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The first sacral spinal nerve, S1, is a mixed nerve that contains both somatic efferent (motor) and somatic afferent (sensory) fibres. It originates from the spinal column below the first body of the sacrum. S1 supplies many muscles, either directly or through nerves originating from S1. It is involved in the innervation of muscles in the pelvis and lower limb, including the gluteus minimus, gluteus medius, and tensor fasciae latae. S1 also provides sensory innervation to the skin overlying the back and the posterior gluteal skin. The anterior ramus of S1 is the larger division of the nerve, while the posterior ramus provides innervation to the multifidus muscle, a deep muscle of the pelvic back region.
| Characteristics | Values |
|---|---|
| Type of nerve | Mixed nerve |
| Nerve roots | Posterior (dorsal) root, Anterior (ventral) root |
| Muscle innervation | Deep muscles of the back, Multifidus muscle, Gluteus minimus, Gluteus medius, Tensor fascia lata, Gluteus maximus, Muscles in the posterior compartment of the thigh, Hamstring component of adductor magnus |
| Sensory innervation | Skin overlying the back, Posterior gluteal skin, Skin on the posterior surface of the thigh and leg, Skin of the perineum, Meninges (protective coverings of the spinal cord) |
| Spinal motion segment | L5-S1 (Lumbosacral joint) |
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What You'll Learn
The S1 spinal nerve is a mixed nerve
The S1 spinal nerve, also known as the first sacral spinal nerve, is a mixed nerve that contains both somatic efferent (motor) and somatic afferent (sensory) fibres. It originates from the spinal cord at the S1 spinal segment. The S1 spinal nerve is formed by the union of two roots: the posterior (dorsal) root and the anterior (ventral) root. The posterior root contains sensory fibres that carry signals from peripheral structures, such as the anterior and posterior rami, to the spinal cord. These fibres enter the dorsal gray horn within the spinal cord. The anterior root, on the other hand, consists of motor fibres that originate from the anterior gray horn within the spinal cord. These motor fibres transmit signals to peripheral muscles through the spinal nerve and its branches.
The anterior ramus of the S1 spinal nerve is the larger division of the nerve and carries both motor and sensory fibres. After exiting through the anterior sacral foramen, it merges with the lumbosacral trunk and anterior rami of sacral nerves S2, S3, and part of S4 to form the sacral plexus. This plexus is located on the posterolateral wall of the pelvic cavity, deep to the internal iliac vessels. The posterior ramus of the S1 spinal nerve emerges through the dorsal sacral foramen and lies deep to the multifidus muscle of the pelvic back region. It then divides into two main branches, innervating the multifidus muscle and providing sensory innervation to the posterior gluteal skin.
The lateral branch of the posterior ramus provides sensory innervation to the posterior gluteal skin. After dividing from the posterior ramus of S1, it unites with branches from the posterior rami of other sacral nerves and receives input from the L5 spinal nerve. Together, these branches form neuronal loops, also known as the posterior sacrococcygeal plexus, located on the dorsal surface of the sacrum, beneath the gluteus maximus muscle. Gluteal branches emerge from these loops, piercing the gluteus maximus to supply sensory innervation to the overlying posterior gluteal skin.
The S1 spinal nerve is involved in providing sensory and motor innervation to various structures, including muscles and skin. It plays a role in transmitting signals between the central nervous system (CNS) and the periphery, contributing to the modulation of motor and sensory information. The S1 spinal nerve is not the only mixed nerve, as several other spinal nerves, such as those in the brachial plexus, also exhibit similar properties. These mixed nerves collectively transmit sensory, motor, and autonomic impulses between the spinal cord and the rest of the body.
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S1 innervates the gluteus minimus, gluteus medius and tensor fascia lata
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It originates from the spinal column below the 1st body of the sacrum. S1 supplies many muscles, either directly or through nerves originating from S1.
The superior gluteal nerve (L4, L5, S1) passes through the sciatic notch above the piriformis muscle. It supplies the gluteus medius and minimus and tensor fasciae latae muscles. The inferior gluteal nerve (L5, S1, S2) supplies the gluteus maximus. The superior gluteal nerve arises from the dorsal branches of the L4-5 and S1-2 ventral rami. It then divides into superior and inferior branches. The superior branch accompanies the upper branch of the deep division of the superior gluteal artery and innervates the gluteus medius and occasionally the gluteus minimus. The inferior branch runs with the lower ramus of the deep division of the superior gluteal artery across the gluteus minimus and innervates the gluteus medius and minimus. It then ends in the tensor fasciae latae.
The lateral branch of S1 provides sensory innervation to the posterior gluteal skin. After its division from the posterior ramus of S1, 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.
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S1 is formed by the union of two roots
The spinal nerve S1 is formed by the union of two roots, the posterior (dorsal) root, and the anterior (ventral) root. These roots serve distinct yet complementary functions in sensory and motor signalling, allowing communication between the brain and the body.
The posterior (dorsal) root contains sensory fibres that carry signals from peripheral structures (innervated by the anterior and posterior rami) to the spinal cord. These fibres enter the dorsal gray horn within the spinal cord. The dorsal root brings sensory information into the spinal cord, allowing signals to enter the spinal cord. Each dorsal root has a structure known as the dorsal root ganglion, which is an enlargement that contains the cell bodies of the sensory neurons.
The anterior (ventral) root consists of motor fibres originating from the anterior gray horn within the spinal cord. These fibres transmit motor commands to peripheral muscles via the spinal nerve and its branches. The ventral root carries motor commands out to the muscles, allowing signals to exit the spinal cord. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord.
The anterior and posterior roots merge within the sacral vertebral canal to form the spinal nerve S1. The anterior ramus of spinal nerve S1 is the larger division of the nerve and functions as a mixed nerve, carrying both motor (somatic efferent) and sensory (somatic afferent) fibres. After exiting through the anterior sacral foramen, it merges with the lumbosacral trunk and anterior rami of sacral nerves S2, 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.
The posterior ramus of the spinal nerve S1 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: this branch innervates the multifidus muscle. The lateral branch provides sensory innervation to the posterior gluteal skin.
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S1 innervates the multifidus muscle
The first sacral spinal nerve, S1, is a mixed nerve that contains both somatic efferent (motor) and somatic afferent (sensory) fibres. It provides motor innervation to the deep muscles of the back and sensory innervation to the skin overlying the back.
The multifidus muscle is located deep to the erector spinae, semispinalis cervicis, and thoracis, and superficial to the rotatores muscles. It is found on either side of the vertebral column, extending from the cervical to the lumbar spine. The multifidus muscle is innervated by the medial branches of the posterior rami of spinal nerves in the corresponding cervical, thoracic, and lumbar regions.
The multifidus muscle plays an important role in stabilising the joints within the spine. It is active in all anti-gravity activities and helps to maintain a pure axial rotation during trunk rotation. The muscle has a high cross-sectional area, giving it a high force-generating capacity, and its deep layer contributes more to spinal strength and stability than the superficial layers.
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S1 innervates the skin on the posterior surface of the thigh and leg
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It originates from the spinal column below the 1st body of the sacrum. S1 supplies many muscles, either directly or through nerves originating from S1. The anterior ramus of spinal nerve S1 is the larger division of the nerve and functions as a mixed nerve, carrying both motor and sensory fibres. The posterior ramus of the spinal nerve S1 emerges through the dorsal sacral foramen, lying deep to the multifidus, an intrinsic deep muscle of the pelvic back region.
The anterior rami of the S1-S4 spinal roots divide into several cords, which then combine to form the five major peripheral nerves of the sacral plexus. These nerves descend down the posterior pelvic wall and have two main destinations. The first is to leave the pelvis via the greater sciatic foramen and enter the gluteal region of the lower limb, innervating the structures there. The second is to remain in the pelvis and innervate the pelvic muscles, organs, and perineum.
The sacral plexus is a network of nerve fibres that supplies the skin and muscles of the pelvis and lower limb. It is located on the surface of the posterior pelvic wall, anterior to the piriformis muscle. The posterior cutaneous nerve of the thigh leaves the pelvis via the greater sciatic foramen, entering the gluteal region inferiorly to the piriformis muscle. It then descends deep to the gluteus maximus and runs down the back of the thigh to the knee.
The posterior cutaneous nerve of the thigh innervates the skin on the posterior surface of the thigh and leg. It also innervates the skin of the perineum.
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Frequently asked questions
The S1 spinal nerve is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres. It originates from the spinal column below the 1st body of the sacrum.
The S1 spinal nerve enervates the gluteus medius, gluteus minimus, tensor fascia lata, multifidus muscle, and the deep muscles of the back.
The S1 spinal nerve provides sensory innervation to the skin overlying the back and the posterior gluteal skin. It also forms the basis of the sacral plexus, which supplies the skin and muscles of the pelvis and lower limb.
The L5-S1 spinal motion segment, also called the lumbosacral joint, is the transition region between the lumbar spine and sacral spine in the lower back. This region is susceptible to traumatic injuries, degeneration, disc herniation, and nerve pain.
Issues with the S1 spinal nerve can result in radiculopathy symptoms or sciatica, characterised by sharp, shooting, or searing pain in the buttock, thigh, leg, foot, and/or toes. It can also lead to cauda equina syndrome, which is caused by an injury to the cauda equina nerves descending from the spinal cord.
