Nervous System Page 4

Nervous System                           Page 4

The Simple Reflex Arc:

  • Brain and spinal cord are the coordination centres.
  • They process the signals received and order the effector to act accordingly.
  • Some of the signals need to be analyzed before a response is made. In such cases brain receives and analyzes the signal.
  • But, there are certain responses which need not be processed by brain and are to be responded immediately.
  • Such responses occur at the level of spinal cord with the aid of sensory and motor nerve fibres of the spinal nerves.
  • Thus, the response that occurs at the level of spinal cord without the brain knowing about it, is called Reflex action (the decision taken by spinal cord without the intervention of the brain is called reflex action).
  • The path through which the stimulus travels is called reflex arc.
  • Neuron is the structural unit of nervous tissue, but the smallest functional unit is a reflex arc.

Some e.g. of reflex action are:

  • Removing your hand or finger from some hot object like burning candle.
  • Blinking eyes, hiccupping, shivering, sneezing or coughing etc…..

Components of Reflex arc:

  • Receptor or sense organ to perceive the stimulus.
  • Sensory or afferent nerve which transmits sensory impulses from receptor to the spinal cord.
  • Association neurons / Interneural neuron / Adjustor neurons in the spinal cord which connect sensory neuron with the motor neuron and transmit impulses from former to latter. These neurons are also called intermediate neurons.
  • Motor nerve or efferent nerve which transmits motor impulses from spinal cord to the effector organ, either muscles or glands.

Mechanism of Reflex action

  • Receptors in the sense organ perceive the stimulus that generates sensory impulses.
  • Sensory impulses thus generated are flashed by the sensory fibres of the spinal nerve to the spinal cord.
  • Through association neuron this impulse is passed on to the motor neuron. The endings of the axon form the synapse with dendrites of an associative neuron. The cell body of the associative neuron is in the dorsal horn of grey matter. The impulse is conducted by the associative neuron to its axon which forms a synapse with the dendrites of a motor (or efferent) neuron.
  • The motor neuron generates motor impulses which are transmitted through the motor nerve fibres of the spinal nerve to the effector muscles of the respected organ. The muscle then contracts pulling away the organ.
  • This reaction is performed in a fraction of a second without the brain even knowing about the event.
  • The cell body of motor neuron is in the ventral horn of the grey matter. The impulse is conducted by the motor axon to its ending in a muscle or a gland. This is the effector. The muscle contracts, causing a action.
  • The reflex arc involves a definite circuit along which the impulses are conducted. For simplicity, only one neuron in each type of nerve has been considered, in an actual reflex arc, many neurons, in the same nerve are involved.
  • The muscular contraction takes place only when the impulse is conducted by the nerve fibres. When the impulse ceases, contraction of the muscle stops. Sometimes an associative neuron is not present.

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