Dog Muscle Anatomy: Origins, Insertions, Actions and Clinical Significance
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Functional muscle groups of the dog — what attaches where, what moves what, nerve supply, and the clinical significance of each group for lameness diagnosis, surgical approaches, and neuromuscular disease recognition.
- The dog has approximately 700 named skeletal muscles, organised into functional groups: axial (epaxial and hypaxial), thoracic limb (extrinsic and intrinsic), pelvic limb (gluteal, quadriceps, hamstring, crural), and head/neck (masticatory, facial, pharyngeal, laryngeal).
- The extrinsic muscles of the thoracic limb (those attaching the limb to the trunk — trapezius, rhomboideus, serratus ventralis, pectorals, latissimus dorsi) form a muscular sling that suspends the trunk between the forelimbs. Damage to any component of this sling causes characteristic gait abnormalities.
- The quadriceps femoris — the sole extensor of the stifle — is innervated by the femoral nerve. Femoral nerve injury (from pelvic fracture, iliopsoas haematoma, or iatrogenic during pelvic surgery) causes inability to bear weight because the stifle cannot be fixed in extension.
- The gastrocnemius muscle is the most powerful extensor of the tarsus (hock) and is innervated by the tibial nerve (a branch of the sciatic). Rupture of the gastrocnemius tendon (Achilles mechanism rupture) causes a 'dropped hock' with the calcaneal tuber displaced distally and the digits curled (from unopposed digital flexor muscle pull).
- The masticatory muscles — temporalis, masseter, and pterygoids — are innervated by the mandibular division of the trigeminal nerve (CN V3). Masticatory muscle myositis (MMM) is a breed-specific (large-breed dogs), immune-mediated myositis that preferentially affects the type 2M myofibers unique to the masticatory muscles, causing trismus (inability to open the jaw) and muscle atrophy.
- A dog that cannot open its mouth (trismus) and has bilaterally atrophied temporalis and masseter muscles has masticatory muscle myositis (MMM) until proven otherwise. The diagnostic test is a type 2M antibody titre — NOT a routine muscle biopsy (which may miss the type 2M fiber involvement if the wrong muscle is sampled).
- A dog with an acute inability to bear weight on a pelvic limb, with the stifle flexed and the hock dropped, has a rupture of the Achilles mechanism (gastrocnemius tendon ± superficial digital flexor tendon ± common calcaneal tendon) until proven otherwise — ultrasound confirms the diagnosis and identifies which components are ruptured.
- A dog with progressive, bilaterally symmetric, exercise-induced collapse and elevated creatine kinase (CK >10,000 U/L) has an exertional myopathy or metabolic myopathy until proven otherwise — especially in Labrador Retrievers (exercise-induced collapse, EIC) and working breeds.
- A dog with a very firm, swollen, painful muscle compartment after trauma has compartment syndrome until proven otherwise — the intracompartmental pressure exceeds capillary perfusion pressure, causing muscle ischaemia and necrosis within 6–8 hours. Fasciotomy is limb-saving.
Muscle Architecture and Functional Organisation
Skeletal muscle is the most abundant tissue in the dog's body, accounting for 40–50 % of body mass. Each muscle is an organ comprising muscle fibers (myofibers), connective tissue (endomysium, perimysium, epimysium), blood vessels, and nerves. The arrangement of muscle fibers within a muscle determines its force-generating capacity and range of contraction.
| Architecture | Fiber Arrangement | Example | Force vs Range | Clinical Significance |
|---|---|---|---|---|
| Parallel (fusiform) | Fibers run parallel to the line of pull | Biceps brachii, sartorius | Long range of contraction; moderate force | Parallel muscles can be transected and repaired with less functional loss than pennate muscles. A biceps brachii tenotomy (surgical release of the tendon of origin) is used to treat biceps tenosynovitis, with minimal loss of elbow flexion strength because the brachialis muscle also flexes the elbow |
| Unipennate | Fibers insert obliquely onto one side of a central tendon | Extensor carpi radialis, gastrocnemius | Shorter range; higher force (more fibers per unit volume) | Pennate muscles produce more force but are more sensitive to changes in fiber length. Gastrocnemius recession (surgical lengthening) compromises force production and is reserved for severe contracture |
| Bipennate | Fibers insert obliquely onto both sides of a central tendon | Infraspinatus, rectus femoris | Shortest range; highest force | Bipennate muscles like the infraspinatus are optimised for stabilisation (shoulder joint) rather than large-range motion. Infraspinatus contracture (fibrosis) causes a severe, permanent gait abnormality because the muscle cannot be surgically lengthened without destroying its architecture |
| Multipennate | Multiple tendons with fibers inserting at multiple angles | Deltoideus, masseter | Complex force vectors; high force | Multipennate muscles are difficult to repair after laceration because the internal tendinous septa must be aligned correctly. Masseter muscle injury heals with fibrosis, which can cause trismus |
Extrinsic Muscles of the Thoracic Limb: The Muscular Sling
The thoracic limb is attached to the trunk by a synsarcosis — a muscular connection without a bony joint. Six muscles form this sling: trapezius, rhomboideus, serratus ventralis, pectorals (superficial and deep), latissimus dorsi, and brachiocephalicus. Together, they suspend the trunk between the forelimbs and allow the scapula to glide over the thoracic wall during locomotion.
| Muscle | Origin | Insertion | Action | Nerve Supply | Clinical Significance |
|---|---|---|---|---|---|
| Trapezius (cervical + thoracic parts) | Median raphe of the neck (cervical); supraspinous ligament from T1–T9 (thoracic) | Scapular spine | Elevates and abducts the forelimb; the cervical part pulls the scapula cranially; the thoracic part pulls it caudally | Accessory n. (CN XI) | Trapezius atrophy is a sign of accessory nerve damage (rare). The cervical trapezius is a superficial muscle that is easily visualised and palpated — atrophy suggests chronic forelimb lameness with disuse |
| Rhomboideus (capitis + cervicis + thoracis) | Nuchal crest of the occipital bone (capitis); median raphe of the neck (cervicis); spinous processes of T1–T7 (thoracis) | Dorsal border of the scapula | Elevates the forelimb and pulls the scapula dorsally and cranially | Dorsal branches of cervical and thoracic spinal nn. | Rhomboideus atrophy occurs with suprascapular nerve injury ('Sweeney') — the scapula abducts (moves away from the body wall) because the rhomboid and serratus ventralis are unopposed |
| Serratus ventralis (cervicis + thoracis) | Transverse processes of C3–C7 (cervicis); first 7–8 ribs (thoracis) | Medial (serrated) surface of the scapula | SUPPORTS THE TRUNK — the primary weight-bearing muscle of the thoracic sling. When the serratus ventralis contracts, it elevates the trunk relative to the forelimb | Ventral branches of cervical spinal nn. and the long thoracic n. (C7) | Serratus ventralis atrophy is the most visible sign of chronic forelimb lameness — the dorsal border of the scapula becomes prominent ('knife-edge scapula') because the muscle can no longer hold the scapula against the body wall |
| Pectorals (superficial + deep) | Sternum (superficial: cranial sternebrae; deep: entire sternum + costal cartilages) | Superficial: crest of the greater tubercle of the humerus; Deep (ascending pectoral): lesser tubercle + greater tubercle of the humerus | Adduct the forelimb; the deep pectoral also pulls the trunk cranially during the stance phase (important for propulsion) | Cranial pectoral nn. (C7–C8) and caudal pectoral nn. (C8–T1) | Pectoral muscle atrophy occurs in chronic forelimb lameness — the ventral thorax between the forelimbs appears hollow. The deep pectoral is the largest pectoral muscle and is used as a muscle flap for reconstructive surgery |
| Latissimus dorsi | Thoracolumbar fascia (spinous processes of T7–L7) | Teres major tuberosity of the humerus (with the teres major muscle) | Retracts the forelimb (pulls it caudally); flexes the shoulder | Thoracodorsal n. (C7–C8) | The latissimus dorsi is a broad, flat muscle that is used as a myocutaneous flap for thoracic wall reconstruction. Its nerve supply (thoracodorsal nerve) must be preserved during axillary surgery |
Intrinsic Muscles of the Thoracic Limb: Shoulder to Digits
The intrinsic muscles of the thoracic limb originate and insert on bones of the limb itself — they are responsible for fine control of joint motion. The key functional groups are the shoulder stabilisers (lateral: supraspinatus, infraspinatus, teres minor; medial: subscapularis, coracobrachialis), elbow extensors (triceps brachii, tensor fasciae antebrachii, anconeus), elbow flexors (biceps brachii, brachialis), carpal and digital extensors (craniolateral antebrachium), and carpal and digital flexors (caudomedial antebrachium).
| Muscle Group | Key Muscles | Primary Action | Nerve Supply | Key Pathology |
|---|---|---|---|---|
| Shoulder stabilisers (lateral) | Supraspinatus (extends shoulder), Infraspinatus (abducts and stabilises shoulder), Teres minor (flexes shoulder) | Shoulder stability during weight-bearing; the lateral stabilisers prevent the shoulder from collapsing medially during the stance phase | Suprascapular n. (C6–C7) | Suprascapular nerve injury → denervation atrophy of the supraspinatus and infraspinatus → 'Sweeney' (the shoulder abducts and the scapula becomes prominent). Infraspinatus contracture → permanent shoulder adduction and limb circumduction |
| Shoulder stabilisers (medial) | Subscapularis (adducts and stabilises shoulder), Coracobrachialis (adducts and extends shoulder) | Shoulder stability; the medial stabilisers prevent the shoulder from abducting during the stance phase | Subscapular n. (C6–C7) and Musculocutaneous n. (C6–C8) | Medial shoulder instability (MSI) is an underdiagnosed cause of forelimb lameness in dogs — the shoulder abduction angle is >30° (normal: <25°). Surgical stabilisation (medial imbrication or thermal capsulorrhaphy) is indicated |
| Elbow extensors | Triceps brachii (long, lateral, medial, and accessory heads), Tensor fasciae antebrachii, Anconeus | Extend the elbow — this is the antigravity muscle group that supports the forelimb during the stance phase | Radial n. (C7–T1) | Radial nerve paralysis → inability to extend the elbow → 'dropped elbow' and non-weight-bearing lameness. The triceps brachii is the largest muscle of the thoracic limb, and its long head is the most commonly affected by exertional rhabdomyolysis |
| Elbow flexors | Biceps brachii, Brachialis | Flex the elbow; the biceps brachii also extends the shoulder (its tendon of origin laces through the intertubercular groove of the humerus) | Musculocutaneous n. (C6–C8) | Biceps tenosynovitis → pain on shoulder extension and elbow flexion; diagnosed by ultrasound of the biceps tendon in the intertubercular groove. Biceps tendon release or tenotomy is performed for refractory cases |
| Carpal/digital extensors | Extensor carpi radialis, Common digital extensor, Lateral digital extensor, Ulnaris lateralis | Extend the carpus and digits; the extensor carpi radialis is the primary carpal extensor | Radial n. (C7–T1) | Radial nerve paralysis also affects the carpal and digital extensors → the dog cannot extend the carpus or digits and walks on the dorsal surface of the paw ('knuckling') |
| Carpal/digital flexors | Flexor carpi radialis, Flexor carpi ulnaris, Superficial digital flexor, Deep digital flexor | Flex the carpus and digits; the deep digital flexor flexes the distal interphalangeal joints (the claws) | Median n. and Ulnar n. (C8–T2) | Flexor tendon laceration → loss of digital flexion. The superficial and deep digital flexor tendons can be repaired surgically if lacerated within 7–10 days of injury |
Pelvic Limb Muscles: Hip to Digits
The pelvic limb is the primary generator of propulsion — the gluteal and hamstring muscles extend the hip and tarsus to push the body forward, while the quadriceps extends the stifle to support weight during the stance phase.
| Muscle Group | Key Muscles | Primary Action | Nerve Supply | Key Pathology |
|---|---|---|---|---|
| Gluteal | Superficial gluteal, Middle gluteal, Deep gluteal | Extend and abduct the hip; the middle gluteal is the largest and most powerful hip extensor | Cranial gluteal n. (L6–S1) — middle + deep gluteals; Caudal gluteal n. (L7–S1) — superficial gluteal | Gluteal atrophy → loss of the normal rounded contour of the hip; occurs with hip dysplasia, sacroiliac disease, and chronic hindlimb lameness. The middle gluteal is the most commonly injected muscle for intramuscular injections in the dog — avoid the sciatic nerve (which runs caudal to the hip joint, deep to the gluteals) |
| Quadriceps femoris | Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius | EXTEND THE STIFLE — the sole antigravity muscle group of the stifle. The rectus femoris also flexes the hip (it originates on the ilium, cranial to the acetabulum) | Femoral n. (L4–L6) | Femoral nerve paralysis → inability to extend the stifle → the dog cannot bear weight on the limb. The quadriceps is also the most commonly affected muscle in exertional rhabdomyolysis of the pelvic limb |
| Hamstring | Biceps femoris, Semitendinosus, Semimembranosus | Extend the hip, flex the stifle, and (via the common calcaneal tendon) extend the tarsus. The hamstrings are the primary hip extensors during propulsion | Sciatic n. (L6–S2) — its tibial and peroneal (fibular) branches innervate all hamstring muscles | Hamstring strain → acute-onset hindlimb lameness in agility and working dogs; diagnosed by ultrasound. Semitendinosus and semimembranosus fibrotic myopathy → mechanical lameness with a short-strided, 'stiff' gait; the affected muscle is palpably firm and fibrous |
| Cranial crural | Cranial tibial, Peroneus (fibularis) longus, Extensor digitorum longus | Flex the tarsus and extend the digits; the cranial tibial is the primary tarsal flexor | Peroneal (fibular) n. (branch of sciatic n., L6–S1) | Peroneal nerve paralysis → inability to flex the tarsus → the dog drags the dorsal paw ('knuckling'). This is the most common peripheral nerve injury in the pelvic limb — from trauma, tight bandaging, or iatrogenic during lateral stifle surgery |
| Caudal crural | Gastrocnemius, Superficial digital flexor, Deep digital flexor, Popliteus | Extend the tarsus (gastrocnemius — the most powerful tarsal extensor) and flex the digits. The gastrocnemius and SDF tendons form the common calcaneal tendon (Achilles tendon) | Tibial n. (branch of sciatic n., L6–S2) | Achilles mechanism rupture → 'dropped hock' (the calcaneal tuber is displaced distally) and curled digits (the digital flexors are unopposed). The gastrocnemius tendon is the most commonly ruptured component; the SDF tendon is less commonly ruptured alone |
Axial Muscles: Epaxial and Hypaxial Groups
The axial muscles support and move the vertebral column. The epaxial muscles (dorsal to the transverse processes) extens the vertebral column; the hypaxial muscles (ventral to the transverse processes) flex it.
| Group | Key Muscles | Action | Nerve Supply | Clinical Significance |
|---|---|---|---|---|
| Epaxial (dorsal) | Longissimus (capitis, cervicis, thoracis, lumborum), Iliocostalis (thoracis, lumborum), Spinalis et semispinalis (capitis, cervicis, thoracis), Multifidus | Extend the vertebral column and maintain posture | Dorsal branches of spinal nerves at each segment | The epaxial muscles are the 'topline' visible on the dog's back. Epaxial muscle atrophy suggests chronic spinal pain (IVDD, discospondylitis, neoplasia) or neuromuscular disease. The multifidus is the deepest epaxial muscle and is the most sensitive to denervation from spinal nerve compression |
| Hypaxial (ventral) | Rectus abdominis, External abdominal oblique, Internal abdominal oblique, Transversus abdominis, Psoas major + minor, Iliacus | Flex the vertebral column; compress the abdominal contents; the psoas muscles flex the hip | Ventral branches of spinal nerves at each segment; the iliopsoas is innervated by the femoral n. (L4–L6) | Iliopsoas strain → acute-onset hindlimb lameness; the muscle is palpable per rectum or via deep abdominal palpation. The abdominal muscles are the layers incised during a flank laparotomy — their fiber directions alternate (external oblique: caudoventral; internal oblique: cranioventral; transversus: dorsoventral) to provide structural integrity to the abdominal wall |
Masticatory Muscles: Unique Type 2M Fibers
The masticatory muscles — temporalis, masseter, and medial and lateral pterygoids — are unique among striated muscles in the dog because they contain a fiber type (type 2M) that is not found in any other skeletal muscle. Type 2M fibers express a unique isoform of myosin heavy chain, which is the target of the autoimmune attack in masticatory muscle myositis (MMM).
| Muscle | Origin | Insertion | Action | Key Pathology |
|---|---|---|---|---|
| Temporalis | Temporal fossa of the skull | Coronoid process of the mandible | Closes the jaw (elevates the mandible) | MMM → bilaterally symmetric temporalis atrophy → the dorsal skull becomes prominent ('skull-head' appearance); trismus (inability to open the jaw) is the cardinal sign |
| Masseter | Zygomatic arch | Masseteric fossa of the mandible (ventrolateral mandibular ramus) | Closes the jaw (elevates and protrudes the mandible) | MMM → masseter atrophy and fibrosis; the zygomatic arch becomes prominent. The masseter is also affected by myositis ossificans (heterotopic bone formation within the muscle) in young dogs |
| Medial pterygoid | Pterygopalatine fossa (pterygoid, palatine, and sphenoid bones) | Medial surface of the mandibular ramus | Closes the jaw | The pterygoid muscles are deep and difficult to palpate. They are also affected by MMM but the temporalis and masseter are the most visibly affected |
| Digastricus | Paracondylar process of the occipital bone | Ventral border of the mandibular body (rostral) | Opens the jaw (depresses the mandible) | The digastricus is a jaw OPENER — it is NOT a masticatory muscle in the embryological sense and does NOT contain type 2M fibers. It is NOT affected by MMM. This is why dogs with MMM can open their jaw slightly (the digastricus still functions) but cannot close it with force |
MMM is diagnosed by a serological test for antibodies against type 2M myosin heavy chain (the type 2M antibody titre). A muscle biopsy can confirm the diagnosis but may be falsely negative if the biopsy is taken from a limb muscle (which does not contain type 2M fibers) — the biopsy must be taken from the temporalis or masseter muscle. Treatment is immunosuppressive doses of prednisolone (1–2 mg/kg q12h, tapering over 4–6 months), and the prognosis is good if treatment is started early, before extensive fibrosis has occurred.
- The iliopsoas muscle (a composite of the psoas major and iliacus) is a powerful hip flexor that is commonly strained in agility and working dogs. The muscle is palpated by deep abdominal palpation or per rectum — a firm, painful, rope-like structure cranial to the pelvic brim on the affected side.
- The infraspinatus muscle is the primary stabiliser of the shoulder joint in the dog. Infraspinatus contracture (a fibrotic myopathy, especially in hunting and working dogs) causes a characteristic gait: the affected forelimb is circumducted (swung outward) during the swing phase and the shoulder is held in adduction with the elbow turned outward.
- The cranial tibial muscle is the primary flexor of the tarsus and is innervated by the peroneal (fibular) nerve. Peroneal nerve injury (from trauma, lateral stifle surgery, or tight bandaging) causes inability to flex the tarsus → the dog drags the dorsal surface of the paw during the swing phase ('knuckling').
- The pectineus muscle is a small adductor of the hip that is routinely transected (myectomy) or partially resected for the treatment of hip dysplasia in young dogs — the goal is to reduce the adduction force that contributes to hip subluxation.
Frequently asked questions
In anatomical terminology for the dog: the origin is the proximal attachment (closer to the axial skeleton or the trunk) — it is generally the less mobile attachment. The insertion is the distal attachment (further from the trunk) — it is generally the more mobile attachment. During contraction, the origin remains relatively fixed, and the insertion is pulled toward it. Example: the biceps brachii originates on the supraglenoid tubercle of the scapula (proximal = origin) and inserts on the radial tuberosity and ulna (distal = insertion). When the biceps contracts, it pulls the radius and ulna cranially, flexing the elbow.
The radial nerve (C7–T1) is the most commonly injured nerve in the thoracic limb because of its superficial course around the lateral aspect of the humerus. Causes: humeral fracture (the radial nerve runs in the spiral groove of the humerus), prolonged lateral recumbency under anaesthesia, and direct trauma. Clinical signs: (1) inability to extend the elbow (triceps brachii paralysis) → 'dropped elbow', (2) inability to extend the carpus and digits → the dog walks on the dorsal surface of the paw ('knuckling'), and (3) loss of sensation on the dorsal surface of the paw (radial nerve sensory distribution). Prognosis: if the nerve is intact (neurapraxia from compression), recovery can occur within weeks to months; if the nerve is transected (neurotmesis from a fracture fragment), surgical repair (neurorrhaphy) is required but the prognosis is guarded.
The gastrocnemius tendon is the primary component of the common calcaneal (Achilles) tendon and is the primary extensor of the tarsus. The superficial digital flexor (SDF) tendon is a thinner, more superficial tendon that runs alongside the gastrocnemius tendon and inserts on the tuber calcanei (calcaneal tuber). The SDF continues distally as the superficial digital flexor tendon of the pes, inserting on the middle phalanges of digits II–V. Rupture of the gastrocnemius tendon alone causes a 'dropped hock' with the digits in a normal position. Rupture of BOTH the gastrocnemius and SDF tendons causes a 'dropped hock' with the digits curled (the deep digital flexor is unopposed). Ultrasound is the diagnostic modality of choice to determine which components of the Achilles mechanism are ruptured.
The safest and most commonly used IM injection sites in the dog are: (1) the lumbar epaxial muscles (longissimus lumborum) — dorsal to the lumbar transverse processes, 2–3 cm lateral to the dorsal midline. This site is safe because no major nerves or vessels run in this region. (2) The quadriceps femoris (cranial thigh) — the vastus lateralis belly. Avoid the caudal thigh (the sciatic nerve runs there). (3) The triceps brachii (caudal brachium) — the long head. Avoid the craniolateral brachium (the radial nerve runs in the brachial groove). (4) The middle gluteal muscle — the large muscle belly between the tuber coxae and the greater trochanter. The sciatic nerve runs caudal to the hip joint, deep to the gluteal muscles — to avoid it, inject into the cranial half of the middle gluteal muscle belly. For all IM injections, always aspirate before injecting (to ensure the needle is not in a vessel) and use a needle of appropriate length (1–1.5 inches for a medium dog).
A muscle injury (strain, tear, rupture) causes pain on palpation of the muscle belly or its tendon, swelling and/or a palpable defect in the muscle/tendon, and weakness without complete paralysis — some voluntary movement is still present. Pain is the dominant clinical sign. A nerve injury (neurapraxia, axonotmesis, neurotmesis) causes flaccid paralysis of the innervated muscles (complete inability to move the affected joint), muscle atrophy (develops within 1–2 weeks of denervation), and loss of sensation in the autonomous zone of the nerve — a specific area of skin supplied exclusively by that nerve with no overlap from adjacent nerves. The autonomous zones for the major limb nerves are: radial nerve — dorsal surface of the paw (digit III); femoral nerve — medial crus (saphenous nerve autonomous zone); peroneal nerve — dorsal surface of the pes; tibial nerve — plantar surface of the pes. If the dog has pain on palpation of a muscle belly but can still move the joint, it is a muscle injury. If the dog has complete paralysis of a joint and loss of sensation in the autonomous zone, it is a nerve injury.
'Sweeney' is the colloquial term for suprascapular nerve paralysis with subsequent atrophy of the supraspinatus and infraspinatus muscles. The suprascapular nerve (C6–C7) wraps around the cranial border of the scapular neck and is vulnerable to stretch or compression injury from: direct trauma to the point of the shoulder, abduction injury (the forelimb is pulled away from the body), or iatrogenic injury during shoulder surgery. Clinical signs: (1) the shoulder abducts (moves away from the body wall) during weight-bearing — this is the most characteristic sign, (2) the scapular spine becomes prominent ('knife-edge scapula') as the supraspinatus and infraspinatus atrophy, and (3) the dog may have a mild, mechanical lameness but CAN bear weight (unlike radial nerve paralysis). The diagnosis is clinical; electromyography (EMG) can confirm denervation. Many dogs recover spontaneously over 2–6 months as the nerve regenerates (neurapraxia); if the nerve is transected, the prognosis is guarded.
Self-check quiz
Test yourself. Answers are below each question — cover them first if you are studying.
- A) Biceps femoris
- B) Semitendinosus
- C) Quadriceps femoris
- D) Gastrocnemius
Show answer
Answer: C) Quadriceps femoris
The quadriceps femoris (rectus femoris + vastus lateralis + vastus medialis + vastus intermedius) is the sole extensor of the stifle. The hamstrings (biceps femoris, semitendinosus, semimembranosus) are stifle FLEXORS. The gastrocnemius extends the tarsus, not the stifle. Damage to the femoral nerve (which innervates the quadriceps) causes inability to bear weight.
- A) Trigeminal neuritis
- B) Masticatory muscle myositis (MMM)
- C) Temporomandibular joint ankylosis
- D) Myasthenia gravis
Show answer
Answer: B) Masticatory muscle myositis (MMM)
MMM is an immune-mediated myositis that specifically targets the type 2M myofibers unique to the masticatory muscles. The cardinal signs are bilaterally symmetric temporalis and masseter atrophy and trismus (inability to open the jaw). Trigeminal neuritis causes dropped jaw (inability to CLOSE the jaw), not trismus. TMJ ankylosis can cause trismus but does not cause muscle atrophy. Myasthenia gravis causes generalised weakness, not specifically masticatory muscle atrophy.
- A) Musculocutaneous nerve
- B) Radial nerve
- C) Median nerve
- D) Axillary nerve
Show answer
Answer: B) Radial nerve
The radial nerve (C7–T1) innervates the triceps brachii (elbow extensors), the carpal and digital extensors, and provides sensory innervation to the dorsal surface of the paw. The musculocutaneous nerve innervates the biceps brachii and brachialis (elbow flexors). The median nerve innervates the carpal and digital flexors. The axillary nerve innervates the deltoideus and teres major.
- A) Gastrocnemius tendon only
- B) Superficial digital flexor tendon only
- C) Both gastrocnemius and superficial digital flexor tendons
- D) Deep digital flexor tendon
Show answer
Answer: C) Both gastrocnemius and superficial digital flexor tendons
Gastrocnemius tendon rupture alone causes a dropped hock without digit curling. SDF tendon rupture alone rarely causes clinical signs. Combined gastrocnemius + SDF tendon rupture (complete Achilles mechanism rupture) causes a dropped hock AND curled digits (because the unopposed deep digital flexor flexes the digits). The DDF tendon is not part of the Achilles mechanism.
- A) Gluteal muscles
- B) Extrinsic muscles of the thoracic limb
- C) Intrinsic muscles of the thoracic limb
- D) Hypaxial muscles
Show answer
Answer: B) Extrinsic muscles of the thoracic limb
The extrinsic muscles — trapezius, rhomboideus, serratus ventralis, pectorals, latissimus dorsi, and brachiocephalicus — attach the thoracic limb to the trunk and form a muscular sling (synsarcosis) that suspends the trunk between the forelimbs. Damage to any component (especially the serratus ventralis) causes a characteristic gait abnormality and muscle atrophy.
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