2_somatosensory system human physiology.pptx

An overview of the human somatosensory system.

1. No great artist ever sees things as they really are . If he did, he would cease to be an artist Oscar Wilde

2. BRAIN PARTS

3. SCHEMATIC DIAGRAM OF CNS

4. Sensory Pathways

5. Review Nerves classification by Erlanger &Gasser , and Lloyd and Hunt

6. Sensory Systems Afferent Division of the Nervous System Sensory receptors Sensory neurons Sensory pathways

7. Afferent Division Somatic Sensory info Sensory cortex of cerebrum Cerebellum Visceral Sensory info Reflex centers in brainstem Reflex centers in diencephalon ( thalamus, the subthalamus, the hypothalamus, and the epithalamus)

8. Sensory Pathways Stimulus as physical energy  sensory receptor Receptor acts as a transducer Intracellular signal  usually change in membrane potential Stimulus  threshold  action potential to CNS Integration in CNS  cerebral cortex or acted on subconsciously

9. Table 10-2 Sensory Receptors

10. Processing of the sensory information Levels of neural integration in sensory systems: Receptor level — the sensor receptors Circuit level — ascending pathways in the CNS Perceptual level — neuronal circuits in the cerebral cortex

11. Figure 13.2 1 2 3 Receptor level (sensory reception and transmission to CNS) Circuit level (processing in ascending pathways) Spinal cord Cerebellum Reticular formation Pons Muscle spindle Joint kinesthetic receptor Free nerve endings (pain, cold, warmth) Medulla Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Processing at the Receptor Level

12. Figure 13.2 1 2 3 Receptor level (sensory reception and transmission to CNS) Circuit level (processing in ascending pathways) Spinal cord Cerebellum Reticular formation Pons Muscle spindle Joint kinesthetic receptor Free nerve endings (pain, cold, warmth) Medulla Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Processing at the circuit Level

13. Processing at the Receptor Level The receptor must have specificity for the stimulus energy (adequate stimulus; law of specific nerve energies) Sensory coding – the processing of a receptor stimulus to a recognizable sensation. Attributes of stimuli coded for are: Modality – type of energy transmitted by the stimulus Location Intensity Duration

14. Receptors adaptation The mechanisms for receptors’ adaptation depends on the receptors: Potassium channels in the receptor’s membrane open causing the membrane repolarization Sodium channels inactivated stopping depolarization Accessory structure may contribute to decrease sensitivity (muscle in the ear contract and limit the movement of the auditory oscicles)

15. Processing at the circuit Level A sensory pathway is a set of neurons arranged in series. The circuit level role is to deliver the impulses to the appropriate region in the cerebral cortex . The ascending tract typically consists of 3 neurons First order neurons cell bodies in a ganglion (dorsal or cranial) Impulses from skin and proprioceptors to spinal cord or brain stem to a 2 nd order neuron Second order neuron In the dorsal horn of the spinal cord or in the medulary nuclei Transmit impulses to thalamus or cerebellum Third order neurons Cell bodies in the thalamus (no 3 rd -order neurons in the cerebellum) Transmit signals to the somatosensory cortex of the cerebrum

16. Pathways for somatic perception In the spinal cord, 1 st order sensory neurons synapse with interneurons – 2 nd order neurons All 2 nd order neurons cross over at some point (sensations are being integrated in the opposite side) The synapse between the 2 nd and the 3 rd happens in the thalamus The axons of the 3 rd order neurons project to the appropriate somatosensory area in the cerebral cortex

17. Thalamic Function The thalamus is the “ gateway to the cerebral cortex ” Major relay station for most sensory impulses that arrive to the primary sensory areas in the cerebral cortex: taste, hearing, equilibrium, vision, touch, pain, pressure, temperature Connects areas of the cerebrum Impulses of similar function are sorted out, edited, and relayed as a group

18. 3 major somatosensory pathways –1) spinothalamic pathway Conscious sensation of poorly localized sensations Anterior spinothalamic tracts – crude touch and pressure Lateral spinothalamic tracts – pain and temperature 1 st order neurons synapse with the 2 nd in the posterior gray horn at the level of entrance 2 nd order neuron crosses before ascending to the thalamus 3 rd order neuron synapses at the level of the primary somatosensory cortex

19. http://webanatomy.net/anatomy/spinothalamic.jpg

20. 3 major somatosensory pathways - 2) Posterior column pathway ( dorsal column medial lemniscal pathway –DCLM) Sensation of precise touch, vibration and proprioception Includes Signals from the upper limb  (T6 and above) – travel in the fasciculus cuneatus (the lateral part of the dorsal column). They then synapse in the nucleus cuneatus of the medulla oblongata. Signals from the lower limb  (below T6) – travel in the fasciculus gracilis (the medial part of the dorsal column). They then synapse in the nucleus gracilis  of the medulla oblongata. 1 st order neurons enter the CNS at the dorsal roots and the sensory roots of cranial nerves. Synapses with 2 nd order in the medulla 2 nd order neurons cross over in the brain stem 3 rd order in the thalamus where the stimuli are sorted by the nature of stimulus and the region of body involved

21. http://webanatomy.net/anatomy/gracilis_cuneatus.jpg

22. 3 major somatosensory pathways – 3) The spinocerebellar pathway Information about muscle spindle, Golgi tendon and joint capsule on position from the spine to the cerebellum This information is subconscious 1 st order neurons synapse in the dorsal horn 2 nd order neurons ascend via anterior, posterior, Cuneocerebellar and Rostral spinocerebellar tracts to the cerebellar cortex Used to coordinate movements In this pathway there is no 3 rd order neuron

23. Types of spinocerebellar tracts Posterior spinocerebellar tract  – Carries proprioceptive information from the lower limbs to the ipsilateral cerebellum. Cuneocerebellar tract  – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum. Anterior spinocerebellar tract  – Carries proprioceptive information from the lower limbs. The fibres decussate twice – and so terminate in the ipsilateral cerebellum. Rostral spinocerebellar tract  – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

24. http://webanatomy.net/anatomy/spinocerebellar.jpg

25. Pathway Sensation 1 st order 2 nd order 3 rd order Final destination Spinothalamic pathway Lateral spinothalamic Pain and temperature Dorsal root ganglion Posterior horn Thalamus Primary sensory cortex (opposite side) Anterior spinothalamic Crude touch and pressure Dorsal root ganglion Posterior horn Thalamus Primary sensory cortex (opposite side) Posterior column pathway Fasciculus gracilis Proprioception, fine touch and pressure from inferior half of the body Dorsal root ganglion Medulla oblongata Thalamus Primary sensory cortex (opposite side) Fasciculus cuneatus Proprioception, fine touch and pressure from superior half of the body Dorsal root ganglion Medulla oblongata Thalamus Primary sensory cortex (opposite side) Spinocerebellar pathway Anterior and posterior Proprioception Dorsal root ganglion Posterior horn Not present Cerebellar cortex

26. Clinical application 1. Brown Sequard syndrome ( hemisection of the spinal cord) 2. What is the expected nature of sensory deficit in a patient with Tuberculosis of the spine?

27. Fine touch, proprioception, vibration Nociception, temperature, coarse touch SPINAL CORD MEDULLA THALAMUS Pain, temperature, and coarse touch cross the midline in the spinal cord. Fine touch, vibration, and proprioception pathways cross the midline in the medulla. Sensations are perceived in the primary somatic sensory cortex. Sensory pathways synapse in the thalamus. Primary sensory neuron Secondary sensory neuron Tertiary neuron KEY 1 1 2 2 3 3 4 4 Somatic Senses Pathways Figure 10-9, steps 1–4

28. Figure 13.2 1 2 3 Receptor level (sensory reception and transmission to CNS) Circuit level (processing in ascending pathways) Spinal cord Cerebellum Reticular formation Pons Muscle spindle Joint kinesthetic receptor Free nerve endings (pain, cold, warmth) Medulla Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Processing at the Perceptual Level

29. Somatic sensory thalamus and cortex organization Is this the end of the line?

30. Processing at the Perceptual Level Interpretation of sensory input occurs in the cerebral cortex The ability to identify the sensation depends on the specific location of the target neurons in the sensory cortex not on the nature of the message (all messages are action potentials)

31. The CNS integrate sensory information Most of the somatic sensory information enters the spinal cord and travels via ascending pathways to the brain Some information goes directly to the brain through the cranial nerves Autonomic sensory information does not arrive conscious perception

32. Main Aspects of Sensory Perception Perceptual detection – detecting that a stimulus has occurred and requires summation Magnitude estimation – the ability to detect how intense the stimulus is Spatial discrimination – identifying the site or pattern of the stimulus Feature abstraction – used to identify a substance that has specific texture or shape Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)

33. Somatosensory Association Cortex Located posterior to the primary somatosensory cortex and has connection with it Integrates sensory information like temperature and pressure coming from the primary somatosensory cortex. Forms understanding of the stimulus like size, texture, and relationship of parts Example.: putting the hand in the pocket and feeling something. The center integrate previous information to identify objects without seeing them

34. The main Sensory Areas in the cerebral cortex Figure 12.8a

35. Sensory Pathways Figure 10-4 3 2 1 1 2 3 Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Equilibrium pathways project primarily to the cerebellum. Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. Eye Nose Tongue Equilibrium Sound Brain stem Visual cortex Auditory cortex Gustatory cortex Primary somatic sensory cortex Olfactory cortex Olfactory bulb Cerebellum Thalamus Somatic senses

36. 1. Discuss Ascend in DC to medulla Laminae 6-9 Laminae 3-5 Large-diameter fiber (A  , A  ) Lamina 1 Lamina 9 2. How would the anterolateral system differ from the above pathway?