Chapter 13 - Peripheral Nervous System and Reflex Activity

Introduction

• PNS – all neural structures outside the brain and spinal cord
• Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings
• Provides links to and from the external environment
• Two subdivisions - Sensory (__________________) and Motor (__________________)
  
  
• Motor further subdivided into _______________ and ________________

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
• Receptor Classification by Stimulus Type
  • Mechanoreceptors - respond to touch, pressure, vibration, stretch, & 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
• 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
  • Interoceptors
    • Respond to stimuli arising within the body
    • Found in internal viscera and blood vessels
    • Sensitive to chemical changes, stretch, and temperature changes
  • Proprioceptors
    • Respond to degree of stretch of the organs they occupy
    • Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
    • Constantly "advise" the brain of one's movements
• 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
  • Simple Receptors: Unencapsulated
    • Free dendritic nerve endings
    • Respond chiefly to temperature and pain
    • Merkel (tactile) discs
    • Hair follicle receptors
  • Simple Receptors: Encapsulated
    • Meissner's corpuscles (tactile corpuscles)
    • Pacinian corpuscles (lamellated corpuscles)
    • Muscle spindles, Golgi tendon organs, and Ruffini's corpuscles
    • Joint kinesthetic receptors
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• Adaptation of Sensory Receptors
  • Adaptation occurs when sensory receptors are subjected to an unchanging stimulus
    • Receptor membranes become less responsive
    • Receptor potentials decline in frequency or stop
  • Receptors responding to pressure, touch, and smell adapt quickly
  • Receptors responding slowly include Merkel's discs, Ruffini's corpuscles, and interoceptors that respond to chemical levels in the blood
  • Pain receptors and proprioceptors do not exhibit adaptation

FROM SENSATION TO PRECEPTION

• Survival depends on sensation & perception
• Sensation is the awareness of changes in the internal and external environment
• Perception is conscious interpretation of stimuli
• The three main levels of neural integration in the somatosensory system are:
  • Receptor level - the sensor receptors
    • Stimulus energy must be converted into a graded potential
    • A generator potential in the associated sensory neuron must reach threshold
  • Circuit level - ascending pathways
    • Chains of three neurons (first-, second-, and third-order) conduct sensory impulses upward to the brain
  • Perceptual level - neuronal circuits in the cerebral cortex
    • The thalamus projects fibers to:
    • The somatosensory cortex
    • Sensory association areas

NERVES

• 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 _____________ _______
    • Perineurium - coarse connective tissue that bundles fibers into ______________
    • Epineurium - tough fibrous sheath around a _________
• Mixed Nerves carry impulses both to and from CNS contain both __________ and __________ axons and are most common.

Regeneration and Repair

• Repair within PNS
  • Axons & dendrites may be repaired if
    • neuron cell body remains intact
    • schwann cells are active & form a tube
    • scar tissue does not form too rapidly
  • Chromatolysis
    • 24-48 hours after injury, Nissl bodies break up into fine granular masses
  • By 3-5 days,
    • wallerian degeneration occurs (breakdown of axon & myelin sheath distal to injury)
    • retrograde degeneration occurs back one node
  • Within several months, regeneration occurs
    • neurolemma on each side of injury repairs tube (schwann cell mitosis)
    • axonal buds grow down the tube to reconnect (1.5 mm per day)
• Neurogenesis in the CNS
  • Plasticity maintained throughout life
    • sprouting of new dendrites
    • synthesis of new proteins
    • changes in synaptic contacts with other neurons
  • Limited ability for regeneration (repair)
  • PNS can repair damaged dendrites or axons
    • CNS no repairs are possible
  • Formation of new neurons from stem cells was not thought to occur in humans
    • 1992 a growth factor was found that stimulates adult mice brain cells to multiply
    • 1998 new neurons found to form within adult human hippocampus
  • There is a lack of neurogenesis in other regions of brain and spinal cord.
  • Factors preventing neurogenesis in CNS
    • inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue

CRANIAL NERVES

• Twelve pairs of cranial nerves arise from the brain
• They have sensory, motor, or both sensory and motor functions
• Classification of Nerves
  • Peripheral nerves originate from the brain (12) or spinal column (31)
  • Mixed nerves (most common) - carry somatic (sensory and motor) and autonomic (visceral sensory and motor) impulses
    • The four types of mixed nerves are:
      • Somatic afferent and somatic efferent
      • Visceral afferent and visceral efferent
  • Sensory (afferent) Nerves
    • only toward the CNS
    • rare - only 3 prs,.cranial nerves
  • Motor (efferent) Nerves
    • only axons away from CNS
    • rare only 5 prs. of cranial nerves
• Each nerve is identified by a number (I through XII) and a name
• Four cranial nerves carry parasympathetic fibers that serve muscles and glands
• Cranial Nerve Exam
• CRAINAL NERVES

  _____I. Olfactory	A. sensory (vision); thalamus; optic chiasma
  _____II. Optic	B. motor (eyes); pupil contraction, lens focusing; midbrain
  ____III. Oculomotor	C. sensory (hearing & balance); pons
  ____IV. Trochlear	D. sensory (taste); motor (face, tears, salvia); pons
  ____V. Trigeminal	E. sensory (taste); motor (swallowing); medulla
  ____VI. Abducens	F. motor (larynx, neck); joins vagus; medulla
  ____VII. Facial	G. motor(eyes); pons
  ____VIII.Vestibulocochlear	H. sensory (smell); cerebral cortex
  ____IX. Glossopharyngeal	I. motor (tongue); medulla
  ____X. Vagus	J. motor (eyes); midbrain
  ____XI. Accessory	K. 3 branches; sensory (teeth); motor(face); pons
  ____XII. Hypoglossal	L. sensory (taste, visera); motor (speech, swallowing, visceral organ function); medulla

SPINAL NERVES

• Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head
• They are named according to their point of issue
  • 8 cervical (C1-C8)
  • 12 thoracic (T1-T12)
  • 5 Lumbar (L1-L5)
  • 5 Sacral (S1-S5)
  • 1 Coccygeal (C0)

Spinal Nerve Roots

• Each spinal nerve connects to the spinal cord via two medial roots
• Each root forms a series of rootlets that attach to the spinal cord
• Ventral roots arise from the anterior horn and contain motor (efferent) fibers
• Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers
• The short spinal nerves branch into three or four mixed, distal rami
  • Small dorsal ramus
  • Larger ventral ramus
  • Tiny meningeal branch
  • Rami communicantes at the base of the ventral rami in the thoracic region
• Spinal Nerve Innervation: Back, Anterolateral Thorax, & Abdominal Wall
  • The back is innervated by dorsal rami via several branches
  • The thorax is innervated by ventral rami T1-T12 as intercostal nerves
  • Intercostal nerves supply muscles of ribs, anterolateral thorax, & abdominal wall
  • The area of skin innervated by the cutaneous branches of a single spinal nerveis called a dermatome.
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• Nerve Plexuses
  • Fibers travel to the periphery via several different routes
  • Each muscle receives a nerve supply from more than one spinal nerve
  • Damage to one spinal segment cannot completely paralyze a muscle
  • Plexuses formed by ventral rami
    • The cervical plexus is formed by the ventral rami of the first four cervical nerves.
    • The brachial plexus is situated partly in the neck and partly in the axilla and gives rise to virtually all the nerves that innervate the upper limb.
    • The sacral and lumbar plexuses overlap and because many fibers of the lumber plexus contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to as the lumbosacral plexus

Reflexes

• Reflexes are unlearned, rapid, predictable motor responses to a stimulus, and occur over highly specific neural pathways called reflex arcs
• Spinal Reflexes - somatic reflexes mediated by the spinal cord
  • In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or an internal stretch.
  • The Golgi tendon reflex produces muscle relaxation and lengthening in response to contraction.
  • The flexor, or withdrawal reflex is initiated by a painful stimulus & causes automatic withdrawal of threatened body part from the stimulus.
  • The crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex.
  • Superficial reflexes are elicited by gentle cutaneous stimulation
    • Example: Plantar reflex is initiated by stimulating the lateral aspect of the sole of the foot
      • The response is downward flexion of the toes
      • Indirectly tests for proper corticospinal tract functioning
      • Babinski's sign: abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally.

Developmental Aspects of the CNS

• CNS is established during the first month of development
• Gender-specific areas appear in response to testosterone (or lack thereof)
• Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the fetus' developing CNS
• Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage
• The hypothalamus is one of the last areas of the CNS to develop
• Visual cortex develops slowly over the first 11 weeks
• Growth and maturation of the nervous system occurs throughout childhood and reflects progressive myelination
• Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s
• Excessive use of alcohol causes signs of senility unrelated to the aging process

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