Peripheral nervous system
The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other body parts (the peripheral system).
Components of the peripheral nervous system
There are four major components of the peripheral nervous system: cranial nerves, spinal nerves, peripheral nerves, and the nerves that comprise the autonomic nervous system.
Cranial nerves
Twelve pairs of nerves arise from the brainstem, providing sensory and motor innervation to the head and neck.
Olfactory nerve - I
- Sensory only
- Function: smell
Optic nerve - II
- Sensory only
- Function: visual acuity
Oculomotor nerve - III
- Motor only
- Function: moves the eye up, down, and in (medially); elevation of the eyelid; constriction of the pupil
Trochlear nerve - IV
- Motor only
- Function: adducts the eye out and in
Trigeminal nerve - V
- Sensory and motor
- Sensory: touch, pain, and temperature to the face; ophthalmic, maxillary, and mandibular regions
- Motor: muscles of mastication
Abducens nerve - VI
- Motor only
- Function: moves the eye out (laterally)
Facial nerve - VII
- Sensory and motor
- Sensory: taste to the anterior two-thirds of the tongue
- Motor: facial expressions, closing eyes tightly
Vestibulocochlear nerve - VIII
- Sensory only
- Function: balance and hearing acuity
Glossopharyngeal nerve - IX
- Sensory and motor
- Sensory: taste to the posterior one-third of the tongue
- Motor: gag reflex, pharynx, and larynx control
Vagus nerve - X
- Sensory and motor
- Sensory: small area of taste on the back of the tongue; primary parasympathetic autonomic nervous system supply to internal organs, assisting with digestion, slowing heart rate, and bronchial constriction
- Motor: pharynx and larynx control (swallowing and speech)
Accessory nerve - XI
- Motor only
- Function: controls the trapezius and sternocleidomastoid muscles
Hypoglossal nerve - XII
- Motor only
- Function: controls tongue movements
Spinal nerves
- Thirty-one 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 (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
Structure of peripheral nerves
Autonomic nervous system
The autonomic nervous system provides motor innervation to smooth muscle, cardiac muscle, and glands, giving unconscious control over these systems. It’s further divided into the sympathetic, parasympathetic, and enteric divisions, each of which works to maintain homeostasis in the body.
Dermatomes and myotomes
Dermatomes and myotomes are used to diagnose potential damage to a spinal nerve root. Review the dermatome map and myotome chart below.
Myotomes
- Upper extremity
- C1: cervical rotation
- C2-C4: shoulder elevation
- C5: shoulder abduction/elbow flexion
- C6: wrist extension
- C7: elbow extension/wrist flexion
- C8: digit flexion/thumb extension
- T1: finger adduction
- Lower extremity
- L1-L2: hip flexion
- L3: knee extension
- L4: knee extension/ankle dorsiflexion
- L5: ankle dorsiflexion/great toe extension
- S1: ankle eversion/ankle plantar flexion
- S2: ankle plantar flexion
Spinal reflexes
- C5: biceps reflex
- C6: brachioradialis reflex
- C7: triceps reflex
- L3-L4: patellar reflex
- L5: semitendinosus reflex
- S1-S2: Achilles reflex
Example of reflexes working in the spinal cord below:
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 the central nervous system. The peripheral nerves innervating all muscles are derived from these three plexuses.
Brachial plexus
Muscles innervated by the brachial plexus
Roots (C5-T1)
- Dorsal scapular nerve (C5):
- Rhomboid major
- Rhomboid minor
- Levator scapulae
- Long thoracic nerve (C5-C7):
- Serratus anterior
Trunks
- Suprascapular nerve (upper trunk: C5-C6):
- Supraspinatus
- Infraspinatus
- Nerve to subclavius (upper trunk: C5-C6):
- Subclavius
Divisions
- No direct muscle innervation from the 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 and 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):
- Subscapularis
- Thoracodorsal nerve (C6-C8):
- Latissimus dorsi
- Lower subscapular nerve (C5-C6):
- Subscapularis
- Teres major
- Axillary nerve (C5-C6):
- Deltoid
- Teres minor
- Radial nerve (C5-T1):
- All muscles in the posterior arm and forearm, recalled using the BEAST acronym:
- B: Brachioradialis
- E: Extensor muscles of the wrist and fingers (e.g., extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris, extensor pollicis muscles)
- A: Abductor pollicis longus, Anconeus
- S: Supinator
- T: Triceps brachii
- All muscles in the posterior arm and forearm, recalled using the BEAST acronym:
- Posterior interosseous nerve (C7-C8):
- Majority of muscles in the posterior (extensor) compartment of the forearm: extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
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
- Anterior interosseous nerve (AIN) (C7-T1):
- Motor nerve branching from the median nerve: flexor pollicis longus, flexor digitorum profundus, and pronator quadratus.
Lumbar plexus
Muscles innervated by the lumbar plexus (L1-L4)
The lumbar plexus gives rise to several nerves that supply the abdomen, groin, and thigh. Refer to the textbook to view the full list of muscles each nerve innervates.
Iliohypogastric nerve (L1)
- Internal oblique
- Transversus abdominis
Genitofemoral nerve (L1-L2)
- Cremaster muscle (in males; part of the spermatic cord)
Lateral femoral cutaneous nerve (L2-L3)
- No motor innervation — sensory only
Femoral nerve (L2-L4)
- Iliacus
- Pectineus (also occasionally innervated 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 the femoral nerve)
Sensory nerves of lumbar plexus
Lateral femoral cutaneous nerve (L2-L3)
- No motor innervation — sensory only, lateral aspect of the thigh
Saphenous nerve (L3-L4)
- No motor innervation — sensory only, medial aspect of the lower leg
Sacral plexus
Muscles innervated by the sacral plexus (L4-S3)
Superior gluteal nerve (L4-S1)
- Gluteus medius and minimus
- Tensor fasciae latae
Inferior gluteal nerve (L5-S2)
- Gluteus maximus
The sciatic nerve splits into tibial and common peroneal divisions that supply the muscles of the lower leg and foot. Refer to the textbook to view the full list of muscles each division innervates.
Sciatic nerve (L4-S3)
- Tibial division
- Major proximal muscles:
- Biceps femoris (long head)
- Semitendinosus
- Semimembranosus
- Below the knee (as the tibial nerve), serving posterior leg and foot muscles:
- Superficial group:
- Gastrocnemius
- Soleus
- Plantaris
- Deep group:
- Tibialis posterior
- Flexor hallucis longus (FHL)
- Flexor digitorum longus (FDL)
- Intrinsic foot muscles (medial and lateral plantar branches)
- Superficial group:
- Major proximal muscles:
- Common peroneal (fibular) division
- Major proximal muscle:
- Biceps femoris (short head)
- Below the knee, by its divisions:
- Deep peroneal (fibular) (anterior compartment):
- Tibialis anterior (TA)
- Extensor hallucis longus (EHL)
- Extensor digitorum longus (EDL)
- Peroneus (fibularis) tertius
- Extensor digitorum brevis (EDB)
- Extensor hallucis brevis (EHB)
- Superficial peroneal (fibular) (lateral compartment):
- Peroneus (fibularis) longus
- Peroneus (fibularis) brevis
- Deep peroneal (fibular) (anterior compartment):
- Major proximal muscle:
Upper limb tension tests
- Upper limb tension test 1 (ULTT 1) — median and anterior interosseous nerve
- Cervical spine: contralateral lateral flexion
- Shoulder: depression and abduction to 110 degrees
- Elbow: extension
- Forearm: supination
- Wrist: extension
- Fingers and thumb: extension
- Upper limb tension test 2 (ULTT 2) — median, axillary, and musculocutaneous nerve
- Cervical spine: contralateral lateral flexion
- Shoulder: depression and abduction to 10 degrees, lateral rotation
- Elbow: extension
- Forearm: supination
- Wrist: extension
- Upper limb tension test 3 (ULTT 3) — radial nerve
- Cervical spine: contralateral lateral flexion
- Shoulder: depression and abduction to 10 degrees; internal rotation
- Elbow: extension
- Forearm: pronation
- Wrist: flexion with ulnar deviation
- Fingers and thumb: flexion
- Upper limb tension test 4 (ULTT 4) — ulnar nerve
- Cervical spine: contralateral lateral flexion
- Shoulder: depression and abduction (10-90 degrees) with hand to ear
- Elbow: flexion
- Forearm: pronation
- Wrist: extension and radial deviation
- Fingers and thumb: extension
Thoracic outlet syndrome
- Adson’s test (compression of the subclavian artery and/or nerves as they pass through the interscalene space)
- Patient seated with the radial pulse palpated; head rotated toward the extremity being tested while the shoulder is extended and externally rotated, then the head is extended
- Positive: reproduction of neurological (pain, weakness, numbness, and loss of hand coordination) and vascular symptoms (loss of radial pulse)
- Patient seated with the radial pulse palpated; head rotated toward the extremity being tested while the shoulder is extended and externally rotated, then the head is extended
- Roos elevated arm test (nerves and/or blood vessels in the space between the clavicle and the first rib are compressed)
- Patient standing with shoulders fully externally rotated, 90 degrees abducted, elbows flexed to 90 degrees; the patient then rapidly opens and closes their hands for 3 minutes
- Positive: reproduction of neurological (pain, weakness, numbness, and loss of hand coordination) and vascular symptoms (loss of radial pulse)
- Patient standing with shoulders fully externally rotated, 90 degrees abducted, elbows flexed to 90 degrees; the patient then rapidly opens and closes their hands for 3 minutes
- Wright’s test (compression at the space behind the pectoralis minor muscle)
- Patient seated with passive movement of the arm into abduction and external rotation
- Positive: reproduction of neurological (pain, weakness, numbness, and loss of hand coordination) and vascular symptoms (loss of radial pulse)
- Patient seated with passive movement of the arm into abduction and external rotation
- Costoclavicular test (compression of the neurovascular bundle between the clavicle and first rib)
- Patient seated; the therapist assists the patient through 4 movements — scapular retraction, scapular depression, elevation, and protraction — holding each position for up to 30 seconds while resting their forearms on their thighs
- Positive: reproduction of neurological (pain, weakness, numbness, and loss of hand coordination) and vascular symptoms (loss of radial pulse)
- Patient seated; the therapist assists the patient through 4 movements — scapular retraction, scapular depression, elevation, and protraction — holding each position for up to 30 seconds while resting their forearms on their thighs