Sequence of a Simple Reflex:

The correct sequence of a simple reflex is as follows:
Receptor - sensory nerves - central nervous system (CNS) - effector.

Explanation:

A simple reflex is an involuntary and automatic response to a particular stimulus. It involves a specific pathway that allows for a rapid response without the involvement of conscious thought or decision-making. Here is a detailed explanation of each step in the sequence:

1. Receptor:
The receptor is a specialized structure, such as a sensory organ or a sensory cell, that detects the stimulus. It can be located in various parts of the body, including the skin, muscles, or internal organs. The receptor converts the stimulus into an electrical signal, which is then transmitted to the sensory nerves.

2. Sensory nerves:
The sensory nerves, also known as afferent nerves, carry the electrical signals generated by the receptors towards the central nervous system (CNS). These nerves are responsible for transmitting sensory information from the receptors to the CNS.

3. Central Nervous System (CNS):
The CNS consists of the brain and the spinal cord. It receives and processes the sensory information transmitted by the sensory nerves. In the case of a simple reflex, the reflex arc does not involve the brain and is instead processed within the spinal cord. The spinal cord plays a crucial role in coordinating the reflex response.

4. Effector:
The effector is the structure or organ that carries out the response to the stimulus. It can be a muscle or a gland. In the case of a simple reflex, the effector is usually a muscle. Once the sensory information reaches the CNS, a motor response is generated and transmitted back to the effector through motor nerves.

5. Response:
The effector carries out the appropriate response to the stimulus. For example, if the stimulus is a hot object, the reflex action may involve the contraction of muscles to withdraw the hand from the object. The response occurs rapidly and without conscious control.

Overall, the correct sequence of a simple reflex is receptor - sensory nerves - CNS - effector. This pathway allows for a quick and automatic response to a stimulus, providing protection and maintaining homeostasis in the body.