PHYL 141 Ch 11 part 1 of 2

Muscles shorten when they contract, pulling on tendons attached to bones to create movement. The attachment of a muscle to a stationary bone is called the origin, while the attachment to a movable bone is the insertion. An example is the biceps brachii, where its origin is on the scapula and its insertion is on the radius. When it contracts, it pulls the forearm towards the origin, causing arm flexion and pronation. For a muscle to move, many things have to occur for movement, such as dancing and making gestures.

Our bones act as levers, and joints as fulcrums. There are three classes of lever systems. First-class levers (like a seesaw) have the fulcrum in the middle, and can be seen in the neck balancing the head. Second-class levers (like a wheelbarrow) have the load in the middle, between the fulcrum and effort, and are powerful but less common, an example being standing on tiptoes. Third-class levers have the effort in the middle, between the fulcrum and load, and are the most common in the body, although mechanically less favorable.

Skeletal muscles are arranged in opposing pairs. The agonist (prime mover) is the main muscle producing a movement, while the antagonist works oppositely. For example, during forearm flexion, the biceps brachii is the agonist, and the triceps brachii is the antagonist. Synergists prevent unwanted movements and aid the prime mover, often located nearby. Fixator muscles stabilize the origin of the prime mover, allowing specific movements without involving other joints, like flexing fingers without moving the wrist.

Muscles are named based on various characteristics. Names like 'rectus' (parallel to midline), 'transverse' (perpendicular to midline), and 'oblique' (diagonal) describe their direction. Size terms include 'maximus' (largest) and 'latissimus' (widest). Shape terms include 'deltoid' (triangular) and 'rhomboid' (diamond-shaped). Actions like 'flexor' or 'extensor' also contribute to naming. The number of origins, such as 'quadriceps' (four origins) or 'biceps' (two origins), is another naming convention. There are also fusiform, circular, and triangular muscle shapes, which means a bundle of muscle fibers or cells.

Muscles of the head include the corrugator supercilii (frowning), levator palpebrae superioris (opening eyes), orbicularis oculi (closing eyes), buccinator (whistling, chewing), platysma (sheet-like muscle from mandible), risorius (grimacing), zygomaticus major (smiling), and frontalis (surprising look). Damage or disease to the facial nerve can lead to facial paralysis, as seen in Bell's Palsy, which mimics stroke symptoms and affects facial muscle movement.

Muscles moving the mandible aid mastication (chewing) and speech. The masseter and temporalis are powerful muscles that help close the mandible. The lateral pterygoid assists with side-to-side grinding, while the medial pterygoid also helps elevate the mandible. These three muscles (masseter, temporalis, medial pterygoid) are crucial for closing the mouth; their relaxation during sleep explains why the mouth may open.

The neck muscles include the splenius capitis and the sternocleidomastoid. The sternocleidomastoid originates from the sternum and clavicle and inserts into the mastoid process, enabling head rotation. The sternocleidomastoid muscle divides the neck into anterior and posterior triangles. These triangles are important anatomical regions containing vital arteries (carotid), veins (internal jugular), and nerves, which makes them clinically significant.

The abdominal muscles include the rectus abdominis (parallel to midline, responsible for 'six-pack' abs due to tendinous intersections), external oblique (diagonal), internal oblique (underneath external oblique, also diagonal), and transversus abdominis (transverse). Hernias, particularly inguinal hernias common in men due to a larger inguinal canal, involve the protrusion of an organ out of its original space, often the small intestine, causing pain and pressure. Surgical intervention can fix it.

The primary muscle for breathing is the dome-shaped diaphragm. During inhalation, the diaphragm contracts and lowers, increasing space in the lungs. The external intercostals, located between the ribs, also aid in inhalation. The internal intercostals run in the opposite direction and are involved in forced exhalation.

The pelvic floor muscles are vital, especially the external anal sphincter, which helps control defecation. The levator ani group, which includes the pubococcygeus muscle, is crucial for pelvic floor support. Kegel exercises, involving contracting and relaxing these muscles, strengthen the pelvic floor and are important for urinary continence, especially with aging.

Muscles of the pectoral girdle include the levator scapulae and trapezius in the back, and the pectoralis minor (underneath pectoralis major) and serratus anterior on the side. The serratus anterior is known as the 'boxer's muscle' for its role in pushing and punching motions. On the back, the rhomboid major and minor muscles are diamond-shaped. The pectoralis major and latissimus dorsi are often called 'swimmer muscles'. The subscapularis, supraspinatus, infraspinatus, and teres minor are collectively known as the rotator cuff muscles, which are crucial for shoulder stability. Overuse of these muscles, especially in overhead motions like sports, can lead to rotator cuff injuries, often affecting the supraspinatus. Rest, ice, and elevation are key for recovery.