The Peripheral Nervous System and Reflex Activity

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Chapter 13

• The Peripheral Nervous System and Reflex Activity

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Peripheral Nervous System

• PNS all neural structures outside the brain and
  spinal cord
• All neural structures outside the brain and
  spinal cord
• Sensory receptors
• Peripheral nerves and associated ganglia
• Efferent motor endings
• PNS all neural structures outside the brain and
  spinal cord

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Sensory Receptors

• Structures specialized to respond to stimuli
• Activation of sensory receptors results in
  depolarizations that trigger impulses to the CNS
• The realization of these stimuli, sensation and
  perception, occur in the brain

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Receptor Classification by Stimulus Type

• Mechanoreceptors respond to touch, pressure,
  vibration, stretch, and itch
• Thermoreceptors sensitive to changes in
  temperature
• Photoreceptors respond to light energy (e.g.,
  retina)
• Chemoreceptors respond to chemicals (e.g.,
  smell, taste, changes in blood chemistry)
• Nociceptors sensitive to pain-causing stimuli

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Receptor Class by Location Exteroceptors

• Respond to stimuli arising outside the body
• Found near the body surface
• Sensitive to touch, pressure, pain, and
  temperature
• Include the special sense organs

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Receptor Class by Location Interoceptors

• Respond to stimuli arising within the body
• Found in internal viscera and blood vessels
• Sensitive to chemical changes, stretch, and
  temperature changes

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Receptor Class by Location Proprioceptors

• Found in skeletal muscles, tendons, joints,
  ligaments, and connective tissue coverings of
  bones and muscles
• Respond to degree of stretch of the organs they
  occupy
• Constantly advise the brain of ones movements

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Receptor Classification by Structural Complexity

• Receptors are structurally classified as either
  simple or complex
• Most receptors are simple and include
  encapsulated and unencapsulated varieties
• Complex receptors are special sense organs

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Simple Receptors of the General Senses

• Involved in
• Tactile sensation
• Temperature monitoring
• Pain
• Muscle sense
• Are either unencapsulated (free nerve endings) or
  encapsulated nerve endings

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Unencapsulated Nerve Endings

• Free Nerve Endings
• Located everywhere
• Respond to pain, temperature, pressure

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Unencapsulated Nerve Endings

• Merkel discs
• Lie in deep layers of epidermis
• Function as light touch receptors

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Unencapsulated Nerve Endings

• Hair follicle receptors
• Wrap around hair follicles
• Respond to light touch

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Encapsulated Nerve Endings

• Meissners corpuscles
• Found just beneath the epidermis
• Numerous on sensitive hairless areas

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Encapsulated Nerve Endings

• Pacinian corupscles
• Found everywhere
• Stimulated by deep pressure

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Encapsulated Nerve Endings

• Ruffinis corpuscles
• Lie deep in dermis, subcutaneous tissue and joint
  capsules
• Respond to deep pressure and stretching

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Encapsulated Nerve Endings

• Muscle spindles
• Found in perimysia of skeletal muscles
• Detect when a muscle is stretched
• Initiated a reflex that resists the stretch

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Encapsulated Nerve Endings

• Golgi tendon organs
• Located in tendons, close to muscle insertion.
• Stimulated when muscle stretches the tendon

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Encapsulated Nerve Endings

• Joint kinesthetic receptors
• Monitor stretch in articular capsules of synovial
  joints
• Provide information on joint position and motion

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Structure of a Nerve

• Nerve cordlike organ of the PNS consisting of
  peripheral axons enclosed by connective tissue
• Connective tissue coverings include
• Endoneurium loose connective tissue that
  surrounds axons
• Perineurium coarse connective tissue that
  bundles fibers into fascicles
• Epineurium tough fibrous sheath around a nerve

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Classification of Nerves

• Sensory and motor divisions
• Sensory (afferent) carry impulse to the CNS
• Motor (efferent) carry impulses from CNS
• Mixed sensory and motor fibers carry impulses
  to and from CNS most common type of nerve

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Regeneration of Nerve Fibers

• Damage is serious because neurons are amitotic
• If the soma of a damaged nerve remains intact,
  damage can be repaired
• Regeneration involves coordinated activity among
• Macrophages remove debris
• Schwann cells form regeneration tube and
  secrete growth factors
• Axons regenerate damaged part

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Cranial Nerves

• Twelve pairs of cranial nerves arise from the
  brain

• They have sensory, motor, or both sensory and
  motor functions
• Each nerve is identified by a number (I through
  XII) and a name
• Four cranial nerves carry parasympathetic fibers
  that serve muscles and glands

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Cranial Nerve I Olfactory

• Origin Receptors are in nasal cavity
• Passes through the cribriform plate of the
  ethmoid bone
• Fibers run through the olfactory bulb and
  terminate in the primary olfactory cortex
• Functions solely by carrying afferent impulses
  for the sense of smell

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Cranial Nerve II Optic

• Origin retina of the eye
• Optic nerves converge at the optic chiasm
• They continue to the thalamus where they synapse
• From there, the optic radiation fibers run to the
  visual cortex
• Functions solely by carrying afferent impulses
  for vision

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Cranial Nerve III Oculomotor

• Fibers extend from midbrain to extrinsic eye
  muscles
• Functions in raising the eyelid, directing the
  eyeball, constricting the iris, pupil, and
  controlling lens shape

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Cranial Nerve IV Trochlear

• Fibers emerge from the dorsal midbrain and
  innervate the superior oblique muscle
• Primarily a motor nerve that directs the eyeball

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Cranial Nerve V Trigeminal

• Fibers extend from pons to face and form
• Mixed cranial nerve
• 3 divisions
• Ophthalmic division sensory fibers from area
  surrounding eye
• Maxillary division sensory fibers from nasal
  cavity, upper teeth

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Cranial Nerve V Trigeminal

• Mandibular division sensory fibers from tongue
  lower teeth, chin, and motor fibers to muscles of
  mastication

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Cranial Nerve VI Abducens

• Motor fibers run from pons to extrinsic muscle
  that abducts eye.

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Cranial Nerve VII Facial

• Fibers leave the pons, travel to the lateral
  aspect of the face
• Mixed nerve with five major branches
• Motor functions include facial expression, and
  the transmittal of autonomic impulses to lacrimal
  and salivary glands
• Sensory function is taste from the anterior
  two-thirds of the tongue

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Cranial Nerve VIII Vestibulocochlear

• Fibers arise from the hearing and equilibrium
  apparatus of the inner ear and enter the
  brainstem at the pons-medulla border

• Two divisions cochlear (hearing) and vestibular
  (balance)
• Functions are solely sensory equilibrium and
  hearing

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Cranial Nerve IX Glossopharyngeal

• Fibers emerge from the medulla, leave the skull
  via the jugular foramen, and run to the throat
• Mixed nerve with motor and sensory functions
• Motor innervates part of the tongue and
  pharynx, and provides motor fibers to the parotid
  salivary gland
• Sensory fibers conduct taste and general
  sensory impulses from the tongue and pharynx

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Cranial Nerve X Vagus

• The only cranial nerve that extends beyond the
  head and neck
• Fibers emerge from the medulla via the jugular
  foramen
• The vagus is a mixed nerve
• Most motor fibers are parasympathetic fibers to
  the heart, lungs, and visceral organs
• Its sensory function is in taste

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Cranial Nerve XI Accessory

• The accessory nerves are unique in that they are
  formed by the union of a cranial and spinal root
• Primarily a motor nerve
• Supplies fibers to the larynx, pharynx, and soft
  palate
• Innervates the trapezius and sternocleidomastoid,
  which move the head and neck

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Cranial Nerve XII Hypoglossal

• Motor fibers arise from the medulla and
  innervates both extrinsic and intrinsic muscles
  of the tongue, which contribute to swallowing and
  speech

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• I. Olfactory
• II. Optic
• III. Oculomotor
• IV. Trochlear
• V. Trigeminal
• VI. Aducens
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• VII. Facial
• VIII. Vestibulocochlear
• IX. Glossopharyngeal
• X. Vagus
• XI. Accessory
• XII. Hypoglossal

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