Somatic Senses PDF

Somatic Senses Kevin Johnston, PhD Department of Physiology and Pharmacology Western University Outline Quick overview Mechanoreceptors Spinal Cord Somatosensory Cortex Temperature & Pain  Somatic Senses 1. Touch 2. Proprioception: awareness of body movement and locat...

Somatic Senses Kevin Johnston, PhD Department of Physiology and Pharmacology Western University Outline Quick overview Mechanoreceptors Spinal Cord Somatosensory Cortex Temperature & Pain  Somatic Senses 1. Touch 2. Proprioception: awareness of body movement and location in space 93 Somatic Senses  Somatic Senses 1. Touch 2. Proprioception: awareness of body movement and location in space 3. Temperature 4. Nociception (Pain ) All the receptors are neurons Receptors are located at skin and viscera The secondary neurons in spinal cord and medulla synapse onto tertiary neurons in the thalamus The information is sent to the Somatosensory cortex 93 Receptor cell is a non-neural cell Receptor cell is a neural cell: primary sensory Primary neuron sensory neuron Secondary Secondary sensory sensory neuron neuron VISION HEARING OLFACTION BALANCE SOMATIC TASTE SENSES 93  Mechanoreceptors Merkel receptors Meissner’s corpuscle responds to flutter sense steady pressure and stroking movements. and texture. Hair Free nerve ending Free nerve ending Free nerve ending of of hair root senses Hair root nociceptor responds hair movement. to noxious stimuli. Sensory nerves Pacinian corpuscle carry signals to senses vibration. spinal cord. Ruffini corpuscle responds to skin stretch. 94 How do mechanoreceptors differ? 1) Adaptation Rate 2) Receptive Field Size 95  Adaptation Rate All sensory receptors quickly respond to the presence of a stimulus. Then they become used to the presence of the stimulus (they adapt to it). Some receptors rapidly adapt to the presence of a stimulus (Pacinian and Meissner’s corpuscles – rapidly adapting), while some are slowly adapting (Merkel’s disks and Ruffini’s endings). 95  Receptive Field Size The area of skin which, when stimulated, influences the firing rate of a given neuron. Small – Merkel (touch) and Meissner (touch) Large – Ruffini (stretch) and Pacinian (vibration) 95/96 Properties of skin receptors Superficial: Meissner Merkel Deep: Pacinian Ruffini 96 Mechanoreceptors  Touch is critical for fine motor control… Normal: Nerve block*: Nerve block into digital nerves of thumb and first through third fingers.  Tactile Acuity and the Body Surface Two-point discrimination: the smallest separation between two points on the skin that is perceived as two points rather than one. Regions with high tactile acuity have small receptive fields. 97  Receptive fields Differences in two-point discrimination arise from the size of the receptive fields, and also from different patterns of convergence of primary sensory neurons onto secondary sensory neuron. calipers Primary sensory neurons Secondary sensory neurons One signal goes to brain Two signals go to brain 97 Two-point thresholds Demo Two-point thresholds 97  Spinal cord Each segment of the spinal cord receives sensory input from a particular region of the body and supplies motor input to a similar region. Grey matter White matter Dorsal root: SENSORY Dorsal root ganglion Primary sensory neuron From sensory receptor Spinal nerve Ventral root: MOTOR 98  Spinal cord Dermatome: A dermatome is an area of skin that is mainly supplied by afferent nerve fibers from a single dorsal root of spinal nerve. But why aren’t there dermatomes on the face? In addition to the 31 pairs of spinal nerves, there are also 12 pairs of cranial nerves that come directly off the brain. 99 Dermatomes Dermatomes Shingles (Herpes Zoster) Ramsay Hunt Syndrome Somatosensory pathways Processed at somatosensory in the CNS cortex Synapse with tertiary sensory THALAMUS neurons at thalamus MEDULLA Synapse with secondary sensory neurons at the medulla Fine touch, Cross the midline proprioception, vibration Synapse with secondary sensory SPINAL CORD Nociception, neurons at the spinal cord temperature, Cross the midline coarse touch 100 Primary Somatosensory Cortex (S1) S1 made up of areas 3, 1 and 2 Located on the postcentral gyrus. Primary motor cortex is located on the precentral gyrus. 101  Body Map in Somatosensory Cortex The somatosensory information goes to specific areas in the cortex. The amount of space on the somatosensory cortex devoted to each body part is proportional to the sensitivity of that part. Thalamus Sensory signals from left side of body 102 But this is how your brain sees you! The Homunculus  Temperature Stimulus Free nerve endings located at epidermis Free nerve endings Thermoreceptors in the brain – homeostasis Unmyelinated axon Cold receptors Cell Warm receptors body Cold and warm receptors are spread across the body They present slow adaption between 20-40 ˚C but they don’t adapt outside this range 103 Pain  Pain Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Pain is a good and very important thing. It protects you from harm. How would your life be if you couldn’t feel physical pain? Congenital Insensitivity to Pain – autosomal recessive disorder Imagine stepping on a nail … http://emedia.rmit.edu.au/infection_control/content/4_PortalEntry/02_entr_ex.htm  Pain fibers Fiber Fiber Type Associated stimuli characteristics Aβ (beta) Large, myelinated Mechanical stimuli (touch) Intense mechanical or Aδ (delta) Small, myelinated mechanothermal stimuli, fast pain C Small, unmyelinated Heat, cold, slow pain 103  Thermoreceptors vs. Nociceptors Nociceptors have a higher threshold than thermoreceptors. In the painful stimulus range, thermoreceptors continue to fire at the same rate as at lower temperatures, whereas the firing rate of nociceptors continues to increase. 104 To review the concept... If someone tossed you a hot potato and you caught it, which information would reach your CNS first — news that the potato was hot or that it was relatively smooth? Why? Next lecture: Vision