BIOL 350 Lec8 Somatosensory System PDF

Lecture 8: Somatosensory system Somatosensory Systems -​ sensation can be divided into 4 types: -​ superficial: concerned with touch, pain, temperature, and 2 point discrimination -​ deep: muscle and joint position sense, deep muscle pain, and vibration...

Lecture 8: Somatosensory system Somatosensory Systems -​ sensation can be divided into 4 types: -​ superficial: concerned with touch, pain, temperature, and 2 point discrimination -​ deep: muscle and joint position sense, deep muscle pain, and vibration -​ contraction of muscles, information we do not need to deal with -​ visceral: autonomic afferent fibers, include hunger, nausea and visceral pain -​ strong stimulation, e.g. feeling something down your throat -​ special: special senses: smell, vision, hearing, taste and equilibrium -​ 5 main sensory systems: -​ touch/ pressure, proprioception, temperature and pain -​ aka somatosensory system -​ vision/ visual system -​ hearing and balance/ auditory and vestibular system -​ gustation/ gustatory system -​ olfaction/ olfactory system -​ order of neurons -​ 1st: cell bodies in the dorsal root ganglia -​ 2nd: cell bodies in the brain stem -​ 3rd: cell bodies in the thalamus, post lateral (cortex) ​ Receptors -​ specialized sensory receptor, afferent axon, and cell body together with the synaptic contacts in the spinal cord are known as the primary afferent -​ exteroceptors (all in the skin): -​ meissner’s corpuscles, merkel’s corpuscles, and hair cells for touch Krause’s end bulbs for cold -​ ruffini's corpuscles for warmth -​ free nerve endings for pain -​ mechanoreceptors = touch, vibration, -​ thermoreceptor = temp, nociceptors = pain -​ proprioceptors: deep sensation, associated w/ deep sensations -​ pacinian corpuscles -​ joint receptors -​ muscle spindle -​ golgi tendon organs ​ Connections -​ anterolateral system -​ 2 main pathways: spinothalamic & dorsal column lemniscus -​ dorsal = somatosensory/ascending -​ ventral = motor/descending -​ aka ventrolateral/ anterolateral -​ myelinated delta fibers convery sharp fast pain  -​ all tracts travel through white matter -​ unmyelinated C fibers transmit chronic pain -​ the primary sensory area is connected to adjacent cortical areas that perform more complex sensory processing (secondary sensory areas) ​ Converge theory of Referred Pain -​ hard to identify where pain is coming from visceral organs (no map/ homunculus) as there are no nociceptors; brain points in their direction but not at them -​ e.g. heart attack like pain in chest, neck, jaw, kidney stones feels like pain in address → legs ​ Somatosensory Pathway: -​ dorsal column-medial lemniscus (DCML) → fine touch, vibration, proprioception -​ 1st order: DRG → Gracilis (lower)/ cuneatus (upper) in medulla -​ 2nd order: decussates in medulla (ML) → thalamus (VPL) -​ 3rd order: thalamus → somatosensory cortex (postcentral gyrus) -​ spinothalamic tract → pain, temperature, crude touch -​ 1st order: DRG → spinal cord -​ 2nd order: decussates immediately in spinal cord → thalamus (VPL) -​ 3rd order: thalamus → somatosensory cortex Motor and descending (efferent) -​ pyramidal tracts -​ lateral corticospinal tract -​ anterior corticospinal tract -​ extrapyramidal tracts -​ rubrospinal, reticulospinal, olivospinal, vestibulospinal ​ Sensory and ascending (afferent) -​ dorsal column medial lemniscus system -​ gracile fasciculus, cuneate fasciculus -​ spinocerebellar tracts -​ posterior spinocerebellar tract -​ anterior spinocerebellar tract -​ anterolateral system -​ lateral spinothalamic tract -​ anterior spinothalamic tract -​ spino-olivary fibers ​ The Visual Pathway -​ sensory transduction: involves the conversion of a stimulus from the external or internal environment into an electrical signal for transmission through the nervous system -​ phototransduction -​ visual pathway: retina → optic nerve → optic chiasm (nasal fibers cross) → optic tract → lateral geniculate nucleus (LGN, thalamus) → optic radiations → primary visual cortex (occipital lobe) -​ dorsal pathway (“where?”) → parietal lobe (motion, spatial awareness) -​ ventral pathway (“what?”) → temporal lobe (object recognition)