Reflex Action:
A reflex action is an automatic and involuntary response to a stimulus. It occurs without conscious thought or decision-making, as it is controlled by the spinal cord and not the brain. Reflex actions are essential for protecting the body from harm and ensuring quick responses to potential dangers.

Reflex Arc:
A reflex arc refers to the neural pathway that mediates a reflex action. It consists of a sensory receptor, a sensory neuron, an interneuron (optional), a motor neuron, and an effector. The reflex arc allows for the rapid transmission of signals from the sensory receptor to the effector, bypassing the brain, and enabling swift responses to stimuli.

Differences between Reflex Action and Reflex Arc:

1. Definition:
- Reflex action: It is an automatic and involuntary response to a stimulus, occurring without conscious thought.
- Reflex arc: It is the neural pathway that mediates a reflex action, allowing for the transmission of signals from the sensory receptor to the effector.

2. Components:
- Reflex action: It involves the stimulation of a sensory receptor, transmission of sensory signals to the spinal cord, and subsequent activation of motor neurons leading to an appropriate response.
- Reflex arc: It comprises a sensory receptor, a sensory neuron, an interneuron (in some cases), a motor neuron, and an effector. These components work together to transmit signals and produce the reflex action.

3. Control:
- Reflex action: It is controlled by the spinal cord and does not require involvement or processing by the brain.
- Reflex arc: It describes the pathway taken by the nerve impulses during a reflex action but does not have control over the action itself.

4. Speed:
- Reflex action: It is extremely rapid, as the signal travels directly from the sensory receptor to the motor neuron without involving the brain. This allows for quick responses to potential dangers.
- Reflex arc: It facilitates the rapid transmission of signals, ensuring swift reflex actions by bypassing the brain and utilizing shorter neural pathways.

5. Complexity:
- Reflex action: It is a simple and innate response that does not involve conscious decision-making or cognitive processing.
- Reflex arc: It can be relatively simple or complex, depending on the number of neurons involved. Some reflex arcs may include interneurons, which allow for more complex reflex responses.

In summary, a reflex action refers to the automatic response to a stimulus, while a reflex arc represents the neural pathway responsible for transmitting the signals involved in the reflex action. The reflex arc enables quick and involuntary responses by bypassing the brain and utilizing shorter pathways.