Masseter Muscle

Overview

Parts and Attachments

Nerve Supply

• Motor nerve: Masseteric nerve - a branch of the mandibular nerve [CN V3]
• The masseteric nerve originates in the infratemporal fossa and passes laterally over the mandibular notch to enter the deep surface of the muscle.
• Blood supply runs alongside: the masseteric artery (from the maxillary artery) travels the same route.

All muscles of mastication (except geniohyoid, which is C1) are innervated by branches of CN V3 arising in the infratemporal fossa. - Gray's Anatomy for Students

Actions

Masseter - Elevation and protrusion of the mandible.

Summary Table

Rima Glottidis

Definition

• Laterally: the vocal folds (true vocal cords) and the mucosa-covered arytenoid cartilages
• Anteriorly: the apex (pointed end, at the anterior commissure)
• Posteriorly: the base, formed by the interarytenoid fold of mucosa at the bottom of the interarytenoid notch

Two Parts

Shape Changes with Function

Opening and Closing

• Posterior cricoarytenoid - the ONLY abductor; widens the entire rima glottidis by rotating the vocal processes laterally
• Lateral cricoarytenoid - adducts (closes) the intermembranous part
• Transverse and oblique arytenoids - adduct the intercartilaginous part
• Thyroarytenoid / vocalis - tenses/relaxes vocal folds

Phonation Mechanism

1. Arytenoid cartilages and vocal folds are adducted - closing the rima glottidis
2. Vocal ligaments are tensed (by cricothyroid muscle)
3. An expiratory stream of air forces the closed rima glottidis open
4. The vocal folds vibrate, producing sound waves
5. Volume depends on force of airstream; pitch depends on vibration frequency, which varies with length, thickness, and tension of the vocal ligaments

Clinical Significance

• Laryngospasm: Involuntary closure of the rima glottidis (e.g. triggered by foreign body, water, or irritants) can cause life-threatening airway obstruction
• Bilateral recurrent laryngeal nerve (RLN) injury: Paralyzed vocal folds meet in the rima glottidis, causing stridor and respiratory distress - may require tracheostomy
• Cricothyrotomy: If the rima glottidis is life-threateningly closed, an airway can be established by incision through the median cricothyroid ligament, which lies just below the level of the rima glottidis
• Laryngoscopy: Direct visualization of the rima glottidis (the image is inverted - anterior structures appear at the top)

Rima Glottidis - Summary

• Narrowest part of the laryngeal cavity
• Opening between the two vocal folds (true vocal cords) and the arytenoid cartilages
• Separates the middle laryngeal chamber (above) from the infraglottic cavity (below)

• Intermembranous part (anterior, longer) - between the two vocal folds, overlying vocal ligaments
• Intercartilaginous part (posterior, shorter) - between the two arytenoid cartilages

• Laterally - vocal folds + mucosa-covered arytenoid cartilages
• Apex (anterior) - anterior commissure
• Base (posterior) - interarytenoid fold at the interarytenoid notch

• Quiet respiration - triangular
• Forced inspiration - rhomboid (wide open)
• Phonation - first closed, then opened by expiratory airstream
• Whispering - only intercartilaginous part open
• Effort closure / swallowing - completely closed

• Opened by - posterior cricoarytenoid (only abductor)
• Closed by - lateral cricoarytenoid, transverse and oblique arytenoids
• Tension adjusted by - cricothyroid and vocalis muscles

• Vocal folds adducted and tensed → expiratory air forces them apart → vibration → sound
• Pitch = depends on length, tension, thickness of vocal ligaments
• Volume = depends on force of airstream

• Laryngospasm - involuntary closure, can be life-threatening
• Bilateral RLN palsy - folds meet in midline → stridor → may need tracheostomy
• Cricothyrotomy - performed just below rima glottidis level through cricothyroid membrane
• Visualized directly by laryngoscopy

Triceps Surae

Definition

• Triceps surae = collective name for the three-headed muscle group forming the bulk of the calf
• Comprises: gastrocnemius (medial head + lateral head) + soleus
• Located in the superficial posterior compartment of the leg
• Plantaris is sometimes considered the "4th head" due to its close association

Components, Origin & Insertion

Actions

• Soleus is the more powerful plantarflexor during walking (acts when knee is extended or flexed)
• Gastrocnemius is most effective as a plantarflexor when the knee is extended (since it crosses both joints)

Nerve Supply

• All three heads: Tibial nerve (S1, S2)

Calcaneal (Achilles') Tendon

• Common tendon of insertion of all three heads
• Average length: 20-25 cm; cross-sectional area ~70-80 mm²
• Breaking strength: 60-100 N/mm² (can bear ~1 ton when healthy)
• Thinnest blood supply at 2-6 cm proximal to calcaneal insertion - most common site of rupture
• Rupture signs: "whiplike snap," loss of active plantarflexion, palpable gap in tendon
• Tendon lies below skin with no muscular protection - vulnerable to direct trauma and overuse

Soleus - Special Features

• Does not cross the knee joint - acts as a plantarflexor regardless of knee position
• Its tendinous arch spans over the popliteal vessels and tibial nerve as they pass into the deep posterior compartment
• Acts as a peripheral venous pump ("peripheral heart") - important in venous return from the lower limb

Clinical Points

• Achilles tendon rupture: Sudden load, chronic overuse, fluoroquinolone use, or steroid injections predispose to rupture - typically 2-6 cm above insertion
• Calcaneus gait: Weakness of triceps surae (gastrocnemius-soleus) causes increased ankle dorsiflexion at heel strike
• DVT risk: Immobilization impairs soleus pump, increasing deep vein thrombosis risk
• Reflex: S1 ankle (Achilles) jerk tests the triceps surae - absence suggests S1 radiculopathy

Triceps Surae - Short Note

• Group of 3 muscles forming the calf bulk, in the superficial posterior compartment of the leg

• Gastrocnemius (medial + lateral heads)
• Soleus
• Plantaris is sometimes considered the 4th head

• Gastrocnemius medial head - medial femoral condyle
• Gastrocnemius lateral head - lateral femoral condyle
• Soleus - soleal line of tibia + posterior head/neck of fibula (joined by a tendinous arch)

• All three - calcaneal (Achilles') tendon → posterior surface of calcaneus

• Plantarflexion of foot (all three)
• Flexion of knee (gastrocnemius only - as it crosses the knee)

• Tibial nerve (S1, S2)

• Common tendon of all three heads
• Most vulnerable at 2-6 cm above insertion (least blood supply - commonest rupture site)
• Rupture presents with a snap, palpable gap, and loss of active plantarflexion

• S1 ankle jerk tests triceps surae
• Soleus acts as a peripheral venous pump - immobility increases DVT risk
• Calcaneus gait = triceps surae weakness
• Fluoroquinolones and steroid injections predispose to Achilles tendon rupture

Biceps Femoris

• Lateral muscle of the posterior compartment of the thigh
• Part of the hamstring group (along with semitendinosus and semimembranosus)
• Has two heads - long head and short head

Origin

Insertion

• Head of the fibula (lateral surface)
• Extensions blend with the fibular collateral ligament and lateral knee joint ligaments

Nerve Supply

Key point: The two heads have different nerve supplies - the only hamstring muscle where this occurs.

Actions

Biceps Femoris

• Long head - ischial tuberosity (common with semitendinosus) + sacrotuberous ligament
• Short head - lateral lip of linea aspera (femoral shaft)

• Head of fibula

• Long head - tibial division of sciatic nerve (L5, S1, S2)
• Short head - common fibular division of sciatic nerve (L5, S1, S2)
• Only hamstring with dual nerve supply

• Knee - flexion + lateral rotation of leg (both heads)
• Hip - extension + lateral rotation of thigh (long head only)

Ejection Fraction (EF)

• The fraction (percentage) of blood in the ventricle that is ejected with each heartbeat
• Measures the effectiveness of ventricular contraction

EF = Stroke Volume / End-Diastolic Volume × 100

• Stroke Volume = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
• So: EF = (EDV - ESV) / EDV × 100

• EDV = 140 mL, ESV = 70 mL
• SV = 140 - 70 = 70 mL
• EF = 70/140 = 0.5 = 50%

• Approximately 55-70% (normal LV EF)
• Meaning: about 55-70% of blood in the ventricle is pumped out each beat; the rest remains

• EF is an indicator of myocardial contractility
• ↑ EF = increased contractility (e.g. sympathetic stimulation)
• ↓ EF = decreased contractility (e.g. heart failure, cardiomyopathy)

• Echocardiography (most common)
• Also used to guide ICD and cardiac resynchronization therapy (CRT) implantation