UB BIO 203 - 4-Somatic Sensation 10-02-2018 (21 pages)

Previewing pages 1, 2, 20, 21 of 21 page document View the full content.
View Full Document

4-Somatic Sensation 10-02-2018



Previewing pages 1, 2, 20, 21 of actual document.

View the full content.
View Full Document
View Full Document

4-Somatic Sensation 10-02-2018

26 views


Pages:
21
School:
University at Buffalo-SUNY
Course:
Bio 203 - General Physiology Lec
Unformatted text preview:

SOMATIC SENSATION 1 Modalities of somatic sensation qualitatively different sensations CUTANEOUS SKIN DEEP touch pressure pain pain warm position cool Separate and independent neural systems for each modality from periphery up to cortex How are the cutaneous modalities distributed across the skin 2 Punctate nature of cutaneous sensation Any point on skin gives rise to one and only one sensation T C W P 1 mm T W C P Why is this true There is a physiological anatomical explanation look at how the skin is innervated 3 Innervation transduction DEFINITION axons that innervate the body are the primary afferent fibers first order fibers Cell bodies are in the DRG Dorsal Root Ganglion axons branch one end to periphery other into the spinal cord Transduction is accomplished by channels at the fiber ending no separate class of receptor cells 4 Shingles disease of DRG Shingles causes a painful rash with blisters It is caused by the varicella virus the same virus that causes chickenpox it is a herpes virus also called herpes zoster The virus lies dormant in the DRG but may reactivate as shingles It travels down the sensory nerve fibers and affects the skin 5 What activates the virus Rash is seen in skin innervated by that nerve sometimes with severe pain Skin can remain painful and sensitive to touch for months or even years after the rash clears up postherpetic neuralgia Conditions that weaken the body s immune system such as aging cancer or certain drugs increase the chance that the virus will become active again Not known why any particular DRG is affected nor what happens in the skin to cause the rash Half of the nearly 1 million shingles cases in the United States each year occur in people aged 60 years and older It is treated with antiviral drugs there is a vaccine 6 Functional classification of primary afferent fibers innervating the skin 4 functional types 1 Pain nociceptors noxious damaging stimuli 2 Touch mechanoreceptors 3 Warm and 4 Cool thermoreceptors 7 Transduction Fibers for the different modalities differ at a microscopic level channels specialized so that they open to different kinds of stimulus energy depolarization if stimulus intense enough action cool fiber potentials touch fiber pain fiber warm fiber Fibers of different types are intermingled in the skin sensation at each point determined by the kind of fiber that is innervating that point 8 Anatomical classification of first order fibers 3 anatomical types fiber diameter and conduction velocity correlate with function myelinated 1 A 6 12 m 30 70 m sec mechanoreceptors touch also pressure and position some have encapsulated endings Ruffini endings Merkel disks Pacinian corpuscles Kraus end bulbs 9 Fiber types 2 A 1 5 m 12 30 m sec nociceptors pain thermoreceptors warm cool all end as free nerve endings unmyelinated 3 C 0 4 1 2 m 5 2 m sec nociceptors thermoreceptors free nerve endings assessing cv is important clinically in cases of nerve damage neuropathy 10 Innervation density Innervation density how many axons area of skin Varies over the body surface and correlates with somatosensory sensitivity fingertips lips back 11 Localization and 2 point threshold Localization varies over body surface 2 point threshold also varies and is better in areas of higher innervation density Receptive fields of axons or neurons in the somatosensory system region of skin where appropriate stimulus affects the activity of axon or cell Receptive field size of all cells in the system varies over the body surface smallest in regions of highest innervation density 12 Basis of 2 point discrimination High innervation density smaller receptive fields better 2 point discrimination Lower innervation density larger receptive fields worse 2 point discrimination 13 Somatosensory pathways Somatosensory information from the skin and deep tissue travels to the brain in 2 anatomical systems Different modalities are carried by the 2 systems Dorsal Column Medial Lemniscal System touch pressure position input from A fibers skin and deep tissue Anterolateral system pain temperature crude touch input from A and C fibers again from skin and pain from deep tissue 14 Organization of dorsal column system There are orderly maps of the body at all levels as in visual system somatotopic organization At the level of the spinal cord and medulla maps are of the IPSILATERAL half of the body At the level of thalamus and cortex maps are of the CONTRALATERAL half of the body Where do axons cross the midline 15 Gracile and cuneate nuclei medulla 16 Dorsal column medial lemniscal system Spinal cord ascending tracts 17 Somatosensory cortex Parietal lobe postcentral gyrus areas 3 1 2 S1 somatosensory area 1 numbered areas from anatomical studies 17 Occipital lobe 4 18 Map in somatosensory cortex Sensory homunculus in somatosensory cortex representation of the contralateral half of the body Map is distorted with an expanded representation of areas with higher innervation density coronal section through left somatosensory cortex 19 Receptive fields in somatosensory cortex Definition area of body from which a single cell gets information stimulus affects firing rate SMALLER for areas with expanded representation and higher peripheral innervation density LARGER for areas with smaller representations and lower peripheral innervation density Receptive fields in cortex are larger than the receptive fields of primary afferent fibers as a consequence of convergence at every level of the system 20 Plasticity in somatosensory cortex Plasticity changes in organization of the map of the body How can this happen 1 denervation of periphery e g amputation of finger 2 use especially with learning and motivation the map can reorganize 21


View Full Document

Access the best Study Guides, Lecture Notes and Practice Exams

Loading Unlocking...
Login

Join to view 4-Somatic Sensation 10-02-2018 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view 4-Somatic Sensation 10-02-2018 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?