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in this video we will discuss the typical anatomy of the brachial plexus
the brachial plexus supplies motor innervation to muscles of the shoulder,
arm, forearm, and hand
the brachial plexus also provides sensory innervation to the bones, skin
muscles, and connective tissue of the shoulder and upper extermity.
let's begin by defining what a nerve plexus is
a nerve plexus is formed when nerve fibers from two or more spinal segments
intermingle and then segregate in order to travel together to a common
anatomical region
for example, fibers from two or three spinal segments that are all going to the
biceps brachii muscle may join together to form a single nerve bundle
the travels to the biceps
other fibers from the same spinal segments maybe destined for the triceps
brachii muscle
and they will join together to form a separate nerve bundle the travels to the
triceps
specifically, the brachial plexus contains motor and sensory fibers for spinal
segments C5 to T1
fibers from the spinal segments mix together and then segregate as they form
first roots, then trunks,
then divisions
then cords, and finally terminal branches.
let's look at how these different parts of the brachial plexus are formed
ventral rami from C5 to T1 form the roots
there is one root from each spinal segment
and they're named according to the spinal segment they represent
example "the C5 root"
more distally the roots form the trunks
the C5 and C6 roots join together to form the upper or superior trunk
likewise the C8 and T1 roots join to form the lower or inferior trunk.
finally the C7 root continues by itself to become the middle trunk
next each trunk divides into an anterior division and a poster division
this is the most important functional segregation that takes place in the
brachial plexus because all of the fibers in the anterior division will
innervate anterior compartment muscles
all of the fibers in the posterior division innervate posterior compartment muscles.
let's look at what this means
in the anatomical position, the arm can be divided by a midaxillary line into an
anterior compartment
and a posterior compartment
these are also called preaxillary and post axillary compartments or
flexor and extensor compartments
you can now understand why the anterior and posterior divisions of the brachial plexus
are formed and how they are segregated to serve different anatomical regions
returning to the brachial plexus we can see that the poster divisions of
each of the three trunks join together to form the posterior cord
anterior division of the upper trunk and the anterior division of the
middle trunk
join together to form the lateral cord
finally, the anterior division of the lower trunk
continues by itself to form the medial cord.
note that these cords, medial, lateral, and posterior,
are named according to their position relative to the axillary artery
in the anatomical position the medial cord lies medial to the artery, the lateral
cord lies lateral to the artery
and the posterior cord lies posterior to the artery
it's just a coincidence that the posterior cord is also formed by the posterior
divisions of the three trunks
finally the brachial plexus will form it's five terminal branches
these are the axillary nerve,
radial nerve, musculocutaneous nerve, ulnar nerve,
and to median nerve.
the posterior cord divides into two terminal branches:
the axillary nerve
and the radial nerve
the lateral cord divides into the musculocutaneous nerve
and contributes to the median nerve by giving off the lateral root of the
median nerve
the medial cord divides into the ulnar nerve
and also contributes to the median nerve
by giving the medial root of the median nerve, which joins with the lateral
root of the median nerve to form the median nerve proper
the terminal branches of the lateral and medial cords form the characteristic "M"
shape anterior to the axillary artery, which is usually the most distinctive
feature of the brachial plexus
and serves as a reference point for locating and identifying the other
components of the plexus
now that we understand the major structural features of the brachial
plexus, it's time to add in important smaller nerves that arise from the roots,
trunk,s and chords.
note that no nerves arise directly from the divisions
two nerves arise from the roots:
the dorsal scapular nerve
which is a branch of the C5 root
and the long thoracic nerve, which is formed by branches from C5, C6,
and C7 roots.
two nerves also arise from the trunks:
both the suprascapular nerve and the nerve to subclavius are branches
from the upper trunk
remember: the divisions do not give rise to any nerves
finally, the cords give rise to seven nerves in addition to their terminal
branches:
the lateral cord gives off the lateral pectoral nerve,
the posterior cord gives off the upper, middle,
and lower subscapular nerves
note that the middle subscapular nerve is also called the
thoracodorsal nerve.
the medial cord gives off the medial pectoral nerve,
medial the medial brachial cutaneous nerve,
and the medial antebrachial cutaneous nerve.
let's conclude by briefly discussing some clinical aspects of the brachial
plexus
because the brachial plexus innervates all the muscles of the upper extremity
injuries to it could have devastating effects on arm and hand function that
are critical for activities of daily living
the brachial plexus can be injured in many ways, of course, including
penetrating knife or bullet wounds,
shoulder injuries,
fractures of the humerus, and surgical procedures
however two of the most common types of injuries are upper brachial plexus injury
and lower brachial plexus injury
upper brachial plexus injury occurs during extreme or violent lateral
flexion of the head
especially if the shoulder is simultaneously depressed
this is typified by a difficult delivery in which the baby's head is pulled out
of the birth canal
while his shoulders are trapped within the mother's pelvis
Upper brachial plexus injuries
can also occur in adults when an oblique force pushes the head and shoulder in
opposite directions
such as might occur during a fall from a motorcycle
such injuries put enormous stretch on the C5 and C6 nerve roots and/or
the upper trunk itself
injury to these structures results in characteristic deficits known as Erb-Duchenne
paralysis
In erb duchenne paralysis,
the arm hangs limply at the side and is internally rotated,
the forearm is also pronated
which slightly flexes the wrist
this produces the characteristic "waiters tip position"
lower brachial plexus injury occurs during extreme or violent abduction of
the arm, such as occurs when a person falling from a ladder or tree attempts
to stop himself by reaching overhead for something to hang on to
additionally, this type of injury can also occur during delivery when the
baby is pulled by the arm from the birth canal
lower brachial plexus injury occurs from extreme stretch placed upon the
C8 root, the the T1 root,
the lower trunk or a combination of these
damage to these structures compromises muscles in the forearm and hand
resulting in Klumke's paralysis
Klumke's paralasis is typified by "claw hand," in which the fingers are flexed