Gross Anatomy
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THE BRACHIAL PLEXUS

Required Reading—336-341

I. INTRODUCTION

  1. The brachial plexus is a somatic nerve plexus formed by intercommunications among the ventral rami of the lower four cervical nerves ( C 5 - C 8) and the first thoracic nerve (T 1). The plexus is responsible for the motor innervation to all of the muscles of the upper limbwith the exception of the trapezius and levator scapula. It supplies all of the cutaneous innervation of the upper limb with the exception of the area of the axilla( armpit) (supplied by the intercostobrachial nerve), an area just above the point of the shoulder (supplied by supraclavicular nerves) and the dorsal scapular area which is supplied by cutaneous branches of dorsal rami. The brachial plexus communicates with the sympathetic trunk by gray rami communicates that join all the roots of the plexus and are derived from the middle and inferior cervical sympathetic ganglia and the first thoracic sympathetic ganglion.
  2. Prefixed Brachial Plexus—Occurs when the C 4 ventral ramus contributes to the brachial plexus. Contributions to the plexus usually come from C 4 - C 8.
  3. Postfixed Brachial Plexus—Occurs when the T 2 ventral ramus contributes to the brachial plexus. Contributions to the plexus usually come from C 6 - T 2.

II. FORMATION OF THE BRACHIAL PLEXUS (Fig. 1)

  1. Roots
    1. The ventral rami of spinal nerves C5 to T1 are referred to as the roots of the plexus.
  2. Trunks
    1. Shortly after emerging from the intervertebral foramina , these 5 roots unite to form three trunks.
    2. The ventral rami of C5 & C6 unite to form the Upper Trunk.
    3. The ventral ramus of C 7 continues as the Middle Trunk.
    4. The ventral rami of C 8 & T 1 unite to form the Lower Trunk.
  3. Divisions
    1. Each trunk splits into an anterior division and a posterior division.
    2. The anterior divisions usually supply flexor muscles
    3. The posterior divisions usually supply extensor muscles.
  4. Cords
    1. The anterior divisions of the upper and middle trunks unite to form the lateral cord.
    2. The anterior division of the lower trunk forms the medial cord.
    3. All 3 posterior divisions from each of the 3 cords all unite to form the posterior cord.
    4. The cords are named according to their position relative to the axillary artery.
  5. Terminal Branches are mixed nerves containing both sensory and motor axons.
    1. Musculocutaneous nerve is derived from the lateral cord.
      1. This nerve innervates the muscles in the flexor compartment of the arm
      2. Carries sensation from the lateral ( radial) side of the forearm. (Figs. 2,3)
    2. Ulnar nerve is derived from the medial cord
      1. Motor innervation is mainly to intrinsic muscles of the hand
      2. Sensory innervation is from the medial ( ulnar) 1 & 1/2 digits ( the 5th. and 1/2 of the 4th. digits). (Figs. 2,3)
    3. Median nerve is derived from both the lateral and medial cords
      1. Motor innervation is to most of the flexors muscles in the forearm and intrinsic muscles of the thumb (thenar muscles).
      2. Sensory innervation is from the lateral ( radial) 3 & 1/2 digits ( the thumb and first 2 and 1/2 fingers). (Figs. 2,3)
    4. Axillary nerve is derived from the posterior cord.
      1. Motor innervation is deltoid and teres minor muscles that act on the shoulder joint.
      2. Sensory innervation is from the skin just below the point of the shoulder. (Figs. 2,3)
    5. Radial nerve is also derived from the posterior cord.
      1. Called “Great Extensor Nerve” because it innervates the extensor muscles of the elbow, wrist and fingers.
      2. Sensory innervation is from the skin on the dorsum of the hand on the radial side. (Figs. 2,3)

III. BRANCHES (Fig. 4) Nerves that are branches from portions of the brachial plexus usually contain only 1 type of axon; either sensory or motor)

  1. From the Roots
    1. Dorsal Scapular nerve
      1. Derived from C5 root
      2. Motor nerve to the Rhomboideus major and minor muscles
    2. Long Thoracic nerve
      1. Derived from C 5,6,7
      2. Innervates the serratus anterior muscle
  2. From the Upper Trunk
    1. Nerve to subclavius muscle
    2. Suprascapular nerve
      1. Innervates supra and infraspinatus muscles
  3. From the Lateral Cord
    1. Lateral Pectoral nerve
      1. Innervates the clavicular head of the pectoralis major muscle
  4. From the Medial Cord
    1. Medial Pectoral nerve
      1. Innervates the sternocostal head of the pectoralis major muscle
      2. Innervates the pectoralis minor muscle
    2. Note : the medial and lateral pectoral nerve often join together to act as a single nerve innervating both the pectoralis major & minor muscles
    3. Cutaneous Branches
      1. Medial brachial cutaneous
        1. Carries sensation from the lower medial portion of the arm
      2. Medial antebrachial cutaneous
        1. Carries sensation from the medial (ulnar portion of the forearm)

IV. DISTRIBUTION OF ROOTS (Fig. 5)

  1. Definitions
    1. Spinal Segment
      1. Region of spinal cord giving origin to a specific spinal nerve
    2. Dermatome
      1. Region on the surface of the skin from which sensation is carried by cutaneous branches of a single spinal nerve
    3. Myotome
      1. Those muscles receiving innervation from axons derived from a single spinal nerve &/or the ventral ramus of a spinal nerve
      2. Most muscles are innervated by axons from more than one spinal nerve
      3. Predominant spinal nerve determines myotome segment
    4. Segmental Innervation
      1. involves understanding the manner in which the ventral rami of spinal nerves are distributed to the various dermatomes and myotomes of a given region such as the upper limb
  2. Lesion
    1. Damage to a structure, in this case a nervous structure. The structure damaged could be the spinal cord, a spinal nerve , a nerve root (ventral ramus ) or a branch of a ventral ramus
    2. Site of a lesion can be determined by the extent of muscle damage and / or loss of sensation
    3. Muscles usually receive their innervation from axons derived from more than one spinal segment. Predominant spinal nerve determines myotome spinal segment
    4. Peripheral Nerve Lesion
      1. Paralysis of muscles supplied by the damaged nerve
      2. Loss of sensation from cutaneous region supplied by the damaged nerve
      3. Examples
        1. Trauma
          1. Usually leads to loss of nerve function
        2. Entrapment
          1. Nerve passing though a muscle or defined space
            1. Musculocutaneous nerve passing through coracobrachialis muscle
            2. Carpal Tunnel Syndrome
          2. Diminished function but no total loss
    5. Spinal Cord lesion
      1. All muscles supplied by spinal nerves below the site of the lesion are paralyzed
      2. Loss of sensation below the site of the lesion
      3. Lesions are referred to as the lowest portion of the spinal cord that functions
        1. a C5 spinal cord lesion means all spinal nerves below the 5th. cervical nerve are no longer able to function.
    6. Nerve Root (Ventral Ramus) Lesion
      1. All muscles supplied by a given nerve root will be weakened
        1. most muscles receive their innervation from axons derived from more than one spinal segment.
      2. Dermatome of that nerve root will have reduced sensation (paraesthesia)
        1. spinal nerves overlap in each dermatome
  3. Root Distribution in Brachial Plexus
    1. Axons within each individual nerve root are distributed to many nerve branches
    2. Axons contributing to the formation of a given nerve form the segment of that nerve
      1. Axillary nerve ( C 5,6) has axons derived from the ventral rami of C 5 and C6.
      2. Ulnar nerve (C8, T1)has axons derived from ventral rami C8 and T 1
      3. Long Thoracic nerve ( C 5,6,7) has axons derived from ventral rami C5, C6, and C7
    3. For convenience, in figure 5 nerve roots C 5,6 are considered together as are nerve roots C8 and T1
    4. C5 & 6
      1. Distributed to muscles acting on the shoulder and elbow
    5. C8,T1
      1. Distributed to the intrinsic muscles of the hand

V. LESIONS OF THE BRACHIAL PLEXUS

  1. Very common
  2. A knowledge of the muscles innervated by branches of the brachial plexus and the action(s) of these muscles and areas of anesthesia &/or paraesthesia will enable the future clinical to determine the localization (site) of a given lesion.
  3. Chart 1 reviews the types of motor and sensory deficits produced as a result of lesions to different parts of the brachial plexus


    BRACHIAL PLEXUS LESIONS-CHART 1
    Nerve ( Segment) Motor Deficit(s) Sensory Deficits
    Long Thoracic
    (C 5,6,7)
    Winged Scapula- Serratus Anterior None
    Suprascapular
    (C 5,6 )
    Hard to start shoulder abduction - Supraspinatus None
    Axillary (C 5,6 ) Difficult abducting arm to horizontal - Deltoid Lateral side of arm below point of shoulder
    Loss of shoulder roundness - Deltoid
    Musculocutaneous
    C 5,6,(7)
    Very weak flexion of elbow joint- Biceps & Brachialis Lateral forearm
    Weak supination of radioulnar joint -Biceps
    Radial (C 5 - T1) Drop Wrist - Extensor carpi radialis longus & brevis, Ext. carpi ulnaris Posterior lateral &arm; dorsum of hand
    Difficulty making a fist - synergy between wrist extensors and finger flexors
    Median C 5 - T1) at Elbow Pronation of radioulnar joints-Pronator teres & quadratus
    Radial portion of palm; palmar surface & tips of radial 31/2 digits
    Weak wrist flexion - Fl. carpi radialis
    Weakened opposition of thumb - thenar muscles
    “Ape Hand”- thumb hyper extended and adducted - thenar muscles
    “Papal Hand” Loss of flexion of I.P. joints of thumb & fingers 1 & 2 - Fl. pollices longus ; Fl. digit. superficialis, Fl. digit profundus
    Median (C 5 - T1) at Wrist Weakened opposition of thumb - thenar muscles Palmar surface & tips of radial 31/2 digits
    “Ape Hand”- thumb hyper extended and adducted - thenar muscles
    Ulnar (C 8, T1) at Elbow “Clawing” of fingers 3 & 4- M.P. joints hyper extended; P.I.P. Flexed - Interossei & Lumbricals Ulnar and dorsal aspect of palm and of ulnar 1 1/2 digits
    Loss of abduction & adduction of M.P joints of fingers -Interossei
    Thumb - abducted and extended - adductor pollices
    Loss of flexion of D.I.P. joints of fingers 4 & 5 - Fl. digit profund.
    Ulnar (C 8, T1) at Wrist “Clawing” of fingers 3 & 4- M.P. joints hyper extended; P.I.P. Flexed - Interossei & Lumbricals Ulnar and dorsal aspect of palm and of ulnar 1 1/2 digits
    Loss of abduction & adduction of M.P joints of fingers - Interossei
    Thumb - abducted and extended - adductor pollices


    UPPER AND LOWER ROOT LESIONS- CHART 2
    Lesion Motor Deficits Sensory Deficits Nerves
    Erb’s Palsy (C 5,6 ) Loss of abduction, flexion and rotation at shoulder ; Weak shoulder extension - deltoid, rotator cuff Posterior and lateral aspect of arm - axillary n. Axillary, Suprascapular, Upper and Lower subscapular
    Very weak elbow flexion and supination of radioulnar joint - biceps brachii & brachialis Radial side of Forearm- musculocutaneous n. Thumb and 1st finger - superficial br. of radial; digital brs. - Median n. Musculocutaneous ; Radial N. brs. to supinator & brachioradialis muscles
    Susceptible to shoulder dislocation - loss of rotator cuff muscles Suprascapular, Upper and Lower subscapular
    “Waiters Tip”position
    Klumke’s Palsy (C8, T1 ) Loss of opposition of thumb -Thenar muscles Ulnar side of forearm , hand & & ulnar 1 1/2 & digits - ulnar and medial antebrachial cutaneous Thenar branch of Median nerve

    Loss of adduction of thumb - Adductor pollices
    Ulnar nerve

    Loss of following finger movements: abduction and adduction of M.P. joints ; flexion at M.P. & extension of I.P.joints. Lumbricals & interossei
    Deep branch of Ulnar & Median

    Very weak flexion of P.I.P.& D.I.P. joints Fl. Digit. Super. & Profund.
    Ulnar and Median



Figure 4 Somatic Nerve Plexi



Figure 5 - Brachial Plexus
1. 5, 6,7, 8, 1 = Nerve roots C5, C6, C7, C8,T1

2. U, M, L = Upper, Middle and Lower Trunks

3. LC, MC, PC = Lateral, Medial, Posterior Cords

4. MCN = Musculocutaneous nerve; MN = Median nerve; UN = Ulnar nerve; RN = Radial nerve; AN = Axillary nerve



VI. OBJECTIVES

  1. Understand embryological origin of the brachial plexus.
    1. Know the basis of a prefixed and postfixed plexus
    2. Describe how the cervical and thoracic ventral rami form the brachial plexus
  2. Know the branches originating from the plexus , their component fibers and their distribution.
  3. Be able to determine the spinal nerve root level(s) that contribute to the major branches of the brachial plexus.
    1. Be able to distinguish the dermatome and myotome patterns of the brachial plexus
  4. Know the muscles innervated by the branches of the brachial plexus and be able to determine the type(s) of motor deficit(s ) that would likely result from a lesion to each major branch.
  5. Know the cutaneous dermatome innervation of the upper limb. Be able to use this knowledge to locate the spinal level of origin of the nerve lesion .
  6. Understand the anatomical basis of Upper and Lower Nerve Root Lesions.
    1. Given a set of symptoms, be able to distinguish between each type of lesion
  7. Be familiar with the muscles that are used to determine if a specific nerve root of the brachial plexus is lesioned.
  8. HGiven a set of symptoms, be able to use your knowledge to localize the specific site of injuries to the brachial plexus
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