Small Animal Forelimb Nerves: Anatomy and Clinical Applications

Learning Objectives and Evaluation

• Target Learning Objectives (LO): $1-3$ based on Module $2$ guidelines.
• Resources: Materials are listed in Module/Chapter $2$.
• Instructor: Dr. Cristian Martonos, DVM, MSc, PhD.
• Assessment Details: Total of $20$ questions relative to this material.
• Primary Academic Goals:
  • Identify and describe the brachial plexus and its associated structures.
  • Identify and describe the specific nerves found in the canine thoracic limb.
  • Understand and describe the relationships between nerves and corresponding muscles.
  • Describe the cutaneous distribution and identified autonomous zones of innervation in the dog's thoracic limb.
  • Describe the clinical dysfunctions that manifest when specific nerves are damaged.

The Brachial Plexus

• Anatomical Composition: Formed by the ventral branches of spinal nerves $C6-T2$.
• Function: Provides the primary innervation (both somatic motor and sensory) to the forelimbs.
• Nature of Nerves: Mixed nerves containing:
  • Somatic motor efferent fibers.
  • Sensory afferent fibers.
• Location: Situated within the axillary region.

Proximal Nerves of the Forelimb

• Cranial Pectoral Nerve (Cr.pectoraln.):
  • Vascular Landmark: Often runs in conjunction with the external thoracic artery (ext.thoracica.).
  • Target Muscle: Superficial pectoral muscle.
• Caudal Pectoral Nerve (Cd.pectoraln.):
  • Target Muscle: Deep pectoral muscle.
• Lateral Thoracic Nerve:
  • Vascular Landmark: Runs alongside the lateral thoracic artery.
  • Proximity: Located near the axillary lymph node.
  • Function: Serves as the motor nerve to the cutaneous trunci muscle.
  • Clinical Significance: This nerve is essential for testing the panniculus reflex.
• Thoracodorsal Nerve:
  • Vascular Landmark: Runs with the thoracodorsal artery.
  • Target Muscle: Latissimus dorsi muscle.
  • Regional View: Best observed in a ventro-medial view of the axillary region against the thoracic wall.

Scapular and Brachial Nerves

• Suprascapular Nerve:
  • Path: Travels laterally between the subscapularis muscle and the supraspinatus muscle.
  • Target Muscles: Infraspinatus and supraspinatus muscles.
  • Protection: Anatomically protected by the acromion process.
• Subscapular Nerve:
  • Variation: May present with multiple visible branches.
  • Target Muscle: Subscapularis muscle.
• Musculocutaneous Nerve:
  • Somatic Motor Targets: Coracobrachialis muscle, biceps brachii muscle, and brachialis muscle.
  • Primary Motor Function: Elbow flexion.
  • Sensory Component: Provides innervation to the medial aspect of the antebrachium via the medial cutaneous antebrachial nerve.
• Axillary Nerve:
  • Path: Enters the musculoskeletal space between the subscapularis and the teres major muscles.
  • Somatic Motor Targets: Teres major muscle, teres minor muscle, deltoideus muscle, and a portion of the subscapularis muscle.
  • Sensory Component: Provides sensation to the caudal scapular region and the craniolateral brachium.

The Radial Nerve

• Profile: The largest nerve of the forelimb.
• Pathology:
  • Enters the triceps between the long head and the medial head.
  • Travels parallel to the deep brachial artery, which serves as an important surgical and anatomical landmark.
  • Spirals caudally around the humerus to emerge on the lateral side.
  • Emerges laterally between the triceps and the brachialis muscles.
• Somatic Motor Targets (Proximal Brachium):
  • Triceps brachii muscle: Crucial for weight bearing capabilities.
  • Tensor fasciae antebrachii muscle.
  • Anconeus muscle.
• Branches:
  • Deep Branch: The primary motor supply to the extensors of the carpus and digits.
  • Superficial Branch: Known as the lateral cutaneous antebrachial nerve; travels with the cephalic vein on the cranial aspect of the antebrachium.
• Sensory Component: Provides sensation to the dorsal manus (the back of the paw).

Median and Ulnar Nerves

• Common Origin: These nerves originate at the brachial plexus as a common trunk.
• General Shared Function: Both nerves innervate the flexors of the carpus and manus (motor function).
• Median Nerve:
  • Anatomy: Positioned in contact with the caudal surface of the brachial artery.
  • Location: Found immediately cranial to the medial epicondyle of the humerus.
  • Motor Targets: Pronator teres muscle, pronator quadratus muscle, flexor carpi radialis muscle, superficial digital flexor ($SDF$) muscle, and parts of the deep digital flexor ($DDF$) muscle.
  • Distal Path: Continues distally with the median artery through the carpal canal.
  • Sensory Component: Shares innervation of the palmar aspect of the paw with the ulnar nerve.
• Ulnar Nerve:
  • Anatomy: In contact with the collateral ulnar artery.
  • Location: Found immediately caudal to the medial epicondyle (colloquially referred to as the "funny-bone").
  • Motor Targets: Flexor carpi ulnaris ($FCU$) muscle and the deep digital flexor ($DDF$) muscle.
  • Path: Passes through the carpal canal.
  • Sensory Component:
    • The palmar aspect of the antebrachium.
    • The palmar aspect of the manus (overlap with the median nerve).
    • Autonomous Zone: Provides the exclusive sensory innervation to the fifth digit ($5^{th}$ digit).

Venous Landmarks of the Thoracic Limb

• Cephalic Vein: Located on the cranial aspect of the antebrachium.
• Accessory Cephalic Vein: Associated with the superficial branches of the radial nerve.
• Median Cubital Vein:
  • Branch given off by the cephalic vein near the cranial (flexor) surface of the elbow.
  • Function: Anastomoses with the brachial vein.

Autonomous Zones

• Definition: An area of skin sensation provided by only ONE specific peripheral nerve, with no overlap from adjacent nerves.
• Contrast: Most areas of the skin are innervated by multiple nerves (overlap zones).
• Specific Canine Autonomous Zones:
  • Ulnar Nerve: The $5^{th}$ digit.
  • Radial Nerve: The dorsum (dorsal surface) of the manus (paw).
  • Axillary Nerve: Lateral aspect of the brachium/caudal scapular region.
  • Musculocutaneous Nerve: Medial aspect of the antebrachium.

Clinical Consequences of Nerve Damage

Radial Nerve Damage

• Proximal Injury (At the Brachial Plexus):
  • Loss of ability to extend the elbow, carpus, and digits.
  • Patient cannot support weight on the forelimb.
• Distal Injury (Distal to Triceps Innervation):
  • Loss of ability to extend the carpus and digits.
  • The animal can still extend and fix the elbow joint.
  • Clinical Presentation: The animal will "knuckle over" at the carpus.
• Sensory Loss: Loss of cutaneous sensation on the cranial aspect of the antebrachium and the dorsal aspect of the paw.

Median and Ulnar Nerve Damage

• Motor Loss: Reduction or complete loss of the ability to flex the carpus and digits.
• Clinical Presentation: Sinking (hyperextension) of the carpus and fetlock.
• Sensory Loss:
  • Loss of sensation to the palmar aspect of the paw.
  • For ulnar nerve damage specifically: Loss of cutaneous sensation to the lateral aspect of the paw and the $5^{th}$ digit.

Axillary Nerve Damage

• Motor Loss: Loss of shoulder flexors (deltoideus, teres major, teres minor, part of subscapularis); decreased shoulder stability.
• Clinical Presentation: Historically referred to as "dropped shoulder."
• Sensory Loss: Lateral aspect of the brachium and caudal scapular region.

Musculocutaneous Nerve Damage

• Motor Loss: Decrease or lack of ability to flex the elbow (affects coracobrachialis, biceps brachii, and brachialis muscles).
• Sensory Loss: Medial aspect of the forearm.

Summary of Thoracic Limb Innervation and Supply