Skeletal Muscle Anatomy

• Skeletal muscle moves bones, enabling animal movement.
• Formerly known as voluntary striated muscle, it combines conscious control with automatic functions.
• Maintaining balance and posture are skeletal muscle movements governed by "cruise control".
• The striated appearance of skeletal muscle comes from alternating dark and light bands visible under magnification.

Gross Anatomy

• Skeletal muscles consist of muscle cells grouped together and surrounded by the epimysium.
• They have a thick belly and attachment sites, connecting them to the structures they move.

Attachments

• Tendons attach muscles to bones, as a continuation of the epimysium.
• Aponeuroses are broad sheets of connective tissue that attach some muscles to bones or other muscles.
• The linea alba is an aponeurosis on the ventral midline, connecting abdominal muscles and serving as a surgical entry point.
• The origin is the more stable attachment site of a muscle.
• The insertion of the muscle moves more when the muscle contracts.
• For example, the gastrocnemius muscle originates on the femur and inserts on the calcaneal tuberosity.
• Contraction of the gastrocnemius muscle flexes the knee and extends the tarsus.

Actions

• Muscles contract when stimulated, causing movement.
• Agonists directly produce movement.
• Antagonists oppose the action of agonists, helping to control movement or create rigidity.
• Synergists aid agonists.
• Fixator muscles stabilize joints during movement.
• Skeletal muscles may act as agonists, antagonists, synergists, or fixators.

Muscle-Naming Conventions

• Muscles are named based on physical characteristics, function, shape, location, fiber direction, number of heads/divisions, or attachment sites.
• Flexor muscles flex joints, while extensor muscles extend joints.
• The deltoid muscle is triangular.
• The biceps brachii muscle is in the brachial region.
• Rectus muscles have straight fibers.
• Bi-, tri-, and quadriceps muscles have two, three, and four heads, respectively.
• The sternocephalicus muscle attaches to the sternum and head.

Selected Skeletal Muscles

• Cutaneous muscles twitch the skin, lying beneath it with little or no bone attachment.
• Head and neck muscles control facial expression, chewing, and head movement.
• The masseter muscle closes the jaw.
• The splenius and trapezius muscles extend the head and neck.
• The brachiocephalicus muscle extends the head and pulls the front leg forward.
• The sternocephalicus muscle flexes the head.
• Abdominal muscles support organs, flex the back, aid in straining (defecation, urination, parturition, vomiting), and assist in respiration.
• The abdominal muscle layers, from outside in, are the external abdominal oblique, internal abdominal oblique, rectus abdominis, and transversus abdominis.
• The oblique muscles' fibers run obliquely.
• The rectus abdominis runs lengthwise on either side of the linea alba.
• The transversus abdominis muscle's fibers run downward.

Thoracic Limb Muscles

• Thoracic limb muscles allow for locomotion.
• Shoulder muscles include the latissimus dorsi, pectoral muscles, and deltoid muscle.
• The latissimus dorsi flexes the shoulder.
• The pectoral muscles adduct the front leg.
• The deltoid muscle abducts and flexes the shoulder.
• The biceps brachii flexes the elbow, while the triceps brachii extends it.
• Distal muscles are carpal and digital flexors and extensors.

Pelvic Limb Muscles

• Gluteal and hamstring muscles extend the hip.
• Hamstring muscles include the biceps femoris, semimembranosus, and semitendinosus.
• The quadriceps femoris extends the stifle joint.
• The gastrocnemius muscle extends the hock, inserting on the calcaneal tuberosity.

Muscles of Respiration

• Muscles of respiration alter the thoracic cavity size for breathing.
• Inspiratory muscles are the diaphragm and external intercostals.
• The diaphragm separates the thoracic and abdominal cavities; its contraction increases thoracic cavity size.
• External intercostal muscles rotate ribs upward and forward during contraction.
• Expiratory muscles are the internal intercostals and abdominal muscles.
• Internal intercostal muscles rotate ribs backward during contraction.
• Abdominal muscles push organs against the diaphragm, decreasing thoracic cavity size.

Microscopic Anatomy

• Skeletal muscle fibers are large, multinucleated cells also known as fibers; they are thin and long.
• Myofibrils are lengthwise bundles within muscle fibers comprised of protein filaments.
• Sarcomeres are the basic contractile units in myofibrils, delineated by Z lines.
• Actin filaments are thin filaments attached to the Z lines, extending towards the sarcomere's center.
• Myosin filaments are thick filaments floating in the sarcomere's middle, between actin fibers.
• I bands are light bands made of actin filaments, spanning from one sarcomere to the next.
• A bands are darker bands of overlapping myosin and actin filaments.
• The H band is a light area in the middle of the A band, containing only myosin filaments.
• Actin consists of two twisted protein strands.
• Myosin has a tail with globular heads that form cross-bridges to actin.

Neuromuscular Junction

• Nerve impulses stimulate skeletal muscle.
• Atrophy occurs if a muscle's nerve supply is interrupted.
• The neuromuscular junction connects nerve fibers to muscle fibers, separated by the synaptic space.
• Synaptic vesicles contain acetylcholine, released by nerve impulses.
• Acetylcholine binds to receptors on the sarcolemma, initiating muscle contraction.
• Acetylcholinesterase removes acetylcholine, ending nerve impulses.
• Motor units consist of one nerve fiber and all innervated muscle fibers.
• Muscles needing precise movements have few muscle fibers per nerve fiber.
• Large muscles have many muscle fibers per motor unit.

Connective Tissue Layers

• The endomysium surrounds individual muscle fibers.
• Fascicles are groups of fibers bound by the perimysium.
• The epimysium surrounds groups of fascicles.
• These layers connect to tendons/aponeuroses, anchoring muscles to structures.
• Connective tissues contain blood vessels, nerve fibers, and fat deposits (marbling).

Skeletal Muscle Physiology

• Acetylcholine release initiates muscle contraction and relaxation.
• Calcium ions (Ca2+) trigger contraction, powered by ATP, and are then pumped back out for relaxation; this also requires ATP.
• Blood calcium levels affect muscle function.
• Cross-bridges pull actin towards the center during contraction, shortening the sarcomere.
• The all-or-nothing principle describes how individual muscle fibers act. Body movements depend on varying the number of fibers stimulated.
• The latent phase is followed by the contracting and relaxation phases in a single muscle fiber twitch.

Chemistry of Muscle Contraction

• ATP is the immediate energy source with glucose and oxygen being the ultimate source in creating ATP.
• Creatine phosphate (CP) “recharges” ADP back to ATP.
• Glucose is stored as glycogen, and oxygen is stored by myoglobin.
• Oxygen is supplied via aerobic metabolism, which depletes during strenuous activity triggering anaerobic metabolism.
• Anaerobic metabolism results in lactic acid buildup, also known as "oxygen debt".

Heat Production

• Muscular activity generates heat.
• Shivering produces heat in cold conditions through spasmodic muscle contractions.

Cardiac Muscle

• Cardiac muscle is involuntary striated muscle that contracts without conscious control
• Cardiac muscle is exclusive to the heart, forming the majority of its walls.

Gross Anatomy

• Cardiac muscle is in the heart and forms networks around cardiac chambers.

Microscopic Anatomy

• Cardiac muscle cells contain myofibrils, they're smaller than skeletal muscle cells, and have one nucleus.
• Cells are connected by intercalated discs.

Physiology

• Cardiac muscle cells contract rhythmically without external stimulation.
• Contraction rates of the rapid cells determine the rate for the cell groups
• Contractions are coordinated by the heart’s impulse conduction system beginning at the sinoatrial node.
• Sympathetic nerves increase heart rate; parasympathetic nerves decrease it.

Smooth Muscle

• Smooth muscle is nonstriated involuntary muscle throughout the body.

Gross Anatomy

• Smooth muscle is in sheets (visceral smooth muscle) or small groups (multi-unit smooth muscle).

Microscopic Anatomy

• Smooth muscle cells are spindle-shaped with a single nucleus without parallel myofibrils.
• Contractile units crisscross the cell and attach to dense bodies which contract more than skeletal or cardiac muscle cells.

Physiology

• Visceral smooth muscle is in organ walls and produces rhythmic contractions in the stomach, intestine, uterus, and bladder.
• Stretching can stimulate contraction.
• Hormones like progesterone prevent contractions in the pregnant uterus.
• Sympathetic nerves decrease activity, and parasympathetic nerves increase activity.
• Multi-unit smooth muscle allows for precise muscle movements that require specific impulses from autonomic nerves to contract.