Also noted along the abscissa at each arrow is the axon diameter in micrometers of axons contributing to the peaks in the whole nerve potential. 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.
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. Somatosensory neurons are topographically i.
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. 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. The posterior dorsal column - medial lemniscal pathway i. 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.
Press PLAY to view the course of the pathway. Click on the structure labels to view their locations in the sections. Click on the label "Cuneate fasciculus" or "Gracile fasciculus" to view the somatotopic organization of the posterior funiculus and the blood supply provided by the posterior spinal artery. Click on the label "Medial lemniscus" to view its somatotopic organization and the blood supply provided by the paramedian branches of the basilar artery. 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 lower part of the body foot and leg are represented in the posterior paracentral lobule, whereas the upper body chest, arm, and hand are represented in the upper postcentral gyrus See Figure 4.
Press PLAY to animate. The flash of light at each synapse represents the release of neurotransmitter by the presynaptic axon terminal. 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. The main sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face Figure 4.
Discriminative touch, vibration and position senses are normal in the waist area and for the rest of the body and face. 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. In syringomyelia , there are cysts that form within the spinal cord near the central canal Figure 5. 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.
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. 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. 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.
Whereas neurons of the spinal trigeminal pathway STP process pain, temperature and crude touch information from the face. Pin pricks into the right side of the face and the left hand do not produce well-localized, sharp pain sensations.
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.
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. 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. 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. Somatosensory Cortex. The sensory loss from head trauma or stroke that damages the somatosensory cortex will.
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.
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. The neurons of the spinal trigeminal pathway STP process all pain, temperature and crude touch information from the face. 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.
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.
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. From this chapter, you should have learned how the somatosensory system is organized from the skin, muscles and joints to the cortex. 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. 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. The posterior parietal lobe is located caudal to the postcentral gyrus and serves as the somatosensory association cortex. The buttock, leg, foot, and genitals are represented in the posterior paracentral lobe, which is located on the medial aspect of the cerebral hemisphere.
This is incorrect, as the posterior funiculus contains first order afferents of the medial lemniscal pathway, which processes discriminative touch and proprioception. The neospinothalamic pathway processes sharp pain sensation from the body and the second order axons of this pathway are in the lateral and anterior funiculi the spinothalamic tract.
This is incorrect, as the first order medial lemniscal afferents do not decussate. Thus, while area 3b acts as a primary area for touch information, that information is then also sent to areas 1 and 2 for more complex processing. Area 1, for example, seems to be important to sensing the texture of an object while area 2 appears to play a role in perceiving size and shape.
Area 2 also is involved with proprioception. Specific lesions to any of these areas of the somatosensory cortex support the roles mentioned above; lesions to area 3b, for example, result in widespread deficits in tactile sensations while lesions to area 1 result in deficits in discriminating the texture of objects. Each of the four areas of the primary somatosensory cortex are arranged such that a particular location in that area receives information from a particular part of the body.
This arrangement is referred to as somatotopic , and the full body is represented in this way in each of the four divisions of the somatosensory cortex.
Because some areas of the body e. Thus, the somatotopic maps found in the somatosensory cortex are distorted such that the highly sensitive areas of the body take up a disproportionate amount of space in them see image to the right. Unconscious proprioception is communicated primarily via the dorsal and ventral spinocerebellar tracts to the cerebellum. An unconscious reaction is seen in the human proprioceptive reflex, or Law of Righting. In the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon.
This is seen even in infants as soon as they gain control of their neck muscles. This control comes from the cerebellum, the part of the brain that affects balance. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of a muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts.
The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles. Muscle spindle : Mammalian muscle spindle showing typical position in a muscle left , neuronal connections in spinal cord middle , and expanded schematic right. The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a primary group Ia afferent and a secondary group II afferent coil around the non-contractile central portions of the intrafusal fibers.
The Golgi organ also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle fibers onto the tendons of skeletal muscle. It provides the sensory component of the Golgi tendon reflex. The Golgi organ should not be confused with the Golgi apparatus—an organelle in the eukaryotic cell —or the Golgi stain, which is a histologic stain for neuron cell bodies.
Golgi tendon organ : The Golgi tendon organ contributes to the Golgi tendon reflex and provides proprioceptive information about joint position. The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system.
In a Golgi tendon reflex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex. Although muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord that supply the muscle are inhibited. However, antagonistic muscles are activated. The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus.
A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary. The first always has its cell body in the dorsal root ganglion of the spinal nerve. Dorsal root ganglion : Sensory nerves of a dorsal root ganglion are depicted entering the spinal cord. The axons of many of these neurons terminate in the thalamus, and others terminate in the reticular activating system or the cerebellum.
In the case of touch and certain types of pain, the third neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe. In the periphery, the somatosensory system detects various stimuli by sensory receptors, such as by mechanoreceptors for tactile sensation and nociceptors for pain sensation.
The sensory information touch, pain, temperature, etc. Generally, there is a correlation between the type of sensory modality detected and the type of afferent neuron involved. For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual touch. In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain.
One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurons of the dorsal column—medial lemniscal pathway and the ventral spinothalamic pathway. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture, are projected to the cerebellum, including the ventral and dorsal spinocerebellar tracts.
Another important target for afferent somatosensory neurons that enter the spinal cord are those neurons involved with local segmental reflexes. Spinal nerve : The formation of the spinal nerve from the dorsal and ventral roots.
The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe. This is the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus at this location. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area.
For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Somatosensory information involved with proprioception and posture also target an entirely different part of the brain, the cerebellum. This is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex; namely, the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body.
The thalamus is a midline symmetrical structure within the brain of vertebrates including humans; it is situated between the cerebral cortex and midbrain, and surrounds the third ventricle. Its function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness.
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