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The Peripheral Nervous System Links the Brain to the “Real”

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The Peripheral Nervous System Links the Brain to the “Real” Peripheral Nervous System Peripheral Nervous System Organization of Nervous System: The peripheral nervous system links Nervous system the brain to the “real” world Integration Ganglia Central nervous system Peripheral nervous system (neuron cell bodies) (CNS) (PNS) Motor Sensory output input Brain Spinal cord Motor division Sensory division (efferent) (afferent) Nerve Types: Nerves 1) Sensory nerves (contains only afferent fibers) (bundles of axons) Autonomic nervous system Somatic nervous system 2) Motor nerves (contains only efferent fibers) (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) 3) Mixed nerves (contains afferent / efferent fibers) Sensory receptors Most nerves in the human body are mixed nerves Sympathetic division Parasympathetic division Motor output Peripheral Nervous System Peripheral Nervous System Nerve Structure: Endoneurium Classification of Nerve Fibers: A. Epineurium: • Classified according to conduction velocity • Outside nerve covering • Dense network of collagen fibers Perineurium Relative Relative conduction Classification Type diameter velocity Myelination B. Perineurium: • Divides nerve into fascicles A alpha (A) Largest Fastest (120 m / s) Yes • Contains blood vessels A beta (A) Medium Medium Yes Epineurium C. Endoneurium: Sensory and A gamma (A) Medium Medium Yes • Surrounds individual axons and ties Motor A delta (A) Small Medium Yes them together (Erlanger & Gasser) B Small Medium Yes C Smallest Slowest (0.2 m / s) No Fascicle Ia Largest Fastest Yes Ib Largest Fastest Yes Sensory only II Medium Medium Yes (Lloyd & Hunt) III Small Medium Yes IV Smallest Slowest No Marieb & Hoehn – Figure 13.26 Peripheral Nervous System Sensory Systems Organization of Nervous System: “Nothing is in the mind that does not pass through the senses” Nervous system Aristotle (~ 350 B.C.) Integration Central nervous system Peripheral nervous system (CNS) (PNS) • Sensory receptors provide the only channels of communication from the external Motor Sensory world to the nervous system: output input • Detections: Electrical impulses - triggered by receptor cells Brain Motor division Sensory division • Sensations: Electrical impulses - reach brain via neurons Spinal cord Subjective (efferent) (afferent) • Perceptions: Interpretation of electrical impulses by brain Autonomic nervous system Somatic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) Sympathetic division Parasympathetic division Sensations / perceptions are not inherent in stimuli themselves; instead they depend on the neural processing of the stimuli 1 Sensory Systems Sensory Systems Modality: Category of sensation Sensory Pathways in NS: Fourth-order Sensory Receptors: midline neuron 1) Receptor: Type of Receptor: Modality: Location: • Converts stimuli to electrochemical energy • Structure variable Cerebral cortex • Skin (touch) • Specialized epithelial cells (e.g., vision) Third-order neuron Mechanoreceptors • Inner ear (audition) • Modified neurons (e.g., olfaction) • Inner ear (vestibular) 2) First-order afferent neuron: Touch Audition Vestibular • Cell body located in ganglion (PNS) Thalamus Second-order Photoreceptors • Eye (vision) 3) Second-order afferent neuron: neuron Vision • Synapse with first-order neuron at relay nuclei • Many first-order synapse with one second-order Brain stem • Tongue (taste) • Interneurons present (signal processing) Chemoreceptors • Nose (olfaction) • Cross at midline (decussate) Taste Olfaction Osmolarity • Vessels (osmolarity) 4) Third-order afferent neuron: • Reside in thalamus (relay nuclei) Thermoreceptors Relay nucleus • Skin (temperature) • Interneurons present (signal processing) Temperature Receptor 5) Fourth-order afferent neuron: Spinal • Reside in appropriate sensory area First-order cord Nociceptors • Skin (pain) neuron Pain Guyton & Hall – Figure 46.2 / 46.3 Costanzo – Figure 3.5 / 3.6 Sensory Systems Sensory Systems Sensory Receptors: Sensory Receptors: Sensory transduction: The process by which environmental stimuli activates a Receptor Field: An area of the body that when stimulated results in a change in receptor and is converted into electrical energy firing rate of a sensory neuron • Receptor excitation = Change in membrane potential (Receptor potential) Receptor fields may be excitatory or inhibitory (usually via opening of ion channels) Receptor fields exist for first-, Excitatory receptor fields increase firing rate second-, third-, and fourth-order neurons Inhibitory receptor fields decrease firing rate Example: Pacinian corpuscle 1) Pressure causes a change in the membrane properties (e.g., mechanical deformation) Receptor Potential > Threshold Action Potential 2) Ion channels open; receptor potential develops • Amplitude correlates with stimulus size The smaller a receptor field, the more Lateral inhibition defines boundaries ( Intensity = Receptor potential = AP frequency) 3) Receptor potential triggers action potential precisely the sensation can be localized and provides a contrasting border Sensory Systems Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory Receptors: Sensory coding: Various aspects of perceived stimuli are encoded and sent to the Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency proper location in the CNS of action potentials generated declines over time Features encoded: Tonic receptors (slowly adapting receptors) 1) Modality 2) Location 3) Threshold Labeled Line Principle: Threshold of Detection: Can encode Receptors respond to Weakest signal that will stimulus intensity single modality produce a receptor response 50% of the time Touch Somatosensory Hearing: receptor cortex Vision: Bending of hair equal to 1 photon of light Will adapt to extinction; Receptor fields diameter of H atom may take hours / days 4) Intensity 5) Duration • Number of receptors activated Length of time a sensory Pattern of adaptation differs among • Transmit impulses as long as • Firing rate of sensory neurons neuron fires different types of receptors stimulus present • Types of receptors activated During prolonged stimulus, receptors “adapt” to the stimulus • Keep brain appraised of body status Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7 2 Costanzo – Figure 3.8 Sensory Systems Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Rapid adapters = Change in velocity Sensory Receptors: Somatosensory System: Slow adapters = Intensity; duration Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency • Processes information about touch, position, pain, and temperature of action potentials generated declines over time Mechanoreceptors: Phasic receptors Two-point discrimination (rapidly adapting receptors) Pacinian corpuscle Meissner’s corpuscle Can not encode stimulus intensity Location = Subcutaneous; intramuscular Location = Non-hairy skin Detection = Vibration; tapping Detection = Tapping; fluttering Adaptation = Very rapidly Adaptation = Rapidly Pattern of adaptation differs among • Transmit impulses only when different types of receptors Hair plexus Ruffini’s corpuscle change is taking place Location = Hairy skin Location = Skin; joint capsules • Important for predicting future Detection = Velocity; movement direction Detection = Stretch; joint rotation position / condition of body Adaptation = Rapidly Adaptation = Slowly Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7 Costanzo – Figure 3.8 / 3.9 Sensory Systems Sensory Systems Rapid adapters = Change in velocity Somatosensory System: Slow adapters = Intensity; duration Somatosensory System: • Processes information about touch, position, pain, and temperature • Processes information about touch, position, pain, and temperature Mechanoreceptors: Nociceptors: • Respond to noxious stimuli that can produce tissue damage Merkel’s receptor / Tactile disc Mechanical nociceptors: • Respond to mechanical stimuli Location = Non-hairy skin / Hairy skin Detection = Vertical indentation • A delta (A) afferent neurons (myelinated) Adaptation = Slowly Polymodal nociceptors: Thermoreceptors: • Respond to multiple stimuli: • High-intensity chemical stimuli • Slowly adapting receptors; located in skin • Hot / Cold stimuli • C afferent neurons (unmyelinated) Chemical cues released at damaged / Hyperalgesia: Intense activity areas include: • Cold receptors vs. warm receptors Process where axons of nociceptors release • H+ • Prostaglandins substances that sensitize the nociceptors to • Overlap in response ranges • K+ • Bradykinins stimuli that were not previously painful • Do not respond at extreme ranges (nociceptors) • ATP • Substance P Sensory Systems Sensory Systems Somatosensory Pathways: Somatosensory Pathways: midline midline 1) Dorsal column system 2) Anterolateral (spinothalamic) system • Information transmitted: • Information transmitted: Cerebral Cerebral a) Discriminative touch cortex a) Pain cortex b) Pressure / Vibration b) Temperature / Light touch c) Two-point discrimination • Consists mainly of groups III and IV fibers d) Proprioception • Fast pain (e.g., pin prick) • Consists mainly of groups I and II fibers Thalamus • A delta fibers; group II & group III fibers Thalamus • Fibers decussate in brain stem (second-order neurons) • Rapid onset / offset; precisely localized • Nucleus gracilis = Info from lower body • Slow pain (e.g., burn) • Nucleus cuneatus = Info from upper body Brain • C fibers; group IV fibers Brain stem
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