**Core Concept:**
The enteric nervous system (ENS) is a self-contained neural network residing within the gastrointestinal tract (GI tract). It is also known as the "motor" plexus as it controls smooth muscles of the GI tract. The ENS plays a crucial role in various functions of the GI tract, including peristalsis, secretion, and absorptive processes.

**Why the Correct Answer is Right:**
The enteric nervous system contains a vast number of neurons, which is approximately **10^12** neurons. This number is significantly higher than the total number of neurons in the peripheral autonomic nervous system (PANS). This high density of neurons allows the ENS to control various functions like peristalsis, secretion, and absorptive processes within the GI tract.

**Why Each Wrong Option is Incorrect:**
A. **10^9**: This number is significantly lower than the actual number of neurons in the ENS, making it incorrect.
B. **10^10**: This number is too high compared to the actual number of neurons in the ENS.
C. **10^8**: This number is too low, and it does not represent the actual number of neurons present in the ENS.
D. **Neurons absent**: This option contradicts the fact that the ENS contains a substantial number of neurons and is crucial for GI tract functions.

**Clinical Pearl / High-Yield Fact:**
The complexity of the ENS is highlighted by the presence of a higher number of neurons compared to the PANS. This demonstrates the autonomous control of GI tract functions by the ENS, emphasizing its vital role in maintaining GI tract health and homeostasis.