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Peripheral Nerve (LMN)
- Motor (efferent) fibers: originate from motor nuclei (cranial nerves) or anterior horn cells (spinal nerves)
- Sensory (afferent) fibers: originate in cells outside brainstem or SC with sensory ganglia (cranial nerves) or dorsal root ganglia (spinal nerves)
- Autonomic nervous system fibers: sympathetic fibers at thoracolumbar spinal segments and parasympathetic fibers at craniosacral segments
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Cranial Nerves
- 12 pairs
- All nerves are distributed to head and neck (except CN X - distributed to thorax and abdomen)
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CN I, II, and VIII
- Pure sensory
- Carry special senses of smell, vision, hearing, and equilibrium
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CN III, IV, and VI
- Pure motor
- Control eye movements and pupillary constriction
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CN XI and XII
- Pure motor
- Innervating SCM, trap and tongue
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CN V, VII, IX, and X
- Mixed motor and sensory
- Involved in:
- Chewing (V)
- Facial expression (VII0
- Swallowing (IX, X)
- Vocal sounds (X)
- Sensations from head (V, VII, IX)
- Alimentary tract, heart, vessels, and lungs (IX, X)
- Taste (VII, IX, X)
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CN III, VII, IX, and X
- Carry parasympathetic fibers of ANS
- Involved in control of smooth muscles of inner eye (III), salivatory and lacrimal glands (VII), parotid gland (IX), muscles of heart, lung, and bowel (X)
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Spinal Nerves
- 31 pairs of spinal nerves
- Spinal nerves divided into groups: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
- Correspond to vertebral segments
- Each has a ventral root and dorsal root
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Spinal Nerves: Ventral (Anterior) Root
- Efferent (motor) fibers to:
- Voluntary muscles (alpha motor neurons, gamma motor neurons)
- Viscera, glands, and smooth muscles (preganglionic ANS fibers)
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Spinal Nerves: Dorsal (Posterior Root)
- Afferent (sensory) fibers from skin, joints, and muscles
- Each dorsal root possesses a dorsal root ganglion (cell bodies of sensory neurons)
- There is no dorsal root for C1
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Dermatome
Refers to a specific segmental skin area innervated by sensory spinal axons
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Myotome
Refers to the skeletal muscle innervated by motor axons in a given spinal root
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Motor Unit
Consists of alpha motor neuron and the muscle fiber it innervates
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Nerve Roots
- Exit from the vertebral column through the intervertebral foramina
- In the c-spine, numbered roots exit horizontally above the corresponding vertebral body, with C8 exiting below C7 and above T1
- In the thoracic, lumbar, and sacral spine, roots exit below the corresponding vertebral body
- Spinal cord ends at level of L1
- In lumbosacral region, nerve roots descend almost vertically below the cord to form caudal equine (horse's tail)
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Spinal Nerve Divisions
- Emerge from intervertebral foramen
- Each spinal nerve divides into large anterior ramus and small posterior ramus
- Each ramus contains motor and sensory fibers
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Anterior Rami
- Supply muses and skin of anterolateral body wall and limbs
- Join at root of the limbs to form nerve plexuses
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Cervical Plexus
Arises from C1 through C4 nerve roots
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Brachial Plexus
- Arise from C5 through T1 nerve roots
- Split into anterior and posterior divisions
- Redistributes fibers into 3 cords (lateral, posterior, and medial)
- Finally branch into the peripheral nerves that supply UE
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Lumbar Plexus
- Arises from T12 through L4 nerve roots
- Nerve fibers redistributed into the peripheral nerves that supply the LE
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Sacral Plexus
- Arises from L4 through S3 nerve roots
- Nerve fibers redistributed into the peripheral nerves that supply the LE
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