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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Nervous Coordination Page 2

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Autonomic Nervous System


  • The part of the peripheral nervous system, which controls the involuntary functions of the body, is called autonomic nervous system.
  • The autonomic pathway involves two neurons, that connect the CNS with the effector organ, which is most often a gland or an involuntary muscle.
  • The neuron, with its cell body in CNS, passes an impulse to the second neuron, the cell body of which is located in certain thickenings called ganglia. The junction between the two neurons through which an impulse is relayed is known as synapse.



Sympathetic Nervous System or Thoracolumbar Outflow


  • The sympathetic nervous system consists of a pair of sympathetic nerve cords.
  • The sympathetic nervous system has sympathetic ganglia and paravertibral ganglia.
  • The sympathetic nervous system mobilises the energy of the body for quick action.
  • It speeds up the heartbeat, dilates the pupil, constricts the blood vessels and inhibits the actions of stomach and intestine.



Parasympathetic System or Craniosacral Outflow


  • The parasympathetic nervous system consists of two parts namely, the cranial part and the sacral part.
  • The cranial part is connected to the posterior part of the brain through cranial nerves, while the sacral part is connected to the spinal cord through spinal nerves.
  • The parasympathetic nervous system slows down the heartbeat, contracts the pupil and stimulates the actions of the stomach and the intestine. Parasympathetic stimulation tends to slow down the body processes except the digestive, urinary and reproductory systems. Their effect is that of a 'peacemaker', calming down the excited state of the organs.





  • Receptors are structures that receive stimuli.
  • There are different kinds of receptors sensitive to different kinds of stimuli. These are: Chemoreceptors - They are sensitive to chemicals (taste and smell). They are located in the taste buds of the tongue and olfactory cells of the nose.
    Photoreceptors - They are sensitive to light. They are located in the retina of eye.
    Thermoreceptors - They are sensitive to change in temperature. They are located in the skin.
    Tangoreceptors or mechanoreceptors - They are sensitive to pressure from sound or touch. They are located in the skin and nerve endings.
    Statoreceptors - They are sensitive to change in position or gravity.
    Phonoreceptors - They are sensitive to sound waves. They are located in cochlea or internal ear.
    Nociceptors - They are sensitive to injury or pain.
    Osmoreceptors - They are sensitive to osmotic concentration.
  • Receptors on the basis of their position in the body can be classified as follows:
    Exteroceptors - These are receptors that receive stimuli from the exterior. They are located at the surface of the body including skin and tongue. Example: eye, nose, ear, cutaneous receptor (touch, pain, etc.) and taste buds.
    Proprioceptors - These are located in the muscles, tendons and joints. They are important for muscular co-ordination, maintenance of correct posture, etc. They usually function at the unconscious level.
    Interoceptors - These are the receptors located in the visceral organs such as respiratory, digestive and reproductory systems.





  • Structures, which respond to the stimuli, are called effectors.
  • Effectors are situated at the ends of efferent nerve fibres.
  • The most common effectors are skeletal muscles, smooth muscles and glands.
  • Pigment cells, bioluminiscent (light producing) organs and electric organs are also effectors present in some animals but not in humans.



Reflex Action


The automatic and rapid response to a stimulus is called reflex action.
Example: the sudden withdrawal of the arm when it is pricked with a pin.



Reflex Arc


The path through which the impulse travels during the reflex action is
called the reflex arc.



Components of Reflex Arc


  • Receptor organs
  • Sensory or afferent neuron
  • Adjustor or associated neuron
  • Motor or efferent neuron
  • Effector organ



Mechanism of Reflex Action


  • During reflex action, the transmission of the impulse takes place through the synaptic gap, which lies between the axon of one neuron and dendron of the adjacent neuron.
  • On receiving the impulse from a receptor organ, the axon of the adjustor neuron secretes acetylcholine, a neurohormone, which fills up the synaptic gap. Owing to this, the impulse is transmitted to the dendron of the motor neuron.
  • The neurohormone, acetylcholine, after the transmission of the impulse, gets neutralised due to the activity of the enzyme called cholinesterase, which is necessary to prevent mixing up or lingering of impulses.
  • The motor neuron transmits the impulse to effector organ, which shows the response.



Importance of Reflex Action


  • The reflex action brings forth an immediate response to a stimulus, and thus, helps an animal to adjust itself to the changing environment.
  • The reflex action helps the body to respond quickly to harmful stimuli, which in turn protects the body from danger.



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