BIOL 2160 Study Guide - Final Guide: Endoplasmic Reticulum, Skeletal Muscle, Calcium Atpase
1 May 2018
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Biology 2160 - Test 4 Notes
Muscle Physiology
Types of Muscle
• Skeletal muscle (striated / voluntary)
• Cardiac muscle (striated / involuntary)
• Smooth muscle (smooth / involuntary)
Skeletal Muscle Structure
• Muscle body connective tissue that holds together fascicles (little bundles) of muscle fibers
(aka muscle cell) myofibril made up of proteins
• Muscle cells and neurons are the only two types of cells that don’t regenerate at adulthood
• Transverse tubules carry the charge deep into the muscle cell to tell myofibrils to shorten to make the
muscle move
• Sarcoplasmic reticulum is the endoplasmic reticulum in a muscle cell (“sarc” means muscle)
o Stores a lot of calcium
• Neuromuscular junction is the most well-studied synapse in the human body; formed by the contact
between a motor neuron and a muscle fiber
• Motor end plate is a part of the muscle cell where there is the highest number of receptors for
Acetycholine
Sarcomere Components
• Thin filaments – made up of two strands of F actin that form a double helix
• Thick filaments – made up of myosin dimers bound together at tails, binding sites on heads (cross
bridges) for actin
• Tropomyosin – extends along thin filaments, masks myosin binding site in absence of calcium
• Troponin – calcium binding site, actin binding site, tropomyosin binding site
Steps in Muscle Contraction (Excitation-Contraction Coupling)
• Motor Neuron Action Potential
o From the moment you decide to move your finger to the moment your finger actually wiggles
• End plate potential (excitation)
o ACh binds to receptors that produces an end plate potential (aka an action potential in a muscle
cell)
• Increase in muscle cell calcium levels
• Troponin and Trypomyosin conformational changes
• Crossbridge cycling sliding filaments (contraction)
Crossbridge Cycling
• The functional unit of contraction is called the sarcomere
• When calcium released by the SR binds to troponin it forms a cross bridge
• Tropomyosin moves away from the binding sites allowing the myosin head to bind actin to form a cross
bridge
• ATP binds to the myosin then is hydrolyzed into ADP and inorganic phosphate
• This activates the myosin head and forces it into the cocked position
• Step 1: Cross Bridge Formation
o Activated myosin head binds to actin forming a cross bridge
o Inorganic phosphate is released
o The bond between and myosin and actin becomes stronger
• Step 2: The Power Stroke
o ADP is released and the activated myosin head pivots sliding the thin myofilaments toward the
center of the sarcomere
• Step 3: Cross Bridge Detachment
o When another ATP binds to the myosin head, the link between the myosin head and actin
weakens
o The myosin head detaches
• Step 4: Reactivation of Myosin Head
o ATP is hydrolyzed to ADP and inorganic phosphate
o The energy released during hydrolysis reactivated myosin head
o Returning it to a cocked position
• The cross bridge cycle ends when calcium ions are transported back into the SR
Termination of Contraction
• Motor neuron input terminates
o Motor neuron stops releasing Ach
• End Plate Potentials terminate
• High myoplasmic (cytoplasm of muscle cell) calcium concentration shuts SR calcium channels
• Active calcium uptake through SERCA pumps on SR membrane
o SERCA – Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase
o Pump that uses ATP to pump calcium back into the SR
• Calcium dissociates from troponin
• Tropomyosin covers myosin binding sites in actin
The Twitch
• Smallest muscle contraction you can have
• The mechanical response or change in force/tension (measured in grams) of an individual muscle fiber,
motor unit, or whole muscle to a single action potential
• Isotonic twitch – muscle-generated force causes muscle shortening and lifts a load (load must be less
than or equal to muscle tension) ex: picking up a book
o If the object moves isotonic
• Isometric twitch – muscle generates force but does not shorten (load/force opposing muscle shortening
greater than muscle tension) ex: pushing on a wall
o If it doesn’t move isometric
Slow vs. Fast twitch fibers
• You can classify twitches based on the latent period (the delay)
• The amount of time in between the stimulus and the muscle contracting
• Three types of muscles:
o Extraocular muscle (allows eye to move) – very short latent period
o Gastrocnemius (calf) – much longer latent period
o Soleus (calf) – mush longer latent period
o The eye is much faster because (from an evolutionary standpoint) we needed to see the danger of
an approaching predator
▪ Has to do with how quickly the myosin head breaks down ATP
• Slow twitch- contain slow myosin, (hydrolyzes ATP to ADP and P slower, myosin head cocking slower)
• Fast twitch – contain fast myosin, (hydrolyzes ATP to ADP and P faster, myosin head cocking faster)
• Skeletal muscles can contain slow twitch fibers, fast twitch fibers, or a mixture of both
Glycolytic vs. Oxidative fibers
• Glycolytic (anaerobic) – generate more ATP through glycolysis
o High concentration of glycolysis enzymes
o Have few mitochondria because they don’t need them
o Used for short bursts of energy
o Lighter in color
• Oxidative (aerobic) – generate more ATP through oxidative phosphorylation
o Low concentration of glycolysis enzymes
o Contains lots of mitochondria
o Used for endurance, low-impact exercise
o Darker in color
Types of skeletal muscle fibers and exercise
• High intensity exercise (anaerobic) (spring, weights)
o Increases glycolytic capacity
o Decreases oxidative capacity
o Fatigue is caused by lactic acid buildup
• Low intensity exercise (aerobic)
o Fatigue is caused by depletion of energy reserves (glycogen)
Smooth Muscle Anatomy
• No striations – no sarcomeres
• Actin and myosin run parallel to each other in several axes (contractions still occur through cross bridge)
• Actin and myosin connect to connective tissue at dense bodies
Smooth Muscle Contraction
• Contracts in several different directions (gets shorter and thinner)
• One difference between skeletal and smooth muscle contraction is the source of calcium
o Skeletal muscle – calcium comes from the SR (inside the cell)
o Smooth muscle – calcium comes from outside the cell
• Other difference is what calcium does once it is inside the cell
o The first thing calcium does is it binds to an enzyme called calmodulin
o Calmodulin activates another enzyme called MLCK (Myosin Light Chain Kinase)
o MLCK (similar to troponin in skeletal muscle) phosophorylates the myosin head and allows
cross-bridges to form, and the muscle to contract
o Troponin and MLCK both keep cross-bridges from forming when there is no calcium around
Shutting off smooth muscle contraction
• Inactivation of myosin by phosphates, which remove phosphate group from myosin light chain
Cardiac Physiology
Cardiovascular System
• Blood (fluid that carries materials to and from cells)
• Blood vessels (series of tubes)
• Heart (pump)
• Things move from high pressure to low pressure but when dealing with these chambers of the heart
(atria or ventricles), there’s only two ways that you can build pressure in a chamber. Add more blood,
or contract that chamber.
Blood
• Blood cells
o Erythrocytes (red blood cells)
▪ Carry oxygen bound to hemoglobin (contains iron)
o Leukocytes (white blood cells)
▪ Mediate immune responses
o Platelets (cell “fragments”)
Document Summary
Types of muscle: skeletal muscle (striated / voluntary, cardiac muscle (striated / involuntary, smooth muscle (smooth / involuntary) Increase in muscle cell calcium levels: troponin and trypomyosin conformational changes, crossbridge cycling sliding filaments (contraction) The twitch: smallest muscle contraction you can have, the mechanical response or change in force/tension (measured in grams) of an individual muscle fiber, motor unit, or whole muscle to a single action potential. Isotonic twitch muscle-generated force causes muscle shortening and lifts a load (load must be less than or equal to muscle tension) ex: picking up a book: if the object moves isotonic. Isometric twitch muscle generates force but does not shorten (load/force opposing muscle shortening greater than muscle tension) ex: pushing on a wall: if it doesn"t move isometric. Smooth muscle anatomy: no striations no sarcomeres, actin and myosin run parallel to each other in several axes (contractions still occur through cross bridge, actin and myosin connect to connective tissue at dense bodies.
