Our senses provide us with wonderful capabilities. If you had to lose one, which would it be?

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Erick Quinn
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1 Our senses provide us with wonderful capabilities. If you had to lose one, which would it be? Neurological disorders take away sensation without a choice! Chris Cohan, Ph.D. Dept. of Pathology/Anat Sci University at Buffalo

2 Objectives 1. Describe the general characteristics of sensory pathways 2. Understand the general organization and numbered areas of spinal cord gray matter 3. Understand dermatomes and be able to name them on the body surface. 4. Understand the general characteristics of tactile sensory receptors. 5. Understand how axon diameter relates to transmission of tactile information. 6. Describe the names and locations of the cell bodies and axons in the pathway that transmits tactile information. Describe the cortical area involved. 7. Understand the concept of somatotopic orientation and describe it for tactile information at the level of the spinal cord, brainstem, and cortex. 8. Understand the term lesion and describe how lesions in the spinal cord, brainstem and cortex affect tactile sensibility. 9. Understand the effects of Tabes Dorsalis and use of Romberg test

3 What are we Doing How is sensory info transmitted from body to brain? Or when I poke you, why do you feel it How are neurons specialized to transmit sensory info? Where are the neurons that transmit tactile sensation? How do diseases impair sensory function/touch?

4 Sensation A 24-year-old man was brought to the ED after he was shot during an argument outside a bar. On exam, he had the following symptoms: Loss of touch, vibration sense and ability to tell position of his leg passively moved all on Left lower limb. Tactile, vibration, position sense normal on Right Pinprick sensation absent on Right lower limb but normal on Left. R L

5 QUESTIONS: Which sensory systems involved Which areas of body affected. Why lost sensation on right and left Is there single location of damage (Lesion)

Body Sensation Stimuli touch, vibration, proprioception, pain, temperature Sensory Receptors modified nerve endings How are stimuli transmitted to the brain: encoding

6 Body Sensation Stimuli touch, vibration, proprioception, pain, temperature Sensory Receptors modified nerve endings How are stimuli transmitted to the brain: encoding by receptor pathway to brain types of neurons - specialized locations injury effects synapses - processing Different types of neurons specialized for different functions

7 Somatosensory Systems Overall Plan: Pathways for reflexes and perception perception Reflexes - clinical tools to assess pathways Perception - how we experience the world? movement sensation

8 General Features of Pathways 3-neuron pathways for perception 1, 2, 3 Locations of axons (tracts), cell bodies (nuclei) Conducting action potentials synapses integrate activity 3 2 1

9 General Features of Pathways 3-neuron pathways for perception 1, 2, 3 Locations of axons (tracts), cell bodies (nuclei) What happens when a pathway is damaged The brain experiences the deficit as a loss from the side where the stimulus originated

10 General Features of Pathways Cell Body Axon 1 Dorsal Root Ganglion Dorsal Root 2 Spinal cord or Brainstem CROSSES CNS 3 Thalamus Internal Capsule Synapses in cortex Download template from web site 3 2 1

11 Spinal Cord Laminae

12 Define the relationship between body regions and the spinal cord segment innervating them. Dermatomes

13 Tactile Sensation Sensory Stimuli: light touch, vibration, pressure, proprioception We call it touch, but it really has many qualities! The different qualities are due to the different nerve endings that encode physical deformation.

14 Tactile Receptors What are they - modified nerve endings! mechanoreceptors cutaneous muscle skeletal Mechanoreceptors in skin differ in: 1. Morphology 2. Depth in skin 3. Axon properties

15 Tactile Receptors Different properties of mechanoreceptors produce the basic sensations of touch. Collectively, these properties allow us to discriminate the tactile qualities of objects. This information eventually arrives at the cortex where features of the stimulus are analyzed and perceived.

16 Pathway Overview thalamus (VPL) Cell Body Axon 1 Dorsal Root Ganglion Dorsal Root 2 Brainstem CROSSES CNS 3 Thalamus Internal Capsule Synapses in cortex midbrain pons medulla nuc gracilis, nuc cuneatus Dorsal root gang spinal cord

17 Diameter of First-Order Axons Nerve Fiber Grp Group A(myelinated) α Axon diameter Conduction Velocity Function 10-20µm m/sec mechanorecep β 5-15 µm m/sec mechanorecep γ 3-10 µm m/sec mechanorecep δ 1-5 µm 6-30 m/sec nociceptors, thermal r. Grp C-unmyelinated µm m/sec nociceptors, thermal r. An action potential traveling from my finger to my spinal cord (1 meter) would require 8 ms for the fastest axon and 100 ms for the slowest mechanoreceptor axon.

18 Reflex Pathways As sensory info enters spinal cord, it immediately is used for reflexes. Basis of stretch reflexes 1 axons make local connections in spinal cord onto motoneurons. The presence and magnitude of reflexes provide a clinical tool to assess pathways. 0, 1+, 2+, 3+, 4+

19 Pathway VPL midbrain fasciculus cuneatus pons medial lemniscus nucleus gracilis nucleus cuneatus fasciculus gracilis caudal medulla

20 Fasciculus gracilis and fasciculus cuneatus travel the length of the spinal cord on its dorsal (posterior) surface on each side of the midline. Consequently, these tracts are often called collectively the dorsal columns. Pathway

21 Cortical Localization Circle this The 3º neuron terminates in the postcentral gyrus. Paracentral lobule Lateral view Medial

22 Paracentral Lobule central sulcus The postcentral gyrus does not continue as a gyrus onto the medial surface of the hemisphere.

23 Somatotopic Orientation SPINAL CORD Relationship between a part of the body and position in a pathway Upper body FC FG FG Above T6 Below T6- Lower body only FG FG

24 Somatotopic Orientation BRAINSTEM medial lemniscus medulla pons Upper body Lower body

25 Somatotopic Orientation CORTEX Paracentral lobule homonculus

26 Cortical Integration Perception of the sensory stimulus occurs when activity reaches the cortex. You feel something touching your body. Postcentral gyrus analyzes simple tactile qualities: dull/sharp, light/heavy Neighboring parietal cortex associates the stimulus with previous experiences (memory) to provide more complex features: stereognosis, graphesthesia, 2-point discrimination, etc.

27 Damage to the Postcentral gyrus loss of tactile sensation Damage to neighboring parietal cortex while postcentral gyrus is intact the complex features are lost but tactile perception is retained (agnosia). FEATURE 2-point discrimination Stereognosis Graphesthesia Double Simultaneous Stim Cortical Integration (Attention to stimulated area) DEFICIT (agnosia) loss of Astereognosis Agraphesthesia Extinction on.. Sensation intact but inability to comprehend Loss of any of the complex features while tactile sensation is intact, indicates a (parietal) cortex lesion.

28 Clinical Applications LESION: tumor, trauma, infection, degenerative/metabolic/genetic disorder An area of localized dysfunction in the nervous system that may be caused by a variety of factors. Symptoms caused by lesion provide clues to its location in the nervous system.

balance - how (visual, tactile, vestibular info)

29 Clinical Applications Tabes Dorsalis (advanced syphilis); vitamin B12 deficiency Romberg Test maintain balance - how (visual, tactile, vestibular info) Vestibular system intact Eyes closed Balance maintained by proprioception

Parietal lobe damage may result in Loss of 2 pt discrim Astereognosis Agraphesthesia

30 Level-Specific Deficits 1. Spinal cord 2. Brainstem 3. Thalamus 4. Internal Capsule 5. Postcentral gyrus Loss of tactile sense Which side? Parietal lobe damage may result in Loss of 2 pt discrim Astereognosis Agraphesthesia Extinction on double simultaneous stimulation Labeled-line encoding: pathways are dedicated to specific sensory info.

31 Testing Intact Pathway On returning home from shopping, a wife finds her 59 yearold husband unconscious on the floor. He has no pulse. After calling 911, she begins CPR. When Mr. T arrives in the ED, he is in ventricular fibrillation. Spontaneous circulation is eventually restored, but Mr. T remains comatose. After 3 days in coma, his wife asks about her husband s condition and the probability of a good outcome. No clinical methods exist to assess cerebral cortex damage/function in unconscious patients. How can a physician assess the extent of CNS damage and chances for recovery?

32 Testing Intact Pathway Coma is caused by damage to the cerebral hemispheres and/or brainstem. Brainstem involvement is much more serious condition and often life-threatening due to autonomic centers. Brainstem involvement can be assessed using knowledge of pathways and reflexes. 1. Somatosensory Evoked Potential

33 Case A 24-year-old man was brought to the emergency room after he was shot during an argument outside a bar. On exam, he had the following symptoms: Loss of touch, vibration sense and ability to tell position of leg passively moved all in Left lower limb. Pinprick sensation absent in Right lower limb.

34 Which sensory systems were affected Why did he lose some senses on Left side, but other senses on Right. What pathway Case Where is the damage (Lesion). R L

35 Check out tutorials on module web site: Review tutorials review each pathway and structure IDs.

1. Somatosensory Pathways

1. Somatosensory Pathways Objectives 1. Describe the general characteristics of sensory pathways 2. Understand the general organization and numbered areas of spinal cord gray matter 3. Understand dermatomes