75 Distribution of Spinal Nerves
Cervical Plexus
The cervical plexus is the plexus of the ventral rami of the first four cervical spinal nerves.
Learning Objectives
Describe the cervical plexus and its function
Key Takeaways
Key Points
- The cervical plexus describes the plexus of the ventral rami of the first four cervical spinal nerves that arise from the cervical spinal column in the neck.
- The cervical spinal nerves that form the cervical plexus are located laterally (farther from the median line) to the transverse processes of the prevertebral skeletal muscles of the neck from the medial side, and vertebral (closer to the vertebral column) to these muscles from the lateral side.
- The cervical plexus forms an anastomosis, a connection, with the accessory nerve, the hypoglossal nerve, and the sympathetic trunk.
- The cervical plexus is located in the neck, internal to the sternocleidomastoid, an anterior neck muscle.
Key Terms
- sympathetic trunk: Also called the sympathetic chain or gangliated cord, these are a paired bundle of nerve fibers that run from the base of the skull to the coccyx.
- plexus: A network or interwoven mass, especially of nerves, blood vessels, or lymphatic vessels.
- platysma: A superficial muscle that overlaps the sternocleidomastoid.
- cervical plexus: A plexus of the ventral rami of the first four cervical spinal nerves that are located from the C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes of the prevertebral muscles from the medial side and vertebral (scalenus, levator scapulae, splenius cervicis muscles) from the lateral side.
Structure and Distribution
The cervical plexus is a plexus of the ventral rami of the first four cervical spinal nerves located from the C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medial side and anteriolateral to the m. scalenus and m. levator scapulae.
There is anastomosis with the accessory nerve, hypoglossal nerve, and sympathetic trunk. It is located deep in the neck, near the sternocleidomastoid muscle.
Nerves formed from the cervical plexus innervate the back of the head, as well as some neck muscles. The branches of the cervical plexus emerge from the posterior triangle at the nerve point, a point that lies midway on the posterior border of the sternocleidomastoid.
Branches and Their Functions
The cervical plexus has two types of branches: cutaneous and muscular.
Cutaneous branches include:
- The lesser occipital nerve, or small occipital nerve, is a cutaneous spinal nerve that arises between the second and third cervical vertebrae, along with the greater occipital nerve. It innervates the scalp in the lateral area of the head posterior to the ear.
- The great auricular nerve originates from the cervical plexus and is composed of branches from spinal nerves C2 and C3. It provides sensory innervation for the skin over the parotid gland and mastoid process, and both surfaces of the outer ear.
- The transverse cervical nerve (superficial cervical or cutaneous cervical) arises from the second and third cervical nerves, turns around the posterior border of the sternocleidomastoideus about its middle, then passes obliquely forward beneath the external jugular vein to the anterior border of the muscle, where it perforates the deep cervical fascia and divides beneath the platysma into ascending and descending branches that are distributed to the antero-lateral parts of the neck.
- The supraclavicular nerves (descending branches) arise from the third and fourth cervical nerves. They emerge beneath the posterior border of the sternocleidomastoideus, and descend in the posterior triangle of the neck beneath the platysma and deep cervical fascia.
Muscular branches include:
- Ansa cervicalis (loop formed from C1–C3), geniohyoid (C1 only), thyrohyoid (C1 only), sternothyroid, sternohyoid, omohyoid: The ansa cervicalis is a loop of nerves that are part of the cervical plexus.
- The phrenic nerve (C3–C5, but primarily C4) is a nerve that originates in the neck and passes down between the lung and heart to reach the diaphragm.
- Segmental branches (C1–C4) innervate the anterior and middle scalenes.
There are two additional branches that are formed by the posterior roots of the spinal nerves:
- Preauricular nerve (from the posterior roots of C2–C3).
- Postauricular nerve (from the posterior roots of C3–C4).
Cervical plexus: Diagram of the cervical plexus.
Brachial Plexus
The brachial plexus is formed by the four lower cervical spinal nerves and the first thoracic spinal nerve.
Learning Objectives
Describe the brachial plexus
Key Takeaways
Key Points
- The nerve bundles of the brachial plexus pass through the cervico-axillary canal to serve the axilla (armpit), brachium (arm), antebrachium (forearm), and hand.
- The brachial plexus provides cutaneous (skin) and muscular innervations for the upper limbs, excluding the trapezius muscle and an area of skin near the axilla that are innervated by the spinal accessory nerve and the intercostobrachial nerve, respectively.
- The brachial plexus is divided into roots, trunks, divisions, cords, and branches.
Key Terms
- spinal accessory nerve: A nerve that controls specific muscles of the shoulder and neck.
- axilla: The armpit, or the cavity beneath the junction of the arm and shoulder.
- brachium: The upper arm.
Formation and Distribution
Right brachial plexus: The right brachial plexus with its short branches, viewed from the front.
The brachial plexus is a network of nerve fibers that run from the spine that are formed by the ventral rami of the lower four cervical and first thoracic nerve roots (C5–C8, T1).
The brachial plexus proceeds through the neck, the axilla (armpit region), and into the arm. It is a collection of nerves passing through the cervico-axillary canal to reach the axilla and innervate the brachium, the antebrachium, and the hand.
The brachial plexus is responsible for cutaneous and muscular innervation of the entire upper limb, with two exceptions: the trapezius muscle is innervated by the spinal accessory nerve (CN XI) and an area of skin near the axilla is innervated by the intercostobrachial nerve. Lesions on the brachial plexus can lead to severe functional impairment.
Branches and Divisions
The brachial plexus is divided into roots, trunks, divisions, cords, and branches. There are five terminal branches and numerous other preterminal or collateral branches that leave the plexus at various points along its length. Its structure includes:
- Five roots: The five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck.
- These roots merge to form three trunks: The superior or upper (C5–C6), the middle (C7), and the inferior or lower (C8, T1).
- Each trunk then splits in two, to form six divisions: The anterior divisions of the upper, middle, and lower trunks and the posterior divisions of the upper, middle, and lower trunks.
- These six divisions will regroup to become the three cords. The cords are named by their position with respect to the axillary artery:
- The posterior cord is formed from the three posterior divisions of the trunks (C5–C8,T1).
- The lateral cord is the anterior divisions from the upper and middle trunks (C5–C7).
- The medial cord is simply a continuation of the anterior division of the lower trunk (C8, T1).
- Most branches branch from the cords, but a few branch directly from earlier structures. The five on the left are considered terminal branches.
Brachial plexus: Diagram of the brachial plexus and its branches.
Lumbar Plexus
The lumbar plexus is formed by the subcostal nerve and divisions of the first four lumbar nerves that arise from the middle to lower back.
Learning Objectives
Describe the lumbar plexus
Key Takeaways
Key Points
- The lumbar plexus passes through the psoas major muscle and innervates the skin and muscles of the abdominal wall, thigh, and external genitalia.
- The largest nerve that forms part of the lumbar plexus is the femoral nerve, which innervates the anterior thigh muscles and some of the skin distal to the inguinal ligament.
- The ilioinguinal nerve pierces the lateral abdominal wall and runs medially at the level of the inguinal ligament. It supplies motor branches to both transversus abdominis and sensory branches (to the skin over the pubic symphysis and the lateral aspect of the labia majora or scrotum ).
- The genital branch of the genitofemoral nerve courses differently in females and males.
Key Terms
- lumbosacral plexus: The anterior divisions of the lumbar nerves, sacral nerves, and coccygeal nerve form this plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic nerve.
- inguinal ligament: A band running from the pubic tubercle to the anterior superior iliac spine. Its anatomy is very important for operating on hernia patients.
- subcostal nerve: The twelfth thoracic nerve, its anterior division is larger than the others; it runs along the lower border of the twelfth rib and often gives a communicating branch to the first lumbar nerve, and then passes under the lateral lumbocostal arch.
- aponeurosis: Layers of flat, broad tendons that have a shiny, whitish-silvery color.
Structure and Distribution
The lumbar plexus is a nerve plexus in the lumbar region of the body that forms part of the lumbosacral plexus. It is formed by the ventral divisions of the first four lumbar nerves (L1–L4) and from contributions of the subcostal nerve (T12), which is the last thoracic nerve.
This plexus lies within the psoas major muscle. Nerves of the lumbar plexus serve the skin and the muscles of the lower abdominal wall, the thigh, and external genitals. The largest nerve of the plexus is the femoral nerve and it supplies the anterior muscles of the thigh and a part of skin distal to the inguinal ligament.
Lumbar plexus: An image of the lumbar plexus with its nerves highlighted in yellow.
Branches of the Lumbar Plexus
Iliohypogastric Nerve
Lumbar plexus: Schematic of the lumbar plexus.
Runs anterior to the psoas major on its proximal lateral border to run laterally and obliquely on the anterior side of the quadratus lumborum. Lateral to this muscle, it pierces the transversus abdominis to run above the iliac crest between that muscle and the abdominal internal oblique. It gives off several motor branches to these muscles and a sensory branch to the skin of the lateral hip. Its terminal branch then runs parallel to the inguinal ligament to exit the aponeurosis of the abdominal external oblique above the external inguinal ring, where it supplies the skin above the inguinal ligament (i.e., the hypogastric region) with the anterior cutaneous branch.
Ilioinguinal Nerve
This nerve closely follows the iliohypogastric nerve on the quadratus lumborum but then passes below it to run at the level of the iliac crest. It pierces the lateral abdominal wall and runs medially at the level of the inguinal ligament where it supplies motor branches to both the transversus abdominis and sensory branches through the external inguinal ring to the skin over the pubic symphysis and the lateral aspect of the labia majora in females, and in males, the scrotum.
Genitofemoral Nerve
Pierces the psoas major anteriorly, below the former two nerves to immediately split into two branches that run downward on the anterior side of the muscle. The lateral femoral branch is purely sensory and pierces the vascular lacuna near the saphenous hiatus and supplies the skin below the inguinal ligament. In males, the genital branch runs in the spermatic cord then sends sensory branches to the scrotal skin and supplies motor innervations to the cremaster muscle. In females, it runs in the inguinal canal together with the teres uteri ligament. It then sends sensory branches to the labia majora in females.
Lateral Cutaneous Femoral Nerve
Pierces the psoas major on its lateral side and runs obliquely downward below the iliac fascia. Medial to the anterior superior iliac spine, it leaves the pelvic area through the lateral muscular lacuna and enters the thigh by passing behind the lateral end of the inguinal ligament. In the thigh, it briefly passes under the fascia lata before it breaches the fascia and supplies the skin of the anterior thigh.
Obturator Nerve
Leaves the lumbar plexus and descends behind the psoas major on its medial side, follows the linea terminalis into the lesser pelvis, then finally leaves the pelvic area through the obturator canal. In the thigh, it sends motor branches to obturator externus before dividing into an anterior and a posterior branch, both of which continue distally. These branches are separated by the adductor brevis and supply all thigh adductors with motor innervations. The anterior branch contributes a terminal, sensory branch that passes along the anterior border of gracilis and supplies the skin on the medial, distal part of the thigh.
Femoral Nerve
This is the largest and longest of the plexus’ nerves. It gives motor innervation to iliopsoas, pectineus, sartorius, and quadriceps femoris, and sensory innervation to the anterior thigh, posterior lower leg, and hindfoot. In the pelvic area, it runs in a groove between the psoas major and iliacus muscles and gives branches to both. It exits the pelvis through the medial aspect of the muscular lacuna. In the thigh, it divides into numerous sensory and muscular branches and the saphenous nerve, its long sensory terminal branch that continues down to the foot.
Sacral and Coccygeal Plexuses
The sacral plexus is the plexus of the three sacral spinal nerves (S2–S4) that arise from the lower back just above the sacrum.
Learning Objectives
Distinguish between the sacral and coccygeal plexuses
Key Takeaways
Key Points
- The sacral plexus, together with the lumbar plexus, forms the lumbosacral plexus.
- The largest nerve in the sacral plexus is the sciatic nerve that innervates the thigh, lower leg, and the foot.
- The coccygeal plexus consists of the coccygeal nerve and the fifth sacral nerve, which innervate the skin in the coccygeal region, around the tailbone (called the coccyx).
Key Terms
- sacral plexus: A nerve plexus that provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis.
- coccygeal nerve: The spinal nerve that corresponds to the coccyx bone.
- sciatic nerve: A large nerve that starts in the lower back and runs through the buttock and down the lower limb.
The sacral plexus is a nerve plexus that provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis. It is part of the lumbosacral plexus and emerges from the sacral vertebrae (S2–S4).
The sacral plexus: The nerves of the sacral plexus are shown.
The largest and longest nerve of the human body, the sciatic nerve, is the main branch and gives rami to the motor innervation of the muscles of the foot, the leg, and the thigh.
The sacral plexus is formed by:
- The lumbosacral trunk.
- The anterior division of the first sacral nerve.
- Portions of the anterior divisions of the second and third sacral nerves.
The nerves forming the sacral plexus converge toward the lower part of the greater sciatic foramen and unite to form a flattened band from the anterior and posterior surfaces, from which several branches arise.
The band itself is continued as the sciatic nerve, which splits on the back of the thigh into the tibial nerve and the common fibular nerve. These two nerves sometimes arise separately from the plexus and, in all cases, their independence can be shown by dissection.
Often, the sacral plexus and the lumbar plexus are considered to be one large nerve plexus, the lumbosacral plexus. The lumbosacral trunk connects the two plexuses.
The coccygeal plexus originates from the S4, S5, and Co1 spinal nerves. It is interconnected with the lower part of the sacral plexus. The only nerve in this plexus is the anococcygeal nerve, which serves sensory innervation of the skin in the coccygeal region.
Sacral plexus diagram: Diagram of the sacral plexus showing the various anterior and posterior nerve branches that arise from each lumbar and sacral nerve.
Sensory and Motor Tracts
The spinothalamic tract is a somatosensory tract and the corticospinal tract is a motor tract.
Learning Objectives
Distinguish between sensory and motor tracts
Key Takeaways
Key Points
- The spinothalamic tract is split into the lateral spinothalamic tract that transmits pain and temperature sensation to the thalamus, and the anterior spinothalamic tract that transmits pressure and crude touch sensation to the thalamus.
- The corticospinal tract is a motor tract comprised of mostly motor axons that carry motor information from the cerebral cortex and brainstem to the musculature for voluntary movement.
- The corticospinal tract is split into the lateral and anterior corticospinal tracts, which decussate, or cross, in the medulla oblongata. Therefore, the right brain controls the left side of the body, and the left brain controls the right side of the body.
- Betz cells, the largest pyramidal cells, are only found in the corticospinal tract.
Key Terms
- spinothalamic tract: A sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch, and crude touch.
- somatosensory tract: The system that reacts to diverse stimuli using thermoreceptors, nociceptors, mechanoreceptors, and chemoreceptors. The transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain.
- Betz cell: Giant pyramidal cells (neurons) located within the fifth layer of the grey matter in the primary motor cortex. They have a rapid conduction rate of over 70m/sec, which is the fastest conduction of any signals from the brain to the spinal cord.
- corticospinal tract: The nervous system tract that conducts impulses from the brain to the spinal cord. It contains mostly motor axons and is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract.
The Somatosensory Tract
The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch, and crude touch. The pathway decussates at the level of the spinal cord.
Somatosensory organization is divided into the dorsal column–medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the pain/temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex.
These neurons are designated primary, secondary, and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.
Function
The types of sensory information transmitted via the spinothalamic tract are described as affective sensation. This means that the sensation is accompanied by a compulsion to act. For instance, an itch is accompanied by a need to scratch, and a painful stimulus makes us want to withdraw from the pain.
There are two subsystems:
- Direct (for direct, conscious appreciation of pain).
- Indirect (for affective and arousal impact of pain).
Indirect projections are further divided into:
- Spino-reticulo-thalamo-cortical (part of the ascending reticular arousal system, also known as ARAS).
- Spino-mesencephalic-limbic (for affective impact of pain).
The Corticospinal Tract
The corticospinal tract conducts impulses from the brain to the spinal cord. It contains mostly motor axons. The corticospinal tract is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract.
The corticospinal tract also contains the Betz cell (the largest pyramidal cells) that are not found in any other region of the body. An understanding of these tracts leads to an understanding of why one side of the body is controlled by the opposite side of the brain.
The corticospinal tract is concerned specifically with discrete, voluntary, skilled movements, such as the precise movement of fingers and toes. The brain sends impulses to the spinal cord that relay the message.
This is imperative in understanding that the left hemisphere of the brain controls the RIGHT side of the body, while the right hemisphere of the brain controls the LEFT side of the body. The signals cross in the medulla oblongata, and this process is also known as decussation.
Spinal cord tracts: Spinal cord tracts are identified.
Function
The primary purpose of the corticospinal tract is to maintain voluntary motor control of the body and limbs. However, connections to the somatosensory cortex suggest that the pyramidal tracts are also responsible for modulating sensory information from the body.
Some of these connections cross the midline; therefore, each side of the brain is responsible for controlling muscles for the limbs on opposite sides of the body. However, control of trunk muscles is on the same side of the body.
After a patient’s pyramidal tracts are injured, the patient is paralyzed on the corresponding side of the body. Fortunately, they can re-learn some crude, basic motions, but not fine movements. This implies that the connections to these tracts are crucial for fine movement, and only partial recovery is possible if they are damaged.
