The Peripheral Nervous Chapter 13 The Peripheral Nervous System (PNS) Part B

Thirty-one pairs of mixed nerves Arise from the spinal cord Spinal Nerves Thirty-one pairs of mixed nerves Arise from the spinal cord Supply all parts of the body except the head Are named according to their point of issue 8 cervical (C1-C8) 12 thoracic (T1-T12) 5 lumbar (L1-L5) 5 sacral (S1-S5) 1 coccygeal (Co1)

Spinal Nerves Figure 13.6

Each nerve connects to the cord via two roots Spinal Nerves: Roots Each nerve connects to the cord via two roots Each root forms rootlets that attach it to the cord The two roots include the ventral & the dorsal roots Ventral roots: Contain motor (efferent) fibers Arise from the anterior horn (spinal cord grey matter) Dorsal roots: Contain sensory (afferent) fibers Arise from sensory neurons (in dorsal root ganglion)

Spinal Nerves: Roots Figure 13.7a

The spinal nerves result from the union of the dorsal & ventral roots Spinal Nerves: Rami The spinal nerves result from the union of the dorsal & ventral roots They are short and branch into three or four mixed rami These rami include: Small dorsal ramus Larger ventral ramus Tiny meningeal branch Rami communicantes

The tiny meningeal branch reenters the canal to innervate: Spinal Nerves: Rami The tiny meningeal branch reenters the canal to innervate: The meninges The blood vessels within the meninges The rami communicantes: Located at the base of the ventral rami Found in the thoracic region Contain autonomic (visceral) nerve fibers

Only & all ventral rami except T2-T12 form: Nerve Plexuses Only & all ventral rami except T2-T12 form: Interlacing nerve networks called plexuses Plexuses are found in the following regions: Cervical Brachial Lumbar Sacral Each resulting plexus contains: Fibers from several spinal nerves

Fibers travel to the periphery via: Nerve Plexuses Fibers travel to the periphery via: Several different routes Each muscle receives: A nerve supply from more than one spinal nerve Therefore, damage to one spinal segment: Cannot completely paralyze a muscle

Spinal Nerve Innervation: The back is innervated by: Dorsal rami (via several branches) The thorax is innervated by: Ventral rami T1-T12 (as intercostal nerves) Intercostal nerves supply: Muscles of the ribs Anterolateral thorax Abdominal wall

The most important nerve of this plexus is: The phrenic nerve: Cervical Plexus The cervical plexus is formed by: Ventral rami of spinal nerves C1-C4 Most branches are cutaneous nerves of: The neck The head The shoulders The most important nerve of this plexus is: The phrenic nerve The phrenic nerve: The major motor & sensory nerve of the diaphragm

Cervical Plexus

Brachial Plexus Formed by: It gives rise to: Spinal nerves C5-C8 and T1 C4 and T2 may also contribute to this plexus It gives rise to: Nerves that innervate the upper limb

Brachial Plexus Comprises four major groups of branches: Rami (misleadingly called roots): Five ventral rami (C5-T1) They unite to form trunks Trunks: Include: Upper trunk Middle trunk Lower trunk Each trunk divides immediately into two divisions

Brachial Plexus: Nerves Figure 13.9b

Brachial Plexus Figure 13.9a

Brachial Plexus Divisions: Include: Anterior division Posterior division Serve the front and back of the limb, respectively Give rise (in axilla) to three large fiber bundles called cords Cords: Lateral Medial Posterior

Brachial Plexus: Upper limb Nerves The brachial plexus ends in the axilla Its three cords give rise to the main nerves of the upper limb The five especially important of these nerves are: Axillary :– innervates: Deltoid Teres minor Musculocutaneous:– Sends fibers to: Biceps brachii Brachialis Coracobrachialis

Brachial Plexus: Upper limb Nerves Median :– branches to: Flexor muscles of anterior arm Ex. Flexor carpi radialis Ulnar :– supplies: Ex. Flexor carpi ulnaris Radial :– innervates: Extensor (posterior) muscles of arm Ex. Extensor carpi radialis

Brachial Plexus: Distribution of Nerves Figure 13.9c

Brachial Plexus: Nerves Figure 13.9b

Lumbar Plexus Arises from L1-L4 Innervates: The major nerves are: Thigh (anterior & medial) Abdominal wall Psoas muscle The major nerves are: The femoral The obturator

Lumbar Plexus Figure 13.10

Arises from spinal nerves L4-S4 Serves: Sacral Plexus Arises from spinal nerves L4-S4 Serves: Buttock Lower limb Pelvic structures The perineum

The sciatic is actually composed of two nerves: Sacral Plexus The major nerve is: The sciatic The longest & thickest nerve of the body The sciatic is actually composed of two nerves: The tibial nerve The common fibular (peroneal) nerve

Sacral Plexus Figure 13.11

Innervation of the Skin (Dermatomes) A dermatome is: The area of skin innervated by: The cutaneous branches of a single spinal nerve All spinal nerves participate in dermatomes except: The 1st cerviacal spinal nerve (C1)

Dermatomes Figure 13.12

Reflexes Reflexes: A reflex is: Reflexes may be: Are a category of control systems in the body A reflex is: A rapid, predictable motor response to a stimulus Reflexes may be: Inborn (intrinsic): Ex. Response to splashing hot water on one’s arm Learned (acquired): Ex. Sequences of reaction during driving

Reflexes Awareness of reflex responses: Reflexes involve only: Acknowledged: Ex. Dropping a pot of a boiling water Passed unnoticed: Ex. Visceral reflexes controlled by brain stem & spinal cord Reflexes involve only: Peripheral nerves The spinal cord Higher brain centers as well

Reflex Arc Highly specific neural paths over which reflexes occur Reflex arc components: Receptor Sensory neuron Integration center Motor neuron: Effector Classification: Somatic reflexes: activates skeletal muscles Autonomic (visceral) reflexe: activates visceral effectors

Reflex Arc Figure 13.14