Peripheral Nervous Systems : Spinal Nerves & Plexuses

As an Endurance Athlete we all suffer ongoing injuries, I have suffered so many that have been cause by overuse and imbalance of my muscles. The problem that I have faced many times, is nerve entrapment and damage resulting from these overuse issues. Here is my best shot at the nerves in their most basic layout - hope it helps.

We have 31 pairs of spinal nerves and they are named after the sec­tion of the spine they come out of.

• 8 pairs of cer­vi­cal nerves (C1-C8)
• 12 pairs of tho­racic nerves (T1 – T12)
• 5 pairs of lum­bar nerves (L1-L5)
• 5 pairs of sacral nerves (S1-S5)
• 1 pair of coc­cygeal nerves (Co1)

Why do we have an 8^th pair of cer­vi­cal nerves if we only have 7 cer­vi­cal ver­te­brae you ask? Well because we start at the top of the atlas and all the nerves are in between the ver­te­brae so the 8^th pair is in between C7 and T1.


Note the dif­fer­ence between roots and rami (ramus is sin­gu­lar). The rootlets extend out of the spinal cord medi­ally and com­bine to form roots. The roots com­bine to form the spinal nerve and then they split apart again and are now called rami (ramus for sin­gu­lar). The dor­sal root is pos­te­rior to the ven­tral (front) root.
The dor­sal rami inner­vate the deep mus­cles of the back for motor con­trol, such as the erec­tor spinae and also a hor­i­zon­tal strip of skin for sen­sory input.
The ven­tral rami in the tho­racic region are known as the inter­costal nerves. They run deep to the ribs and inner­vate the inter­costal mus­cles and pro­vide sen­sory input for the over­ly­ing hor­i­zon­tal strips of skin there as well as the abdom­i­nal wall mus­cles (motor) and skin (sen­sory). The inner­va­tion of the back, ribs, and abdom­i­nal area is shown here:

Nerve Plexuses

A nerve plexus is a net­work of nerves that seem to be tan­gled that mostly serve the limbs. Each net­work is a net­work of nerves that come together and then redis­trib­ute them­selves out with a dif­fer­ent dis­tri­b­u­tion of nerves into the limbs. They are made from the rest of the ven­tral rami (that is, from other than the tho­racic region that we haven’t talked about yet). Each end of each plexus con­tains fibers from sev­eral spinal nerves. The fibers from each ven­tral ramus trav­els along dif­fer­ent routes so that each limb mus­cle receives inner­va­tion from more than 1 spinal nerve to have a backup plan in case of injury.

We have four plexuses: Cer­vi­cal, Brachial, Lum­bar, and Sacral

Cer­vi­cal Plexus

This plexus is located under­neath the ster­n­oclei­do­mas­toid mus­cle (from C1 – C4). Most of the branches inner­vate the skin of neck and deep neck mus­cles. The Phrenic nerve (C3, C4, C5) gets spe­cial men­tion that inner­vates the top of the diaphragm (after trav­el­ing down through tho­racic cav­ity, along either side of the heart). Note that if both phrenic nerves are cut or if the spinal cord is sev­ered above C3, breath­ing stops.

The Brachial Plexus

This plexus gets con­tri­bu­tion from the tran­si­tional area from neck into armpit (axilla) between C5 and T1. It pro­vides almost all the inner­va­tion of the upper limb (this includes back and scapu­lar mus­cles that con­trol the limb). The orga­ni­za­tion of this plexus is also very messy. There’s mix­ing of branches and then a branch­ing of branches and then mix­ing again. Note how there’s divi­sions, trunks and roots.

Five nerves inner­vate the mus­cles of the arm:

1. Mus­cu­lo­cu­ta­neous inner­vates the biceps brachii and some over­ly­ing skin. (C5, C6, C7)
2. Median inner­vates ante­rior fore­arm mus­cles, some intrin­sic hand mus­cles, and skin of lat­eral (anatom­i­cal position/toward thumb) palm. (C5, C6, C7, C8, T1)
3. Ulnar inner­vates intrin­sic hand mus­cles and skin of the medial hand. (C8, T1)
4. Radial trav­els along radius and inner­vates Tri­ceps brachii and mus­cles of the pos­te­rior com­part­ment, and over­ly­ing skin. (C5, C6, C7, C8, T1)
5. Axil­lary inner­vates the del­toid and teres minor, and over­ly­ing skin. (C5, C6)

The Lum­bar Plexus

This plexus lies within the iliop­soas mus­cle and comes from between L1 and L4. The main branches inner­vate the ante­rior thigh. Smaller branches inner­vate some of the abdom­i­nal wall and iliop­soas muscle.

Femoral nerve — inner­vates quadri­ceps femoris mus­cle and over­ly­ing skin

Obtu­ra­tor nerve — inner­vates adduc­tor mus­cles and over­ly­ing skin

The Sacral Plexus

The sacral plexus lies cau­dal to the lum­bar plexus (stems from L4 to S4) and is often referred together with­the lum­bar plexus as the lum­bosacral plexus. The branches inner­vate but­tocks, pelvis, per­ineum and lower limb (except for ante­rior and medial thigh).

Sci­atic nerve, the largest nerve of the sacral plexus is actu­ally two nerves wrapped in one sheath:

Tib­ial nerve (L4,L5,S1,S2,S3) inner­vates pos­te­rior thigh, pos­te­rior leg (lower leg), and intrin­sic mus­cles of foot

Com­mon fibu­lar (per­oneal) nerve (L4,L5,S1,S2) inner­vates mus­cles of the antero­lat­eral leg (lower leg)