31 Pairs of the Spinal Nerve

Spinal nerves

  • There are 31 pairs of spinal nerves.
  • Mixed nerve fibers are formed by the joining of the anterior motor and posterior sensory roots.
  • Posterior roots contain afferent (sensory) nerve fibers.
  • Anterior roots contain efferent (motor) nerve fibers.


Autonomic nervous system

  • Sympathetic (adrenergic) fibers dilate pupils, increase heart rate and rhythm, contract blood vessels, and relax smooth muscles of the bronchi.
  • Parasympathetic (cholinergic) fibers produce the opposite effect.



  • Acetylcholine
  • Norepinephrine
  • Dopamine
  • Serotonin
  • Amino acids
  • Polypeptides


  • The neuron consists of the cell body, axons, and dendrites.
  • The cell body contains the nucleus.
  • Neurons carrying impulses to the central nervous system (CNS) are called sensory neurons.
  • Neurons carrying impulses away from the CNS are called motor neurons.
  • Synapse is the chemical transmission of impulses from one neuron to another.

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