The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other body parts (peripheral system).
Definitions
Somatic nervous system
Innervate the skin and muscle of voluntary control
Autonomic nervous system
Innervate the visceral organs such as heart, stomach, liver; control is involuntary
Lower motor neuron lesion
Nerve damage to nerve fibers that connect the spinal cord to the peripheral muscles; common symptoms will be muscle weakness, muscle atrophy, fasciculations, hypotonia, and hyporeflexia
Fasciculations
Involuntary muscle twitching that occurs when peripheral nerve is overactive and damaged
Components of peripheral nervous system
There are four (4) major components of the peripheral nervous system– cranial nerves, spinal nerves, peripheral nerves, and the nerves that comprise the autonomic nervous system.
Cranial nerves
12 pairs of nerves arising from the brainstem responsible for sensory and motor functions of the head and neck
List of cranial nerves
Olfactory neve-I:
Optic nerve - II:
Oculomotor nerve- III:
Motor only
Function: move eye up, down, and in (medially)
Trochlear nerve -IV:
Motor only
Function: move adducted eye down (torsional movement)
Trigeminal nerve - V:
Sensory and motor
Function: sensory -touch, pain and temperature to face; motor - muscles of mastication
Abducens nerve - VI:
Motor only
Function: moves out out (laterally)
Facial nerve - VII:
Sensory and motor
Function: sensory- taste to anterior two-thirds of tongue; motor- facial expressions
Vestibulocochlear nerve- VIII:
Sensory only
Function: balance and hearing acuity
Glossopharyngeal nerve - IX:
Sensory and motor
Function: sensory- taste to posterior one-third of tongue; motor- gag reflex, pharynx and larynx control
Vagus nerve- X:
Sensory and motor
Function: sensory- autonomic nervous system; motor- pharynx and larynx control, assist with digestion, aids in slowing heart rate, and bronchial constriction
Accessory nerve- XI:
Motor only
Function: control trapezius and sternocleidomastoid muscle
Hypoglossal nerve - XII:
Motor only
Function: controls tongue movements
Image #19
https://open.oregonstate.education/app/uploads/sites/157/2021/02/1320_The_Cranial_Nerves.jpg
Spinal nerves
Thirty-one (31) pairs of nerves exiting the varying vertebral areas- divided into dorsal (sensory) and ventral (motor) roots
Each spinal nerve corresponds with a segment of the spinal cord (i.e. 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
Structure of peripheral nerves
Consists of the following:
Nerve fibers
Axons send electrical signals from dendrites to cell body
Connective tissue
The endoneurium, perineurium, and epineurium surrounding and binding the nerves providing protection from injury
Blood vessels
Aid in vascularizing nerve
Myelin
Fatty tissue surrounding and protecting the nerve
Image #25
https://open.oregonstate.education/app/uploads/sites/157/2021/02/1319_Nerve_Structure.jpg
Image #26
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Definitions
Axon
Projection that conducts electrical impulses away from the nerve cell body
Dendrite
Receiving connection of the nerves
Nerve fascicle
Bundle of nerve fibers that are targeting the same anatomical location- located inside of dendrite
Perineirium
Innermost layer of connective tissue that surrounds a nerve fascicle
Endoneurium
Layer of connective tissue associated with the peripheral nerve composed of collagen, fibroblasts, macrophages, or mast cells that provinces protection to nerve fiber
Epineurium
Outermost layer of connective tissue that surrounds a nerve fascicle
Autonomic nervous system
Provides motor innervation to smooth muscle, cardiac muscle, and glands to provide unconscious control
Further divided into sympathetic, parasympathetic, and enteric divisions- each division works to maintain homeostasis within the body
Sympathetic nervous system
Works to manage individual’s response to anxiety, stress
Parasympathetic
Works to manage individual at rest
Enteric system
Management of gastrointestinal system
Image #76
https://upload.wikimedia.org/wikipedia/commons/c/c5/The_Autonomic_Nervous_System.jpg
Dermatomes and myotomes
A dermatome is an area on the skin associated with a specific spinal nerve root (31 spinal nerve roots) in which sensory information is carried. A myotome is the link between a group of peripheral muscles and a specific spinal nerve root (31 spinal nerve roots) in which motor information is carried. Dermatomes and myotomes are used to diagnose the potential damage of a spinal nerve root. Review the below dermatome map and myotome chart.
Image #27
Dermatome Map
https://upload.wikimedia.org/wikipedia/commons/9/93/Grant_1962_663.png
Myotomes
Upper extremity
C1- cervical rotation
C2-C4- shoulder elevation
C5- shoulder abduction/elbow flexion
C6- wrist extension
C7- elbow extension/wrist flexion
C8- thumb extension
T1- finger adduction
Lower extremity
L1-L2: hip flexion
L3: knee extension
L4: ankle dorsiflexion
L5: great toe extension
S1: ankle plantar flexion
S2: knee flexion
Spinal reflexes
Spinal reflexes (also called deep tendon reflexes) are automatic, involuntary responses of the motor system in response to sensory stimuli. Spina reflexes occur in the spinal cord and thus allow the body to respond quickly without conscious thought. Review the spinal reflexes below.
Spinal reflexes
C5: biceps reflex
C6: brachioradialis reflex
C7: triceps reflex
L3: patellar reflex
S1: achilles reflex
Example of reflexes work in spinal cord below:
Image #28
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Brachial, lumbar, and sacral plexuses
The brachial, lumbar, and sacral plexuses are a network of nerves that form a connection between the peripheral nerves and central nervous system. The peripheral nerves innervating all muscles are derived from the brachial, lumbar, and sacral plexuses.
The brachial plexus provides innervation to the chest and upper limbs.
The lumbar plexus provides innervation to the abdomen, groin, anterior thigh and leg, and feet.
The sacral plexus provides innervation to the pelvis, buttocks, feet, and genitals.
Brachial plexus
Image #29
https://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Brachial_plexus_2.svg/1500px-Brachial_plexus_2.svg.png
Muscles Innervated by the Brachial Plexus
Roots (C5–T1)
Dorsal scapular nerve (C5):
Rhomboid major
Rhomboid minor
Levator scapulae
Long thoracic nerve (C5–C7):
Trunks
Suprascapular nerve (Upper trunk: C5–C6):
Supraspinatus
Infraspinatus
Nerve to subclavius (Upper trunk: C5–C6):
Divisions
No direct muscle innervations from divisions.
Lateral Cord
Lateral pectoral nerve (C5–C7):
Pectoralis major (clavicular head)
Musculocutaneous nerve (C5–C7):
Biceps brachii
Brachialis
Coracobrachialis
Medial Cord
Medial pectoral nerve (C8–T1):
Pectoralis major (sternal head)
Pectoralis minor
Medial cutaneous nerves (arm & forearm):
No motor function (sensory only)
Ulnar nerve (C8–T1):
Flexor carpi ulnaris
Medial half of flexor digitorum profundus
Most intrinsic hand muscles:
Hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi)
Adductor pollicis
3rd and 4th lumbricals
Palmar and dorsal interossei
Posterior Cord
Upper subscapular nerve (C5–C6):
Thoracodorsal nerve (C6–C8):
Lower subscapular nerve (C5–C6):
Subscapularis
Teres major
Axillary nerve (C5–C6):
Radial nerve (C5–T1):
All muscles in the posterior arm and forearm:
Triceps brachii
Anconeus
Brachioradialis
Extensor muscles of wrist and fingers (e.g., extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris, extensor pollicis muscles)
Supinator
Median Nerve (from both Lateral and Medial Cords)
Most anterior forearm muscles:
Pronator teres
Flexor carpi radialis
Palmaris longus
Flexor digitorum superficialis
Lateral half of flexor digitorum profundus
Flexor pollicis longus
Pronator quadratus
Thenar muscles:
Abductor pollicis brevis
Flexor pollicis brevis (superficial head)
Opponens pollicis
1st and 2nd lumbricals
Lumbar plexus
Image #30
https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Lumbar_plexus.svg/1920px-Lumbar_plexus.svg.png
Muscles Innervated by the Lumbar Plexus (L1–L4)
Iliohypogastric Nerve (L1)
Internal oblique Transversus abdominis
Genitofemoral Nerve (L1–L2)
Cremaster muscle (in males; part of spermatic cord)
Lateral Femoral Cutaneous Nerve (L2–L3)
No motor innervation – sensory only
Femoral Nerve (L2–L4)
Iliacus Pectineus (also sometimes by the obturator nerve)Sartorius Quadriceps femoris group:
Rectus femoris Vastus lateralis Vastus medialis Vastus intermedius
Obturator Nerve (L2–L4)
Obturator externus
Adductor longus
Adductor brevis
Adductor magnus (adductor portion)
Gracilis
Pectineus (occasionally – shared with femoral nerve)
Lumbosacral Trunk (L4–L5)
No direct motor innervation
Contributes to sacral plexus (sciatic nerve, etc.)
Sacral plexus
Imager #31
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