The Muscular System is part of the Anatomy and Physiology section which provides High Yield information for the MCAT exam needed for Medical School.
Muscle Basics
- – Three types (cells are long and thin, called fibers)
- 1. skeletal
- a. striated
- b. voluntary control
- c. many nuclei per cell
- d. longest fibers (extend the length of the whole muscle
- 2. cardiac
- a. striated
- b. involuntary control
- c. one nucleus per cell
- 3. smooth
- a. not striated
- b. involuntary control
- c. one nucleus per cell
- 1. skeletal
- – Functions
- 1. movement
- a. of whole body or body parts (skeletal)
- b. of substances within body (cardiac – pumps blood, smooth – substances move through hollow organs)
- 2. heat production (mostly skeletal)
- 3. maintain posture and stabilize joints (skeletal)
- 1. movement
- – Characteristics
- 1. excitability – respond to stimuli like neurotransmitters (from neurons) or hormones with
electrical signals - 2. contractility – ability to develop tension (muscle fiber may shorten)
- 3. extensibility – can stretch
- 4. elasticity – assumes original length after stretching
- 1. excitability – respond to stimuli like neurotransmitters (from neurons) or hormones with
Skeletal Muscle
- – Associated Connective Tissue
- 1. superficial fascia (subcutaneous layer or hypodermis)
- a. areolar & adipose
- 1) stores water and fat
- 2) decreases heat loss
- 3) protects underlying tissues
- a. areolar & adipose
- 2. deep fascia
- a. dense irregular
- 1) holds together functional groups of muscle
- 2) allows free movement of muscles
- 3) packs spaces between muscles, nerves and blood vessels pass through
- a. dense irregular
- 3. less coarse CT layers
- a. protect and support muscle cells, reinforce whole muscle, provide elasticity
- 1) epimysium – dense irregular CT, wraps whole muscle
- 2) perimysium – dense irregular CT, wraps bundles of fibers called fascicles
- 3) endomysium – similar to areolar CT, lots of reticular fibers, wraps each fiber
- a. protect and support muscle cells, reinforce whole muscle, provide elasticity
- 4. all the CT layers are continuous with one another and with the tendons that attach the muscle to the periosteum of bone
- a. tendons are dense regular CT
- b. a flattened tendon is called an aponeurosis (may attach to bone, skin or another muscle)
- 1. superficial fascia (subcutaneous layer or hypodermis)
- – Skeletal muscle cells
- 1. very large
- a. 10 – 100 μm in diameter, may be many cm long
- 2. plasma membrane called sarcolemma
- 3. cytoplasm called sarcoplasm
- a. lots of glycogen (stored form of glucose) and myoglobin (a protein that binds O2)
- b. contains the usual organelles plus some modified ones
- 4. myofibrils
- a. specialized organelles that run the length of the cell (100s-1000s/cell)
- b. made up of contractile units called sarcomeres
- 1) sarcomeres are made up of myofilaments
- 2) the arrangement of myofilaments causes the striations
- c. myofilaments
- 1) thick filaments- made of the protein myosin, often called cross-bridges because they can bind with the thin filaments
- 2) thin filaments- made of the proteins actin (where myosin binds), tropomyosin and troponin
- 5. sarcoplasmic reticulum
- a. specialized smooth ER that stores calcium and releases it when signaled by a nerve impulse (an electrical signal from a neuron)
- 6. T tubules
- a. the sarcolemma penetrates into deeper parts of the cell, forming hollow tubes surrounding all the myofibrils
- b. conducts electrical signals throughout the cell so all myofibrils contract at once
- 7. Sliding Filament Mechanism
- a. when a nerve impulse signals the muscle cell, calcium is released from the SR
- b. this allows myosin to bind to actin and pull the thin filaments toward the center of the sarcomeres
- c. ATP required
- 1. very large
- – Blood supply
- 1. lots of blood needed to supply oxygen and carry away wastes from these very active cells
- 2. vessels penetrate CT layers, lot of capillaries in endomysium
- – Nerve supply
- 1. each muscle served by at least one motor nerve containing 100s of motor neurons
- a. a motor unit is one motor neuron plus all the muscle fibers it innervates
- b. a motor unit may have only a few muscle fibers or 1000+
- c. fewer muscle fibers per motor unit where fine, delicate control needed (eyes, fingers)
- d. more muscle fibers per motor unit where more power needed (limbs)
- e. activating more motor units at one time means a more powerful contraction
- 2. neuromuscular junction
- a. area where a neuron meets a muscle fiber
- b. separated by a gap called synaptic cleft
- c. when an electrical signal (action potential) travels to the end of a neuron, the neuron releases a chemical message called a neurotransmitter (specifically, acetylcholine at the neuromuscular junction, also known as ACh)
- d. the ACh binds to the muscle cell, and initiates an electrical signal (action potential) there
- e. this ultimately results in the muscle fiber contracting
- 1. each muscle served by at least one motor nerve containing 100s of motor neurons
- – Muscle tone
- 1. small groups of motor units are periodically activated involuntarily
- 2. this keeps the muscle ready to contract
- – Fiber types
- 1. red slow twitch (a.k.a. slow oxidative)
- a. small
- b. contract slowly (use ATP at a slow rate), have lots of mitochondria, myoglobin, good capillary supply (for using O2 to make ATP)
- c. resist fatigue, good for low intensity endurance activity
- d. postural muscles in back and lower limbs have lots
- 2. white fast twitch (a.k.a. fast glycolytic)
- a. large
- b. contract quickly, with lots of power (use ATP at a fast rate)
- c. fewer oxygen use components (generate most ATP anaerobically – without O2)
- d. fatigue quickly, good for high intensity activity
- e. lots in arms for lifting
- 3. intermediate fast twitch (a.k.a. fast oxidative)
- a. medium sized
- b. contract quickly, with lots of power (like white fast twitch)
- c. have component for making ATP with oxygen (like red slow twitch)
- d. fatigue resistant, good for intermediate activities
- e. muscles used for walking have lots
- 4. each muscle has a mix of the three types, but has a greater proportion of the type used most often
- 5. exercise can change fiber types
- a. endurance activities(e.g., running long distances): white fast twitch → intermediate fast twitch
- b. intense activities (e.g., weight lifting): intermediate fast twitch → white fast twitch
- c. changes occur in size of fiber, blood supply, number of mitochondria, etc.
- d. generally cannot convert between slow and fast fibers (depends on nerve supply)
- 1. red slow twitch (a.k.a. slow oxidative)
Skeletal Muscular Anatomy
Anterior
By Mikael Häggström, used with permission. / Public domain
Posterior
By Mikael Häggström, used with permission. / Public domain
Cardiac Muscle
- – 100 μm long, 15 μm diameter
- – Basically the same set-up of myofilaments, etc.
- – Cells connected by intercalated discs
- 1. desmosomes and gap junctions
- 2. cells contract as a unit
- – Main electrical stimulation from specialized cells that spontaneously activate (autorhythmicity)
- – Use oxygen to make ATP
OpenStax / CC BY
Smooth Muscle
- – 30 – 200 μm long, 2 – 10 μm diameter at middle
- – Arranged in sheets – Has thick and thin filaments, but not in the same pattern as other muscle types
- – Two basic types
- 1. multiunit
- a. groups of cells function independently
- b. innervated by autonomic (involuntary) nervous system
- c. found in large blood vessels, large airways, eye (for adjusting lens and iris), arrector pili
- 2. single unit (visceral)
- a. cells electrically linked by gap junctions and contract as a unit
- b. clusters of cells are self-excitable
- 1) pass electrical signal to other cells
- 2) also influenced by ANS
- c. most smooth muscle in the body is this type (hollow organs)
- d. uses oxygen to make ATP
- e. can be influenced by local metabolic changes or hormones
- 1. multiunit
Lever Systems
- – A lever is a rigid bar that moves on a fixed point (the fulcrum) when a force is applied to it; the force (effort) applied is used to move a resistance (load)
- 1. bones = levers
- 2. joints = fulcrum
- 3. muscles provide the effort
- – Levers operate in one of two ways
- 1. mechanical advantage
- a. load is closer to fulcrum, effort farther from fulcrum
- b. little effort moves a large load over a small distance
- 2. mechanical disadvantage
- a. load is farther from fulcrum, effort is closer to fulcrum
- b. lots of effort moves a load rapidly over a large distance
- 1. mechanical advantage
- – Types of levers
- 1. first-class
- a. fulcrum between load and effort
- b. seesaws, scissors, lifting head off chest
- c. can be mechanical advantage or disadvantage
- 2. second-class levers
- a. load between fulcrum and effort
- b. wheelbarrow, standing on toes
- c. mechanical advantage
- 3. third-class levers (most muscles in the body are set up this way)
- a. effort between load and fulcrum
- b. tweezers, lifting using biceps
- c. mechanical disadvantage
- 1. first-class
Arrangement of Fascicles
- – Influences range of motion and power
- 1. longer fibers can shorten more and have greater range of motion
- 2. a greater number of shorter fibers means more power
- – Types of arrangements
- 1. parallel (tend to be less powerful)
- 2. fusiform (nearly parallel)
- 3. circular
- 4. convergent
- 5. pennate (tend to be the most powerful)
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Skeletal Muscle Fibers
- Myofibrils
- repeating contractile (sarcomeres) which are made of two protein myofilaments
- Myosin – thick filament
- Actin – Thin filament
- Thick filament (myosin) – small protruding heads which bind to regions of the thin filament (actin)
- Movement of these two filaments relative to one another causes the lengthening and shortening of the sarcomere
- Sarcomere
- Z lines
- – Each individual sarcomere is flanked by dense protein discs, which hold the myofilaments in place
- Z discs
- – The actin filaments radiate out from the Z discs and help to anchor the central myosin filaments in place
- Striated pattern
- – produced by recurring sarcomeres along the length of the skeletal muscle fibers
- A band
- – center of the sarcomere appears darker due to the overlap of both actin and myosin filaments
- I band
- – peripheries of the sarcomere appear lighter as only actin is present in this region
- H zone
- – dark A band may also contain a slightly lighter central region where only the myosin is present
- Z lines
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Group Actions
- – Functional types of muscles
- 1. prime mover/agonist: the muscle that has the main responsibility for a particular movement
- 2. antagonist: opposes the action of the agonist
- 3. synergist: helps the agonist
- a. add extra force
- b. stabilize joint and prevent undesired movement
- 4. fixator: stabilizes prime mover
- – One muscle may act as any of the functional types
Origin and Insertion
- – Origin: the attachment point on the more stationary (less movable) bone, usually proximal
- – Insertion: the attachment point on the more movable bone, usually distal
- – The insertion moves toward the origin when the muscle shortens
Naming Muscles
- – Names may be based on…
- 1. location
- 2. shape
- 3. relative size
- 4. direction of fascicles and fibers
- 5. location of attachments
- 6. number of origins
- 7. action
Muscles Anatomy
References
- Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain.
- Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain.
- Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
