Somatosensory System Chapter 12 PDF

Somatosensory System Chapter 12  – Discuss the somatosensory senses of: – Touch – Pain Learning – Temperature Pressure objectives – – Proprioception – And their pathways from the...

Somatosensory System Chapter 12  – Discuss the somatosensory senses of: – Touch – Pain Learning – Temperature Pressure objectives – – Proprioception – And their pathways from the receptors all the way up to the brain  – Somatic sensation – Responsible for touch and pain – Somatic sensory system: different from other systems – Not considered one of the “special senses” – Receptors: broadly distributed – Responds to many kinds of stimuli—at least four senses rather than one What is – The five key somatosensory sensations: somato- – Touch – Pain sensation? – Temperature – Pressure – Proprioception  – Types and layers of skin – Hairy and glabrous (hairless—e.g., palms) – Epidermis (outer) and dermis (inner) – Functions of skin – Protects Touch – Prevents evaporation of body fluids – Provides direct contact with world – Mechanoreceptors – Most somatosensory receptors are mechanoreceptors.  – Pacinian corpuscles – Also called “lamellar corpuscles” – High & low frequency vibrations, pressure – Ruffini's endings – Stretching & sliding Mechano- – Meissner's corpuscles – Also called “tactile receptors corpuscles” – Pressure, force – Fine touch – Merkel's disks – Light touch – Krause end bulbs – Temperature…? Sexual stimulation??  – Pacinian corpuscles – Also called “lamellar corpuscles” – High & low frequency vibrations, pressure – Ruffini's endings – Stretching & sliding Mechano- – Meissner's corpuscles – Also called “tactile receptors corpuscles” – Pressure, force – Fine touch – Merkel's disks – Light touch – Krause end bulbs – Temperature…? Sexual stimulation??  – Receptive field size indicates how fine of a sense of touch you can perceive – Small receptive field = fine touch, like feeling the edges of a head of a small nail Receptive – Large receptive field = broad touch, like grabbing a basketball with one hand fields of mechano- receptors  – Receptive field size indicates how fine of a sense of touch you can perceive – Small receptive field = fine touch, like feeling the edges of a head of a small nail Receptive – Large receptive field = broad touch, like grabbing a basketball with one hand fields of mechano- receptors  – Receptive field size indicates how fine of a sense of touch you can perceive – Small receptive field = fine touch, like feeling the edges of a head of a small nail Receptive – Large receptive field = broad touch, like grabbing a basketball with one hand fields of mechano- receptors  – Adaptation rates are different between the various mechanoreceptors – Advantageous for feeling static vs. dynamic pressure, movement Receptive fields of mechano- receptors  – Adaptation rates are different between the various mechanoreceptors – Advantageous for feeling static vs. dynamic pressure, movement Receptive fields of mechano- receptors  – Mechanoreceptors have unmyelinated axon terminals. – Mechanosensitive ion channels convert mechanical force into change of ionic current. – Mechanical stimuli may trigger release of second messengers. Mechano- – Specific types of channels in most somatic sensory receptors still unidentified sensitive ion – Some ion channels are sensitive to physical stretching of the lipid channels of membrane: mechano- receptors  – Mechanoreceptors have unmyelinated axon terminals. – Mechanosensitive ion channels convert mechanical force into change of ionic current. – Mechanical stimuli may trigger release of second messengers. Mechano- – Specific types of channels in most somatic sensory receptors still unidentified sensitive ion – Some ion channels open with force applied to extracellular channels of structures: mechano- receptors  – Mechanoreceptors have unmyelinated axon terminals. – Mechanosensitive ion channels convert mechanical force into change of ionic current. – Mechanical stimuli may trigger release of second messengers. Mechano- – Specific types of channels in most somatic sensory receptors still unidentified sensitive ion – Some ion channels open with deformation/stress on the cell’s channels of cytoskeleton: mechano- receptors  – Primary afferent axons of the somatosensory system are: – Aa, Ab, Ad, C axons – C fibers mediate pain, temperature, and itch. – Ab mediates touch sensations. Primary afferent axons  – Primary afferent axons of the somatosensory system are: – Aa, Ab, Ad, C axons – C fibers mediate pain, temperature, and itch. – Ab mediates touch sensations. Primary afferent axons  – Spinal segments (30)—spinal nerves within four divisions of spinal cord Segmental – Cervical organization – Head/neck – Thoracic of the spinal – Chest/abdomen – Lumbar cord – Lower abdomen, upper leg – Sacral – Lower limbs  – Dermatomes make up a map indicating the specific areas of your skin where sensory information is relayed to 1 Dermatomes specific spinal segment nerve – Shingles virus travels along the nerves/dermatomes!  – Dermatomes make up a map indicating the specific areas of your skin where sensory information is relayed to 1 Dermatomes specific spinal segment nerve – Shingles virus travels along the nerves/dermatomes!  – Dermatomes make up a map indicating the specific areas of your skin where sensory information is relayed to 1 Dermatomes specific spinal segment nerve – Shingles virus travels along the nerves/dermatomes!  – Divisions of spinal gray matter: – Dorsal horn Ascending – Intermediate zone somato- – Ventral horn sensory information in the nervous system routes through the spinal cord  – Conduct sensory pathways upward through a chain of three neurons: – First-order neuron – Conducts impulses from cutaneous receptors and proprioceptors – Branches diffusely as it enters spinal cord or medulla Key features – Synapses with second-order neuron – Second-order neuron of ascending – Interneuron – Cell body in dorsal horn of spinal spinal tracts cord or medullary nuclei – Axons extend to thalamus or cerebellum – Third-order neuron – Also an interneuron – Cell bodies in thalamus – Axon extends to somatosensory cortex – No third-order neurons in cerebellum Dorsal – Dorsal column–medial lemniscal pathway column – Transmit input to medial somatosensory cortex for discriminative touch, lemniscus vibrations, pressure, and proprioception pathway – Decussates (crosses over) at the medulla (DCML) oblongata Dorsal – Dorsal column–medial lemniscal pathway column – Transmit input to medial somatosensory cortex for discriminative touch, lemniscus vibrations, pressure, and proprioception pathway – Decussates (crosses over) at the medulla (DCML) oblongata  – Trigeminal touch pathway conveys somatosensory Trigeminal information from the face – Some information from touch the dura mater is conveyed as well pathway – Decussates at same level of entry of cranial nerve V (trigeminal nerve) at the trigeminal nuclei in brainstem  – Primary somatosensory cortex is called S1 (Brodmann areas 1, 2, 3) – Area 3a: Somatic – Proprioception – Area 3b: sensory – Basic touch sensations areas of – Area 1: cortex – Texture – Area 2: – Shape & size; proprioception as well  – Remember that our bodies are disproportionally represented in our cortical maps (cortical somatotopy) Cortical somatotopy – homunculus  – Other animals, such as cats and mice who have whiskers (vibrissae), have cortical somatotopy as well Somatotopy in other animals  – Some animals have somatotopic maps Somatotopy along more than 1 in other cortical region – Areas 3b and 1 of owl animals monkey—hand area with mirror image maps  – Somatotopic maps will do their best to adjust if digits are removed (such as Somatotopic amputation) so that brain space is not “wasted” map – Remove digits or overstimulate— neuroplasticity examine somatotopy before and after – Maps are dynamic.  – Beyond S1, posterior parietal cortex is involved in somatic sensation, visual stimuli, movement planning, attentiveness – Damage to posterior parietal areas causes neurological disorders. – Agnosia Posterior – Inability to recognize stimuli from senses parietal cortex – Astereognosia – Inability to identify things by feeling them with hands (without using eyes) – Neglect syndrome – One side of hemisphere is just completed “neglected”; can’t process information from it at all  – Nociceptors are the receptors for pain – Pain and nociception – Pain—feeling of sore, aching, throbbing sensations – Nociception—sensory process, provides signals that trigger pain – Nociceptors: transduction of pain Pain – Ion channels opened by: – Strong mechanical stimulation, temperature extremes, oxygen deprivation, chemicals – Substances released by damaged cells – Proteases (-> bradykinin), ATP, K+ ion channels – Histamine  – Types of nociceptors—most are polymodal – Mechanical – Physical forces like stabbing, punching, stretching – Thermal – Hot & cold – Chemical Nociception & – pH-sensitive or ligand sensitive, e.g. chemical burns transduction of painful stimuli  – Hyperalgesia – sensation of pain with mild stimulus that normally doesn’t cause pain (like light touch) Peripheral – Primary – Actual damaged tissue site chemical hurts (like poking a bruise) mediators of – Secondary – Non-damaged tissue feels pain & pain (due to nerves sharing pathways) hyperalgesia – Inflammation – one of the 4 cardinal signs is pain – Many chemicals can mediate the sensation of pain  – Itchiness is a disagreeable sensation that induces desire or reflex to scratch – Usually brief, minor annoyance—can become chronic, debilitating condition – Triggered by skin conditions or non-skin disorders – Can be considered the “inverse” of pain sometimes (a desire to inflict temporary pain to relieve an itch) but also shares similarities to pain in terms of neural processing Itch – Signaling molecules and receptors mediating itch not yet identified  – First pain and second pain – First pain = sharp, sudden, brief, precisely-localized; Aδ fibers – Second pain = dull, achy, long lasting, broad localization; C fibers – Referred pain, e.g., angina – Sometimes you’ll feel pain in an undamaged region because of the shared neural pathways of pain-transmitting fibers Primary afferents & spinal mechanisms  – Touch and pain pathways differ: – Nerve endings in the skin; different receptors – Diameter of axons; pain axons a bit thinner/slower – Connections in spinal cord – Touch—ascends ipsilaterally – Pain—ascends contralaterally – Brown–Séquard syndrome—damage to one side of spine Ascending pain pathways  – Spinothalamic pathway – Lateral and ventral spinothalamic tracts Spinothalamic – Transmit pain, temperature, coarse pathway touch, and pressure impulses within lateral spinothalamic tract  – Spinothalamic pathway – Lateral and ventral spinothalamic tracts Spinothalamic – Transmit pain, temperature, coarse pathway touch, and pressure impulses within lateral spinothalamic tract  – Spinothalamic pathway – Lateral and ventral spinothalamic tracts Spinothalamic – Transmit pain, temperature, coarse pathway touch, and pressure impulses within lateral spinothalamic tract  – Main takeaways: – DCML carries touch & proprioception – Crosses midline at DCML vs. medulla (at the brainstem) spino- – Spinothalamic carries thalamic temperature and pain – Crosses midline at entry tracts spinal level (crosses then stays at the same side) – Both carry pressure information  – Afferent regulation – Pain signals can be modulated on the way up to the brain – Gate theory of pain: – Substantia gelatinosa region of spinal cord can use inhibitory interneurons to “shut off” pain signals from traveling further up – ”Gate” can be open or closed depending on a LOT of factors, even emotional state Pain regulation  – Descending regulation – Brain is providing instructions & chemicals downstream to “shut up” pain receptors and neurons – Endogenous opioids Pain – Powerful chemicals that can regulation essentially ”mute” first and second-order neurons by slowing or stopping their neurotransmitter release – Prevents pain signals from reaching the brain from a top- down direction  – Temperature is modulated through “hot” and “cold” receptors called TRP channels – Varying sensitivities of TRP channels to temperature ranges; also, they can be “tripped” by chemicals such as capsaicin or mint Temperature sensation  – The onset of temperature change is usually felt quite sharply (high frequency of APs), but temperature adapts within 2 seconds (frequency of APs will drop although not completely stop) Adaptation of thermo- receptors  – Organization of temperature pathway – Identical to pain pathway – spinothalamic tract Temperature – Cold receptors coupled to Ad and C fibers pathway – Hot receptors coupled to C fibers – Axons of second-order neurons decussate  – Sensory systems exhibit similar organization and function. – Topographic maps of sensory information preserved at the brain level Summary of – Higher level neurons process more complicated sensory information sensory – Somatic sensory information is segregated within the systems spinal cord and cerebral cortex. – Parallel processing of information – Perception of handled object involves seamless coordination of all facets of somatic sensory information. Quiz hint! – Signals picked up by mechanoreceptors in the skin Questions?