Reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain.

Reflex actions are involuntary responses to stimuli that occur without conscious thought or decision-making. They are rapid, automatic, and protective actions that help organisms respond to potential harm or danger. Reflex actions are mediated through the spinal cord.

The spinal cord is a long, thin bundle of nerves that extends from the base of the brain down the vertebral column. It serves as a communication pathway between the brain and the rest of the body. The spinal cord is responsible for transmitting nerve signals to and from the brain and coordinating reflex actions.

Key Points:
1. Role of Receptors: When a stimulus is detected by specialized sensory receptors in the body, such as the skin, muscles, or organs, they generate electrical signals that are transmitted to the spinal cord.

2. Role of Spinal Cord: The spinal cord acts as a relay station for these signals. It receives the sensory information from the receptors and sends motor commands to the effectors.

3. Spinal Reflex Arc: The pathway through which reflex actions are mediated is known as the spinal reflex arc. It involves a series of steps:
a. Stimulus: A stimulus is detected by the sensory receptors.
b. Sensory Neurons: The sensory neurons transmit the electrical signals to the spinal cord.
c. Interneurons: Within the spinal cord, the signals are relayed to interneurons, which process the information and determine the appropriate response.
d. Motor Neurons: The interneurons then send signals to motor neurons, which carry the response commands from the spinal cord to the effectors.
e. Effectors: The effectors, which can be muscles or glands, carry out the necessary action in response to the stimulus.

4. Speed of Reflex Actions: Reflex actions mediated through the spinal cord are extremely rapid because the signals do not have to travel all the way to the brain for processing. Instead, the spinal cord can quickly initiate a response to the stimulus, allowing for immediate protection or adjustment.

In conclusion, reflex actions are mediated through the spinal cord. The spinal cord acts as a relay station, receiving signals from sensory receptors and coordinating the appropriate response through motor neurons. This allows for rapid and automatic reactions to stimuli without the need for conscious thought or decision-making.