12-1 Spinal Nerves AP 150 Chapter 12
12-1
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