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Along with all the complex mechanical activities involved in movement, adequate sensory input by the nerves must be maintained to all points of the upper limb (Kumar, 1999). The hands must perform the activities of daily life, along with the ability to differentiate between most environmental surfaces and temperatures instantaneously, allowing for immediate motor response to the sensations being felt.

Skeletal muscles are well supplied with nerves. Some nerve endings are responsive to the degree of movement and stretch at a joint while others convey information on pain. All information from the nerves is relayed to the brain via the spinal cord which may respond by sending signals to the muscles via different nerves to adjust body movement (Tortora, Grabowski, 2003).

As an individual assumes a posture or movement, the structural and biomechanical properties of peripheral nerves in the upper limb including the median nerve are modified as the nerve responds to the physical stresses placed upon it (Dilley, et al 2003). The peripheral nerves employ axons to conduct impulses that facilitate the bodies interactions with the world and tolerate and adapt to the everyday movements and postures (Topp, Boyd, 2006). Axons are the lengthy extensions of cell bodies and are insulated from each other, bundled together, and protected by layers of connective tissue known as the endoneurium, the perineurium, and the epineurium. Figure 1 illustrates the components of a peripheral nerve.

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Figure 2-15: The components of a peripheral nerve. (www.vanat.cvm.umn.edu. Last

imaged 01-10-10).

The endoneurium is the layer where the myelin of each myelinated axon is formed from the plasma membrane of a Schwann cell, with a single Schwann cell enveloping a myelinated axon to form an internode. The points of separation between myelinating Schwann cells are called nodes of Ranvier. Unmyelinated axons do not have a

Schwann cell plasma membrane. The myelinated axon or group of unmyelinated axons is surrounded by type IV collagen (Thomas, 1963), which is similar to the collagen found in tendons and discussed in the previous sub-section.

A number of axons are grouped into nerve fascicles in the perineurium. These layers of type I and type II collagen fibres (and also include fibroblasts, a few mast cells and macrophages, and endoneurial fluid) and perineurial cells provide mechanical

strength, resulting in the perineurium being the primary load-bearing portion of the nerve (Rydevik, et al 1990). Nerve fascicles are held together and surrounded by a connective tissue layer, termed the epineurium. The epineurial layer includes bundles of type I and type III collagen fibrils and elastic fibres (as well as fibroblasts, mast

cells, and fat cells) (Rydevik, et al 1990). The epineurium is loosely attached to the perineurium which allows for sliding of one fascicle independently of an adjacent fascicle (Millesi, et al 1995). The outermost tissue of the epineurium is attached to the surrounding connective tissue, which contains a significant amount of adipose tissue to protect the nerve at sites of recurring compression and facilitate the transverse and longitudinal sliding of the nerve (Millesi, et al 1995).

Under normal physiological conditions imposed by posture and movement, nerves are exposed to various mechanical stresses such as tensile or compressive stress (Driscoll, et al 2002).

Tensile stress on a nerve occurs during movement of a joint, with the nerve

accommodating the stress by both elongating and sliding (Millesi, et al 1995). The displacement or sliding of a nerve relative to the surrounding structures is called excursion with the direction and magnitude of nerve excursion is dependent on the anatomical position between the nerve and the axis of rotation in the moving joint (Erel, et al 2003). During limb movement, excursion of the nerve occurs first in the nerve segment immediately adjacent to the moving joint and develops progressively in a more distal direction (Dilley et al 2003).

When a load is first applied, the nerve lengthens relative to the applied load and results in straightening of the connective tissue and axons in the endoneurial

compartment. This causes a reduction in the cross-sectional area, a property termed transverse contraction (Millesi, et al 1995). Increased pressure in the endoneurial

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resisting the transverse contraction (Millesi, et al 1995). When the tensile stress is removed, it is likely that a combination of elasticity of the connective tissues and pressure within the neural core will allow recoiling of the nerve to the original cross- sectional area and length (Millesi, et al 1995).

In addition to tensile stress, nerves are exposed statically and dynamically to compressive stresses. Nerves may be compressed externally by approximation to adjacent tissues, such as muscle, tendon, or bone, or by pressure increases in the extraneural environment. Compression of a nerve segment causes displacement of its internal contents in transverse and longitudinal directions (Millesi, et al 1995). Low compressive stress for a short duration may result in reversible physiological and minor structural changes, however low compressive stress applied over a long period of time may cause significant structural changes in the nerve due to impairment of blood flow and ischemia. High compressive stress may result in structural alterations in myelin sheaths and disruption of axons (Millesi, et al 1995).

Common functional positions may result in compression pressures that approach or exceed the 20 to 30 mm Hg demonstrated to impair nerve blood flow (Rydevik, et al 1981). Carpal tunnel pressure in subjects who have normal median nerve function measured at 3 to 5 mm Hg with the wrist in a neutral position (Rojviroj, et al 1990). Placing the hand on a computer mouse was shown to increase the tunnel pressure from the resting 5 mm Hg to 16 to 21 mm Hg, (Rojviroj, et al 1990) and actively using the mouse to point and click increased the tunnel pressure to between 28 and 33 mm Hg, a pressure high enough to reduce nerve blood flow (Rojviroj, et al 1990).

In addition, carpal tunnel pressure was shown to increase to 63 mm Hg with 40 degrees of wrist extension and 0 degrees of metacarpophalangeal flexion,80 and the anatomy of adjacent structures is thought to play a large role in these increases in carpal tunnel pressure (Rojviroj, et al 1990).

The wrist and hand are innervated for motor and sensory function by three peripheral nerves; the radial, ulnar and median nerve which are illustrated in Figure 2-16. The radial nerve innervates muscles that facilitate extensions of the wrist and fingers, and the ulnar nerve innervates the muscles that power grip actions of the hand (Omer, et al 1998). The median nerve is the only one of the three nerves that travels through the carpal tunnel and is critical for fine motor hand function concerning both motor and sensory supply (Omer, et al 1998).

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All the nerves in the upper arm originate from the brachial plexus. The brachial

plexus is an arrangement of nerve fibers, running from the spine, through the neck and into the arm and is responsible for most of the skin surface and muscular innervations of the upper limb. The median nerve is formed by the lateral and medial cords of the brachial plexus and also carries fibres originating from the 6th, 7th and 8th cervical and the 1st thoracic spinal segments. The origins of the three nerves of the upper limb from the brachial plexus are illustrated in Figure 2-17.

Figure 2-17: The brachial plexus (www.eorthopod.com/content/quadrilateral-space. Last viewed 04/05/10)

The median nerve carries both motor and sensory fibres. The motor fibres supply most of the muscles on the front of the forearm along with four small muscles in the hand. The sensory function of the median nerve includes the palmar aspect of the

thumb, the index and middle finger and one half of the ring finger (Omer, et al 1998). Appendix A details median nerve innovation in the hand and arm.

During movement, the peripheral nerves of the upper limb not only have to conduct sensory and mechanical information to and from the central nervous system but also have to adapt mechanically. This adaptation of the median nerve has been reported as increasing in length by as much as 20% between movements (Butler, 1991).