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Appendicular Skeleton: Forelimb Muscles and Joints - Vocabulary Flashcards

Joints and Joint Angles in the Appendicular Skeleton

  • Some joints have angles facing caudally and some facing cranially; the direction of the angle is determined by the anatomy of the bones that form the joint.
  • The schematic in the lecture emphasizes how the angle direction relates to movement: applying force to increase or decrease the angle results in extension or flexion, respectively, depending on the side of the joint the muscle is arranged on.
  • Elbow anatomy reminder from the lab model context:
    • The ulna has a deep \text{semilunar notch} which, per the transcript, is described as fitting into the \text{femoral condyle} (note: this is likely a transcription error; in actual anatomy the ulna articulates with the humeral trochlea at the elbow).
    • The elbow joint predominantly moves in one plane; although some overextension can occur, it is limited by the bony anatomy.
  • The position of the joint and the direction of movement determine whether a muscle action increases or decreases the joint angle.
  • Overview of the major joints of the appendicular skeleton from the schematic: the angles of these joints are oriented to help determine how muscle contraction will affect joint motion when a force is applied to the limb.
  • Concept recap: if a muscle contracts and pulls the origin and insertion closer, it tends to move the limb in a direction that changes the joint angle in the indicated way (extension vs. flexion).

Forelimb and Thoracic Girdle: Key Concepts and Attachments

  • The forelimb is not directly attached to the axial skeleton via a joint; it is attached by muscles.
  • Major muscles attaching the forelimb to the body (blue = attach to the scapula; green = attach to the humerus):
    • Trapezius (numbered as 5)
    • Latissimus dorsi (numbered as 12)
    • Brachiocephalicus (numbered as 6)
    • Deep pectoral (numbered as 26)
    • Serratus ventralis (numbered as 25)
  • These muscles provide both stabilization of the limb to the body and the ability to advance or draw the limb back:
    • The trapezius and the caudal part of the trapezius together with the latissimus dorsi form antagonistic groups to move the limb forward and backward.
    • The cranial (proximal) attachments tend to pull the limb forward when contracting; the caudal parts and latissimus dorsi pull the limb backward.
  • The thoracic girdle is the muscular support system that holds the forelimbs to the body and also helps support the thoracic cavity; this is especially important in quadrupeds to bear weight.
  • Serratus ventralis:
    • Described as the largest and most important muscle of the thoracic girdle for holding the trunk to the limb.
    • Multiple origins: from the cervical vertebrae down to 10^{\text{th}} costal cartilage, with tendinous slips at each costal cartilage.
    • All fibers insert on the medial aspect of the scapula.
    • Because of its broad origin and insertion, serratus ventralis is often highlighted in diagrams as a large, significant muscle for the thoracic girdle.
  • Brachial plexus and neurovascular considerations:
    • A complex network of nerves crossing the axillary region (the armpit) to form the final peripheral nerves that innervate the limb.
    • The course of these nerves can complicate clinical lymphatics, vascular supply, and innervation; a thorough review of the brachial plexus will be revisited later.
  • Other soft tissues to consider include tendons, blood vessels, nerves, and skin; these are important for surgical and clinical considerations when moving or manipulating the limb.

Major Muscle Groups of the Forelimb: Attachments and Actions

  • The forelimb muscles discussed are grouped by their attachments to the scapula (blue) versus the humerus (green).
  • Major muscles to know for lab labeling:
    • Trapezius (blue region, numbered 5) – attaches to the scapula and contributes to limb positioning.
    • Latissimus dorsi (blue region, numbered 12) – large back muscle contributing to limb movement.
    • Brachiocephalicus (blue region, numbered 6) – extends forward and helps advance the limb.
    • Deep pectoral (blue/green region, numbered 26) – between sternum and humerus; contributes to adduction of the limb.
    • Serratus ventralis (blue region, numbered 25) – key thoracic girdle muscle; origin from cervical vertebrae and ribs; inserts on the medial scapula.
  • Functional implications:
    • These muscles hold the forelimb to the body and are involved in advancing the limb forward or drawing it back.
    • Antagonistic pairs in action: the cranial (or rostral) group tends to pull the limb forward; the caudal (or caudal) group and latissimus dorsi tend to pull the limb back.
  • Thoracic girdle overview:
    • The thoracic girdle comprises the muscles around the scapula and rib cage that support the forelimb attachment and thorax.
    • In upright bipeds (humans), gravity reduces immediate thorax loading compared to quadrupeds, but in quadrupeds, these muscles are essential to support weight and maintain posture.
  • Targeted notes on serratus ventralis:
    • Anatomy: large, multi-segment origin from the cervical vertebrae to the 10^{\text{th}} costal cartilage with multiple slips; inserts on the medial border of the scapula.
    • Clinical/labeling note: serratus ventralis is often a key muscle you will be asked to label in diagrams.

Shoulder (Glenohumeral) Joint: Extensors and Flexors

  • The glenohumeral joint is the main shoulder joint; the major muscles shown here extend or flex the shoulder.
  • Extensors of the shoulder (or muscles on the cranial side of the joint):
    • Brachiocephalicus (number 6): inserts on the humerus; contraction pulls the limb forward and helps extend the shoulder.
    • Supraspinatus (originates on the scapula; inserts on the humerus): when contracted, helps to extend the shoulder by increasing the joint angle.
    • Supraspinatus as well as other posterior muscles contribute to shoulder extension.
    • Deltoid region is mentioned in a broader context of abduction/adduction and shoulder mechanics; note: in the transcript the deltoid is described as an adductor, which is atypical (the deltoid is generally an abductor in typical anatomy).
  • Flexors of the shoulder (or muscles on the caudal side of the joint):
    • Latissimus dorsi (number 12) is described in the transcript as a major flexor; functionally, latissimus dorsi is typically an adductor and extensor of the shoulder in many species, but the transcript places it among the caudal-side flexors for teaching purposes.
    • Other muscles listed as flexors include the dorsal scapular and related structures in the caudal aspect.
  • Supporting muscles influencing shoulder motion:
    • Deltoid region: described as aiding limb elevation and abduction when it contracts.
    • Deep pectoral contributes to adduction of the limb toward the body.
    • Subscapularis (medial side of scapula to humerus) assists with adduction.
  • General rule from the lecture:
    • Extensors are positioned on the cranial (rostral) side of the joint to increase the angle of the shoulder when contracted.
    • Flexors are positioned on the caudal side to decrease the shoulder angle when contracted.

Elbow Joint: Flexors and Extensors; Antagonistic Pairs

  • Elbow extension:
    • Triceps brachii (noted as having 3 heads in the typical anatomy; some sources mention “three heads” or “three heads (triceps)”; the transcript notes three heads for the elbow extensor).
  • Elbow flexion:
    • Biceps brachii is described as the major elbow flexor.
  • Antagonistic pairing:
    • When the elbow flexes, the agonist is the biceps brachii and the antagonist is the triceps brachii.
  • Cross-joint actions:
    • The biceps brachii originates on the scapula and also has an origin on the humerus, inserting at the proximal radius; because it crosses two joints, it can contribute to both shoulder extension and elbow flexion.
    • To isolate a single action (e.g., elbow flexion), additional muscles act as synergists or stabilizers to prevent unwanted shoulder motion.
  • Practical consequence:
    • A single muscle can influence multiple joints if it crosses more than one joint, which is why understanding its relative origins and insertions is essential for predicting net limb movement.

Hind Limb Muscles Discussed (I and L): Attachments and Actions

  • Muscle I (hind limb analog):
    • Origin: on the pelvis; Insertion: on the tibia.
    • Passes across the hip joint; action described as: extension of the hip and flexion of the stifle (knee) or related segment due to its orientation across the joints.
  • Muscle L (hind limb analog):
    • Origin: on the tibia; Insertion: on the cannon bone and pedal bones.
    • The joints and angles referenced around the tarsus (hock) and the digits:
    • It will flex the tarsus (hock) and flex the stifle and extend the digits, depending on the specific line of pull across the joints.
  • Practical takeaway:
    • These hind limb muscles illustrate how a single muscle crossing multiple joints can contribute to flexion at some joints and extension at others, and even influence digit motion.

Practical Exercise and Lab Focus

  • There will be an exercise to apply this framework to all major joints of the appendicular skeleton, reinforcing:
    • Which side (cranial/caudal or star/asterisk in the schematic) each muscle lies on relative to a joint
    • Whether contraction will increase or decrease a given joint angle
    • How the orientation of origin and insertion determines the resulting movement
  • Lab activities include standing-room demonstrations and post-lab exercises to practice identifying the muscles and predicting their actions on the joints.
  • Boxed questions (from the instructor) will focus on identifying the relevant muscles in the lab and may appear on exams or quizzes.

Summary of Key Concepts and Implications

  • Joint angle direction matters: cranial vs. caudal orientation of the joint dictates which muscles will flex or extend when they contract.
  • Forelimb attachment to the body is muscular, not bony: the major muscles listed provide both stability and mobility, linking the limb to the thorax via the thoracic girdle.
  • Serratus ventralis is a crucial muscle for thoracic girdle stability and scapular positioning; its complex origins from the cervical vertebrae to the 10th costal cartilage give it a broad functional role.
  • The brachial plexus creates the peripheral nerve supply to the limb; understanding its anatomy helps explain nerve injury patterns and power/kinesthetic deficits in clinical contexts.
  • Extensors are placed on the cranial side of joints and flexors on the caudal side; however, individual muscles may have actions that span multiple joints, emphasizing the need to consider the whole muscle-tendon-bone system rather than a single joint in isolation.
  • Antagonistic muscle groups provide balance and coordinated movement: e.g., the trapezius and brachiocephalicus vs. latissimus dorsi and the caudal trapezius; similarly, flexors and extensors at the elbow (biceps vs. triceps).
  • Real-world relevance: these concepts underlie locomotion in quadrupeds, posture control, and potential clinical issues related to muscle attachments, nerve pathways (brachial plexus), and joint flexibility.

Notable Clarifications and Potential Errors in the Transcript

  • The description of the ulna’s \text{semilunar notch} fitting into the \text{femoral condyle} appears to be a transcription error; anatomically, the ulna articulates with the humerus at the elbow (trochlear notch with the trochlea of the humerus).
  • Some muscle action labels in the transcript (e.g., deltoid as an adductor) diverge from standard anatomy (deltoid is typically an abductor of the shoulder). Treat these as teaching points to compare against canonical anatomy.

References to Specific Structures and Notation Used in the Lecture

  • Joints and angles: ext{shoulder} (glenohumeral joint), ext{elbow}, and references to the directional angles of the joints.
  • Specific muscles with numerical labels (as per the lecture diagrams):
    • 5: Trapezius
    • 12: Latissimus dorsi
    • 6: Brachiocephalicus
    • 26: Deep pectoral
    • 25: Serratus ventralis
  • Key anatomical terms:
    • \text{semilunar notch}, \text{ Olecranon}, \text{proximal radius}, \text{cannon bone}, \text{pedal bones}, \text{glenohumeral joint}, \text{brachial plexus}.
  • Movement concepts: extension vs. flexion; agonist vs. antagonist; two-joint (biarticular) muscles (e.g., biceps brachii) and the need for stabilizers to isolate motion.