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Partial Chapters 12, 13 & 16 Circuits, Receptors, and Reflexes Postsynaptic potentials LaPointe Fall ’11 Slide # 2 • Most important determinants of neural activity are EPSP / IPSP interactions • EPSP (excitatory postsynaptic potential) = depolarization • IPSP (inhibitory postsynaptic potential) = hyperpolarization • EPSPs and IPSPs can combine through summation • Temporal summation • Spatial summation • facilitation / inhibition ESPS and ISPS LaPointe Fall ’11 Slide # 3 Temporal and Spatial Summation LaPointe Fall ’11 Slide # 4 Summation (facilitation) LaPointe Fall ’11 Slide # 5 EPSP – IPSP Interactions (inhibition) LaPointe Fall ’11 Slide # 6 Martini Figure 12.23 LaPointe Fall ’11 Slide # 7 Summation Seeley, Stephens and Tate Spatial summation Mixed summation Temporal summation Presynaptic inhibition and facilitation LaPointe Fall ’11 Slide # 8 • Inhibition • GABA release at axoaxonal synapse inhibits opening calcium channels in synaptic knob • Reduces amount of neurotransmitter released when action potential arrives • Facilitation • Activity at axoaxonal synapse increases amount of neurotransmitter released when action potential arrives • Enhances and prolongs the effect of the neurotransmitter Presynaptic Inhibition LaPointe Fall ’11 Slide # 9 Information processing LaPointe Fall ’11 Slide # 10 • Determination of the strength of a stimulus can be coded through recruitment (more neurons fire); or • By the rate of generation of action potentials are often used to interpret the signal. Neuronal pools • Functional group of interconnected neurons • Neural circuit patterns • Divergence • Convergence • Reverberation • Serial processing • Parallel processing LaPointe Fall ’11 Slide # 11 Simple circuits (also see Saladin figs 12.29 & 12.30) LaPointe Fall ’11 Slide # 12 Martini Figure 13.15 Sensory receptors LaPointe Fall ’11 Slide # 13 • Receptors are specialized cells or cell processes that monitor specific conditions (respond to stimuli) • Act as the interface between the CSN and the internal and external environments. • Arriving information into the CNS is a sensation. • Awareness of a sensation is a perception. • not all sensations are perceived • neural input must go to the primary sensory areas for conscious awareness Receptor classification LaPointe Fall ’11 Slide # 14 • Exteroceptors - provide information about the external environment • Interoceptors - provide information about visceral organs and functions • Propioceptors - provide information about positions and tension of the joints and skeletal muscles. Classification of receptors LaPointe Fall ’11 Slide # 15 • Receptors can also be classified based on the type of stimulus they respond to • nociceptors - pain • thermoreceptors - temperature • mechanoreceptors - physical distortion • chemoreceptors - chemical concentrations • photoreceptors - light Sensory receptors LaPointe Fall ’11 Slide # 16 • Nerve fibers fire when an action potential is generated regardless of what caused it. Thus, they are nonspecific. • Specificity comes from specialized receptors. • Receptor cells are generally sensitive to limited types of stimuli (modality) known as receptor specificity. • Each receptor cell monitors a specific receptive field. • The larger the receptive field the less precise localization is. Receptors and Receptive Fields LaPointe Fall ’11 Slide # 17 Figure 15.2 Receptor fields and 2 point discrimination LaPointe Fall ’11 Slide # 18 Interpretation of sensory information LaPointe Fall ’11 Slide # 19 Nerve fibers are non-specific !!! - but they go to specific regions of the brain along specific tracts or bundles. • Information is interpreted based on the labeled line that it travels as to what type of sensation it is (qualitative processing). • True sensations cannot be distinguished from false sensations. • All other characteristics of a stimulus (strength, duration, variability, etc) are conveyed by the frequency and pattern of the action potentials (quantitative processing). Receptors LaPointe Fall ’11 Slide # 20 • Tonic receptors • Always active • Frequency of firing determines information • Slow to adapt • Phasic receptors • Are normally inactive • Give a burst of activity when stimulated • Provide information about the intensity and rate of change of a stimulus • Combined receptors Adaptation LaPointe Fall ’11 Slide # 21 • Reduction in sensitivity of a receptor in the presence of a constant stimulus. • Peripheral adaptation occurs at the level of the receptor. • Reduces sensory information entering the CNS • fast-adapting receptors - phasic receptors, e.g. temperature • slow-adapting receptors - tonic receptors, e.g. pain, proprioception Adaptation LaPointe Fall ’11 Slide # 22 • Central adaptation occurs within the CNS and often involves inhibitory interneurons within the pathway. • CNS can inhibit sensory pathways for example to “filter noise” • Reduces information reaching the cerebral cortex • Awareness is reduced even though the receptors are still active. • Responses may still occur via lower level circuits. • CNS output can also facilitate transmission i.e. increase sensory transmission Sensory receptors LaPointe Fall ’11 Slide # 23 • Free nerve endings • dendrites • not protected by accessory structures • sensitive to many stimuli (pain, temperature, pressure, trauma) • Complex receptors • Are often not neural cells • Merkel cells • rods and cones Mechanoreceptors LaPointe Fall ’11 Slide # 24 • Sensitive to distortion of their membrane • Mechanically sensitive ion channels • Tactile receptors (six types) - touch, pressure, vibration • touch - shape and texture • pressure - mechanical distortion • vibration - pulsing or oscillating pressure • Baroreceptors - monitor pressure changes • Proprioceptors (three groups) - joint and muscle movement, position and location Tactile receptors LaPointe Fall ’11 Slide # 25 • Crude touch and pressure - have large receptor fields • Tactile receptors - more narrow fields provide more information 1. Free nerve endings 2. Root hair plexus- rapid respond to movement 3. Tactile discs (Merkel discs) fine touch myelinated fibers 4. Tactile corpuscles (Messner’s corpusules and Krause end bulbs) - fine touch and pressure, low frequency vibrations. Myelinated fibers adapt within 1 second after contact Tactile receptors LaPointe Fall ’11 Slide # 26 5. Lamellated corpuscles (Pacinian corpuscles) Large structures, deep pressure, fast adapting so more sensitive to vibrations. Seen in viscera such as mesentaries, in the pancreas, urethra, and urinary bladder, as well as skin 6. Ruffini corpuscles - pressure and distortion of skin, located in deep dermis, show little adaptation. • Itch and tickle sensations use free nerve endings Proprioception • Muscle spindle, • Golgi tendon organs, • joint kinesthetic receptors • stretch receptors or joint pressure LaPointe Fall ’11 Slide # 27 Chemoreceptors LaPointe Fall ’11 Slide # 28 • Chemoreceptors of the general senses do not send information to the primary sensory cortex. Thus, there is no conscious awareness (sensation without perception). • Exhibit peripheral adaptation after a few seconds and may exhibit central adaptation • Carotid bodies and Aortic bodies are sensitive to pH, CO2 and O2 • chemoreceptors may respond to chemicals released by damaged tissue Summary slide LaPointe Fall ’11 Slide # 29 Summary slide LaPointe Fall ’11 Slide # 30 LaPointe Fall ’11 Slide # 31 • Note: Somatosensory projection pathways and pain pages 587-591 will be covered in the pathways and tracts lecture Reflexes LaPointe Fall ’11 Slide # 32 • Reflexes are rapid automatic responses to stimuli • Neural reflex involves sensory fibers to CNS and motor fibers to effectors Reflex arc LaPointe Fall ’11 Slide # 33 • Five steps • Arrival of stimulus and activation of receptor • Activation of sensory neuron • Integration / Information processing (interneurons) • Activation of motor neuron • Response by effector (muscle or a gland) Reflex classification LaPointe Fall ’11 Slide # 34 • Named several ways i.e. according to: • Development (innate or acquired i.e. learned) • Site of information processing (cranial or spinal) • Nature of resulting motor response (e.g. flexor reflex) • Complexity of neural circuit Reflex classifications LaPointe Fall ’11 Slide # 35 • Innate reflexes - Result from connections that form between neurons during development (e.g. chewing, sucking, tracking). • Acquired reflexes - Learned, and typically more complex (e.g. driving skills, bell ringing and leave class, typing) • Cranial reflexes - Reflexes processed in the brain (e.g. startle reflex) • Spinal reflexes - Interconnections and processing events occur in the spinal cord (e.g. knee jerk reflex) More reflex classifications LaPointe Fall ’11 Slide # 36 • Somatic reflexes • Control skeletal muscle • They are imprecise and crude (e.g. the knee jerk reflex) • Provide a rapid response (e.g. pull away from a hot surface) • often modified by higher centers • Visceral reflexes (autonomic reflexes) • Control activities of other systems (e.g. blood pressure, urination, defecation) and even more reflex classifications LaPointe Fall ’11 Slide # 37 • Monosynaptic reflex • Sensory neuron synapses directly on a motor neuron (there is no interneuron) • Polysynaptic reflex • At least one interneuron between sensory afferent and motor efferent • Because of synaptic delay, the more interneurons there are the slower the reflex i.e. the longer delay between stimulus and response Monosynaptic Reflexes LaPointe Fall ’11 Slide # 38 • Stretch reflex automatically monitors skeletal muscle length and tone • Patellar (knee jerk) reflex • Sensory receptors are muscle spindles • Postural reflexes maintains upright position Stretch Reflex (e.g. patellar reflex) LaPointe Fall ’11 Slide # 39 Also see Saladin fig 13.21 with steps involved Figure 13.16 Muscle spindles LaPointe Fall ’11 Slide # 40 • Specialized muscle regions used as sensory stretch receptors. • Extrafusal muscle fibers • alpha (a) motor neurons • Intrafusal muscle fibers • gamma (g) motor neurons Muscle spindles (also see Saladin fig 13.20) LaPointe Fall ’11 Slide # 41 Figure 13.15 Intrafusal Fibers LaPointe Fall ’11 Slide # 42 Figure 13.17 Golgi Tendon Reflex (also see Saladin fig 13.23) LaPointe Fall ’11 Slide # 43 •Prevents contracting muscles from applying excessive tension to tendons •Produces sudden relaxation of the contracting muscle and activation of the antagonistic muscles Flexor and Inhibitory Reflexes Also see Saladin fig 13.21 LaPointe Fall ’11 Slide # 44 Withdraw and crossed extensor reflexes Also see Saladin fig 13.22 LaPointe Fall ’11 Slide # 45 Reinforcement and inhibition LaPointe Fall ’11 Slide # 46 • Brain can facilitate or inhibit motor patterns based in spinal cord • Complex movements such as walking can work by having the brain initiate reflex movements • Reinforcement - facilitation that enhances spinal reflexes • Spinal reflexes can also be inhibited • Babinski reflex replaced by the Planter reflex The Plantar and Babinski Reflexes LaPointe Fall ’11 Slide # 47 Figure 13.23