Neural Control and Coordination - Notes | Class 11 | Part 4: Human Neural System - PNS, Reflex Action



It includes cranial nerves and spinal nerves.Nerve fibres of PNS are 2 types:
  • Afferent (sensory) fibres: Carry impulses from sense organs to CNS.
  • Efferent (motor) fibres: Carry impulses from CNS to muscles and glands.
PNS has 2 divisions. They are
  • Somatic neural system: Relays impulses from the CNS to skeletal muscles.
  • Autonomic neural system (ANS): Transmits impulses from CNS to involuntary organs & smooth muscles. It includes sympathetic & parasympathetic nerves. Sympathetic system prepares body to cope with emergencies, stresses & dangers. It increases heartbeat, breathing rate, constricts arteries and elevates BP. Parasympathetic system returns the body to a resting state after stressful situations and slows down heartbeat, dilates arteries, lowers BP etc.
Visceral nervous system is the part of PNS. It includes nerves, fibres, ganglia & plexus by which impulses travel from CNS to the viscera and from viscera to CNS.


It is the rapid, involuntary and unconscious actions of body in response to a stimulus. E.g.
  • Withdrawal of the hand when it touches a hot object.
  • Touching lips of a nursing baby evokes sucking reflex.
  • Closing of the eyelids when light falls on them.
  • Knee jerk phenomenon.
  • If a child sees or smells a food unknown to him, he does not salivate. But if he sees or smells that food every time before tasting it, he salivates (conditioned reflex).
The pathway of impulses in a reflex action is called Reflex arc. It consists of
  • A receptor organ: It receives the stimulus.
  • Sensory (afferent) neuron: It transmits impulses from sense organ to CNS.
  • Intermediate (connector) neuron: It connects sensory and motor neurons.
  • Motor (efferent/effector/excitor) neuron: It conducts impulse from the CNS to effector organ.
  • An effector organ (muscle/gland): It responds to impulse.

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