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Viewpoint: The Embryological Development of
Sternalis Muscle and Implications for Trigger Point
Pain
AUTHOR NAMES: Laurie Y Hung1* DC, MSc, Octavian C Lucaciu1 MD, PhD
AUTHOR AFFILIATIONS:
1Department
of Anatomy, Canadian Memorial Chiropractic College, 6100 Leslie Street, Toronto,
Ontario, Canada, M2H 3J1
CORRESPONDING AUTHOR EMAIL ADDRESS: laurieykhung@gmail.com
ABSTRACT
Sternalis muscle (SM) is a recognized variant muscle of the anterior thoracic wall. Although the
morphology of SM is agreed upon, the function is unknown and the innervation and
embryological origin of this muscle are areas of debate. No existing theories regarding the
origin of sternalis muscle explains the variability in innervation that sternalis muscle presents
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with (i.e. anterior branches of
1st-5th
intercostal nerves vs. pectoral nerves). Most old and
modern anatomists argue that SM is innervated by either the pectoral nerves (lateral or medial
or both) or the 1st-5th intercostal nerves (anterior branches). It is also argued that SM is derived
from either the pectoralis major muscle or the rectus abdominis. Several manual therapy books
have proposed trigger point pain referral patterns based on innervation from both nerves
(pectoral and intercostal). The authors of this paper propose that there are two types of SM,
resolving apparent disputes in the SM focused literature, and offer an explanation of existing
composite pain referral patterns of SM based on innervation.
KEYWORDS: embryology, myofascial trigger point, Pectoral nerves, intercostal nerves,
pectoralis muscles
Introduction
Sternalis muscle (SM) is recognized by anatomists as a variant muscle of the anterior
thoracic wall (Demirpolat G et al., 2010; Raikos A, et al., 2011). Although the morphology of SM
is agreed upon, the function is unknown and the innervation and embryological origin of this
muscle are areas of debate. There are six main theories that have been proposed regarding the
origin of sternalis muscle: 1) It is a cranial extension of rectus abdominis (Humphry, 1873;
Sadler, 2004) 2) it is a caudal continuation of sternocleidomastoid (Parsons, 1893) 3) it is a
bridging between sternocleidomastoid superiorly and external oblique inferiorly (Barlow, 1935)
4) it develops from the pectoral mass from fibers that were displaced at about a right angle from
the fibers of pectoralis major and minor (Huntington, 1904; Kida et al., 2000) 5) it is a remnant of
panniculus carnosus (Humphry, 1873) and 6) it is a muscle peculiar to humans (Parsons, 1893).
Not one of these theories alone explain the variability in innervation that sternalis muscle
presents with (i.e. anterior branches of 1st-5th intercostal nerves vs. pectoral nerves). Most old
and modern anatomists argue that SM is innervated by either the pectoral nerves (lateral or
medial or both) or the 1st-5th intercostal nerves (anterior branches) (Humphry, 1873; Parsons,
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1893; Barlow, 1935; Kida et al., 2000). Trigger point pain referral patterns for SM have been
described by many manual therapy books, most notably Travell and Simons’ Trigger Point
Manual (1999). The authors of this paper propose that there are two types of SM, resolving
apparent disputes in the SM focused literature, and offer an explanation of existing composite
pain referral patterns of SM based on innervation.
Discussion
Embryology and Innervation
In contrast to the traditional view on the embryology of the sternalis muscle, some
anatomists suggested that the sternalis muscle develops from an abnormal migration of the
pectoral mass. The pre-pectoral mass is located chiefly in the lower cervical region and is
located initially in the region of its nerve supply (Lewis, 1910). Cunningham and Huntington
(1904) believed that the pre-pectoral mass cleaves into a superficial (pectoralis major) and deep
layer (subclavius, pectoralis minor, pectoralis abdominalis [abdominal part of pectoralis minor]).
This mass then gradually migrates caudally to the costal region where it splits into two bundles:
the clavicular portion and the sternocostal portion (Huntington, 1904). The sternocostal portion
eventually differentiates into the sternocostal part of pectoralis major and pectoralis minor
(Huntington, 1904). An abnormal migration or cleavage of the pectoral mass may lead to the
formation of SM (Huntington, 1904).
According to traditional embryology, myotomes differentiate into a dorsal myogenic
column, the epimeres, and a ventral myogenic column, the hypomeres (Sadler, 2004). The
epimeres give rise to the deep muscles of the back (i.e. erector spinae) and are innervated by
the posterior primary rami of the spinal nerves (Sadler, 2004). The hypomeres form the lateral
and anterior muscles of the thorax and abdomen (i.e. external, internal, and innermost
intercostal muscles at the level of the thorax) and the flat muscles of the anterolateral abdominal
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wall (i.e. external oblique, internal oblique, transverse abdominis) (Sadler, 2004). The ventral
most tip of the hypomeres form the rectus abdominis muscle at the level of the abdomen and in
the cervical region, the scalene and infrahyoid strap muscles, occasionally they might develop at
the level of the thorax forming the sternalis muscle (Sadler, 2004). The hypomeric muscle
derivatives are supplied by the anterior branches of the 1st -5th thoracic nerves (Sadler, 2004).
From the perspective of nerve supply determining muscle origin, our review of the
literature suggests that there are two types of SM topographically occupying a similar body
region but differing in both their nerve supply and caudal attachments. Based on the theory that
a muscle is to be regarded as the end organ of a nerve, the homology of the muscle may be
investigated by tracing its nerve (Shinohara, 1996). In a correspondence by Shinohara (1996),
he summarized the concept of nerve-muscle specificity in to three laws of separation, fusion,
and migration: 1) when a single nerve supplies two different muscles, these muscles are
considered to have derived from a single muscle mass 2) when two different nerves supply a
single muscle, this muscle is considered to be a fusion of two muscle masses and 3) The route
of migration and origin of a muscle can be traced using the supplying nerve as an indicator. The
application of these laws to the SM suggests that there are, and accounts for, two types of SM
with different innervation, as well as for SMs that have been found to have two innervation
sources. This concept had also been suggested previously by Shepherd (1889).
This approach to rationalizing the origin of muscles utilizes the muscle as an end organ
of a nerve, the nerve-muscle specificity theory proposed by Fürbringer in 1888. Although there
is a lack of evidence to support this and existing evidence to refute it, there is no alternative
theory presently (Shinohara, 1996). As well, even if the theory is disproven, the concept is still
valuable to anatomists in determining muscle homology.
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In light of the two different approaches to the embryological development of SM, it may
be suggested that two types of SM can be formed, each with a well defined innervation: 1)
Innervation from the 1st-5th intercostal nerves suggests that the homology is related to the
anterior most hypomeric mass that at the level of the anterior abdominal wall forms rectus
abdominis 2) Innervation from the pectoral nerves, either the lateral pectoral nerve [C5, C6, C7]
or medial pectoral nerve [C8, T1], or both suggests that the homology is related to the flexor
group of muscles developing from the upper limb bud (Figure 1. Myotome masses Sternalis
Muscle may develop from). Integrating these two types of SM accounts for both innervation
sources that may otherwise be perceived as conflicting anatomical observations.
Figure 1. Myotome masses Sternalis Muscle may develop from (indicated by lines – upper line
depicts upper limb bud, lower line depicts hypomeric mass at the level of the anterior abdominal
wall)
Trigger Point Pain Referral Pattern
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Travell and Rinzler (1948) proposed a pain referral pattern of SM based on intercostal
innervation, while other authors later suggested trigger point referral charts that are a sum of all
SM’s studied, encompassing referral patterns from pectoral and 1st-5th intercostal innervation
sources of SM (Figure 1. Sternalis Muscle trigger point pain referral pattern) (Travell & Simons’,
1999; Davies & Davies, 2004). An important note made by some authors is the potential for SM
referral pain to mimic myocardial ischemic pain (Travell & Rinzler, 1948). Where these pain
referral patterns are described (ex. Manual therapy books), there is no clear evidence that an
SM was identified while charting trigger points; the only data provided are SM trigger points.
The authors consider it is appropriate to presume the presence of SM, and consequently pain
referral from SM, only if SM is clearly identified upon visual inspection, palpation, muscle testing,
sonography, mammography, CT, or surgery. A review of the old (1867-1930) literature reveals
only four SM observed in a living person (Cunningham, 1888; Kirk, 1925). Kirk (1925) observed
SM in a living male patient. According to Shepherd (1885) and Cunningham (1888), in 1867,
Malbrane was able to identify SM in two living subjects using electrical stimuli. In one of the
subjects, stimulation of the pectoral nerves caused contraction of SM, and in the other subject,
contraction of SM was brought on by stimulation of the intercostal nerves. Cunningham also
noted that Hallet’s 1848 paper quoted the existence of a well developed SM in an old man
whose inspiratory muscles had degenerated and been replaced with fat. Arraez-aybar (2003)
reported that Pichler (1911) described muscle testing SM by having the subject make a
scratching motion with their hand over the contralateral anterior superior iliac spine.
Based on the duality of nerve supply of SM, the authors suggest that pathology or
conditions arising from this muscle may be referred to its respective dermatomes. In the case of
trigger point pain referral, SMs innervated by the 1st-5th intercostal nerves will have a parasternal
referral pattern, while SMs innervated by the pectoral nerves (medial, lateral, or both) will have a
chest and upper limb referral pattern.
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The original pain referral pattern proposed by Travell and Rinzler (1948) can be
rationalized using the above approach. The pain referral pattern proposed by Travell and
Rinzler (1948) was T shaped extending laterally to both anterior shoulders from the manubrial
region of the thoracic wall and then caudally to the uppermost part of the epigastric region of the
abdominal wall (intercostal anterior cutaneous branch nerve supply). We divide this pain
referral pattern for SM into 2 parts: 1) from manubrium out laterally to the anterior shoulders and
2) from manubrium to epigastric region. More recently, in the Trigger Point Manual by Travell
and Simons’ (1999), the presented pain referral pattern of SM extends from the manubrium
laterally to the anterior shoulders, caudally to the epigastric region and caudally to the medial
epicondyle along the medial arms. This is an example of a composite illustration that blends
both types of sternalis muscle innervations, 1st-5th intercostal anterior branches and pectoral
nerves respectively, into one image.
Figure 2. Sternalis Muscle trigger point pain referral pattern
Another issue of debate concerns the 1st intercostal nerve; some anatomy textbooks
describe the 1st intercostal nerve as having no anterior cutaneous branch, while other authors
document the presence of an anterior cutaneous branch of the 1 st intercostal nerve in about 75
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% (Miyawaki, 2006) of the population. To support Travell and Simons’ (1999) pain referral
pattern for SM (Part 1 from above), there must be the presence of a T1 dermatome including the
anterior cutaneous branch of the 1st intercostal nerve. The parasternal part (Part 2) of Travell
and Simons’ (1999) pain referral pattern reflects innervation by anterior cutaneous branches of
T1-T5 intercostal nerves. This referral pattern is in agreement with an SM that originates as a
cranial expansion of the rectus abdominis column with according innervations from 1st-5th
intercostal nerves.
Summary
Applying the nerve-muscle specificity theory, the authors propose that there are two
types of SM: 1) SM derived from the pectoral mass and innervated by pectoral nerves (lateral
[C5, C6, C7] or medial [C8, T1] and 2) SM derived from the ventral most tip of the hypomeric
mass and innervated by anterior branches of 1st-5th intercostal nerves. The trigger point pain
referral patterns put forth by Travell and Simons’ (1999) can be rationalized by innervation. The
parasternal referral pattern occurs with anterior branch of 1st-5th intercostal nerves innervation,
and the manubrium to anterior shoulders referral pattern occurs with pectoral nerve and T1
intercostal nerve innervation. The concept of two types of SM explains apparent conflicting
anatomical observations and trigger point pain referral patterns that have been charted for SM.
Funding Sources and Conflicts of Interest
The authors have no funding sources or conflicts of interest to declare.
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