THE BRACHIAL PLEXUS

Required Reading—336-341

I. INTRODUCTION

A. 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.
B. 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.
C. 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)

A. Roots
1. The ventral rami of spinal nerves C5 to T1 are referred to as the roots of the
plexus.
B. 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.
C. 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.
D. 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.
E. Terminal Branches are mixed nerves containing both sensory and motor axons.
1. Musculocutaneous nerve is derived from the lateral cord.
 a. This nerve innervates the muscles in the flexor compartment of the arm
b. Carries sensation from the lateral ( radial) side of the forearm. (Figs. 2,3)
. Ulnar nerve is derived from the medial cord

a. Motor innervation is mainly to intrinsic muscles of the hand

b. Sensory innervation is from the medial ( ulnar) 1 & 1/2 digits ( the 5th.
and 1/2 of the 4th. digits). (Figs. 2,3)
. Median nerve is derived from both the lateral and medial cords

a. Motor innervation is to most of the flexors muscles in the forearm and

intrinsic muscles of the thumb (thenar muscles).
b. Sensory innervation is from the lateral ( radial) 3 & 1/2 digits ( the thumb
and first 2 and 1/2 fingers). (Figs. 2,3)
. Axillary nerve is derived from the posterior cord.

a. Motor innervation is deltoid and teres minor muscles that act on the
shoulder joint.
b. Sensory innervation is from the skin just below the point of the shoulder.
(Figs. 2,3)
. Radial nerve is also derived from the posterior cord.

a. Called “Great Extensor Nerve” because it innervates the extensor muscles

of the elbow, wrist and fingers.
b. 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)

A. From the Roots

1. Dorsal Scapular nerve

a. Derived from C5 root

b. Motor nerve to the Rhomboideus major and minor muscles
. Long Thoracic nerve

a. Derived from C 5,6,7

b. Innervates the serratus anterior muscle
B. From the Upper Trunk
. Nerve to subclavius muscle
. Suprascapular nerve

a. Innervates supra and infraspinatus muscles

B. From the Lateral Cord
. Lateral Pectoral nerve
 a. Innervates the clavicular head of the pectoralis major muscle
B. From the Medial Cord
. Medial Pectoral nerve

a. Innervates the sternocostal head of the pectoralis major muscle

b. Innervates the pectoralis minor muscle
. Note : the medial and lateral pectoral nerve often join together to act as a single
nerve innervating both the pectoralis major & minor muscles
. Cutaneous Branches

a. Medial brachial cutaneous

i. Carries sensation from the lower medial portion of the arm

b. Medial antebrachial cutaneous

i. Carries sensation from the medial (ulnar portion of the forearm)

IV. DISTRIBUTION OF ROOTS (Fig. 5)

A. Definitions
1. Spinal Segment

a. Region of spinal cord giving origin to a specific spinal nerve

. Dermatome

a. Region on the surface of the skin from which sensation is carried by

cutaneous branches of a single spinal nerve
. Myotome

a. Those muscles receiving innervation from axons derived from a single

spinal nerve &/or the ventral ramus of a spinal nerve
b. Most muscles are innervated by axons from more than one spinal nerve
c. Predominant spinal nerve determines myotome segment
. Segmental Innervation

a. 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
B. Lesion
. 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
. Site of a lesion can be determined by the extent of muscle damage and / or loss of
sensation
. Muscles usually receive their innervation from axons derived from more than one
spinal segment. Predominant spinal nerve determines myotome spinal segment
 . Peripheral Nerve Lesion

a. Paralysis of muscles supplied by the damaged nerve

b. Loss of sensation from cutaneous region supplied by the damaged nerve
c. Examples
1. Trauma

a. Usually leads to loss of nerve function

. Entrapment

a. Nerve passing though a muscle or defined space

i. Musculocutaneous nerve passing through

coracobrachialis muscle
ii. Carpal Tunnel Syndrome
b. Diminished function but no total loss
. Spinal Cord lesion

a. All muscles supplied by spinal nerves below the site of the lesion are
paralyzed
b. Loss of sensation below the site of the lesion
c. Lesions are referred to as the lowest portion of the spinal cord that
functions
. a C5 spinal cord lesion means all spinal nerves below the 5th.
cervical nerve are no longer able to function.
. Nerve Root (Ventral Ramus) Lesion

a. All muscles supplied by a given nerve root will be weakened

. most muscles receive their innervation from axons derived from
more than one spinal segment.
b. Dermatome of that nerve root will have reduced sensation (paraesthesia)
. spinal nerves overlap in each dermatome
B. Root Distribution in Brachial Plexus
. Axons within each individual nerve root are distributed to many nerve branches
. Axons contributing to the formation of a given nerve form the segment of that
nerve

a. Axillary nerve ( C 5,6) has axons derived from the ventral rami of C 5 and
C6.
b. Ulnar nerve (C8, T1)has axons derived from ventral rami C8 and T 1
c. Long Thoracic nerve ( C 5,6,7) has axons derived from ventral rami C5,
C6, and C7
. For convenience, in figure 5 nerve roots C 5,6 are considered together as are
nerve roots C8 and T1
. C5 & 6
 a. Distributed to muscles acting on the shoulder and elbow
. C8,T1

a. Distributed to the intrinsic muscles of the hand

V. LESIONS OF THE BRACHIAL PLEXUS

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

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

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 =
 Musculoc
utaneous
nerve;
MN =
Median
nerve; UN
= Ulnar
nerve; RN
= Radial
nerve; AN
= Axillary
nerve

VI. OBJECTIVES

A. 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
B. Know the branches originating from the plexus , their component fibers and their
distribution.
C. 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
D. 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.
E. 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 .
F. 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
G. Be familiar with the muscles that are used to determine if a specific nerve root of the
brachial plexus is lesioned.
H. HGiven a set of symptoms, be able to use your knowledge to localize the specific site of
injuries to the brachial plexus