Midterm 3

• somatic nervous system involves sensory nerves receiving information from external environment and body senses and motor nerves sending information to skeletal muscles
• Somatic sensation responsible for touch, itch, pain. Group many senses instead of 1. Unique in 2 ways. 1) receptors broadly distributed, everywhere in skin. 2) responds to many kind of stimuli
• major subsystems of somatic sensation: 1) mechanosensory: touch and body position (proprioception) 2) temperature and pain
• Sensory Neurons: 1 axon extends to periphery, another into CNS (brain or spinal cord) via dorsal root. Pseudounipolar neurons with cell body in dorsal root ganglion. Peripheral process on one end, cell body, central process on other end.
• Somatosensory Receptors: divided based on function. 1) Mechanoreceptors (touch) 2) Nociceptors (pain) 3) Pruriceptors (itch) 4) thermoreceptors (temperature)
• Somatosensory receptors divided based on morphology. 1) free nerve endings present in skin (unmyelinated terminal branches) include nociceptors, thermoreceptors, pruriceptors. 2) encapsulated, different types: pacinian, ruffini, meissner, etc. includes mechanoreceptors for touch.
• Sensory Transduction: 1) stimuli deform skin. 2) receptor membrane permeability altered. 3) Depolarizing current generated. 4) action potential triggered -> central process -> brain. Detect quality and quantity of stimulus
• Quality of stimulus: (What and where it is) determined by nature of receptors themselves: encapsulation affects what stimuli respond to. Quantity: determined by rate of action potential discharge: adaption, neurons adjust response in continued stimulus and decrease action potentials
• Mechanoreceptors: encapsulated nerve endings. "cutaneous mechanoreceptors". Sensitive to physical distortion: low threshold (high sensitivity) and will fire to small stimulus
• Types of corpuscles: meissner, pacinian, ruffini's, merkel's disks, free nerve endings
• Receptive fields of mechanoreceptors: vary in preferred stimulus. Receptive field is specific area of skin where can transduce pressure or vibration. Meisnner's corpuscles and Merkel's disks: small receptive fields. Pacinian corpuscles and ruffini's endings: large
• Experiments to determine receptive fields of mechanoreceptors: place probe in palm and electrode in nerve to see if stimulation causes action potential change
• adaption of mechanoreceptors: vary in persistence of response of long lasting stimuli. Meissner and Pacinian corpuscles: respond quickly but cease firing stimulus continues. Radpidly adapting. Merkel's disks and Ruffini's endings: generate sustained response to long stimulus. Slowly adapting.
• Meissner's corpuscle receptive field: small, rapid. initial compression causes action potentials. stop. fire once removed.
• Merkel's disks receptive field: small, slow. initial compression causes increased action potentials. Decrease but continue until removed
• Pacinian corpuscle receptive field: large, rapid. dispersed action potentials with initial compression. stop. continue once removed
• Ruffini's ending receptive field. large, slow. continued action potentials with and without compression.
• Pacinian corpuscles: football shaped capsule with connective tissue layers like onion. In-tact corpuscles: generate large receptor potential at stimulus onset and offset. layers requried for sensitivity to vibration, pressure, etc. bare axons prolonged potentials and slower adaption
• Mechanosensitive Ion Channels: convert mechanical force (pushing/pulling) into change of ionic current. Specific types of channels in most somatic sensory receptors are still unidentified. Some sensitive to stretching, force from extracellular or intracellular proteins, deform cytoplasm
• Two Point Discrimination: measure spatial resolution: fingertips work best because: 1) increase density of mechanoreceptors 2) enriched in receptor types (Merkel's disks) with small receptive fields and high resolution 3) more brain tissue devoted to each square mm of fingertip. 4) special neural mechanisms devoted to high resolution discrimination
• Mapping of 2 point discrimination: where in body 2 points received as 1 vs. 2. Mapped onto primary somatosensory cortex. Motor homunculus and primary somatosensory cortex (posterior to central sulcus). Body not represented evenly, on graph much more for tips.
• Primary Afferent Axons: axons that bring information from somatic sensory receptors to spinal cord or brain PNS -> CNS. Enter spinal cord through dorsal roots and cell bodies in dorsal root ganglion. Neurons differ in axon diameter and myelination
• Varying of primary afferent axons: diameter size correlates to type of sensory receptor from which receive information, determinant of conduction velocity due to resistance. C fibers have the smallest diameter and unmyelinated-slowest. Mediate temperature, throbbing, pain, itch. ABeta axons larger, conduct touc sensations via cutaneous mechanoreceptors
• Segmental Organization of Spinal Cord: most peripheral nerves communicate with the CNS via spinal cord. 30 spinal segments of paired dorsal and ventral roots. Correspond to vertebrae through which they pass. Spinal nerves are further diveded into 4 subdivision within spinal cord
• Dermatomes: segmental organization of spinal nerves and sensory innervation of skin related. Dermatome: area of skin innervated by right and left dorsal roots of spinal segment. Set of overlapping bands on the surface of body.
• Shingles: herpes zoster virus (chicken pox) remains dormant in primary sensory neurons. Reactivation later in life when neurons of one dorsal root ganglion infected, affecting one dermatome. Increases excitability of sensory neurons. Result is constant burning sensation and skin becomes inflames nad blistered (scaly). Vaccine if over 50 for those who had pox.
• Sensory Organization of Spinal Cord: divisions of spinal gray matter: dorsal horn, intermediate zone, ventral horn. ABeta axons from cutaneous mechanoreceptors enter dorsal horn. One branch synapses in dorsal horn on second order neurons to initiate or modify reflexes, info stays within spinal cord. Other branch ascends straight to brain ipsilateral to stimulus, responsible for perception.
• Touch pathway: begins at skin (largest sensory organ in body); 2 types: hair and glabrous/hairless. 2 layers: epidermis (outer) and dermis (inner). Function of skin: protects, prevents evaporation of body fluids, provides direct contact with world
• Dorsal Column Medial Lemniscal Pathway: mediates tactile sensation, vibration, proprioception, major route for information to cortex. ABeta axons enter and ascend via ipsilateral dorsal column. Axons terminate in dorsal column nuclei in medulla. Axons from dorsal column decussate and ascend via medial lemniscus, synapse in ventral posterior nucleus (VPN thalamus) send axons to primary somatosensory cortex (S1)
• Trigeminal Touch Pathway: somatosensory information from face supplied by trigeminal nerve (CNV) Innervates face, mouth area, outer 2/3 of tongue, dura. Trigeminal nerve has 3 branches: 1) opthalmic (V1): in and around eye, top of head. 2) Maxillary (V2): midface. 3) Mandibular (V3): jawbone. Enters brainstems at pons (principle sensory trigeminal nucleus) ipsilateral. Decussates and sends projections to medial VPN and then to s1
• Somatosensory Cortex (SMC): located in parietal lone postcentral gyrus: BA1, 2, 3, 3A, 3B. 3B is primary SMC because 1) receives input form VPN of thalamus. 2) Responsive to somatosensory stimuli. 3) lesions impair somatic sensation. 4) Stimulation evokes somatic sensory experiences.
• Laminar structure of SMC: layer IV recieves thalamic input. S1 neurons with similar inputs are stacked vertically into columns
• Somatotopic Mapping: mapping of body's surface sensations onto structure in brain. relative to size of cortex devoted to each body part correlated with density of sensory input received from that part. Phantom limbs, loose limb that had associated areas in S1, remapping, may feel pain. Fingers, lips, tongue overrepresented
• Somatotopic Map Plasticity: Compare somatotopy before and after. Map regions of s1 sensitive to stimulation of hand. Removal vs. Addition of stimulation.
• Effect of removing one finger on map: remap hand representation months later. Cortex devoted to removed finger now respond to stimulation of adjacent digits. Missing digit leads to reorganization
• Effect of increasing activity to digits on map: experiment by training monkeys to use selected digitis. remapping of S1 found representation of stimulated digits expanded. can happen to guitar players.
• Posterior Parietal Cortex: BA5 and 7. Somatic sensation, visual stimuli, movement planning, attentiveness. Neurons large receptive fields with elaborate stimulus preferences. Damage causes interesting neurological disorders.
• Agnosia: inability to identify object even though simple sensation normal. Damage to Posterior parietal cortex.
• Astereognosia: normal sense of touch but lack ability to identify objects by feeling them, without visual input. Damage to Posterior parietal cortex.
• Neglect syndrome: a part of the body or visual field is ignored or its existence is denied, not consciously aware. Damage to Posterior parietal cortex, more common for right hemisphere damage. Often imporves over time.