and his eyes closed).

You conclude that the somatosensory losses in his body (Figure 5.24) involve a "dissociate anesthesia"; that is, loss of

pain and temperature sensations on the left lower body
discriminative touch and proprioception on the right lower body

Figure 5.24
The patient exhibits "dissociate anesthesia"; i.e., a loss of discriminative touch and proprioception on one side of the body and a loss of pain on the opposite side of the body.

Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.25) may have been affected

the spinothalamic pathways
the medial lemniscal pathway

Figure 5.25
Neurons in the medial lemniscal pathway (MLP) process discriminative touch and proprioception from the body, whereas those in the neospinothalamic pathway (NSTP) process sharp pain and temperature information from the body. The right half of the spinal cord contains the uncrossed 1° afferents of the medial lemniscal pathway, which are in the right posterior funiculus, and the crossed 2° afferents of the neospinothalamic pathway, which are in the right lateral funiculus.

Side & Level of Damage: Motor functions are involved and the sensory losses (Figure 5.26)

do not involve the face
involve both the spinothalamic and medial lemniscal pathways
start at the nipples and extend to the feet
are different for each side of the body (i.e., pain on the left and vibration on the right)

Figure 5.26
The results of testing somatosensory sensation for Example 5. Neither a vibrating tuning fork applied to the right foot nor a pin prick applied to the left foot result in the appropriate sensations. Press FOOT to view the course of action potentials generated in response to the tuning fork on the right foot and a pin prick to the left foot. Vibration and pain sensations are normal for the rest of the body. Press HAND to view the course of action potentials generated in response to the tuning fork on the right hand and a pin prick to the left hand.

So, you conclude that

damage involves one half of the spinal cord (i.e., hemisection of the spinal cord, Figure 5.27)
the fifth thoracic segment of the spinal cord may be involved (Figure 5.11)
the symptoms are bilateral but involve damage to the right half of the spinal cord (i.e., The symptoms are contralesional (on the left side) for pain and ipsilesional (on the right side) for vibration with a hemisection of the (right) spinal cord, Figure 5.27)

Figure 5.27
Damage of the right half of the spinal cord at upper thoracic levels (T5) produces the Brown-Sequard syndrome that starts below the nipples and extends down to include the feet. The symptoms are bilateral - with discriminative touch and proprioception lost on the ipsilesional side and pain and temperature affected on the contralesional side.

Hemisection of the Spinal Cord. The symptoms resulting from hemisection of the spinal cord (i.e., damage to the right or left half of the spinal cord) are collectively called the Brown-Sequard syndrome (Figure 5.27). There are both motor and sensory losses: for now learn that the motor losses involve weakness, loss of fine motor control, and abnormal reflexes (which are characteristic of "upper motor" neuron damage) on the ipsilesional side starting at the level of the lesion and extending down the body. For example, if the right spinal cord is sectioned, say at T5, the motor effect is on the right side starting at the chest and extending down to and including the right leg and foot. Because spinal cord hemisection interrupts both the posterior column and spinothalamic tracts, there will be sensory losses that are bilateral: ipsilesional for the posterior column (discriminative touch and proprioception) and contralesional for the spinothalamic tracts (pain and temperature). As the sensory losses in each half of the body differ, they are sometimes referred to as "dissociate anesthesia."

Example 1

Example 2

Example 3

Example 4

Example 5

Example 6

Example 7

Example 8

5.16 Clinical Examples:
Central Nervous System: The Spinal Cord (continued)

The patient suffers from loss of pain and temperature sensations that wrap around his body at his waist.

Symptoms: The patient exhibits loss of pain and temperature sensations that are bilateral and limited to his waist area (i.e., like a cummerbund, Figure 5.28). While pain sensation is diminished around the waist, it is normal above and below the waist. Discriminative touch, vibration and position senses are normal in the waist area and for the rest of the body and face.

You conclude that the somatosensory losses in his body involve

pain sensations bilaterally around his waist

Pathway(s) Affected: You conclude that structures in the following somatosensory pathway (Figure 5.29) may have been affected

the spinothalamic pathways

Figure 5.29
Neurons in the neospinothalamic pathway process sharp pain and cool/cold information from the body. Notice that the 2° neospinothalamic afferents decussate in the spinal cord within the anterior white commissure.

Side & Level of Damage: The sensory losses (Figure 5.30)

do not involve the face
involve the spinothalamic pathways
have a segmental (dermatomal) pattern
are bilateral (analgesia on both sides of the body)

Figure 5.30
The results of testing somatosensory sensation for Example 6. Pin pricks applied anywhere around the waist do not produce well-localized, sharp pain sensations. Press WAIST to view the course of action potentials generated in response to a pin prick to the right and left side of the body at the waist. Pin pricks applied to the feet produce well-localized sensations of sharp pain. Press FOOT to view the course of action potentials generated in response to a pin prick to the right and left feet. Pin pricks applied to the hands produce well-localized sensations of sharp pain. Press HAND to view the course of action potentials generated in response to a pin prick to the right and left hands.

So, you conclude that

damage involves neospinothalamic structures in the spinal cord (Figure 5.31)
the ninth or tenth thoracic segment of the spinal cord is involved (Figure 5.11)
symptoms are bilateral and segmental as they involve damage to the decussating spinothalamic fibers in the anterior white commissure (Figure 5.31)

Figure 5.31
Cavitation of the spinal cord central canal (syringomyelia) at lower thoracic levels (T9 or T10) produces a bilateral loss of pain and temperature that is segmental and localized around the waist area.

In syringomyelia, there are cysts that form within the spinal cord near the central canal (Figure 5.31). As the cyst grows, it first compresses and then destroys the decussating fibers in the anterior white commissure. Many of these fibers belong to the spinothalamic tracts and the resulting sensory loss involves pain and temperature sensation bilaterally and segmentally. The bilateral loss is described to form a belt or girdle pattern - if the damage involves the lower thoracic segments, and does not involve sensation below and above the cyst (i.e., it is segmental). As the cyst grows, it may involve anterior horn motor neurons and produce such "lower motor" signs as weakness, muscle wasting, and loss of reflexes.

Example 1

Example 2

Example 3

Example 4

Example 5

Example 6

Example 7

Example 8

5.17 Clinical Examples:
Central Nervous System: The Brain

Brain Stem. Trauma, stroke, multiple sclerosis (a disease of myelin), and brain tumors are the major causes of brain stem lesions. The location of the lesion site can often be deduced by the loss in cranial nerve function.

Eleven of the thirteen cranial nerves are associated with the brain stem and may be damaged with brain stem lesions.
Most of the 1° somatosensory afferents carrying crude touch, pain and temperature information from the face enter the brain stem at mid pontine levels and descend down to second cervical segment of the spinal cord.
Axons in the spinothalamic pathway decussate in the spinal cord. The decussation in the medial lemniscal pathway occurs in the lower medulla. Consequently, both the spinothalamic tract and medial lemniscus represent the contralateral side of the body at and above the upper (open) medulla.
The spinal trigeminal 2° afferents decussate upon leaving the spinal trigeminal nucleus in the medulla and lower pons, whereas the main sensory trigeminal nucleus axons decussate at mid pons levels immediately upon leaving the nucleus.
Above the level of the pons, all of the somatosensory pathways have decussated and are traveling in close proximity.
The initial higher order processing the discriminative touch and proprioception occurs exclusively within the primary somatosensory cortex. In contrast, pain information is processed in multiple cortical areas.

The patient suffers from a decrease in pain and temperature sensations involving the left side his body and the right side of his face.

Symptoms: The patient exhibits decrease in pain and temperature sensations that involve the left side of his body and right side of his face (Figure 5.32). Discriminative touch, vibration and position senses are normal in these areas. Touch, vibration, position, temperature, and pain sensations are normal for the rest of the body and face.

You conclude that the somatosensory losses involve

pain and temperature sensations on the left body
pain and temperature sensations on the right side of the face

Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.33) have been affected

the spinothalamic pathways
the spinal trigeminal pathway

Figure 5.33
Neurons of the spinothalamic pathways (NSTP, neospinothalamic and PSTP, paleospinothalamic) process pain, temperature and crude touch information from the body. Whereas neurons of the spinal trigeminal pathway (STP) process pain, temperature and crude touch information from the face.

Side & Level of Damage: The sensory losses (Figure 5.34)

involve pain sensation in the body and face
involve both the spinothalamic and spinal trigeminal pathways
are bilateral (analgesia in the left body and the right face)

Figure 5.34
Results of testing somatosensory sensation for Example 7. Pin pricks into the right side of the face and the left hand do not produce well-localized, sharp pain sensations. Press PIN PRICK to view the course of action potentials generated in response to pin pricks into the right side of the face and the left hand. The vibration of a tuning fork applied to the right jaw and left hand, as well as manipulation of the jaw and fingers of the left hand produce normal vibration and proprioceptive sensations. Press TOUCH to view the course of action potentials generated in response to a vibrating tuning fork applied to the right side of the face and the left hand.

So, you conclude that

the damage must be in the brain stem (Figure 5.35)
the spinothalamic and spinal trigeminal tracts are involved
the symptoms are bilateral but the damage is unilateral and involves the right half of the medulla. That is, the symptoms are ipsilesional (right) for the face and contralesional (left) for the body

Figure 5.35
There is damage, colored black, involving the right side of the medulla. Damage to the posterolateral medulla will destroy the uncrossed descending 1° afferents of the spinal trigeminal pathway (STP - colored violet) and the crossed ascending 2° afferents of the neospinothalamic pathway (NSTP - colored red). Notice that the medial lemniscus and ventral trigeminal lemniscus, which are located in the anteromedial medulla, have been spared by this infarct.

Wallenberg's Syndrome. In the medulla, both the spinothalamic tracts and the spinal trigeminal tracts are located posteriorly in the area that normally receives blood via branches of the posterior inferior cerebella artery (PICA) (Figure 5.36). Consequently, an obstruction of the PICA blood supply to the medulla will result in analgesia and thermo-anesthesia of the contralesional body (spinothalamic tracts) and of the ipsilesional face (spinal trigeminal tract). Branches of the anterior spinal and vertebral arteries supply more anterior areas of the upper medulla. Therefore, an infarct involving the PICA blood supply will not affect the medial lemniscus or ventral trigeminal lemniscus. Consequently, discriminative touch and proprioception from the body and pain, temperature and crude touch in the contralesional half of the face will not be affected with an infarct involving PICA.

Figure 5.36
Obstruction of the posterior inferior cerebellar artery (PICA) will result in damage to the posterior quadrant of the medulla. The descending spinal trigeminal tract and nucleus and the ascending spinothalamic tract would be damaged, whereas the medial lemniscus and ventral trigeminal lemniscus would be spared.

Above the level of the pons (Figure 5.11), all of the major somatosensory tracts are crossed and located in close proximity. Consequently, if the brain stem were hemisected above the pons, there would be anesthesia of the contralesional body (section of the spinothalamic tracts which decussate in the spinal cord, and the medial lemniscus, which decussates in the medulla) and contralesional face (section of the ventral trigeminal lemniscus which consists of 2° afferents of the spinal and main sensory trigeminal nuclei that decussate in the medulla and pons).

Example 1

Example 2

Example 3

Example 4

Example 5

Example 6

Example 7

Example 8

5.18 Clinical Examples:
The Cortex

Somatosensory Cortex. The sensory loss from head trauma or stroke that damages the somatosensory cortex will

be contralesional, as decussations in all somatosensory pathways occur below the thalamus
be more complex, as somatosensory information is integrated at cortical levels
not result in analgesia, as pain is represented by multiple pathways (e.g. the neospinothalamic and paleospinothalamic pathways) and in multiple cortical areas
often be accompanied by motor deficits, as the somatosensory cortex provides input to the motor cortex and motor control signals to the brain stem and spinal cord.

The patient suffers from deficits in discriminative touch and proprioceptive sensations involving the right side of his body and face. Tactile and pain sensations are also poorly localized on his right side. He has difficulty walking and controlling his right arm and hand and the right side of his face.

Symptoms: The patient exhibits deficits in fine motor control and in discriminative touch and proprioception on the right side of his body and face (Figure 5.37). He has problems manipulating and identifying objects placed in his right hand (stereognosis). He is unable to identify letters or numbers written on the skin of the right face and the palm of his right hand (graphesthesia). He also has difficulty in judging weight differences (baragnosis) and cannot appreciate textures with his right hand. He is unable to detect the passive movement of his right foot and the fingers of his right hand. Compared with the left side of his body pain sensations are not as sharp, well defined or easily localized on the right side of his body. Touch, vibration, position, thermal, and pain sensations are normal for the rest of the body and face. The patient has difficulty walking and the Romberg test is positive.

You conclude that the somatosensory losses in his body involve

discriminative touch, proprioception and sharp pain on the right body
discriminative touch, proprioception and sharp pain on the right side of his face

Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.38) may have been affected

the medial lemniscal pathway
the main sensory trigeminal pathway

Figure 5.38
the neospinothalamic and spinal trigeminal pathways Neurons of the medial lemniscal pathway (MLP) process discriminative touch and proprioception information from the body, whereas those of the main sensory trigeminal pathway (MSTP) process discriminative touch and proprioception information from the face. The neurons of the neospinothalamic pathway (NSTP) process sharp, cutting pain and cool/cold information from the body, whereas those of the archispinothalamic and paleospinothalamic pathways (PSTP) processes dull and aching pain, warm/hot and crude touch from the body. The neurons of the spinal trigeminal pathway (STP) process all pain, temperature and crude touch information from the face. Notice that thalamic neurons in the paleospinothalamic (PSTP) and spinal trigeminal pathways (STP) send axons to multiple cortical areas.

Side & Level of Damage: The sensory losses (Figure 5.39)

involve the body and face
are unilateral and complex in nature
involve higher order cortical functions, e.g., graphesthesia
do not include complete loss of pain information
are accompanied by motor deficits

Figure 5.39
The results of testing somatosensory sensation in Example 8. The vibration of a tuning fork applied to the right jaw or right hand, as well as manipulation of the right foot, produce no vibration or proprioceptive sensations. Press TOUCH to view the course of action potentials generated in response to a vibrating tuning fork applied to the right jaw and the right hand. Pinching the right cheek or right hand produce pain sensations. Press PINCH to view the course of action potentials generated in response to pinching the right side of the face and the right hand.

So, you conclude that the damage

involves the left somatosensory parietal cortical area
may involve a branch of the middle cerebral artery (Figure 5.40)

Figure 5.40
Subdural hemorrhage involving a parietal branch of the middle cerebral artery injured somatosensory areas of the parietal lobe.

Somatosensory Cortex. Hemorrhage limited to somatosensory parietal areas produces contralesional astereognosis, baragnosis, and losses in the ability to discriminate object size and texture. Also decreased or lost on the contralesional side of the body are the ability to discriminate position and movement of body parts and the control of fine movements. The hemorrhage would not produce a total loss of pain sensation as other cortical areas are also involved in the perception of painful stimuli. For example, the cingulate gyrus in the frontal lobe and part of the insular cortex appear to be involved in the perception of, and emotional reaction to, painful stimuli (Figure 5.41).

Figure 5.41
Cortical areas involved in pain sensation. The thalamic neurons of the spinothalamic pathways and spinal trigeminal pathway that are involved in processing pain information send their axons to the cingulate gyrus and insular cortex. Consequently, damage limited to the somatosensory parietal cortex will not result in the loss of all pain sensation.

Example 1

Example 2

Example 3

Example 4

Example 5

Example 6

Example 7

Example 8

5.19 Summary

From this chapter, you should have learned how the somatosensory system is organized from the skin, muscles and joints to the cortex. You have learned that stimulus features extracted by the somatosensory receptors are kept segregated in separate "information channels" and processed in parallel by different chains of neurons. Information coded and carried by thousands of spinal cord and cranial ganglion cells are distributed to millions of cortical neurons in the parietal lobe. The perceptions of coherent somatosensory stimuli and body image are recomposed out of these fragments of information by the simultaneous activation of large areas of cortex. You have learned how to use the somatotopic organization and the modality specificity of the different somatosensory pathways to determine the location and extent of damage to the somatosensory structures.

Test Your Knowledge

Make the best match between the named cortical functional area and the cortical structure area.

Primary somatosensory cortex
A
B
C
D
E

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula This is an INCORRECT match.

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus This is the CORRECT match!

The precentral gyrus (selection C) is the motor cortex.

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus This is an INCORRECT match.

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe This is an INCORRECT match.

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis This is an INCORRECT match.

Secondary somatosensory cortex
A
B
C
D
E

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula This is an INCORRECT match.

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus This is an INCORRECT match.

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus This is an INCORRECT match.

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe This is an INCORRECT match.

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis This is the CORRECT match!

The pars opercularis of the parietal lobe forms the "upper lip" of the lateral fissure and contains both visceral sensory cortex and the secondary somatosensory cortex. The insula is the site of the gustatory cortex and more visceral cortex.

Somatosensory association cortex
A
B
C
D
E

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula This is an INCORRECT match.

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus This is an INCORRECT match.

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus This is an INCORRECT match.

D. Posterior parietal lobe

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe This is the CORRECT match!

The posterior parietal lobe is located caudal to the postcentral gyrus and serves as the somatosensory association cortex.

E. Parietal lobe pars opercularis

A. Insula

B. Postcentral gyrus

C. Precentral gyrus

D. Posterior parietal lobe

E. Parietal lobe pars opercularis This is an INCORRECT match.

Question 1
A
B
C
D
E

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue

B. hand

C. arm

D. chest

E. foot

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue This answer is INCORRECT.

The tongue is represented in the postcentral gyrus near the lateral sulcus.

B. hand

C. arm

D. chest

E. foot

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue

B. hand This answer is INCORRECT.

The hand is represented in the lateral aspect of the postcentral gyrus.

C. arm

D. chest

E. foot

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue

B. hand

C. arm This answer is INCORRECT.

The arm is represented superior to the hand in the lateral aspect of the postcentral gyrus.

D. chest

E. foot

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue

B. hand

C. arm

D. chest This answer is INCORRECT.

The chest is represented in the superior aspect of the postcentral gyrus.

E. foot

Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.

A. tongue

B. hand

C. arm

D. chest

E. foot This answer is CORRECT!

The buttock, leg, foot, and genitals are represented in the posterior paracentral lobe, which is located on the medial aspect of the cerebral hemisphere.

Question 2
A
B
C
D
E