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Somatic senses

Somatic senses. There are 4 somatosensory modalities Touch Temperature Nociception (pain and itch) Proprioception. Pathways for somatic perception – 1 st and 2 nd neurons. Receptors for the somatic sensations are found both un the skin and viscera

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Somatic senses

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  1. Somatic senses • There are 4 somatosensory modalities • Touch • Temperature • Nociception (pain and itch) • Proprioception

  2. Pathways for somatic perception – 1st and 2nd neurons • Receptors for the somatic sensations are found both un the skin and viscera • Receptor activation triggers AP in the 1st order neuron • In the spinal cord, sensory neurons synapse with interneurons – 2nd order neurons • All 2nd order neurons cross over at some point (sensations are being integrated in the opposite side) • Neurons associated with nociception, temperature and coarse touch synapse with 2nd neurons shortly after entering the spinal cord and cross over in the spinal cord • Most fine touch, vibration and proprioceptive neurons have very long axons that project all the way to the medulla where they synapse with the 2nd neuron and cross over

  3. Pathways for somatic perception – 2nd and 3rd neurons • The synapse between the 2nd and the 3rd happens in the thalamus • The axons of the 3rd order neurons project to the appropriate somatosensory area in the cerebral cortex

  4. Thalamic Function • The thalamus is the “gateway to the cerebral cortex” • Major relay station for mostsensory impulses that arrive to the primary sensory areas in the cerebral cortex: • taste, smell, hearing, equilibrium, vision, touch, pain, pressure, temperature • Contributes to motor functions by transmitting information from the cerebellum and basal ganglia to the cerebral primary motor area • Connects areas of the cerebrum • Impulses of similar function are sorted out, edited, and relayed as a group

  5. http://medinfo.ufl.edu/year2/neuro/review/images/thalamus.jpghttp://medinfo.ufl.edu/year2/neuro/review/images/thalamus.jpg

  6. 3 major somatosensory pathways –1) spinothalamic pathway • Conscious sensation of poorly localized sensations • Anterior spinothalamic tracts – crude touch and pressure • Lateral spinothalamictracts – pain and temperature • 1st order neurons synapse with the 2nd in the posterior gray horn at the level of entrance • The 2nd cross before ascending to the thalamus • 3rd order synapse at the level of the primary somatosensory cortex

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

  8. Spmatosensation perception • The specific sensation depends on the 2nd and 3rd neurons • The ability to localize the specific location of a stimulus depends on the stimulation of a specific area in the primary somatosensory cortex • A sensory “homunculus” (little human) is a functional map of the primary somatosensory cortex

  9. 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 • Ex.: putting the hand in the pocket and feeling something. The center integrate previous information to identify objects without seeing them

  10. Pain pathways • Pain is a subjective perception • It is individual and can vary depending on emotional state • Types of pain sensations: • Fast pain – sharp and localized • Rapidly transferred to CNS by small myelinatedfibeers • Slow pain – more diffused pain • Carried by small unmyelinated fibers • Often slow pain will follow a fast one • Pain from the body – via spinal cord • Pain from face – via trigeminal (V) that enters the pons, descend to the medulla where they cross over and ascend to the thalamus

  11. Pain pathways • The ascending pathway sends branches not only to thalamus and the cerebral cortex but also to the limbic system (emotions) and hypothalamus (autonomic reaction) • The result is that pain may be accompanied by emotional distress and autonomic reactions such as nausea, vomiting or sweating

  12. Pain perception • Pain can be felt in skeletal muscle when anaerobic metabolism • In cardiac muscle, pain is a result of ischemia (lack of oxygen due to reduced blood flow) during myocardial infraction (heart attack) • Visceral pain is poorly localized and called referred pain

  13. 3 major somatosensory pathways - 2) Posterior column pathway • Sensation of precise touch, vibration and proprioception • Includes • Left and right fasciculus gracilis(inferior part of the body) • Left and right fasciculus cuneatus(superior part of the body) • First order neurons enter the CNS at the dorsal roots and the sensory roots of cranial nerves. • Synapse with 2nd order in the medulla • 2nd order neurons cross over in the brain stem • 3rd order in the thalamus where the stimuli are sorted by the nature of stimulus and the region of body involved

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

  15. 3 major somatosensory pathways – 3) The spinocerebellar pathway • Information about muscle, tendon and joint position from the spine to the cerebellum • This information is subconscious • 1st order neurons synapse in the dorsal horn • 2nd order neurons ascend via anterior and posterior spinocerebellar tracts to the cerebellar cortex • Used to coordinate movements • In this pathway there is no 3rd order neuron

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

  17. Visceral sensory pathways • Collected by interoceptors within the closed ventral body cavities • The interoceptors include nociceptors, thermoreceptors, tactile receptors, baroreceptors and chemoreceptors • The axons of the 1st order neuron usually travel with the autonomic motor fibers innervating the same visceral structures • 2nd order neurons within the spinal cord use the spinothalamic pathway and arrive to the medulla oblongata • Cranial nerves V, VII, IX and X carry visceral sensory information also to the medulla • (all parasympathetic and will be discussed with the ANS)

  18. Cranial Nerve V: Trigeminal Fibers run from the faceto the pons, to the thalamus and to the primary somatosensory cortex Three divisions: ophthalmic (V1), sensory from face maxillary (V2), mandibular (V3) supplies motor fibers (V3) for mastication

  19. Cranial Nerve VII: Facial Fibers leave the pons to the lateral aspect of the face Mixed nerve with five major branches Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands (subconscious) Sensory function is taste from the anterior two-thirds of the tongue (taste buds to pons, to the thalamus, to the insula and parietal cortex for taste perception)

  20. Cranial Nerve IX: Glossopharyngeal Fibers emerge from the medulla and run to the throat Nerve IX is a mixed nerve Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland (autonomic) Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

  21. Cranial Nerve X: Vagus The only cranial nerve that extends beyond the head and neck Fibers emerge from the medulla The vagus is a mixed nerve Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs Its sensory function is in taste

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