LAB 9 GEN PROPERTIES OF REFLEXES IN THE FROG

• Spinal Shock


  Refers to the period following transaction of the spinal cord during which all spinal reflex responses below the level of section are profoundly depressed
• Spinal shock has been attributed to sudden cessation of tonic bombardment of spinal motor neurons by excitatory impulses in descending pathways
• Recovery from spinal shock may possibly be due to the development of denervation sensitivity to the mediators chronically released by the remaining spinal excitatory endings
• The exact cause of spinal shock is not known
• Materials and equipment

  • Stimulating apparatus
  • Dissecting set
  • Sulfuric acid (0.2% soln.)
  • Strychnine sulfate soln.
• Localization of Reflex Function

  1. Normal Frog

  2. Spinal Frog

  3. Double-pithed Frog
• When the toes of the right leg of the frog are dipped in sulfuric acid, the reflex time would be longer than when electrical stimulation is applied concurrently with dipping in sulfuric acid. This is called Reinforcement and is somewhat similar in principle to temporal summation
• When a single stimulus is applied on a nerve fiber, the nerve fiber may or may not become excitable depending on whether the stimulus is at threshold value or not
• The duration of spinal shock is proportionate to the degree of encephalization of motor function in the various species of animal
• If the stimulus of subthreshold value is applied successively on the same nerve, then the impulse is summated and becomes a propagated action potential. Such type of summation is called Temporal Summation
• The state of transient partial depolarization produced by a subthreshold stimulus is called an Excitatory Postsynaptic Potential
• Strychnine sulfate is a central nervous system stimulant specifically acting on the spinal cord by lowering synaptic resistance. This impulse reaches the synapse without encountering any inhibition and the overall effect of which gives rise to convulsions even with subminimal stimulation
• Preparation


  Define a reflex action and state briefly the importance of reflexes in general
• If the stimulus is ineffective or of subthreshold value, then a propagated action potential is not produced but instead, the stimulus produces only a transient partial depolarization
• Experiment 9

  1. To differentiate between a spinal animal, a decerebrate animal and a decorticate animal as to reflex responses

  2. To find out how spinal reflexes may be reinforced or summated

  3. To find out the effect of strychnine sulfate on synapses