Lec 13: PNS

• Sensory Receptors


  Respond to changes in their environment called stimuli

  -Stimulus initiates grades potential (local depolarization) that then in turn triggers nerve impulses along afferent PNS fibers to the CNS
• Mechanoreceptors


  respond to touch/pressure
• Thermoreceptors


  response to temperature changes
• Photoreceptors


  respond to light energy
• Chemoreceptors


  respond to chemicals
• Nociceptors


  respond to damaging stimuli
• Exteroceptors


  1) Located at or near body surface and respond to stimuli outside of the body

  e.g. touch, pressure, pain, vibrations, skin temperature, limb motion, and special senses (vision, hearing, equilibrium, taste, smell)
• Interoceptors (visceroceptors)


  1) Internal; respond to stimuli inside the body such as internal viscera and blood vessels

  e.g. receptors for stretch, pain, temperature, chemical changes (nausea, hunger, etc.)
• Proprioceptors


  1)Respond to internal stimuli like above, but location is restricted to skeletal muscle, tendons, joints, and ligaments

  e.g. Constantly advise brain on body movement
• Simple Receptors


  1) Modified dendritic endings of sensory neurons

  2) Located throughout body and monitor general sensory info (touch, stretch, temp, pressure, chemicals, etc.)
• Complex Receptors


  1)Clusters of multiple cell types; usually considered complex sensory organs2)Associated with special senses: vision (eyes), hearing (cochlea), equilibrium (vestibule), smell, and taste
• Endoneurium


  Bundles individual fibers,

  Located around Schwann cells and Myelin Sheath

  
• Perineurium


  Bundles fibers into fascicles

  
• Epineurium


  Bundles fascicles into a nerve

  
• What disease that could cause at trigeminal nerves?

  HSV-1 (herpes simplex virus)

  
• Dorsal Rami


  Serve the skin and muscles of the posterior trunk

  
• Ventral Rami


  Extended anterior to recombine with other ventral rami superior and inferior to it to form a complex network called a plexus

  
• Ventral rami note:

  A advantage of this fiber regrouping is that damage to one spinal segment or root cannot completely paralyze any limb muscle

  
• Plexus


  Formed by multiple ventral rami that will then give rise to individual spinal nerves
• Dorsal Ramus


  Innervates posterior trunk (the back),

  Each dorsal ramus remains posterior to innervate narrow strip of muscle and skin in line with its emergence point from spinal column

  
• Intercostal Nerves


  1)Ventral rami arranged in simple segmental pattern corresponding to dorsal rami (only location that doesn't recombine into a plexus) and extend anterior to form intercostal nerves.

  2) Intercostal nerves and their branches then extend anterior to supply intercostal muscles, muscle and skin of anterolateral thorax, and most of abdominal wall

  
• Cervical Plexus


  Originated between C3-C5

  1) Mostly cutaneous nerve innervation of neck and shoulder with exception of phrenic nerve

  
• Phrenic Nerve


  Supply both sensory and motor innervation to diaphragm

  
• Lumbar Plexus


  Proximal branches innervate parts of abdominal wall muscles but major branches descend to innervate anterior and medial thigh

  
• Femoral Nerve


  Innervates skin and muscles (quadriceps) of anterior thigh and medial surface of leg from knee to foot

  
• Sacral Plexus


  Branches serve buttocks, lower limbs, pelvic structures and perineum

  
• Sciatic Nerve


  1) Thickest and longest nerve in body

  2) Innervates posterior thigh to supply entire lower limb except anteromedial thigh

  3) Has branches that innervate muscles and skin of perineum, help stimulate erection, and maintain voluntary control of urination

  
• Dermatomes


  1) Cutaneous branches of a single spinal nerve that innervate all areas of the skin.

  2) Fairly uniform in width in body trunk and in direct line with their spinal nerves; different in lower limbs

  -Lumbar nerves supply anterior leg surfaces

  -Sacral nerves supply posterior leg surfaces
• Efferent nerves in the PNS

  1) Ach in the Somatic Nervous system

  2) Typically Ach or NE in the ANS

  
• Somatic nerves to voluntary skeletal muscles (effectors)

  Motor endings at the neuromuscular junction concentrate neurotransmitters

  - Very fast transmission of impulse across the synapse

  
• Parasympathetic Nerves


  1)Routine daily activities with the ability to relax and slow down organ function

  2)Local response (one foot on the brake)

  
• Sympathetic Nerves


  1) Vigorous exercise, excitement, emergency response.

  2)Body wide mobilization (one foot on the gas)

  
• Reflex Arc

  
  

  1) Basic functional unit of the nervous system (the neuron)

  2) Capable of receiving a stimulus, integrating the information and sending out motor commands based on that stimulus

  3)Seek to establish homeostatic balance and/or protection and are an automatic (involuntary) reaction to stimuli

  4) Can be inborn (intrinsic) or learned (acquired from practice or repetition)

  
• Components of Reflex Arc

  Receptor,

  Sensory (afferent) Neuron,

  Integration Center (usually with an interneuron),

  Motor (efferent) Neuron,

  Effector

  
• Functional classifications of Reflex Arcs

  1) Somatic (activate skeletal muscle),

  2) Autonomic (activate visceral effectors... smooth or cardiac muscle or glands)

  
• The Stretch Reflex


  Muscle spindle receptors send action potentials in response to fibers stretching

  
  

  ex: Knee jerk reflex-- As your knees begin to buckle, stretch receptor is activated and reflex causes quadriceps to contract without you having to think about it. Produces extension at the knee.

  
• The Withdrawal Reflex


  Causes automatic withdrawal of threatened body part from painful or unpleasant stimulus

  ex) arm withdrawal reflex

  
• Cross-Extensor Reflex


  1) One body part withdraws (flexion) while the opposite side of the body extends

  2) Particular important in maintaining balance

  ex: stepping on broken glass (rapid lifting of cut foot while activation of extensor muscles on opposite leg to support weight suddenly shifted to it)

  
• Reflexes interact with other nerve pathways

  In the withdrawal reflex, signals from pain receptors continue on to the brain to be interpreted as pain even though the reflex has already removed the body art

  
• Types of Nerves

  • mixed nerves
  • sensory nerves
  • motor nerves