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

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The Peripheral Nervous System Links the Brain to the “Real” World Peripheral Nervous System Organization of Nervous System: Nervous system Integration Central nervous system Peripheral nervous system (CNS) (PNS) Motor Sensory output input Brain Spinal cord Motor division Sensory division (efferent) (afferent) Autonomic nervous system Somatic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) Sympathetic division Parasympathetic division Peripheral Nervous System The peripheral nervous system links the brain to the “real” world Ganglia (neuron cell bodies) Nerve Types: Nerves 1) Sensory nerves (contains only afferent fibers) (bundles of axons) 2) Motor nerves (contains only efferent fibers) 3) Mixed nerves (contains afferent / efferent fibers) Sensory receptors Most nerves in the human body are mixed nerves Motor output 1 Peripheral Nervous System Nerve Structure: Endoneurium A. Epineurium: • Outside nerve covering • Dense network of collagen fibers Perineurium B. Perineurium: • Divides nerve into fascicles • Contains blood vessels Epineurium C. Endoneurium: • Surrounds individual axons and ties them together Fascicle Marieb & Hoehn – Figure 13.26 Peripheral Nervous System Classification of Nerve Fibers: • Classified according to conduction velocity Relative Relative conduction Classification Type diameter velocity Myelination A alpha (A) Largest Fastest (120 m / s) Yes A beta (A) Medium Medium Yes Sensory and A gamma (A) Medium Medium Yes Motor A delta (A) Small Medium Yes (Erlanger & Gasser) B Small Medium Yes C Smallest Slowest (0.2 m / s) No Ia Largest Fastest Yes Ib Largest Fastest Yes Sensory only II Medium Medium Yes (Lloyd & Hunt) III Small Medium Yes IV Smallest Slowest No 2 Peripheral Nervous System Organization of Nervous System: Nervous system Integration Central nervous system Peripheral nervous system (CNS) (PNS) Motor Sensory output input Brain Spinal cord Motor division Sensory division (efferent) (afferent) Autonomic nervous system Somatic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) Sympathetic division Parasympathetic division Sensory Systems “Nothing is in the mind that does not pass through the senses” Aristotle (~ 350 B.C.) • Sensory receptors provide the only channels of communication from the external world to the nervous system: • Detections: Electrical impulses - triggered by receptor cells • Sensations: Electrical impulses - reach brain via neurons Subjective • Perceptions: Interpretation of electrical impulses by brain Sensations / perceptions are not inherent in stimuli themselves; instead they depend on the neural processing of the stimuli 3 Sensory Systems Sensory Pathways in NS: Fourth-order midline neuron 1) Receptor: • Converts stimuli to electrochemical energy • Structure variable Cerebral cortex • Specialized epithelial cells (e.g., vision) Third-order neuron • Modified neurons (e.g., olfaction) 2) First-order afferent neuron: • Cell body located in ganglion (PNS) Thalamus Second-order 3) Second-order afferent neuron: neuron • Synapse with first-order neuron at relay nuclei • Many first-order synapse with one second-order Brain stem • Interneurons present (signal processing) • Cross at midline (decussate) 4) Third-order afferent neuron: • Reside in thalamus (relay nuclei) Relay nucleus • Interneurons present (signal processing) Receptor 5) Fourth-order afferent neuron: Spinal • Reside in appropriate sensory area First-order cord neuron Sensory Systems Modality: Category of sensation Sensory Receptors: Type of Receptor: Modality: Location: • Skin (touch) Mechanoreceptors • Inner ear (audition) • Inner ear (vestibular) Touch Audition Vestibular Photoreceptors • Eye (vision) Vision • Tongue (taste) Chemoreceptors • Nose (olfaction) Taste Olfaction Osmolarity • Vessels (osmolarity) Thermoreceptors • Skin (temperature) Temperature Nociceptors • Skin (pain) Pain 4 Guyton & Hall – Figure 46.2 / 46.3 Sensory Systems Sensory Receptors: Sensory transduction: The process by which environmental stimuli activates a receptor and is converted into electrical energy • Receptor excitation = Change in membrane potential (Receptor potential) (usually via opening of ion channels) 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 ( Intensity = Receptor potential = AP frequency) 3) Receptor potential triggers action potential Costanzo – Figure 3.5 / 3.6 Sensory Systems Sensory Receptors: Receptor Field: An area of the body that when stimulated results in a change in firing rate of a sensory neuron Receptor fields may be excitatory or inhibitory Receptor fields exist for first-, Excitatory receptor fields increase firing rate second-, third-, and fourth-order neurons Inhibitory receptor fields decrease firing rate The smaller a receptor field, the more Lateral inhibition defines boundaries precisely the sensation can be localized and provides a contrasting border 5 Sensory Systems Sensory Receptors: Sensory coding: Various aspects of perceived stimuli are encoded and sent to the proper location in the CNS Features encoded: 1) Modality 2) Location 3) Threshold Labeled Line Principle: Threshold of Detection: Receptors respond to Weakest signal that will 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 Receptor fields diameter of H atom 4) Intensity 5) Duration • Number of receptors activated Length of time a sensory • Firing rate of sensory neurons neuron fires • Types of receptors activated During prolonged stimulus, receptors “adapt” to the stimulus Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency of action potentials generated declines over time Tonic receptors (slowly adapting receptors) Can encode stimulus intensity Will adapt to extinction; may take hours / days Pattern of adaptation differs among • Transmit impulses as long as different types of receptors stimulus present • Keep brain appraised of body status Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7 6 Sensory Systems Adaptation depends on how quickly receptor potential drops below threshold Sensory Receptors: Sensory adaptation: When a constant stimulus is applied to a receptor, the frequency of action potentials generated declines over time Phasic receptors (rapidly adapting receptors) Can not encode stimulus intensity Pattern of adaptation differs among • Transmit impulses only when different types of receptors change is taking place • Important for predicting future position / condition of body Guyton & Hall – Figure 46.5 Costanzo – Figure 3.7 Costanzo – Figure 3.8 Sensory Systems Rapid adapters = Change in velocity Somatosensory System: Slow adapters = Intensity; duration • Processes information about touch, position, pain, and temperature Mechanoreceptors: Two-point discrimination Pacinian corpuscle Meissner’s corpuscle Location = Subcutaneous; intramuscular Location = Non-hairy skin Detection = Vibration; tapping Detection = Tapping; fluttering Adaptation = Very rapidly Adaptation = Rapidly Hair plexus Ruffini’s corpuscle Location = Hairy skin Location = Skin; joint capsules Detection = Velocity; movement direction Detection = Stretch; joint rotation Adaptation = Rapidly Adaptation = Slowly 7 Costanzo – Figure 3.8 / 3.9 Sensory Systems Rapid adapters = Change in velocity Somatosensory System: Slow adapters = Intensity; duration • Processes information about touch, position, pain, and temperature Mechanoreceptors: Merkel’s receptor / Tactile disc Location = Non-hairy skin / Hairy skin Detection = Vertical indentation Adaptation = Slowly Thermoreceptors: • Slowly adapting receptors; located in skin • Cold receptors vs. warm receptors • Overlap in response ranges • Do not respond at extreme ranges (nociceptors) Sensory Systems Somatosensory System: • Processes information about touch, position, pain, and temperature Nociceptors: • Respond to noxious stimuli that can produce tissue damage Mechanical nociceptors: • Respond to mechanical stimuli • A delta (A) afferent neurons (myelinated) Polymodal nociceptors: • Respond to multiple stimuli: • High-intensity chemical stimuli • Hot / Cold stimuli • C afferent neurons (unmyelinated) Chemical cues released at damaged / Hyperalgesia: Intense activity areas include: Process where axons of nociceptors release • H+ • Prostaglandins substances that sensitize the nociceptors to • K+ • Bradykinins stimuli that were not previously painful • ATP • Substance P 8 Sensory Systems Somatosensory Pathways: midline 1) Dorsal column system • Information transmitted: Cerebral a) Discriminative touch cortex b) Pressure / Vibration c) Two-point discrimination d) Proprioception • Consists mainly of groups I and II fibers Thalamus • Fibers decussate in brain stem (second-order neurons) • Nucleus gracilis = Info from lower body • Nucleus cuneatus = Info from upper body Brain stem Fasciculus Fasciculus cuneatus gracilis Receptor Spinal cord Marieb & Hoehn – Figure 12.33 Sensory Systems Somatosensory Pathways: midline 2) Anterolateral (spinothalamic) system • Information transmitted: Cerebral a) Pain cortex b) Temperature / Light touch • Consists mainly of groups III and IV fibers • Fast pain (e.g., pin prick) • A delta fibers; group II & group III fibers Thalamus
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