The Peripheral Nervous System

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The Peripheral Nervous System
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Peripheral Nervous System

• 31 pairs of spinal nerves
• 12 pairs of cranial nerves
• All of the smaller nerves that branch from larger
  nerves

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Afferent and Efferent Pathways
Afferent Pathways
Efferent Pathways
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Spinal Nerves

• 31 Pairs
• Numbered according to the level of the vertebral
  column at which they emerge from the spinal
  cavity
• Lumbar, Sacral, and Coccygeal nerves descend from
  their point of origin at the lower end of the
  spinal cord
• Lower end of the cord is called the cauda equina,
  which means horses tail in Latin

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Structure of Spinal Nerves

• Mixed Nerves (contain motor and sensory fibers)
• Attach by means of two short roots
• Dorsal root
• Has ganglion (cell bodies)
• Carries sensory neruons
• Ventral root
• Carries motor neurons

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Dorsal and Ventral Rami

• Soon after each spinal nerve emerges from the
  spinal cavity, it forms several large branches,
  each of which is called a ramus.
• Dorsal Ramus
• Supplies somatic motor and sensory fibers to
  serveral smaller nerves
• Ventral Ramus
• More complex

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Rami of Spinal Nerves
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Nerve Plexuses

• Ventral rami of most spinal nerves subdivide to
  form complex networks called plexuses.
• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus

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• Damage to one spinal nerve does not mean
  complete loss of function in any one region.

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Dermatomes and Myotomes

• Each skin surface area supplied by sensory fibers
  of a given spinal nerve is called a dermatome.
• A myotome is skeletal muscle or group of muscles
  that receives motor axons from a given spinal
  nerve.

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Dermatome Distribution of Spinal Nerves
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Myotomes and Body Movement
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Cranial Nerves

• 12 pairs
• Connect to the undersurface of the brain
• Pass through small foramina (holes) in the
  cranial cavity and skull
• Identified by names and numbers
• 3 Types
• Mixed
• Sensory
• Motor

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Divisions of the Peripheral Nervous System
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Somatic Motor Nervous System

• All the voluntary motor pathways outside the CNS
  (pathways to skeletal muscles)
• Single motor neuron whose axon stretches from the
  cell body in the CNS all the way to the effector
• Stimulates effectors by means of the
  neurotransmitter acetylcholine

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Somatic Reflexes

• The action that results from a nerve impulse
  passing over a reflex arc is called a reflex.
• Predictable response to a stimulus
• May or may not be conscious
• Somatic reflexes are contraction of skeletal
  muscle
• Autonomic reflexes consist of contractions of
  smooth or cardiac muscle, or secretion by glands
• Used in the diagnosis of disease

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Knee Jerk Reflex
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Autonomic Nervous System

• Regulates involuntary effectors
• Major function of the ANS is to regulate
  heartbeat, smooth muscle contraction, and
  glandular secretion in ways to maintain
  homeostasis.

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Sympathetic and Parasympathetic Divisions

• Sympathetic and Parasympathetic have separate
  pathways
• Effectors may have dual innervation, that is they
  have input from both types of pathways
• Parasympathetic rest-and-repair
• Sympathetic fight-or-flight

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Autonomic Conduction Paths
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