Psyc week 3
Popular in Introduction to Neuroscience
Popular in Psychology (PSYC)
This 8 page Class Notes was uploaded by Sabrina Straus on Friday September 30, 2016. The Class Notes belongs to PSYC 6 at Dartmouth College taught by Catherine Cramer in Fall 2016. Since its upload, it has received 3 views. For similar materials see Introduction to Neuroscience in Psychology (PSYC) at Dartmouth College.
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Date Created: 09/30/16
2. 09-28-16 Intro to Neuro class notes SENSORY PROCESSES AND SOMATOSENSATION ~sensory surface is skin- > goes through spinal cord -> brainstem->midbrain to thalamus or directly to thalamus-> primary sensory cortical area or cortex and then to secondary I. General characteristics of sensory systems A. Transduction: something turned into a neural signal generator potentials: response to stimulus can reach threshold and result in AP (and sent to central nervous system) but doesn’t have to B. Labeled lines-what info travels on/ segregated by type of info C. Receptotopic organization receptive fields: area of sensory surface to which a given upstream neuron responds (produce different firing neurons) D. Feature coding and perception E. Psychophysical (how the whole organism responds to something) versus neurophysiological (record how nervous system responds to a particular stimulus~you control stimulus) approaches II. Somatosensation A. Specificity of receptors Proprioceptors (muscle spindles, Golgi tendon organs) Contact group Pacinian corpuscles-deepest layer } sensitive to vibration Large vague borders Fast-adapting response Ruffini’s endings-deepest layer } sensitive to stretch Large borders Slow-adapting Meissner’s corpuscles-touch Small borders fast -adapting Merkel’s discs-touch Sharp borders slow-adapting Hair follicle cells-touch Thermal group (free nerve endings~ pain and temp) } close to surface TRPV-type (warm) 1 C fiber: thin } second pain} remember+re-cooperative PAIN: 2 A8 fiber: ticker faster and myelinated} sharp first pain~reaction Spinothalamic pathway: decussate at spinal level > medulla through anterolateral tracts in grey matter -> pons + reticular formation -> periaqueductal gray-> thalamic nuclei AB: mechanoreceptors, myelinated faster Axon to grey matter which is the intermediate zone and send through white matter (myelinated axons) which goes to the brain Aa: proprioceptors, myelinated, fastest TRPM-type (cool) } menthol C fiber Pain group (nociceptors) Mechanical and thermal types B. Fast- vs slow-adapting C. Spinal inputs dorsal root ganglia (unipolar neurons) and dorsal horn -> go into gray matter -> first synapse in medulla and decussate (cross over) -> midbrain -> cortex Dorsal column sensation Speed varies on axon A-fibers: myelinated C-fibers Ipsilateral - same side decussates Contralateral -opposite side } all sensory info Dermatomes: sacral, lumbar, thoracic, cervical D. Ascending somatosensory pathways Dorsal column system Anterolateral (aka spinothalamic) system E. Processing somatosensory information homunculus receptive fields: small in finger tips but large in other places intensity cortical columns Chapter Notes Chapter 12: Somatic Sensory System ~somatic sensation: enables our body to feel >distributed receptors >responds to many different kinds of stimuli >4 senses: touch, temperature, pain, and body position >a single sensory receptor can determine intensity, duration, position, and direction -Touch: begins at the skin ● Skin: hairy + glabrous ○ Layers ■ Epidermis: outer layer ■ Dermis: inner layer ○ Function ■ Protection ■ Keep in fluids ■ Direct contact with world ● Mechanoreceptors of the skin: sensitive to physical distortion such as bending or stretching ○ Monitor skin contact, pressure in the heart and blood vessels, stretching of the digestive organs and urinary bladder, and force against teeth ○ Have unmyelinated axon branches that are sensitive to stretching, bending, pressure, or vibrat ○ Examples: each sensitive around different vibratios Large receptive fields ■ Pacinian corpuscle: esp in fingers } rapidly adapting receptors ■ Ruffini’s endings } slowly adapting ■ Krause end bulbs: nerve terminals look like knotted balls of string Small receptor fields ■ Meissner’s corpuscles: located in the ridges of glabrous skin } rapidly adapting receptors ■ Merkel’s disks: consist of a nerve terminal and a flattened, non-neural epithelial cell (the Merkel cell) } slowly adapting ~slowly adapting: sustained response during long stimulus as opposed to the short one w/ rapidly adapting receptors >hairs grow from follicles which are innervated by free nerve endings (terminations of single axons -> when hair is bent, there is a deformation of the follicle and surrounding skin tissues, which affects nerve endings (increasing or decreasing their action potential firing frequency) ● Vibration and pacinian corpuscle: selectivity of mechanoreceptive axon depends on structure of special ending-> compressed capsule results in energy transfer to nerve terminal-> membrane is deformed-> mechanosensitive channels open-> current runs through and generates a receptor potential (depolarization) -> action potential or capsle layers can transfer energy so axon terminal isn’t deformed ○ More sensitive to steady pressure w/out capsle -> capsle helps Pacinian corpuscles to communicate down their large fast axons ● Mechanosensitive ion channels ○ Unmyelinated axons w/ mechanosensitive ion channels on membranes that convert mechanical force into a change of ionic current ○ Forces ■ Alter gating ■ Stretched or bent membrane ■ connections between the channels and extracellular proteins or intracellular cytoskeletal components ■ Second messenger ● Two-point discrimination: measure of spatial resolution ○ Fingertips have the highest resolution bc ■ Higher density of mechanoreceptors ■ Enriched in receptor types that have small receptive fields ■ More brain tissue devoted to the sensory info of fingertip ■ Special neural mechanisms devoted to high-resolution discriminations ● Primary afferent axons ○ Afferent: bring info from somatic sensory receptors to spinal cord or brainstem ■ Enter through dorsal roots (lie in ganglia) ■ Varying diameters ● Aa - larges / proprioceptors of skeletal muscle ● Ab - mechanoreceptors of skin ● A8 - pain temperature ● C - smallest for temperature/ pain/ itch ● Spinal cord: encased in bony vertebral column ○ Segmental organization of the spinal cord: each spinal nerve passes through backbones of spinal column ■ Cervical-top ■ Thoracic ■ Lumbar ■ Sacral-bottom ~dermatome: area of skin innervated by right and left dorsal roots of a single spinal segment ~shingles: skin innervated by axons of one dorsal root -> all neurons of a single dorsal root ganglion become infected ~cauda: bundles of spinal nerves streaming down within the lumbar and sacral vertebral column Lumbar puncture- used to collect CSF for medical diagnostic tests ○ Sensory Organization of spinal cord: inner core of gray matter, surrounded by a thick covering of white matter tracts that are often called columns ■ Dorsal horn ■ Intermediate zone ■ Ventral horn ■ second -order sensory neurons: receive sensory input from primary afferents (most in dorsal) ● Dorsal column medial lemniscal -pathway for touch ○ ascending branch of the large sensory axons (Ab) enters the ipsilateral dorsal column of the spinal cord, the white matter tract medial to the dorsal horn ->The dorsal columns carry information about tactile sensation (and limb position) toward the brain ■ Composed of primary sensory axons ○ Terminate in dorsal column nuclei ○ ipsilateral (right info-> right side) ○ Axons of dorsal column nuclei ascend in medial lemniscus-> goes thru medulla, pons, and midbrain -> axons synapse upon neurons of ventral posterior of thalamus-> project to specific regions of primary somatosensory cortex ● Trigeminal touch pathway: for face ○ Enter the brain at pons ○ three peripheral nerves that innervate the face, mouth areas, the outer two-thirds of the tongue, and the dura mater covering the brain ○ Larger diameter: sensory which synapse onto second-order neurons in ipsilateral trigeminal nucleus-> project into the medial VP of thalamus -> somatosensory cortex ● Somatosensory cortex-mostly in parietal lobe + posterior parietal cortex ○ 3b= primary somatic sensory cortex bc 1. receives dense inputs from VP nucleus of thalamus 2. Neurons are responsive to somatosensory stimuli 3. Lesions impair somatic sensation 4. Sensory experiences can be evoked thru electrical stimulation ○ 3a:sense of body position ○ Area 1(texture) and 2(size and shape) receives input from 3b ○ Cortical somatotopy: mapping of the body’s surface sensations onto a structure in the brain ■ Generated by electrical stimulation+neuronal recordings ■ Shows where most sensory input is from ○ Cortical map plasticity ■ Adjust depending on amt of sensory experience ○ Posterior parietal cortex:essential for the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space ■ neurons below the cortex and in cortical areas 3a and 3b are not sensitive to the direction of stimulus movement across the skin, but cells in areas 1 and 2 are ■ Neurons have large receptive fields with stimulus preferences that are a challenge to characterize because they are so elaborate->area is concerned not only with somatic sensation but also with visual stimuli, movement planning, and even a person’s state of attentiveness ■ Damage ● Agnosia:inability to recognize objects ● Neglect syndrome: by parietal cortical lesions which suppresses part of the body -Pain ~somatic sensation depends on mechanosensitive touch receptors and nociceptors (free, branching, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged/ sensory process that provides signals that trigger pain) ~takes dif. Path to brain and causes dif. response ● Nociceptors+transduction of painful stimuli ○ nociceptors : activated by stimuli that can cause tissue damage + activate ion channels ■ Strong mechanical stimulation ■ Extreme temp ■ No O2 ■ Exposure to chemicals ○ Types: ■ Polymodal nociceptors: respond to mechanical, thermal, and chemical stimuli ■ Mechanical nociceptors: selective responses to strong pressure ■ Thermal nociceptors: selective responses to extreme temps ■ Chemical nociceptors: showing selective responses to histamine etc. ~absent in the brain except meninges ○ Hyperalgesia + inflammation: reduced threshold for pain, increased intensity of stimuli, or spontaneous pain ■ Primary: occurs in damaged tissue and also can affect surrounding tissue w/ secondary hyperalgesia ■ Inflammatory soup: substances released when skin is damaged ● Contain neurotransmitters, peptides, lipids, proteases, neurotrophins, cytokines, and chemokines ● Trigger inflammation: attempts to eliminate injury and stimulate healing ● Some can make nociceptors more sensitive ○ Bradykinin ○ Prostaglandins: chemicals generated by enzymatic breakdown of lipid membrane ○ Substance P: can be released and cause other branches of axon to have swelling blood capillaries and release histamine ○ Cross-talk between touch and pain pathways ● Treatment: aspirin ○ Inhibit enzymes required for prostaglandin synthesis ● Itch ○ Chronic: allergic reactions, infections, infestations, and psoriasis; it can also be triggered by non-skin disorder such as cancers, iron deficiency, hyperthyroidism, liver disease, stress, and psychiatric conditions ○ mediated by thin sensory axons+triggered by various types of stimuli, including chemicals and touch. Some of the drugs and compounds that regulate pain can also trigger itch, and some signaling molecules transduce both sensations. Pain and itch also interact ○ Histamine: itching producing substance released during inflammation ■ mediates itch by binding to histamine receptors, which then activate TRPV1 channels ○ Endogenous and exogenous substances can also trigger itch ● Primary afferents+spinal mechanisms ○ Sharp pain: activation of A8 ■ Cell bodies in segmental dorsal root ganglia ■ Enter dorsal horn of spinal cord ■ Fibers branch and travel in a region called zone of Lissauer ■ Synapse on cells of outer part of dorsal horn in substantia gelatinosa region ○ Glutamate: neurotransmitter + substance P(contained within storage granules in axon terminals and can be released by high-frequency trains of action potentials) ■ Referred pain: where visceral nociceptor activation is perceived as cutaneous sensation because of cross-talk ● Ex: angina (heart doesn’t get enough O2), appendicitis ○ Dull long pain: activation of C fibers ■ Ascending pain pathways ○ Differences between touch and pain ■ Touch: characterized by specialized structures in the skin / diameter of axons=swift, fat, myelinated AB fibers (terminate in deep dorsal horn) / info ascends ipsilaterally ■ Pain: free nerve endings/ diameter of axons=slow, thin, lightly myelinated A8 fibers and unmyelinated C fibers (run within zone of Lissauer and terminate within substantia gelatinosa) / info ascends contralaterally (affects opposite side) ○ Spinothalamic pain pathway: conveys pain from spinal cord to brain ■ Axons of second-order neurons immediately decussate and ascend through tract-> synapse @ thalamus ○ Trigeminal pain pathway: pain from face and head ○ Thalamus+cortex ■ Some pain tracts end in intralaminar nuclei (as well as VP nucleus) ● Regulation of pain ○ Afferent regulation: pain evoked by activity in nociceptors can also be reduced by simultaneous activity in low-threshold mechanoreceptors (A fi bers). ■ gate theory of pain: certain neurons of the dorsal horns, which project an axon up the spinothalamic tract, are excited by both large-diameter sensory axons and unmyelinated pain axons->projection neuron is also inhibited by an interneuron, and the interneuron is both excited by the large sensory axon and inhibited by the pain axon ->activity in the pain axon alone maximally excites the projection neuron, allowing nociceptive signals to rise to the brain-> if the large mechanoreceptive axon fires concurrently, it activates the interneuron and suppresses nociceptive signals. ○ Descending regulation ■ Periaqueductal gray matter: send descending axons into various midline regions of the medulla (esp. Raphe nuclei)->project axons down to dorsal horns to depress activity of nociceptive neurons ○ Endogenous opioids: produce analgesia ■ Act by binding tightly to opioid receptors-> brain manufactures endogenous morphine like substances} endorphins (small proteins or peptides) ● Endorphins: suppress the release of glutamate from presynaptic terminals and inhibit neurons by hyperpolarizing their postsynaptic membranes -Temperature: depend on neocortex ● Thermoreceptors: neurons that are exquisitely sensitive to temp because of specific membrane mechanisms ○ Esp in hypothalamus and spinal cord ○ Some are sensitive to either hot or cold ○ Adapt during long stimuli ● Temperature pathway: same as pain pathway ○ Cold: A8 and C ○ Warm: C ○ Small diameter axons synapse within the substantia gelatinosa of the dorsal horn ○ Axons of second-order neurons immediately decussate and ascend in the contralateral spinothalmic tract
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