45 Somatosensation November 8 & 11, 2013 Chapter 14: 540 – 544, Chapter 16: Dr. Diane M. Jaworski

46 What sensations does the brain detect? Special senses –Olfaction (smell) –Gustation (taste) –Audition (hearing) –Vestibular (balance) –Vision (sight) Somatosensations – Superficial cutaneous senses: light touch and pressure, vibration, hot, cold, pain, itch, tickle – Deep senses: position sense, pain, intense pressure – Visceral senses: visceral pain, hunger, nausea, thirst

47 Somatosensation utilizes many receptors Somatosenstation = Senses detected throughout body Many different receptor types –Nociceptors (pain) Free nerve endings sense tissue damage, chemical signals, temp extremes –Thermoreceptors Free nerve endings sense temperature –Chemoreceptors Respond to chemicals in interstitial fluid (H +, O 2, CO 2 ) –Mechanoreceptors Superficial cutaneous - highly sensitive fine touch and pressure for specific localization Deep cutaneous - less sensitive crude touch and pressure for less specific localization Baroreceptors detect internal pressure (blood vessels, bladder, GI) Proprioceptors/Joint receptors (monitor limb position)

48 Receptor characteristics Receptive field - part of the body from which the receptor can be stimulated (e.g., area of skin for touch sensation) - Small receptive fields can discriminate finer sensations - When receptors synapse in spinal cord, many receptors converge; therefore, spinal cord neuron receptive fields are much more complex © McKinley et al. A & P 2013

49 Receptor Characteristics Slowly adapting receptors respond throughout the stimulus duration Rapidly adapting receptors only respond at the beginning of the stimulus Adaptation - reduction in sensation with repeated stimulation some receptors adapt whereas others do not Why would you want a receptor to adapt?

Receptor characteristics Amplitude of receptor potential is due to intensity of the stimulus Once of sufficient intensity the receptor fires an action potential Action potential firing rate codes stimulus intensity R Record APs externally Apply rapid pressure to skin to deform mechanoreceptor S - 2 S - 3 S - 1 Receptor Potentials differ from Action Potentials: - Action Potentials are All-or-None - Receptor Potentials are Graded and Summed 50

51 Meissner’s corpuscle - rapidly adapting, low frequency vibration Merkel cell - slowly adapting, pressure Superficial Cutaneous Mechanoreceptors Monitor small areas of skin (small receptive field) to detect very fine sensations

52 Pacinian corpuscle - rapidly adapting, vibration Ruffini ending - slowly adapting, pressure Deep Cutaneous Mechanoreceptors Monitor larger areas of skin (large receptive field) to detect crude sensations

53 Muscle Spindle Muscle Receptor Monitors muscle length and rate of change Organized in parallel with extrafusal muscle fibers

Sensory receptor is associated with intrafusal muscle Requires two types of motor neurons: Alpha motor neurons innervate extrafusal muscle (muscle bulk) Gamma motor neurons innervate intrafusal muscle Both motor neurons must fire simultaneously!! Extrafusal versus Intrafusal Muscle Fibers 54

55 Golgi Tendon Organ (GTO) Joint Receptor Monitors muscle tension Organized in series with extrafusal muscle fibers

56 What happens after a receptor is activated?

57 Ultimately, somatosensory info is transmitted to the Postcentral Gyrus in the Parietal Lobe © McKinley et al. A & P 2013

Mechanoreceptors Joint & muscle receptors Nociceptors (pain) Thermoreceptors (temperature) Chemoreceptors (H+, O2, CO2) Spinocerebellar Cuneocerebellar unconscious sensations Anterolateral (also called Spinothalamic) Pain & Temp The 3 Main Somatosensory Pathways can be understood by receptor types 58 The same proprioceptive info about muscle length & tension is used by two pathways. The dorsal column-medial lemniscus path goes to the cortex unmodified, while the spino/cuneocerebellar pathways go to the cerebellum for fine tuning before going to the cortex. Dorsal Column - Medial Lemniscal conscious sensations

59 Spinocerebellar pathway in spinal cord © McKinley et al. A & P 2013 This pathway provides proprioceptive information (position sense of body) to the cerebellum. “what you are doing”

Dorsal Spinocerebellar pathway Fu nction - Provides cerebellum with info about muscle length and tension (unconscious proprioception) “what am I doing?” Receptors - Muscle spindles and Golgi tendon organs Ascending pathway - Primary afferent neuron synapse within the dorsal horn gray matter - Second order neuron enters ipsilateral cerebellum via inferior cerebellar peduncle - After synapsing in cerebellum, info exits to contralateral red nucleus & thalamus; therefore, INFO FROM THIS PATHWAY CROSSES IN CEREBELLUM!!! 60

61 Dorsal Spinocerebellar pathway © McKinley et al. A & P 2013

62 Cuneocerebellar Pathway Uses the same receptors and conveys the same info as Dorsal Spinocerebellar Pathway except for arm

63 Dorsal Column pathway in spinal cord © McKinley et al. A & P 2013 This pathway also provides proprioceptive information, but the axons go to the thalamus. The unconscious info that went to the cerebellum exits the cerebellum and goes to the thalamus. The thalamus then compares how the cerebellum “fixed” the uncoordinated movement.

Dorsal Column-Medial Lemniscus Pathway Fu nction - Touch (pressure, vibration) - Conscious proprioception - sense of body in space Receptors - Cutaneous mechanoreceptors - Muscle spindles, Golgi tendon organs Ascending pathway - Primary afferent ascends in dorsal white matter WITHOUT SYNAPSING IN SPINAL CORD FIRST SYNAPSE IS IN MEDULLA - leg info synapses in N. gracilis - arm info synapses in N. cuneatus - Second order neuron crosses and synapses in thalamus INFO FROM THIS PATHWAY CROSSES IN MEDULLA!!! 64 “what you are doing”

65 Dorsal Column- Medial Lemniscus Pathway © McKinley et al. A & P 2013

Have you ever felt Pain? Pain has been defined as an “unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” INTERNATIONAL ASSOCIATION FOR THE STUDY OF PAIN Merskey H. Qual Life Res. 1994;3(suppl 1):S69-S76 Does Pain have a Purpose? How do we measure Pain? 66

67 Nociceptive pain “Normal” pain caused by tissue damage

68 Two Anterolateral Pathways exist to convey Different types of Somatic Pain Acute pain - Derived from skin - Has 2 components controlled by different afferents: - Sharp and localizable initial pain - Delayed pain (dull or burning) Chronic pain - Derived from muscles, joints, bones or connective tissue - Poorly localized, dull pain which tends to radiate into surrounding tissues

Anterolateral pathway in spinal cord 69 © McKinley et al. A & P 2013

Anterolateral or Spinothalamic Pathway 70 This pathway is referred to by two names: 1) Based on its location in the spinal cord “Anterolateral” 2) Based on where it starts & stops “Spinothalamic” © McKinley et al. A & P 2013

Fu nction - ACUTE pain (sharp, critical) - Temperature extremes - Crude touch Receptors - Myelinated nociceptors in skin Ascending pathway - Primary afferent synapses in dorsal gray matter - Second order neuron crosses in spinal cord, then synapses in thalamus INFO FROM THIS PATHWAY CROSSES IN THE SPINAL CORD!!! NEOspinothalamic Tract 71

PALEOspinothalamic Tract Fu nction - Chronic pain (dull) - Middle Range Temperature - Crude touch Receptors - Unmyelinated nociceptors Ascending pathway - Same as Neospinothalamic tract except second order neuron sends info to Raphe nucleus and Periaqueductal gray. These structures are involved in modulation of chronic pain. INFO FROM THIS PATHWAY CROSSES IN THE SPINAL CORD!!! 72

73 Central Pain Modulation

74 Neuropathic pain Pain that persists long after all tissue injury is healed Hyperalgesia an exaggerated painful response to a normally noxious stimulus Allodynia a painful response to a normally non-noxious stimulus (e.g., cloths touching skin after the sunburn is gone)

75 Referred pain – Visceral pain Generally dull and diffuse Can be produced by multiple stimuli – Cessation of blood flow – An irritant chemical – Tissue stretching – Muscle spasms within an organ Often accompanied by autonomic responses Since most viscera is not mapped in the somatosensory cortex, pain is localized (“referred”) to nearby somatic area

76 Mechanism of referred pain Axons carrying organ pain info and axons carrying skin pain info synapse on the same spinal cord neuron. So the brain cannot distinguish where pain is coming from. © McKinley et al. A & P 2013

Common Areas of Referred Pain 77 © McKinley et al. A & P 2013

78 Phantom Limb Pain Up to 80% of amputees have some phantom sensation Pain is usually neuropathic Etiology unclear, but believed to be due to retention of the motor and sensory maps in the cerebral cortex

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80 © McKinley et al. A & P 2013

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