Muscular System: Gross Anatomy Notes

Muscular System: Gross Anatomy (Chapter 10)

• Chapter covers gross anatomy of the muscular system; labs cover many of these concepts, and there will be exam questions linked to Chapter 11 and 12 (mechanisms of muscle contraction at cellular/molecular level). Chapter 10 emphasizes structure and organization, with Chapter 11–12 going into physiology.
• The muscular system is closely related to other systems studied earlier; you’ll see overlap in labs and clinical questions.
• The body contains about 600 skeletal muscles, which constitute roughly half of body weight.
• Three kinds of muscle:
  • Skeletal muscle: voluntary
  • Cardiac muscle: involuntary
  • Smooth muscle: involuntary
• Myology: the study of the muscular system; primary function is movement.
• Major functional areas to learn in this section: structural/functional organization of muscle, muscles of the head and neck, trunk, shoulder/upper limb, hip/lower limb.

Key Concepts and Definitions

• Specialized for one major purpose: convert chemical energy to mechanical energy for movement.
• Heat production: skeletal muscles generate up to $85\%$ of body heat.
• Myogenic or glucose regulation (glucose homeostasis) is another function attributed to muscle activity.
• Internal vs external sphincters:
  • Internal sphincters (smooth muscle): involuntary control.
  • External sphincters (skeletal muscle): voluntary control (e.g., urinary and anal sphincters).
• Breathing and circulation involve skeletal muscle (e.g., diaphragm for breathing).
• Muscle contraction is responsible for movement of the skeleton via shortening of sarcomeres inside muscle fibers (topics to be covered in Chapter 11).

Muscle Tissue Organization and Histology

• Connective tissue layers of a muscle:
  • Epimysium: dense fibrous connective tissue surrounding the entire muscle; continuous with tendons and periosteum of bone.
  • Perimysium: surrounds bundles of muscle fibers called fascicles.
  • Endomysium: thin connective tissue around each muscle fiber; contains capillaries and nerves; provides extracellular environment for the muscle fiber and its nerve ending.
• Muscle fiber anatomy:
  • Sarcolemma: plasma membrane surrounding the muscle fiber.
  • Multinucleate cells (nuclei located just beneath the sarcolemma).
  • Myofibrils: long, rod-like elements inside the fiber containing two filament types—actin (thin) and myosin (thick).
  • Myofilaments organized into sarcomeres, the contractile units.
  • Sliding of actin and myosin filaments shortens the sarcomere, leading to contraction and movement of the skeleton.
• Cellular components around a muscle fiber:
  • Fascicle: a bundle of fibers surrounded by perimysium.
  • Nerves and blood vessels travel through the connective tissue to reach fibers.
  • Endomysium provides extracellular environment and houses nerve endings.
• Shape and architecture influence function:
  • Muscles can be named by shape (e.g., deltoid is triangular), size (major/minor), location, number of heads (biceps=2, triceps=3), and fiber orientation (rectus=straight, oblique, transverse).
  • Examples:
  • Orbicularis: circular/sphincter-like shape; orbicularis oculi encircles the eye.
  • Deltoid: triangular shape.
  • Bipennate vs unipennate vs multipennate: reflect fiber orientation and strength distributions.
• Compartmentalization:
  • Muscle compartments are functional groups enclosed by fascia, separated from neighboring groups by connective tissue septa (fascia, deep fascia).
  • Compartments often contain major nerves and vessels; examples include thoracic, abdominal, pelvic, and limb compartments.
• Attachments to bone:
  • Tendons: dense connective tissue that attaches muscle to bone.
  • Aponeuroses: broad, flat tendinous sheets.
  • Direct attachment (fleshy): muscle attaches almost directly to bone with little intervening tendon.
• Tendonitis:
  • Inflammation of a tendon; suffix -itis indicates inflammation.
• Muscles and attachments continuity:
  • Collagen fibers of the endomysium, perimysium, and epimysium continue into the tendon, passing through the periosteum and into the bone matrix for strong continuity between muscle and bone.

Origin, Insertion, and Movement

• Definitions:
  • Origin: the stationary bone or proximal attachment.
  • Insertion: the moving bone or distal attachment when the muscle contracts.
• Typical rule: the insertion moves when the muscle contracts.
• Example: Biceps brachii (two heads): origin is at the proximal arm/shoulder region; insertion is at the radial tuberosity; contraction shortens the muscle and causes flexion of the elbow.
• Practical note on naming attachments:
  • Some muscles insert on fascia or the skin (e.g., facial muscles), not just bones.
  • Some muscles cross multiple bones; naming can reflect attachment sites (proximal/distal or superior/inferior).
• Examples of naming by origin/insertion:
  • Sternocleidomastoid: origin on sternum and clavicle; insertion on the mastoid process of the skull; contraction flexes the neck.

Naming of Muscles (Latin and Beyond)

• Latin-based naming system has several axes:
  • Size: major (large), minor (small), maximus (largest), minimus (smallest).
  • Shape: e.g., deltoid (triangular), trapezius (trapezoid), rectus (straight).
  • Location: e.g., cervicis (neck), thoracis (thorax), abdominalis (abdomen).
  • Number of heads: biceps (two heads), triceps (three heads), quadriceps (four heads).
  • Fascicle orientation: rectus (straight), obliquus (slanted/oblique), transversus (across).
  • Action: elevatus/elevator (elevate), extendens (extend), etc.
• Practical clinical note:
  • Knowing the Latin roots helps locate and identify muscles in clinical settings (e.g., deltoid for shoulder, rectus abdominis for anterior abdominal wall).
  • Examples discussed: pectoralis major (major = large; pectoralis = chest), latissimus dorsi (widest back muscle).
• Pregnancy-related anatomy term:
  • Linea alba: tendinous seam in the midline of the abdomen.
  • Linea nigra: darkened line that can appear during pregnancy.

Functional Groups of Muscles

• Four main functional categories based on action:
  • Prime mover (agonist): the main muscle producing a movement.
  • Synergist: assists the prime mover and can stabilize nearby joints, sometimes modifying movement direction.
  • Antagonist: muscle that opposes the prime mover.
  • Fixator: stabilizes the origin of the prime mover (keeps the bone still so the prime mover can act effectively).
• Analogy used in lectures:
  • Prime mover = mother (does most of the movement)
  • Synergist = father (assists)
  • Antagonist = kids (oppose/limit movement)
  • Fixator = stabilizes the position like anchoring a tool
• Example with arm muscles:
  • Biceps brachii (agonist for elbow flexion) vs brachialis (also flexes, serves as a major contributor); they can be synergists, but brachialis is often the primary mover for elbow flexion.
  • Triceps brachii acts as an antagonist to the biceps during elbow flexion.

Intrinsic vs Extrinsic Muscles

• Intrinsic muscles: originate and insert within the same region (e.g., hand muscles start and end in the hand).
• Extrinsic muscles: have origins outside the region and insert within it (e.g., forearm muscles that act on the hand).
• Examples in the hand:
  • Intrinsic: small muscles within the hand (start and end in the hand).
  • Extrinsic: muscles like flexor digitorum superficialis (originate in forearm, insert in fingers).

Innervation, Neuromuscular Junction, and Blood Supply

• Innervation: nerves that stimulate muscle function; essential for diagnosis of nerve injuries.
  • Spinal nerves arise from the spinal cord and exit via foramina; plexuses form networks near the vertebral column.
  • Cranial nerves innervate head/neck muscles; 12 pairs (I–XII).
• Neuromuscular junction: relationship between a motor neuron and a muscle fiber; critical for signal transmission to initiate contraction.
• Cranial nerve innervation (examples mentioned):
  • Facial expression muscles: innervation referenced as facial nerve; transcript notes CN V in one place (trigeminal) and CN VII later—cross-check with course materials for accuracy.
  • Muscles of mastication: innervated by mandibular branch of trigeminal (CN V3).
  • Facial expression: CN VII (facial nerve) is the typical innervation; note the transcript’s discrepancy for CN numbering.
• Blood supply:
  • Resting muscle receives about $1.24\,\text{L/min}$ of blood; about $25\%$ of cardiac output goes to muscles at rest.
  • During heavy exercise, muscle blood flow can rise to more than $75\%$ of cardiac output (i.e., the share can exceed $\frac{3}{4}$ of cardiac output).
  • Total cardiac output during heavy exercise can reach about $11.6\,\text{L/min}$.
  • Capillaries are extensively distributed through the endomysium to reach every muscle fiber.

Muscles of the Head and Neck (Overview of Major Groups)

• Muscles of facial expression:
  • Attach to the dermis/subcutaneous tissue; move the skin to create facial expressions.
  • Innervated by the facial nerve (CN VII) in standard anatomy; transcript notes CN V in one place—verify with course material.
  • Key muscles mentioned: frontalis (raises eyebrows), orbicularis oculi (blinks/winks), levator palpebrae superioris (lifts the eyelid), nasalis (nasal region), orbicularis oris (around the mouth), zygomaticus major/minor (smiling), mentalis (chin), depressor anguli oris, platysma (neck expression).
• Muscles of mastication and swallowing:
  • Extrinsic tongue muscles (e.g., genioglossus) and intrinsic tongue muscles (vertical, transverse, longitudinal fascicles).
  • Intrinsic tongue muscles enable fine tongue movements; extrinsic muscles move the tongue and contribute to swallowing and speech.
  • Chewing muscles (mandibular): temporalis, masseter, medial pterygoid, lateral pterygoid; innervated by the mandibular branch of the trigeminal nerve (CN V).
  • Swallowing anatomy includes pharyngeal constrictors and muscles forming the pharynx; deglutition (swallowing) is a crucial function for digestion and speech.
• Hyoid and supra-/infra-hyoid muscles:
  • Suprahyoid group: digastric, stylohyoid, mylohyoid, geniohyoid (depresses mandible or elevates/hyoid region depending on action).
  • Infrahyoid group: sternohyoid, thyrohyoid, sternothyroid, omohyoid (stabilize hyoid bone during swallowing and speech).
  • Some descriptions in the transcript mix terms (e.g., geniohyoid, digastric, stylohyoid) and refer to supra-/infra-hyoid groups.
• Neck and head motion muscles:
  • Sternocleidomastoid: flexes the neck when contracted; named for sternum, clavicle, and mastoid process of the skull.
  • Trapezius (neck and upper back) and other neck extensors/rotators are described.

Muscles Involved in Respiration

• Diaphragm:
  • Dome-shaped skeletal muscle; separates thoracic and abdominal cavities.
  • Central tendon with muscle fascicles; contraction flattens the diaphragm, increasing thoracic cavity volume to drive inspiration.
  • Relaxation raises the diaphragm, decreasing thoracic volume and promoting expiration.
  • Important for breathing (and is a skeletal muscle; not smooth).
• Intercostal muscles:
  • External intercostals: elevate ribs, expanding the thoracic cavity for inspiration.
  • Internal intercostals: depress/retract ribs, aiding expiration.
  • Innermost intercostals: assist with rib movements during respiration.

Abdominal Wall Muscles

• External abdominal oblique: directions described in the transcript; contribute to trunk movement and increasing abdominal pressure.
• Internal abdominal oblique: fibers run opposite to external oblique; contributes to trunk rotation and flexion.
• Transverse abdominis: deepest lateral abdominal muscle; runs horizontally; helps compress abdominal contents.
• Rectus abdominis: runs anteriorly, responsible for flexing lumbar spine; can form a six-pack with tendinous intersections.
• Linea alba: a tendon running down the midline; becomes darker during pregnancy (Linea nigra).
• Abdominal function: supports abdominal contents and aids in movements of the vertebral column; also contributes to forced expiration and core stability.

Muscles of the Back

• Superficial back muscles:
  • Latissimus dorsi: broad back muscle; actions include extension, adduction, and medial rotation of the arm; important in throwing movements.
  • Trapezius: elevates the shoulder (and other actions depending on the region of fibers).
• Deep back muscles (erector spinae group):
  • Erector spinae (group including spinalis, longissimus, iliocostalis): extend and laterally flex the vertebral column.
  • Semispinalis capitis, multifidus: contribute to extension, rotation, and stabilization of the spine.
  • Other deep muscles mentioned: splenius capitis, levator scapulae, rhomboid major/minor, quadratus lumborum, serratus posterior superior and inferior.
• General spine actions described:
  • Flexion, extension, lateral flexion, and rotation of the vertebral column.
  • Postural support and spinal stabilization via deep back muscles like multifidus.

Muscular System in Practice: Study Tips and Clinical Connections

• Direct vs indirect attachment to bone is important for clinical anatomy and injections:
  • Intramuscular injections are done into skeletal muscle because of the rich vascular supply (ample capillaries in endomysium).
  • Tendonitis: inflammation of a tendon (suffix -itis).
• Practical clinical notes:
  • Naming and location knowledge is useful for locating muscles quickly (e.g., deltoid for shoulder injections, quadriceps in pediatric injections).
  • Understanding origin/insertion helps predict movement and posture changes during contraction.
• Learning strategies mentioned in the transcript:
  • Use Latin names to aid memorization and cross-reference with anatomical origins/insertions.
  • For exam preparation, review muscle names by location and function; pronounce and spell them aloud; create mental images of their origins/insertions.
  • The APU view (interactive viewer) allows tapping to name muscles; practice with the visual resources in the course materials.
  • Palpate muscles on yourself when possible to reinforce identification and movement.
  • Focus on origin, insertion, articulation, and action for each muscle; reinforce with repeated verbalization.
  • A sample mnemonic approach involves using a household analogy for agonists, synergists, antagonists, and fixators.

Quick Reference: Notable Numbers, Terms, and Concepts

• Major numerical figures:
  • Skeletal muscles: ≈ $600$ muscles; about $50\%$ of body weight.
  • Resting blood flow to muscles: $1.24\ \text{L/min}$.
  • Blood supply during rest: about $25\%$ of cardiac output.
  • Blood supply during heavy exercise: >$75\%$ of cardiac output; total cardiac output around $11.6\ \text{L/min}$.
  • Heat production: skeletal muscle accounts for up to $85\%$ of body heat.
• Key anatomical terminology (examples):
  • Epimysium, perimysium, endomysium (tissue layers surrounding muscle, fascicles, and fibers respectively).
  • Fascicle: bundle of muscle fibers.
  • Sarcomere: contractile unit; sliding filament theory underlies contraction.
  • Tendon: direct attachment to bone via connective tissue continuity; aponeurosis: broad tendon sheet.
  • Linea alba and linea nigra: midline abdominal tendon and pregnancy-associated dark line, respectively.
  • Major muscle shapes: deltoid (triangular), trapezius (trapezoidal), orbicularis (circular).
  • Terms for muscle action or location: elevate (levator), extend (extensor), flex (flexor).

Notes on Terminology Variations in the Transcript

• In the section on facial expression innervation, the transcript states the facial muscles are innervated by the facial nerve (cranial nerve number 5). The standard anatomical reference is cranial nerve VII. Be aware of this discrepancy when studying from this transcript.
• The tongue and hyoid-related muscles list several terms that appear slightly garbled (e.g., "genioarid", "styroglossus", etc.). The intended muscles include genioglossus, styloglossus, hyoglossus, geniohyoid, mylohyoid, digastric, etc. Use standard anatomical nomenclature when studying these muscles and confirm with your course resources.
• Some phrases in the transcript mix intrinsic/extrinsic tongue muscles, suprahyoid, and infrahyoid groups. The general concepts (intrinsic muscles originate and insert within the same region; extrinsic muscles originate outside and insert in the region) remain valid.

Quick Visual and Practice Prompts

• For origin vs insertion practice, use the biceps brachii example: origin (proximal, near shoulder) vs insertion (distal, at radial tuberosity); contraction shortens the muscle and pulls the insertion toward the origin, causing elbow flexion.
• If you’re preparing for exams, review muscle lists (List 1 and List 2 in your textbook or lab manual) to focus on major muscles and their actions.
• When studying respiration, remember diaphragm as the primary skeletal muscle of respiration with external intercostals aiding inspiration and internal intercostals aiding expiration.
• For the abdominal wall, recall the layered arrangement: external oblique, internal oblique, transverse abdominis, and rectus abdominis with the linea alba running along the midline.
• For the back, memorize the major superficial vs deep muscles and their primary actions in terms of spine movement (extension, rotation, lateral flexion) and shoulder girdle movements.

Summary Takeaways

• The skeletal muscular system comprises ~600 muscles, about half your body weight, and is responsible for movement, posture, contact with the environment (breathing, speech), and several autonomic processes (e.g., digestion via movement of contents and sphincters).
• Muscles are organized in hierarchical connective tissue layers (endomysium, perimysium, epimysium) and attach to bone via tendons or aponeuroses.
• Muscle fibers contain sarcomeres, the microscopic contractile units formed by actin and myosin filaments; contraction shortens sarcomeres via sliding filament mechanism (details in Chapter 11).
• Naming muscles involves multiple axes (size, shape, location, number of heads, fiber orientation, action), aided by Latin terminology.
• Functional muscle groups include agonists, synergists, antagonists, and fixators; intrinsic vs extrinsic muscles define whether a muscle acts within the same region or from outside a region.
• Innervation and blood supply are essential for function and clinical diagnosis; muscles receive substantial blood flow especially during exercise, which is reflected in notable numbers for resting and active states.
• Practical study tips emphasize lab review, palpation, origin/insertion practice, verbal naming, and using visuals (APU view or atlas) to reinforce memory.