Nerve supply and action of masseter
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Here is a complete account of the masseter muscle's nerve supply and actions, drawn from Gray's Anatomy for Students and Cummings Otolaryngology:
Masseter Muscle
Fig. 8.144 - Masseter muscle (Gray's Anatomy for Students)
Overview
The masseter is a powerful, quadrangular muscle of mastication overlying the lateral surface of the ramus of the mandible. It is anchored above to the zygomatic arch and below to the lateral surface of the ramus.
Parts and Attachments
| Part | Origin | Insertion |
|---|---|---|
| Superficial | Maxillary process of zygomatic bone + zygomatic arch | Angle of mandible + posterior ramus (lateral surface) |
| Deep | Medial aspect of zygomatic arch + posterior inferior margin | Central and upper ramus up to the coronoid process |
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
The masseter performs two main movements of the mandible:
| Action | Notes |
|---|---|
| Elevation (jaw closure) | Primary and most powerful action - closes the mouth, generates biting force |
| Protrusion | Contraction also protrudes the mandible (forward movement) |
This is contrasted with the temporalis, which also elevates but retracts (not protrudes) the mandible, and the medial pterygoid, which only elevates.
From Cummings Otolaryngology (Table 86.2):
Masseter - Elevation and protrusion of the mandible.
The masseter, temporalis, and medial pterygoid are the three primary jaw-closing muscles. The masseter is especially adapted for sustained loading during mastication, with a high proportion of slow and hybrid myosin heavy-chain (MHC) isoforms including MHC-fetal and MHC-cardiac-α.
Summary Table
| Feature | Detail |
|---|---|
| Nerve | Masseteric nerve (CN V3 - mandibular division of trigeminal) |
| Nerve pathway | Infratemporal fossa → over mandibular notch → deep surface of masseter |
| Primary action | Elevation of mandible (jaw closure) |
| Secondary action | Protrusion of mandible |
| Blood supply | Masseteric artery (from maxillary artery) |
Sources: Gray's Anatomy for Students, p. 1117; Cummings Otolaryngology Head and Neck Surgery, p. 1560
Rima glottidis
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Here is a complete account of the rima glottidis from Gray's Anatomy for Students and Color Atlas of Human Anatomy:
Rima Glottidis
Fig. 8.224 - Laryngeal Cavity. (C) Superior view showing rima vestibuli and rima glottidis. (D) Laryngoscopic photographs - the rima glottidis is the opening between the true vocal cords.
Definition
The rima glottidis is the narrowest part of the laryngeal cavity - a triangular opening (when open) bounded by:
- 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
It separates the middle chamber of the larynx (above) from the infraglottic cavity (below).
Two Parts
The rima glottidis is divided into two portions:
| Part | Also Called | Boundaries | Notes |
|---|---|---|---|
| Anterior (longer) | Intermembranous part | Between the two vocal folds (overlying the vocal ligaments) | Involved in phonation |
| Posterior (shorter) | Intercartilaginous part | Between the two arytenoid cartilages | Involved in respiration |
Shape Changes with Function
The shape of the rima glottidis changes dynamically depending on the activity:
| Activity | Shape / State of Rima Glottidis |
|---|---|
| Quiet respiration | Triangular; arytenoids abducted; intercartilaginous part open |
| Forced/deep inspiration | Wide open - rhomboid shape; vocal folds fully abducted by posterior cricoarytenoid muscles |
| Phonation | First closed (vocal folds adducted); then opened by expiratory airstream causing vocal fold vibration |
| Whispering | Intermembranous part closed; intercartilaginous part forms a small triangular opening |
| Effort closure (e.g. coughing, lifting) | Completely closed - both rima glottidis and rima vestibuli shut |
| Foreign body entry / laryngospasm | Involuntary closure |
Opening and Closing
Both the rima glottidis and the rima vestibuli can be opened and closed by movement of the arytenoid cartilages and the associated fibro-elastic membranes. The key muscles involved are:
- 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
When phonating:
- Arytenoid cartilages and vocal folds are adducted - closing the rima glottidis
- Vocal ligaments are tensed (by cricothyroid muscle)
- An expiratory stream of air forces the closed rima glottidis open
- The vocal folds vibrate, producing sound waves
- 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)
Sources: Gray's Anatomy for Students, pp. 1199-1204; Color Atlas of Human Anatomy Vol. 2 (Internal Organs), p. 202
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Rima Glottidis - Summary
Definition
- 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)
Parts
- Intermembranous part (anterior, longer) - between the two vocal folds, overlying vocal ligaments
- Intercartilaginous part (posterior, shorter) - between the two arytenoid cartilages
Boundaries
- Laterally - vocal folds + mucosa-covered arytenoid cartilages
- Apex (anterior) - anterior commissure
- Base (posterior) - interarytenoid fold at the interarytenoid notch
Shape Changes
- 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
Opening & Closing
- Opened by - posterior cricoarytenoid (only abductor)
- Closed by - lateral cricoarytenoid, transverse and oblique arytenoids
- Tension adjusted by - cricothyroid and vocalis muscles
Phonation
- 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
Clinical Points
- 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
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