Download neuroanatomy of respiratory muscles

Neuro-anatomy of
Respiratory Muscles
BY
AHMAD YOUNES
PROFESSOR OF THORACIC MEDICINE
Mansoura faculty of medicine
Neuro-anatomy of respiratory Muscles
• The principal function of the lung is to ventilate the blood.
The alternating air flow to and from the alveolar surface is
driven by pressure gradients generated by the respiratory
muscles. In spite of their specific task which does not
allow them to rest during their entire life, the respiratory
muscles have the same structure and function as all other
limb and trunk muscles.
• The specialization of the respiratory muscles derives
directly from the characteristics of the fibers of which they
are composed.
• Respiratory muscle fibers, however, are not only highly
specialized for their functional tasks but are also able to
modify their properties to adapt to new requirements which
might arise from physiological conditions such as physical
exercise or from lung or respiratory diseases .
Structural properties of the respiratory muscles
• Skeletal muscles are composed of several motor units,
each with hundreds of muscle fibers.
• Three types of muscle fibers are usually present. They are
classified according to which isoform of myosin heavy
chain (MHC) is expressed.
• Slow-twitch (ST or Type I) fibers are identified by a slow
contraction time and a high resistance to fatigue.
Structurally, they have a small motor neuron and fiber
diameter, a high mitochondrial and capillary density, and a
high myoglobin content, they contain few of the enzymes
involved in glycolysis, but contain many of the enzymes
involved in the oxidative path ways. Functionally, ST fibers
are used for aerobic activities requiring low-level force
production .
Structural properties of the respiratory muscles
• Fast-twitch (FT or Type II) fibers are identified by a quick
contraction time and a low resistance to fatigue. Fast-twitch
fibers are further divided into fast-twitch A (FT-A or Type IIA)
and fast- twitch B (FT-B or Type IIB) fibers.
• FT-A fibers have a moderate resistance to fatigue and
represent a transition between the two extremes of the ST
and FT-B fibers. Structurally, FT-A fibers have a large motor
neuron and fiber diameter, a high mitochondrial density, a
medium capillary density, and a medium myoglobin content.
They have both a high glycolytic and oxidative enzyme
activity. Functionally, they are used for prolonged anaerobic
activities with a relatively high-force output .
• Fast-twitch B fibers, are very sensitive to fatigue and are used
for short anaerobic, high force production activities, Like the
FT-A fibers, FT-B fibers have a large motor neuron and fiber
diameter, but a low mitochondrial and capillary density and
myoglobin content. They also are high in creatine phosphate
and glycogen, but low in triglycerides. They contain many
glycolytic enzymes but few oxidative enzymes
Structural properties of the respiratory muscles
•The respiratory muscles are mixed muscles containing both
fast-twitch and slow-twitch fibers.
•In diaphragm; type I fiber represents approximately 50% of
the muscle fibers, type IIA about 20%, and type IIB about 30%.
• In intercostal muscles, the proportion of slow fibers is above
60% (that is, slightly higher than in the diaphragm) in both the
internal and the external intercostal muscles . The respiratory
muscles thus seem to be generally well equipped to sustain
continuous rhythmic contraction.
•The density of mitochondria in each of the three fiber types
tends to be greater than in the same fiber types in limb
muscles. The volume density of mitochondria in the
diaphragm is twofold greater than in the limb muscles.
Therefore, the oxygen uptake capacity of the diaphragm is
considerably greater than that of limb muscles.
Functional Properties of the respiratory muscles
• The maximal blood flow also considerably exceeds that of
limb muscles because of the greater capillary density, which
is about twice the capillary density in the limb muscle .
• The length of the muscle prior to the contraction affect the
strength of contraction as it determines how much overlap
there will be between actin and myocin and, thus, how many
cross-bridges can be formed .
• Maximal tension is generated at the optimal length. So, with
acute hyperinflation, the diaphragm shortens and its
capacity to generate force is concomitantly reduced .
• With fibers at the optimal length, the force developed by a
skeletal muscle is the function of the frequency of
stimulation. The frequency-force relationship results from
the summation of twitch force during repeated stimulation.
Slow muscles will show summation at lower frequency of
stimulation than fast muscles.
Functional Properties of the respiratory muscles
• The velocity of contraction is a direct function of myosin
ATPase activity, and, hence, the force-velocity curve is
primarily determined by the muscle fiber composition. The
respiratory muscles normally function at a low afterload, but
with increasing loads; as in case of increased resistance to
airflow, the velocity of contraction is reduced.
• The production of airflow into the lungs requires power
output by the muscles of respiration; consequently, the
ability to develop and sustain power is the most important
characteristic of respiratory muscle function. Power may be
calculated as the product of the values of velocity and force
according to the force-velocity relationship. Instantaneous
peak power occurs at 30 percent of maximal force and at 30
percent of maximal velocity. The frequency-power
relationship shows a similar dependency of force and power
upon frequency of stimulation.
Anatomy and action of the respiratory muscles
• The group of inspiratory muscles includes the
diaphragm, external intercostals, parasternal,
sternomastoid and scalene muscles.
• The group of expiratory muscles includes the
internal intercostal, rectus abdominis, external
and internal oblique and transverse abdominis
muscles .
• During low breathing effort (i.e. at rest) only the
inspiratory muscles are active. During high
breathing effort (i.e. exercise) the expiratory
muscles become active as well
The diaphragm
• The diaphragm muscle is composed of two domains .
The costal diaphragm is a thin domed sheet of muscle
composed of a radial array of myofibers extending
laterally from the ribs and medially to a central tendon. It
arises from the inner surfaces and upper margins of the
lower six ribs and sternum.
• The crural diaphragm is thicker and located more
posteriorly, where it attaches to the first three lumbar
vertebrae and the medial and lateral arcuate ligaments .
Medially, the myofibers of both the costal and crural
muscles insert into the central tendon. The central
tendon is located at the apex of the domed diaphragm,
holding the diaphragm muscle domains together.
Mechanism of CSA
The diaphragm
• Under normal circumstances, a zone of
apposition exists around the outside of the
diaphragm where it is in direct contact with the
internal aspect of the rib-cage, with fibers
arranged in a cranial-caudal direction, with no
lung in between, but the parietal pleura still
allowing free movement of the diaphragm .
• At upright functional residual capacity (FRC) in
humans, approximately 55% of the diaphragm
surface area is in the zone of apposition .
The diaphragm
• The diaphragm receives its entire motor supply from the
phrenic nerve from cervical segments 3, 4, and 5.
• The sensory nerve fibers from the central part of the
diaphragm also run in the phrenic nerve, while the
peripheral part, including the crura, receives sensory fibers
from the lower intercostal nerves .
• The diaphragm has an abundant blood supply derived from
the phrenic and intercostal arteries and from branches of the
internal thoracic (mammary) arteries. Flow can increase to
approximately 250 mL/min/100 g of muscle during maximal
activation (about half of the maximal blood flow to the
heart).In comparison with other skeletal muscles, the
diaphragm is extremely active.
• Muscle fibers within the diaphragm can reduce their length
up to 40% between RV and TLC .
Diaphragmatic contraction increases chest wall
dimensions because of three distinct reasons.
• First, diaphragmatic descent increases the craniocaudal
dimensions of the thorax. This may be considered using
a "piston in a cylinder" analogy, the trunk representing
the cylinder and the diaphragm the piston .
• The first possible mechanism is involving downward
movement of the diaphragm simply by shortening the
zone of apposition around the whole cylinder and
leaving the dome shape unchanged.
• This is a pure "piston-like" action and has the advantage
of being the most energy efficient way of converting
diaphragm contraction into lung expansion.
“Piston in a cylinder” analogy of the mechanisms of diaphragm actions on the
lung volume. (A) Resting end-expiratory position. (B) Inspiration with pure
piston-like behavior. (C) Inspiration with pure non-piston-like behavior. (D)
Combination of piston-like and non-piston-like behavior in an expanding
cylinder, which equates most closely with inspiration in vivo. ZA, zone of
apposition.
Non-piston-like" behavior
• "Non-piston-like" behavior is the second possible
mechanism in which zone of apposition remains
unchanged but an increase in tension of the diaphragm
dome reduces its curvature, so expanding the lung . This
is likely to be less efficient than piston-like behavior
because much of muscle tension developed simply
opposes the opposite side of the diaphragm rather than
moving the diaphragm downward, such that in theory,
when the diaphragm becomes flat, further contraction will
have no effect on lung volume .
• Finally, both types of behavior already described but also
includes expansion of the lower ribcage (known as "piston
in an expanding cylinder") that occurs with diaphragmatic
contraction particularly in supine position .
Third, because the muscle fibers of the costal diaphragm insert
onto the upper margins of the lower six ribs, they also apply a
force on these ribs when they contract, and the cranial
orientation of these fibers is such that this force is directed
cranially. It therefore has the effect of lifting the ribs and
rotating them outward . This is the insertional component of
diaphragmatic contraction.
• During inspiration, as the fibers of the
costal diaphragm contract, they exert
a force on the lower ribs (arrow). If
the abdominal visceral mass
effectively opposes the descent of
the diaphragmatic dome (open
arrow), this force is oriented cranially.
Consequently, the lower ribs are lifted
and rotate outward .
Intercostal Muscles
• The external intercostal muscle forms the most superficial
layer. Its fibers are directed downward and forward from the
inferior border of the rib above to the superior border of the
rib below . The lower insertion of the external intercostals
muscles is more distant from the ribs axis of rotation than
the upper one, and as a result, contraction of this muscle
exerts a larger torque acting on the lower rib which raises of
the lower rib with respect to the upper one. The net effect of
the contraction of these muscles raises the rib cage.
• The internal intercostal muscle forms the intermediate layer.
Its fibers are directed downward and backward from the
subcostal groove of the rib above to the upper border of the
rib below.
Intercostal Muscles
• The innermost intercostal muscle forms the deepest layer.
It is an incomplete muscle layer and crosses more than
one intercostal space within the ribs. It is related internally
to endothoracic fascia and parietal pleura and externally to
the intercostal nerves and vessels .
• Between the chondral portions of the ribs and the
sternum; only one layer of intercostal muscles, the
parasternal intercostals, is present. Dorsally, from the
angles of the ribs to the vertebrae, the only fibers come
from the external intercostals muscles. These latter,
however, are duplicated in each interspace by a thin,
spindle-shaped muscle that runs from the tip of the
transverse process of the vertebra cranially to the angle of
the rib caudally; this muscle is the “levator costae”.
• All intercostal muscles are innervated by the intercostals
nerves .
LEVATORES COSTARUM
• Origin: Vertebrae (C7, T1-11
transverse processes)
• Insert: Ribs (below origin)
• Action: Raise ribs in
Inspiration
• Innervation: Dorsal primary
rami of thoracic spinal
nerves
Intercostal Muscles
• External intercostals are inspiratory in action, and the
internal intercostals are expiratory in action.
• As the fibers of the external intercostal muscle slope
downward and forward from the rib above to the rib
below, their lower insertion is more distant from the
center of rotation of the ribs (the costovertebral
articulations) than their upper insertion. Consequently,
when this muscle contracts, the torque acting on the
lower rib is greater than that acting on the upper rib, so
its net effect would be to raise the ribs and to inflate the
lung . The elevation of the ribs in this way increases
both the lateral and anteroposterior diameter of the
ribcage resulting in a ‘bucket handle’ action .
Diagram illustrating the actions of the intercostal
muscles. The hatched area in each panel represents the
spine, and the two bars oriented obliquely represent
two adjacent ribs, these are linked to each other by the
sternum (right).
• The external (A) and
internal (B) intercostal
muscles are depicted as
single bundles, and the
torques acting on the ribs
during contraction of
these muscles are
represented by arrows .
Intercostal Muscles
• As the fibers of the internal intercostals muscle
slope downward and backward from the rib
above to the rib below, their lower insertion is
less distant from the center of rotation of the
ribs than the upper one. As a result, when this
muscle contracts, the torque acting on the lower
rib is smaller than that acting on the upper rib,
so its net effect would be to lower the ribs and
to deflate the lung .
Bucket handle movement of ribs
The scalene muscles
• Three pairs of muscles in the lateral neck, namely
the anterior scalene, middle scalene, and posterior scalene.
• They originate from the transverse processes from
the cervical vertebrae of C2 to C7 and insert onto the first
and second ribs. Thus they are called the lateral vertebral
muscles.[3]
• They are innervated by the fourth, fifth, and sixth
cervical spinal nerves (C4-C6).
• The action of the anterior and middle scalene muscles is to
elevate the first rib and laterally flex (bend) the neck to the
same side;the action of the posterior scalene is to elevate
the second rib and tilt the neck to the same side.
• They also act as accessory muscles of inspiration, along
with the sternocleidomastoids.
The scalene muscles
• The action of these muscles is to raise the first two ribs.
The orientation of their axis in the neck causes upward
motion of these ribs (“pump handle” motion) . Moreover,
the scalenes are consistently active during quiet
breathing in normal individuals and contribute to chest
wall expansion.
• They may be very important in the case of spinal cord
injury. When the injury is below C4-C8, the scalenes’
function is entirely or partially preserved, and they
contribute importantly to upper rib cage motion in these
patients.
Pump handle movement of ribs and sternum
Sternocleidomastoid
• The muscle is attached inferiorly by two heads. The
medial or sternal head, arises from the upper part of the
anterior surface of the manubrium sterni. The lateral or
clavicular head, ascends almost vertically from the
superior surface of the medial third of the clavicle.
• It inserts superiorly by a strong tendon into the lateral
surface of the mastoid process from its apex to its
superior border, and by a thin aponeurosis into the
lateral half of the superior nuchal line.
• It is supplied by the spinal part of the accessory nerve.
Branches from the ventral rami of the second, third, and
sometimes fourth, cervical spinal nerves also enter the
muscle .
Sternocleidomastoid
• In humans, these muscles are electrically silent during
quiet breathing, but they may be recruited with increased
ventilatory load.
• These muscles are particularly important in high
quadriplegics. They also may be recruited in patients with
poliomyelitis and diaphragmatic dysfunction.
• These muscles are thought to be important in moving the
upper rib cage in patients with COPD, even though a
clinical experimental study failed to demonstrate
consistent activity in these muscles in these patients
The Shoulder Girdle and Neck Muscles
• Several shoulder girdle and neck muscles may contribute
to inspiration under particular circumstances.
• Most of these muscles run from the rib cage to an
extrathoracic extension. When the rib cage is fixed in the
lean-forward position— a position commonly employed
by patients with COPD—these muscles contribute to
expansion of the rib cage during inspiration.
• Muscles that may contribute to inspiration include the
trapezius, latissimus dorsi, pectoralis major and minor,
erector spinae, teres major, serratus anterior, platysma,
mylohyoid, and sternohyoid.
• Since these muscles commonly contribute to inspiration
in patients with severe airflow obstruction, using these
muscles for other activities, such as hair combing, may
considerably increase dyspnea in these patients .
Transversus thoracis
• Transversus thoracis (also called the triangularis sterni)
spreads over the internal surface of the anterior thoracic wall
• It arises from the lower one-third of the posterior surface of
the sternum, the xiphoid process and the costal cartilages of
the lower three or four true ribs near their sternal ends.
• Its fibres diverge and ascend laterally as slips that pass into
the lower borders and inner surfaces of the costal cartilages
of the second, third, fourth, fifth and sixth ribs.
• The lowest fibres are horizontal, and are contiguous with the
highest fibres of transversus abdominis; the intermediate
fibres are oblique; the highest are almost vertical.
• It is supplied by the adjacent intercostal nerves.
Transversus thoracis
• The transversus thoracis is the most important
expiratory muscle of the rib cage, and its action
is to lower the ribs relative to the sternum and
thus to cause expiration.
• It is electrically silent in humans breathing
quietly, but it is recruited during speech,
laughing, or expiration below FRC .
The muscles of the anterior abdominal wall
• The rectus abdominis arises from the 5th, 6th and 7th
costal cartilages and is inserted into the crest of the pubis.
• The external oblique arises from the outer surfaces of the
lower eight ribs and fans out into the xiphoid, linea alba,
the pubic crest, pubic tubercle and the anterior half of the
iliac crest.
• The internal oblique arises from the lumbar fascia, the
anterior two-thirds of the iliac crest and the lateral twothirds of the inguinal ligament. It is inserted into the lowest
six costal cartilages, linea alba and the pubic crest.
• The transversus abdominis arises from the lowest six
costal cartilages (interdigitating with the diaphragm), the
lumbar fascia, the anterior two thirds of the iliac crest and
the lateral one-third of the inguinal ligament; it is inserted
into the linea alba and the pubic crest.
The muscles of the anterior abdominal wall
• With the exception of gas within the bowel
lumen, the abdomen is an incompressible
volume held between the diaphragm and the
abdominal muscles.
• Contraction of either will cause a corresponding
passive displacement of the other. Thus
abdominal muscles are generally expiratory.
Contraction of these muscles results in an
increase in abdominal pressure displacing the
diaphragm in a cephalad direction.
• Their insertion into the costal margin results in
a caudal movement of the ribcage, so assisting
expiration by opposing the ribcage muscles .
The Shoulder Girdle and Neck Muscles
• Several shoulder girdle and neck muscles may contribute to
inspiration under particular circumstances. Most of these
muscles run from the rib cage to an extrathoracic extension.
• When the rib cage is fixed in the lean-forward position— a
position commonly employed by patients with COPD—these
muscles contribute to expansion of the rib cage during
inspiration.
• Muscles that may contribute to inspiration include the
trapezius, latissimus dorsi, pectoralis major and minor,
erector spinae, teres major, serratus anterior, platysma,
mylohyoid, and sternohyoid.
• Since these muscles commonly contribute to inspiration in
patients with severe airflow obstruction, using these
muscles for other activities, such as hair combing, may
considerably increase dyspnea in these patients .
Extrinsic Shoulder Muscles
Extrinsic Shoulder Muscles
Muscles of the Pelvic Floor: Pelvic Diaphragm