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Anatomy & Physiology: Spinal Nerves

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31 pairs of spinal nerves:
- Cervical 8 pairs
Thoracic 12 pairs
Lumbar 5 pairs
Sacral 5 pairs
Coccygeal 1 pair
The thoracic region:
ventral rami of the spinal nerves split off another ramus carrying autonomic nerve fibers. These are called the rami communicantes, and their fibers enter the sympathetic trunk located on both sides of the vertebral column. The sympathetic trunk is comprised of ganglia (clusters of sympathetic neuron cell bodies) connected to each other forming a chain.
Motor fibers:
- Somatic motor fibers leaving neurons of the ventral horn enter the rootlets which combine to form ventral roots.

After fusing to form the spinal nerve, the motor fibers enter the dorsal or ventral rami. The dorsal ramus of each spinal nerve carries fibers to the posterior trunk where they innervate a narrow band of skeletal muscle and skin (somite) in line with its emergence from the spinal cord.

- The ventral ramus is larger, and with the exception of segments T2-T12, all ventral rami branch and fuse with one another lateral to the vertebral column forming complicated nerve plexuses. The preganglionic visceral motor fibers enter the white (preganglionic ramus) of the rami communicantes. From here, they can synapse with a postganglionic neuron at the same level to enter nerves innervating smooth muscle, glands and viscera of the thoracic cavity, or travel in the sympathetic chain to synapse in a ganglion at a different level and enter nerves innervating the abdominopelvic viscera. In any case, the postganglionic fibers seen in black can travel up or down the chain in the gray ramui to reach the nerves of either the dorsal ramus (innervating smooth muscle and glands of the back) or ventral ramus (innervating smooth muscle and glands of the limbs, body wall).
Spinal nerves: sensory
- Sensation is divided into somatic and visceral sensations. The somatic sensations emanate from exteroceptors and proprioceptors of the body wall, back or limbs depending on the ramus.
- Visceral sensation comes from interoceptors from the same regions and from the viscera. These are shown in lavender. Notice that both somatic and visceral neurons have their cell bodies located in the dorsal root ganglion and they enter the dorsal horn of the spinal cord. The visceral fibers from abdominopelvic organs travel in the rami communicantes to enter the spinal cord at the appropriate level.
neurovascular bundle :
This is the name for a group of structures that includes an artery, a vein, and a nerve. Typically these structures run together as a group.
no nerve plexus:
Spinal nerves T1-T12 have a simple, segmental pattern of distribution. These nerves pass directly out of spinal cord to the periphery and supply structures in a narrow band corresponding to the body somite. These include structures located in the intercostal spaces, axilla, chest, muscles of thorax, deep back and some abdominal muscles. Everywhere else, the ventral rami form nerve plexuses.
nerve plexuses:
Nerves emanating from regions of the spinal cord that control the muscles of the neck, upper limbs, and lower limbs
Cervical plexus:
- formed by the ventral rami of the first four cervical nerves. Most of the branches are superficial cutaneous and supply only the skin around the neck, ear and back of the head and shoulder. These include the Lesser occipital, transverse cervical, supraclavicular and great auricular nerves supplying the skin of upper chest, shoulder and neck.
-The other branches innervate muscles of the anterior neck. The deep motor branches include the Ansa cervicalis (originating from C1-C3) which innervates the infrahyoid muscles (sternothyroid, sternohyoid, omohyoid).
- the segmental cervical nerves (C1-C5) and other branches innervating the deep neck muscles like the geniohyoid and thyrohyoid, as well as the levator scapulae, scalene, sternocleidomastoid, and trapezius muscles.
*******But the most important nerve of all is the phrenic nerve which originates from C3-C5. The phrenic nerve is the only nerve innervating the diaphragm.
phrenic nerve:
: irritation of the phrenic nerve causes spasms of the diaphragm or hiccups.
If both phrenic nerves are severed, or if there is severe injury to the spinal cord at level (C3-C5), the diaphragm is paralyzed. This is can be fatal unless the victim is placed on an artificial respirator.
brachial plexus:
- consists of ventral rami C5-T1; innervates the pectoral girdle and upper limbs; nerves in this plexus originate from cords or trunks
- can be palpated just superior to the clavicle at the lateral border of the sternocleidomastoid muscle
brachial subdivisions:
The brachial plexus has four main subdivisions. Moving from medial (closest to the spinal cord) to lateral are the roots which are the ventral rami of C5-T1 spinal nerves. At the lateral border of the sternocleidomastoid muscle, the roots unite to form 3 trunks (upper, middle and lower). The trunks are large bundles of axons from several levels. The trunks divide into anterior and posterior divisions. In general, the anterior divisions serve the front of the limb and the fibers from the posterior divisions supply the back part of the limb. The divisions pass deep to the clavicle and enter the axilla where they give rise to three large fiber cords (lateral, medial and posterior). Cords are named based on their relative position to the axillary artery.
5 main brachial plexus nerves:
Axillary nerve
Radial nerve
Musculocutaneous nerve
Median nerve
Ulnar nerve

-Axillary C5-C6: Deltoid and Teres minor
-Musculocutaneous (C5-C7): arm and forearm flexors.
-Radial nerve (C5-C8, T1): arm and forearm extensors and sensation to dorsum of hand, thumb (except dorsum of fingers 2 & 3).
-Ulnar nerve (C8-T1): some of the wrist flexors and finger flexors. Sensation to medial two fingers
-Median nerve (C5-C8, T1): forearm flexors except flexor carpi ulnaris. Sensation on palm side, lateral 2/3.


*. Damage to the brachial plexus can have devastating effects on upper limb function (paresis, paralysis, and loss of sensation) . Sports injuries to the plexus can occur when the upper limb is pulled hard and the plexus is stretched as when being yanked by the arm. Swinging children by their arms can result in permanent damage to the plexus with life-long effects. Falling hard on the top of the shoulder such that the humerus is forced inferiorly can also damage the plexus.
brachial plexus disorder:
Erb's palsy: 60% of brachial plexus cases.
Bending or stretching of the neck in a direction away from side of injury.
Lower roots can be damaged by pulling up of the arm above the head which stretches C8-T1.
axillary nerve:
The axillary nerve branches off the posterior cord and runs posterior to the surgical neck of the humerus. It innervates the [deltoid and teres minor muscles].
musculocutaneous :
- The musculocutaneous nerve is the end branch of the lateral cord. It courses inferiorly supplying the [biceps brachii and brachialis muscles]. Distal to the elbow, it provides for[ cutaneous sensation of the lateral forearm].
Median nerve:
-The median nerve runs through the anterior forearm where it supplys the [skin and most of the flexor muscles]. On reaching the hand, it innervate [five intrinisic muscles of the lateral palm]. The median nerve [activates muscles that pronate the forearm, flex the wrist and fingers and oppose the thumb] . The median nerve supplies sensation to the lateral palm, thumb, digits II-III and half of IV on the anterior surface, and the dorsum of digits II-IV.

* Inability to pronate arm. Difficulty flexing wrist
Carpal tunnel syndrome caused by repetitive flexion pressing nerve against the flexor retinaculum.
Sensory deficit & palsy I-lateral half of IV
Ulnar nerve:
- The ulnar nerve arises from the medial cord and descends toward the elbow where it goes behind the medial epicondyle (forming the "funny bone"), and then follows the ulna. The ulnar nerve supplies the flexor carpi ulnaris and the medial part of the flexor digitorum profundus (in other words, the flexors not supplied by the median nerve). It produces wrist and finger flexion, adduction and abduction of the medial fingers. It sensory distribution includes the remainder of the skin on the anterior surface of the hand, the medial dorsum and digits V and half of IV.
Problems adducting or abducting fingers.
Sensory deficit to half of III & all of IV.
Palsy of same fingers due to paralysis of flexors
Radial nerve:
The radial nerve is the largest branch of the brachial plexus and is a continuation of the posterior cord. The radial nerve wraps around the humerus in the radial groove, then runs around the lateral epicondyle of the elbow. It divides into a superficial and deep branches. The motor fibers innervate all the extensors of the upper limb, forearm supination, wrist and finger extension and thumb abduction. It is also sensory to the skin overlying the back of the metacarpals and thumb.
Trauma to the radial nerve results in weakness to the forearm extensors because their innervation has been compromised. This leads to the inability to extend the wrist, a condition known as wrist drop. Another common disorder involving the radial nerve is referred to as "Saturday night paralysis". Recall that the radial nerve wraps around the humerus in the radial groove. When the arm is slung over the back of a hard bench or chair, the nerve is compressed against the humerus.
winging of scapula:
Compression of C5-C7
Compression of long thoracic nerve to the serratus anterior.
Caused by contraction of middle scalene (military press)
Lumbar plexus:
The lumbar plexus arises from roots L1-L4 and actually lies within the psoas major muscle. The proximal branches innervate the anterolateral abdominal wall muscles, genitals, and psoas major.
- The ilioinguinal nerve L1 provides sensation to skin of the genitals and proximal medial aspect of the thigh (inner thigh).
- Genitofemoral nerve L1-L2 is sensory to the scrotum in males and labia majora in females (these structures are homologous, meaning that they have the same embryological origin as we will learn later on), and the motor division innervates the cremaster muscle in males. Stimulation of this nerve causes the cremaster to contract which elevates the testes (cremasteric reflex). This is a means of regulating temperature for sperm production.
- The obturator is motor to the adductor magnus, gracilis and obturator externus muscles. The Femoral nerve (L2-L4) is the biggest of the plexus. It is motor to the quadriceps and sartorius muscles of the thigh and to the pectineus and iliacus. Damage to this nerve results in an inability to extend the leg at the knee (quadriceps & sartorius).
Sacral plexus :
The sacral plexus arises from spinal roots L4-S4. Notice that the sacral and lumbar plexuses overlap. Because of this relationship, many fibers of the lumbar plexus contribute to the sacral plexus. Consequently, these two plexuses are frequently referred to as the lumbosaral plexus. The sacral plexus supplies the gluteal region, floor of the pelvis and the lower extremities. The largest nerve of the sacral plexus, and for that matter, the largest nerve in the body, is the sciatic nerve. It is formed from spinal roots L4-S3, and supplies the entire lower limb except the anteromedial thigh (femoral nerve). The sciatic nerve is actually two nerves, the tibial and common fibular (AKA common peroneal) which are wrapped together into a common sheath by connective tissue.

Sciatic: semimembranosus & semitendinosus
A. Tibial: long head biceps femoris, knee flexors, plantar flexors.
B. Fibular (peroneal): short head biceps femoris,fibularis muscles, tibialis anterior, toe extensors.
2. Pudendal (S2-S4): muscles of perineum, urogenital diaphragm, anal & urethral sphincters.