Which of the following neurons may not have myelin sheath?

Analyze the question: The question asks which type of neuron may not have a myelin sheath. Myelin is a fatty insulating layer that speeds up the transmission of nerve impulses.

Concept: There are three types of neurons:

• Sensory neurons carry impulses from receptors to the central nervous system (CNS). They are usually myelinated, especially those carrying fast signals such as touch and pain.
• Intermediate (relay/association) neurons are found entirely within the CNS (brain and spinal cord), where they connect sensory and motor neurons. Many of them are short, and they are often unmyelinated or only lightly myelinated. Grey matter, which is rich in cell bodies and short unmyelinated fibres, is where most of them are found.
• Motor neurons carry impulses from the CNS to effectors (muscles and glands). They are usually myelinated, since they need to transmit signals quickly to muscles.

Step-by-step reasoning:

1. Effector cells (A) are muscle or gland cells, not neurons, so they are ruled out.
2. Motor neurons (C) have long, myelinated axons for rapid response, so they usually have a myelin sheath.
3. Sensory neurons (D) also have long, usually myelinated fibres, so they typically have a myelin sheath.
4. Intermediate neurons (B) are short and located within the CNS, and they often lack a myelin sheath.

The correct answer is **B.

Tip: Remember that myelin is mostly for long-distance, fast transmission. Short connector neurons in the CNS (intermediate neurons) are the ones that may lack it. Also, always check whether an option is actually a neuron. "Effector cells" is a distractor because those are muscles or glands.