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

Reflex is a special ability that evolution gifted us to facilitate our survival. Whenever part of your body comes in contact with an object capable of causing you harm, you tend to quickly withdraw that part of the body. This happens before your brain gets the time it needs to process the threat.

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

Contact with the hot pot triggers the start of a series of events in the body to evoke a response.

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 impulses are processed and a response is relayed back.

In the spinal cord, the interneurons (also known as relay neurons) make the connections between the sensory neurons (bringing the message from hand) and the correct motor neurons (taking the response back to the hand). It would not be useful if the response was sent to the wrong part of the body—in this case, a response sent to the leg wouldn't be too helpful as the stimulus is coming from the hand.

From the interneurons, the response is relayed to the motor neurons which 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 pathway taken by nerve impulses to elicit a response is known as a ‘reflex arc’.

This process happens so fast that the response occurs before the message reaches the brain. This results to a quicker time-to-response as the thinking process of the brain may be relatively time consuming.

The stimulus

In the above example, 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. Another example of a stimulus is an object (e.g. an insect) approaching your eye causing you to blink before you know it.

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 neuron (the interneuron) with the help of a neurotransmitter (acetylcholine).

The spinal cord

Interneurons (also known as relay neurons) are fully contained in the central nervous system (the spinal cord and the brain). 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

In the 'hot pot' example above, motor neurons send nerve impulses away from the central nervous system to effector organs or muscle fibers. This causes the muscle fibers to contract, resulting in you snatching your hand away from the hot pot.

If the stimulus was at the sight of an insect flying towards your eyeball, then the motor neurons would relay the response back to your eyelids (to close) to protect them from the approaching threat.

The response

This occurs when the motor neurons deliver nerve impulses from the spinal cord to the part of the body where a response to the stimulus is needed. In the above example, the response is the muscle contraction to quickly pull the hand away from the hot pot. In the second example, the response is to blink to prevent the insect from making contact with the eye.

A reflex arc, an article on how a stimulus elicits a response