Define synaptic plasticity.

Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time, which is crucial for learning, memory, and overall neural adaptability. This phenomenon allows the brain to modify its connections based on experiences, which can enhance the efficiency of communication between neurons. When two neurons are repeatedly activated together, the synaptic connection between them can become stronger, facilitating more efficient transmission of signals. Conversely, if certain synaptic pathways are less used, their connections may weaken, adapting the neural circuitry to better fit the organism's needs and experiences.

This concept is fundamental in understanding how learning occurs at a cellular level, as it explains how memories can be formed and retained through changes in synaptic strength. The other choices do not capture the essential dynamic nature of synapses. For instance, forming new connections refers to synaptogenesis, rigid structures do not reflect the adaptable nature of synapses, and sequential firing of neurons does not encompass the broader concept of synaptic changes over time.