Question

What is a reflex arc? Draw a neat labeled diagram of the components in a reflex arc. Why do impulses flow only in one direction in a reflex arc?

Answer 1

 
Hint: The stimuli that are sudden and unpredictable and that may harm an organism either by burning, pinching, etc. are escaped by our body through a developed mechanism that is rapid, automatic, short-lived response.

Complete answer: The reflex arc is the neural pathway that controls the reflex action. The process carried out during a reflex arc is:
$The \ stimulus \ is \ detected \ by \ sensory \ receptors \ present \ in \ the \ skin\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \downarrow \\ These \ initiate \ nerve \ impulses \ in \ the \ sensory \ or \ afferent \ neurons \ leading \ from \ them \ to \ the \ spinal \ cord. \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \downarrow \\ These \ impulses \ enter \ the \ spinal - cord \ and \ initiate \ impulses \ in \ one \ or\quad more \ interneurons.\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \downarrow \\ Interneurons \ initiate \ impulses \ in \ one \ or \ more \ efferent \ neurons.\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \downarrow \\ The \ motor \ neurons \ then \ carry \ these \ impulses \ to \ the \ effectors \ or \ the \ skeletal \ muscles \ in \ which \ response \ will \ be \ shown.$

The muscles called the flexors (which bend a part towards the body) are stimulated to contract so that they withdraw the body part whereas the muscles called extensors (which extend apart) are inhibited so that they do not extend the body part. The result is that the body part is suddenly, involuntarily withdrawn from the irritating stimulus. This entire pathway from the sensory neuron to the spinal cord to motor neuron to effector is called reflex pathway or reflex arc.
Neurotransmitters and their receptors are just like the lock and key mechanism. A specific neurotransmitter binds to the specific receptors only. Neurotransmitters are stored in vesicles going only in one direction. The receptors for the respective neurotransmitter are present in the next neurons located. Thus, the impulses in the nerve flow in one direction only and don’t flow backward.

Note:
- The entire reflex arc process is due to the response to peripheral nerve stimulation that occurs involuntarily, i.e., without conscious efforts or thoughts.
- The axon terminals of the sensory neurons (bringing the impulses from the peripheries) form synapses with several interneurons in the spinal cord. And a single interneuron may, in turn, have hundreds of different axons (both sensory neurons and other interneurons) converging upon it. Hence, it is a multitude of interconnections that provide for complex responses and which also enable us to keep informed of what our body is doing.