Final answer:
Sensory neuron signals reach motor neurons through a sequence of electrical impulses and chemical synapses, traveling from the site of sensory stimulus to the spinal cord and brain, and then back down to cause a motor response in the target muscle.
Step-by-step explanation:
Signals from sensory neurons reach motor neurons through a complex process involving electrical and chemical signals. When a stimulus, such as water temperature, activates the sensory neuron's endings, the signal's strength is initiated based on the stimulus intensity. A graded potential, if strong enough, will initiate an action potential that travels through the axon of the sensory neuron to the spinal cord, making a synaptic contact in the gray matter.
The signal is then transmitted via neurotransmitters across the synapse, generating a new graded potential and subsequent action potential in the subsequent neuron. This action potential moves up the sensory pathway, reaching the brain, specifically the thalamus, before reaching the cerebral cortex. After processing the information, the brain sends a motor command down through upper motor neurons to the spinal cord, where it targets the lower motor neuron. The axon of the lower motor neuron extends out to the muscle and connects through a neuromuscular junction, prompting muscle contraction. This entire sequence allows the body to react to sensory information.