Nerves Flashcards
Why does the injury to the radial nerve lead to wrist drop?
Injury to the radial nerve leads to wrist drop because the radial nerve innervates the muscles responsible for extending the wrist and fingers, primarily the extensor group (e.g., extensor carpi radialis, extensor carpi ulnaris, extensor digitorum). Here’s why wrist drop occurs:
1. Loss of Extension: The radial nerve supplies the muscles that extend the wrist and fingers. Injury to the radial nerve results in paralysis of these extensor muscles, leading to the inability to extend the wrist.
2. Flexion Dominance: With the extensor muscles paralyzed, the flexor muscles (innervated by other nerves like the median and ulnar nerves) act unopposed, causing the wrist to fall into flexion. This results in the characteristic “wrist drop” posture, where the wrist is unable to remain extended.
3. Loss of Finger Extension: In severe cases of radial nerve injury, finger extension is also impaired, further contributing to the appearance of wrist drop.
Explain why Median Nerve Injury Causes Ape Hand Deformity.
Median nerve injury leads to ape hand deformity due to the specific muscles innervated by the median nerve and their loss of function:
1. Loss of Function in Thenar Muscles: The median nerve innervates the thenar muscles (opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis), which are responsible for thumb opposition, abduction, and flexion. When the median nerve is damaged, these muscles become paralyzed.
2. Loss of Thumb Opposition: The loss of thumb opposition is a key feature of ape hand deformity. The thumb cannot move across the palm to touch the other fingers, as the opponens pollicis muscle, which facilitates this movement, is paralyzed.
3. Atrophy of Thenar Eminence: The thenar eminence (the muscular bulge at the base of the thumb) atrophies due to the loss of innervation, resulting in a flattened palm and the characteristic appearance of the “ape hand.”
4. Motor Dysfunction in the First and Second Digits: In addition to the thenar muscles, the median nerve innervates the lateral half of the flexor digitorum profundus (which flexes the distal interphalangeal joints of the index and middle fingers) and the lumbricals for the index and middle fingers. Loss of function here leads to an inability to flex these fingers fully, causing the fingers to remain extended.
Thus, ape hand deformity is characterized by thumb adduction (inability to oppose the thumb), atrophy of the thenar eminence, and inability to flex the first and second fingers, all resulting from median nerve injury.
Why does the ulnar nerve injury result in claw hand deformity?
Ulnar nerve injury leads to claw hand deformity because the ulnar nerve innervates the intrinsic hand muscles, which are responsible for fine motor control and maintaining the normal posture of the fingers. Here’s why the deformity occurs:
1. Loss of Function in Intrinsic Muscles: The ulnar nerve innervates important intrinsic hand muscles, such as the lumbricals (for the 4th and 5th fingers), interossei (responsible for finger abduction and adduction), and the adductor pollicis. These muscles help flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints.
2. Imbalance Between Flexors and Extensors: When the ulnar nerve is injured, the lumbricals and interossei muscles are paralyzed, leading to a loss of flexion at the MCP joints and extension at the IP joints. This causes the fingers to adopt a claw-like position where the MCP joints are hyperextended and the IP joints are flexed.
3. Loss of Fine Motor Control: The ulnar nerve injury particularly affects the 4th and 5th fingers, which become more pronounced in the deformity, as these fingers depend heavily on the ulnar innervation for fine motor control and positioning.
In summary, claw hand deformity occurs due to ulnar nerve injury because the loss of intrinsic hand muscle function results in hyperextension of the MCP joints and flexion of the IP joints, especially in the 4th and 5th fingers. This gives the characteristic claw-like appearance of the hand.
What is the anatomical basis of Erb-Duchenne Palsy, Waiter’s Tip Deformity?
Erb-Duchenne Palsy (also known as Waiter’s Tip Deformity) results from injury to the upper trunk of the brachial plexus, specifically affecting the C5 and C6 nerve roots. This injury typically occurs due to traction on the arm, such as during a difficult delivery or trauma (e.g., motorcycle accidents). Here’s the anatomical basis:
1. Injury to C5 and C6 Nerve Roots: The upper trunk of the brachial plexus is formed by the union of the C5 and C6 nerve roots. These roots give rise to several important nerves, including those innervating muscles responsible for shoulder abduction, elbow flexion, and forearm supination.
2. Motor Dysfunction:
• Shoulder Muscles: The injury affects the supraspinatus and infraspinatus muscles (responsible for shoulder abduction and external rotation), leading to loss of shoulder abduction and external rotation.
• Biceps and Brachialis Muscles: The biceps brachii and brachialis muscles (responsible for elbow flexion) are also affected, resulting in weak or absent elbow flexion.
• Forearm Supinators: The supinator muscle, responsible for forearm supination, is also paralyzed, causing the forearm to remain pronated.
3. Clinical Presentation (Waiter’s Tip Deformity): The combination of muscle paralysis leads to the classic “Waiter’s Tip” position:
• The arm is held adducted (due to paralysis of the deltoid and supraspinatus),
• The elbow is extended (due to paralysis of the biceps and brachialis),
• The forearm is pronated (due to loss of supinator function),
• The hand is flexed (due to lack of innervation to the wrist and hand flexors).
How does Klumpke’s Palsy occur due to lower brachial plexus injury?
Klumpke’s Palsy (or Klumpke’s paralysis) occurs due to injury to the lower trunk of the brachial plexus (typically involving C8 and T1 nerve roots), which affects the intrinsic muscles of the hand. Here’s the mechanism:
1. Injury to Lower Trunk (C8-T1): The lower trunk of the brachial plexus, formed by the C8 and T1 nerve roots, gives rise to nerves that innervate the intrinsic hand muscles (like the lumbricals, interossei, and thenar muscles), which control fine motor movements and finger positioning.
2. Motor Dysfunction:
• Loss of Intrinsic Hand Muscles: The lumbricals and interossei (responsible for finger flexion and extension) are paralyzed, leading to hyperextension of the MCP joints and flexion of the IP joints.
• Claw Hand: This causes the claw hand deformity, where the 4th and 5th fingers are affected, resulting in hyperextension of the MCP joints and flexion of the IP joints. In severe cases, the 1st dorsal interosseous and adductor pollicis are also paralyzed, impairing thumb function.
3. Clinical Presentation: The characteristic claw hand deformity results from the loss of motor control in the ulnar-innervated muscles, leading to finger flexion and extension issues.
In summary, Klumpke’s Palsy causes claw hand deformity due to damage to the C8-T1 nerve roots, affecting the intrinsic hand muscles and leading to loss of finger control.
Why does long thoracic nerve injury cause winging of the scapula?
Long thoracic nerve injury causes winging of the scapula due to the paralysis of the serratus anterior muscle, which is innervated by the long thoracic nerve. Here’s why it happens:
1. Function of Serratus Anterior: The serratus anterior muscle is responsible for protracting (pulling the scapula forward) and holding the scapula against the thoracic wall. It also helps in upward rotation of the scapula, essential for raising the arm above the head.
2. Effect of Injury: When the long thoracic nerve is injured, the serratus anterior muscle becomes weak or paralyzed, which allows the scapula to move away from the thoracic wall. This results in the scapula winging outward, as the muscle can no longer hold it flat against the chest.
3. Clinical Presentation: The winging is most apparent when the patient pushes against a surface (e.g., wall push-up), and the scapula protrudes posteriorly, creating a “winged” appearance.
In summary, long thoracic nerve injury causes scapular winging due to the paralysis of the serratus anterior, leading to the scapula protruding away from the thoracic wall.
Explain the significance of the ulnar nerve’s superficial location at the medial epicondyle
The ulnar nerve is particularly vulnerable at its superficial location near the medial epicondyle of the humerus because:
1. Superficial Position: The ulnar nerve runs just beneath the skin and passes around the medial epicondyle, an area that is unprotected by muscle or other structures. This makes it susceptible to injury or compression.
2. Vulnerability to Trauma: The nerve is prone to direct trauma (such as a blow to the elbow), often referred to as the “funny bone” sensation, which results in a tingling or electric shock-like feeling along the ulnar distribution (the pinky and half of the ring finger).
3. Compression and Cubital Tunnel Syndrome: The ulnar nerve passes through a narrow space called the cubital tunnel at the medial epicondyle. Prolonged pressure or repetitive elbow flexion can lead to cubital tunnel syndrome, causing symptoms like numbness, tingling, and weakness in the hand, especially the pinky and ring fingers.
Why is carpal tunnel syndrome associated with median nerve compression?
Carpal tunnel syndrome is associated with median nerve compression because the median nerve passes through the carpal tunnel, a narrow passageway in the wrist. Here’s why compression occurs and its significance:
1. Anatomy of the Carpal Tunnel: The carpal tunnel is formed by the carpal bones on the bottom and the flexor retinaculum (a thick ligament) on the top. The median nerve, along with the tendons of the flexor muscles, passes through this tunnel.
2. Narrowing of the Tunnel: Any condition that causes swelling or thickening of the tendons (such as inflammation from overuse) or the flexor retinaculum (e.g., in conditions like arthritis or pregnancy-related fluid retention) can reduce the space available in the carpal tunnel.
3. Compression of the Median Nerve: When the tunnel narrows, the median nerve becomes compressed, leading to nerve dysfunction. This results in symptoms like numbness, tingling, and pain in the thumb, index, middle, and half of the ring finger, which are the areas innervated by the median nerve.
In summary, carpal tunnel syndrome occurs due to compression of the median nerve as it passes through the narrow carpal tunnel, leading to sensory and motor dysfunction in the hand.
Clinical relevance of the quadrangular and triangular spaces.
The quadrangular and triangular spaces are anatomical regions in the shoulder that are clinically significant because they contain important neurovascular structures. Here’s their clinical relevance:
- Quadrangular Space
• Boundaries:
• Superior: Teres minor
• Inferior: Teres major
• Medial: Long head of triceps
• Lateral: Humerus
• Contents:
• Axillary nerve
• Posterior circumflex humeral artery
• Clinical Relevance:
• Quadrangular space syndrome: Compression of the axillary nerve and posterior circumflex humeral artery within this space can lead to shoulder pain, weakness in shoulder abduction, and atrophy of the deltoid.
• Injury to the axillary nerve can also result in loss of sensation over the lateral shoulder and weakness in shoulder movements. - Triangular Space
• Boundaries:
• Superior: Teres minor
• Inferior: Teres major
• Lateral: Long head of triceps
• Contents:
• Scapular circumflex artery
• Clinical Relevance:
• While less commonly affected than the quadrangular space, compression or injury to the scapular circumflex artery can impact the blood supply to the scapula, potentially leading to ischemic damage in certain cases.
In summary, both spaces are important for the passage of vital neurovascular structures, and their clinical relevance lies in the potential for nerve compression and vascular compromise, which can lead to functional deficits in the shoulder.
How does the injury to the posterior cord of brachial plexus affect the shoulder and arm movements?
Injury to the posterior cord of the brachial plexus affects several important muscles and can significantly impact shoulder and arm movements. Here’s how:
- Muscles Affected
The posterior cord gives rise to several key nerves:
• Axillary nerve (innervates the deltoid and teres minor)
• Radial nerve (innervates the triceps brachii, anconeus, and muscles of the forearm and hand)
- Impact on Shoulder Movements
• Deltoid muscle: The axillary nerve innervates the deltoid, which is responsible for shoulder abduction, flexion, and extension. Injury leads to weakness or paralysis of the deltoid, impairing shoulder abduction.
• Teres minor: Also innervated by the axillary nerve, the teres minor assists with external rotation of the shoulder. Injury causes weakness in this movement. - Impact on Arm Movements
• Triceps brachii: The radial nerve innervates the triceps, which is responsible for elbow extension. Injury causes weakness or paralysis of the triceps, impairing the ability to extend the elbow.
• Forearm and hand muscles: The radial nerve also innervates the forearm extensors and wrist extensors. Injury can lead to wrist drop (inability to extend the wrist) and weakness in forearm extension. - Clinical Manifestation
• Shoulder instability due to weakness in the deltoid and teres minor muscles.
• Weakness or loss of elbow extension (due to triceps paralysis).
• Wrist drop (due to radial nerve involvement).
• Loss of sensation over the posterior arm, forearm, and part of the hand (due to radial nerve sensory branches).
Why is the musculocutaneous nerve spared in upper trunk injuries?
The musculocutaneous nerve is often spared in upper trunk brachial plexus injuries because of its unique origin and anatomical pathway:
1. Origin: The musculocutaneous nerve arises from the lateral cord of the brachial plexus, which is formed by the C5, C6, and C7 nerve roots. The upper trunk injury typically involves C5 and C6 roots, which affects nerves originating from the upper trunk (such as the suprascapular nerve and axillary nerve), but the musculocutaneous nerve primarily originates from the lateral cord and is therefore less commonly affected by upper trunk lesions.
2. Anatomical Pathway: The musculocutaneous nerve branches off the lateral cord and travels down the arm to innervate the coracobrachialis, biceps brachii, and brachialis muscles, which are responsible for flexion at the elbow. Because the lateral cord is not usually directly impacted by an upper trunk injury, the musculocutaneous nerve is typically preserved.
What is the significance of the axillary nerve close relation to the surgical neck of the humerus?
The axillary nerve‘s close relation to the surgical neck of the humerus is significant because it makes the nerve highly vulnerable to injury in cases of fracture or dislocation at this location. Here’s why:
- Anatomical Proximity
• The axillary nerve runs around the surgical neck of the humerus, passing through the quadrangular space. This area is superficial and unprotected by muscle or other structures, making it prone to damage during trauma. - Injury Mechanisms
• Fracture of the Surgical Neck: A fracture at the surgical neck of the humerus can directly injure the axillary nerve, leading to deltoid muscle weakness, loss of shoulder abduction, and sensory loss over the lateral shoulder.
• Shoulder Dislocation: A dislocation of the shoulder, especially anterior dislocation, can stretch or compress the axillary nerve, resulting in similar deficits. - Clinical Consequences
• Deltoid Muscle Dysfunction: The axillary nerve innervates the deltoid and teres minor muscles. Injury leads to weakness or paralysis of the deltoid, impairing shoulder abduction and causing difficulty with raising the arm.
• Sensory Loss: Damage to the axillary nerve also causes numbness or tingling in the lateral shoulder (the “regimental badge” area).
