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Case Study 13 Nervous Tissue Structure

REFERENCES

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NERVE

Injuries to nerve tissue are an important source of soft-tissue pain and dysfunction. Due to their anatomical locations in the extremities, numerous nerve trunks are particularly susceptible to mechanical trauma. Both sensory and motor nerve fibers are at risk, so symptoms from nerve injury can involve both motor and sensory disturbances.

The nerves that leave the spinal cord have a dorsal root that carries sensory information and a ventral root that carries motor signals; these are called the spinal nerve roots. The two sections of the nerve root blend together shortly after leaving the spinal cord, and converge to make the major trunks of the peripheral nerves that travel down the upper and lower extremities and to all other areas of the body (Fig. 2.5).

Within the major nerve trunks there are individual nerve fibers that transmit nerve signals and several connective tissue layers. A connective tissue layer called the endoneurium surrounds each individual nerve fiber. The nerve fibers are collected into bundles called fascicles, and another connective tissue layer called the perineurium surrounds the fascicles. The fascicles are collected in bundles, which are all enclosed within another connective tissue layer called the epineurium (Fig. 2.6). It is the numerous bundles of fascicles that make up an entire peripheral nerve.

The different layers of connective tissue within a nerve play an important role in nerve tissue pathologies. Peripheral nerves are provided more support and protection from the connective tissue layers than the spinal nerve roots. The epineurium surrounding the spinal nerve fibers are poorly developed or non-existent.42 This lack of protection makes the spinal nerves more susceptible to injury, particularly compression trauma.

The nerve needs adequate blood circulation to function properly so there is an intricate vascular supply to each nerve. There is a complex web of tiny blood vessels within the nerve. Neural ischemia due to compression is a known cause of neurological symptoms.43 Most people are familiar with the experience of pins and needles that comes with neural ischemia from holding a limb or sitting in a static position for a length of time. Once circulation returns the sensations recede. Nerve tissue is generally pliable and resilient to mechanical injury, but excessive force can produce tissue damage.

In addition to carrying sensory and motor impulses, the nerve fiber serves another important function. The nerve carries its own nutrient proteins necessary for proper function. These substances are carried through the nerve by a slow flowing cytoplasm within the nerve cell called axoplasm. This two-way flow of axoplasm inside the nerve is called the axoplasmic flow. Disturbances to the axoplasmic flow affect the nerve in the local area as well as throughout the entire length of the nerve.

Most major nerve trunks carry sensory and motor fibers so pathology can produce both sensory and motor symptoms. Some nerve pathologies are more likely to produce sensory symptoms while others produce motor dysfunction due to the number of sensory and motor fibers in their associated nerve trunks. In the upper extremity, for example, the median nerve has a greater percentage of sensory fibers than motor fibers, so symptoms of carpal tunnel syndrome tend to be sensory before there is motor impairment. In contrast, compression of posterior interosseous nerve, which has mostly motor fibers, creates weakness before there is sensory impairment.43

Dermatomes, cutaneous innervation, myotomes

Becoming familiar with dermatomes, cutaneous innervation, and myotomes will help practitioners better understand nerve injury symptoms. A dermatome is an area of skin supplied by fibers from a single nerve root. The fibers from that one nerve root make up several peripheral nerves and innervate specific areas of the body. For example, the C8 nerve root has branches that make up portions of several upper extremity nerves, such as the median and ulnar nerves. Sensory symptoms from C8 nerve root irritation are felt in the ulnar side of the hand and medial side of the forearm and arm (Fig. 2.7).

Each peripheral nerve is made up of fibers deriving from the different nerve roots. A skin region supplied by a peripheral nerve is called the cutaneous innervation of that nerve (Fig. 2.8). There can be some overlap between the dermatome and the area of cutaneous innervation. For example, the ulnar nerve in the arm has fibers from C7, C8, and T1, yet its sensory fibers only supply the ulnar aspect of the hand and the last two fingers, whereas the C8 dermatome covers the ulnar aspect of the hand, as well as the entire medial forearm and arm.

Information about dermatomes and regions of cutaneous innervation is valuable in the clinical evaluation process. Using the example above, if sensory symptoms are felt throughout the C8 dermatome, one would suspect nerve root involvement rather than a problem with the ulnar nerve. This is because the symptoms extend outside the ulnar nerve's cutaneous innervation area on the ulnar side of the hand. If symptoms are only in the ulnar side of the hand, ulnar nerve pathology is more likely. Keep in mind that symptoms may appear in only a part of the dermatome. Whenever sensory symptoms are reported in an extremity one should consider the possibility of the nerve root being the source of the problem.

Myotomes are somewhat similar to dermatomes. A myotome is a group of muscles that are innervated by the same nerve root. A single muscle can have fibers that come from several different nerve roots. Each peripheral nerve also has a number of muscles that it innervates. If there is weakness apparent in a group of muscles innervated by the same nerve root, i.e. the same myotome, then the problem is most likely at the nerve root level. Muscle weakness in a myotome is not always easy to detect because the muscle may only have a small number of fibers that come from the affected nerve root. For more on these conditions see the text, Orthopedic Assessment in Massage Therapy.

Nerve injuries

Nerve injuries generally develop from compressive loads, such as a direct blow to the nerve or a chronic low level of compression. With excess tensile stress the overall diameter of the nerve decreases compressing the fibers within the nerve; this condition is called adverse neural tension.44–46 The symptoms of compression or tension pathology are similar because both cause degeneration of the nerve. Compressive and tensile forces can occur in numerous locations along the nerve, but several sites are most vulnerable for nerve damage.46

Radiculopathy, neuropathy

There are two terms that describe nerve injuries and which indicate the location of the pathology in the nerve. A radiculopathy is a nerve pathology that occurs at the nerve root. A common radiculopathy is the herniated nucleus pulposus (HNP) or herniated disc, in which the disc presses on a nerve root. Pathology farther along the length of the nerve is a neuropathy. It is also called a peripheral neuropathy indicating that the injury is in the peripheral nerves, distant from the nerve roots and spinal cord. Many nerve compression syndromes, such as thoracic outlet and carpal tunnel syndrome, are examples of peripheral neuropathies.

Box 2.4

Areas Where Nerves Are Most Vulnerable

Tunnels – soft or bony tissues may create tunnels the nerve must travel through, compression is the risk. Ex: carpal tunnel in the wrist, the cubital tunnel in the elbow or the tarsal tunnel in the foot.

Nerve branches – anywhere nerve tissue branches out to other areas there is the potential for increased neural tension. Ex: where the posterior interosseous nerve branches from the main radial nerve near the elbow.

Nerve is fixed – anywhere the nerve is tethered to an adjacent structure for stability is a region for potential compressive or tensile stress on the nerve. Ex: deep peroneal nerve where it is attached to the upper region of the fibula in the lower extremity.

Nerve passes by unyielding surface – passing close to unyielding surfaces like bone there is a greater chance of compression or tension. Ex: brachial plexus as it goes over the first rib in the upper thoracic region.

Tension points – the mid-point of a stretched nerve fiber, called the tension point of the nerve, is more susceptible to pathology.

Nerve degeneration

Nerve degeneration results from mechanical forces or from systemic disorders that attack the nerve, such as multiple sclerosis. Nerve injuries are classified by severity into three levels: neurapraxia, axonotmesis, and neurotmesis.47,48 Either compression or tension injuries can produce these levels of nerve injury.

When there is impairment of the axoplasmic flow in one part of a nerve, the remainder of the nerve becomes nutritionally deficient and thus more susceptible to pathological changes. With additional regions of the nerve more susceptible to degenerative changes, a client may have symptoms of more than one nerve pathology. The presence of multiple sites of neurological pathology is called the double or multiple crush phenomenon.49–51

The nerves of the upper extremity provide a good illustration of double crush pathology. Brachial plexus compression near the thoracic outlet impairs axoplasmic flow and subsequent function of the distal regions of the upper extremity nerves. As a result simultaneous symptoms of thoracic outlet syndrome and carpal tunnel syndrome can develop.

Box 2.5

Nerve Injury Levels

Neurapraxia – mild sensory and motor deficits

Axonotmesis – sensory and motor dysfunction, significant pain

Neurotmesis – altered sensation or function, or no recovery

Neurapraxia, axonotmesis, neurotmesis

Neurapraxia is the least severe nerve injury and involves the blocking of axon conduction. The nerve continues to conduct some signals above and below the primary area of compression or injury, but conduction velocity slows. Common symptoms include mild sensory and motor deficits, which usually decrease when the nerve is no longer compressed.

The next level of nerve damage is called axonotmesis. There is a loss in continuity of the axon, but the surrounding endoneurium may still be intact. The outer layers of connective tissue are still intact as well. Typical symptoms include sensory and motor dysfunction, as well as significant pain. If the connective tissue layers are intact the nerve axon is likely to regenerate, although slowly. The rate of regeneration of nerve axons is estimated to be 1 mm per day or 1 inch per month, but in certain cases slower.46

A severe nerve injury is neurotmesis. At this level, damage affects not only the axons, but also their connective tissue layers. Because these layers are damaged, recovery from neurotmesis may not be possible. Neurotmesis occurs in severe crush injuries or situations where the nerve is severed. Axons can regenerate once severed, but because the connective tissue template is disrupted the axons may not grow back in their original locations. This is one reason some surgical repairs produce altered sensation or function in the region when the individual regains use.

Box 2.3

Nerve Compression/Tension Signs/Symptoms

Reduced sensory input

Reduced motor impulses

Pain in a specific dermatome

Motor weakness in a specific myotome

Hyperesthesia or paresthesia sensations

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