Muscle Tissue

Question 1

Characteristics of ALL muscles

Answer 1

1. Contractility: ability to contract (shorten) and relax to produce movement
2. Extensibility: ability to extend, or stretch, to allow muscles to return to their resting length
3. Excitability (irritability): ability to be stimulated and respond to regulatory signals from nerves, hormones & local stimuli
4. All muscles are also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal

Question 2

Functions of skeletal muscles

Answer 2

1. Posture: Continuous partial contraction of some skeletal muscles lead to sitting, standing and staying still
2. Heat production: Catabolic process which produces body heat and maintains homeostasis
3. Movement: pulls on bones (other muscles) to move the body as a whole or its parts
4. Protection: covers internal organs, supports weight of organs, keeps joints and bones from being over stressed

Question 3

Types of Muscle Tissue

Answer 3

Skeletal muscle, Cardiac muscle and smooth muscle

Question 4

Structure, function and location of Skeletal muscle

Answer 4

Structure (anatomy): multinucleated, Regular arrangement of actin and myosin fibers into bands of light and dark (striated)

Function (physiology): move the skeleton, especially the limbs

Location: usually connected to bones or fascia

Question 5

Structure, function and location of Cardiac muscle

Answer 5

Structure (anatomy): 1-2 nuclei, branching cells with intercalated disks organized as a syncytium to allow for coordinated contraction

Function (physiology): pump blood through the circulatory system

Location: Heart

Question 6

Structure, function and location of Smooth muscle

Answer 6

Structure (anatomy): 1 nucleus, no regular arrangement of actin and myosin proteins in cytoplasm (non-striated/smooth)

Function (physiology): Goosebumps, moves food through digestive tract, blood through circulatory system

Location: parts of viscera (organs, ducts) throughout the body

Question 7

Characteristics of cardiac muscle

Answer 7

1. Highly coordinated contractions of cardiac muscle pump blood into blood vessels of the circulatory system. Pacemaker cells control rate of cardiac contractions
2. Similarity with skeletal muscle: striated, organized into sarcomeres
3. Differences with skeletal muscle: only 1-2 nuclei, multiple mitochondria and myoglobin, extensively branched fibers cells, intercalated discs.
4. Intercalated discs consist of sarcolemma with gap junctions & desmosomes which allow heart to work as a pump by coordinating cardiac contraction
5. Gap junctions: channels between adjacent cells that allow ions to flow from one cell to another quickly. Depolarization spreads quickly between cells to allow for coordinated contraction of entire heart. This electric coupling creates a syncytium (functional unit of contraction).
6. Desmosomes anchor the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting

Question 8

Smooth Muscle

Answer 8

1. Similarity with skeletal muscle: actin & myosin contractile proteins, thick & thin filaments.
2. Differences with skeletal muscle: 1 nucleus, spindle-shaped , no striations, sarcomere, troponin, tropomyosin
3. Thin filaments are anchored by dense bodies (similar to Z-discs) attached to sarcolemma.
4. Ca++ enters sarcoplasm from SR and ECF and binds to regulatory protein calmodulin

Question 9

Structure of a skeletal muscle

Answer 9

Three layers of connective tissue enclose a muscle to provides structure while compartmentalizing fibers within it:
Epimysium, Perimysium and Endomysium

Question 10

Epimysium (top level)

Answer 10

sheath of dense, irregular connective tissue around each muscle organ
allows a muscle to contract/move while maintaining structural integrity
separates muscle from other regional tissues/organs in the area, allowing independently movement

Question 11

Perimysium (middle level)

Answer 11

middle layer of connective tissue

allows nervous system to trigger a specific movement of a muscle by activating fascicle

Question 12

Endomysium (bottom level)

Answer 12

thin layer of collagen and reticular fibers around each muscle fiber
organizes muscle fibers into fascicle (individual bundles)
contains extracellular fluid and nutrients supplied by blood

Question 13

Skeletal muscle fibers

Answer 13

Skeletal muscle cells are also called muscle fibers as they are long and cylindrical
During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers (myofibrils) with multiple copies of genes to allow bulk production of proteins and enzymes for muscle contraction.

Question 14

Skeletal Muscle Fibers (Cells)

Answer 14

Sarcolemma, Sarcoplasm, Sarcoplasmic reticulum (SR), Sarcomere

Question 15

Sarcolemma:

Answer 15

plasma membrane of muscle fibers

Question 16

Sarcoplasm:

Answer 16

cytoplasm of muscle fibers

Question 17

Sarcoplasmic reticulum (SR):

Answer 17

pecialized smooth endoplasmic reticulum: stores, releases, and retrieves calcium ions (Ca++)

Question 18

Sarcomere:

Answer 18

functional unit of skeletal muscle fibre: highly organized arrangement of contractile protein (actin, myosin myofilaments) and regulatory proteins (troponin, tropomyosin)

Question 19

The Sarcomere (functional unit of skeletal muscles)

Answer 19

1. The sarcomere is the functional unit of skeletal muscles: 3D cylinders with striations (bands of light and dark due to arrangement of actin and myosin myofilaments)
2. Each myofibril can contain 100-1000s sarcomeres connected end to end
3. All sarcomeres within a myofibril contracts (and relaxes) simultaneously, contracting (and relaxing) the entire myofibril & muscle cell

Question 20

Thin filament

Answer 20

Starts from Z-discs and projects partway to the center consists of thinner actin strands and its troponin-tropomyosin complex

Question 21

Thick filament

Answer 21

Starts from the center and projects partway to the Z-discs

consists of thicker strands and their multiple heads

Question 22

Z-discs (Z-lines)

Answer 22

Forms the boundary of sarcomeres at both ends

Anchored to actin myofilaments

Question 23

Myofilaments

Answer 23

Each myofibril contains 1000s of thick and thin myofilaments

Four different kinds of protein molecules make up myofilaments

Question 24

Protein molecules

Answer 24

1. Actin (thin filaments): contains active sites (myosin binding sites) which bind to myosin heads
2. Myosin (thick filament): Contains myosin heads that are chemically attracted to actin and forms cross bridges with actin
   Tropomyosin (regulatory protein): at rest, it blocks the myosin
3. binding sites on actin molecules when
   Troponin (regulatory protein): at rest, it holds tropomyosin in place, can bind to calcium (Ca2+) ions

Question 25

The Neuromuscular Junction

Answer 25

1. Location: site where nerve ending meets the muscle fiber
2. All living cells have membrane potentials (electrical gradients across their membranes): -60 to -90 mV
3. When the membrane potential becomes LESS negative, depolarization occurs and an action potential can start
4. Membrane potentials change when ions either enter or leave the cell through ion channels which can open and close depending on the stimuli. This change generates electrical signals (action potential) which travel quickly over long distances. An action potential in a nerve at the NMJ releases a neurotransmitter which leads to the start of an action potential in the muscle. This action potential in the muscle causes muscle contraction (Excitation-contraction coupling)
5. Every skeletal muscle fiber is innervated by a motor neuron at the NMJ
   A signal from the motor neuron can cause the contraction of skeletal muscle fibers
   Each motor neuron can innervate from 10s to 1000s skeletal muscle fibers

Question 26

Process of Contraction of a Skeletal muscle

Answer 26

1. Action potential (AP) reaches the end of the motor neuron. Neurotransmitter (acetyl choline or ACh) is released into the NMJ
2. ACh binds to specific receptors on ligand gated ion channels for sodium on the skeletal muscle fiber
3. Sodium channels open: sodium enters sarcoplasm of muscle fiber. Membrane potential of muscle fiber changes
4. AP starts along the sarcolemma of muscle fiber: AP travels into the interior of the cell via T-tubules (extensions of the sarcolemma). AP starts along the sarcolemma of muscle fiber: AP travels into the interior of the skeletal muscle cell via T-tubules (extensions of the sarcolemma)
5. Action potential depolarizes the cell membrane. Voltage-gated Ca++ channels in SR. Ca++ diffuses out of SR into sarcoplasm.
6. Ca++ binds to troponin on thin filament. Troponin-tropomyosin complex moves to expose myosin-binding sites
7. Myosin binds actin at its myosin-binding site to form cross-bridge. Adenosine diphosphate (ADP) and inorganic phosphate (Pi) generated in the previous contraction cycle are released
8. Myosin head pivots toward M-line at center of the sarcomere- power stroke
9. New ATP attaches to the myosin head
10. Cross-bridge is detached. ATPase in myosin head hydrolyzes ATP to ADP and Pi, releasing energy. Angle of myosin head moves into a cocked position (re-cock), ready to form another crossbridge with next myosin -binding site

Question 27

Sliding Filament Model of Contraction

Answer 27

Overview: contraction of skeletal muscle fiber contracts as the thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres

Requires Ca++ and ATP
Ca++ initiates contraction by exposing actin-binding site to form myosin crossbridges
ATP sustains contraction: Each cycle in cross-bridge cycling requires energy provided by hydrolysis of ATP
Without ATP, the myosin head remains attached to actin: rigor mortis
Myosin is in a high-energy configuration when myosin head is cocked: this energy is used during the power stroke

Question 28

Muscle contraction usually stops when

Answer 28

Nerve signal stops,

Muscle runs out of ATP and becomes fatigued

Question 29

Relaxation of a muscle fiber process

Answer 29

1. Nerve signal stops
2. Release of ACh stops
3. Ligand gated Na+ channels close
4. Sarcolemma and T-tubules repolarizes
5. Voltage-gated Ca++ channels in the SR close
6. Ca++ ions are pumped back into SR using ATP
7. Tropomyosin moves to cover myosin-binding sites
8. Thick and thin filament interaction relaxes

Question 30

Sources of ATP in skeletal muscles:

Answer 30

Skeletal muscle only has a small amount of ATP stored

In order to sustain contraction, ATP must be replaced quickly

Question 31

Creatine phosphate:

Answer 31

Excess ATP transfers energy by producing ADP and creatine phosphate. When energy is needed, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. Can only provide 15 seconds worth of energy

Question 32

Glycolysis

Answer 32

anaerobic breakdown of glucose to produce ATP, at a slower rate than creatinine phosphate. Provides 1 minute burst of energy

Question 33

Aerobic respiration

Answer 33

anaerobic breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. More efficient, produces 95% of ATP

Question 34

Motor Units

Answer 34

group of muscle fibers innervated by a single motor neuron
Small motor units can innervate less than 10 muscle fibers and permit very fine motor control of the muscle, eg eyeball movements. have smaller, lower-threshold motor neurons that are more excitable
Larger motor units can supply 1000s of muscle fibers in a muscle are concerned with simple, or “gross,” movements, eg thigh muscles. bigger, higher-threshold motor neurons

Question 35

Recruitment process

Answer 35

where smaller motor units tend to be recruited first before larger ones, increasing the muscle contraction. Recruitment of more motor units will increase the strength of muscle contraction: allows for variation in picking up a feather vs a heavy weight.