Extrinsic muscles of the back – Anatomy Tutorial

Extrinsic muscles of the back – Anatomy Tutorial


Okay so this is a tutorial on the extrinsic
muscles of the back. The extrinsic muscles of the back are those muscles which are superficial
and are responsible for movements at the shoulder and upper limb. These are different to the
intrinsic muscles of the back which are deep, and which are responsible for controlling
posture and movement of the spine and head. So the extrinsic muscles of the back are the
trapezius, the latissimus dorsi, the rhomboid major and minor, and the levator scapulae.
So just working superficially to deep I’ll just show you these muscles. So I’ll start
with the trapezius, which is this muscle here which you probably all know – big trapezium
shaped muscle, which is why it’s called the trapezius. And it basically extends from the
external occipital protuberance here, all the way down to the lowest thoracic vertebra
T12 – the spine of T12. And laterally it inserts along the spine of the scapula and posteriorly
on the distal end of the clavicle, which you can’t quite see, but it inserts just at the
back of the clavicle here, and also on the acromion of the scapula. So this muscle is
responsible for elevating and depressing the scapula, and it can also retract the scapula.
It’s innervated by the axillary nerve, sorry, the accessory nerve, which is the eleventh
cranial nerve and it receives sensory innervation from the ventral rami of C3 and C4. All the
extrinsic muscles are innervated by the ventral rami, as opposed to the dorsal rami, because
these muscles insert onto the…insert onto the upper limbs, whereas the intrinsic muscles,
which are primarily innervated by the posterior rami, are deeper and are just responsible
for attaching to, and controlling the posture and movement of the spine. So that’s the trapezius,
this huge muscle here, just get rid of that. The next muscle is the latissimus dorsi, and
it’s called that because in Latin, “latissimus” means “the broadest”, and “dorsi” means “back”,
so it’s basically the “broadest muscle of the back”. So this muscle inserts into the
humerus in between the insertion points of the pec major and…what’s this one…the
teres major, and for that reason it’s often referred to as the “lady between two majors”,
so it inserts in the intertubercular groove of the humerus, in between the teres major
and the pectoralis major. So this huge muscle, the latissimus dorsi adducts the arm, extends,
and internally rotates the humerus. It’s innervated by spinal nerves C6, 7 and 8. So moving on,
just underneath the trapezius, which I’ve removed here, we now have, we can now see
the rhomboid muscles. So this is the rhomboid minor here…zoom in a bit closer…so the
rhomboid minor and the rhomboid major. So these just lie deep to the trapezius muscle,
and as you can see here they insert onto, or sorry, they originate on the spines of
the vertebra. So the rhomboid minor here originates on the spine of C7 and T1, and inserts onto
the medial border of the scapula. And the rhomboid major originates on the spinous processes
of T2 to T5 and inserts medially on the scapula. So these muscles basically keep the scapula
pressed against the thoracic wall and they can retract the scapula when the trapezius
is contracted. So they act as antagonists to the trapezius muscle. They are both innervated
by the dorsal scapula nerve, which comes of C3, spinal nerves C3 and C4. So these are
the rhomboids. The last extrinsic muscle of the back is the levator scapulae, or well,
collectively the levator scapulae. So I’ll just get rid of the rhomboids, so had a good
look at those, and underneath them you can see the levator scapulae. So it’s this muscle
here. And as you can tell by the name, this muscle elevates the scapula, so levator is
Latin I guess, for lifting, lifter, or something like that, so it elevates the scapula. So
we can see here, just at its origin, it’s covered by the sternocleidomastoid and the
splenius capitis. Okay so if we zoom in, you can see exactly where it originates on the
transverse process of the first vertebra, and then if we follow the path down, we can
see that it inserts on the superior angle of the scapula and the adjacent medial border.
So that’s the levator scapulae and it’s innervated by branches of C4 and C5, and it elevates
the scapula. So just to recap, the extrinsic muscles, you’ve got the trapezius, the latissimus
dorsi, the rhomboid major, the rhomboid minor, and the levator scapulae, so these two muscles
here.