what brain region routes sensory information to the proper areas of the cortex?
Chapter 5: Somatosensory Processes
Previous chapters described the ways in which the different somatosensory receptors reply to specific types of somatosensory stimuli and that the receptors, by virtue of their selective sensitivities, excerpt specific information nigh 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 hurting is mediated in the neospinothalamic (information aqueduct) 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 answer to muscle wrinkle) and the processing of this extracted information is kept separate forth most of the ascending pathway. In addition to this parsing of stimulus information, the somatosensory organization is also organized to provide a somatotopic representation of the torso 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 feel into stimulus fragments spread over torso pieces. Then how does ane have a sense of "oneness" of the body and how does one identify an object by handling it? Ane can do so considering somatosensory data converges in the parietal lobe of the cognitive cortex to provide a cohesive perception of the body and of somatosensory stimuli.
The first role of this chapter will present boosted 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 full general organization of the somatosensory pathways will be used in the clinical assessments of somatosensory role.
v.i Sensory Pathways Decussate earlier Reaching the Thalamus
In each of the somatosensory pathways covered thus far, afferent axons decussate (cross the midline) one time on their form to the thalamus (Figure five.1).
| Figure 5.ane |
Above the level of decussation, the neurons in a somatosensory pathway represent the contralateral (i.e., opposite) side of the trunk or confront. 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.two Modality Specificity is Maintained up to the Cortex
The sensory data necessary for discriminative affect, proprioception, pain and thermal sensations are kept split within the somatosensory pathways (Figure 5.two).
| Figure v.2 |
To ensure the fidelity of stimulus representation in the discriminative touch-proprioceptive pathways, there is minimal convergence of modality specific data along the pathways. That is, within the medial lemniscal and main sensory trigeminal pathways, there is piffling mixing of information from different receptor types and from afferents with different adaptive backdrop upward 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., ane° afferents with minor "bear on" receptive fields and rapidly adapting discharges). Inside VPL and VPM, the medial lemniscal and ventral trigeminal lemniscal fibers end in different regions based on the sensory data they are carrying. Fibers carrying cutaneous data terminate within the core of the nucleus, whereas those conveying proprioceptive data cease in the surrounding, peripheral shell of the nucleus.
Inside the cerebral cortex, at that place is a convergence of modality specific data and the response properties of cortical neurons go more than complex. The primary somatosensory cortex is responsible for the offset stage of cortical processing. Within the primary sensory cortex, discriminative touch and proprioceptive data 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 |
five.3 Somatosensory Neurons accept 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.east., the fiber tract) that responds to tactile stimulation of the right alphabetize finger pad will not respond to tactile stimulation of any other expanse in the hand, body or face. The stimulated area producing the response is called the neuron's receptive field (Figure 5.3). The neuron'south receptive field tin besides be defined anatomically equally that area of the sense organ (i.e., pare, muscles or joints) innervated direct or indirectly by the neuron. Consequently, a somatosensory neuron can be described to channel information about stimulus location - equally well as stimulus modality. Furthermore, the size of a neuron's receptive field is related to the body expanse innervated/represented. The receptive fields of neurons innervating/representing the finger pads, lips, and natural language 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 command, equally in playing the piano or speaking, modest proprioceptive receptive fields are required.
5.four Spatial Data is Topographically Mapped in Sensory Pathways
Within each somatosensory structure, neurons are organized to provide a spatial representation of the body and confront called the somatotopic map (Figure 5.4). That is, inside the spinal string, brain stem, thalamus and postcentral gyrus, the location of a neuron is related to its receptive field. Consequently, body and confront (i.due east., the receptive fields) are represented spatially (topographically) within nuclei and cortex such that, neurons with face-to-face receptive fields are located adjacent to ane another inside a given structure. For example, bordering areas of the body are represented in adjoining areas of the cortex (Effigy v.4). The resulting neural maps of the body and face are not isomorphic representations and announced distorted because of the disproportionate representation of the hand and face up areas (Figure v.4). That is, every bit the neurons representing the hand and face have pocket-sized receptive fields (Effigy v.3), a greater number of neurons are required to represent the hand and face. Because somatosensory neurons correspond specific stimulus features and specific areas of the body or face up, electric stimulation of a restricted area of the postcentral gyrus (e.1000., the area representing the tongue) will produce a somatic (and not gustatory) sensation that is perceived as arising from the specific region of the trunk (i.east., the natural language).
| Figure 5.4 |
5.5 The Somatosensory Cortex
Somatosensory information converges in the parietal lobe of the cognitive cortex where it is candy to provide a cohesive perception of your torso 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 & v.v).
| Figure 5.v |
SI extends from the depths of the cardinal sulcus up superiorly to grade the posterior lip of the fundamental sulcus. SI is considered the primary somatosensory cortex considering 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 trunk 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 paw in the upper half of the postcentral gyrus. The face is represented in the lower one-half of the postcentral gyrus (Figure 5.iv).
Differential projections to the SI areas ascend from the central core and crush 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 data carried past 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 get larger, modality specificity diminishes, and the cortical neural responses get more complex.
SI neurons ship their axons to the secondary somatosensory cortex, adjacent areas of the parietal lobe, and to cortical motor areas (Effigy 5.half-dozen) as well as to subcortical nuclei, brain stalk and spinal cord. Unilateral destruction of SI produces severe deficits in all aspects of discriminative touch and proprioception on the contralesional side of the trunk. 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, in that location are deficits in fine motor coordination.
| Figure 5.half dozen | |
v.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 transport their axons to SI, association cortex, motor cortex, and insula (Effigy five.vi). The latter project, 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 social club processing required for recognizing manus-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 arrears called "fail", wherein there is a partial neglect (inattention) to tactile, proprioceptive and/or visual stimuli delivered contralateral to the lesion site. The patient is described equally ignoring the contralesional one-half of her/his trunk and space. The perception of a "whole" body is lost and the trunk parts affected may be considered to belong to someone else. Visual stimuli on the contralesional side may besides be ignored.
5.ix 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 awareness is non lost when the chief somatosensory cortex is damaged. Principal 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 volition affect the quality of pain sensations and the power to localize the verbal location of the painful stimulus.
5.10 Clinical Examples
An splendid mode to test your noesis of the material presented thus far is by examining the furnishings of damage to structures inside the somatosensory pathways. The observed sensory loss(southward) provide clues to the pathway(s) affected; and the area(southward) and side of the body/confront affected provide clues to the level of the damage. The following section should help you determine how well yous can utilize what you have learned thus far about the somatosensory organisation.
| Example 1 | Case 2 | Case iii | Example four | Example 5 | Example 6 | Example 7 | Example 8 |
v.11 Peripheral Nervous Organisation
Peripheral Nervus Damage: Impairment 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 peel, muscles and joints of a given body part (e.chiliad., the manus 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 role.
The patient reports a loss of all sensation from his left hand.
Symptoms: The patient complains of loss of awareness and weakness involving his left hand (Figure 5.vii). The physical examination determines that he is insensitive to hurting, affect, vibration and finger position in his left hand. However, bear on, vibration, position and pain sensations are normal in the residual of his body and face.
You conclude that the somatosensory losses in his left mitt include
- Loss of discriminative touch and proprioception (touch on, vibration & position).
- Astringent decrement in pain awareness (analgesia).
Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.viii) may have been affected
- the medial lemniscal (touch, vibration & position) pathway
- the spinothalamic (pain and temperature) pathway
| Figure 5.8 |
Side & Level of Damage: The sensory losses (Figure 5.9)
- are express to the left hand
- involve all somatosensory pathways
- are associated with motor dysfunction
| Effigy 5.9 Printing 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.ten)
- symptoms are ipsilesional (i.due east., on the same (left) side every bit the damaged nervus)
| Figure five.10 |
Impairment to peripheral nerves results in a loss of all somatosensory modalities and motor function in a restricted area of the torso defined by the nervus distribution. Electrophysiological methods can exist used to determine the nerves involved and the caste of nervus damage (Refer to the section "Peripheral Somatosensory Axons" in the chapter on Somatosensory Pathways).
Posterior or Cranial Nervus Root Damage: The central processes of the i° somatosensory afferents collect to grade a posterior root prior to entering the spinal string. Consequently, the area of the body supplied by a single posterior root is represented by the sum of receptive fields of the ane° afferents in the root. The surface area of the trunk innervated by a posterior root is called a dermatome (Figure 5.11). Posterior root damage would effect in somatosensory losses in the dermatome supplied by the root. All sensations would exist lost in the primal expanse of the dermatome. The more than peripheral areas of the dermatome volition take some awareness, albeit less than normal, as consecutive roots accept partially overlapping dermatomes.
| Figure v.eleven |
The symptoms produced past cranial nervus root harm depend upon the cranial nervus involved. For case, the trigeminal nerve root contains somatosensory (major) and chemosensory (minor) one° afferent axons innervating the face, every bit well every bit efferent (motor) axons controlling the jaw muscles (Refer to Tabular array 2 in the affiliate on Somatosensory Pathways for the cranial nerves providing somatosensory innervation of the face up and dura).
| Case one | Example 2 | Example 3 | Example four | Case 5 | Example 6 | Example 7 | Example 8 |
5.12 Clinical Examples:
Peripheral Nervous Organization (continued)
The patient reports a loss of sensation forth 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 downward to include the thumb of his left mitt (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 on and pain sensations along the lateral border of the left arm down to the pollex. Touch, vibration, position, and pain sensations are normal for the rest of the body and face up.
Yous conclude that the somatosensory losses in his left arm involve
- discriminative touch and proprioception
- pain sensation
Pathway(due south) Affected: Yous conclude that structures in the following somatosensory pathways (Figure v.eight) accept been affected
- the medial lemniscal pathway
- the spinothalamic pathway
| Figure 5.thirteen |
Side & Level of Impairment: As the sensory losses (Figure five.thirteen)
- are limited to the lateral attribute of the left arm and extended down to include the left thumb
- involve both somatosensory pathways
- involve a dermatomal blueprint
- do not involve motor function
You conclude that
- impairment may involve the fifth, sixth and seventh cervical posterior roots (Figure 5.eleven)
- symptoms are ipsilesional (i.e., involve the left posterior roots, Effigy five.14)
| Figure five.14 |
Section of a Posterior Root results in the loss of all somatosensory modalities in a restricted area of the body divers past the root dermatome (Figure 5.11). Consequently, the damaged posterior root can exist identified by the dermatomal pattern of sensory loss. Radiographic methods tin can be used to make up one's mind if the roots are being compressed past abnormalities in the vertebra.
| Instance 1 | Case two | Example 3 | Example 4 | Instance 5 | Instance half-dozen | Example seven | Example eight |
5.thirteen Clinical Examples:
Central Nervous Arrangement: The Spinal Cord
Spinal Cord Impairment: 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 impairment to it does non effect in sensory losses.
- When a somatosensory tract is sectioned in the spinal cord, the sensory losses commencement at the level of the lesion and extend downward to embrace lower parts of the body. Such a loss occurs because the period 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 fretfulness, which are associated with the encephalon stem, supply the motor and sensory innervation of the confront. Consequently, spinal cord lesions exercise not produce sensory or motor losses involving the confront. 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 well-nigh obvious motor deficits (i.e., deficits in voluntary motor control beneath the level of the lesion).
The patient suffers from loss of discriminative touch and proprioception (i.due east., vibration and position sensations) from the right one-half of the body starting just below the correct nipple and extending downward to and including his right human foot.
Symptoms: The patient complains of problems with walking, particularly at night when there is petty low-cal. He also reports a loss of awareness in his right human foot. Concrete examination determines that there are decreases in vibration and position sensations and poor localization of tactile stimuli involving the right half of his trunk starting simply below the right nipple and extending downwards to include his right foot (Figure 5.15). Hurting awareness is normal in the correct torso, leg and human foot. Bear on, vibration, position and pain sensations are normal for the remainder of the body and face. The Romberg test is positive. (i.east., The patient has difficulty continuing upright with his anxiety together and his eyes airtight.)
Yous conclude that the somatosensory losses in his right lower body involve
- discriminative touch and proprioception
Pathway(due south) Affected: You conclude that structures in the post-obit somatosensory pathway may have been afflicted (Figure 5.16)
- the medial lemniscal pathway
| Effigy 5.16 |
Side & Level of Damage: The sensory losses (Effigy 5.17)
- do not involve the face
- involve just the medial lemniscal pathway
- offset at the nipple and extend to the human foot
- are express to the correct side of the trunk
| Figure five.17 |
So, you conclude that
- damage is limited to the posterior cavalcade of the spinal cord equally hurting awareness is non affected (Figure five.eighteen)
- the fifth thoracic segment of the spinal cord may be involved (Figure five.xi)
- symptoms are ipsilesional (i.e., involve damage to the correct half of the spinal cord, Figure v.18)
| Figure 5.eighteen |
When just the posterior column of the spinal string is damaged, at that place are losses involving discriminative touch and proprioception, only no loss of hurting, temperature or crude touch on sensitivity. The arrears is ipsilesional and extends down the body from the level of the lesion. At that place is an disability to capeesh vibrating stimuli and the position and motility in the ipsilesional lower body. The remaining tactile sense in the ipsilesional lower body is poorly localized every bit 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 airtight.
| Example 1 | Case 2 | Example 3 | Example 4 | Instance 5 | Example half dozen | Example vii | Case eight |
5.14 Clinical Examples:
Cardinal Nervous Organization: The Spinal String (connected)
The patient suffers from loss of hurting and temperature sensations from the left half of the torso starting just beneath the left nipple and extending downwardly to and including his left pes.
Symptoms: The patient presents with a complaint of repeatedly injuring his left foot. Physical examination determines that there are losses of hurting and temperature sensations involving the left half of his body starting just beneath the left nipple and extending down to include his left foot (Effigy 5.19). However, discriminative touch, and position sensations are normal in the left trunk, leg and foot. Touch, vibration, position, hurting, and temperature sensations are normal for the rest of the body and face. The outcome 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 five.20) accept been affected
- the spinothalamic pathways
| Figure 5.20 |
Side & Level of Damage: The sensory losses (Figure 5.21)
- do non involve the face
- involve the spinothalamic pathways
- start at the nipple and extend to the foot
- are limited to the left side of the torso
| Figure 5.21 |
Then, you conclude that
- damage is in the lateral funiculus (lateral spinothalamic tract) of the spinal cord (Figure five.22)
- the fifth thoracic segment of the spinal cord may exist involved (Figure five.11)
- symptoms are contralesional (i.e., the damage is to the right side of the spinal cord)
| Effigy 5.22 |
Anterolateral cordotomy has been used to relieve intractable pain. When the cut is limited to section of the spinothalamic tract, at that place is a subtract in pain and temperature sensitivity. As the posterior funiculus is not involved in the section, discriminative touch and proprioception remain intact. The arrears in hurting and temperature sensitivity is contralesional and extends down the length of the body from the site of the lesion. However, pain sensation often returns, admitting in a unlike course, following the surgical section of the spinothalamic tract.
| Example 1 | Case 2 | Example 3 | Example 4 | Example five | Instance 6 | Example 7 | Example viii |
5.xv Clinical Examples:
Cardinal Nervous Organisation: The Spinal Cord (connected)
The patient suffers from loss of hurting and temperature sensations from the left one-half of the trunk starting just beneath the left nipple and extending down to and including his left foot. He likewise exhibits loss of discriminative touch and proprioception in a corresponding area on the correct 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 in that location are losses of pain and temperature sensations involving the left half of his trunk starting just beneath the left nipple and extending downward to include his left foot. At that place are as well loss of vibration and position sensations and poor localization of tactile stimuli on the correct side of his body starting just below the right nipple and extending down to include his right foot. Touch on, vibration, position and pain sensations are normal for the residue of the trunk and face. The Romberg test is positive (i.e., The patient has difficulty continuing upright with his feet together and his optics airtight).
You conclude that the somatosensory losses in his body (Figure five.24) involve a "dissociate anesthesia"; that is, loss of
- pain and temperature sensations on the left lower torso
- discriminative impact and proprioception on the right lower body
| Figure v.24 |
Pathway(s) Affected: You lot conclude that structures in the following somatosensory pathways (Figure five.25) may have been affected
- the spinothalamic pathways
- the medial lemniscal pathway
| Effigy 5.25 |
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 |
And then, you conclude that
- damage involves one one-half of the spinal string (i.e., hemisection of the spinal cord, Effigy five.27)
- the fifth thoracic segment of the spinal string may be involved (Figure 5.11)
- the symptoms are bilateral but involve damage to the correct half of the spinal string (i.e., The symptoms are contralesional (on the left side) for hurting and ipsilesional (on the right side) for vibration with a hemisection of the (right) spinal cord, Figure v.27)
| Figure 5.27 |
Hemisection of the Spinal Cord. The symptoms resulting from hemisection of the spinal string (i.e., damage to the correct or left one-half of the spinal string) are collectively called the Brown-Sequard syndrome (Figure five.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 upshot is on the right side starting at the chest and extending downwards to and including the right leg and pes. Because spinal cord hemisection interrupts both the posterior column and spinothalamic tracts, at that place 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). Every bit the sensory losses in each half of the body differ, they are sometimes referred to as "dissociate anesthesia."
| Example 1 | Example 2 | Example 3 | Case 4 | Example 5 | Case half-dozen | Example 7 | Example 8 |
v.16 Clinical Examples:
Central Nervous System: The Spinal Cord (continued)
The patient suffers from loss of pain and temperature sensations that wrap around his torso at his waist.
Symptoms: The patient exhibits loss of hurting and temperature sensations that are bilateral and limited to his waist area (i.eastward., like a cummerbund, Figure 5.28). While pain sensation is diminished around the waist, it is normal higher up and below the waist. Discriminative touch, vibration and position senses are normal in the waist area and for the rest of the body and confront.
Yous 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 v.29 |
Side & Level of Harm: The sensory losses (Figure v.30)
- exercise non involve the face
- involve the spinothalamic pathways
- accept a segmental (dermatomal) design
- are bilateral (analgesia on both sides of the trunk)
| Figure 5.30 |
Then, yous conclude that
- impairment involves neospinothalamic structures in the spinal cord (Figure v.31)
- the ninth or 10th thoracic segment of the spinal cord is involved (Effigy 5.11)
- symptoms are bilateral and segmental every bit they involve impairment to the decussating spinothalamic fibers in the anterior white commissure (Figure 5.31)
| Effigy 5.31 |
In syringomyelia, there are cysts that form within the spinal cord near the central culvert (Figure v.31). As the cyst grows, information technology offset compresses so destroys the decussating fibers in the inductive 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 blueprint - if the harm involves the lower thoracic segments, and does not involve awareness beneath and above the cyst (i.e., it is segmental). Every bit the cyst grows, it may involve anterior horn motor neurons and produce such "lower motor" signs as weakness, musculus wasting, and loss of reflexes.
| Example 1 | Example 2 | Instance 3 | Example iv | Example 5 | Example 6 | Example 7 | Case eight |
5.17 Clinical Examples:
Central Nervous System: The Encephalon
Brain Stem. Trauma, stroke, multiple sclerosis (a affliction of myelin), and brain tumors are the major causes of encephalon stem lesions. The location of the lesion site can often be deduced by the loss in cranial nervus role.
- 11 of the thirteen cranial nerves are associated with the brain stem and may exist damaged with brain stalk lesions.
- Most of the 1° somatosensory afferents conveying crude impact, hurting and temperature information from the face up enter the brain stalk at mid pontine levels and descend down to 2nd 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 up) medulla.
- The spinal trigeminal 2° afferents decussate upon leaving the spinal trigeminal nucleus in the medulla and lower pons, whereas the chief 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 college society processing the discriminative touch and proprioception occurs exclusively within the principal somatosensory cortex. In contrast, hurting data 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 up.
Symptoms: The patient exhibits subtract in pain and temperature sensations that involve the left side of his body and right side of his face up (Figure five.32). Discriminative touch, vibration and position senses are normal in these areas. Affect, vibration, position, temperature, and pain sensations are normal for the rest of the body and face.
You conclude that the somatosensory losses involve
- hurting and temperature sensations on the left body
- pain and temperature sensations on the right side of the face
Pathway(south) Affected: You conclude that structures in the following somatosensory pathways (Effigy v.33) have been affected
- the spinothalamic pathways
- the spinal trigeminal pathway
| Figure v.33 |
Side & Level of Impairment: The sensory losses (Figure 5.34)
- involve pain awareness in the torso and face
- involve both the spinothalamic and spinal trigeminal pathways
- are bilateral (analgesia in the left body and the right face)
| Figure five.34 |
So, you conclude that
- the damage must be in the encephalon stem (Figure five.35)
- the spinothalamic and spinal trigeminal tracts are involved
- the symptoms are bilateral but the damage is unilateral and involves the right one-half of the medulla. That is, the symptoms are ipsilesional (right) for the confront and contralesional (left) for the body
| Figure 5.35 |
Wallenberg'southward Syndrome. In the medulla, both the spinothalamic tracts and the spinal trigeminal tracts are located posteriorly in the area that unremarkably receives blood via branches of the posterior inferior cerebella artery (PICA) (Effigy 5.36). Consequently, an obstacle of the PICA blood supply to the medulla will result in analgesia and thermo-anesthesia of the contralesional trunk (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 torso and pain, temperature and rough touch in the contralesional half of the face will non be affected with an infarct involving PICA.
| Figure 5.36 |
Above the level of the pons (Figure 5.11), all of the major somatosensory tracts are crossed and located in shut proximity. Consequently, if the brain stem were hemisected in a higher place the pons, in that location would exist anesthesia of the contralesional body (department 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 primary sensory trigeminal nuclei that decussate in the medulla and pons).
| Instance 1 | Example ii | Example iii | Case 4 | Example five | Example 6 | Instance 7 | Example 8 |
5.18 Clinical Examples:
The Cortex
Somatosensory Cortex. The sensory loss from caput trauma or stroke that damages the somatosensory cortex volition
- be contralesional, as decussations in all somatosensory pathways occur below the thalamus
- be more circuitous, every bit somatosensory data is integrated at cortical levels
- non result in analgesia, as pain is represented by multiple pathways (e.yard. the neospinothalamic and paleospinothalamic pathways) and in multiple cortical areas
- often be accompanied by motor deficits, equally the somatosensory cortex provides input to the motor cortex and motor control signals to the encephalon stem and spinal string.
The patient suffers from deficits in discriminative affect and proprioceptive sensations involving the right side of his torso and face. Tactile and pain sensations are also poorly localized on his right side. He has difficulty walking and decision-making 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 trunk and confront (Figure five.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 capeesh textures with his right hand. He is unable to discover the passive movement of his right pes and the fingers of his correct paw. Compared with the left side of his torso pain sensations are not as sharp, well defined or easily localized on the correct side of his torso. Bear on, vibration, position, thermal, and hurting sensations are normal for the rest of the torso and face. The patient has difficulty walking and the Romberg test is positive.
You conclude that the somatosensory losses in his body involve
- discriminative bear on, proprioception and abrupt pain on the right body
- discriminative touch, proprioception and sharp pain on the right side of his confront
Pathway(s) Affected: You lot conclude that structures in the following somatosensory pathways (Figure 5.38) may accept been affected
- the medial lemniscal pathway
- the primary sensory trigeminal pathway
| Figure five.38 |
Side & Level of Damage: The sensory losses (Effigy five.39)
- involve the body and face
- are unilateral and complex in nature
- involve college gild cortical functions, east.g., graphesthesia
- practice not include complete loss of hurting information
- are accompanied past motor deficits
| Figure v.39 |
And then, yous conclude that the damage
- involves the left somatosensory parietal cortical surface area
- may involve a branch of the middle cerebral artery (Figure 5.40)
| Figure 5.twoscore |
Somatosensory Cortex. Hemorrhage limited to somatosensory parietal areas produces contralesional astereognosis, baragnosis, and losses in the ability to discriminate object size and texture. Too decreased or lost on the contralesional side of the body are the power to discriminate position and motion of trunk 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 exist involved in the perception of, and emotional reaction to, painful stimuli (Figure 5.41).
| Effigy 5.41 |
| Instance 1 | Example 2 | Example 3 | Case 4 | Example v | Case 6 | Example seven | Case 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 dissimilar 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 prototype are recomposed out of these fragments of information past the simultaneous activation of big areas of cortex. You have learned how to use the somatotopic organization and the modality specificity of the unlike somatosensory pathways to determine the location and extent of damage to the somatosensory structures.
Test Your Cognition
Make the all-time match between the named cortical functional area and the cortical structure surface 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 friction 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 lucifer!
The precentral gyrus (choice 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 Wrong 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 friction match.
E. Parietal lobe pars opercularis
A. Insula
B. Postcentral gyrus
C. Precentral gyrus
D. Posterior parietal lobe
Due east. Parietal lobe pars opercularis This is an INCORRECT match.
- Secondary somatosensory cortex
- A
- B
- C
- D
- East
A. Insula
B. Postcentral gyrus
C. Precentral gyrus
D. Posterior parietal lobe
East. Parietal lobe pars opercularis
A. Insula This is an Wrong lucifer.
B. Postcentral gyrus
C. Precentral gyrus
D. Posterior parietal lobe
E. Parietal lobe pars opercularis
A. Insula
B. Postcentral gyrus This is an Wrong 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 lucifer.
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 cleft and contains both visceral sensory cortex and the secondary somatosensory cortex. The insula is the site of the gustatory cortex and more than visceral cortex.
- Somatosensory clan cortex
- A
- B
- C
- D
- Due east
A. Insula
B. Postcentral gyrus
C. Precentral gyrus
D. Posterior parietal lobe
E. Parietal lobe pars opercularis
A. Insula This is an INCORRECT friction match.
B. Postcentral gyrus
C. Precentral gyrus
D. Posterior parietal lobe
East. Parietal lobe pars opercularis
A. Insula
B. Postcentral gyrus This is an Wrong 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 lucifer!
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
East. Parietal lobe pars opercularis This is an Wrong match.
- Question i
- 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. natural language This answer is INCORRECT.
The natural language is represented in the postcentral gyrus nearly the lateral sulcus.
B. paw
C. arm
D. chest
E. pes
Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.
A. natural language
B. hand This answer is Wrong.
The hand is represented in the lateral aspect of the postcentral gyrus.
C. arm
D. chest
E. pes
Select the best answer: Electrical stimulation of the posterior paracentral lobe volition 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 all-time answer: Electric stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the _______.
A. tongue
B. mitt
C. arm
D. chest This reply is INCORRECT.
The breast 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 Right!
The buttock, leg, foot, and genitals are represented in the posterior paracentral lobe, which is located on the medial aspect of the cognitive hemisphere.
- Question 2
- A
- B
- C
- D
- East
Select the best answer: Impairment to the posterior funiculus at spinal string level T6 produces a loss ______.
A. of sharp, cut hurting sensation
B. that is contralesional
C. of sensation in the arms and hands
D. that produces a positive Rhomberg sign
E. that is called the Brown-Sequard syndrome
Select the best reply: Harm to the posterior funiculus at spinal string level T6 produces a loss ______.
A. of sharp, cutting pain sensation This reply is INCORRECT.
This is incorrect, as the posterior funiculus contains commencement order afferents of the medial lemniscal pathway, which processes discriminative bear upon and proprioception. The neospinothalamic pathway processes sharp pain sensation from the torso and the 2nd order axons of this pathway are in the lateral and anterior funiculi (the spinothalamic tract).
B. that is contralesional
C. of sensastion in the arms and hands
D. that produces a positive Rhomberg sign
E. that is chosen the Brownish-Sequard syndrome
Select the all-time respond: Damage to the posterior funiculus at spinal string level T6 produces a loss ______.
A. of abrupt, cut pain sensation
B. that is contralesional This answer is INCORRECT.
This is incorrect, every bit the first club medial lemniscal afferents do not decussate. Consequently, the sensory loss is ipsilesional when these afferents are destroyed.
C. of sensation in the arms and hands
D. that produces a positive Rhomberg sign
Due east. that is chosen the Brown-Sequard syndrome
Select the all-time reply: Damage to the posterior funiculus at spinal cord level T6 produces a loss ______.
A. of sharp, cutting pain awareness
B. that is contralesional
C. of sensation in the arms and hands This answer is INCORRECT.
This is incorrect, every bit the medial lemniscal first order afferents innervating the arm and hand enter the spinal string posterior funiculus via posterior roots above T6.
D. that produces a positive Rhomberg sign
E. that is called the Brown-Sequard syndrome
Select the best answer: Damage to the posterior funiculus at spinal cord level T6 produces a loss ______.
A. of sharp, cutting pain sensation
B. that is contralesional
C. of sensation in the arms and easily
D. that produces a positive Rhomberg sign This answer is Correct!
The lesion produces a positive Rhomberg sign as in that location is a loss of proprioception in the ipsilesional leg and the patient is unable to maintain his residue when his eyes are closed and his feet are shut together.
E. that is called the Brown-Sequard syndrome
Select the best respond: Damage to the posterior funiculus at spinal string level T6 produces a loss ______.
A. of sharp, cut pain sensation
B. that is contralesional
C. of sensation in the arms and hands
D. that produces a positive Rhomberg sign
E. that is chosen the Brown-Sequard syndrome This reply is Wrong.
This is wrong as the Dark-brown-Sequard syndrome results from hemisection of the spinal cord. The syndrome includes ipsilesional loss of discriminative touch and proprioception and contralesional loss of hurting and temperature sensations.
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