Which of the following are the first and last elements in a spinal reflex?

Options

A. Axon and dendrite
B. Sense organ and muscle effector
C. Ventral horn and dorsal horn
D. Motor neuron and sensory neuron

Explanation

To determine the first and last elements in a spinal reflex, we can break down the process step by step.

### Step 1: Understand the Components of a Spinal Reflex
A spinal reflex involves several key components that work together to produce a quick response to a stimulus. The main elements include:
- Receptors (sense organs)
- Sensory neurons
- Interneurons (in the spinal cord)
- Motor neurons
- Effectors (muscles or glands)

### Step 2: Identify the First Element
The first element in a spinal reflex is the **receptor**, which is typically a sense organ. This is where the stimulus is detected. For example, if you touch something hot, the receptors in your skin sense the heat.

### Step 3: Identify the Last Element
The last element in a spinal reflex is the **effector**, which is usually a muscle or gland that carries out the response. In the case of touching something hot, the muscle would contract to pull your hand away.

### Step 4: Match the Elements to the Options Given
Now, looking at the options provided:
1. Axon and dendrite
2. Sense organ and muscle effector
3. Ventral horn and dorsal horn
4. Motor neuron and sensory neuron

From our analysis:
- The first element (receptor) corresponds to the **sense organ**.
- The last element (effector) corresponds to the **muscle effector**.

### Conclusion
Therefore, the correct answer is **sense organ and muscle effector** as they represent the first and last elements in a spinal reflex.

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