Ch.11 Muscles Notes

How Skeletal Muscles Produce Movement

• Skeletal muscles, attached to bones via tendons, produce movements by exerting force. This force leads to the bones moving relative to each other at joints.

• Tendons, composed of dense connective tissue, transmit the force of muscle contraction to bones, causing movement. The efficiency and strength of muscle action depend on the coordination between muscles, bones, and joints.

Origin and Insertion

• Most muscles cross at least one joint, attaching to the articulating bones that form the joint. This arrangement allows muscles to facilitate movement across the joint.

• When a muscle contracts, it draws one articulating bone toward the other. The specific movement depends on the muscle's location relative to the joint and the type of joint.

  • Origin: The attachment of the muscle to the stationary bone, typically proximal. The origin provides a stable base during muscle contraction.

  • Insertion: The attachment of the muscle to the movable bone, typically distal. The insertion point is where the muscle's force is applied to create movement.

Lever Systems and Leverage

• Bones act as levers, and joints serve as fulcrums in the musculoskeletal system. This lever system amplifies the effect of muscle contraction.

• Lever: A rigid structure (bone) that can move around a fixed point (fulcrum/joint). The lever system enhances the distance and speed of movement produced by muscles.

• The lever is acted on by:

  • Load (Resistance): The weight or resistance that the lever must overcome. This can include the weight of the body part being moved or an external load.

  • Effort: The force exerted by the muscle contraction to move the lever. The amount of effort required depends on the placement of the fulcrum, load, and effort.

Types of Levers

• First-class lever: Fulcrum (F) is positioned between the effort (E) and the load (L), like a seesaw. Examples in the body are rare but include the atlanto-occipital joint controlling head movement.

• Second-class lever: Load (L) is between the fulcrum (F) and the effort (E), like a wheelbarrow. These levers favor force production over speed and range of motion. An example is the calf muscle lifting the body onto the toes.

• Third-class lever: Effort (E) is between the fulcrum (F) and the load (L), like tweezers. This is the most common type of lever in the body, favoring speed and range of motion over force. An example is the biceps brachii flexing the forearm.

Effects of Fascicle Arrangement

• Muscle fibers are arranged in parallel bundles within fascicles, but the arrangement of fasciculi in relation to the tendon can vary. This arrangement influences the muscle's force and range of motion.

• Fascicular arrangement is correlated with:

  • The amount of power a muscle can produce: Pennate arrangements (unipennate, bipennate, multipennate) generally produce more force due to a higher density of muscle fibers.

  • The range of motion a muscle can produce: Parallel and fusiform arrangements allow for a greater range of motion because the muscle fibers can contract over a longer distance.

Arrangement of Fascicles

• Parallel:

  • Muscle fascicles run parallel to the longitudinal axis of the muscle, terminating at either end in flat tendons. These muscles are designed for speed and range of motion.

  • Example: Sternohyoid muscle (see Figure 11.8a)

• Fusiform:

  • Muscle fascicles are nearly parallel to the longitudinal axis of the muscle, tapering toward the tendons, where the diameter is less than at the belly. These muscles combine good power with moderate range of motion.

  • Example: Digastric muscle (see Figure 11.8a)

• Circular:

  • Muscle fascicles form concentric circular arrangements, creating sphincter muscles that enclose an orifice (opening). These muscles control the opening and closing of body passages.

  • Example: Orbicularis oculi muscle (see Figure 11.4a)

• Triangular (Convergent):

  • Muscle fascicles spread over a broad area and converge at a thick central tendon, giving the muscle a triangular appearance. These muscles allow for versatile force application.

  • Example: Pectoralis major muscle (see Figure 11.3a)

• Unipennate:

  • Muscle fascicles are arranged on only one side of the tendon. These muscles are strong for their size.

  • Example: Extensor digitorum longus muscle (see Figure 11.22b)

• Bipennate:

  • Muscle fascicles are arranged on both sides of a centrally positioned tendon. These muscles are even stronger than unipennate muscles.

  • Example: Rectus femoris muscle (see Figure 11.20a)

• Multipennate:

  • Muscle fascicles attach obliquely from many directions to several tendons. These muscles generate the highest force but have a limited range of motion.

  • Example: Deltoid muscle (see Figure 11.10a)

Coordination Within Muscle Groups

• Most muscle movements are coordinated by several skeletal muscles acting in groups rather than individually. This ensures smooth, controlled, and efficient movements.

• Most skeletal muscles are arranged in opposing pairs at joints, providing balance and control over movements.

  • Agonist/prime mover: The muscle primarily responsible for producing a specific movement. e.g., Biceps brachii during elbow flexion

  • Antagonist: The muscle that opposes or reverses the action of the agonist. It relaxes or contracts eccentrically to control the movement. e.g., Triceps brachii during elbow flexion

  • Synergist: Muscles that assist the agonist by stabilizing the joint or providing additional force, fine-tuning the direction of movement. e.g., Brachialis assisting biceps brachii during elbow flexion

How Skeletal Muscles are Named

• A muscle may be named based on:

  • Location: Reflects the anatomical region where the muscle is found. e.g., Temporalis (near the temporal bone)

  • Size: Indicates the relative dimensions of the muscle. e.g., Gluteus maximus (largest of the gluteal muscles)

  • Number of origins: Specifies how many tendons of origin the muscle has. e.g., Biceps brachii (two origins)

  • Appearance: Describes the shape or visual characteristics of the muscle. e.g., Deltoid (triangular)

  • Direction of fibers: Indicates the orientation of the muscle fibers relative to the body's midline. e.g., Rectus abdominis (fibers run parallel to the midline)

  • Origin and insertion: Details the sites where the muscle originates and inserts. e.g., Sternocleidomastoid (originates on the sternum and clavicle, inserts on the mastoid process)

  • Muscle action: Describes the primary action of the muscle. e.g., Flexor carpi ulnaris (flexes the wrist)

Naming Muscles - Direction

• Rectus: Parallel to midline; e.g., Rectus abdominis (Figure 11.10b). The fibers run straight up and down.

• Transverse: Perpendicular to midline; e.g., Transversus abdominis (Figure 11.10b). The fibers run horizontally across the abdomen.

• Oblique: Diagonal to midline; e.g., External abdominal oblique (Figure 11.10a). The fibers run at an angle to the midline.

Naming Muscles - Size

• Maximus: Largest; e.g., Gluteus maximus (Figure 11.20c). This muscle is the largest in the gluteal region.

• Minimus: Smallest; e.g., Gluteus minimus (Figure 11.20d). This muscle is the smallest in the gluteal region.

• Longus: Long; e.g., Adductor longus (Figure 11.20a). This muscle is long relative to other adductor muscles.

• Brevis: Short; e.g., Adductor brevis (Figure 11.20b). This muscle is short relative to other adductor muscles.

• Latissimus: Widest; e.g., Latissimus dorsi (Figure 11.15b). This muscle covers a wide area of the back.

• Longissimus: Longest; e.g., Longissimus capitis (Figure 11.19a). This muscle is the longest of the capitis muscles.

• Magnus: Large; e.g., Adductor magnus (Figure 11.20b). This muscle is large and powerful.

• Major: Larger; e.g., Pectoralis major (Figure 11.10a). This muscle is larger than the pectoralis minor.

• Minor: Smaller; e.g., Pectoralis minor (Figure 11.14a). This muscle is smaller than the pectoralis major.

• Vastus: Huge; e.g., Vastus lateralis (Figure 11.20a). This muscle is a large component of the quadriceps femoris.

Naming Muscles - Shape

• Deltoid: Triangular; e.g., Deltoid (Figure 11.15b). This muscle has a triangular shape.

• Trapezius: Trapezoid; e.g., Trapezius (Figure 11.3b). This muscle has a trapezoidal shape.

• Serratus: Saw-toothed; e.g., Serratus anterior (Figure 11.14b). This muscle has a serrated, or saw-toothed, appearance.

• Rhomboid: Diamond-shaped; e.g., Rhomboid major (Figure 11.15c). This muscle has a diamond shape.

• Orbicularis: Circular; e.g., Orbicularis oculi (Figure 11.4a). This muscle has a circular arrangement around the eye.

• Pectinate: Comblike; e.g., Pectineus (Figure 11.20a). This muscle has a comblike appearance.

• Piriformis: Pear-shaped; e.g., Piriformis (Figure 11.20d). This muscle has a pear shape.

• Platys: Flat; e.g., Platysma (Figure 11.4c). This muscle is a flat, sheet-like muscle.

• Quadratus: Square, four-sided; e.g., Quadratus femoris (Figure 11.20d). This muscle has a square shape.

• Gracilis: Slender; e.g., Gracilis (Figure 11.20a). This muscle is slender and thin.

Naming Muscles - Action

• Flexor: Decreases joint angle; e.g., Flexor carpi radialis (Figure 11.17a). This muscle flexes the wrist.

• Extensor: Increases joint angle; e.g., Extensor carpi ulnaris (Figure 11.17d). This muscle extends the wrist.

• Abductor: Moves bone away from midline; e.g., Abductor pollicis longus (Figure 11.17e). This muscle abducts the thumb.

• Adductor: Moves bone closer to midline; e.g., Adductor longus (Figure 11.20a). This muscle adducts the thigh.

• Levator: Raises or elevates body part; e.g., Levator scapulae (Figure 11.14a). This muscle elevates the scapula.

• Depressor: Lowers or depresses body part; e.g., Depressor labii inferioris (Figure 11.4a). This muscle depresses the lower lip.

• Supinator: Turns palm anteriorly; e.g., Supinator (Figure 11.17c). This muscle supinates the forearm.

• Pronator: Turns palm posteriorly; e.g., Pronator teres (Figure 11.17a). This muscle pronates the forearm.

• Sphincter: Decreases size of an opening; e.g., External anal sphincter (Figure 11.12). This muscle constricts the anal opening.

• Tensor: Makes body part rigid; e.g., Tensor fasciae latae (Figure 11.20a). This muscle tenses the fascia lata.

• Rotator: Rotates bone around longitudinal axis; e.g., Rotatore (Figure 11.19b). These muscles rotate the vertebral column.

Naming Muscles - Number of Origins

• Biceps: Two origins; e.g., Biceps brachii (Figure 11.16a). This muscle has two heads of origin.

• Triceps: Three origins; e.g., Triceps brachii (Figure 11.16b). This muscle has three heads of origin.

• Quadriceps: Four origins; e.g., Quadriceps femoris (Figure 11.20a). This muscle group has four heads of origin.

Naming Muscles - Location

• Temporalis: Muscle near temporal bone (Figure 11.4c). This muscle is located in the temporal region of the skull.

Naming Muscles - Origin and Insertion

• Sternocleidomastoid: Originating on sternum and clavicle and inserting on mastoid process of temporal bone (Figure 11.3a). This muscle’s name reflects its origin and insertion points.

Superficial/Anterior Skeletal Muscles

• Listing of muscles including: Occipitofrontalis (frontal belly), Nasalis, Orbicularis oris, Depressor anguli oris, Platysma, Epicranial aponeurosis, Temporalis, Orbicularis oculi, Masseter, Omohyoid, Sternohyoid, Rectus abdominis, Brachioradialis, External abdominal oblique, Tensor fasciae latae, Iliacus, Sternocleidomastoid, Trapezius, Scalenes, Deltoid, Pectoralis major, Serratus anterior, Biceps brachii, Brachialis, Triceps brachii, Pronator teres, Brachioradialis, Flexor carpi radialis, Flexor digitorum superficialis, Flexor carpi ulnaris, Psoas major, Pectineus, Adductor longus, Sartorius, Gracilis, Vastus lateralis, Rectus femoris, Vastus medialis, Tendon of quadriceps femoris, Patella, Gastrocnemius, Soleus, Iliotibial tract, Patellar ligament, Tibialis anterior, Fibularis longus, Tibia, Thenar muscles, Hypothenar muscles

Superficial/Posterior Skeletal Muscles

• Listing of muscles including: Epicranial aponeurosis, Occipitofrontalis (occipital belly), Splenius capitis, Trapezius, Deltoid, Temporalis, Sternocleidomastoid, Brachialis, Triceps brachii, Anconeus, Extensor carpi radialis brevis, Extensor digitorum, Extensor carpi ulnaris, Flexor carpi ulnaris, Abductor pollicis longus, Extensor pollicis brevis, Infraspinatus, Teres minor, Teres major, Latissimus dorsi, External abdominal oblique, Gluteus medius, Flexor carpi ulnaris, Brachioradialis, Gluteus maximus, Plantaris, Gastrocnemius, Soleus, Flexor digitorum longus, Calcaneal tendon, Gracilis, Adductor magnus, Semitendinosus, Biceps femoris, Semimembranosus, Popliteal fossa, Sartorius.

Muscles of Facial Expression

• Lie within the subcutaneous layer, originating on the fascia or bones of the skull and inserting into the skin. These muscles are responsible for a wide range of facial movements that convey emotions and expressions.

• When these muscles contract, they move the skin rather than a joint. This unique arrangement allows for subtle and nuanced facial expressions.

Scalp Muscles

• Occipitofrontalis:

  • Frontal belly: Originates from the epicranial aponeurosis and inserts into the skin superior to the supraorbital margin. Action: draws scalp anteriorly, raises eyebrows, and wrinkles skin of forehead horizontally. Innervated by Facial (VII) nerve. This muscle is responsible for expressions of surprise or attention.

  • Occipital belly: Originates from the occipital bone and mastoid process of temporal bone and inserts into the epicranial aponeurosis. Action: Draws scalp posteriorly. Innervated by Facial (VII) nerve. This muscle works in coordination with the frontal belly to control scalp movement.

Mouth Muscles

• Orbicularis oris:

  • Origin: Muscle fibers surrounding opening of mouth.

  • Insertion: Skin at corner of mouth.

  • Action: Closes and protrudes lips, as in kissing; compresses lips against teeth; and shapes lips during speech. This muscle is crucial for facial expressions and speech articulation.

  • Innervation: Facial (VII) nerve.

• Zygomaticus major:

  • Origin: Zygomatic bone.

  • Insertion: Skin at angle of mouth and orbicularis oris.

  • Action: Draws angle of mouth superiorly and laterally, as in smiling. This muscle is a key player in expressing happiness and joy.

  • Innervation: Facial (VII) nerve.

• Zygomaticus minor:

  • Origin: Zygomatic bone.

  • Insertion: Upper lip.

  • Action: Raises (elevates) upper lip, exposing maxillary (upper) teeth. This muscle contributes to expressions of sadness or contempt.

  • Innervation: Facial (VII) nerve.

• Levator labii superioris:

  • Origin: Superior to infraorbital foramen of maxilla.

  • Insertion: Skin at angle of mouth and orbicularis oris.

  • Action: Raises upper lip. This muscle is involved in expressions of disdain or sneering.

  • Innervation: Facial (VII) nerve.

• Depressor labii inferioris:

  • Origin: Mandible.

  • Insertion: Skin of lower lip.

  • Action: Depresses (lowers) lower lip. This muscle contributes to expressions of sadness or disappointment.

  • Innervation: Facial (VII) nerve.

• Depressor anguli oris:

  • Origin: Mandible.

  • Insertion: Angle of mouth.

  • Action: Draws angle of mouth laterally and inferiorly, as in opening mouth. This muscle is important for both facial expressions and basic functions like speaking and eating.

  • Innervation: Facial (VII) nerve.

• Levator anguli oris:

  • Origin: Inferior to infraorbital foramen.

  • Insertion: Skin of lower lip and orbicularis oris.

  • Action: Draws angle of mouth laterally and superiorly. This muscle contributes to expressions of confidence or smugness.

  • Innervation: Facial (VII) nerve.

• Buccinator:

  • Origin: Alveolar processes of maxilla and mandible and pterygomandibular raphe.

  • Insertion: Orbicularis oris.

  • Action: Presses cheeks against teeth and lips, as in whistling, blowing, and sucking; draws corner of mouth laterally; and assists in mastication (chewing) by keeping food between the teeth. This muscle is vital for eating, speaking, and various facial expressions.

  • Innervation: Facial (VII) nerve.

• Risorius:

  • Origin: Fascia over parotid (salivary) gland.

  • Insertion: Skin at angle of mouth.

  • Action: Draws angle of mouth laterally, as in grimacing. This muscle expresses feelings of discomfort or displeasure.

  • Innervation: Facial (VII) nerve.

• Mentalis:

  • Origin: Mandible.

  • Insertion: Skin of chin.

  • Action: Elevates and protrudes lower lip and pulls skin of chin up, as in pouting. This muscle shows feelings of doubt or uncertainty.

  • Innervation: Facial (VII) nerve.

Neck Muscles

• Platysma:

  • Origin: Fascia over deltoid and pectoralis major muscles.

  • Insertion: Mandible, blends with muscles around angle of mouth, and skin of lower face.

  • Action: Draws outer part of lower lip inferiorly and posteriorly as in pouting; depresses mandible. This muscle expresses tension or stress.

  • Innervation: Facial (VII) nerve.

Extrinsic Eye Muscles

• Muscles that move the eyeballs originate in the orbit and insert on the sclera. Their precise control enables us to track objects, maintain focus, and coordinate eye movements.

• 3 pairs of extrinsic eye muscles:

  • Superior and inferior recti: Primarily responsible for vertical movements.

  • Lateral and medial recti: Responsible for horizontal movements.

  • Superior and inferior obliques: Assist in rotational movements and stabilize the eyeball.

• Levator palpebrae superioris: Insertion allows it to raise the eyelid. This muscle works to keep the eye open

Eye Muscles - Origins, Insertions, Actions & Innervation

• Superior rectus:

  • Origin: Common tendinous ring.

  • Insertion: Superior and central part of eyeballs.

  • Action: Moves eyeballs superiorly (elevation) and medially (adduction), and rotates them medially. This muscle helps in looking upwards.

  • Innervation: Oculomotor (III) nerve.

• Inferior rectus:

  • Origin: Common tendinous ring.

  • Insertion: Inferior and central part of eyeballs.

  • Action: Moves eyeballs inferiorly (depression) and medially (adduction), and rotates them laterally. This muscle helps in looking downwards.

  • Innervation: Oculomotor (III) nerve.

• Lateral rectus:

  • Origin: Common tendinous ring.

  • Insertion: Lateral side of eyeballs.

  • Action: Moves eyeballs laterally (abduction). This muscle abducts or directs the eye away from the nose.

  • Innervation: Abducens (VI) nerve.

• Medial rectus:

  • Origin: Common tendinous ring.

  • Insertion: Medial side of eyeballs.

  • Action: Moves eyeballs medially (adduction). This muscle adducts or directs the eye towards the nose.

  • Innervation: Oculomotor (III) nerve.

• Superior oblique:

  • Origin: Sphenoid bone, superior and medial to common tendinous ring in orbit.

  • Insertion: Eyeball between superior and lateral recti (via tendon through trochlea).

  • Action: Moves eyeballs inferiorly (depression) and laterally (abduction), and rotates them medially. This muscle depresses and rotates the eye when it is adducted.

  • Innervation: Trochlear (IV) nerve.

• Inferior oblique:

  • Origin: Maxilla in floor of orbit.

  • Insertion: Eyeballs between inferior and lateral recti.

  • Action: Moves eyeballs superiorly (elevation) and laterally (abduction), and rotates them laterally. This muscle elevates and rotates the eye when it is adducted.

  • Innervation: Oculomotor (III) nerve.

• Levator palpebrae superioris:

  • Origin: Roof of orbit (lesser wing of sphenoid bone).

  • Insertion: Skin and tarsal plate of upper eyelids.

  • Action: Elevates upper eyelids (opens eyes). This muscle plays a critical role in controlling eyelid movement and keeping the eyes open.

  • Innervation: Oculomotor (III) nerve.

Orbit and Eyebrow Muscles

• Orbicularis oculi:

  • Origin: Medial wall of orbit.

  • Insertion: Circular path around orbit.

  • Action: Closes eye. This muscle is crucial for blinking, squinting, and protecting the eye.

  • Innervation: Facial (VII) nerve.

• Corrugator supercilii:

  • Origin: Medial end of superciliary arch of frontal bone.

  • Insertion: Skin of eyebrow.

  • Action: Draws eyebrow inferiorly and wrinkles skin of forehead vertically as in frowning. This muscle is essential for expressing concern or concentration.

  • Innervation: Facial (VII) nerve.

Muscles of Mastication

• Muscles that move the mandible. These muscles facilitate chewing by controlling the opening, closing, and lateral movements of the jaw.

• Masseter:

  • Origin: Maxilla and zygomatic arch.

  • Insertion: Angle and ramus of mandible.

  • Action: Elevates mandible, as in closing mouth. This is one of the strongest muscles in the body and is primarily responsible for closing the jaw.

  • Innervation: Mandibular division of trigeminal (V) nerve.

• Temporalis:

  • Origin: Temporal bone.

  • Insertion: Coronoid process and ramus of mandible.

  • Action: Elevates and retracts mandible. This muscle assists in closing the jaw and pulling it back.

  • Innervation: Mandibular division of trigeminal (V) nerve.

• Medial pterygoid:

  • Origin: Medial surface of lateral portion of pterygoid process of sphenoid bone; maxilla.

  • Insertion: Angle and ramus of mandible.

  • Action: Elevates and protracts (protrudes) mandible and moves mandible from side to side. This muscle works with the masseter to elevate the mandible and contributes to lateral movements.

  • Innervation: Mandibular division of trigeminal (V) nerve.

• Lateral pterygoid:

  • Origin: Greater wing and lateral surface of lateral portion of pterygoid process of sphenoid bone.

  • Insertion: Condyle of mandible; temporomandibular joint (TMJ).

  • Action: Protracts mandible, depresses mandible as in opening mouth, and moves mandible from side to side. This muscle is essential for opening the mouth and for lateral chewing movements.

  • Innervation: Mandibular division of trigeminal (V) nerve.

Muscles of the Tongue

• The tongue is attached to the mandible, styloid process of the temporal bone, and the hyoid bone. These attachments provide the tongue with a stable base for complex movements involved in speech and swallowing.

• Genioglossus:

  • Origin: Mandible.

  • Insertion: Undersurface of tongue and hyoid bone.

  • Action: Depresses tongue and thrusts it anteriorly (protraction). This muscle is responsible for most of the tongue's forward movement.

  • Innervation: Hypoglossal (XII) nerve.

• Styloglossus:

  • Origin: Styloid process of temporal bone.

  • Insertion: Side and undersurface of tongue.

  • Action: Elevates tongue and draws it posteriorly (retraction). This muscle retracts the tongue and is crucial for swallowing.

  • Innervation: Hypoglossal (XII) nerve.

• Hyoglossus:

  • Origin: Greater horn and body of hyoid bone.

  • Insertion: Side of tongue.

  • Action: Depresses tongue and draws down its sides. This muscle lowers the tongue, important for speech and swallowing.

  • Innervation: Hypoglossal (XII) nerve.

• Palatoglossus:

  • Origin: Anterior surface of soft palate.

  • Insertion: Side of tongue.

  • Action: Elevates posterior portion of tongue and draws soft palate down on tongue. This muscle elevates the back of the tongue and closes off the oral cavity, assisting in swallowing.

  • Innervation: Pharyngeal plexus (Vagus X).

Anterior Neck Muscles

• Suprahyoid muscles elevate the hyoid bone, floor of the oral cavity, and tongue during swallowing. These muscles are located above the hyoid bone and play a crucial role in the initial stages of swallowing.

• Infrahyoid muscles depress the hyoid bone and some move the larynx. Located below the hyoid bone, these muscles assist in later stages of swallowing and vocalization.

Suprahyoid Muscles

• Digastric:

  • Origin: Anterior belly from inner side of inferior border of mandible; posterior belly from temporal bone.

  • Insertion: Body of hyoid bone via an intermediate tendon.

  • Action: Elevates hyoid bone. Depresses mandible, as in opening mouth. This muscle has two bellies and plays a key role in both swallowing and jaw movement.

  • Innervation: Anterior belly: mandibular division of trigeminal (V) nerve. Posterior belly: facial (VII) nerve.

• Stylohyoid:

  • Origin: Styloid process of temporal bone.

  • Insertion: Body of hyoid bone.

  • Action: Elevates hyoid bone and draws it posteriorly. This muscle elevates and retracts the hyoid bone, aiding in swallowing.

  • Innervation: Facial (VII) nerve.

• Mylohyoid:

  • Origin: Inner surface of mandible.

  • Insertion: Body of hyoid bone.

  • Action: Elevates hyoid bone and floor of mouth and depresses mandible. This muscle supports the floor of the mouth and elevates the hyoid bone during swallowing.

  • Innervation: Mandibular division of trigeminal (V) nerve.

• Geniohyoid:

  • Origin: Inner surface of mandible.

  • Insertion: Body of hyoid bone.

  • Action: Elevates hyoid bone, draws hyoid bone and tongue anteriorly. Depresses mandible. This muscle elevates the hyoid bone and protracts the tongue, crucial for swallowing and speech.

  • Innervation: First cervical spinal nerve (C1).

Infrahyoid Muscles

• Omohyoid:

  • Origin: Superior border of scapula and superior transverse ligament.

  • Insertion: Body of hyoid bone.

  • Action: Depresses hyoid bone. This muscle helps to depress the hyoid bone and larynx, assisting in swallowing and speech.

  • Innervation: Branches of spinal nerves C1–C3.

• Sternohyoid:

  • Origin: Medial end of clavicle and manubrium of sternum.

  • Insertion: Body of hyoid bone.

  • Action: Depresses hyoid bone. This muscle depresses the hyoid bone, aiding in the final stages of swallowing.

  • Innervation: Branches of spinal nerves C1–C3.

• Sternothyroid:

  • Origin: Manubrium of sternum.

  • Insertion: Thyroid cartilage of larynx.

  • Action: Depresses thyroid cartilage of larynx. This muscle depresses the larynx, which assists in vocalization and swallowing.

  • Innervation: Branches of spinal nerves C1–C3.

• Thyrohyoid:

  • Origin: Thyroid cartilage of larynx.

  • Insertion: Greater horn of hyoid bone.

  • Action: Elevates thyroid cartilage of larynx. Depresses hyoid bone. This muscle elevates the larynx and depresses the hyoid bone, facilitating both speech and swallowing.

  • Innervation: Branches of spinal nerve C1 via the descending hypoglossal (XII) nerve.

Muscles That Move the Head

• Sternocleidomastoid:

  • Origin: Sternal head: manubrium of sternum; clavicular head: medial third of clavicle.

  • Insertion: Mastoid process of temporal bone and lateral half of superior nuchal line of occipital bone.

  • Action: Acting together (bilaterally), flex cervical portion of vertebral column, extend head at atlanto-occipital joints; acting singly (unilaterally), laterally flex neck and head to same side and rotate head to side opposite contracting muscle. This muscle is essential for head and neck movement, and its actions depend on whether it contracts unilaterally or bilaterally.

  • Innervation: Accessory (XI) nerve, C2, and C3.

• Semispinalis capitis:

  • Origin: Articular processes of C4–C6 and transverse processes of C7–T7.

  • Insertion: Occipital bone between superior and inferior nuchal lines.

  • Action: Acting together, extend head and vertebral column; acting singly, rotate head to side opposite contracting muscle. It’s involved in maintaining posture and head movements.

  • Innervation: Cervical spinal nerves—posterior rami.

• Splenius capitis:

  • Origin: Ligamentum nuchae and spinous processes of C7–T4.

  • Insertion: Occipital bone and mastoid process of temporal bone.

  • Action: Extend head; acting together, muscles of each region (cervical and thoracic) extend vertebral column of their respective regions.

  • Innervation: Cervical spinal nerves—posterior rami.

• Longissimus capitis:

  • Origin: Articular processes of T1–T4.

  • Insertion: Mastoid process of temporal bone.

  • Action: Acting together, extend head and vertebral column; acting singly, laterally flex and rotate head to same side as contracting muscle. this muscle assists in head extension, lateral flexion, and rotation.

  • Innervation: Cervical spinal nerves—posterior rami.

• Spinalis capitis:

  • Origin: Often absent or very small; arises with semispinalis capitis.

  • Insertion: Occipital bone.

  • Action: Extends head and vertebral column.

  • Innervation: Cervical spinal nerves—