Chapter 32

Skeletal System
Introduction

I-Bones

A. Function
B. Structure

II-Skeletal System

III-Joints

Muscular System
I-Introduction

II-Skeletal Muscle

A. Structure
B. Sliding Filament Mechanism

Skeletal System

Introduction
Skin, muscle and bones allow movement. Skin – pliable covering. Muscles do actual moving. Bones give anchor to move against.

I-Bones

A. Functions of Bones

1. Support. Provide a hard framework.
2. Protection of many vital organs.
3. Movement. Act as levers with skeletal muscles moving them. Joints control possible movements.
4. Mineral storage. Especially calcium and phosphate, critical minerals for cellular function. Continuous deposition and withdrawal.

Exquisite control of Ca++ (calcium ions) levels necessary for function of nerves, muscles, blood coagulation and other functions. Most of Ca++ in body in bones. Osteoclasts & osteoblasts controlled by hormones which regulate blood levels of Ca++.
5. Blood cell formation. Certain bones have active marrow.

B. Structure

1. Compact-Dense outer layer, looks smooth and solid. Contains cylinder of concentric layers with central canals.

a. Haversian system = circles of bone (lamella) with central canal (Haversian canal)
b. Central canal contain blood vessels & nerves. Connected at right angles to network.
c. Perforating small canals - blood vessels & nerves go through lamellar bone to supply osteocytes. Connect to periosteum.
d. Osteocytes live in bone, maintain it. Live in holes called lacunae. Connect to each other and central canal via canaliculi, little canals. Pass nutrients, waste products

2. Spongy- honeycombed, open spaces. Same structure as compact but less regular.

Withstand maximum stress with least weight.
In bone interiors & weird weight bearing bones like head of femur.
Not organized in lamella.
Trabeculae are arranged along lines of stress.
Osteocytes interconnected by canaliculi. Nutrients reach osteocytes by diffusing through the canaliculi from capillaries.

Osteoporosis - More bone resorption than deposition, very weak bone.

II-Skeletal System

A. Axial skeleton

Principal supportive structure of the body
Includes skull, vertebrae, sternum & ribs.
Central column of the skeleton from which arms and legs & bones that help them hang.

B. Appendicular skeleton

Provides fairly freely movable frame for upper & lower limbs.
Includes pectoral (shoulder) & pelvic (hip) girdles, arms, forearms, wrists, hands, thighs, legs & feet.

III-Joints

Bones -> framework; muscles -> power; joints provide mechanism that allows body to move

A joint is where 2 adjacent bones or cartilages or combination thereof meet.
Most joints movable, some not.

General Structure

1. Articular cartilage
2. Joint (synovial) cavity
3. Articular capsule – external layer = fibrous capsule, inner layer is a synovial membrane
4. Synovial fluid-occupies all free spaces within the joint capsule, fluid derived by filtration from blood flowing thorough the capillaries in the synovial membrane
5. Reinforcing ligaments

Muscular System

I-Introduction

II-Skeletal Muscle

A. Structure
B. Sliding Filament Mechanism

I-Introduction
Skeleton provide system of levers. Joints allow skeleton to move. Muscles provide force for movement.
Muscle can only contract, it cannot push. Antagonistic pairs.

The crucial properties of muscle tissue are its ability to:

1. to contract
2. be excited by a stimulus
3. return to its original shape after stretching or contracting.

II-Skeletal Muscle
Skeletal muscles are usually contracted voluntarily & consciously.

A. Structure

Composed of bundles of muscle cells (called muscle fibers). Muscle cells contain thick (myosin) & thin (actin) protein threads (filaments) that slide over each other. Series of proteins in register -> striated appearance.

Network of tubes (sarcoplasmic reticulum) running through muscle contain Ca++ (calcium ions) that are necessary for control of muscle. Capillaries surround muscle cells, deliver nutrients & O2, remove wastes & CO2.

B. Sliding Filament Mechanism
Sliding filaments -> contraction. Shortening of muscle cell is consequence of thin filaments sliding between the thick filaments in muscle cell. When muscle relaxes or is stretched, the overlap is reduced. Length of filaments (myofilaments) does not change.

1. Muscle resting-myosin heads are blocked by tropomyosin from actin.

2. Action potential (nerve) -> depolarization of muscle fiber, spreads rapidly throughout muscle, -> Ca ++ (calcium ions) release

3. Ca++ exposes actin. Tropomyosin moved. Myosin heads bind to actin and slide over it. Forces actin filaments to come close together, pulls Z lines closer together, -> muscle shorter

4. Cross-bridge binding allows myosin head to tilt & pull actin over myosin -> muscle contraction. Happens several times -> ratchet action.

5. Muscle relaxation - Ca++ pumped out of muscle cell -> myosin blocked from actin again. Tropomyosin moves back. ATP binds to myosin again & it regains resting position.

Rigor mortis – ATP depleted & Ca++ cannot be pumped out of cell. Myosin heads cannot detach from actin -> rigidity.

3 sources of ATP – different amounts and accessibility

1. creatine phosphate -> nearly instant, direct phosphorylation of ADP, lasts about 15 sec
2. anaerobic glycolysis of stored glycogen or glucose from blood -> lasts about 30-40 secs, lactic acid formation
3. aerobic respiration – requires O2, -> lasting, efficient supply of energy. Glucose -> pyruvic acid via glycolysis, then combines with O2 in mitochondria -> lots of ATP.

Strength of muscle contraction depends on the frequency of stimulation & on how many motor units are activated.