This chapter describes the general organization of somatosensory pathways and the anatomy of the somatosensory pathways involved in processing discriminative touch and proprioceptive information, and those involved with sharp pain and cool/cold information. Discriminative touch and proprioceptive information allow for the recognition of objects by touch, provide for a sense of our body image and is used for maintaining balance and posture. Sharp, pricking pain and cool/cold information allows for the detection and localization of potential tissue-damaging stimuli in a timely manner. Show 4.1 General Organization of Somatosensory Pathways Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations. 4.2 Common Anatomical Features Somatosensory stimuli activate a chain of neurons starting with the peripheral first-order (1°) afferent and ending in the cerebral cortex (e.g., Figure 4.1).
Within each somatosensory pathway,
Each somatosensory pathway is named after a major tract or nucleus in the pathway. In general, conscious perception of sensory stimuli requires the involvement of neurons in the thalamus and cerebral cortex. For example, electrical stimulation of a structure in pathways connecting muscle and joint receptors to the cerebellum (e.g., electrical stimulation of the anterior spinocerebellar tract) will not produce a sensation of limb movement, as these pathways do not include the thalamus or cortex. In contrast, electrical stimulation of a structure in the posterior column-medial lemniscal pathway (e.g., electrical stimulation of the medial lemniscus) may result in a sensation of limb movement, as this pathway includes the thalamus and terminates in the cerebral cortex. 4.3 Peripheral Somatosensory Axons The morphology of the peripheral somatosensory axon is related to the receptor it innervates or forms and to the sensory information it carries (Figure 4.2).
The morphology of the peripheral somatosensory axon is also related to the conduction velocity of the action potentials generated by the axon. The conduction velocity of an axon is determined by electrically stimulating the axon and recording the time (latency) it takes the electrically elicited action potential to reach a recording electrode (Figure 4.3). The distance traveled from the electrical stimulating site to the recording site divided by the latency provides the conduction velocity of the axon. As discussed in earlier chapters, the larger and more heavily myelinated the axon, the greater its conduction velocity (Figure 4.3). Consequently, the 1° afferent axons carrying information required for fine motor control and rapid reflex responses (i.e., those forming body proprioceptors) conduct action potentials rapidly, whereas those carrying information about body and object temperature conduct action potentials at a much slower rate. The whole nerve potential or compound action potential (CAP) is recorded extracellularly from an electrically stimulated nerve and is the sum of the signals produced by each of the individual action potentials of the axons forming the nerve. (Figure 4.3) The mixed nerve (afferent and efferent axons) compound action potential has three prominent peaks that are called A, B and C. The conduction velocity of an axon determines the axon's contribution to the compound action potential peaks. Specifically, the faster the axon conduction velocity, the shorter the latency of axon response and the greater the axon's contribution to the shorter latency peaks (e.g., compare columns CAP Peak and Conduction Velocity in Table I). The axons contributing to a given compound action potential peak (e.g., peak A) are named according to the peak name (e.g., Type A axon). When the relative amplitudes of the peaks differ from those generated by "normal" nerves, the types of damaged axons can be assessed by determining which peaks are abnormal. Consequently, the compound action potential is used clinically to detect nerve damage and to monitor the progress of the regeneration of damaged nerves.
For historical reasons, the terminology based on axon conduction velocity (Group I, II, III and IV) is used for afferent and efferent axons innervating muscles and tendons. And the terminology based on the compound action potential (Type A, B or C) is used for afferent axons innervating the skin, joints and viscera. Note that the fastest conducting somatosensory 1° afferents (Group Ia) innervate skeletal muscle and the slowest (C-fibers) form the receptors of the pain systems. While one might expect painful, tissue damaging stimuli to have priority over all other somatosensory stimuli, the afferent information required to control the reaction to the painful stimuli are conveyed by the faster conducting muscle and joint afferents. Even afferents providing more exact information about the location of a cutaneous stimulus, the Aβ axons, conduct at a faster rate than the Aδ and C axons carrying information about painful stimuli. 4.4 Somatotopic Organization Somatosensory neurons are topographically (i.e., spatially) organized so that adjacent neurons represent neighboring regions of the body or face (Figure 4.4). This organization is preserved by a precise point-to-point somatotopic pattern of connections from the spinal cord and brain stem to the thalamus and cortex. Consequently, within each somatosensory pathway there is a complete map (spatial representation) of the body or face in each of the somatosensory nuclei, tracts, and cortex. Additional information on somatotopic organization is presented in Chapter 5 of Section II.
4.5 Somatosensory Pathways The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face. Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face. 4.6 Medial Lemniscal Pathway: Body Discriminative Touch and Proprioception The posterior (dorsal) column - medial lemniscal pathway (i.e., the medial lemniscal pathway) carries and processes discriminative touch and proprioceptive information from the body (Figure 4.5). It is important to keep in mind that within the medial lemniscal pathway, the afferents carrying discriminative touch information are kept separate from those carrying proprioceptive information up to the level of the cerebral cortex.
The peripheral axons of the 1° afferents are myelinated, large or medium diameter axons. Each axon travels via a posterior root, spinal nerve and peripheral nerve to skin, muscle or joint- where it forms or innervates a somatosensory receptor. The 1° medial lemniscal afferent peripheral process that end in the
The 1° medial lemniscal afferent central axons
In the medulla,
The 2° medial lemniscal afferents
The axons of the VPL 3° afferent neurons
The postcentral gyrus and posterior paracentral lobule
The action potentials ascend the spinal cord via the central process of the 1° afferent in the fasciculus gracilis of the posterior column until they reach the medulla. In the medulla, the action potentials initiate the release of neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the gracile nucleus. The 2° afferent generates action potentials that are conducted by its axons, which decussate to form the medial lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the foot where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the core of the VPL of the thalamus. The action potentials generated by the 3° VPL afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the posterior paracentral lobule of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the Meissner corpuscle. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as tactile and from a Meissner corpuscle. 4.7 Main Sensory Trigeminal Pathway: Face Discriminative Touch and Proprioception The main sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face (Figure 4.7). Consequently, it is the cranial homologue of the medial lemniscal pathway.
The cranial 1° main sensory trigeminal afferent neurons
The main sensory trigeminal 2° afferent axons
The 2° main sensory trigeminal afferents in the ventral trigeminal lemniscus
The axons of the 3° main sensory trigeminal afferents (VPM neurons)
The postcentral gyrus
Figure 4.8 illustrates the course of action potentials generated in response to touching the left cheek with a wisp of cotton. A Merkel receptor in the left cheek is stimulated, and its 1° afferent generates action potentials that are conducted by the 1° afferent Ab axon, past its pseudounipolar soma, into the brain stem.
The 1° afferent central process conducts the action potentials into the pons where they initiate the release neurotransmitter from the 1° afferent axon terminals. The neurotransmitter is released onto 2° afferents within the main sensory trigeminal nucleus. The 2° afferent generates action potentials that are conducted along its axon, which decussates in the pons to join the ventral trigeminal lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the site where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the core of the VPM of the thalamus. The action potentials generated by the 3° VPM afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the postcentral gyrus of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the Merkel cell. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as tactile and from a Merkel cell. There is a minor proprioceptive component for the jaw in cranial nerve V that has 1° afferent cell bodies located in the mesencephalic trigeminal nucleus. The peripheral axons of these afferents travel in the mandibular branch of the trigeminal nerve and end in the jaw muscles and joint. The central processes of most of these afferents end in the trigeminal motor nucleus that controls the muscles of the jaw. Few synapse in the main sensory trigeminal nucleus. 4.8 Neospinothalamic Pathway: Body - Sharp Prickling Pain and Cool/Cold The neospinothalamic pathway carries and processes sharp, pricking pain and dropping temperature (cool/cold) information from the body (Figure 4.9). The pain information carried by the neospinothalamic pathway is well localized and the sensations are the short lasting “fast” or “first” pain elicited by tissue-damaging cutaneous stimuli. The neospinothalamic pathway is also characterized by somatotopic representation, which allows for accurate localization of the painful stimulus. Recall that there are multiple spinal pathways processing pain information (see Somatosensory Systems Table I). Most of the ascending afferents of the spinal pain pathways travel with the neospinothalamic afferents in a fiber tract called the "spinothalamic tract" or "anterolateral spinothalamic tract". Elements of these other pain pathways will be mentioned below to help you understand how pain sensations may remain after damage to the neospinothalamic pathway. The pain pathways will be covered in greater detail in later chapters.
The 1° neospinothalamic afferents
The 2° neospinothalamic afferent axons
The crossed 2° neospinothalamic afferent axons
The spinothalamic afferent axons from the thalamus
The postcentral gyrus and posterior paracentral lobule are
The insula and rostral cingulate gyrus
Figure 4.10 illustrates the course of action potentials generated in response to a pin prick into the left foot. Free nerve endings in the left foot are stimulated by the pin prick. Action potentials are generated and conducted by the 1° afferent Aδ axon, past the pseudounipolar soma, and into the spinal cord (Figure 4.10). The action potentials enter the spinal cord via the central process of the 1° afferents to initiate the release neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the posterior marginal nucleus. The 2° afferent generates action potentials that are conducted by its axon, which decussates in the anterior white commissure of the spinal cord. The crossed 2° neospinothalamic afferent axons form the lateral component of the spinothalamic tract. The action potentials conducted by the crossed 2° afferent axon are contralateral to their site of origin and contralateral to the foot where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the VPL of the thalamus. The action potentials generated by the 3° VPL afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the posterior paracentral lobule of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the free nerve ending. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as a sharp pinprick. 4.9 Spinal Trigeminal Pathway: Face Pain, Temperature and Crude Touch The spinal trigeminal pathway carries and processes crude touch, pain and temperature information from the face (Figure 4.11) Consequently, it is the cranial homologue of the spinothalamic pathways i.e., homologous to all the spinothalamic pathways, the archi-, paleo- and neospinothalamic pathways. As in the spinothalamic pathways, the afferents carrying crude touch information are kept separate from those carrying temperature information and from others carrying pain information. Also the trigeminal afferents carrying sharp, cutting pain information are segregated from those carrying dull, burning pain and deep aching pain information.
The 1° spinal trigeminal afferents
The spinal trigeminal tract
The 2° spinal trigeminal afferent axons
Multiple thalamic nuclei process information in this pathway
The 3° spinal trigeminal afferent axons from the thalamus:
The spinal trigeminal pathway terminates in multiple cortical areas:
Figure 4.12 illustrates the course of action potential generated in response to a pin prick to the left cheek. Free nerve endings in the left cheek are stimulated by the pin prick. Action potentials are generated and conducted by the 1° afferent Aδ axon, past the pseudounipolar soma, and into the brain stem The 1° afferent central process bypasses the main sensory trigeminal nucleus and descends the brain stem in the spinal trigeminal tract. The action potentials are conducted in this descending tract to the spinal trigeminal nucleus, where they initiate the release neurotransmitter from the 1° afferent axon terminals. The neurotransmitter is released onto 2° afferents within the spinal trigeminal nucleus. The 2° afferent generates action potentials that are conducted along its axon, which decussates to form the ventral trigeminal lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the cheek where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the VPM. The action potentials generated by the 3° VPM afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the postcentral gyrus of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the free nerve ending. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as a sharp pinprick. 4.10 Concluding Remarks Clinically, it
is important to remember what information is carried by a particular pathway and the level of the pathway at which decussation occurs (Figure 4.13).
For example, if the posterior funiculus were sectioned, sparing the rest of the spinal cord, discriminative touch and proprioception would be affected but not pain, temperature, or crude touch. Also, the loss of discriminative touch and proprioception would be ipsilesional (e.g., on the right side of the body with section of the right posterior column) because the 1° afferent axons in the posterior columns do not decussate. Consequently, damage to the posterior column in the spinal cord would be suspected if a patient presented with a loss of discriminative touch and proprioception in the right leg and foot, with no change in pain or temperature sense in the body or face. This area of loss resulted because the ascending medial lemniscal 1° afferent axons from coccygeal to lower thoracic levels were cut off from the brain stem, and the information they carried could not be sent on to the thalamus and cortex. In contrast, a stroke affecting the posterior paracentral lobule would produce sensory deficits in discriminative touch, proprioception and sharp pricking pain contralateral to the site of stroke. However such a stroke would affect other pain, temperature and crude touch sensations less than a large spinal cord lesion because these somatic sensations are represented in diffuse areas of the cortex.
Test Your Knowledge Make the best match between the 1° somatosensory axon type and the sensations carried by the axon.
The neospinothalamic tract crosses the midline in which of the following structures?
The neospinothalamic tract crosses the midline in which of the following structures?
The neospinothalamic tract crosses the midline in which of the following structures?
The neospinothalamic tract crosses the midline in which of the following structures?
The neospinothalamic tract crosses the midline in which of the following structures?
The neospinothalamic tract crosses the midline in which of the following structures?
The medial lemniscus crosses the midline at which level of the nervous system?
The medial lemniscus crosses the midline at which level of the nervous system?
The medial lemniscus crosses the midline at which level of the nervous system?
The medial lemniscus crosses the midline at which level of the nervous system?
The medial lemniscus crosses the midline at which level of the nervous system?
The medial lemniscus crosses the midline at which level of the nervous system?
What system transmits sensory information to the brain?The spinal cord is a vital aspect of the CNS found within the vertebral column. The purpose of the spinal cord is to send motor commands from the brain to the peripheral body as well as to relay sensory information from the sensory organs to the brain.
Which nervous system transmits sensory information to the brain and spinal cord?The somatic system includes the nerves that transmit motor and sensory information to and from the CNS.
Which body system sends and receives messages to the brain spinal cord?The nervous system uses tiny cells called neurons (NEW-ronz) to send messages back and forth from the brain, through the spinal cord, to the nerves throughout the body.
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