**Core Concept**
Long-term potentiation (LTP) is a persistent strengthening of synaptic connections between neurons, leading to enhanced signal transmission in the brain. This phenomenon is a key component of learning and memory, and is thought to underlie the consolidation of information from short-term to long-term memory.

**Why the Correct Answer is Right**
LTP occurs when high-frequency stimulation of one neuron leads to a long-lasting increase in the strength of synaptic transmission between that neuron and its postsynaptic partner. This is achieved through a complex interplay of various signaling pathways, including the activation of NMDA receptors, which allow calcium ions to flow into the postsynaptic neuron. The influx of calcium ions triggers a cascade of intracellular signaling events, ultimately leading to the strengthening of synaptic connections. The enhanced signal transmission is a direct result of the increased efficacy of neurotransmitter release and the potentiation of postsynaptic receptors.

**Why Each Wrong Option is Incorrect**
**Option B:** Increased number of receptors is not the primary mechanism underlying LTP. While changes in receptor density can occur, the primary effect of LTP is on the efficacy of neurotransmitter release and the potentiation of existing receptors.

**Option C:** Increased number of neurons is not a direct result of LTP. LTP occurs within the existing neural circuitry, and does not involve the growth of new neurons.

**Option D:** Increased muscle tone is not related to LTP, which is a phenomenon specific to the brain and nervous system.

**Clinical Pearl / High-Yield Fact**
LTP is thought to be one of the cellular mechanisms underlying the development of learning and memory disorders, such as Alzheimer's disease and amnesia. Understanding the underlying mechanisms of LTP is essential for the development of novel therapeutic strategies for these conditions.

**✓ Correct Answer: A. Enhancement of signal transmission**