<P> Upper motor neurons release a neurotransmitter, acetylcholine, from their axon terminal knobs, which are received by nicotinic receptors of the alpha motor neurons . In turn, alpha motor neurons relay the stimulus . </P> <P> From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers . </P> <P> A reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output . Reflex circuits vary in complexity--the simplest spinal reflexes are mediated by a two - element chain, of which in the human body there is only one, also called a monosynaptic reflex (there is only one synapse between the two neurones taking part in the arc: sensory and motor). The singular example of a monosynaptic reflex is the patellar reflex . The next simplest reflex arc is a three - element chain, beginning with sensory neurons, which activate interneurons in the spinal cord, which then activate motor neurons . Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex ("knee jerk") activated by tapping the patellar tendon, contribute to ordinary behaviour . </P> <P> In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory . For vertebrates, however, the response of a skeletal striated muscle fiber to a neurotransmitter--always acetylcholine (ACh)--can only be excitatory . </P>

Where are the cell bodies located for the efferent neurons that supply skeletal muscles