Chapter 10: Muscular Tissue Flashcards
Three types of muscular tissue
- skeletal
- cardiac
- smooth (visceral)
Functions of muscular tissue
- producing body movements
- stabilizing body positions
- storing and mobilizing substance within the body
- generating heat ex; “shivering”
Properties of muscular tissue
- electrical excitability (when its “on” muscle shorter, when its “off” it relaxes and lengthens
- contractility (when you contract biceps; triceps relax and vice versa)
- extensibility
- elasticity
How are muscles formed?
fusion of myoblasts into skeletal muscle
Muscle cell nucleus and length
- muscles cells are very long and therefore are multinucleated.
Myofibrils
- protein filaments that run parallel along the length of the muscle cell
Muscle connects to
bones by tendons
Muscle fascicle
muscle cells are arranged in bundles called muscle fascicle
Endomysium
layer of connective tissue that wraps around the sarcolemma (membrane that surrounds muscle cells)
Perimysium
surrounds each muscle fascicle
Epymysium
a whole muscle is surrounded by epimysium
The more muscle cells a person has…
the more strength the person has (myofibrils in specific)
Function of T-tubules
they fold inward to allow membrane proteins to get close to the sarcoplasmic reticulum
Glycogen granules in muscle cells
- muscle cells carry their own energy all the time
Myoglobin
protein found only in muscle cells and caries oxygen
Dystrophin
- accessory protein
- functions to anchor the length of the cells
Muscular hypertrophy
- enlargement of EXISTING muscle fibers
- due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles
Muscular atrophy
decrease in size of muscle fibers due to loss of myofibrils
- occurs as a result of aging or disuse
What causes striations in muscle cells
- there are regions in skeletal fibers where it looks darker and lighter through microscope because i band has only thin filaments whereas A band has thick filaments
Z discs
Narrow, plate-shaped regions of dense material that separate one sarcomere from the next
A band
dark, muddle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments
I (i) band
lighter, less dense area of sarcomere that contains remainder of thin filaments but no thick filaments.
- a Z disc passes through the center of each i band
H band
narrow region in center of each A band that contains thick filaments but no thin filaments
M line
region in center of H band that contains proteins that hold thick filaments together at the center of the sarcomere
As a sarcomere shortens…
the zone of overlap increases and i band disappears
Thick filaments
myosin
Thin filaments
actin
- connected to Z discs and myosin
Contraction simple explained
myosin pulls actin filaments therefore, pulling z discs causing sarcomere and entire myofibril to shorten (contract)
What are contractile proteins
myosin and actin
Myosin descrption
contractile protein that makes up thick filament; molecule consists of a tail and two myosin heads, which bind to myosin-binding sites on actin molecules of thin filament during muscle contraction
Actin description
Contractile protein that is the main component of thin filament; each actin molecule has a myosin-binding site where myosin head of thick filament binds during muscle contraction
Regulatory proteins in myofibrils
- Tropomyosin
- Troponin
Tropomyosin description
regulatory protein that is a component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin
Troponin description
Regulatory protein that is a component of thin filament; when calcium ions (Ca+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin
Structural proteins of myfibrils
- Titin
- a (alpha) actinin
- Myomesin
- Nebulin
- Dystrophin
Titin (structural protein)
structural protein that connects Z discs to M line of sarcomere, thereby helping to stabilize thick filament position; can stretch and then spring back unharmed, and thus accounts for much of the elasticity and extensibility of myofibrils
(holds sarcomere structure)
a Actinin (alpha)
Structural protein of Z discs that attaches to actin molecules of thin filaments and to titin molecules
Dystrophin
- links edge of sarcomere to sarcolemma
- muscular dystrophy lacks dystrophin
The contraction cycle; step 0
- before you contract a sarcomere you must release calcium to each myofibril
The contraction cycle; step 1
- myosin head hydrolyzes ATP and becomes energized and oriented
- myosin head winds back (changes orientation) using ATP hydrolysis
The contraction cycle; step 2
- myosin head binds to actin, forming a cross bridge (connection between myosin + actin)
- phosphate leaves and myosin head goes back and pulls actin back with it
The contraction cycle: step 3
- myosin head pivots, pulling the thin filament past the thick filament toward center of the sarcomere (power stroke)
- ADP leaves
The contraction cycle; step 4
- as myosin head binds ATP, the cross bridge detaches from actin
and the cycle repeats
When the contraction cycle repeats the myosin grabs..
a new position making the sarcomere shorter until it can’t contract anymore
ATP hydrolysis?
when ATP turns into ADP
What happens when there is no ATP you cannot release the cross bridge
the muscle stays contracted (occurs when a person dies)
- after 24 hours the body starts to relax because the proteins get broken down.
Rigor mortis (what happens)
state of rigidity in muscles that occurs after 3-4 hours after death
- calcium leaks out of sarcoplasmic reticulum
- myosin heads to bind to actin forming cross-bridge
- cross bridge can’t detach since ATP synthesis has ceased
When does rigor mortis disappear
disappears after 24 hours as proteolytic enzymes digest the cross-bridge
Length-tension relationship
- the force of a muscle contraction depends on the length of the sarcomeres in a muscle prior to contraction
What does neuromuscular junction or neuromuscular synapse produce
produces a muscle action potential
Steps of NMJ or NMS
- voltage-gated calcium channels in a neurons synaptic end bulb open
- Ach binds to Ach receptor
- Muscle depolarization and calcium releases from the sarcoplasmic reticulum
- Ach gets broken down by acetylcholinesterase
Step 1 in NMJ or NMS
1: voltage-gated calcium channels in a neuron’s synaptic end bulb open
- resulting in calcium influx
- this causes exocytosis of the neurotransmitter acetylcholine (Ach) into synaptic cleft
What does acetylcholine do
turns on muscle cells
Step 2 in NMJ or NMS
Ach binds to Ach receptor on the motor endplate, which causes an influx of NA+ into muscle (action potential)
- sodium rushes from outside of muscle cell into the cell. Eventually you reach action potential
Step 3 in NMJ or NMS
The muscle is now depolarized and it results in Ca to be released from the sarcoplasmic reticulum
Step 4 in NMJ or NMS
Ach gets broken down by acetylcholinesterase
- should be able to turn off the cell and not be contracted all the time
What does acetylcholinesterase do
breaks down Ach and it prevents prolonged muscle contraction
Motor neuron to skeletal muscle cell ratio
1 motor neuron can connect to several skeletal muscle cells
Only contraction uses..
ATP not relaxing because relaxing is passive
3 ways that muscles produce ATP
- Creatine phosphate (least)
- Anaerobic glycolysis (middle)
- Aerobic respiration (most)
Creatine phosphate explained
- happens during periods of rest
- Creatine kinase catalyzes the transfer of a phosphate group from creatine phosphate to ADP to rapidly yield ATP
- myosin is always hydrolyzing ATP
Anaerobic glycolysis explained
- when creatine phosphate stores are depleted, glucose is converted into pyruvic acid to generate ATP
- glycolysis produces 2 ATP by pushing pyruvate away from mitochondria through the lactate dehydrogenase reaction
Aerobic respiration explained
- under aerobic conditions, pyruvic acid can enter the mitochondria and undergo a series of oxygen-requiring reactions to generate large amounts of ATP
- 4 molecules of ATP are produced per glucose
Central fatigue
- occurs due to changes in the central nervous system (brain) and generally results in cessation of exercise
- protective mechanism to prevent the overuse of muscles
Muscle fatigue
- inability to maintain force of contraction after prolonged activity
Reason for onset of muscle fatigue
- inadequate release of Ca+ from the sarcoplasmic reticulum
- depletion of creatine phosphate, oxygen and nutrients
- build up of lactic acid and ADP
- insufficient release of acetylcholine at the neuromuscular junction
Why do you continue to breathe heavily for a period of time after stopping exercise
to allow your body to recover
The extra oxygen when breathing heavily after exercise goes toward
- replenishing creatine phosphate stores
- converting lactate into pyruvic acid (pyruvic acid is used for ATP production via aerobic respiration)
- reloading O2 onto myoglobin
Other reasons the body needs extra oxygen after exercise
- increased rate of chemical reactions that is accompanied by elevated body temperature
- increased consumption of oxygen from the heart and respiratory muscle tissues
- tissue repair processes are occurring at an increased pace.
The strength of a muscle contraction depends on
how many motor units are activated
What does a motor unit consist of
consists of a somatic motor neuron and the muscle fiber it innervates
- activating only few motor units will generally result in a weak contraction, activating many will generally result in a strong muscle contraction
Wave summation
occurs when a second action potential triggers muscle contractions before the first contraction has finished and the skeletal muscle fiber has relaxed
- the second stimulus occurs after the refractory period but before the skeletal muscle has relaxed
Unfused tetanus
when the muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate
Fused tetanus
no relaxation period between muscle contractions
Motor unit recruitment
- weakest are recruited first, followed by stronger motor units
- motor units contract alternately to sustain contractions for longer periods of time (they do not all relax at the same time)
Muscle tone; even when at rest, a skeletal muscle exhibits
a small amount of taughtness or tension, called muscle tone
What is muscle tone established by
neurons in the brain and spinal cord that excite the muscle’s motor neurons
What happens when a motor neuron thats serving skeletal muscle gets damaged or cut
the muscle becomes flaccid
Isotonic contractions
tension is constant while muscle length changes
- concentric: muscle length shortens
- eccentric: muscle length lengthens
Isometric contractions
- does not change length of the muscle
- tension generated is not enough to exceed the resistance so the muscle length does not change
(generates just enough to keep balance of objects)
Intercalated discs function in cardiac muscle
- contain desmosomes that hold the cardiac muscle fibers together
- contain gap junctions that allow muscle action potentials to spread from one cardiac muscle fiber to another
Cardiac muscle cells have more…
mitochondria and their contractions last 10-15 times longer than skeletal muscle contractions
What type of ATP does cardiac muscle use
uses aerobic respiration
Somatic motor neurons connect to
skeletal muscle to provide voluntary movements
Autonomic motor neurons connect to
smooth muscle and allows involuntary movements
Smooth muscle contractions
start more slowly and last longer than skeletal and cardiac muscle contractions
Smooth muscle can shorten and stretch to a greater extent than
skeletal and cardiac muscle
