Reflex Action and Reflex Arc: What Happens When You Accidentally Touch a Hot Pot

Whenever part of your body comes close to an object that is capable of causing you harm, you tend to quickly withdraw that part of the body.

If you were cooking and you accidentally touched a hot pot on your stove, you would involuntarily (and nearly instantaneously) snatch your hand away from the pot. This response is called a ‘reflex action'.

When you touch the hot pot, a response is created in the body. At the point of contact with the hot pot, skin receptors quickly send nerve impulses (electrical) to the spinal cord (central nervous system) via sensory neurons.

In the spinal cord, the nerve impulses move from sensory neurons to the interneurons (also known as relay neurons). The impulses are then sent to motor neurons that project out of the spinal cord to stimulate your muscles (effector) to contract, causing you to snatch your hand away from the hot pot. This is known as a ‘reflex arc’. This process happens so fast that the response occurs before the message reaches the brain or the message may not be sent to the brain at all.

Stimulus: In the example above, the stimulus is the contact with the hot pot. This contact causes a nerve impulse that will travel to the spinal cord via the sensory neurons.

Sensory neurons: These neurons carry the nerve impulse to the spinal cord. Similar to the interneuron and motor neuron, sensory neurons receive incoming impulses at the dendrites. The impulses move away from the cell body along the axon to the synaptic terminal where the impulse is sent to the next interneuron with the help of a neurotransmitter (acetylcholine).

Interneurons: The interneuron is also known as relay neuron. These neurons are fully contained in the central nervous system. The interneuron serves as the connection between the sensory neurons and the motor neurons.

The synapse is a tiny space between two neurons. When an impulse gets to the end of one neuron and has to be sent down the next neuron, the synapse acts as a bridge. The signal arrives at the end of one neuron (close to the synapse) as an electrical signal, crosses the synapse as a chemical signal (with the help of a neurotransmitter known as acetylcholine released by the synaptic vesicles at the synaptic terminal) and continues as an electrical signal in the next neuron.

Motor neurons: These neurons send nerve impulses away from the central nervous system to effector organs or muscle fiber in our example above. This causes the muscle fiber to contract, resulting in you snatching your hand away from the hot pot.

Response: To respond to the stimulus of the reflex arc, the muscle needs to contract to pull the hand quickly away from the hot pot. For this to happen, the impulse travels to the synaptic terminal of the motor neuron. Synaptic vesicles at the synaptic terminal will then release acetylcholine which will cross the synapse and bind to the receptors on the muscle fibers to trigger the muscle contraction known as the ‘response’.