Beneath this is a layer of fat cells, the perineurium, which forms a complete sleeve around a bundle of axons.

This forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors.

The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer delicate meshwork of collagen fibres.

Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid.

Nerve growth normally ends in adolescence but can be re-stimulated with a molecular mechanism known as "notch signaling".

The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to all remaining body parts.

The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems.

Neurologists usually diagnose disorders of nerves by a physical examination, including the testing of reflexes, walking and other directed movements, muscle weakness, proprioception, and the sense of touch.

This initial exam can be followed with tests such as nerve conduction study, electromyography (EMG), and computed tomography (CT).

Symptoms can be felt in areas far from the actual site of damage, a phenomenon called referred pain.

It occurs when the macrophages of an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve.

[11]: 38–44  Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right.

Each Mauthner cell has an axon that crosses over, innervating (stimulating) neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes.

The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself rapidly forward.

There is no brain or centralised head region, and instead there are interconnected neurons spread out in nerve nets.