I believe the answer is at least one type of neuropeptide neurotransmitter. Neurotransmitters are the chemicals which allow the transmission of signals from one neuron to the next across synapses. They are also found at the axon endings of motor neurons, where they stimulate the muscle fibers. Neuropeptides are one type of neurotransmitters and they are composed only of peptides with larger molecular weights, a key difference between neurotransmitters and neuropeptides.