musculoskeletal system

skeletal muscle

responsible for voluntary movements; innervated by the somatic nervous system

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**striated** due to the arrangement of actin and myosin into **sarcomeres**

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contains red fibers, white fibers, and myoglobin

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Ca+2 required for contraction

red fibers

slow-twitch fibers

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high myoglobin

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derive energy aerobically; require high levels of mitochondria and use oxidative phosphorylation to make ATP

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contract slowly, but can sustain activity (ex. muscle that support posture)

myoglobin

oxygen carrier that uses iron in a heme group to bind oxygen, imparting a red color

white fiber

fast-twitch fiber

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less myoglobin compared to red fibers

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less mitochondria than red fibers; relies on fermentation and glycolysis to make ATP

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contract rapidly, but fatigue faster

smooth muscle

responsible for involuntary movement; controlled by autonomic nervous system

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have a single nucleus located in the center of the cell

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contains actin and myosin

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not striated

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capable of more-sustained contractions; a constant state of low-level contraction, as seen in blood vessels, is called a **tonus**

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can contract with myogenic activity'; contract directly in response to stretch or other stimuli

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Ca+2 required for contraction

myogenic activity

ability for smooth muscle to contract without nervous system input

muscular system

consists of smooth, cardiac, and skeletal muscle

cardiac muscle

characteristics of both smooth and skeletal muscle

* striated (skeletal)
* involuntary movement; innervated by autonomic nervous system (smooth)

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primarily uninucleated, but some cells can contain 2 nuclei

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cells are connected by **intercalated discs** which contain **gap junctions**

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able to define and maintain their own rhythm; myogenic activity

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Ca+2 required for contraction

gap junctions

connections between the cytoplasm of adjacent cells, allowing for the flow of ions directly between cells; allows for rapid and coordinated depolarization of muscle cells and efficient contraction of cardiac muscle

sarcomere

basic contractile unit of skeletal muscle

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composed of thick and thin filaments

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divided into different lines, zones, and bands

* during contraction, these lines become smaller
* A band size remains constant

thick filaments

organized bundles of myosin

thin filaments

organized bundles composed of actin, troponin, and tropomyosin

titin

protein that acts as a spring and anchors the actin and myosin filaments together, preventing excessive stretching of the skeletal muscle

Z line

defines the boundary of each sarcomere

M line

runs down the middle of the sarcomere, through the middle of the myosin filaments

I band

region containing exclusively thin filaments

H zone

contains only thick filaments

A band

contains thick filaments in their entirety, including any overlap with thin filaments

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size does not change during contraction

myofibril

composed of sarcomeres

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surrounded by **sarcoplasmic reticulum (SR)** that contains a high concentration of Ca+2

sarcolemma

cell membrane of a myocyte

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capable of propagating an action potential and can distribute the action potential to all sarcomeres in a muscle using a system of **T-tubules**

myocyte

muscle cell

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composed of many myofibrils arranged in parallel

muscle contraction

1. starts at the **neuromuscular junction**, where the NS communicates with muscle via efferent (motor) neurons
2. signal travels down the neuron to the **nerve terminal (synaptic bouton)** where acetylcholine is released into the synapse; nerve terminal also called the **motor end plate**
3. Acetylcholine binds to receptors on the sarcolemma, causing depolarization
4. depolarization triggers action potential that spreads down the sarcolemma to the T-tubules
5. action potential travels down the T-tubules into the muscle tissues to the SR where Ca+2 is released
6. Ca+ binds to a regulatory subunit in troponin, triggering a change in the confirmation of tropomyosin to which troponin binds
7. this change exposes **myosin-binding sites** on the actin thin filaments
8. free globular heads of the myosin molecules move toward and bind with the exposed sites on actin to form cross bridges
9. cross bridges allow myosin to pull on actin, which draws the thin filaments toward the M line, resulting shortening of the sarcomere
10. Acetylcholine is degraded in the synapse by **acetylcholinesterase** which results in termination of the signal at the neuromuscular junction and allows the sarcolemma to repolarize
11. ATP binds to the myosin heads, freeing them from actin

actin-myosin cross bridge cycle

1. myosin carrying hydrolyzed ATP (ADP + inorganic Phosphate) is able to bind with the myosin-binding site
2. release of inorganic phosphate and ADP in rapid succession provides energy for the powerstroke and results in sliding of the actin filament over the myosin filament
3. ATP binds to the myosin head, releasing it from actin
4. This ATP is hydrolyzed to ADP + inorganic phosphate which recocks the myosin head so that it is in position to initiate another cross-bridge cycle

motor unit

myocytes + nerve terminal

latent period

time between reaching threshold and the onset of contraction

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action potential spreads along the muscle and allows for calcium to be released from the SR

frequency summation

if a muscle fiber is exposed to frequent and prolonged stimulation, it will have a insufficient time to relax

tetanus

results when muscle contractions become so frequent that the muscle is unable to relax at all

creatine phosphate

oxygen reserve in muscles that is created by transferring a phosphate group from ATP to creatine during times of rest

oxygen debt

difference between the amount of oxygen needed by the muscles and the actual amount present

exoskeleton

encase whole organisms and are usually found in arthropods

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must be shed and regrown to accommodate growth

endoskeletons

internal skeleton that is found in vertebrates

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not able to protect soft tissue structures

skeletal structure

**axial skeleton:**

* consists of the skull, vertebral column, rib cage, and hyoid bone
* provides for basic central framework for the body

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**appendicular skeleton:**

* consists of the upper limbs, pectoral girdle, and pelvis

skeletal system

created from 2 major components: bone and cartilage

macroscopic bone structure

connective tissue derived from embryonic mesoderm

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contains a marrow cavity

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outermost portion of bone are composed of compact bone

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internal core is made of spongy bone

Long bones are found in the appendicular skeleton

compact bone

dense and strong bone

spongy bone (cancellous)

lattice structure consists of bony spicules (joints) known as **trabeculae**

red marrow

type of bone marrow filled with hematopoietic stem cells which are responsible for the generation of all the cells in our blood

yellow marrow

type of bone marrow that contains fats and is relatively inactive

long bone

bones in the appendicular skeleton

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have cylindrical shafts (**diaphyses)** that swell at the end to form **metaphyses,** and terminate in **epiphyses**

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diaphyses and metaphyses are full of bone marrow

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epiphyses use spongy cores for more effective dispersion of forces and pressure at joints

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surrounded by **periosteum** to protect it as well a serve as a site for muscle attachment

epiphyseal plate

growth plate

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cartilaginous structure and the site of longitudinal growth

tendons

attach muscle to bone

ligaments

hold bones together at joints

bone matrix

strength of compact bones come from here

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has organic and inorganic compounds

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organic compounds: collagen, glycoproteins, and other peptides

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inorganic compounds: calcium, phosphate, and hydroxide ions

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ordered into structural units known as **osteons/ Haversian systems**

osteons

contain concentric circles of bony matrix called **lamellae** surrounding a central microscopic channel

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longitudinal channels = **Haversian canals**

transverse channels = **Volkmann’s canals**

lacunae

small spaces between the lamellar rings

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contain mature bone cells (**osteocytes)**

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interconnected by **canaliculi** that allow for the exchange of nutrients and wastes between osteocytes and the Haversian and Volkmann’s canals

osteoblasts

cell type that builds bone

osteoclasts

polynucleated resident macrophages of bone that resorb it

parathyroid hormone

peptide hormone released by the parathyroid gland in response to low blood calcium promotes bone resorption

vitamin D

activated in parathyroid gland

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promotes the resorption of bone and encourages the growth of new, stronger bone

calcitonin

peptide hormone released by the parafollicular cells of the thyroid in response to high blood calcium promote bone formation, lowering blood calcium levels

cartilage

composed of chondrin

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avascular and not innervated

chondrin

firm but elastic matrix found in cartilage

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composed from chondrocytes

endochondral ossification

hardening of cartilage into bone

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responsible for the formation of most of the long bones in the body

intramembranous ossification

undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into, and replaced by, bone

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occurs in bones of the skull

immovable joints

consists of bones that are fused together to form sutures or similar fibrous joints

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found primarily in the head

movable joints

structures that include hinge joints, ball-and-socket joints, and others

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permit bones to shift relative to one another

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strengthened by ligaments

ligaments

pieces of fibrous tissue that connects bones to one another

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consists of **synovial capsule**, which encloses the actual **joint cavity (articular cavity)**

synovium

secretes synovial fluid which lubricates the movement of structures in the joint space

articular cartilage

contributes to the joint by coating the articular surfaces of the bones so that impact is restricted to the lubricated joint cartilage, rather than to the bones

origin

end of the muscle with a larger attachment

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proximal connection

insertion

end of the muscle with a smaller attachment to bone

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distal connection

antagonistic pairs

one relaxes while the other contracts

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Ex. bicep and tricep

synergetic pair

working together to accomplish the same function

flexor

muscle that decreases the angle across a joint

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Ex. biceps brachii

extensor

muscle that increases or straightens the angle across a joint

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Ex. tricep brachii

abductor

muscle that moves a part of the body away from the midline

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Ex. deltoid

adductor

muscle that moves a part of the body toward the midline

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Ex. pectoralis major

Medial rotation

rotation that rotates the axis of the limb toward the midline

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Ex. subscapularis

lateral rotation

rotation that rotates the axis of the limb away from the midline

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Ex. infraspinatus