Human Morph ch 10

What are the types of muscle?

skeletal, cardiac, smooth

What are some functions of skeletal muscle?

- Move bones, make facial expressions, speak, breathe, swallow

- Stabilize joints, maintain body position

- Package internal organs and hold them in place

- Circular sphincters control passage of material

- Help maintain body temperature

Describe excitability in skeletal muscle.

ability to respond to a stimulus by changing electrical membrane potential

Describe conductivity in skeletal muscle.

involves sending an electrical change down the length of the cell membrane

Describe contractility in skeletal muscle.

exhibited when filaments slide past each other, enables muscle to cause movement

Describe extensibility in skeletal muscle.

ability to be stretched

Describe elasticity in skeletal muscle.

ability to return to original length following a lengthening or shortening

Each skeletal muscle is an ______.

organ

Muscle fibers bundled within a _______.

fascicle

A fascicle consists of many _______ _______.

muscle fibers

What are muscle fibers?

a muscle cell

What type of tissue is involved in skeletal muscle?

skeletal muscle fibers, connective tissue, nerve tissue, and blood vessels

Describe Epimysium of skeletal muscle.

Dense irregular connective tissue wrapping whole muscle

Describe Perimysium of skeletal muscle.

Dense irregular connective tissue wrapping fascicle that houses many blood vessels and nerves

Describe Endomysium of skeletal muscle.

Areolar connective tissue wrapping individual fiber, delicate layer for electrical insulation, capillary support, binding of neighboring cell

Describe tendon in skeletal muscle.

cordlike structure of dense regular connective tissue

Describe Aponeurosis in skeletal muscle.

thin, flattened sheet of dense irregular tissue

Describe deep fascia of skeletal muscle.

Dense irregular CT superficial to epimysium that separates individual muscles; binds muscles with similar functions

Describe superficial fascia of skeletal muscle.

Areolar and adipose CT superficial to deep fascia that separates muscles from skin

Is skeletal muscle vascularized?

yes

What is skeletal muscle innervated by?

somatic motor neurons

Describe the somatic motor neurons in skeletal muscle.

- cxons of neurons branch, terminate at neuromuscular junctions

- considered voluntary muscle, because contraction is voluntarily controlled

Describe Sarcoplasm (cytoplasm) of skeletal muscle.

Has typical organelles pluscontractile proteins and other specializations

Describe the nuclei of skeletal muscles.

- individual cells are multinucleated

- Cell is formed in embryo when multiple myoblasts fuse

- Some nearby myoblasts become undifferentiated satellite cells for support and repair of muscle fibers

Describe Sarcolemma (plasma membrane) of skeletal muscle.

- has voltage-gated ion channels that allow for conduction of electrical signals

- has T-tubules (transverse tubules) that extend deep into the cell

- contain voltage-sensitive calcium channels

Describe Myofibrils of skeletal muscle.

Bundles of myofilaments enclosed in sarcoplasmic reticulum

Describe Sarcoplasmic reticulum in skeletal muscle.

Internal membrane complex w/ Terminal cisternae, calcium pumps that import calcium, and calcium release channels

What are terminal cisternae in skeletal muscle?

close-ended sacs of sarcoplasmic reticulum that serve as reservoirs for calcium ions

• Two cisternae with T-tubule in between = triad

What are calcium release channels triggered by?

electrical signal traveling down T-tubule; calcium released into sarcoplasm

What are Myofilaments?

contractile proteins within myofibrils; two types: thick and thin

Describe thick filaments.

Consist of bundles of many myosin protein moleculs w/ myosin heads pointing toward ends of the filament

Describe thin filaments.

- Twisted strands of actin (each F-actin composed of G-actin monomers)

- G-actin has myosin binding site where myosin heads attach

- Tropomyosin and troponin are regulatory proteins

Myofilaments are arranged in repeating units called __________.

sarcomeres

What are sarcomeres?

Composed of overlapping thick and thin filaments, delineated at both ends by Z discs w/ positions of thin and thick filaments giving rise to alternating I- bands and A-bands

Describe function of Z discs.

Specialized proteins perpendicular to myofilaments that anchors for thin filaments

Describe I bands in sarcomeres.

• Light-appearing regions that contain only thin filaments that are bisected by Z disc and get smaller when muscle contracts (can disappear with maximal contraction)

Describe A bands in sarcomeres.

Dark-appearing region that contains thick filaments and

overlapping thin filaments, housing H zone and M line which makes up central region of sarcomere

Describe function of Connectin.

Extends from Z disc to M line, stabilizes thick filaments and has "springlike" properties

Describe Dystrophin.

Anchors some myofibrils to sarcolemma proteins

- Abnormalities of this protein cause muscular dystrophy

Why do Muscle fibers have abundant mitochondria?

for aerobic ATP production

What does Myoglobin within cells allow for?

allows storage of oxygen used for aerobic ATP production

Why is Glycogen stored in skeletal muscle?

for when fuel is needed quickly

What is the purpose of Creatinine phosphate in muscle tissue?

can quickly give up its phosphate group to help replenish ATP supply

Describe the Motor unit of skeletal muscle.

a motor neuron and all the muscle fibers it controls

What do axons of motor neurons from spinal cord (or brain) innervate?

numerous muscle fibers

Why do small motor units have less than five muscle fibers?

Allow for precise control of force output

Why do large motor units have thousands of muscle fibers?

Allow for production of large amount of force (but not precise control)

Fibers of a motor unit are dispersed _______ the muscle.

throughout

Describe the Neuromuscular junction (NMJ).

• Location where motor neuron innervates muscle

• Usually mid-region of muscle fiber

• Has the following parts: synaptic knob (axon terminal),

synaptic cleft, motor end plate

Describe the Axon terminal of NMJ.

• Expanded tip of the motor neuron axon

• Houses synaptic vesicles

• Has Ca2+ pumps in plasma membrane

• Has voltage-gated Ca2+ channels in

membrane

What is the purpose of synaptic vesicles in the axon terminal of NMJ?

Small sacs filled with neurotransmitter acetylcholine (ACh)

Describe the Motor end plate of NMJ.

• Specialized region of sarcolemma with numerous folds

• Has many ACh receptors, synaptic cleft, narrow fluid-filled space

• Synaptic cleft separates axon terminal from motor end plate

• Acetylcholinesterase resides in synaptic cleft

What is the purpose of ACh receptors in the motor end plate?

Plasma membrane protein channels opened by binding ACh and allows Na+ in and K+ out

What is Acetylcholinesterase?

Enzyme that breaks down ACh molecules

Describe the resting membrane potential (RMP) of muscle fibers.

• Fluid inside cell is negative compared to fluid outside cell

• RMP of muscle cell is about −90 mV

• RMP established by leak channels and Na+/K+ pumps

(voltage-gated channels are closed)

How does muscle contraction occur in skeletal muscle?

1. nerve signal travels down axon, opening ca2+ channels so ca2+ can diffuse into oxon terminal and bind to synaptic vesicles which merge w/ synaptic knob, prompting ACh to exocytosize to the synaptic cleft at the motor end plate, binidng w/ ACh receptors.

2. Na+ gates open so Na+ rushes in and K+ out, depolarizing the cell along the sarcolemma & T-tubule

3. Ca2+ channels open in SR terminal cisternae, releasing Ca2+ from the SR, interacting with myofilaments to facilitate contraction

4. Ca2+ binds to troponin, triggering cross-bridge cycling

   • myosin head attaches to exposed actin'

   • Power Stroke: myosin head pulls thin filaments towards center of sarcomere, releasing ADP & Pi

   • ATP binds to myosin head releasing actin

   • ATP splits back into ADP & Pi by myosin ATPase

5. Sarcomere shortens as Z discs move closer together, H band & I zone narrow, thick & thin filaments slide past each othe

Describe the Calcium entry at axon terminal.

Nerve signal travels down axon, opens voltage-gated Ca2+ channels where Ca2+ diffuses into axon terminal and binds to proteins on surface of synaptic vesicles

Describe Release of ACh from terminal.

vesicles merge with cell membrane at terminal via exocytosis, releasing thousands of ACh molecules from about 300 vesicles

Describe Binding of ACh at motor end plate.

ACh diffuses across cleft, binds to receptors, excites fiber

Describe Excitation-contraction coupling.

Stimulation of the fiber is coupled with the sliding of filaments for the end-plate potential (EPP), muscle action

potential, and release of Ca2+ from the sarcoplasmic reticulum

Describe function of End-plate potential (EPP).

ACh receptors are chemically gated channels that open when ACh binds to them

- Na+ diffuses into the cell through the channels (while a little K+ diffuses out), making the cell membrane less

negative at the end plate region

Does EPP lead to the opening of voltage-gated ion channels in the adjacent region of the sarcolemma?

no

Describe the initiation and propagation of action potential along the sarcolemma and T-tubules.

- EPP reaches threshold (AP) by causing nearby voltage-gated Na+ channels to open

- Na+ diffuses into the cell through voltage-gated channels -> cell depolarizes -> opening entry for Na2+

- voltage-gated K+ channels open -> K+ diffuses out -> cell repolarizes

- both = refractory period

What is an action potential (AP)?

a rapid rise (depolarization) and fall (repolarization) in the charge of the membrane

What is the voltage of the cell when Na2+ diffuses into the membrane?

becomes less negative, eventually becomes +30 mV

What is the voltage of the cell when K+ diffuses out the cell membrane?

returns to -90mV

What is refractory period?

a period of inactivity after a neuron has fired

What happens once AP travels down T-tubules?

• Causes voltage-sensitive calcium channels in T-tubule membrane to trigger the opening of calcium release

channels in SR terminal cisternae

• Release of Ca2+ from the sarcoplasmic reticulum

• Ca2+ interacts with myofilaments triggering contraction

What happens when Ca2+ binds to troponin in the sarcomere?

it triggers crossbridge cycling

• Troponin and tropomyosin move so actin is exposed

What are the four steps of Crossbridge cycling?

1) Crossbridge formation

2) Power stroke

3) Release of myosin head

4) Reset myosin head

Describe the first step of crossbridge cycling: Crossbridge formation.

Myosin head attaches to exposed binding site on actin

Describe the second step of crossbridge cycling: Power stroke.

Myosin head pulls thin filament toward center of sarcomere where ADP and Pi are released

Describe the third step of crossbridge cycling: Release of Myosin Head.

ATP binds to myosin head causing its release from actin

Describe the fourth step of crossbridge cycling: Reset of Myosin Head.

ATP split into ADP and Pi by myosin ATPase which provides energy to reset the myosin head

What is the result of cross-bridge cycling?

Results in sarcomere shortening as Z discs move closer together, narrowing (or disappearance) of H zone and I band but thick and thin filaments remain the same length but slide past each other

Describe muscle relaxation.

- termination of nerve signal and ACh release from motor neuron prompts hydrolysis of ACh by acetylcholinesterase, leading to closure of ACh receptor that causes cessation of end plate potential for no further action potential generation.

- closure of calcium channels in sarcoplasmic reticulum, prompts return of Ca2+ to sarcoplasmic reticulum by pumps, return of troponin to original shape, return of tropomyosin blockade of actin's myosin binding sites, and return of muscle to original position due to its elasticity

What is the effect of large amounts of ATP being used in Ca2+ pumps?

releases free phosphates that could possibly bind with

calcium

• To form hydroxyapatite -> calcification

• Calcium levels in sarcoplasm must be kept low to prevent this from happening

What happens to ATP after death?

becomes insufficient -> muscle relaxation cannot

occur -> muscles remain in contracted state due to this

• Rigor mortis

Describe myokinase role in producing more ATP in muscle cells.

Phosphate transferred from one ADP to another to make ATP

What are three additional ways to generate ATP in muscle cells?

• Creatine phosphate

• Glycolysis

• Aerobic cellular respiration

Describe role of creatine phosphate in skeletal muscle metabolism.

• Contains a high-energy bond between creatine and phosphate

• Phosphate can be transferred to ADP to form ATP

• Catalyzed by creatine kinase

• 10-15 seconds of additional energy

Describe the role of glycolysis in skeletal muscle metabolism.

• Does not require oxygen

• Glucose (from muscle's glycogen or through blood) is

converted to two pyruvate molecules

• 2 ATP released per glucose molecule

• Occurs in cytosol

Describe the role of Aerobic cellular respiration in skeletal muscle metabolism.

• Requires oxygen

• Pyruvate oxidized to carbon dioxide

• Energy used to generate ATP by oxidative phosphorylation

• Pyruvate, fatty acids, and amino acids can also be used as fuel to produce ATP

• More ATP from triglycerides with longer fatty acid chains

• Essential for longer, moderate levels of activity

Describe lactate formation in skeletal muscle metabolism.

• Lactate formation from pyruvate occurs under conditions of low oxygen availability

• Pyruvate converted to lactate by lactate dehydrogenase

• Lactate can be used as fuel by skeletal muscle fiber or enter blood and taken up by cardiac muscle or liver

• Lactic acid cycle

Describe lactic acid cycle.

cycling of lactate to liver where it's converted to glucose, and transport of glucose back to muscle

What is oxygen debt?

Amount of additional oxygen needed after exercise to

restore pre-exercise conditions:

• Replace oxygen on hemoglobin and myoglobin

• Replenish glycogen

• Replenish ATP and creatine phosphate

• Convert lactic acid back to glucose

Describe muscle tone.

• Resting tension in a muscle, generated by involuntary nervous stimulation of muscle

• Do not generate enough tension for movement

• Decreases during deep sleep

How are skeletal muscle fibers classified?

1. Type of contraction generated

2. Means for supplying ATP

Describe power in terms of contraction.

related to diameter of muscle fiber

Describe speed in terms of contraction.

related to type of myosin ATPase, quickness of action potential propagation, and quickness of Ca2+ release and

reuptake by sarcoplasmic reticulum

Describe fast-twitch fibers in terms of contraction.

more powerful and have quicker and briefer contractions than slow twitch fibers

How do Oxidative fibers (fatigue-resistant) supply ATP?

use aerobic cellular respiration via extensive capillaries, many mitochondria, and a large supply of myoglobin

• Red fibers

How do Glycolytic fibers (fatigable) supply ATP?

use anaerobic cellular respiration via fewer capillaries, fewer mitochondria, small supply of myoglobin, and large

glycogen reserves

• White fibers

What are the three types of skeletal fibers?

1. slow oxidative

2. fast oxidative glycolytic

3. fast glycolytic

Describe slow oxidative skeletal fiber.

• Contractions are slower and less powerful w/ high endurance since ATP supplied aerobically

• About half the diameter of other fibers, red in color due to myoglobin

Describe fast oxidative glycolytic skeletal fiber.

• Contractions are fast and powerful w/ primarily aerobic respiration, but delivery of oxygen lower

• Intermediate size, light red in color

Describe fast glycolytic skeletal fibers.

anaerobic contractions are fast, powerful, but brief as ATP production is primarily anaerobic

• Largest size, white in color due to lack of

myoglobin

What kind of skeletal muscle fibers do hand muscles have?

fast glycolytic fibers for quickness

What kind of skeletal muscle fibers do back muscles have?

low oxidative fibers to continually maintain postural support

What kind of skeletal muscles do long-distance runners have?

Higher proportion of slow-oxidative fibers in legs