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NBIO 401

Fall 2009

Somatosensory Tracts & Maps

Class 7 ­ Wednesday, October 14, 2009 Robinson

Objectives - Be able to describe the pathways for each of the three types of somatosensory information, from primary afferents to the pathway destination in the cerebral cortex or cerebellum. You should also be able to explain where the pathway starts, where it synapses, and where fibers cross the midline. The three types of somatosensory information are: 1) touch and conscious proprioception 2) pain & temperature 3) unconscious proprioception - Be able to describe the somatotopic organization of somatosensory information in: 1) the dorsal columns, 2) the spinothalamic tract, 3) the VP thalamus, 4) the somatosensory cortex. - Be able to describe the relationship between particular somatosensory regions of thalamus and somatosensory regions in the cerebral cortex. - Know and be able to explain the difference in physiology of neurons in S1 and higher somatosensory cortical areas. I. Overview Mechanical and chemical receptors transduce a variety of signals. These include touch, temperature, mechanical and chemical pain, and the mechanical state (positions, movements, and forces) of the musculoskeletal system. This last type of signal, i.e., about the state of the musculoskeletal system, is called proprioception, or literally, selfsensing, because it tells you about the mechanical state of your body. Together all of these signals constitute somatosensory information. The somatosensory system is composed of three sub-systems that are distinct both functionally and anatomically. They are: 1) conscious discriminative touch and proprioception ­ Conscious touch is the sensation that allows you to, for example, know if something touching your skin is big or small or if it is rough or smooth. Conscious proprioception is the sensation that allows you to know and describe the position of your limbs and body in the dark. 2) conscious pain and temperature ­ conscious pain tells you about aversive stimuli that have damaged, or may damage, your body. Conscious temperature allows you to feel and describe the level of heat or cold on or near your body. 3) unconscious proprioception ­ you are completely unaware of this information but it is extremely important in making your movements smooth and accurate.

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Each of these three types of somatosensory information come from every part of the body. Below we divide the body into three large parts: 1) the lower body, i.e., leg and lower trunk 2) the upper body, i.e., arm and upper trunk, neck, and back of the head 3) head, i.e., the top of the head and face We will review, and you should learn, the anatomical pathway via which each of the three types of somatosensory information travels from each of these three parts of the body to its destination inside the CNS. Thus, you will learn the information that fits into each cell of a 3 X 3 matrix like the one to the right.

conscious touch & proprioception pain & temperature unconscious proprioception

Note the rightmost cell in the top row, for the head, is subdivided into two parts. This is because unconscious proprioceptive information from the head arises from two main sources, the muscles of the face and the muscles of the jaw. Signals from these two sources take separate paths to the cerebellum. Below we describe the somatosensory pathways represented by each cell in this matrix. For each pathway we describe, in order, the pathway's 1st, 2nd, & 3rd neuron. II. Conscious Touch and Proprioception to A. Lower body & upper body dorsal column nuclei 1. 1st NEURON (periphery to DRG Neuron dorsal column nuclei) - Axons carrying touch and proprioceptive signals from receptors in the lower and upper from receptor body are major components of axon on right the dorsal roots entering the branch arm spinal cord. The cell bodies for enters axon dorsal these axons are in the dorsal branches to columns root ganglion (DRG) just outside motoneurons & interneurons the cord. Central to the DRG axons enter the cord and branch. a. Some branches synapse on interneurons or motoneurons in the ipsilateral gray matter. These branches terminate in the same spinal segment where the axon entered and also in segments rostral and caudal to where the axon entered. These branches mediate reflexes and combine with descending information. b. Another branch of the incoming axon enters the large fiber tracts in the dorsomedial part of the cord, the dorsal columns.

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i. As the picture above shows, the arrangement of axons in the dorsal columns represents the arrangement of the receptors that the peripheral end of the axon contacts. ii. Axons that enter the cord at successively more rostral levels lay progressively more laterally in the dorsal columns. Thus, in the upper cervical cord, afferents from the sacral regions lie closest to the midline, and those from the neck lie most laterally. iii. The figure above also shows that by mid-thoracic segments, dorsal column fibers are divided into two clear bundles, or fasciculi. The more medial bundle, carrying information from the sacral, lumbar, and lower thoracic segments, is called the gracile fasciculus. The more lateral bundle, carrying information from the upper thoracic and cervical segments, is called the cuneate fasciculus. iv. The relative spatial position of receptors is preserved throughout the somatosensory system. This type of organization, i.e., an orderly representation inside the CNS of the spatial relationship of peripheral receptors is called a topographic, or somatotopic organization, and is exhibited in most sensory, motor, and associative systems. c. The axons in the gracile and cuneate fasciculi terminate on neurons in nuclei at the top of the dorsal columns. Axons in the gracile fasciculus terminate in gracile nucleus. Those in the cuneate fasciculus terminate in the cuneate nucleus. Together the gracile and cuneate nuclei are called the dorsal column nuclei. They are in the dorsal part of the caudal medulla.

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2. 2nd NEURON (dorsal column nuclei to VP nucleus of the thalamus) ­ As the red (gracile) and green (cuneate) neurons in the picture below illustrate, the axons of neurons in the dorsal column nuclei travel ventrally and medially from the nuclei. The bundle in which these travel immediately after leaving the nuclei is called the internal arcuate fasciculus (not labeled below). Ventral and medial to the dorsal column nuclei these axons cross the midline and form a large fiber bundle called the medial lemniscus.

a. The medial lemniscus ascends through the medulla, pons, and midbrain. b. The medial lemniscus is broad and narrow, like a ribbon. Its long axis is oriented dorsal-ventral in the medulla but medial-lateral in the pons. c. Cuneate (upper body) fibers are dorsal to gracile (lower body) fibers in the medulla and medial to them in the pons. 3. The axons in the medial lemniscus terminate on neurons in the ventral posterolateral nucleus (VPL) of the thalamus. a. Axons originating in the gracile nucleus (lower body) terminate laterally in VPL. b. Axons originating in the cuneate nucleus (upper body) terminate medially in VPL. c. Axons originating in the sensory trigeminal nucleus (face), which we describe below, terminate more medially still, in the nucleus just medial to VPL, the ventral posteriomedial (VPM) nucleus.

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3. 3rd NEURON (VPL thalamus to cerebral cortex) ­ As the red and green neurons in the picture above illustrate, neurons in VPL that receive input from the gracile (lower body) and cuneate (upper body) nuclei send their axons to terminate in the cerebral cortex. a. Axons carrying signals from the lower body terminate in the dorsal and medial parts of the cortex. b. Those carrying signals from the upper body terminate more laterally and ventrally. c. The axons of blue neurons in VPM, carrying signals from the face, terminate more laterally still. d. This orderly arrangement results in an organized somatotopy, i.e., map of the body surface in somatosensory cortex, with lower body represented dorsal and medial and upper body represented ventrally and laterally. We will describe somatosensory cortex in more detail below. B. Face 1. 1st NEURON (peripheral receptor to main sensory trigeminal nucleus) ­ Axons carrying touch and proprioceptive information from receptors in the face enter the brain via the trigeminal nerve. The cell bodies of these neurons are in the trigeminal ganglia outside the brain. These ganglia are like the DRGs of the cord because they contain the cell bodies of primary somatosensory axons. a. After entering the brain, these axons branch. Some branches terminate in the motor trigeminal nucleus to mediate reflexes. b. Another branch terminates on neurons in the ipsilateral main sensory trigeminal nucleus. This nucleus is analogous to the dorsal column nuclei because it receives input from primary sensory axons.

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2. 2nd NEURON (sensory trigeminal nucleus to VPM thalamus) ­ The drawing below represents the brainstem as viewed from above. The cerebellum has been removed. a. It illustrates that the axons of neurons in the main sensory trigeminal nucleus travel medially from the nucleus. They cross the midline and join the ascending fibers in the medial lemniscus. b. Trigeminal fibers join the medial lemniscus at the mid pons level and form its most medial part. Thus, there is an orderly array of fibers in the medial lemniscus with lower body represented most laterally and face most medially. c. Like the other fibers in the medial lemniscus, those originating in the sensory trigeminal nucleus terminate in the thalamus. Trigeminal fibers terminate in the VPM nucleus as illustrated above. rd 3. 3 NEURON (VPM nucleus of the thalamus to cerebral cortex) ­ As the picture on the previous page shows, the axons of neurons in the VPM nucleus of the thalamus send their axons to the ventrolateral part of primary somatosensory cortex. III. Pain and Temperature A. Lower body & upper body 1. 1st NEURON (periphery to dorsal horn of spinal gray matter) - Axons carrying pain and temperature signals from receptors in the lower and upper body are a major component of the dorsal roots entering the spinal cord. The cell bodies of these neurons are in the DRG. These incoming axons branch. a. Some branches ascend and descend to other spinal levels. As the picture to the right illustrates axon branches reach other spinal segments via a tract in the most dorsal and lateral part of the dorsal horn called Lissauer's tract. These branches mediate pain-related reflexes such as withdrawal. b. After axons carrying pain and temperature signals have entered the cord via the dorsal root, they terminate on neurons in the dorsal horn of the spinal gray matter.

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2. 2nd NEURON (from dorsal horn of the spinal gray matter to the contralateral VPL thalamus) ­ As the picture on the previous page shows, the neuron in the dorsal horn that receives input from primary pain & temperature neurons sends its axon ventrally and medially. That axon crosses the midline and enters the lateral spinothalamic tract in the ventrolateral part of the spinal white matter. a. There is a somatotopic arrangement of fibers in the lateral spinothalamic tract in the spinal cord. The first fibers that enter the tract at the sacral level of the cord are in the most dorsolateral part of the tract. As fibers from more rostral spinal segments join the tract, they occupy successively more ventromedial positions. The picture on the previous page illustrates this arrangement broadly by showing the fibers carrying pain and temperature signals from the lower body in the dorsolateral part of the tract and those for the upper body in the ventromedial MEDIAL part of the tract. LEMNISCUS b. As the picture to the right shows, on its SPINOTHALAMIC TRACT route from the spinal cord to the VPL thalamus, the spinothalamic tract occupies different parts of sections through the medulla and pons. In the INFERIOR medulla it is lateral to the dorsal part of OLIVE the inferior olive. In the pons it is lateral to PYRAMIDS MID the, now medial lemniscus, oriented at MEDULLA this level with its long axis oriented mostly MEDIAL medially and laterally. LEMNISCUS SPINOTHALAMIC TRACT c. The axons in the spinothalamic tract terminate in the VPL nucleus of the thalamus in the somatotopic map established by incoming medial lemniscus fibers. (See the picture on page 4). PYRAMIDAL TRACT 3. 3rd NEURON (from the VPL nucleus of the MID thalamus to the somatosensory cortex) ­ PONTINE PONS NUCLEI VPL neurons that receive pain and temperature input from the spinothalamic tract send their axons to the somatosensory region of the cerebral cortex. This projections follows the same somatotopic pattern as that of the conscious touch and proprioception pathway. (See the picture on page 5). B. Face 1. 1st NEURON (from periphery to the spinal trigeminal nucleus) - Axons carrying pain and temperature information from receptors in the face enter the brain via the trigeminal nerve. The cell bodies of these neurons are in the trigeminal ganglia outside the brain. These ganglia are like the DRGs of the cord because they are they contain the cell bodies of primary somatosensory axons. a. After entering the brain, these axons branch. Some branches terminate in the facial nucleus to mediate pain-related reflexes.

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b. Another branch descends in a large bundle of similar fibers in the dorsolateral part of the brainstem, adjacent and just lateral to the spinal trigeminal nucleus. This bundle is the spinal trigeminal tract. This tract is analogous to Lissauer's tract because it carries a branch of a primary pain and temperature axon to another rostralcaudal level of the CNS. c. These descending fibers terminate in the caudal half of the adjacent spinal trigeminal nucleus. This nucleus is analogous to the region of the dorsal horn of the spinal cord that receives input from primary pain and temperature axons. Like that part of the cord's gray matter, the spinal trigeminal nucleus receives input from primary sensory axons. d. As the picture above illustrates, approximately the caudal half of the spinal trigeminal nucleus represents pain and temperature information from the face. The region just rostral to that represents pain and temperature in the teeth. The region rostral to that represents unconscious proprioception from the facial muscles. We will discuss this pathway in the next section. e. There is a somatotopic arrangement of pain and temperature information from the face in the caudal part of the spinal trigeminal nucleus. The face is represented upside down with the forehead and upper part of the face are represented most caudally and the chin and lower part of the face represented most rostrally. f. The spinal trigeminal tract and nucleus extend caudally all of the way to the spinal cord. Their location in the cross sections of the pons and medulla are illustrated in the picture to the right. INFERIOR OLIVE g. At the junction of the caudal medulla and rostral spinal cord the spinal trigeminal tract is PYRAMIDS CAUDAL continuous with Lissauer's Tract. The spinal MEDULLA trigeminal nucleus is continuous with the dorsal horn of the spinal cord. The long caudal extent of the spinal trigeminal tract and nucleus and their continuity with the cord are PYRAMIDAL the reason that they have the word "spinal" in TRACT their names.

CAUDAL PONS

PONTINE NUCLEI

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2. 2nd NEURON (from the spinal trigeminal nucleus to the VPM nucleus of the thalamus) ­ As the picture on the top of page 8 shows, the axons of neurons in the caudal two parts of the spinal trigeminal nucleus (caudal - face; middle ­ teeth) exit the nucleus and travel almost directly medially. They cross the midline and join the ascending spinothalamic tract. a. There is a somatotopic arrangement of fibers in the tract above the mid pons such that the foot is represented most laterally and the chin and teeth are represented most medially. b. These fibers travel to the VPM thalamus and terminate on neurons there in the same somatotopic arrangement as exhibited by the conscious touch and proprioception terminations. (See the picture at the bottom of page 4.) rd 3. 3 NEURON (from the VPM thalamus to the somatosensory cortex) ­ VPM neurons receive pain and temperature signals via axons originating in the contralateral spinal trigeminal nucleus. These VPM neurons, in turn, send their axons to the ventrolateral part of somatosensory cortex fitting into the same somatotopy exhibited by fibers carrying touch and conscious proprioception signals. (See the picture at the top of page 5.) C Other Targets of Axons Carrying Pain & Temperature Signals 1. Secondary axons carrying pain and temperature signals from the body (cell bodies in the dorsal horn of the cord) and face (cell bodies in the spinal trigeminal nucleus) terminate in the reticular formation of the medulla, pons, and midbrain. a. These projections mediate the changes in alertness and general arousal caused by pain & temperature stimulation. b. Many people think that the name "spinothalamic tract" is misleading because secondary pain & temperature fibers terminate in structures other than the thalamus (as well as also terminating in the thalamus). In this strict sense, it is more appropriate to call this tract the "anteriolateral tract" because of its location in the spinal white matter. 2. There is no direct pain & temperature input to the cerebellum. IV. Unconscious Proprioception A. Lower Body 1. 1st NEURON (from the periphery in the leg and lower trunk to Clarke's column in the spinal gray matter) ­ Axons carrying proprioceptive information from the lower body enter the sacral, lumbar, and lower thoracic segments of the cord via the dorsal roots. The cell bodies for these axons are in the DRG. These are the same axons and cell bodies that carry conscious proprioception. a. After entering the cord these axons branch with one branch going to the dorsal columns. b. Another branch runs ventrally and medially to a group of large neurons in the dorsomedial part of the spinal gray matter. These

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neurons stain deeply with Nissl stains. Together these neurons are called Clarke's column, a cigar-shaped column of neurons extending the length of the thoracic cord. c. CLARKE'S COLUMN EXISTS ONLY IN THE THORACIC CORD. (Remember this.........really.) d. Axons carrying proprioceptive information into the sacral and lumbar segments of the cord ascend to the thoracic segments in the dorsal columns. Once they have reached the thoracic segments they branch and collaterals exit the dorsal columns and terminate on Clarke's column neurons. 2. 2nd NEURON (from Clarke's Column to the cerebellum) ­ The axons of Clarke's column neurons run laterally to the dorsolateral edge of the spinal white matter where they join the dorsal spinocerebellar tract (DSCT). a. Clarke's Column axons in the DSCT travel up the length of the cord and enter the cerebellum via the inferior cerebellar peduncle, a large fiber tract carrying many types of fibers into the caudal side of the cerebellum. We will describe the inferior peduncle in more detail in a little over 2 weeks in the cerebellar anatomy lecture on October 27. b. Once they enter the cerebellum fibers from the DSCT terminate in several places in the cerebellar cortex but primarily in the anterior lobe. These fibers terminate as mossy fibers, one of two types of input to the cerebellar cortex. Again, we will cover this and other inputs to the cerebellum in more detail in the upcoming cerebellar anatomy lecture. B. Upper Body 1. 1st NEURON (from the periphery to the external cuneate nucleus) ­ Axons carrying proprioceptive signals from the arm, neck, and upper trunk enter the cervical cord via dorsal roots. As usual, the cell bodies for these neurons are in the DRGs for these roots. After entering the cord these fibers branch. a. A branch enters the dorsal columns. b. This branch ascends to the level of the cuneate nucleus. There it branches again. One branch terminates on neurons in the cuneate nucleus. The other terminates in the external cuneate nucleus which is in the caudal medulla just lateral to the cuneate nucleus. (It is also called the lateral cuneate nucleus or accessory cuneate nucleus). Neurons in the external cuneate nucleus are large and stain densely with Nissl stains, like the cells in Clarke's column. nd 2. 2 NEURON (from the external cuneate nucleus to the cerebellum) ­ Neurons in the external cuneate nucleus send their axons ventrally and laterally to the DSCT where they join the fibers from the lower body and travel into the cerebellum via

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the inferior cerebellar peduncle and terminate as mossy fibers mostly in the anterior lobe. a. Above the external cuneate nucleus the DSCT contains axons transmitting proprioceptive signals from both the lower and upper body. As it travels rostrally, it runs into and becomes the major component of the inferior cerebellar peduncle. b. The external cuneate nucleus is analogous to Clarke's Column for the upper body. C. Face, part 1: Facial Muscles 1. 1st NEURON (from periphery to rostral part of the spinal trigeminal nucleus) ­ Axons carrying signals from receptors in facial muscles enter the brain in the trigeminal nerve. The cell bodies for these axons is in the trigeminal ganglia. The central process of these axons may branch after they enter the brain. a. If so, one branch probably enters the facial nucleus, which contains the motoneurons for facial muscles. If it exists, this branch would mediate stretch reflexes for facial muscles. Still, we currently know little about these reflexes or the putative projection from the trigeminal nerve that would carry the sensory signals to drive them. b. As the picture above and to the right illustrates, another branch enters the rostral part of the ipsilateral spinal trigeminal nucleus. This part of the trigeminal nucleus is distinguished by the large neurons there that stain deeply with Nissl stains. Thus, in these characteristics, these neurons resemble those in Clarke's column and the external cuneate nucleus. 2. 2nd NEURON (from the rostral spinal trigeminal nucleus to the cerebellum) ­ The axons of the large neurons in the rostral part of the spinal trigeminal nucleus send their axons into the cerebellum via the inferior cerebellar peduncle where they join the fibers carrying signals for unconscious proprioception from the lower (Clarke's column) and upper (external cuneate nucleus). The picture above and to the right show this projection for the left spinal trigeminal nucleus.

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D. Face, part 2: Jaw Muscles 1. 1st NEURON (from periphery to mesencephalic trigeminal nucleus and THEN to motor trigeminal nucleus and cerebellum) ­ Axons carrying signals from receptors in the jaw muscles enter the brain in the trigeminal nerve. The cell bodies for these axons are NOT, repeat NOT, in the trigeminal ganglia. They are in the mesencephalic trigeminal nucleus. THESE ARE THE ONLY PRIMARY SENSORY NEURONS INSIDE THE CNS. All other primary sensory neurons are in ganglia outside the CNS, e.g., in the DRG or the trigeminal ganglia. The central process of these neurons (i.e., the process on the other side of the cell body from the process that carries information from the periphery) branches. a. One branch goes to the ipsilateral motor trigeminal nucleus. This branch mediates stretch reflexesfor jaw muscles. b. Another branch enters the cerebellum, probalby via the inferior cerebellar peduncle. We know that the mesencephalic trigeminal nucleus projects to the cerebellum because when we inject HRP into the cerebellum we see filled cells in the mesencephalic nucleus. Still the pathway that these axons travel is not well described. USEFUL GENERALIZATIONS: 1) With one exception, all somatosensory pathways to the cerebral cortex contain 3 neurons. The first is outside the CNS in a ganglion, (DRG or trigeminal). The second is inside the CNS and its axon crosses the midline. The third enters the cerebral cortex contralateral to the side of the pathway's origin in the periphery. 2) The exception to 1 above rules is the pathway carrying unconscious proprioception signals to the cerebellum. With one exception it contains two neurons. The first is outside the CNS in a ganglion (DRG or trigeminal). The second is in the CNS. Its axon does NOT cross the midline but enters the cerebellum on the same side as the pathway's origin in the periphery. 3) The exception to 2 above is the unconscious proprioception for the jaw muscles. This pathway, at least the shortest version of it, contains only one neuron whose cell body is in the mesencephalic trigeminal nucleus.

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V. Sensory Processing in the Somatosensory System ­ The one or two relay neurons in somatosensory pathways provides an opportunity for processing, or changing, ascending signals. A. Examples of information processing in "relay" nuclei. 1. Control of pain in the dorsal horn ­ sensory gating a. Non-nociceptive primary afferents can inhibit the firing of dorsal horn neurons in the pain pathway, possibly by activating local (i.e., also in the dorsal horn) inhibitory interneurons. b. Descending pathways from the brainstem also regulate pain signals in the dorsal horn. i. Electrical stimulation in the periaqueductal gray (PAG) or raphe nucleus produces strong analgesia, without affecting other sensory modalities, by inhibiting nociceptive neurons in the dorsal horn. ii. This regulation is mediated by the action of opioid peptides. Nociceptive neurons in the dorsal horn, and neurons in the PAG and nucleus raphe, are responsive to the actions of endogenous (enkephalins, endorphins, and dynorphins) and nonendogenous (morphine, codeine) opioids. iii. These painsuppression pathways are activated in various behavioral contexts (e.g., stress, concentration) by the hypothalamus and cerebral cortex.

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2. Local processing in pre-cerebellar nuclei ­ sensory convergence a. Clarke's column and external cuneate nucleus process proprioceptive information before it reaches the cerebellum. b. For example, the activity of single primary afferents from proprioceptors is related to features of individual muscles or joints (e.g., length, force, angle of rotation). However, the activity of many neurons in Clarke's column and the external cuneate nucleus is better correlated with movement of the entire hindlimb relative to the body than with any single muscle or joint feature. This is due to the convergence of information from multiple receptors throughout the limb onto these neurons. The combined information must be more "useful" for the cerebellum. For example, in the graph to the right, the top trace shows the angle of a monkey's shoulder as the monkey tracks a target with its arm movements. The bottom trace shows the firing rate of a single neuron in the ipsilateral external cuneate nucleus during this movement. Note how accurately the neurons firing rate represents the movement. c. Note: Although somatotopic maps are preserved through multi-synaptic pathways, they may not represent the same information. 3. Regulation of ascending transmission in thalamus ­ state dependence a. Neurons in VP and other thalamic nuclei can fire in either burst or transmission mode. b. When VP neurons are hyperpolarized by inhibitory inputs from the reticular nucleus of the thalamus, a voltage-gated calcium channel is activated and the neurons fire together in rhythmic bursts. In this mode, firing rate is independent of synaptic input and sensory information is not relayed to the cerebral cortex.

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c. When the reticular input is suppressed, the calcium channels are inactivated and the VP neurons generate action potentials in patterns that reflect the medial lemniscal input. d. Switching between burst and transmission modes is controlled by projections to the thalamic reticular nucleus from brainstem and basal forebrain regions that regulate the level of arousal and sleep. VI. Somatosensory Cortex A. VP thalamus projects to the primary somatosensory cortex, or S1, located in the postcentral gyrus of the parietal lobe. 1. Note: no sensory information, of any modality, reaches the cerebral cortex without first synapsing in the thalamus. 2. Note: S1, and all other primary sensory cortical areas, do not receive sensory information from more than one sensory modality. B. Somatosensory perception 1. Lesions of S1 impair tactile object discrimination and recognition, two-point tactile discrimination thresholds, and localization of pain. 2. Perception of pain and some aspects of tactile discrimination remain intact after lesions to S1, suggesting a significant overlap of function with other cerebral cortical areas. C. Sensory maps 1. The orderly projection of somatotopically organized thalamocortical axons produces sensory maps, or homunculi, of the contralateral body in S1. 2. Toes are most medial, deep within the interhemispheric fissure, and tongue and throat are most lateral. 3. Portions of the body are not represented in the sensory map in proportion to body mass: the fingers, lips and tongue have the largest representations (~100 times more cortical territory is devoted to a sq. cm of skin on the finger than on the abdomen). This is due to the higher receptor density in the skin of the fingers and lips, and consequently better two-point discrimination, than in the skin of the back and abdomen. 4. S1 neurons respond to simple somatosensory stimuli, like movement of a single joint or touch on a small region of skin.

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D. Higher order sensory processing 1. S1 projects to secondary somatosensory cortex, S2, on the superior bank of the lateral fissure, and areas in the posterior parietal cortex. 2. Neurons in higher-order somatosensory areas respond to complex features of a stimulus, such as surfaces of specific shapes or textures, movement of tactile stimuli across the skin in a specific direction, or manipulation of an object under visual inspection.

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