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The S1, or the sacral spinal nerve 1, is a spinal nerve of the sacral segment. It is formed by the union of two roots: the anterior (ventral) root and the posterior (dorsal) root. The anterior root transmits motor commands to peripheral muscles, while the posterior root carries signals from peripheral structures to the spinal cord. The S1 nerve supplies many muscles, either directly or through nerves originating from S1. It also helps transfer loads from the spine into the pelvis and legs. The L5-S1 spinal motion segment, where the S1 nerve is located, is susceptible to degeneration, herniation, and nerve pain due to its distinctive anatomy and higher degree of mechanical stress and loads.
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What You'll Learn
S1 is a spinal nerve of the sacral segment
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It originates from the spinal column, specifically from below the 1st body of the sacrum. S1 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres. It supplies and innervates many muscles, either directly or through nerves originating from S1. These muscles include the gluteus maximus, gluteus medius, gluteus minimus, and tensor fascia lata.
The spinal nerve S1 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 to the spinal cord, specifically to the dorsal gray horn within the spinal cord. The anterior root consists of motor fibres that originate from the anterior gray horn within the spinal cord and transmit motor commands to peripheral muscles via the spinal nerve and its branches.
The anterior and posterior roots merge within the sacral vertebral canal to form the spinal nerve S1. This nerve then divides into two primary rami: the anterior ramus and the posterior ramus. The anterior ramus is the larger division and functions as a mixed nerve, carrying both motor and sensory fibres. It contributes to the sacral plexus, which provides motor and sensory innervation to structures, including the lower limbs and gluteal regions.
The posterior ramus of the spinal nerve S1 emerges through the dorsal sacral foramen, lying deep within the multifidus muscle of the pelvic back region. It then divides into two main branches, providing innervation to the multifidus muscle and the posterior gluteal skin. The posterior ramus is also believed to provide innervation to the posterior sacroiliac joint. Additionally, the recurrent meningeal nerve, which provides sensory innervation to the meninges (protective coverings of the spinal cord), originates from either the S1 spinal nerve or one of its rami.
Injuries to the sacral nerve roots, including S1, can result in some issues with controlling bowel and bladder function. However, patients with sacral nerve injuries are expected to retain functional autonomy and the ability to perform self-care. Treatment options for sacral nerve injuries include non-steroidal anti-inflammatory drugs, surgical decompression of the nerves, and physical therapy.
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S1 supplies many muscles
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres. The S1 nerve root supplies many muscles, either directly or through nerves originating from S1.
The S1 nerve root innervates the medial head of gastrocnemius, soleus, and abductor hallucis muscles. It also provides motor innervation to the deep muscles of the back, such as the multifidus muscle. The anterior ramus of the S1 nerve functions as a mixed nerve, carrying both motor and sensory fibres. It provides motor innervation to the muscles of the lower limbs and gluteal regions. This includes muscles such as the gluteus minimus, gluteus medius, and tensor fascia lata.
The S1 nerve also contributes to the sacral plexus, which is a network of nerve fibres supplying some of the lower limb skin and musculature. The sacral plexus is located on the posterolateral wall of the pelvic cavity, anterior to the piriformis muscle. The superior gluteal nerve, one of the major branches of the sacral plexus, innervates the gluteus minimus, gluteus medius, and tensor fascia lata muscles.
The posterior ramus of the S1 nerve emerges through the dorsal sacral foramen and divides into two main branches. This branch innervates the multifidus muscle, an intrinsic deep muscle of the pelvic back region. The lateral branch of the posterior ramus provides sensory innervation to the posterior gluteal skin. It unites with branches from the posterior rami of other sacral nerves, forming neuronal loops beneath the gluteus maximus muscle.
The S1 nerve root is involved in the innervation of muscles, along with the L5 nerve root. The L5-S1 spinal motion segment helps transfer loads from the spine into the pelvis and legs. This region is susceptible to degeneration, herniation, and nerve pain due to the higher degree of mechanical stress and loads it receives.
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S1 is formed by the union of two roots
The first sacral spinal nerve S1 is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres. It originates from the spinal cord at the S1 spinal segment.
Like other nerves, the spinal nerve S1 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 (innervated by the anterior and posterior rami) to the spinal cord. These fibres enter the dorsal gray horn within the spinal cord. The anterior 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 anterior and posterior roots merge within the sacral vertebral canal to form the spinal nerve S1. This mixed nerve, containing both sensory and motor fibres, divides into two primary rami: the anterior ramus and the posterior ramus. 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. 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.
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: one that innervates the multifidus muscle, and the other that provides sensory innervation to the skin overlying the back.
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S1 innervates the deep muscles of the back
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres. The S1 nerve originates from the spinal column from below the 1st body of the sacrum.
The S1 nerve supplies many muscles, either directly or through nerves originating from S1. These muscles are not innervated with S1 as a single origin but are partly innervated by S1 and partly by other spinal nerves.
The anterior ramus of the S1 nerve is the larger division of the nerve and functions as a mixed nerve, carrying 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, lying on the anterior surface of the piriformis muscle and deep to the internal iliac vessels.
The posterior ramus of the S1 nerve 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, one of which innervates the multifidus muscle. The multifidus muscle's main function is to stabilise the vertebrae during movements of the spine. The deep back muscles act together to provide support and maintain the body's posture, as well as to produce movements of the head, neck, and trunk.
The splenius muscles are also innervated by the posterior rami of the middle and lower cervical spinal nerves. These muscles are located on the posterolateral aspect of the neck and the posterior upper thorax, overlying the deep muscles of the neck.
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S1 innervates the medial head of gastrocnemius, soleus, and abductor hallucis
The sacral spinal nerve 1 (S1) is a spinal nerve of the sacral segment. It is not a muscle, but a nerve that supplies and innervates muscles, either directly or through nerves originating from S1. These muscles include the medial head of gastrocnemius, soleus, and abductor hallucis.
The gastrocnemius muscle is a powerful plantar flexor of the foot at the talocrural joint. It also flexes the leg at the knee. The muscle is innervated by the anterior rami of S1 and S2 spinal nerves, carried by the tibial nerve into the posterior compartment of the leg. The medial head of the gastrocnemius is one of the two heads of the gastrocnemius, with the other being the lateral head. The medial head originates from the popliteal surface of the femoral shaft, and the posterior surface of the medial condyle. The lateral head, meanwhile, originates from a facet on the upper posterolateral surface of the lateral condyle of the femur, where it joins the lateral supracondylar line. Both heads also take origin from the capsule of the knee joint.
The tendon of biceps femoris partially covers the lateral head, and semimembranosus partially covers the medial head. The small saphenous vein and accompanying sural nerve run along the superficial surface of the muscle, separated from it by the deep fascia. Deep to the gastrocnemius are the soleus, popliteus, and plantaris muscles, the popliteal vessels, and the tibial nerve. The deep flexor muscles, flexor digitorum longus, flexor hallucis longus, and tibialis posterior, also lie deep to the gastrocnemius.
The soleus muscle, together with the gastrocnemius, is a component of a composite, three-headed group of muscles referred to as triceps surae. They act in many basic activities, such as walking, running, and leaping. At the inferior margin of the popliteal fossa, the two heads of the gastrocnemius come together and join to form a single, elongated, muscle belly. This forms most of the bulge of soft tissue on the posterior leg, referred to as the calf. The fleshy part of the muscle extends to approximately the midpoint of the calf. In the lower leg, the muscle fibres of the gastrocnemius gradually form a broad aponeurosis. The aponeurosis gradually narrows, and fuses with the fibres of the soleus to form a large tendon called the calcaneal (Achilles) tendon.
The abductor hallucis is one of the muscles supplied by the S1 nerve root. This muscle is involved in the movement of the big toe.
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Frequently asked questions
S1 is the first sacral spinal nerve. It is a mixed nerve, containing both somatic efferent (motor) and somatic afferent (sensory) fibres.
S1 provides motor innervation to the deep muscles of the back and sensory innervation to the skin overlying the back. It also provides innervation to the lower limbs and gluteal regions.
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. It helps transfer loads from the spine into the pelvis and legs.
