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Unformatted text preview: Somatosensory System Page 14-1 Unlike other sensory systems we will study in this course, the somatosensory system does not have a single receptor sheet where somatosensory information is transduced. Instead, somatosensory receptors are found throughout our bodies, located on our skin surfaces, in our muscles and joints, and in our internal organs. There are several different types of somatosensory receptors which transduce different modalities of somatosensory sensation. The different receptor types have different mechanisms of transduction, different types of axons, and carry information to the brain by different pathways. Somatosensory receptors Mechanoreceptors in skin (1) Free nerve endings Free nerve endings all look the same anatomically (no specializations) but have four different functions. Two types of free nerve endings are thermoreceptors (transducing changes in temperature) and two other types are nociceptors (transducing pain). (A) "Cold" receptors (i) Respond to sudden decreases of temperature, adapt over the course of a few seconds. Transduction mechanism unknown. (ii) Have small diameter myelinated axons. @ How fast will action potentials travel in these axons? (iii) Send information to the brain via the spinothalamic pathway. (B) "Hot" receptors (i) Respond to sudden increases in temperature, adapt over the course of a few seconds. Transduction mechanism unknown. (ii) Have very small diameter unmyelinated axons. @ How fast will action potentials travel in these axons? (iii) Send information to the brain via the spinothalamic pathway An experiment: We put our right hand in cold water for 10 seconds and at the same time put our left hand in hot water for 10 seconds. Then we put both hands in the same bucket of lukewarm water. @ What do we feel on our left hand? @ What do we feel on our right hand? @ Which sensation do we feel first? (C) "Sharp pain" receptors (i) Respond to stimuli causing a shearing force on the skin such as cuts or bruises. Transduction probably chemical. (ii)Have small diameter myelinated axons. @ How fast will action potentials travel in these axons? (iii) Send information to the brain via the spinothalamic pathway. (D) "Burning pain" receptors (i) Respond to many painful stimuli including damaging heat, sharp blows, and noxious chemicals. Transduction probably chemical. (ii) Have very small diameter unmyelinated axons. @ How fast will action potentials travel in these axons? (iii) Send information to the brain via the spinothalamic pathway. Page 14-2 (2) Mechanoreceptors Mechanoreceptors have specialized anatomically distinct endings which cause them to transduce different kinds of touch stimuli. Some free nerve endings may also respond to touch stimuli, particularly light touch resulting in a sensation of "tickle". (A) Slowly adapting mechanoreceptors (Merkel's nerve complex, located superficially, and Ruffini's end organs, located deep) (i) Respond to changes in skin pressure. Transduction by stretch-sensitive channels. (ii) Have large, myelinated axons. @ How fast will action potentials travel in these axons? (iii) Send information to the brain via the lemniscal pathway. Page 14-3 Mechanoreceptors in skin Meissner's corpuscle Merkel's disk Ruffini's capsule Sweat gland Pacinian's corpuscle (B) Rapidly adapting mechanoreceptors (Meissner's corpuscles, located superficially, and Pacinian corpuscles, located deep). (i) Respond to vibrations or very rapid changes in skin pressure. Transduction by stretch sensitive channels. (ii) Have large, myelinated axons. @ How fast will action potentials travel in these axons? (ii) Send information to the brain via the lemniscal pathway. @ Which mechanoreceptors have a phasic response? @ Which mechanoreceptors have a tonic response? Somatosensory Pathways As we have seen, there are two different somatosensory pathways from the receptors to the somatosensory cortex where somatosensory information is processed. (1) Spinothalamic pathway (Pain and temperature) (i) Cell bodies in the Dorsal Root Ganglion (DRG) just outside of the spinal cord. (ii) Axons enter the cord and synapse ipsilaterally (on the same side) with spinal cord neurons. (iii) These neurons have axons which immediately cross over to the contralateral (opposite) side of the cord and ascend to the brain via the spinothalamic tract. (iv) These neurons synapse onto neurons in the thalamus, and the thalamic neurons project up to primary somatosensory cortex. Spinothalamic pathway Page 14-4 (2) Lemniscal pathway (Touch and vibration) (i) Cell bodies in the Dorsal Root Ganglion (DRG) just outside of the spinal cord. (ii) Axons enter the cord but do not synapse there; instead they continue up the spinal cord ipsilaterally to the medulla where they synapse onto cells in the dorsal column nuclei. (iii) The dorsal column nuclei neurons send axons which cross the midline in the medulla and ascend to the contralateral thalamus. (iv) The thalamic neurons then project to the primary somatosensory cortex. Lemniscal pathway Page 14-5 (3) The somatosensory system (and as we will see later other sensory systems) has parallel pathways carrying different modalities of information. The fact that such parallel pathways differ anatomically may have important clinical significance. @ If a patient received a knife wound cutting through the left half of the spinal cord anywhere above where the nerves serving the foot enter the cord, what sensory deficits would s/he have in his/her left foot? @ What deficits would s/he have in his/her right foot? Somatosensory Cortex (1) A striking feature of the connections of the sensory system is its topographic organization. We talked in the last lecture about the "homunculus" (a little man) or somatotopic map of the body which is found in somatosensory cortex. (2) Remember that certain areas of the somatosensory map are magnified. That is, there are proportionally more cells in somatosensory cortex which respond to stimuli on areas of your body like your fingers or your lips than there are to areas like your arms or your back. Homunculus Page 14-6 Somatosensory cortex (3) This magnification has perceptual consequences, as can be seen in a "two point discrimination task". Take an object like a pair of tweezers which can have either two points, or if you squeeze them together only one point. Close your eyes and touch your finger or your lips with either the one point or the two points. You should be able to tell the difference between the two sensations. If you touch your arm or your back, however, you will most likely not be able to discriminate the difference between one point and two points. If you bend the tweezers so that the two points are very far apart, you should be able to discriminate two points even on the less sensitive skin of your arm or back. (4) Generally the areas of a sensory map which are magnified correspond to areas which are behaviorally important to an animal. Our hands need to be sensitive because we use them to manipulate and explore objects, and our mouths need to be sensitive because we use them for eating and for communication. Other animals may have different portions of their bodies which are particularly important, and these areas will have magnified maps in the animals somatosensory cortex. Page 14-7 (5) We have already talked about the fact that in the human somatosensory map the face and hands are magnified. A similar, though less pronounced pattern is seen in the monkey. The cat has very magnified Rabbit Cat Human Monkey representation of the whiskers, as well as magnified representation of the rest of the face and the paws, while the rabbit representation is very magnified for nose, whiskers and jaws, and not magnified for paws. (6) Cortical somatosensory maps are "plastic", that is they can be modified by experience. In experiments in monkeys where the monkey was trained to use one finger over and over again in a discrimination task, the area of cortex responding to that finger was found to become larger over time. Cortical columns (Columnar organization) (1) In addition to having maps of the receptor sheet across the cortical surface, sensory cortical areas tend to also have the cells arranged in columns with cells within a given column having similar response properties. It is a functional column. There is no discernible anatomical differences between columns. (2) In any given region of somatosensory cortex there are columns of cells with the same receptive field (that is responding to stimulation of the same region of the body) but with different modalities. (3) If we look at a small piece of cortex taken from the thumb region of the map, all cells will have receptive fields on the thumb, but cells will be arranged in touch, vibration, hot and cold columns. White matter Pial surface h v c t ...
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