Somatic Sensations I General Organization, the Tactile and Position Senses

1. Somatic Sensations IGeneral Organization, the Tactile and Position Senses Prof. Dr. Bayram Yılmaz Yeditepe University Faculty of Medicine Department of Physiology

2. Classification of somatic senses • Mechanoreceptive somatic senses • Which includes tactile and position senses • Thermoreceptive senses • Pain senses

3. Detection and transmission of somatic senses • Interrelations among the tactile sensations of touch, pressure and vibration • Touch, pressure and vibration are all detected by the same types of receptors • Tactile receptors: There are at least six entirely different types of tactile receptors

4. Tactile Receptors • 1) Free nerve endings (detects touch and pressure) • 2) Meissner’s corpuscle (found mostly in nonhairy parts of the skin and particularly in the fingertips, lips) • Localization of touch sensations • 3) Merkel’s discs (also found in the fingertips and other areas • Responsible for steady-state signals • Iggo dome receptor

5. Iggo Dome Receptor and Multiple Numbers of Merkel’s Discs

6. Tactile Receptors • 4) Hair-end organ (detects movement of objects on the body surface and initial contact with the body) • 5) Ruffini’s end organs (located in deep layers of the skin and internal tissues) • Important for signalling continuous states of deformation of tissues, such as prolonged touch and pressure signals • 6) Pacinian corpuscles (located immediately beneath the skin and deep in fascial tissues) • They are stimulated by rapid local compression of the tissues, and adapt fast.

7. Transmission of tactile signals in peripheral nerve fibers • Almost all specialized sensory receptors transmit their signals in type Ab nerve fibers (30-70 m/sec) • Free nerve ending tactile receptors transmit signals by mainly Ad myelinated fibers (5-30 m/sec) • Detection of vibration and Ab nerve fibers • Free nerve endings, sensation of tickle, itch and C fibers • Crude types of signals are transmitted by way of slower and small nerve fibers

8. Sensory Pathways for Transmitting Somatic Signals • Almost all sensory information from the somatic segments enters the spinal cord through the dorsal roots of the spinal nerves • 1) Dorsal column-medial lemniscal system • 2) Anterolateral system • These two systems come back together at the thalamus • DC-MLS is composed of large, myelinated fibers (30-110 m/sec) • ALS is composed of smaller fibers (40 m/sec)

9. Sensory Pathways for Transmitting Somatic Signals • DC-MLS has a high degree of spatial orientation of the nerve fibers with respect to their origin • ALS has much less spatial orientation • The ALS has an ability to transmit a broad spectrum of sensory modalities (pain, warmth, cold and crude tactile sensations) • DC-MLS is limited to discrete types of mechanoreceptive sensations

10. Dorsal Column-Medial Lemniscal System • Touch sensations with high degree of localization • Touch sensations requiring fine gradations of intensity • Vibratory sensations • Position sensations • Pressure sensations having to do with fine degree of judgement of pressure intensity

11. Anterolateral System • Pain • Thermal sensations • Crude touch and pressure sensations • Tickle and itch sensations • Sexual sensations

12. Transmission in the DC-MLS • Dorsal root and forming medial and lateral branches • The medial branch turns medially and upward in the dorsal column all the way up to the brain • The lateral branch enters the dorsal horn of the cord gray matter, then divides many times to provide terminals that synapse with local neurons: • 1) Most of these fibers enter the dorsal column and then travel upward • 2) Many of the fibers are short and terminate locally in the spinal cord to elicit spinal cord reflexes • 3) Others give rise to spinocerebellar tratcs

13. Transmission in the DC-MLS

14. Transmission in the DC-MLS

15. Spatial orientation of nerve fibers in the DC-MLS • In the dorsal column of the spinal cord, fibers from the lower parts of the body lie toward the center of the cord • Whereas those fibers that enter the cord at progressively higher segmental levels form successive layers laterally • Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented in the right side of the thalamus • And the right side of the body in the left side of the thalamus

16. Spatial orientation of nerve fibers in the DC-MLS

17. Somatosensory Cortex • About 50 distinct areas (Brodmann’s areas) based on histological structural differences • Sensory signals from all modalities terminate in the cerebral cortex immediately posterior to the central fissure • Anterior half of the parietal lobe: reception and interpretation • Posterior half of the parietal lobe: still higher levels of interpretation

18. Somatosensory Cortex

19. Specialized Areas of the Cerebrum

20. Somatosensory Areas I and II

21. Spatial Orientation of Signals from Different Parts of the Body in Somatosensory Area I

22. SENSORY HOMUNCULUS of SI & LOCATION OF SII

23. Layers of the Somatosensory Cortex and Their Function • The cerebral cortex contains six layers of neurons beginning layer I next to the brain surface • The sensory cortex is organized in vertical columns of neurons • Each column detects a different sensory spot on the body with a specific sensory modality (some columns respond to stretch receptors, some to tactile hairs) • Columns in the posterior part of SSA I respond to slowly adapting cutaneous receptors and deep pressure

24. Layers of the Somatosensory Cortex and Their Function Layers I and II receive nonspecific signals from lower brain centers Neurons in layers II and III send axons to the opposite side of the brain through C callosum Incoming sensory signal excites neuronal layer IV then spreads to the surface and deeper layers Neurons in layer V project to distant areas such as basal ganglia, brain stem and spinal cord where they control signal transmission From layer VI, large numbers of axons extend to the thalamus that help to control excitatory levels of incoming sensory signals

25. Functions of Somatosensory Area I • Widespread bilateral excision of the SSA I causes; • Inability to localize different sensations in different parts of the body • Inability to judge critical degrees of pressure against the body, texture of materials • The person is unable to judge the weights of objects • Astereognosis: inability to judge shapes or forms of objects • Senses of pain and temperature are NOT lost!

26. Somatosensory Association Areas • Brodmann’s areas 5 and 7 of the cerebral cortex • It is important in deciphering deeper meanings of the sensory information • Somatosensory association area combines information arriving from multiple points in the SSA • It receives signals from • Somatosensory area, Ventobasal nuclei and other areas of the thalamus, visual cortex and auditory cortex • Amorphosynthesis; If the somatosensory association area is removed, the person cannot recognize complex objects felt on the opposite side of the body

27. Somatosensory Association Areas • Brodmann’s areas 5 and 7 of the cerebral cortex

28. Characteristics of signal transmission and analysis in the DC-MLS • Basic neuronal circuit in the dorsal column-medial lemniscal system:

29. Two - Point Discrimination • Two peaks, separated by a valley, allow the sensory cortex to detect the presence of two stimulatory points, rather than a single point

30. Two point discrimination test

31. Effect of Lateral Inhibition • Almost every sensory pathway, when excited, gives rise simultaneously to lateral inhibitory signals • Lateral inhibition blocks lateral spread of the excitatory signals and thus increases the degree of contrast in the sensory pattern perceived in the cerebral cortex • In the case of dorsal column system, lateral inhibitory signals occur at each synaptic level • The dorsal column system is of particular importance in apprising the sensorium of rapidly changing conditions • Vibratory signals are rapidly repetetive and can be detected vibration up to 700 cycles per second (these signals are transmitted only in the dorsal column system)

32. Position Senses • The position senses are frequently also called proprioceptive senses • Somatic position sense • Rate of movement sense (kinesthesia or dynamic proprioception) • Both skin tactile receptors and deep receptors near the joints are used for sensation of position • Muscle spindles are the most important receptors for determining joint angulation in mid ranges of motion • At the extremes of joint angulation, additional receptors such as pacinian corpuscle and Ruffini’s endings are also involved

33. Processing of position sense information in the DC-MLS • Thalamic neurons responding to joint rotation • at maximal rotation • and at minimal rotation

34. Anterolateral Pathway • The spinal cord anterolateral fibers originate mainly in dorsal horn laminae I, IV, V and VI • These terminate after entering the dorsal root • Anterior SpinoThalamic Tract • Lateral SpinoThalamic Tract • These tracts terminate in the reticular formation of the brain stem and ventrobasal complex and intralaminar nuclei of the thalamus

35. Anterolateral Pathway

36. Anterolateral Pathway

37. Characteristics of Transmission in the Anterolateral Pathway • Velocity of transmission if one third of the DC-MLS • Degree of spatial localization is poor • Gradations of intensities are far less accurate • Ability to transmit rapidly changing signals is poor • This system is a cruder type of transmission (pain, temperature, tickle, itch, crude touch and sexual sensations • The thalamus has a slight ability to discriminate tactile sensation, it mainly functions as a relay station for sensory information to the cerebral cortex

38. Corticofugal Signals • In addition to the sensory signals from the periphery to the brain, corticofugal signals are transmitted backward from the cortex to lower sensory relay stations • To control intensity and sensitivity of the sensory input • Corticofugal signals are almost entirely inhibitory • They automatically decrease transmission when the sensory input intensity becomes too great

39. Dermatomes • A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve • All spinal nerves except C1 participate in dermatomes

40. AXONS OF CUTANEOUS RECEPTORS ENTER THE DORSAL ROOTS AND DERMATOMES ARE FORMED