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Chapter 6: Somatosensory System Copyright © 2013 by Saunders, an imprint of Elsevier Inc. Introduction Somatosensation is sensory information from the skin and musculoskeletal systems. Information from the skin is superficial (e.g., touch, pain, temperature) or cutaneous. Information from the musculoskeletal system includes proprioception and pain. Information in the somatosensory system proceeds from the receptor through a series of neurons to the brain. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 2 Introduction—cont’d Speed of information processing is determined by the following: Diameter of the axons Degree of axonal myelination Number of synapses in the pathway Distinction is between sensory information (nerve impulses generated from the original stimuli) and sensation (awareness of stimuli from the senses). Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 3 Sensory Receptors Are specialized and respond only to a specific type of stimulus, adequate stimulus, and under normal conditions. Mechanoreceptors are the mechanical deformation of the receptor by touch, pressure, stretch, or vibration. Chemoreceptors are substances released by cells, including damaged cells after injury or infection. Thermoreceptors transmit information regarding heat or cold. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 4 Sensory Receptors: Nociceptors Are a subset of somatosensory receptors. Stimulation results in the sensation of pain. Example: Pressure mechanoreceptors are stimulated by stubbing a toe; the sensation experienced is pain rather than pressure. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 5 Somatosensory Peripheral Neurons Peripheral sensory neurons have two axons. Distal: Conduct messages from the receptor to the cell body. Proximal: Project from the cell body into the spinal cord or brainstem. Afferents are classified according to axon diameter (Ia, Ib, II or A-β, A-δ, C). Larger-diameter axons transmit information faster than smaller-diameter axons. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 6 Cutaneous Innervation Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 7 Cutaneous Innervation—cont’d Receptive fields tend to be smaller distally and larger proximally. Distal regions of the body have a greater density of receptors than proximal areas. Example: The combination of smaller receptive fields and greater density of receptors distally enables us to distinguish between two closely applied stimuli on a fingertip; however, the same stimuli cannot be distinguished on the trunk. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 8 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 9 Cutaneous Innervation—cont’d Touch is categorized as fine or coarse. Fine touch includes a variety of receptors and subsensations. Coarse touch is mediated by free endings throughout the skin. Cutaneous receptors respond to touch, pressure, vibration, stretch, noxious stimuli, and temperature. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 10 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 11 Cutaneous Innervation—cont’d Note differences between nerve root and peripheral nerve Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 12 Muscle Spindles Are the sensory organs in muscle. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 13 Intrafusal and Extrafusal Fibers Intrafusal fibers are contractile only at the ends; the central region cannot contract. Two types of intrafusal fibers include: Nuclear bag fibers, which have clumps of nuclei. Nuclear chain fibers, which have nuclei arranged in single file. Two different sensory endings include: Primary endings of type Ia neurons wrap around the central region of each intrafusal fiber. Secondary endings of type II afferents end mainly on nuclear chain fibers adjacent to the primary endings. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 14 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 15 Golgi Tendon Organs Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 16 Joint Receptors Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 17 Joint Receptors—cont’d Muscle spindles respond to quick and prolonged stretches of the muscle. Tendon organs signal the force generated by the muscle contraction or by a passive stretch of the tendon. Joint receptors respond to mechanical deformation of joint capsules and ligaments. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 18 Summary: Function of Different-Diameter Axons Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 19 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 20 Pathways to the Brain Are important distinctions among the types of pathways; fidelity of information is conveyed. Have high-fidelity transmission that provides accurate details regarding the location of the stimulation. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 21 Pathways to the Brain—cont’d When describing pathways in the nervous system, only the neurons with long axons that connect distant regions of the nervous system (projection neurons) are counted. A tract is the bundle of axons with the same origin and a common termination. Somatosensory pathways are often named for the origin and termination of the tract that contains the second neuron in the series. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 22 Pathways to the Brain—cont’d Three types of pathways bring sensory information to the brain: Conscious relay Divergent (diffuse systems) Unconscious relay Table 6-2 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 23 Conscious Relay Pathways Transmit information to many locations in the brainstem and cerebrum and use pathways with varying numbers of neurons (though usually three) Information in conscious relay pathways is transmitted with high fidelity. Information in these pathways allows individuals to make fine distinctions about stimuli. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 24 Divergent Pathways Information is transmitted to many locations in the brainstem and cerebrum and use pathways with varying numbers of neurons. Sensory information is used at both the conscious and unconscious levels. Aching pain is a form of sensation that is transmitted via divergent pathways in the central nervous system. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 25 Unconscious Relay Pathways Unconscious proprioceptive and other movement-related information is carried to the cerebellum. Does not reach consciousness Information plays an essential role in automatic adjustments of our movements and posture. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 26 Conscious Relay Pathways to the Cerebral Cortex All four types of somatosensation reach conscious awareness. Touch Proprioception Temperature Pain Pathways to consciousness travel upward in the spinal cord via two routes: Dorsal columns Anterolateral tracts Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 27 Rules for Conscious Relay Pathways to the Cerebral Cortex 1. There are 3 neurons in the pathway 1. 2. Only count neurons with long axons The first neuron is in the peripheral nervous system 1. 2. DRG Trigeminal ganglia Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 28 3. The second neuron is in the segmental nervous system 1. spinal cord 2. Brainstem 4. The axon of the second neuron crosses the midline 1. DC-ML: Caudal medulla 2. ALS: Spinal cord Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 29 5. The third neuron is located in the thalamus 1. VPL for the body 2. VPM for the head 6. The third neuron terminates in a specific region of somatosensory cortex (post-central gyrus) Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 30 Discriminative Touch and Conscious Proprioception Discriminative touch is the localization of touch and vibration and the ability to discriminate between two closely spaced points touching the skin. Conscious proprioception is the awareness of the movements and relative position of body parts. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 31 Discriminative Touch and Conscious Proprioception Stereognosis: the ability to use touch and proprioceptive information to identify an object; for example, a key in the hand can be identified without vision. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 32 Discriminative Touch and Conscious Proprioception Pathways for discriminative touch and conscious proprioception use a three-neuron relay. Primary conveys information from the receptors to the medulla. Secondary conveys information from the medulla to the thalamus. Tertiary conveys information from the thalamus to the cerebral cortex. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 33 Discriminative Touch and Conscious Proprioception Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 34 Dorsal Column/Medial Lemniscus System Primary neurons include: Many collateral branches entering the gray matter of the spinal cord • Some collaterals that contribute to motor control, some that influence activity in neurons in other sensory systems, and others that influence autonomic regulation Axons in gracile fascicle (<T6) or cuneate fascicle (>T6) Secondary neurons include: Cell bodies located in the nucleus gracilis or cuneatus Axons that cross the midline as the internal arcuate fibers, then ascend to the thalamus Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 35 Dorsal Column/ Medial Lemniscus System Third-order neurons include: Those that connect the thalamus to the sensory cortex Those that have axons forming part of the thalamocortical radiations, which are fibers connecting the thalamus to the cerebral cortex Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 36 Discriminative Touch Information from the Face Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 37 Somatotopic Arrangement of Information Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 38 Somatosensory Areas of the Cerebral Cortex Primary sensory cortex discriminates among the size, texture, or shape of objects. What is the object like? Somatosensory association areas analyze the information from the primary sensory area and the thalamus and provide stereognosis and memory of the tactile and spatial environment. What is the object? Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 39 Temperature Sensation Heat and cold are detected by specialized free nerve endings of small myelinated and unmyelinated neurons. A-delta fibers carry impulses produced by cooling. C fibers carry information regarding heat. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 40 Pain Pain is composed of both sensation and the emotional response to the sensation. Nociceptive: receptors or neurons that receive or transmit information about stimuli that damage or threaten to damage tissue. Fast pain (spinothalamic pain) is the initial and immediate sharp sensation that indicates the location of the injury. Slow pain (spinolimbic pain) is the dull, throbbing ache following fast pain that is not well localized. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 41 Fast, Localized Nociception: Lateral System Fast pain uses a three-neuron system: Primary neurons bring information into the dorsal horn of the spinal cord. Axons of secondary neurons cross the midline and project from the spinal cord to the thalamus. Tertiary neurons project from the thalamus to the cerebral cortex. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 42 Fast, Localized System: Lateral System—cont’d Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 43 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 44 Dorsal Column/Medial Lemniscus and Spinothalamic Systems Both systems consist of three-neuron relay pathways. In contrast to the discriminative touch and conscious proprioceptive information, anterolateral white matter contains axons transmitting information about pain, temperature, and coarse touch. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 45 Dorsal Column/Medial Lemniscus and Spinothalamic Systems—cont’d Functions of the dorsal and anterolateral columns are not rigidly segregated; information about nondiscriminative (coarse) touch travels in the anterolateral system, and some pain and temperature information ascends in the dorsal columns. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 46 Fast Pain Information From the Face Travels in the trigeminal nerve. Lesions that interrupt the pathways conveying nociceptive information produce analgesia Analgesia is the absence of pain in response to stimuli that would normally be painful. Crossed analgesia: a single lesion can cause pain sensation to be lost on the side of the face ipsilateral to the lesion and the contralateral side of the body Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 47 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 48 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 49 Fast Versus Slow Pain When fast pain information reaches the somatosensory cortex, the individual is consciously aware of sharp pain in a specific location. If tissue damage has occurred, fast pain is followed by a slow, aching pain. Onset of slow pain is later than fast pain because the impulses travel on smaller, unmyelinated axons. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 50 Medial Pain System Medial pain system: divergent ascending network of neurons Uses several pathways with variable numbers of projection neurons, not a threeneuron pathway like fast pain. Activity of the medial pain system elicits affective, motivational, withdrawal, arousal, and autonomic responses. Information is not somatotopically organized, so slow pain cannot be precisely localized. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 51 Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 52 First Neuron First neuron: a small, unmyelinated C fiber Receptors are free nerve endings, sensitive to noxious heat, chemical, or mechanical stimulation Has high-threshold C fiber endings that become sensitized with repeated stimulation. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 53 Ascending Projection Neurons Axons of ascending projection neurons reach the midbrain, reticular formation, and limbic areas via three tracts in the anterolateral spinal cord. Spinomesencephalic Spinoreticular Spinolimbic Tracts are parallel ascending tracts. Only information in the spinolimbic tract is perceived as pain. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 54 Spinomesencephalic Tract Carries nociceptive information to the superior colliculus and periaqueductal gray. Both are in the midbrain • Superior colliculus – tectum (visual map of world) • Periaqueductal gray – around cerebral aqueduct Periaqueductal gray is part of the descending pain control system. Is involved in turning the eyes and head toward the source of noxious input and in activating descending tracts that control pain. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 55 Spinoreticular Tract These ascending neurons synapse in the reticular formation. Reticular formation: A neural network in the brainstem that includes the reticular nuclei and their connections Modulates arousal, attention, and sleep-waking cycles. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 56 Spinolimbic Tract Axons of the spinolimbic tract transmit slow pain information to the medial and intralaminar nuclei in the thalamus. Slow pain pathways provide information that produces automatic movements and autonomic and emotional responses to noxious stimuli. Activity in the spinoreticular and spinolimbic tracts result in arousal, withdrawal, autonomic, and affective responses to pain. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 57 Trigeminoreticulolimbic Pathway Slow pain information is transmitted via this pathway from the face. Although intact sensory and parietal cortex is required for the localization of pain, crude awareness of slow pain can be achieved in many cortical areas and possibly in the thalamus and basal ganglia. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 58 Temperature Information Information is transmitted in phylogenetically older pathways to the following: Reticular formation Nonspecific nuclei of the thalamus Subcortical nuclei Hypothalamus Information that does not reach conscious awareness contributes to arousal, provides gross localization, and contributes to autonomic regulation. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 59 Unconscious Relay Tracts to the Cerebellum Transmit information from proprioceptors and information about activity in spinal interneurons. Relay information critical for adjusting movements. Inadequate proprioceptive input can cause ataxia because the loss of sensory feedback disrupts movement control. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 60 High-Fidelity Pathways Two pathways relay high-fidelity, somatotopically arranged information to the cerebellar cortex. Posterior spinocerebellar pathway • Transmits information from the legs and the lower half of the body. Cuneocerebellar pathway • Begins with primary afferents from the arm and upper half of the body; the central axons travel via the posterior columns to the lower medulla. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 61 Internal Feedback Tracts Tracts monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brainstem: Anterior spinocerebellar tract • Transmits information from the thoracolumbar spinal cord. Rostrospinocerebellar tract • Transmits information from the cervical spinal cord to the ipsilateral cerebellum. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 62 Function of Spinocerebellar Tracts Information in the spinocerebellar tracts comes from the proprioceptors, spinal interneurons, and descending motor pathways. Information, which does not reach conscious awareness, contributes to automatic movements and postural adjustments. Copyright © 2013 by Saunders, an imprint of Elsevier Inc. 63