Muscular System

Thursday, November 3, 2022

Functions of the muscular system:

• Movement: bones or fluids/blood
• Balance: maintaining posture and body position
• Stabilizing joints
• Heat generation (skeletal)

Characteristics of muscle tissue

• Excitability (responsiveness/irritability): receive and respond to stimuli
• Contractility: shorten when stimulated
• Extensibility: ability to be stretched
• Elasticity: recoil to resting length

Types of muscular tissue

• Skeletal muscle: attached to bones and skin; striated; voluntary
• Cardiac muscle: only in the heart; striated; involuntary; insulated discs
• Smooth muscle: walls of hollow organs; not striated; involuntary

Characteristics of skeletal muscle:

• “myo”/ “mys” = muscle 

• “Sarco” = flesh

• A single muscle fiber is enclosed by the endomysium

• A muscle fiber is the same thing as a muscle cell

• The perimysium wraps around a bundle of muscle fibers(fascicle)

• The entire muscle is covered by the epimysium which blends into muscle attachment

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Friday, November 4, 2022

Microscopic anatomy of skeletal muscles:

Outside if the muscle cell:

• Sarcolemma: specialized plasma membrane

Inside of the muscle cell:

• Myofibrils: long organelles
• Sarcoplasm: cytoplasm
• Sarcoplasmic reticulum: specialized smooth ER; stores and releases calcium
• T-tubules: allow electrical nerve transmissions to reach deep interior of muscle fiber

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Sarcomere: contractile unit of a muscle fiber

• Myofibrils align to give distinct banding
• I band = light; thin filaments
• A band = dark; thick filaments
• H zone = bare sone, not thin filaments at rest

Monday, November 7, 2022

Sarcomere = contractile unit of a muscle fiber

Myofilaments (proteins)

• Myosin: thick filaments, ATPase enzymes, myosin heads & tails
• Actin: thin filaments, anchored to z discs

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Myofibrils thick & thin filaments

Myosin - thick filaments

• During contraction, the heads link thick and thin filaments together, forming cross bridges
• Offset from each other -> staggered array of heads at different points along thick filament

Actin - thin filaments

• G actin subunits bear sites for myosin head
• G actin subunits link together to form long, fibrous F (filamentous) actin -> two F actin strands twist together to form a thin filament

Wednesday, November 9, 2022

Sliding filament theory

• Nerve stimulus --> action potential
• Skeletal muscles stimulated by motor neuron(nerve associated with motor cells)
• Motor unit = one motor neuron and all he skeletal muscle cells stimulated by that neuron
• Neuromuscular junction = association site of axon terminal of motor neuron and muscle
• Synaptic cleft = gap between nerve and muscle
• Activation by nerve causes myosin heads to attach to binding site on the actin(cross-bridge)

Action potential

1. Action potential reaches axon terminal of motor neuron (phone ringing)
2. Calcium (Ca+2) channels open and Ca2+ enters the axon terminal (someone picks up the phone)
3. Ca2+ entry causes some synaptic vesicles to to release their contents(acetylcholine, a neurotransmitter) by exocytosis (calcium coming in, forces acetylcholine out)
4. Acetylcholine diffuses across the synaptic cleft and binds to the receptors in the sarcolemma
5. When ACh binds to the receptors and the channel opens which allows sodium to comes in and potassium leaves; more sodium comes in than potassium leaving(more positive coming in then leaving) making the inside the of the cell positive; the change in charge between the outside and inside of the cell 
6. ACh effects are ended by its breakdown in the synaptic cleft by the enzyme acetylcholinesterase

Tuesday, November 15, 2022

Skeletal muscle contraction

• Muscle fiber contraction id “all or none”
• Not all muscle fibers/cells may be stimulated at the same interval
• Different combinations of muscle fibers contracting may give differing responses (graded responses; as in a scale)
• Graded responses depend on: the frequency of muscle stimulation(how often are we getting simulation); and number of stimulated muscle cells at one time(how many are being stimulated)

Types of graded responses

Types of muscle contractions

Isotonic contractions:

• Myofilaments slide past each other

• Muscle shortens and movement occurs

• Ex: you pick up, put down; includes reps;

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Isometric contractions:

• Increased tension in muscles
• Muscle is unable to shorten or produce movement
• Ex: wall sit; pushing 

Thursday, November 17, 2022

Muscle tone and exercise

Fibers contract at different times to provide muscle tone and be constantly ready (some fibers are contracted even in a relaxed muscle)

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Common challenges for low muscle tone:

• Poor posture
• Limited muscle strength
• Poor core stability
• Increased joint flexibility
• Gross motor delays
• Easily fatigued

Aerobic/endurance

• Stronger, more flexible, resistance to fatigue
• Makes body metabolism more efficient
• Improves digestion and coordination

Resistance/isometric

• Increases size, increase strength
• Strengthen and stabilizes
• Helps posture

Energy in Muscles

Direct Phosphorylation

• Muscle cells store CP (high energy molecule)
• After ATP is depleted, ADP is left
  • CP transfers a phosphate group to ADP, to regenerate ATP
  • CP supplies are exhausted in less than 15 seconds
  • About 1 ATP is created per CP 

Anaerobic Respiration & Lactic Acid Fermentation

• Reaction that breaks down glucose without oxygen
  • Glucose broken down to pyruvic acid to produce about 2 ATP
  • Pyruvic acid is converted to lactic acid
• Reaction is fast but not efficient, requires huge amounts of glucose, and leads to muscle fatigue

Aerobic Respiration

• Slower reaction 

• Requires continuous oxygen

  • Glucose is broken down
  • A series of metabolic pathways occur in the mitochondria
  • Releases lots of energy (about 32 ATP)
  • CO2 and H2O are produced

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Muscle Fatigue and Oxygen Deficit

• Common cause for muscle fatigue is oxygen debt
• Oxygen is required to get rid of accumulated lactic acid
• Increasing acidity(from lactic acid + lack of ATP = less muscle contraction
• Fatigued muscles cannot contract even with a stimulus

Rules of Skeletal Muscle Activity

• Muscles cross at least one joint
• Bulk of muscle proximal to joint crossed
• At least 2 attachments (origin & insertion)
• Muscles can only pull
• Insertion moves toward origin

Types of muscles

• Prime mover: major responsible for movement
• Antagonist: opposes or reverses prime mover
• Synergist: aids/in sync with prime mover and prevents rotation
• Fixator: stabilizes origin of prime mover

Friday, November 18, 2022

Special characteristics of muscles

Recruitment - is the ability to activate more motor units as more force (tension) needs to be generated

Atrophy - muscles will not get or stay big if stress is not apple(if you don’t workout)

Hypertrophy - occurs when a muscle is “stressed”(strong demand on the muscle)

• Causes more myofilaments/myofibrils to be produced within muscle fibers

• Allows for more “cross bridges” resulting in more force (strength) as well as larger size

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Muscle tear: tear a muscle cell from the tendon

Rigor mortis

• Literal translation: rigor = intense/hard; mortis = mortal/intense: = hard death
• 3 - 12 hours after death
  • Body stiffens
  • Dying cells let Ca2+ levels rise → muscles contract
  • ATP production stops at death → actin & myosin are locked; muscles stay contracted
• 48-60 hours after death
  • Muscle proteins breakdown
  • Rigor mortis ends

Curare

• Arrowhead poison used by South American Indians
• Binds to ACH receptors on motor end plate (muscles cannot contract): the nerve knows we have to move, the nerve sends the action potential but it doesn't allow the action potential to reach the sarcomere so muscles don’t contract
• Causes cardiac and respiratory arrest
• Similar chemicals are used in medicine for procedures like intubation
• Many nerve gasses have a similar effect by inhibiting acetylcholinesterase

Myalgia

• Acute(small) muscle pain
  • Immediate pain (during or directly after use)
  • Longer contraction
  • Due to deficient blood supply
• Delayed muscle pain
  • Delayed 24-48 hours
  • Do activity that hasn’t been done in a while
  • Tear muscle cells and fibers
  • Tissue swells

Duchenne muscular dystrophy

• Sex-linked recessive disorder
• Usually diagnosed around 2 years old
• Lack dystrophin (protein that helps stabilize the sarcomere)
• Initially muscles enlarge due to fat and connective tissue deposit
• Muscle fibers degenerate

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