Chapter 11.1-11.4 and 11.7: Muscular Tissue

Question 1

What are the universal characteristics of muscle?

Answer 1

• excitability
  
  • to chemical signals / electrical changes across a plasma membrane
• conductivity
  
  • local electrical change triggers a wave of excitation that travels along the muscle fiber
• contractility
  
  • shortens when stimulated
• extensibility
  
  • capable of being stretched between contractions
• elasticity
  
  • returns to its original rest length after being stretched

Question 2

What are the connective tissue wrappings around skeletal muscle?

Answer 2

• Endomysium
  
  • around muscle cell
• Perimysium
  
  • around muscle fasicle
• Epimysium
  
  • around entire muscle
• Tendons
  
  • attachments between muscle and bone matrix

Question 3

What are the names for the plasma membrane and cytoplasm of a muscle cell and what are their components?

Answer 3

• Sarcolemma: plasma membrane
• Sarcoplasm: cytoplasm
  
  • myofibrils: protein cords
  • glycogen: stores glucose
  • myoglobin: stores some oxygen
  • multinucleate
  • mitochonrdia

Question 4

What are(other) distinct, physical components of a muscle cell?

Answer 4

• Sacroplasmic Reticulum (SR): smooth ER
  
  • terminal cisternae: dilated end sacs
  • stores calcium
• T tubules
  
  • tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side
• Triad
  
  • a T tubule and two terminal cisternae

Question 5

What are the 3 types of proteins that make up sarcomeres?

Answer 5

• contractile proteins
• regulatory proteins
• structural proteins

Question 6

What are the types of myofilaments?

Answer 6

• Thick filaments
  
  • made of several hundred myosin molecules
• Thin filaments (actin)
  
  • contain active site that binds to the head of myosin
• Regulatory proteins (turn contraction on and off)
  
  • tropomyosin: block active sites
  • troponin: small protein on each tropomyosin molecule

Question 7

What are the given types of structural proteins?

Answer 7

• Elastic filaments
  
  • titin (huge, springy protein)
    
    • helps stabilize and position thick filament
    • prevent overstretching and provide recoil
• Dystrophin
  
  • links outermost actin to membrane proteins which link to endomysium
  • transfers forces of muscle contraction to connective tissue
  • genetic defects produce muscular dystrophy
    
    • used for diagnosis

Question 8

What are striations?

Answer 8

• result from precise organization of myosin and actin in cardiac and skeletal muscles
• they are alternating A bands and I bands
• A band
  
  • anisotropic
  • H band: middle of A, thick filaments only
  • M line: middle of H band
• I band
  
  • isotropic
  • Z disc: provides anchorage for thin filaments and elastic filaments
    
    • bisects I band

Question 9

What is sliding filament theory?

Answer 9

• Thin filaments slide past the thick filaments, increasing amount of overlap
• ATP needed
  
  • energizes myosin heads for power stroke
• Calcium needed
  
  • uncovers attachment sites on actin

Question 10

What is the nerve/muscle relationship?

Answer 10

• skeletal muscle doesn’t contract unless stimulated by a nerve
• if nerve connections are severed or poisoned, muscle is paralyzed
• Denervation atrophy: shrinkage of paralyzed muscle when nerve remains disconnected

Question 11

What are the motor neurons and motor units and their basic characteristics?

Answer 11

• Somatic motor neurons
  
  • cell bodies in brain stem and spinal cord
  • axon lead to skeletal muscle
  • one motor neuron branches out to a number of muscle fibers
• Motor unit
  
  • one nerve fiber and all the muscles innervated by it
  • average unit contains 200 muscle fibers
  • small units
    
    • fine degree of control
    • 3-6 fibers per neuron
    • eye and hand muscles
  • Large units
    
    • more strength than control
    • powerful contractions with hundreds of fibers
    • e.g. gastrocnemius (1000 muscle fibers per neuron)

Question 12

What do the muscle fibers of one motor neuron do?

Answer 12

• they are dispered throughout muscle
• contract in unison
• produce weak contraction over wide area
• able to sustain long term contraction
  
  • motor units take turns contracting
    
    • partial contraction creates firmness (muscle tension)
• effective contraction usually requires contraction of several motor units at once

Question 13

What are the main components of the neuromuscular junction?

Answer 13

• Synaptic knob
  
  • swollen end of nerve fiber
  • contains synaptic vesicles with acetylcholine
• Synaptic cleft
  
  • gap between synaptic knob and sarcolemma
• Schwann cell
  
  • envelopes and isolates Neuromuscular junction

Question 14

What is the function of the neuromuscular junction?

Answer 14

• nerve impulse from axon opens calcium channels
• Ca+2 enters and causes synaptic vesicles to undergo exocytosis, releasing ACh into synaptic cleft
  
  • Muscle cell has millions of ACh receptors
• Acetylcholine Esterase breaks down ACh, leading to relaxation

Question 15

Define the following: voltage potential and resting membrane potential

Answer 15

• Voltage potential (electrical potential)
  
  • a difference in electrical charge from one point to another
• Resting membrane potential
  
  • About -90Mv in skeletal muscle cells
    
    • maintained by sodium/potassium pump
• muscle fibers and neurons are excitable: their membranes exhibit voltage changes in response to stimuli

Question 16

Describe the state of an unstimulated (resting) cell.

Answer 16

• more anions on the inside of the membrane than the outside
• make the inside of plasma membrane negatively charged
  
  • plasma membrane is electrically polarized with negative Resting Membrane Potential (RMP)
• There are excess sodium ions in the ECF and excess potassium ions in the ICF

Question 17

Describe the state of a stimulated (active) muscle fiber.

Answer 17

• Sodium ion gates open into plasma membrane
• Sodium flows into the cell down its gradient
  
  • These cations override negative charges in the ICF
• Depolarization: inside of membrane becomes positive
• Immediately, Sodium gates closed and Potassium gates open
• Potassium rushes out of cell
  
  • concentration gradient and positive sodium charge
• Repolarization–membrane is negative again
• Overall (quick up and down) shift is Action Potential

Question 18

What is the difference between Resting Membrane Potential and Action Potential?

Answer 18

• RMP
  
  • seen in a waiting excitable cell
• Action Potenital
  
  • a quick event in a stimulated, excitable cell
  • self sustains down the length of a cell membrane
  • action potential at one point causes another to happen immediately in front of it, which triggers another (and so on)
    
    • this wave of excitation is called an “impulse”

Question 19

What are the 4 major phases of contraction and relaxation?

Answer 19

• Excitation
  
  • process in which nerve action potentials lead to muscle action potentials
• Excitation-contraction coupling
  
  • events that link action potentials on the sarcolemma to the activation of myofilaments therby preparing them to contract
• Contraction
  
  • step in which the muscle fiber develops tension and may shorten
• Relaxation
  
  • when stimulation ends, a muscle fiber relaxes and returns to its resting length

Question 20

What occurs during the excitation phase of contraction?

Answer 20

• nerve signals open up voltage gated calcium channels in the synaptic knob
• Calcium enters the knob and stimulates the release of ACh from synaptic vesicles onto synaptic celft
• ACh diffuses across the cleft
• two ACh molcules bind to each receptor and open its channels
• Sodium enters, depolarization of membrane potential
  
  • -70 mV to +90 mV
• Then Potassium leaves and potential hyperpolarizes
  
  • -90mV (hyperpolarization)
  • the quick voltage shift is called “end plate potential”
• Voltage gate in end plate region opens nearby voltage gate channels producing an action potential that spreads over muscle surface

Question 21

What occurs during excitation/contraction coupling?

Answer 21

• Action potential spreads down the t-tubules
• Opens voltage-gated ion channels in t tubules and calcium channels in SR
• Calcium leaves the SR and enters cytosol
• Calcium binds to troponin in thin filaments
• Troponin-tropomyosin complex changes shape and exposes active sites on actin

Question 22

What occurs during the contraction phase?

Answer 22

• ATPase in Myosin head hydrolizes as an ATP molecule
• Activates the head (“cocking it”)
  
  • ADP and P1 remain attached
• Head binds to actin active site forming a myosin-actin cross bridge
• Myosin releases ADP and P and flexes pulling thin filament over it (powerstroke)
• Upon binding more ATP, myosin releases actin
  
  • process can be repeated
  • recovery stroke recocks head
  • Each stroke utilizes one molecule of ATP

Question 23

What occurs during the relaxation phase?

Answer 23

• Nerve stimulation and ACh release stop
• ACHhE breaks down ACh and fragments are reabsorbed into knob
• Stimulation by ACh stops
• Calcium pumped back into the SR by active transport
• Tropomyosin reblocks the active sites of actin
• Muscle fiber ceases to produce or maintain tension
• Muscle fiber returns to its resting length
  
  • recoil of elastic components
  • contraction of antagonistic muscles

Question 24

Define the following: length/tension relationship and muscle tone (and tension)

Answer 24

• the length/tension relationship means the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated
  
  • ^{<em>If overly shortened before stimulated, a weak contraction results, as thick filaments just butt against Z discs </em>}
  • ^{<em>If too stretched before stimulated, a weak contraction results as minimal overlap between thick and thin filaments results in minimal cross bridge formation</em>}
  • ^{<strong>Optimum resting length</strong> produces greatest force when muscle contracts}
• Muscle tone is partial contraction to ensure resting muscles are near this optimal length (via nervous system)
• Tension is force exerted by muscle as it contracts

Question 25

What is rigor mortis?

Answer 25

• It is the hardening of muscles and stiffening of body beginning 3 to 4 hours after death
  
  • ^{deteriorating SR releases calcium}
  • ^{deteriorating sarcolemma lets calcium enter cytosol}
  • ^{calcium activates myosin acting cross bridge}
  • ^{<strong>muscle contracts but cannot relax</strong>}
    
    • ^{relaxation requires ATP (body cannot make any)}
    • ^{fibers remain contracted until myofilaments decay}
• Peaks abut 12 hours after death, then diminishes in next 48 to 60 hours

Question 26

What cells make up cardiac and smooth muscle? Where do they receive innervation from?

Answer 26

• myocytes
• autonomic nervous system
  
  • (NOT somatic motor neurons)

Question 27

What is the tissue makeup of cardiac tissue?

Answer 27

• Striated, short, thick cells
• SR is less developed so must use Ca+2 from ECF
• Cardiocytes joined by intercalated discs
  
  • Gap junctions and desmoses
• Damaged cardiac muscle cells repair by fibrosis
• Can contract without nervous stimulation
  
  • autorhythmic/autonomic
  • NS increases/decreases heart rate
• slow contractions (to expel blood)
• Almost all aerobic respiration

Question 28

What is the tissue makeup of smooth muscle?

Answer 28

• No striations
• Some lack nerve supply
• Others use autonomic fibers with varicosities including synaptic vesicles
• Myocytes connected by gap junctions
• Capable of mitosis and hyperplasia
• Injured smooth muscle regenerates well
• Smooth muscle is slower than skeletal and cardiac
• Form the walls of hollow organs
• Acquires calcium from ECF