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Nerve

Also called nerves, peripheral nerve, peripheral nerves, nerve tissue

The peripheral nerves that carry signals between the spinal cord and the body. How they are built, how they slide and stretch with movement, how they get injured and regrow, what strength training changes, and how they age.

Region
Across the body
Updated
September 30, 2026
Reviewed
September 30, 2026 · how entries are checked

What it is

Spinal nerves carry both sensory and motor axons1. A peripheral nerve wraps its axons in three layers of connective tissue, a family of body tissues that also forms tendons1. These layers resemble the sheaths that wrap muscle1.

LayerWhat it wraps
EpineuriumThe whole nerve1
PerineuriumEach bundle of axons inside the nerve1
EndoneuriumEach single axon1

A review states that the outer layer makes up about half of a nerve’s cross-section6. The nerve’s blood supply runs just outside it6. The review calls that supply fragile and easily disrupted by injury, or by tension at a nerve repair6.

In peripheral nerves, Schwann cells wrap axons in myelin (the fatty, insulating wrap around many nerve fibres)1. Each Schwann cell covers a stretch of just one axon1. Signals travel faster along axons with myelin, jumping from node to node, than along axons without it1. A review gives about 30 to 120 metres a second for the fibres with the most myelin, and 1 to 2 for fibres without it3.

A review states that injured peripheral nerves, unlike nerves in the brain and spinal cord, can regrow and reconnect with the organs they supply7.

How it responds to load

A review describes peripheral nerves as built to keep working while they bear and adapt to the stresses of posture and movement2. Pulling, shearing and squeezing make a nerve slide, lengthen and narrow2. The review rests on the Physical Stress Theory: the level of physical stress on a tissue decides how it adapts2.

How far nerves slide

A review of 12 studies in living people measured how far joint movement slid four nerves8. Strain means stretch as a share of resting length. Just one of those studies measured it, finding at most 2.0% in the median nerve8.

NerveSlide with joint movement (mm)
Median nerve (a main nerve of the forearm and hand)0.1 to 12.58
Ulnar nerve (to the little-finger side of the hand)0.1 to 4.08
Tibial nerve (the main nerve at the back of the lower leg)0.7 to 5.28
Sciatic nerve (the large nerve down the back of the thigh)0.1 to 3.58

A systematic review of 22 studies in living people found a lot of sliding and change in nerve width with limb movement9. Raised strain was not a main finding9.

What stretch and pressure do

The stretch limits here come from two older rabbit studies, from 1992 and 1993, and they point in different directions10,11. In 24 rabbits, an hour of 6% tibial nerve strain cut the nerve’s signal by 70%, which then returned to normal10. In that study, 12% strain blocked the signal completely, with minimal recovery10. In 30 rabbits, 8% strain held for two hours made no significant dent in the muscle’s electrical response11. Strain of 15%, with the ankle bent up, cut it by 99%11.

A 1982 study tested pressure in 16 volunteers, over the median nerve at the carpal tunnel (a narrow passage at the wrist)12. At 30 mmHg there were mild changes and tingling in the hand12. At 60 and 90 mmHg, sensory signals were quickly blocked, 10 to 30 minutes before motor signals12. In three volunteers, a tourniquet version pointed to reduced blood supply, not crushing, as the main cause12.

How it gets injured

Grades of nerve injury

Seddon’s classification sorts nerve injuries into three grades by how much myelin, axon and connective tissue is damaged3. Sunderland’s scheme has five grades, splitting the middle grade by how many layers are also damaged3,6.

Sunderland gradeWhat is damagedOutcome, per one review
I (neurapraxia)Myelin only, in one spot3,6Recovers completely6
IIThe axon3Recovers completely6
IIIThe axon and endoneurium3Recovers partly6
IVThe axon, endoneurium and perineurium3Usually needs surgery6
V (neurotmesis)Axons and all layers, cut through3,6Usually needs surgery6

A review states that neurapraxia typically follows mild squeezing or pulling of a nerve and slows its signals3. At worst, signals are blocked, causing weakness3. Another review describes the block as temporary, clearing completely within 12 weeks once myelin is restored6.

Squeezed nerves

A review states that most nerve compressions are neurapraxia, often where nerves pass through narrow openings3. In the arm, it names the carpal tunnel and the cubital tunnel at the elbow as the commonest sites3.

One review describes two proposed ways squeezing harms a nerve3. Higher pressure presses on its blood vessels and starves it of blood3. Lower pressure can slow blood draining out, leading over time to swelling and scar tissue around and then inside the nerve3. In long-lasting squeezing, the same review reports, the axons remain but the myelin thins and breaks down3.

Nerve injuries in sport

One lab looked back at 180 athletes referred for nerve tests over 23 years: 86% of 216 nerve injuries were in the arm13. A review names squeezing, pulling, cuts and loss of blood supply as common causes in athletes14. A systematic review of reports from 1965 to 2003 found nerve injuries in 53 sports15.

How a nerve heals

Within the first week, an axon cut off by injury breaks down beyond the injury, which clears the way for regrowth3. Schwann cells clear the debris, then line the empty endoneurial tubes in columns that guide the regrowing axons6. A review states that regrowth goes best when these tubes are intact, as in grade II injuries3. Grade III and IV injuries disrupt the tubes and form more scar, so regrowth is disorganised3.

When about 20 to 30% of axons are damaged, one review states, healthy axons mainly sprout new branches3. When over 90% are damaged, recovery depends on the injured axons regrowing3. Two reviews give different regrowth speeds3,6. One gives an average of about 1 mm a day in humans6. The other says the rate may vary, about 2 to 3 mm a day nearer the spinal cord and 1 to 2 mm further out3.

Time also limits recovery. A review states that nerve cells lose their ability to regrow the longer they stay cut off from their targets7. Another states that muscle without its nerve can start to waste as early as 3 weeks, and scars for good after 2 years3. A third review states that fewer than half of people who have a nerve repaired regain good to excellent function6. In one meta-analysis of 623 repaired median or ulnar nerve injuries, each month of delay lowered the odds of motor recovery16.

How training builds it

Early strength and neural drive

The authors of one motor unit study note that strength typically starts to rise within a few training sessions17. They say this is usually put down to changes in neural drive from the brain or spinal cord17. A review cites strength rising without noticeable muscle growth as the first evidence for neural change18. The same review calls the role of changes outside the brain and spinal cord much more uncertain18.

Reviews read early gains differently. One concludes that gains come from a wide mix of neural and muscle factors, and that muscle growth also starts early19. Another argues that early rises in muscle size, up to about 4 sessions, are mostly swelling from muscle damage20.

A meta-analysis of 30 randomised trials, with 623 people, looked at where resistance training changes the nervous system4. Neural drive rose on two measures taken during contraction, but not on a spinal reflex measure4. The authors described the changes as subtle4.

Motor units and nerve speed

Fourteen people did 4 weeks of isometric training lifting the foot, alongside 14 controls17. In the trained group, the shin muscle’s motor units (a nerve cell and the muscle fibres it switches on) fired faster in steady efforts17.

In 48 adults aged 18 to 84, 4 weeks of handgrip training sped up motor signals in the median nerve21. Speed rose in both the younger and older training groups, but not in the control groups21.

Training one side strengthens the other

A meta-analysis of 31 randomised trials with 785 people measured cross-education, an untrained limb getting stronger when only the other one trains22. The untrained limb gained 11.9% on average, though the authors rated the risk of bias as high22.

Mean gain in the untrained limb (%)Isometric (joint not moving)8.2 Concentric (muscle shortening)11.3 Dynamic15.9 Eccentric (muscle lengthening)17.7
Mean gain in the untrained limb (%). Bars start at zero; the table below gives each value with its source.
Type of training in the trained limbMean gain in the untrained limb (%)
Isometric (joint not moving)8.222
Concentric (muscle shortening)11.322
Dynamic15.922
Eccentric (muscle lengthening)17.722

A meta-analysis of 29 leg studies in healthy people split them into eccentric and concentric training23. Only eccentric training significantly raised isometric strength in the untrained leg23. An earlier meta-analysis judged the gain likely to come from more output from motor nerve cells, not muscle change24. Its authors noted that most methods could miss small muscle changes24. A panel of 32 experts named changes in brain circuits that damp down signals as the main cause25.

Across the lifespan

Children

In a study of 5077 normal nerve tests, nerve signal size and speed rose with age in people under 185. In adults, signal size fell with age in every nerve tested5. A 2008 Canadian position paper states that children’s strength gains have been put down mainly to neural changes26. Its reason is that strength rises far more than muscle size26.

Newer work also points to changes in muscle. A 2016 overview states that gains in children rely partly on better nerve-muscle control and partly on changes in muscle and tendon27. A 2026 meta-analysis of 23 studies in 1443 people under 18 found training gave a small rise in muscle size against untrained controls28. The rise was larger against active controls, and largest after puberty28.

Older adults

The authors of the same large study conclude that advanced age goes with smaller, slower nerve signals5. A review names slower responses as a hallmark of ageing, from slower signals in both motor and sensory fibres29. Ageing also brings a net loss of motor units29. Motor nerve cells in the spinal cord can start dying slowly early in life, and faster after about age 6029.

A review reports that motor units typically fire more slowly in older adults29. Studies disagree on whether older adults switch their muscles on less fully, or as fully as young adults after practice29. A systematic review of 53 articles found that inactivity sped the decline of nerve-muscle junctions in ageing30. Resistance and endurance training showed potential to preserve or restore them30.

In 13 older adults, 4 weeks of isometric foot-lifting training made shin motor units fire faster, in step with their strength gains31. A review states that efforts to improve steadiness in older adults have usually used practice and strength training29. It reports large gains in force steadiness after under 4 weeks of light-load training and practice, put down to neural sources29.

Connections

The library’s first entry on a single nerve is the median nerve. It passes through several narrow passages, such as the carpal tunnel, where it can be squeezed32. The carpal tunnel syndrome page covers the commonest of these sites in the arm3.

Nerve and muscle meet in the motor unit, which ageing slowly loses29. A meta-analysis of neural change pooled trials of resistance training4. A study also covers isometric training17, and a meta-analysis eccentric training23.

Related exercise pages: the tibialis raise, where one-leg training in 19 people strengthened both legs33. Also the wrist curl: in 40 people who trained one wrist or one ankle, one-arm training raised the other arm’s strength by 6%34.

Nerve has an entry on

Nerve is compressed in

Nerve switches on

Nerve adapts to

Sources

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  5. Shelly S, Ramon-Gonen R, Paul P, Klein CJ, Klang E, Rahman N, Nikitin V, Ben David M, Dori A (2023). Nerve Conduction Differences in a Large Clinical Population: The Role of Age and Sex.. Journal of Neuromuscular Diseases. doi:10.3233/JND-230052 PMID 37545257
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  31. Casolo A, Del Vecchio S, Goodlich BI, Schrader B, Nuccio S, Lecce E, Bazzucchi I, Angius L, Felici F, Schrader J, Farina D, Del Vecchio A (2026). Ageing does not impair motor neuron adaptations: comparable motor unit responses to strength training in young and older adults.. The Journal of Physiology. doi:10.1113/JP290541 PMID 41823343
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Every source the library cites

Changes

  • First published.