Innervation of the Back — MCQs
Which of the following is NOT a hybrid muscle?
What is the nerve supply of the shown muscle?
During a knife fight, a person is injured in the neck region and presents with weakness in raising the right arm above the head. On further examination, winging of the right scapula is noted. The injury has damaged:
A patient diagnosed with sciatica has tender hamstrings. Which of the following nerves supplies a hybrid muscle that is partially spared in this patient?
Which of the following is not a branch of the posterior cord of the brachial plexus?
Which of the following is not a branch of the cervical plexus?
Which muscle paralysis can cause winging of the scapula?
Traumatic anterior dislocation of shoulder with sensory loss in lateral side of forearm and weakness of flexion of elbow joint, most likely injured nerve is:
In a diving accident that severed the spinal cord below the sixth cervical vertebra, which of the following muscles would be affected?
All of the following muscles have dual nerve supply, EXCEPT?
Innervation of the Back Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is NOT a hybrid muscle?
- A. Sternocleidomastoid
- B. Flexor pollicis brevis
- C. Brachialis
- D. Adductor pollicis (Correct Answer)
Explanation: ***Adductor pollicis*** - The adductor pollicis is solely innervated by the **deep branch of the ulnar nerve (C8, T1)**, making it a non-hybrid muscle. - Its primary actions are **adduction, opposition, and flexion of the thumb**. *Sternocleidomastoid* - This muscle is considered hybrid because it is innervated by two different nerves: the **spinal accessory nerve (CN XI)** and branches from the **cervical plexus (C2-C3)**. - The spinal accessory nerve innervates primarily the motor function, while the cervical plexus provides proprioceptive fibers. *Flexor pollicis brevis* - This muscle often has a dual innervation, with its superficial head supplied by the **median nerve** and its deep head by the **ulnar nerve** [1]. - This dual innervation pattern qualifies it as a hybrid muscle [1]. *Brachialis* - The brachialis muscle is typically innervated by the **musculocutaneous nerve (C5, C6)**, but it also receives a small contribution from the **radial nerve (C7)**. - This additional supply from the radial nerve makes it a hybrid muscle.
Question 2: What is the nerve supply of the shown muscle?
- A. Suprascapular
- B. Dorsal scapular (Correct Answer)
- C. Dorsal rami of C1
- D. Subscapular
Explanation: ***Dorsal scapular*** - The image points to the **levator scapulae muscle**, which elevates and rotates the scapula. - The **dorsal scapular nerve** (C5 root, with contributions from C3-C4) innervates the levator scapulae, as well as the rhomboid major and minor muscles. - This nerve arises from the C5 root of the brachial plexus and pierces through the middle scalene muscle. - Clinically, injury to the dorsal scapular nerve can cause **medial scapular winging** and difficulty elevating the shoulder. *Suprascapular* - The suprascapular nerve (C5-C6) primarily innervates the **supraspinatus and infraspinatus muscles**, which are involved in rotator cuff function. - It does not supply the levator scapulae muscle. - This nerve passes through the suprascapular notch beneath the superior transverse scapular ligament. *Dorsal rami of C1* - The **dorsal ramus of C1** (suboccipital nerve) primarily innervates the muscles of the suboccipital triangle: rectus capitis posterior major and minor, obliquus capitis superior and inferior. - These nerves are involved in fine head and neck movements but do not innervate the levator scapulae. - The levator scapulae receives segmental innervation from C3-C4 cervical nerves directly, in addition to the dorsal scapular nerve. *Subscapular* - The subscapular nerves (upper and lower, from C5-C6) innervate the **subscapularis muscle**, which is part of the rotator cuff. - They also innervate the **teres major muscle**, but not the levator scapulae. - These are branches from the posterior cord of the brachial plexus.
Question 3: During a knife fight, a person is injured in the neck region and presents with weakness in raising the right arm above the head. On further examination, winging of the right scapula is noted. The injury has damaged:
- A. Long thoracic nerve of Bell
- B. Dorsal scapular nerve
- C. Suprascapular nerve
- D. Spinal accessory nerve (Correct Answer)
Explanation: ***Spinal accessory nerve*** - **Weakness in raising the arm above the head** and **winging of the scapula** are characteristic signs of **trapezius muscle dysfunction**, which is supplied by the **spinal accessory nerve (CN XI)**. - The trapezius is essential for **upward rotation of the scapula** during overhead arm abduction (>90°). - Injury to the spinal accessory nerve in the posterior triangle of the neck causes **lateral winging** of the scapula (inferior angle moves laterally), which is most prominent when attempting to raise the arm overhead. - The combination of **scapular winging** + **inability to abduct the arm above horizontal** is pathognomonic for trapezius paralysis. *Long thoracic nerve of Bell* - Damage to the long thoracic nerve causes paralysis of the **serratus anterior muscle**, leading to **medial winging** of the scapula (medial border lifts away from chest wall). - While scapular winging occurs, it is most prominent during **forward flexion** or **pushing movements** (e.g., push-ups, pushing against a wall), not specifically when raising the arm overhead. - Patients can usually still abduct the arm overhead, though with altered scapular mechanics. *Dorsal scapular nerve* - The dorsal scapular nerve innervates the **rhomboid major and minor muscles** and the **levator scapulae**. - Injury primarily causes difficulty **retracting the scapula** (pulling shoulders back) and weakness in shoulder elevation. - Does **not** cause scapular winging or significant weakness in overhead arm movement. *Suprascapular nerve* - The suprascapular nerve innervates the **supraspinatus** and **infraspinatus muscles**. - Damage causes weakness of shoulder **initiation of abduction** (first 15° by supraspinatus) and **external rotation** (infraspinatus). - Does **not** cause scapular winging, as these are rotator cuff muscles, not scapular stabilizers.
Question 4: A patient diagnosed with sciatica has tender hamstrings. Which of the following nerves supplies a hybrid muscle that is partially spared in this patient?
- A. Femoral
- B. Common peroneal nerve (Correct Answer)
- C. Obturator
- D. Tibial
- E. Superior gluteal
Explanation: ***Common peroneal nerve*** - The **short head of the biceps femoris** is a unique "hybrid" muscle, supplied by the **common peroneal nerve**, while the **long head** is supplied by the **tibial nerve**. - If the hamstrings are tender and sciatica is present, but this specific muscle's function is spared, it points towards the common peroneal nerve being the relevant nerve for the spared portion. *Femoral* - The **femoral nerve** primarily innervates the **anterior compartment of the thigh**, including the quadriceps femoris. - It does not supply any part of the hamstring muscles. *Obturator* - The **obturator nerve** primarily innervates the **medial compartment of the thigh**, which consists of adductor muscles. - It does not contribute to the innervation of the hamstring muscles. *Superior gluteal* - The **superior gluteal nerve** innervates the **gluteus medius**, **gluteus minimus**, and **tensor fasciae latae** muscles. - It does not supply any hamstring muscles. *Tibial* - The **tibial nerve** innervates most of the hamstring muscles (semitendinosus, semimembranosus, and the long head of the biceps femoris). - If the hamstrings are tender, involvement of the tibial nerve would likely lead to more widespread hamstring weakness rather than a partially spared scenario involving the short head of the biceps femoris.
Question 5: Which of the following is not a branch of the posterior cord of the brachial plexus?
- A. Thoracodorsal nerve
- B. Axillary nerve
- C. Long thoracic nerve (Correct Answer)
- D. Radial nerve
Explanation: ### Long thoracic nerve - The **long thoracic nerve** originates directly from the **nerve roots C5, C6, C7** of the brachial plexus, **NOT from the posterior cord**. - It innervates the **serratus anterior muscle**, crucial for scapular protraction and upward rotation. - Damage causes **winged scapula** deformity. ### Axillary nerve - The **axillary nerve** is a **terminal branch of the posterior cord**, formed from C5-C6. - It supplies the **deltoid** and **teres minor muscles**, and provides sensory innervation to the lateral shoulder (regimental badge area). - Commonly injured in anterior shoulder dislocations or humeral surgical neck fractures. ### Thoracodorsal nerve - The **thoracodorsal nerve** (nerve to latissimus dorsi) is a **branch of the posterior cord**, deriving from C6-C8 [1]. - It exclusively innervates the **latissimus dorsi muscle**, responsible for shoulder adduction, extension, and internal rotation [1]. - Important in breast reconstruction surgery (latissimus dorsi flap). ### Radial nerve - The **radial nerve** is the **largest terminal branch of the posterior cord**, formed from C5-T1. - It innervates the **triceps brachii** and all extensor muscles of the forearm (wrist and finger extensors). - Provides sensory innervation to the posterior arm, forearm, and anatomical snuffbox. - Most commonly injured nerve of the upper limb (spiral groove fractures).
Question 6: Which of the following is not a branch of the cervical plexus?
- A. Suprascapular nerve (Correct Answer)
- B. Supraclavicular nerve
- C. Lesser occipital nerve
- D. Greater auricular nerve
Explanation: ***Suprascapular nerve*** - The **suprascapular nerve** originates from the **brachial plexus** (specifically the upper trunk), not the cervical plexus. - It primarily innervates the **supraspinatus** and **infraspinatus muscles**. *Lesser occipital nerve* - The **lesser occipital nerve** is a cutaneous branch of the **cervical plexus** (C2) that supplies the skin behind the ear. - It provides sensory innervation to the **scalp posterior to the auricle**. *Greater auricular nerve* - The **greater auricular nerve** is a branch of the **cervical plexus** (C2, C3) and provides sensory innervation to the skin over the parotid gland, mastoid process, and auricle. - It supplies sensation to the **external ear** and the **angle of the mandible**. *Supraclavicular nerve* - The **supraclavicular nerves** (C3, C4) are cutaneous branches of the **cervical plexus** that provide sensory innervation to the skin over the shoulder and upper chest. - They provide sensory innervation to the skin overlying the **clavicle** and the **pectoral region**.
Question 7: Which muscle paralysis can cause winging of the scapula?
- A. Teres minor
- B. Deltoid
- C. Serratus anterior (Correct Answer)
- D. Supraspinatus
Explanation: ***Serratus anterior*** - Paralysis or weakness of the **serratus anterior muscle**, innervated by the **long thoracic nerve**, causes winging of the scapula. - This muscle is responsible for **protraction and upward rotation of the scapula**, keeping it flat against the chest wall; without its function, the medial border of the scapula protrudes posteriorly. *Teres minor* - The **teres minor** is part of the rotator cuff and is involved in **external rotation** and **adduction of the arm**. - Its paralysis would primarily affect shoulder movement and stability, but not directly lead to **scapular winging**. *Deltoid* - The **deltoid muscle** is the primary muscle for **abduction of the arm** beyond the first 15 degrees. - Paralysis of the deltoid would result in significant difficulty lifting the arm, but it does not cause the **scapula to wing**. *Supraspinatus* - The **supraspinatus muscle** initiates **abduction of the arm** (first 15 degrees) and helps stabilize the shoulder joint. - Its paralysis would impair arm elevation but does not directly result in **scapular winging**.
Question 8: Traumatic anterior dislocation of shoulder with sensory loss in lateral side of forearm and weakness of flexion of elbow joint, most likely injured nerve is:
- A. Ulnar nerve
- B. Axillary nerve
- C. Radial nerve
- D. Musculocutaneous nerve (Correct Answer)
Explanation: ***Musculocutaneous nerve*** - The **musculocutaneous nerve** innervates the biceps brachii and brachialis muscles, responsible for **elbow flexion**. - It also provides sensory innervation to the **lateral forearm** via the **lateral cutaneous nerve of the forearm**, explaining the sensory loss described. *Ulnar nerve* - The ulnar nerve primarily innervates muscles of the **hand** and gives sensory supply to the medial 1 and 1/2 digits. - Its injury would typically lead to weakness in **finger adduction/abduction** and sensory loss in the medial hand, not the lateral forearm. *Axillary nerve* - The axillary nerve innervates the **deltoid** and **teres minor** muscles, causing weakness in **shoulder abduction** and external rotation upon injury. - Sensory loss would be over the **regimental badge area** (lateral shoulder), not the lateral forearm. *Radial nerve* - The radial nerve innervates the **extensor muscles of the wrist and fingers**, and the triceps. - Injury would result in **wrist drop** and sensory loss over the **posterior arm, forearm, and hand**, not lateral forearm sensory loss.
Question 9: In a diving accident that severed the spinal cord below the sixth cervical vertebra, which of the following muscles would be affected?
- A. Deltoid
- B. Infraspinatus
- C. Levator Scapulae
- D. Latissimus Dorsi (Correct Answer)
Explanation: ***Latissimus Dorsi*** - The **latissimus dorsi muscle** is primarily innervated by the **thoracodorsal nerve**, which arises from the **C6, C7, and C8** nerve roots (with C7 and C8 being the predominant contributors) [1]. - A spinal cord injury below the sixth cervical vertebra would affect the C7 and C8 segments, thereby disrupting the nerve supply to the latissimus dorsi, leading to weakness or paralysis. - This muscle is responsible for adduction, extension, and internal rotation of the shoulder. *Deltoid* - The **deltoid muscle** is innervated by the **axillary nerve**, which arises predominantly from the **C5 and C6** nerve roots. - Since the injury is below the C6 vertebra, the upper cervical segments (C5 and C6) would remain intact above the level of injury. - Therefore, deltoid function would be preserved. *Infraspinatus* - The **infraspinatus muscle** is innervated by the **suprascapular nerve**, which arises from the **C5 and C6** nerve roots. - Similar to the deltoid, its innervation originates above the level of the spinal cord injury and would be spared. *Levator Scapulae* - The **levator scapulae muscle** receives innervation from the **C3, C4, and C5** spinal nerves, as well as contributions from the dorsal scapular nerve (predominantly C5). - All of these nerve roots originate well above the level of injury, so this muscle would not be affected.
Question 10: All of the following muscles have dual nerve supply, EXCEPT?
- A. Flexor digitorum profundus
- B. Pectineus
- C. Brachialis
- D. Flexor digitorum superficialis (Correct Answer)
Explanation: No changes were made to the original explanation because none of the provided references met the relevance criteria for the specific muscles and nerves discussed. ***Flexor digitorum superficialis*** - This muscle is solely innervated by the **median nerve**. - It works to **flex the middle phalanges** of the medial four digits. *Flexor digitorum profundus* - The medial half of the muscle, which supplies the ring and little fingers, is innervated by the **ulnar nerve**. - The lateral half, which supplies the index and middle fingers, is innervated by the **anterior interosseous nerve** (a branch of the median nerve). *Pectineus* - This muscle typically receives innervation from both the **femoral nerve** and the **obturator nerve**. - Its primary action is **adduction and flexion of the hip**. *Brachialis* - While primarily innervated by the **musculocutaneous nerve**, a small component also receives innervation from the **radial nerve**. - It is a powerful **flexor of the elbow joint**.
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