Spinal nerve S2
Nervus spinalis S2
- Related terms: Second sacral nerve; S2
Definition
The second sacral spinal nerve S2 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibers. It originates from the spinal cord at the S2 spinal segment.
a) Roots
Like other nerves, the spinal nerve S2 is formed by the union of two roots:
1. Posterior (dorsal) root:
This root contains sensory fibers that carry signals from peripheral structures (innervated by the anterior and posterior rami) to the spinal cord.
These fibers enter the dorsal gray horn within the spinal cord.
2. Anterior (ventral) root:
This root consists of motor fibers originating from the anterior gray horn within the spinal cord.
These fibers transmit motor commands to peripheral muscles via the spinal nerve and its branches.
b) Mixed spinal nerve and rami
The anterior and posterior roots merge within the sacral vertebral canal to form the spinal nerve S2. This mixed nerve, containing both sensory and motor fibers, divides into two primary rami:
1. Anterior (ventral) ramus:
This ramus emerges via the anterior sacral foramen and contributes to the sacral plexus, which provides motor and sensory innervation to structures, including the lower limbs and gluteal regions. The anterior ramus further subdivides into ventral and dorsal divisions.
It also forms connections with the sympathetic trunk via the gray rami communicantes, which carry postganglionic sympathetic fibers. It is important to note that white rami communicantes are absent at this level. Postganglionic parasympathetic fibers travel through the pelvic splanchnic nerves, which originate from the anterior divisions of the anterior rami of the S2 to S4 spinal nerves.
2. Posterior (dorsal) ramus:
This ramus provides motor innervation to the deep muscles of the back.
It also provides sensory innervation to the skin overlying the back.
A. ANTERIOR RAMUS OF SPINAL NERVE S2:
The anterior ramus of spinal nerve S2 is the larger division of the nerve and functions as a mixed nerve, carrying both motor (somatic efferent) and sensory (somatic afferent) fibers. After exiting through the anterior sacral foramen, it merges with the nerve fibers from the lumbosacral trunk and anterior rami of sacral nerves S1, S3, and part of S4 to form the sacral plexus. This plexus is located on the posterolateral wall of the pelvic cavity, lying on the anterior surface of the piriformis muscle and deep to the internal iliac vessels.
Key functions and innervations
Motor Innervation (Somatic Efferent Fibers): | Sensory Innervation (Somatic Afferent Fibers): | ||
The anterior ramus of S2 contains motor (somatic efferent) fibers that travel through branches of the sacral plexus to provide innervation to various muscles in the gluteal, hamstring, leg, and foot regions. The key areas of innervation include: 1- Gluteal region: Motor fibers from S2 contribute to the innervation of the following muscles via these nerves: a) Nerve to piriformis (L5, S1, S2): Piriformis. b) Inferior gluteal nerve (L5 to S2): Gluteus maximus muscle. c) Nerve to obturator internus (L5 to S2): Obturator internus and superior gemillus muscles. 2- Hamstrings, knee, and leg regions: The S2 motor fibers travel via the sciatic nerve, branching into the tibial and common (superficial and deep) peroneal nerves to reach the following muscles: a) The hamstring muscles, via the tibial part of sciatic nerve: Hamstring part of adductor magnus, long head of biceps femoris, semimembranosus, and semitendinosus (all L5 to S2). b) The hamstring muscles, via the common peroneal part of sciatic nerve: Short head of biceps femoris c) Lateral leg muscles, via the superficial peroneal nerve: Peroneus longus and peroneus brevis (all L5 to S2) d) Posterior leg muscles, via the tibial nerve: Gastrocnemius, soleus, plantaris (all S1, S2), Flexor hallucis longus, flexor digitorum longus (both S2, S3). 3- Foot region: The efferent neurons from the S2 anterior ramus reach the intrinsic muscles of the foot primarily via the following nerves: a) Medial and lateral plantar nerves (branches of the tibial nerve): Abductor hallucis, abductor digiti minimi, flexor digitorum brevis, quadratus plantae; (all S1, S2, S3), Flexor hallucis brevis muscles (S1, S2), Lumbricals, adductor hallucis, flexor digiti minimi brevis, interossei of foot (plantar and dorsal) (all S2, S3) b) Deep peroneal nerve: Extensor digitorum brevis (S1, S2). | The somatic sensory fibers of the anterior ramus of spinal nerve S2 travel through the deep and superficial peroneal nerves, the sural nerve, the medial and lateral calcaneal nerves, the posterior femoral cutaneous nerve of the thigh (S1 to S3), and the perforating cutaneous nerve (S2, S3). These fibers transmit sensory information from the skin of the anterior lower leg, the heel region, the dorsum and the lateral side of the sole of foot, the posterior thigh area, and the skin over the gluteal fold. | ||
B. POSTERIOR RAMUS OF SPINAL NERVE S2:
The posterior ramus of the spinal nerve S2 emerges through the dorsal sacral foramen, lying deep to the multifidus, an intrinsic deep muscle of the pelvic back region. It then divides into two main branches:
a) Medial branch:
This branch innervates the multifidus muscle.
b) Lateral branch:
The lateral branch provides sensory innervation to the posterior gluteal skin. After its division from the posterior ramus of S2, it unites with branches from the posterior rami of other sacral nerves and receives contributions from the L5 spinal nerve. Together, these branches form neuronal loops (a.k.a. the posterior sacrococcygeal plexus) on the dorsal surface of the sacrum, beneath the gluteus maximus muscle. From these loops, gluteal branches emerge, piercing the gluteus maximus to supply sensory innervation to the overlying posterior gluteal skin.
Some studies also suggest that the posterior ramus provides innervation to the posterior sacroiliac joint.
C. ADDITIONAL STRUCTURES ASSOCIATED WITH SPINAL NERVE S2:
a) Recurrent Meningeal Nerve (Meningeal branch):
The recurrent meningeal nerve (or meningeal branch) originates from either the S2 spinal nerve or one of its rami. This small nerve re-enters the vertebral canal and provides sensory innervation to::
The meninges (protective coverings of the spinal cord),
The vertebral bodies and periosteum,
Adjacent blood vessels.
b) Sympathetic connections:
Through the gray rami communicantes, the sympathetic trunk connects to the spinal nerve S2, enabling postganglionic sympathetic fibers to reach peripheral targets such as sweat glands and blood vessels.
c) Parasympathetic fibers: the pelvic splanchnic nerves:
The pelvic splanchnic nerves, originating from the S2, S3, and S4 spinal roots, are composed of preganglionic parasympathetic fibers. These nerves travel to the inferior hypogastric plexus, where they synapse with postganglionic neuron cell bodies. From there, they provide parasympathetic innervation to the pelvic viscera and genitalia.
References
Snell, R.S. (2010). ‘Chapter 14: The spinal cord and the ascending and descending tracts’, in Clinical Neuroanatomy. (7th ed.) Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp. 137.
Drake, R.L., Vogl, A.W., and Mitchell, A.W.M. (2010). ‘Chapter 2: The Back’, in Gray’s anatomy for students. (2nd ed.) Churchill Livingstone Elsevier, Philadelphia PA 19103, pp.96-99 & Fig. 2.44.
Drake, R.L., Vogl, A.W., and Mitchell, A.W.M. (2010). ‘Chapter 5: Pelvis’, in Gray’s anatomy for students. (2nd ed.) Churchill Livingstone Elsevier, Philadelphia PA 19103, pp.462-468.
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Fortin JD. The anatomy of the lateral branches of the sacral dorsal rami: implications for radiofrequency ablation. Pain Physician. 2014 Sep-Oct;17(5):459-64.
