Thoracic Nerves (Small Animal) Quiz

Brachial Plexus

• Composed of ventral branches of spinal nerves from C6-T2
• Innervation to the forelimb is mixed with sensory afferent and motor efferent
• Located in the axillary region

Proximal Nerves of the Forelimb

• Cranial pectoral nerve:
  • Runs with the external thoracic artery
  • Targets the superficial pectoral muscle
• Caudal pectoral nerve:
  • Targets the deep pectoral muscle
• Lateral thoracic nerve:
  • Runs with the lateral thoracic artery, near the axillary lymph node
  • Targets the motor neuron to the cutaneous trunci muscle, involved in the panniculus reflex
• Thoracodorsal nerve:
  • Runs with the thoracodorsal artery
  • Targets the latissimus dorsi muscle

Nerves of the Forelimb

• Suprascapular nerve:
  • Travels laterally between the subscapularis muscle and the supraspinatus muscle
  • Targets the infraspinatus muscle and supraspinatus muscle
  • Protected by the acromion of the scapula
• Subscapular nerve:
  • May have multiple visible branches
  • Targets the subscapularis muscle
• Musculocutaneous nerve:
  • Provides somatic motor innervation to the coracobrachialis muscle, biceps brachii muscle, and brachialis muscle
  • Provides sensory to the medial aspect of the antebrachium via the medial cutaneous antebrachial nerve
  • Motor function is elbow flexion
• Axillary nerve:
  • Enters the space between the subscapularis muscle and the teres major muscle
  • Provides somatic motor innervation to the teres major muscle, teres minor muscle, deltoideus muscle, and part of the subscapularis muscle
  • Provides sensory to the caudal scapular region and craniolateral brachium
• Radial nerve:
  • Enters the triceps brachii muscle between the long head and medial head
  • Spirals caudally around the humerus and emerges laterally between the triceps brachii muscle and the brachialis muscle
  • Provides somatic motor innervation to the triceps brachii muscle, tensor fascia antebrachii muscle, and anconeus muscle
  • Necessary for weight bearing
  • Deep branch provides somatic motor innervation to the extensors of the carpus and digits
  • Superficial branch (lateral cutaneous antebrachial nerve) provides sensory innervation to the dorsal manus
• Median nerve:
  • Both the median nerve and the ulnar nerve start as a common trunk
  • Provides sensory innervation to the palmar aspect of the manus
  • Targets and provides sensory innervation to the pronator teres muscle, pronator quadratus muscle, flexor carpi radialis muscle, superficial digital flexor muscle, and deep digital flexor muscle
  • Continues distally with the median artery through the carpal canal
• Ulnar nerve:
  • In contact with the collateral ulnar artery and runs towards the caudal aspect of the elbow
  • Lateral to the medial epicondyle of the humerus
  • Provides somatic motor innervation to the flexor carpi ulnaris muscle and deep digital flexor muscle
  • Provides sensory innervation to the palmar aspect of the antebrachium and palmar aspect of the manus

Autonomous Zones

• An autonomous zone refers to an area of sensation provided by only one nerve without overlap from adjacent nerves
• Examples of autonomous zones include:
  • Sensory innervation to the 5th digit, which is an autonomous zone for the ulnar nerve
  • Most areas of the skin are innervated by multiple nerves
  • Nerves supplying autonomous zones in the forelimb include the ulnar nerve, radial nerve, and musculocutaneous nerve

Radial Nerve Damage

• Proximal injury:
  • Results in loss of ability to extend the elbow, carpus, or digits
  • Causes loss of ability to support weight on that forelimb
  • Refers to damage occurring at the brachial plexus prior to innervation to the triceps muscle
• Distal injury:
  • Refers to damage occurring after innervation to the triceps muscle
  • Results in loss of cutaneous sensation (superficial branch) on the cranial aspect of the antebrachium and the dorsal aspect of the paw

Median and Ulnar Nerve Damage

• Results in:
  • Reduction or complete loss of the ability to flex the carpus and digits
  • Sinking (hyperextension) of the carpus and fetlock
  • Loss of sensation to the palmar aspect of the paw
  • Loss of cutaneous sensation to the lateral aspect of the 5th digit specifically linked to ulnar nerve damage

Axillary Nerve Damage

• Results in:
  • Loss of motor innervation to the flexors of the shoulder (deltoideus, teres major, teres minor, and part of the subscapularis muscle), causing a "dropped shoulder" appearance
  • Loss of sensation to the lateral aspect of the brachium and caudal scapular region