A nerve is a connected bundle of peripheral axons. Nerves transmit electrochemical signals between points in the body.
An external stimulus (sound, touch, taste, smell, sight) triggers the sensory receptor, which generates a low-voltage signal by transferring sodium and potassium ions through the neuron membrane. The signal travels along the bundle of axons until it reaches the spinal cord. From there, the impulse travels through the white matter (axon bundles) to the gray matter (neurons and synapses), which transmits very quickly to the brain. Once the stimulus reaches the brain, the appropriate brain lobe makes a "decision," which can be conscious or unconscious, and sends information back down the spinal cord, reversing the electrochemical process. Upon leaving the spinal cord at the proper point, the low-voltage signal travels back along a peripheral nerve to a neuromuscular junction, a place where the muscle tissue is highly excitable. The signal leaves the nerve and spreads around the muscle, causing contraction or expansion.
The pathway is as follows:
Stimulus -> Sensory Receptor -> Sensory Neuron -> Spinal Cord -> Brain -> Spinal Cord -> Sensory Neuron -> Neuromuscular Junction -> Skeletal Muscle