Skeletal System

• Human skeleton is internal, which is actually pretty rare in the animal kingdom, where most skeletons are either external or hydrostatic

• Primary functions of the human skeleton

  • Support

  • Movement

  • Protection

  • Production of blood

• Subdivisions of the human skeleton

  • Axial Skeleton: skull, sternum, ribs, spine

  • Appendicular skeleton: limbs and girdles

• Bone types:

  • Flat: sternum and ribs, cranium; protective

  • Long: primarily in the limbs; longer than wide; diaphysis and epiphysis

  • Short: cuboid; primarily in the wrists and ankles

  • Irregular: specialized shapes such as vertebrae and pelvis

  • (Sesamoid): shaped like a sesame seed; patella

  • (Sutural): bones of the skull, vary in size and shape

• Bone markings

  • Fossae:

    • Depression of bone that articulates with other bones to form joints

  • Processes:

    • A projection of a bone

  • Foramen:

    • A round hole where blood vessels or ligaments pass

  • (Condyle):

    • The round part on end of bone that articulates with fossa

• Skull bones: structurally supportive and protective. Mostly fused. Made up of flat bone which consists of a layer of spongy bone, lined on either side by a layer of compact bone. The two layers of compact bone and the interior spongy bone work together to protect the internal organs.

• Spine:

  • Cervical:

    • Atlas and axis

  • Thoracic:

    • Form the back of the ribcage

  • Lumbar:

    • Supports the whole upper body

  • Sacral:

    • The backside of the pelvic girdle

• Ribs are derived from the dorsal ribs of fish and are flat bone:

  • True ribs:

    • connect directly to the costal cartilage that attaches to the sternum

  • False ribs:

    • attach indirectly to the sternum

  • Floating ribs:

    • are not attached to the sternum at all

• Limb girdles

  • Pelvic girdle: pubis, ilium, ischium. Attaches the legs to the axial skeleton; differences between male and female pelvises

  • Shoulder girdle: scapula, clavicle. Attaches the arms to the axial skeleton

• Common pattern in upper and lower limbs: “one bone (humerus, femur), two bones (radius/ulna, tibia/fibula), lots of small bones (carpals, tarsals), digits (metacarpals/phalanges, metatarsals/phalanges)”

• Bone shape and growth:

  • Epiphysis (end) and diaphysis (long part of bone)

  • Growth occurs in the epiphyseal plates during childhood and fully calcifies in mature, adult bone

• Bone tissue

  • Bone cells:

    • osteogenic cells become osteoblasts, which form bone tissue.

    • Osteoblasts, once trapped in the bone matrix, become osteocytes, which are involved in maintaining bone tissue.

    • Osteoclasts are involved in reabsorbing bone tissue during growth and/or repair.

  • Bones are covered by a tough membrane called the periosteum

  • Spongy bone is found in the epiphysis (end) of long bones, is porous, and contains red marrow which produces red blood cells

  • Compact bone:

    • Found on the outside surface of bones

    • Filled with osteons, which conduct nerves and vessels through the bone

• Joints:

  • Synovial joint: fluid-filled, allow for movement (there are many types that allow for different angles of motion)

  • Cartilaginous joint: filled with cartilage, allows for some movement, but limited

  • Fibrous joint: these are bones that are sutured together and do not allow movement

  • Tendons connect muscles to bone (origin vs. insertion)

  • Ligaments attach bone to bone, often these limit movement and increase stability

• Calcium homeostasis:

  • calcium levels in the blood are important, and if levels are low, the parathyroid gland will stimulate the breakdown of bone to release calcium into the blood.

  • When calcium levels are high, the thyroid will stimulate the bone to deposit calcium.

Muscular System

• Functions of the muscular system:

  • Cause motion (including voluntary and involuntary movements)

  • Restrict motion (maintenance of posture, controlling joint angles, etc)

  • Generate heat (shivering)

  • Protection of organs, specifically in the abdomen and pelvis

• Types of muscle tissue

  • Skeletal:

    • Epimysium covers the whole muscle, which is composed of fascicles

    • Fascicles are covered by the perimysium and consist of bundles of muscle fibers (myocytes/muscle cells)

    • The endomysium is found in between muscle fibers and carries nerves and vessels

    • Also surrounding each myocyte/muscle cell/muscle fiber is the sarcolemma, or cell membrane of each multinucleated muscle cell

    • Muscle cells contain many myofibrils, which in turn are composed of repeating units called the sarcomere

    • Sarcomere structure:

      • Actin

        • Thin filament

      • Myosin

        • Thick filament

      • Troponin

        • changes the shape of the tropomyosin molecule when calcium binds to it

      • Tropomyosin

        • blocks myosin binding sites

      • Calcium

        • binds to troponin

      • Acetylcholine

  • Cardiac: cells are branched and linked together via desmosomes and gap junctions that allow the contraction signal (action potential) to pass easily from one cell to another. Which allows the heart to contract as one unit.

  • Smooth: cells are much shorter, and contract as a single unit, often triggered by local factors such as the contraction of nearby smooth muscle cells, stretching, or hormone action

• Muscle fiber types

  • Slow oxidative: produce ATP primarily by aerobic respiration, contract slowly, fatigue slowly, and contain a high density of mitochondria and blood vessels (so they’re red).  Best for slow actions.

  • Fast oxidative/glycolytic: produce ATP either by aerobic or anaerobic respiration, contract quickly, and fatigue moderately.  Best for either fast or slow contraction.

  • Fast glycolytic: produce ATP primarily by anaerobic respiration (glycolysis), contract very quickly, fatigue quickly, and contain a low density of mitochondria and blood vessels (so they’re white).  Best for fast contraction.

• Major muscle groups:

  • Neck and shoulders: trapezius, deltoids

  • Arms: biceps, triceps

  • Back: rhomboids, latissimus dorsi

  • Abdomen: abdominals, abdominal oblique

  • Legs: gluteus maximus, hamstrings, quadriceps, adductors, calves, soleus

Muscle Tissue Types

Skeletal muscle: voluntary, multinucleated (makes protein synthesis more efficient), striated

• Found attached to the bones, used for movement of the body

• controlled by the somatic nervous system (conscious mind)

• cells are large and very long, striated

• Epimysium: connective tissue that covers the whole muscle

• Periomysium: surrounds each fascicle

• Endomysium; around individual muscle fibers (aka muscle cells, these also contain bundles of myofibrils) (myofibrils are composed of repeating units of actin and myosin- one unit is a sarcomere.)

Actin and Myosin:

1. Atp is attached to the myosin head, it is hydrolyzed, causing a conformational change that “cocks” the myosin head so it is ready to bind to actin

2. Myosin binds to actin (with adp/p still attached)

3. The release of ADP/P causes another conformational change, this one known as “the power stroke” which moves myosin in relation to the actin filament

4. A new ATP molecule binds to a myosin head, causing a conformational change that detaches myosin and returns it to the “rest” position.

   *Ca2+ and Atp must be present in high concentrations for this process to take place

As myosin filaments move along actin, the whole sarcomere shortens, which in turn shortens the whole myofibril, muscle fiber, fascicle, and muscle

Actin filaments are covered by tropomyosin, which blocks the myosin binding sites UNTIl Calcium(Ca2+) released by neurons binds to troponin, which changes the shape of the tropomyosin molecule.

Cardiac muscle: striated, controlled by the involuntary autonomic nervous system, single (or double) nucleated, branched, smaller cells than skeletal muscle

(striated- sarcomeres, the contractive unit of overlapping actin and myosin)

• Special adaptions of cardiac muscle:

  • Intercalated discs: finger-like projections that overlap at cell boundaries

  • Desmosomes: intermembrane proteins on the surface of both cells that anchor the two and keep them from pulling apart during contractions.

  • Gap junctions: proteins that form a channel between two cells and pass Ca2+ from the cytoplasm of one to the next.

smooth muscle:

• smaller, single-nucleated,” football-shaped”, involuntary

• Found in the internal organs, blood vessels, ureter, iris, etc.

• Controlled by the involuntary autonomic nervous system as well as by local stimuli (i.e. the contraction of nearby cells) and hormones

• Smallest of the muscle cells, non-striated

Chemical reactions with proteins can cause them to change shape (conformational change)