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The Nervous System Peripheral Nervous System Chapter 13

Peripheral Nervous System All structures outside the CNS Sensory receptors Peripheral nerves/ganglia Motor neuron endings

Sensory receptors Respond to specific stimuli, environmental changes Classification based on Stimulus type Location of receptor Structural complexity

Stimulus type Mechanoreceptor-touch, pressure, vibration, stretch Thermoreceptor-temperature Photoreceptor-light Chemoreceptor-molecules Nociceptors-pain

Location of receptor Exteroceptor-stimuli from outside the body Interoceptor (visceroceptor)-internal stimuli from viscera Proprioceptor-internal stimuli from muscles and joints

Structural complexity Simple-majority, general senses Complex-highly modified dendrites, clustered in head, special senses (later chapter)

General senses Free dendritic endings-pain and temp Merkel discs, root hair plexus Encapsulated dentritic endings-mech Meissner’s, Krause’s, Pacinian, Ruffini’s corpuscles, Muscle spindles, Golgi tendon organs, and joint receptors

Receptor potentials Stimulus energy transduced by cell into a change in membrane potential Causes a local graded potential May generate a threshold response to open Na+ channels on axon Generates an action potential to CNS Stimulates a CNS event

Adaptation Unchanging stimulus is ignored with time ?membrane becomes desensitized? Pain and proprioception do not adapt Rate of adaptation varies among cells

Nerves and ganglia Nerves are bundles of axons (cables) Wrapped in CT sheaths Epineurium, perineurium, endoneurium Motor (efferent) fibers from CNS Sensory (afferent) fibers to CNS Mixed (both) have motor and sensory Cranial and spinal nerves

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Ganglia Ganglia-soma of neuron in nerve swellings Dorsal root ganglia are sensory cell bodies in spinal nerves Efferent motor ganglia in ANS Cranial nerve ganglia

Regeneration Soma damage may kill neuron and its neighbors Cut/compressed axons may recover Regeneration requires specific steps and cell actions Regen only in PNS, not in CNS Molecular factors may inhibit or enhance chance for regeneration

Motor neuron endings Somatic fibers-axonal terminals (boutons); release Ach at site, rapid Autonomic terminals (varicosities) are more like beads spread across target cell surface (synapse en passant); release NE or Ach widely, diffuses more slowly to cause effect

Cranial nerves Twelve pairs of nerves serving brain, some motor some sensory, some mixed (both) I olfactory--sensory II optic--sensory III oculomotor--motor

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Cranial nerves IV  trochlear--motor V trigeminal--mixed VI Abducens--motor VII Facial--mixed VII Vestibulocochlear--sensory

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Cranial nerves IX Glossopharyngeal--Mixed X Vagus--Mixed XI Spinal accessory--Motor XII Hypoglossal--Motor

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Cranial nerves, mnemoics On Old Olympus’s Towering Tops, A Finn and Visiting German Viewed Some Hops Some Say Marry Money, But My Brothers Say, Bad Business Marry Money

Spinal Nerves 31 pairs of nerves 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

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Spinal Nerves 31 pairs of nerves 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal Peripheral distribution dorsal root and dorsal root ganglion (S) Ventral root (M)

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Rami Branches of spinal nerve outside vert Dorsal ramus to dorsal skin/muscle Meningeal branch to meninges Ramus communicantes--ANS Ventral ramus to ventral side of body

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Ventral rami T1-T12 intercostal nerves in thorax

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Ventral rami T1-T12 intercostal nerves in thorax Plexus (braid) intertwine diff branches Cervical plexus--C1-C4 Brachial plexus--C5-T1 Lumbosacral plexus L1-L4 Sacral plexus L4-S4

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Spinal Nerves, cont Joint innervation is provided by the same nerves serving the muscles that move that joint Hilton’s Law Innervation of the skin is segmental in distinct regions of the trunk and limbs called dermatomes

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Reflexes Reflex is a rapid, involuntary, stereotyped response to stimulus Neural pathways which carry a reflex are called reflex arcs Some pre-programmed, for survival Some can be learned or acquired Regulated by brain & spinal cord

Elements of Reflex Arc Receptor of some sort A sensory (afferent) neuron Integration center synapse interneuron(s) A motor (efferent neuron) Effector of some sort (usually muscle)

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Reflexes Somatic reflexes--skeletal muscle Autonomic reflexes--viscera and glands Some only involve spinal cord; others involve brain integration Ipsilateral or contralateral Used to evaluate nervous system

Spinal Reflexes Stretch reflex--cause muscle to contract to avoid overstretch pateller (kneejerk)-monosynaptic & ipsilateral Deep tendon-polysynaptic, ipsilateral Flexor (withdrawal)-polysynaptic, ipsilateral Crossed extensor-poly/contra

Superficial reflexes Stimulus is gentle cutaneous Checks upper motor pathways and cord reflexes Babinski (plantar)-sacral plexus abdominal reflexes-lower thoracic