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Spinal Nerves

AP 150Chapter 12

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Spinal Nerves

• Spinal nerves attach to the spinal cord via roots

• Dorsal root– Has only sensory neurons

– Attached to cord via rootlets

– Dorsal root ganglion• Bulge formed by cell bodies of unipolar sensory neurons

• Ventral root– Has only motor neurons– No ganglion - all cell bodies of motor neurons found in gray matter of spinal cord

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Spinal Nerves• 31 pair

– each contains thousands of nerve fibers

– All are mixed nerves have both sensory and motor neurons)

• Connect to the spinal cord• Named for point of issue from the spinal cord– 8 pairs of cervical nerves (C1-C8)– 12 pairs of thoracic nerves (T1-T12)– 5 pairs of lumbar nerves (L1-L5)– 5 pairs of sacral nerves (S1-S5)– 1 pair of coccygeal nerves (Co1)

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Formation of Rami• Rami are lateral branches

of a spinal nerve• Rami contain both sensory

and motor neurons• Two major groups

– Dorsal ramus• Neurons innervate the dorsal regions of the body

– Ventral ramus• Larger• Neurons innervate the ventral regions of the body

• Braid together to form plexuses (plexi)

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Dermatomal Map• Spinal nerves indicated by capital letter and number

• Dermatomal map: skin area supplied with sensory innervation by spinal nerves

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Introduction to Nerve Plexuses

• Nerve plexus– A network of ventral rami

• Ventral rami (except T2-T12)– Branch and join with one another – Form nerve plexuses

•In cervical, brachial, lumbar, and sacral regions

•No plexus formed in thoracic region of s.c.

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The Cervical Plexus

• Buried deep in the neck– Under the sternocleidomastoid muscle

• Formed by ventral rami of first four cervical nerves

• Most are cutaneous nerves• Some innervate muscles of the anterior neck

• Phrenic nerve – the most important nerve of the cervical plexus

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Branches of

Spinal Nerves

• Dorsal Ramus– Neurons within muscles of trunk and back

• Ventral Ramus (VR)– Braid together to form plexuses• Cervical plexus - VR of C1-C4

• Brachial plexus - VR of C5-T1

• Lumbar plexus - VR of of L1-L4

• Sacral plexus - VR of L4-S4• Coccygeal plexus -VR of S4 and S5

• Communicating Rami: communicate with sympathetic chain of ganglia– Covered in ANS unit

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Cervical Plexus• Buried deep in the neck

– Under the sternocleidomastoid muscle

• Formed by ventral rami of first four cervical nerves (C1-C4)

• Most are cutaneous nerves• Some innervate muscles of the anterior neck, posterior portion of head

• Phrenic nerve – the most important nerve of the cervical plexus

• Phrenic nerve– Innervate diaphragm

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Brachial Plexus• Formed by ventral rami of spinal nerves C5-T1

• Five ventral rami form – three trunks that separate into

– six divisions that then form – cords that give rise to nerves

• Major nerves– Axillary– Radial– Musculocutaneous– Ulnar– Median

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Brachial Plexus: Axillary Nerve

• Motor neurons stimulate– Deltoid, teres minor• Abducts arm- deltoid• Laterally rotate arm-teres minor

• Sensory neurons– Skin: inferior lateral shoulder

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Brachial Plexus: Radial Nerve• Motor components stimulate

– Posterior muscles of arm, forearm, and hand• Triceps, supinator, brachioradialis, ECR, ECU, extensor digitorum

• Cause extension movements at elbow and wrist, thumb movements

• Sensory components– Skin on posterior surface of arm and forearm, hand

• Damage due to compression results in crutch paralysis

• Major symptom is ‘wrist drop’– Failure of extensors of wrist and fingers to function

– Elbow, wrist, and fingers constantly flexed

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Brachial Plexus:Musculocutaneous Nerve

• Motor components stimulate– Flexors in anterior upper arm: (biceps brachii, brachialis)• Cause flexion movements at shoulder and elbow

• Sensory: Skin along lateral surface of forearm

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Brachial Plexus: Ulnar Nerve

• Motor components stimulate– Flexor muscles in anterior forearm (FCU, FDP, most intrinsic muscles of hand)

– Results in wrist and finger flexion

• Sensory: Skin on medial part of hand

• Most easily damaged– Hitting the “funny bone” excites it

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Brachial Plexus: Median Nerve

• Motor components stimulate– All but one of the flexors of the wrist and fingers, and thenar muscles at base of thumb (Palmaris longus, FCR, FDS, FPL, pronator)

– Causes flexion of the wrist and fingers and thumb

• Sensory components– Stimulate skin on lateral part of hand

• Damaged in carpal tunnel and suicide attempts

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Lumbosacral Plexus• Lumbar plexus: formed by ventral rami of L1-L4– Major nerves include

• Femoral nerve• Obturator nerve

• Sacral plexus: formed by ventral rami of L4-S4– Major nerve = Sciatic nerve (actually 2 nerves in one sheath)• Tibial nerve• Common fibular (peroneal) n.

• Usually considered together because of their close relationship

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Lumbar Plexus:Obturator Nerve• Motor components

– Innervate adductor group and gracilis in thigh

– Causes adduction of the thigh and knee (gracilis)

• Sensory: Skin of the superior middle side of thigh

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Femoral Nerve• Motor components

– Innervates flexors of hip• Iliopsoas (Iliacus and psoas), rectus femoris

• Cause flexion of the hip

– Innervates extensors of knee• Quadriceps group-Vastus and rectus femoris

• Cause extension of the knee

• Sensory: Skin- anterior and lateral thigh; medial leg and foot

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Sciatic Nerve• Thickest and longest nerve of the body

• Composed of 2 nerves in one sheath– Tibial nerve– Common fibular nerve– Leaves pelvis via greater sciatic notch

– Courses deep to gluteus and enters posterior thigh just medial to the hip joint

• The 2 divisions diverge just above the knee.

• Innervates posterior thigh and entire lower leg

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Sciatic Branches: Tibial Nerve

• Tibial nerve– Innervates

– Hamstring muscles –knee flexors, hip extensors

– Posterior leg muscles – gastrocnemius, soleus

– plantar flexors– FDL, FHL

–toe flexors– Branches in foot to form

–medial plantar nerve –lateral plantar nerve

–If injured, paralyzed calf muscles cannot plantar flex foot; shuffling gait develops

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Common Fibular

(Peroneal) Nerve

• Common fibular– Branches are deep and superficial fibular (peroneal) nerves

– Innervates • anterior and lateral muscles of the leg and foot – (extensors that dorsiflex the foot- Tibialis anterior, EDL, EHL)

– Skin distribution: lateral and anterior leg and dorsum of the foot

– susceptible to injury because of its

superficial location at the head

and neck of the fibula

- Foot drop (unable to hold foot horizontal)

- Toes drag while walking

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Other Nerves of the Lumbosacral Plexus

• Nerves that innervate the skin of the suprapubic area, external genitalia, superior medial thigh, posterior thigh– Iliohypogastric nerve

• Innervates muscles of abdominal wall and pubic region

– Genitofemoral nerve• Skin of scrotum (males) and labia (females); inferior abdominal muscles

– Pudendal nerve• Innervates muscles and skin of the perineum (see Fig 10.21, p. 346)– region encompasssing external genitalia and anus

• external anal sphincter• Stimulates muscle involved in developing an erection• Involved in voluntary control of urination• the “shameful” nerve

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Coccygeal Plexus

• S4-S5; coccygeal nerve• Muscles of pelvic floor• Sensory information from skin over coccyx