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. 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.
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.
They do not carry sharp "fast" pain information. Reviewed and revised 07 Oct Vibration B. Static muscle stretch C. Dull, burning pain D. Isotonic muscle contraction E. Sharp "fast" pain F. Dynamic muscle stretch. Anterior white commissure B. Internal arcuate fibers C. Spinal trigeminal tract D. Dorsal columns E. Medial lemniscus. The neospinothalamic tract crosses the midline in which of the following structures? Spinal cord B. Medulla C. Pons D. Mesencephalon E. Skip to main content. Sensory Systems.
Search for:. Sensory Processes. Reception Reception is the first step in the processing of sensation and is dependent on the receptor type, stimulus, and receptive field. Learning Objectives Explain the process of sensory reception. Key Takeaways Key Points Reception is the process of activating a sensory receptor by a stimuli. Sensory transduction is the process of converting that sensory signal to an electrical signal in the sensory neuron.
The process of reception is dependent on the stimuli itself, the type of receptor, receptor specificity, and the receptive field, which can vary depending on the receptor type.
Key Terms somatosensation : general senses which respond to stimuli like temperature, pain, pressure, and vibration reception : the act or ability to receive signals from stimuli. Transduction and Perception Transduction is the process that converts a sensory signal to an electrical signal to be processed in a specialized area in the brain. Learning Objectives Explain how stimuli are converted to signals that are carried to the central nervous system.
Key Takeaways Key Points Sensory signals are converted to electrical signals via depolarization of sensory neuron membranes upon stimulus of the receptor, which causes opening of gated ion channels that cause the membrane potential to reach its threshold.
The receptor potentials are classified as graded potentials; the magnitude of these potentials is dependent on the strength of the stimulus. The sensory system shows receptor specificity; although stimuli can be combined in processing regions of the brain, a specific receptor will only be activated by its specific stimulus. The four major components of encoding and transmitting sensory information include: the type of stimulus, the stimulus location within the receptive field, the duration, and the intensity of the stimulus.
Key Terms membrane potential : the difference in electrical potential across the enclosing membrane of a cell action potential : a short term change in the electrical potential that travels along a cell transduction : the translation of a sensory signal in the sensory system to an electrical signal in the nervous system.
Licenses and Attributions. CC licensed content, Shared previously. Unilateral destruction of SI produces severe deficits in all aspects of discriminative touch and proprioception on the contralesional side of the body. 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. 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.
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. The somatosensory association cortex is located in the superior parietal lobe a. 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. 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. 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.
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 following section should help you determine how well you can utilize what you have learned thus far about the somatosensory 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. The motor losses may be severe i. 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. 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. Pathway s Affected: You conclude that structures in the following somatosensory pathways Figure 5. 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. 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.
The area of the body innervated by a posterior root is called a dermatome Figure 5. 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.
T4 for the fourth thoracic root. A given dermatome e. T4 posterior root. The symptoms produced by cranial nerve root damage depend upon the cranial nerve involved. 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. 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.
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. 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.
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. 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.
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.
The patient suffers from loss of discriminative touch and proprioception i. 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.
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. All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex.
Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory as well as motor signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex Figure How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems; however, there are some individual differences.
A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ. It is easy to differentiate between a one-pound bag of rice and a two-pound bag of rice.
There is a one-pound difference, and one bag is twice as heavy as the other. However, would it be as easy to differentiate between a and a pound bag? Question: What is the smallest detectible weight difference between a one-pound bag of rice and a larger bag?
What is the smallest detectible difference between a pound bag and a larger bag? In both cases, at what weights are the differences detected? This smallest detectible difference in stimuli is known as the just-noticeable difference JND. You will be testing JND of different weights of rice in bags. Choose a convenient increment that is to be stepped through while testing. For example, you could choose 10 percent increments between one and two pounds 1.
So, 20 pounds feels the same as 22 pounds or 23 pounds, but 20 pounds feels less than 24 pounds. Test the hypothesis: Enlist 24 participants, and split them into two groups of To set up the demonstration, assuming a 10 percent increment was selected, have the first group be the one-pound group. As a counter-balancing measure against a systematic error, however, six of the first group will compare one pound to two pounds, and step down in weight 1. Apply the same principle to the pound group 20 to 40, 20 to 38, and so on, and 20 to 22, 20 to 24, and so on.
Given the large difference between 20 and 40 pounds, you may wish to use 30 pounds as your larger weight.
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