Nerves are cable-like structures containing bundles of fibers that transmit electrochemical impulses between the central nervous system and peripheral organs. There are sensory nerves which transmit impulse from sensory organs to the central nervous system responsible for feeling touch, pain, temperature and position. There are also motor nerves which transmit impulses from the central nervous system to muscles to tell them when to contract. 

Each nerve is composed of axons which are individual fibers surrounded by a layer of connective tissue called the endoneurium. The axons are bundles into groups called fascicles which are wrapped in a layer of connective tissue called the perineurium. Several fascicles are grouped together to form a nerve which is wrapped in the epineurium.

When a nerve is injured, it is unable to transmit these electrical impulses. Depending on which nerve is affected, there is varying degrees of numbness and weakness. 

There are several types of nerve injuries. 

Neuropraxia is the least severe. Here the axons are intact but the its sheath is damaged leading to an interruption of electrical conduction along the fiber. This is usually a result of compression of the nerve affecting its blood supply. There is temporary loss of function but most patients go on to recover within hours to months after the injury.

Axonotmesis is more severe and is usually due to crush or stretch of the nerve. Here the axons are interrupted but the connective tissue sheath is intact. The nerve fiber will undergo degeneration before regenerating at a rate of about 1mm a day over weeks to years.

Neurotmesis is the most severe and the axons as well as connective tissue covering is interrupted, as happens when the nerve is transected. There is no potential of full recovery.

If a nerve is transected and its ends can be freshened and brought together to be sutured without tension, then regeneration along the nerve can happen at a rate of 1mm a day. This is known as primary nerve repair.

If a segment of the nerve has been lost and the ends cannot be brought together without tension, then nerve grafting is required. Here a segment of donor nerve is harvested and used to bridge the gap of the defect. The sural nerve which is a sensory nerve of the lower leg that supplies the outer part of the foot is often used as a donor nerve as its function is considered expendable. There will be some numbness in the donor area which would become smaller over several years. The nerve graft acts as a conduit for the axonal fiber to regenerate within across the gap. 

An example of its use is a cross facial nerve graft in facial paralysis. The nerve graft is connected to branches of the facial nerve on the opposite (good) side and runs across to the paralysed side. Over months, the nerve regenerates and the electrical impulses from the opposite (good) facial nerve travel along the nerve graft until it reaches the paralysed side. 

The patient is now ready for the second stage surgery in which a gracilis muscle flap is harvested together with its nerve and blood vessels. This is transferred to the site of the facial paralysis and the blood vessels connected to recipient vessels in the area and the nerve graft is hooked up to the nerve of the gracilis muscle. Eventually after recovery, when the patient smiles, electrical impulses from the opposite (good) facial nerve travel along the nerve graft to the gracilis muscle flap which contracts to cause a smile. 

Nerve transfer involves taking a donor nerve branch with a less important role and transferring it to restore function in a more crucial nerve nearby that has been severely damaged. 

An example is the masseteric nerve transfer for the treatment of facial paralysis. Here the facial nerve is damaged; if left untreated over a few months, the paralysed facial muscles would have wasted and the paralysis becomes irreversible. The nerve to the masseter that is one of the muscles for chewing is nearby and transferred to supply the paralysed facial muscles before wasting ensues. The contractions are stronger as the nerve to the masseter is a powerful nerve and the time to recovery is faster as the donor nerve is nearby and only has a short distance to regenerate across. Moreover a second stage surgery is not needed. 

When the nerve to the masseter has regenerated, its electrical impulses will reach the facial muscle, so when the patient chews, his facial muscles will contract. Initially the patient has to consciously chew to move his face, but over time his brain adapts and soon the patient is able to move his facial muscles subconsciously without thinking of chewing.