Anatomy and Physiology Notes: Calcium Homeostasis, Lower Limb, Joints, and Muscles

Calcium Homeostasis

• Your body utilizes negative feedback loops to maintain calcium homeostasis.

• High Blood Calcium Level:

  • The thyroid gland releases a hormone called calcitonin.

  • Mnemonic: "Calcium in, calcitonin" means it takes calcium from the bloodstream and puts it into the bones for storage.

  • Calcitonin decreases the rate of bone matrix breakdown (decreases osteoclast activity).

  • It increases the rate of bone buildup (increases osteoblast activity).

  • This action reduces the calcium level in the bloodstream, bringing it back to the set point and discontinuing the feedback loop.

• Low Blood Calcium Level:

  • The parathyroid gland releases parathyroid hormone (PTH).

  • PTH takes calcium from the bone and puts it back into the bloodstream.

  • It increases the rate of bone matrix breakdown (increases osteoclast activity).

  • It decreases the rate of bone buildup (decreases osteoblast activity).

  • This action increases the calcium level in the bloodstream, bringing it back to the set point and stopping the feedback loop.

• Bone Cells:

  • Osteoclasts: Cells that break down bone (think "C" for cutting/cleaving).

  • Osteoblasts: Cells that build up bone (think "B" for building).

• Textbook Reference: Page $219$ for calcium homeostasis.

Hip and Lower Limb Anatomy

• Coxal Bone (Hip Bone):

  • Formed by the fusion of three bones: the ilium, ischium, and pubis.

  • These bones are fused and do not move independently within the coxal bone.

  • Ilium: The large, upper portion.

  • Pubis: The medial anterior portion, joined at the front by the pubic symphysis.

  • Ischium: The lower posterior aspect, also known as the "sit bones."

    • Ischial Tuberosity: A large, rough bony process that forms the inferior part of the ischium; palpable on oneself.

    • Ischial Spine: A pointed projection located just above the ischial tuberosity.

• Pelvic Girdle:

  • Comprises the two coxal bones combined with the sacrum (the lower portion of the spinal column).

  • The sacrum is formed by about five fused vertebrae, through which the spinal cord passes.

  • Ligaments bind the coxal bones to the sacrum posteriorly and to each other anteriorly (e.g., the pubic symphysis).

• Acetabulum: A crucial cavity or socket in the coxal bone that articulates with the head of the femur, forming the hip's ball-and-socket joint. There are two acetabula, one on each side of the pelvic girdle.

Leg Bones

• The leg consists of three main bones: the femur (thigh), and the tibia and fibula (lower leg).

• Femur (Thigh Bone):

  • The largest and longest bone in the body.

  • Head of the Femur: The giant, rounded top on the proximal end, covered in hyaline cartilage, sits within the acetabulum.

  • Greater Trochanter: A large bony projection situated lateral to the head of the femur. It's a palpable landmark used in nursing for intramuscular injections and serves as a muscle attachment point for gluteal muscles.

  • Lesser Trochanter: Located on the medial side of the femur, also serving as a muscle attachment point.

  • Distal Femur Features:

    • Lateral Condyle & Medial Condyle: Two rounded projections at the distal end, covered in hyaline cartilage, which articulate with the tibia.

    • Trochlea: A groove between the condyles on the anterior view, where the patella (kneecap) sits.

    • Intercondylar Fossa: A shallow groove on the posterior view between the condyles.

• Patella (Kneecap): A separate bone that slides within the trochlea of the femur; its bottom surface is also covered in hyaline cartilage. Dislocation can occur when it shifts laterally.

• Tibia (Shin Bone):

  • The larger and more medial bone of the lower leg.

  • It is the weight-bearing bone of the lower leg because the femur articulates directly with it.

  • Its shaft is more triangular, with a broad proximal end to support the femur.

  • It has a noticeable crest on its anterior surface.

  • Medial Malleolus: A smaller bony projection at the ankle, on the medial side.

• Fibula:

  • The thinner and more lateral bone of the lower leg.

  • It is non-weight-bearing but provides additional support for the ankle joint and serves as an attachment point for muscles.

  • Lateral Malleolus: A prominent bony projection at the ankle on the lateral side. It helps protect the outside of the ankle joint, which is why ankle sprains often involve an inward roll of the foot, potentially leading to torn ligaments or even a fracture of the malleolus.

Foot Anatomy

• Tarsal Bones: Equivalent to the carpal bones of the hand, forming the ankle and upper foot structure.

  • Talus (or Talus): This bone directly articulates with the distal end of the tibia, forming the ankle joint; covered in hyaline cartilage.

  • Calcaneus: The large heel bone at the back of the foot, where the Achilles (calcaneal) tendon inserts.

  • Navicular: A curved, boat-shaped bone located below the cuneiforms. Mnemonic: "Navicular is navigating the horses (cuneiforms)."

  • Cuneiform Bones: Three bones (medial, intermediate, lateral) located distal to the navicular.

  • Cuboid: A cube-shaped bone located on the lateral side of the foot, lateral to the cuneiforms.

• Metatarsals: Five long bones (numbered $1$ through $5$ from the big toe to the pinky toe) that form the main body of the foot, distal to the tarsals.

• Phalanges (Toes):

  • The big toe (first phalange) has two phalanges: a proximal and a distal.

  • The other four toes each have three phalanges: a proximal, a middle, and a distal.

Review of Previous Concepts

Layers of the Skin

• Acronym: "Corey lotioned grandma's spine before papa returned home."

  • Corneum (Corey): Outermost layer, dead cells filled with keratin, forming a tough protective layer.

  • Lucidum (Lotioned): Found in thick skin (palms, soles), closely packed cells losing their nuclei.

  • Granulosum (Grandma's): $2$ to $4$ layers of tightly packed cells, important for preventing water loss from the body.

  • Spinosum (Spine): $8$ to $10$ layers of irregularly shaped, "spiny" cells, rich in RNA for protein synthesis (e.g., keratin).

  • Basale (Before): The deepest layer of the epidermis, containing melanocytes for pigment and undergoing active cell division (mitosis). Together with the stratum spinosum, it forms the stratum germinativum, the growing layers.

• Dermis (Papa Returned): Located below the epidermis.

  • Papillary Layer: Contains dermal papillae (wavy formations) creating ridges for fingerprints and increasing surface area at the dermo-epidermal junction for stronger adhesion.

  • Reticular Layer: Contains sweat and oil (sebaceous) glands, hair follicles, blood vessels, nerve cells, and various sensory cells.

• Hypodermis (Home): Found below the dermis, connecting the skin to underlying structures; primarily composed of loose adipose tissue, blood vessels, and lymphatic vessels.

• Melanocytes: Produce colored skin pigments (melanin) like eumelanin (dark black) and pheomelanin (reddish-orange); located in the stratum basale.

• Glands:

  • Sebaceous Glands: Associated with hair follicles, secrete oil (sebum) for protection and lubrication.

  • Eccrine Sweat Glands: Most numerous, found on the skin surface.

  • Apocrine Sweat Glands: Found in specific locations (armpits, genitals, areolas), secrete a thicker substance.

  • Ceruminous Glands: Found only in the ear, produce cerumen (earwax).

Structural Unit of Compact Bone

• The structural unit is the Osteon, also known as the Haversian System.

  • Lamellae: Concentric rings of tough, compact bone material, primarily minerals like calcium.

  • Lacunae: Small structures within the lamellae (think "lagoon"), containing osteocytes (living bone cells).

  • Central (Haversian) Canals: Channels running through each osteon, containing blood vessels and nerves.

  • Transverse (Volkmann's) Canals: Link different central canals horizontally.

• Chondrocytes: Living cells found in cartilage tissue.

Long Bone Anatomy

• Diaphysis: The long, main shaft of the bone.

• Epiphysis: The expanded ends of the bone (both proximal and distal).

• Metaphysis (Growth Plate/Epiphyseal Plate): The region between the diaphysis and epiphysis, composed of hyaline cartilage, crucial for longitudinal bone growth during childhood.

• Periosteum: A layer of connective tissue that covers the outer surface of the entire bone.

Axial vs. Appendicular Skeleton

• Axial Skeleton: Comprises the central part of the body, including the skull, vertebral column, rib cage, and sacrum.

• Appendicular Skeleton: Consists of the limbs and the girdles that attach them to the axial skeleton (e.g., femur, clavicles, ilium).

  • Note: The clavicles (collarbones) are part of the appendicular skeleton, not the axial.

Skull Bones

• Temporal Bone: Forms the side of the head (temple region).

• Occipital Bone: Forms the back and base of the skull, containing the foramen magnum (large hole for the spinal cord).

• Parietal Bone: The large bone forming the top and sides of the skill.

• Frontal Bone: Forms the forehead.

• Sphenoid Bone: A butterfly-shaped bone at the base of the skull.

Bone Surface Features for Joints

• Facet: A flat, smooth surface on a bone that articulates with another flat surface, allowing gliding movement (e.g., facet joints between vertebrae).

• Condyle: A rounded, curved articular projection typically found at the ends of long bones (e.g., femoral condyles).

• Crest: A prominent ridge or border on a bone (e.g., tibial crest, iliac crest).

• Process: Any bony prominence or projection (e.g., spinous process of a vertebra).

Joints (Articulations)

• Definition: Any point of contact between bones.

• Classification Schemes:

  • Functional Classification (Degree of Movement):

    • Synarthrosis: Immovable joints (no movement).

    • Amphiarthrosis: Slightly movable joints.

    • Diarthrosis: Freely movable joints.

  • Structural Classification (Type of Tissue):

    • Fibrous Joints: Bones joined by dense connective tissue.

    • Cartilaginous Joints: Bones joined by cartilage.

    • Synovial Joints: Characterized by a joint cavity containing synovial fluid.

Synarthrosis Joints (Immovable)

• Generally fibrous joints, formed by dense connective tissue primarily composed of collagenic fibers.

• Types of Fibrous Joints (Synarthrosis):

  • Syndesmosis: Bones are united by a fibrous ligament (e.g., interosseous ligament between tibia and fibula, or radius and ulna), allowing very little movement.

  • Sutures: Found exclusively in the skull, where tooth-like projections of adjacent bones interlock tightly. They become ossified with age, forming strong fusion lines (initially flexible in infancy for birth).

  • Gomphosis: Specialized joint connecting the root of a tooth to the alveolar process of the mandible or maxilla (jawbone).

Cartilaginous Joints (Amphiarthrosis - Slightly Movable)

• Bones are bound together by cartilage (either hyaline cartilage or fibrocartilage).

• Types of Cartilaginous Joints:

  • Synchondrosis: Characterized by the presence of hyaline cartilage between bones. Examples:

    • Between the bony end of a rib and the sternum.

    • The epiphyseal plate (growth plate/metaphysis) in long bones, allowing for childhood growth.

  • Symphysis: Characterized by fibrocartilage (tougher cartilage) between bones. Examples:

    • Pubic Symphysis: Joins the anterior portions of the pubic bones in the pelvic girdle, permitting slight movement, especially during childbirth.

    • Intervertebral Discs: Dense layers of fibrocartilage forming the vertebral discs in the spine, allowing for slight spinal movement and acting as shock absorbers.

Synovial Joints (Diarthrosis - Freely Movable)

• These are the most common type of joint, allowing for a wide range of movement (e.g., hip, shoulder, knee, elbow).

• Common Components of Synovial Joints:

  • Articular Cartilage: A thin layer of hyaline cartilage covering the ends of the articulating bones, providing a smooth, low-friction surface and cushioning.

  • Joint Capsule: A sleeve-like extension of the periosteum that encloses the joint.

  • Synovial Membrane: A moist membrane lining the inner surface of the joint capsule, which secretes synovial fluid.

  • Synovial Fluid: A viscous fluid within the joint cavity that lubricates the joint, reducing friction and nourishing the articular cartilage.

  • Joint Cavity: The unique space within the synovial joint, filled with synovial fluid.

• Additional Features in Some Synovial Joints (e.g., Knee Joint):

  • Menisci (plural of meniscus): Pads of fibrocartilage (e.g., medial and lateral menisci in the knee). They deepen the articular surfaces, provide cushioning, and enhance joint stability. (A torn meniscus is a common injury).

  • Ligaments: Strong cords of dense white fibrous tissue that further reinforce and support synovial joints by connecting bones to other bones (e.g., ACL and PCL in the knee, often prone to injury).

  • Bursae (plural of bursa): Closed, pillow-like structures filled with synovial fluid, located in areas subject to friction (e.g., suprapatellar, prepatellar, infrapatellar bursae in the knee). They facilitate gliding of adjacent structures. Inflammation of a bursa is called bursitis.

• Types of Synovial Joints (Based on Structure and Movement):

  • Uniaxial Joints (Movement in one plane around one axis):

    • Hinge Joint: Permits flexion and extension only (e.g., elbow joint, knee joint).

    • Pivot Joint: Allows rotation around an axis (e.g., atlanto-axial joint between C1 and C2, head of the radius rotating on the ulna).

  • Biaxial Joints (Movement in two perpendicular planes/axes):

    • Saddle Joint: Articulating surfaces are saddle-shaped (e.g., carpometacarpal joint of the thumb).

    • Condyloid Joint: Oval condyle of one bone fits into an oval depression of another (e.g., atlanto-occipital joint between the atlas (C1) and the occipital bone).

  • Multiaxial Joints (Movement in multiple planes/axes, most freely movable):

    • Ball and Socket Joint: A spherical head of one bone fits into a cup-like socket of another (e.g., glenohumeral joint of the shoulder, acetabulofemoral joint of the hip).

    • Gliding Joints (Plane Joints): Flat or nearly flat articular surfaces allow limited gliding movements (e.g., articular processes between vertebrae (facets), intercarpal and intertarsal joints).

Joint Movements

• Angular Movement: Changes the angle between articulating bones.

  • Flexion: Decreases the angle between bones (e.g., bending the elbow).

  • Extension: Increases the angle between bones (e.g., straightening the elbow).

  • Dorsiflexion: (Specific to the ankle) Tilting the foot upwards, bringing toes towards the shin (decreases angle between foot and shin, like a shark's dorsal fin).

  • Plantar Flexion: (Specific to the ankle) Pointing the toes downwards (increases angle between foot and shin, like pressing a gas pedal).

  • Abduction (AB-duction): Moving a limb away from the midline of the body (e.g., lifting arm out to the side).

  • Adduction (AD-duction): Moving a limb towards the midline of the body (e.g., bringing arm back to the side).

• Circular Movement: Arc-like rotation of a structure around an axis.

  • Rotation: Turning a bone around its own axis (e.g., shaking head no at the C1/C2 joint).

  • Pronation: (Specific to the forearm) Twisting the forearm so the palm faces posteriorly or the thumb points medially (e.g., placing hand palm-down).

  • Supination: (Specific to the forearm) Twisting the forearm so the palm faces anteriorly or the thumb points laterally (e.g., placing hand palm-up, holding "soup").

  • Circumduction: A combination of flexion, extension, abduction, and adduction, where the distal end of a limb moves in a circle while the proximal end remains relatively stable (e.g., making circles with the arm at the shoulder).

• Gliding Movement: Flat articular surfaces slide over each other without angular or circular movement (e.g., between carpal bones, or the facet joints of vertebrae).

• Special Movements: Movements unique to specific joints or body parts.

  • Depression: Downward movement of a body part (e.g., opening the mouth by lowering the mandible).

  • Elevation: Upward movement of a body part (e.g., closing the mouth by raising the mandible).

  • Inversion: (Specific to the ankle) Moving the sole of the foot inwards, towards the midline (a common mechanism for ankle sprains).

  • Eversion: (Specific to the ankle) Moving the sole of the foot outwards, away from the midline.

  • Protraction: Moving a body part forward (e.g., jutting the jaw forward).

  • Retraction: Moving a body part backward (e.g., pulling the jaw back).

Muscles

• Three Categories of Muscle Tissue:

  • Skeletal Muscle: Striated (striped), voluntary control; responsible for moving bones and limbs.

  • Smooth Muscle: Non-striated, involuntary control; found in walls of internal organs, glands, and blood vessels (e.g., intestines, erector pili muscle).

  • Cardiac Muscle: Striated, involuntary control; exclusive to the heart.

Muscle Terminology (Microscopic & Connective Tissue)

• Fascicle: A bundle or group of muscle fibers.

• Myofibrils: Small, contractile subunits within muscle fibers, containing sarcomeres.

• Sarcomere: The basic contractile unit of a myofibril, containing thick and thin myofilaments, joined end-to-end.

• Myofilaments: The individual thick (myosin) and thin (actin) protein filaments that make up myofibrils.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell.

• Sarcoplasmic Reticulum: A specialized endoplasmic reticulum surrounding myofibrils, involved in calcium storage and release for muscle contraction.

• T-tubules (Transverse Tubules): Inward extensions of the sarcolemma that help transmit electrical impulses (action potentials) deep into the muscle fiber, coordinating calcium release for contraction.

• Connective Tissue Layers of Muscle:

  • Endomysium: Surrounds each individual muscle fiber.

  • Perimysium: Surrounds a group of muscle fibers, forming a fascicle.

  • Epimysium: Surrounds the entire muscle.

• Tendons: Strong, fibrous connective tissue that attaches muscle to bone.

• Ligaments: Connective tissue that attaches bone to bone.

• Fascia: A layer of fibrous connective tissue that surrounds the entire muscle (superficial to the epimysium) and can also be found underlying the skin (in the hypodermis).

Muscle Shape and Fiber Arrangement

• Parallel Muscles: Fibers (fascicles) run parallel to the long axis of the muscle. They can be long and strap-like (e.g., sartorius) or have tendinous intersections (e.g., rectus abdominis, forming a "six-pack").

• Convergent Muscles: Broad origin, but fibers converge at a single, narrow insertion point (e.g., pectoralis major).

• Pennate Muscles (Feather-like appearance):

  • Unipennate: Fascicles are anchored to only one side of a central tendon (e.g., extensor digitorum longus).

  • Bipennate: Fascicles are anchored to both sides of a central tendon (e.g., rectus femoris).

  • Multipennate: Fascicles from multiple directions converge onto one or more tendons (e.g., deltoid muscle of the shoulder, which has anterior, middle, and posterior fibers).

• Fusiform Muscles: Similar to parallel, with fascicles running parallel in the belly of the muscle but converging to tendons at one or both ends (e.g., biceps brachii).

• Spiral Muscles: Fibers twist between points of attachment (e.g., latissimus dorsi, wrapping around the torso).

• Circular Muscles (Sphincters): Fibers are arranged in a circle around an opening, allowing for its closure or opening (e.g., orbicularis oculi around the eye, orbicularis oris around the mouth, anal sphincter).

Muscle Actions and Roles

• Origin: The point of attachment of a muscle that remains relatively stationary or does not move significantly during contraction (the fixed anchor point).

• Insertion: The point of attachment of a muscle that moves when the muscle contracts.

• Prime Mover (Agonist): The muscle that directly and primarily performs a specific movement (e.g., brachialis flexing the elbow).

• Antagonist: A muscle whose action opposes that of the prime mover (e.g., triceps brachii extends the elbow, opposing the brachialis).

• Synergist: A muscle that assists the prime mover in performing a specific movement, often by adding force or stabilizing the joint (e.g., biceps brachii and brachialis both contribute to elbow flexion).

• Fixator: A muscle that stabilizes the origin of a prime mover or maintains posture, providing a solid anchor point for other muscles to act upon (e.g., deltoid and trapezius stabilizing the shoulder for arm movements, core muscles stabilizing the trunk).

Lever Systems in the Body

• Every lever system involves a fulcrum (pivot point), a load (resistance), and a pull (muscle force).

• First Class Lever:

  • The fulcrum is located between the load and the pull.

  • Analogy: A teeter-totter.

  • Example: Nodding the head. The neck joint acts as the fulcrum, the weight of the head acts as the load, and the posterior neck muscles provide the pull to lift the head. This system allows muscles to counteract head weight.

• Second Class Lever:

  • The load is located between the fulcrum and the pull.

  • Analogy: A wheelbarrow.

  • Example: Performing a calf raise. The ball of the foot acts as the fulcrum, the body's weight is the load, and the calf muscles pulling via the Achilles tendon provide the pull. This provides a mechanical advantage, allowing a large load to be moved with less force.

• Third Class Lever:

  • The pull is located between the fulcrum and the load.

  • This is the most common lever system in the human body.

  • Example: Biceps curl. The elbow joint is the fulcrum, the object being lifted (e.g., a coffee cup) is the load, and the biceps muscle insertion is the pull, which is closer to the fulcrum than the load. This system favors speed and range of motion over force.

Muscle Naming Conventions

• Muscles are often named based on descriptive characteristics, aiding in memorization:

  • Number of Heads: E.g., Biceps (two heads), Triceps (three heads).

  • Location: E.g., Rectus Abdominis (straight muscle in the abdomen).

  • Function: E.g., Adductor muscles (adduct the leg).

  • Shape: E.g., Deltoid (triangular, like the Greek letter delta).

  • Direction of Fibers: E.g., Rectus (straight, such as rectus femoris).

  • Point of Attachment (Origin & Insertion): E.g., Sternocleidomastoid (originates on the sternum and clavicle, inserts on the mastoid process of the temporal bone).

  • Size: E.g., Gluteus Minimus, Medius, and Maximus (smallest, medium, and largest gluteal muscles).

Axial Muscles (Head, Face, Rib Cage, Spine)

• These muscles are central to the body and primarily move the axial skeleton.

Muscles of the Face (Facial Expressions, Chewing, Speech)

• Occipital Frontalis: Covers the forehead and frontal bone; raises eyebrows, wrinkles forehead, draws scalp back.

• Corrugator Supercilii: Draws eyebrows together and wrinkles the nose.

• Orbicularis Oculi: A circular muscle around the eye socket; contracts to squeeze eyes shut tightly.

• Orbicularis Oris: A circular muscle around the mouth; works with the buccinator to pucker the mouth and press lips against teeth.

• Zygomaticus Major: Attaches to the zygomatic arch (cheekbones); draws the corner of the mouth upward (e.g., smiling).

Muscles of Mastication (Chewing)

• These are powerful muscles for their size, generating significant jaw force.

• Masseter: Located on the side of the mandible; clenches the jaw.

• Temporalis: Originates from the temporal bone; clenches the jaw (can be felt contracting in the temple region).

• Pterygoids (Medial and Lateral): Attach to the sphenoid bone; help move the mandible sideways.

• Buccinator: Important for puckering the mouth (with orbicularis oris) and helps fold food between the teeth during chewing.