The sack-like structures found inside the synaptic knob containing chemicals are called synaptic vesicles.

Here's a breakdown:

* Synaptic knob: This is the bulb-shaped terminal of a neuron where neurotransmitters are released.

* Synaptic vesicles: These are tiny, membrane-bound sacs within the synaptic knob that store neurotransmitters.

* Neurotransmitters: These are chemical messengers that transmit signals across the synapse (the gap between neurons).

When a nerve impulse reaches the synaptic knob, it triggers the release of neurotransmitters from the synaptic vesicles. These neurotransmitters then diffuse across the synapse and bind to receptors on the next neuron, continuing the signal transmission.