Which type of chemical messenger alters the strength of a synapse by modifying neurotransmitters?

A neuromodulator is a type of chemical messenger that plays a critical role in modulating the strength of synaptic transmission. Unlike traditional neurotransmitters, which facilitate direct signaling between neurons, neuromodulators can influence the efficacy of neurotransmitters either by altering their release or by changing the response of the receiving neuron to those neurotransmitters. This modulation can enhance or inhibit synaptic transmission, thereby affecting communication between neurons over longer timescales.

For example, neuromodulators can adjust the amount of a specific neurotransmitter released into the synaptic cleft or may act on different receptors to enhance or dampen neurotransmitter effects. This capability makes them essential for processes such as learning, memory, and mood regulation, allowing the nervous system to adapt to varying conditions and demands.

Other options bear importance but serve different functions. Hormones act more broadly across the body rather than focusing specifically on synaptic functions. Enzymes, while crucial for biochemical reactions, do not directly modify neurotransmitters in the context of synaptic transmission. Growth factors are vital for cell growth and differentiation, but they do not play a direct role in changing synaptic strength or neurotransmitter dynamics.