20 May Commonly injured structures in the foot from running #3 Peripheral Nerves
The peripheral nervous system includes all neural structures outside the brain and spinal cord. They include sensory receptors, peripheral nerves and their associated ganglion, and efferent motor endings.1
Sensory receptors are specialised to respond to changes in their environment, which are called stimuli.1 These sensory receptors are linked to nerves whose stimulus triggers a nerve impulse along the nerves back to the central nervous system.
Nerves provide the pathway for signals to be transmitted to and from the central nervous system to contract skeletal muscle as well as provide sensory feedback. There are two types of fibers within a nerve which are categorised based off the direction of the signals. Sensory or afferent fibers conduct signals from sensory receptors to the central nervous system (brain and spinal cord). Motor or efferent fibers conduct signals from the central nervous system to muscles and glands. The majority of nerves are mixed nerves as pure sensory or motor nerves are rare.1
Peripheral motor endings are the terminals where motor nerves innervate voluntary muscles into each individual muscle fibre at their neuromuscular junction and activate them (muscle fiber contraction) by releasing what are called neurotransmitters.
An important peripheral nervous system mechanism to also mention is what is called the reflex arc which can be simply described as a rapid, predictable motor response to stimulus.1 It is unlearned, unpremeditated, and involuntary and can be considered as built into our neural anatomy.1 For example reflexes we use for running include our body correcting itself if we mis-step on uneven surface. This adjustment obviously needs to happen very quickly and in most cases we may not even remember these small adjustments.
A common nerve injury that occurs in the feet is what is called Morton’s neuroma. It can be thought simply as a thickening of a plantar digital nerve affecting the web spaces of the toes, most commonly the third web space (between the 3rd and 4th toes). The complaint is often sharp pain potentially accompanied by burning, numbness and tingling in the toes. The irritation can occur suddenly while walking or running to the point of having to remove your running shoe on the affected foot.
There are various potential contributors to this condition that have been suggested in particular in the most common site i.e. the 3rd webspace.2 These include a naturally larger nerve compared to the other digital nerves due to a double origin of the digital nerve coming from both the medial and lateral plantar nerves.2 As a result this larger nerve may be more susceptible to trauma including compression from tight fitting shoes.2
Another theory is an inflammation of the intermetatarsal bursa (bursitis) causing it to thicken and therefore compress the close lying digital nerve. The bursa in the 2nd and 3rd webspaces lye particularly close to the neurovascular bundle hence why it has been suggested these are the most common sites for irritation.2
Potential disruption in the blood supply to the nerve from micro trauma to the digital artery has also been suggested.2 This would have the effect of starving the nerve from oxygen (ischemia). It was proposed ‘the most likely factor would be repeated minor trauma to the digital artery from pressure transmitted through the sole.2 This may be in line with occurring while running with the high impact forces exhibited. Especially in those cases where it doesn’t seem there has ever been a shoe fitting width issue with any of the runner’s shoes.
A mechanical stress theory has been suggested which describes the digital nerve being irritated by a potentially unstable transverse arch.2 This instability being from laxity or hypermobility that is displayed in some foot types causing a flattening of the transverse arch and subsequent widening of the foot. If wide enough this makes for potential difficulties in fitting every day, work, casual and running shoes properly. The resultant tight fit may cause compression and shearing of the digital nerve between the bone and bursa leading to irritation and subsequent fibrosis of the plantar digital nerve.2
1 MARIEB, E. N.; HOEHN, K. Human Anatomy & Physiology. 8th. San Francisco, CA, USA: Pearson Benjamin Cummings, 2010.
2 LORIMER, D. L. et al. Neale’s Disorders of the Foot. 6th Edition. Churchhill Livingstone, 2003.
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Aleks Baruksopulo is a Podiatrist on the Gold Coast with a special interest in Sports & Running injuries of the feet and lower limbs, he has over 13 years of experience working in Physiotherapy & Sports Medicine Clinics and has performed over 22,500 patient consultations within this setting.
His business is called SportsMed Podiatry which consults out of Physiotherapy practices across the Gold Coast including at Broadbeach, Burleigh Heads, Robina and the SCU health clinic.
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