Peripheral nerves

The brain and spinal cord are called the central nervous system. Peripheral nerves branch out from beyond this central system throughout the body. Nerve roots emerging from the spine come together to form networks such as the brachial plexus and lumbar plexus. These then divide into major nerves, including the Median nerve, Ulnar nerve, Radial nerve, and Sciatic nerve, which extend all the way to the fingertips and toes.

A single peripheral nerve usually carries three types of fibers together.

• Sensory fibers (afferent) — Carry information about touch, pain, temperature, and position from the skin, muscles, and joints up to the central nervous system.
• Motor fibers (efferent) — Carry commands from the central nervous system down to the muscles to make them move.
• Autonomic fibers — Regulate the functions of sweat glands, blood vessels, and internal organs without our conscious control.

Nerves containing a mixture of these three types of fibers are called mixed nerves. The median, ulnar, and sciatic nerves belong to this group. Most peripheral nerves are mixed nerves.

A cross-section of a nerve looks much like a bundle of electrical wires. At the center are axons, the long projections of nerve fibers, surrounded by Myelin, or the myelin sheath. Each axon is wrapped in the endoneurium. A bundle of these axons is wrapped in another layer, the perineurium, which also acts as a barrier that filters substances entering and leaving. The outer layer that binds multiple bundles into a single nerve trunk is the epineurium.

Small blood vessels that nourish the nerve, called the vasa nervorum, run through the epineurium. An outer vascular network along the nerve's surface and an inner network penetrating the nerve work together to supply blood.

This multilayered structure allows nerves to stretch and slide to some extent as joints move (Nerve gliding). Whenever you reach out with your arm or bend your neck forward, your nerves stretch and move too. However, they are more vulnerable to compression and friction where they pass through narrow channels formed by bones, ligaments, and fascia, or through spaces between muscles (Nerve entrapment syndrome).

The types of signals nerve fibers carry differ according to their thickness and whether they have Myelin. Thick fibers with thick myelin carry signals that need to travel quickly, such as movement commands and position sense. Thin fibers with thin myelin or no myelin mainly carry signals that can travel a little more slowly, such as pain and temperature. This distinction is key to understanding which functions change first when a nerve is damaged.

Most nerves handle both sensation and movement, but some do only one job.

Purely sensory nerves

These nerves consist only of sensory fibers, with no motor fibers. Typical examples include the Lateral femoral cutaneous nerve (skin sensation over the front and outer thigh), the saphenous nerve (skin sensation along the inner shin and foot), and the sural nerve (skin sensation over the back and outer calf).

Because they have no motor fibers, compression or irritation of these nerves does not cause weakness or muscle wasting. The main symptoms are tingling, prickling, and reduced sensation. This is why Meralgia paresthetica, in which the lateral femoral cutaneous nerve is compressed, changes sensation in the thigh while leaving leg strength unchanged.

Purely motor nerves

Conversely, some nerves or nerve branches have only motor fibers, with no sensory fibers. A typical example is the anterior interosseous nerve (Anterior interosseous nerve), which branches from the Median nerve. It passes deep within the forearm and sends only movement signals to the muscles that bend the thumb and index finger, without supplying skin sensation.

When this nerve is compressed, specific movements become weak without tingling. Difficulty making a circle with the thumb and index finger in a pinching motion is a typical feature of Anterior interosseous nerve syndrome. The absence of sensory changes helps distinguish it from other nerve compressions.

Nerve conduction is the process by which electrical signals (action potentials) travel along a nerve cell's axon from one end to the other.

In fibers wrapped in Myelin, signals travel quickly by jumping between gaps in the myelin sheath. In fibers with thin myelin or no myelin, signals travel relatively slowly, progressing sequentially along the axon. In other words, thick fibers responsible for movement and position sense send signals quickly, while thin fibers responsible for pain and temperature send them slowly. This difference forms the basis for distinguishing Large fiber neuropathy from Small fiber neuropathy.

Compression or irritation of a nerve affects this transmission process. With mild, temporary compression, transmission may simply slow down or become blocked briefly without the nerve fibers being severed (temporary conduction block). In such cases, function is known to return relatively well once the compression is relieved (Peripheral nerve injury). If compression or injury is prolonged or severe, changes may occur in the nerve's connections themselves, and recovery may take longer. When nerve transmission slows or stops, tingling, reduced sensation, or weakness may develop in the area supplied by that nerve (see also Radiculopathy).

You usually do not notice your peripheral nerves until they are compressed or irritated.

• The electric sensation shooting down to your little finger when you hit the bony area on the inside of your elbow happens because the Ulnar nerve passing through that area is irritated.
• Pins and needles in your foot when you stand up after kneeling for a long time also occur as a nerve is briefly compressed and then released.
• Tingling along the outside of your leg after sitting with your legs crossed for a long time can also be understood as the result of pressure along a nerve's path.

• Avoid staying in the same position for too long. Get up about once an hour and shake out your arms and legs.
• Avoid keeping your elbows bent for long periods. Holding a phone for a long time or resting your elbows on a desk can make nerves more susceptible to compression.
• Keep your wrists straight when possible. Adjust the height of your keyboard and mouse so that your wrists do not bend while you use them.
• Move gently. Slowly circling your shoulders, elbows, and wrists and taking easy walks help blood circulation and nerve gliding.
• Do not forcefully stretch or knead the side that is tingling. This can further irritate an already sensitive nerve.

• Tingling or reduced sensation continues for several days and gradually spreads
• Your hand becomes weak, causing you to drop things often, or your toes drag when you walk
• Tingling wakes you from sleep, or the muscles in an arm or leg on one side become noticeably thinner

If you notice these changes, have your condition assessed by a professional who can check which nerve is compressed and where.

These are individually named nerves that branch from peripheral nerves. Posterior interosseous nerve · Anterior interosseous nerve · Long thoracic nerve · Lateral plantar nerve · Medial plantar nerve · Intercostal nerves · Saphenous nerve · Cervical plexus · Superior gluteal nerve · Inferior gluteal nerve · Sural nerve · Pudendal nerve · Ilioinguinal and genitofemoral nerve entrapment · Plexopathy