Muscular System

Anatomy and Physiology| Tutorial Notes

Muscular System

Learning objectives

After study of this chapter the student should be able to:

1.Describe the etymology of the term “muscle”

2.List the functions of muscles

3.Compare and contrast smooth muscle, cardiac muscle, and skeletal muscle tissue

4.Distinguish between the structures, functions, and locations of multi-unit smooth muscle and visceral smooth muscle

5.Describe the structure of a skeletal muscle attached to bone

6.Name the components of a muscle fiber (cell) and describe the function of each part

7.Name the proteins that make up a sarcomere and indicate the function of each protein

8.Describe the neural control of skeletal muscle contraction.

9.Describe the major events that occur during excitation-contraction coupling.

10.Describe the major events of cross-bridge cycling.

11.Describe the major events that cause muscle relaxation.

12.Explain why rigor mortis occurs after death.

13.List the energy sources for skeletal muscle contraction and explain when each energy source is used.

14.Describe oxygen debt

15.Define muscle cramp and muscle fatigue

16.Name the three parts of a muscle twitch

16.Distinguish between a twitch, summation, and a sustained contraction

17.Explain how “recruitment” increases the strength of a skeletal muscle contraction

18.Distinguish between isotonic and isometric contractions

19.Distinguish between fast and slow twitch muscle fibers

tutorial outline

I.Etymology

A.muscle – from “Mus musculus” for the house mouse.

Contracting muscles appear as if mice are running underneath the skin.

II.Functions of Muscles

A.Movement – muscles attach to bone moving the skeleton

B.Posture –muscles pull on bones keeping the skeleton upright

C.Heat – Heat is generated as a byproduct of cell respiration. Muscle tissue is a major source of body heat.

D.Communication – facial muscles allow us to speak and give us facial expressions

E.Heart Beat – The walls of the heart are composed of cardiac muscle

F.Digestion – Muscles move the mandible. Smooth muscles lining the digestive tract propel food through the body.

G.Breathing – We breathe by contracting our diaphragm along with intercostal muscles (between ribs)

II.Muscle Histology

A.Smooth Muscle

1. Characteristics

  • Tapered Cells with a single central nucleus
  • Non-striated
  • Involuntary Control

2.Types of Smooth Muscle

a.Multi-unit Smooth Muscle

  • Cells contract individually
  • Located in the walls of blood vessels and iris of the eye

b.Visceral Smooth Muscle

  • Cells are connected by gap junctions
  • Cells contract together
  • Lines visceral organs: stomach, intestines, uterus, urinary bladder, etc.

B.Cardiac Muscles

1.Characteristics

  • Branching and Non-branching cells with a single central nucleus
  • Striated
  • Involuntary Control
  • Cells are connected by intercalated discs

2.Location – only in the wall of the heart

C.Skeletal Muscle

1.Characteristics

  • Thread-like cells with several peripheral nuclei
  • Striated
  • Voluntary Control

2.Location – attached to bones or other connective tissue

III.Parts of a skeletal muscle

A.Attachments

1.Periosteum – dense CT covering the bone. Gives rise to the…

2.Tendon – dense CT attaching muscle to bone. Near the muscle the tendon becomes…

3.Fascia – dense CT surrounding muscle.

  • Subcutaneous Fascia – beneath the skin
  • Deep Fascia – covers the muscle
  • Subserous Fascia – lines serous membranes

4.Aponeurosis – broad flat sheet of CT attaching muscle other muscles or to bone.

B.Organization of the Muscle

1.Muscle – bundle of fascicles

2.Fascicle – bundle of muscle fibers (cells)

3.Muscle Fiber – bundle of myofibrils

4.Myofibril – bundle of Myofilaments

5.Myofilaments – filament proteins, 2 types

  • Thick Filaments = myosin
  • Thin Filaments = actin

C.CT Coverings of the Muscle

1.Epimysium – surrounds the skeletal muscle, just beneath the deep fascia

2.Perimysium – surrounds each fascicle. Divides the muscle into several compartments.

3.Endomysium – surrounds each muscle fiber.

*Epimysium & Perimysium serve as passageways for blood vessels and nerves. Endomysiumcontains capillaries that nourish muscle fibers.

IV.Components of a muscle fiber (myocyte)

A.Sarcolemma – cell membrane

B.Sarcoplasm – cytoplasm

C.t-tubules (transverse) – channels that extend from the sarcolemma deep into the sarcoplasm

D.Sarcoplasmic Reticulum – Modified ER that stores Calcium

E.Cisternae – enlarged ends of the SR, adjacent to t-tubules

F.Myofibril – bundles of myofilaments organized into sarcomeres

V.Components of a Sarcomere

A.Z-lines (z-discs)

  • Attachment sites for actin filaments.
  • Forms the edges of the sarcomere.

B.actin (thin) filaments

  • Chain of globular proteins attached to the z-lines
  • Actin proteins have myosin binding sites, where myosin heads attach during contraction.

C.myosin (thick) filaments

  • Overlaps actin filaments
  • Myosin heads attach to actin filaments during a muscle contraction
  • During contraction, myosin alternates between 2 conformations:
  • Cocked position ______/
  • Sprung position ______\
  • Power Stroke – myosin heads move from the “cocked” to the “sprung” position go through a power stroke
  • Recovery stroke – myosin heads move back from the “sprung” position to the “cocked” position.
  • Energy is required to pull back the myosin head. Energy comes from the breakdown of ATP on the myosin heads.
  • ATPase - enzyme on myosin heads that breakdown ATP → ADP + P. Energy released is used for the recovery stroke.

D.Tropomyosin

  • Filament protein that covers the actin filament when the muscle is at rest
  • Muscle contraction begins when Troponin repositions the tropomyosin filaments, exposing myosin binding sites on the actin filaments

E.Troponin

  • Contains Calcium Binding Sites
  • When Calcium binds to troponin, troponin repositions tropomyosin filaments initiating a muscle contraction.

VI.Striations

A.I-bands

  • Contains only actin filaments
  • Light bands
  • Z-lines form the center of A-bands

B.A-bands

  • Contains myosin filaments + overlapping actin filaments
  • Dark bands

C.During muscle contraction I-bands become narrow while A-bands remain the same width.

VII.Skeletal Muscle Contraction

A.Motor Neurons – initiate muscle contraction

1.Upper Motor Neuron – extends from brain to spinal cord

2.Lower Motor Neuron – extends from spinal cord to the muscle

3.Synapse – functional unit between neurons

  • Neurons communicate by releasing chemicals called neurotransmitters via exocytosis at the synaptic knob.
  • Acetylcholine – neurotransmitter that initiates muscle contractions.

B.Motor Unit – motor neuron + all of the muscle fibers it innervates

  1. Small motor units
  2. Have few muscle fibers per motor neuron.
  3. Allows fine control, such as muscles that move the eye.
  4. Large motor units
  5. Have hundreds or thousands of muscle fibers per motor neuron
  6. Examples include large muscles, like those on the back.

C.Neuromuscular Junction (NMJ) - synapse between a motor neuron and a muscle fiber.

1.Synaptic Knob of motor neuron

2.Synaptic Cleft = gap between cells

3.Motor End Plate – specialized region of muscle fiber at the NMJ

a.Contains Ach receptors.

b.ACh receptor – is a sodium ion channels that opens whenever ACh binds to it.

VIII.Excitation-Contraction Coupling:

Excitation

A.Decide to move.

B.Action Potential (electrical current) along axon of motor neuron

C.Calcium diffuses into synaptic knob.

D.Exocytosis of ACh from synaptic knob into synaptic cleft.

E.ACh binds to ACh receptors on motor end plate.

F.Sodium diffuses into muscle fiber, initiating a muscle impulse (electrical current)

G.Muscle impulse travels across sarcolemma, down t-tubules, and into Cisternae of the SR.

H.Sarcoplasmic Reticula (SR) release Calcium into sarcoplasm

I.Calcium binds to troponin. Troponin repositions tropomyosin, revealing myosin binding sites on actin filaments.

Contraction = Cross-Bridge Cycling

J.Cross-Bridge Formation - Myosin heads binds to actin filaments, forming a cross-bridge

  • Phosphate is released from myosin

K.Power Stroke - Myosin head springs forward through a power stroke, pulling the z-lines closer together.

  • ADP is released from myosin.

L.Cross-Bridge Release – As a new molecule of ATP binds to myosin filament, myosin heads are released from the actin filaments.

M.Recovery Stroke – ATP is broken down into ADP + P. The energy is used to pull the myosin head back to the “cocked” position.

N.Steps J-M are repeated until Calcium is removed from the sarcoplasm:

Cross-Bridge Formation, Power Stroke, Cross-Bridge Release, Recovery Stroke

IX.Sliding Filament Theory of Contraction – actin and myosin filaments do not shorten during contraction. Instead, they slide across one another.

X. Rigor mortis – sustained contraction after death

A.ATP supplies are quickly used up after death.

B.Calcium leaks out of SR into the Sarcoplasm initiating cross-bridge formation.

C.Cross-Bridges cannot be released because there’s no ATP available.

D.Myosin remains attached to actin until the proteins themselves begin to break down

(about 72 hours)

XI.Muscle Relaxation

A.Calcium pumps on the SR return calcium from the sarcoplasm to the sarcoplasmic reticulum. Without Calcium, troponin positions tropomyosin back into place covering acting filaments.

B.Acetylcholinesterase – enzyme that breaks down Acetylcholine in the synapse.

XI.Energy for Muscle Contractions – energy sources are used to replenish ATP supplies

A.Creatine Phosphate

1.CreatinePhosphate is broken down into Creatine + Phosphate.

2.The phosphate is transferred to ADP, quickly replenishing ATP supplies.

3.Creatine Phosphate provides energy for about 10 seconds of intense activity.

4.Whenever ATP supplies are plentiful, ATP is used to reattach phosphate to creatine, replenishing Creatine Phosphate.

B.Anaerobic Respiration (Glycolysis)

1.Does not require oxygen

2.Occurs in the cytosol of the cell.

3.Glycolysis produces 2 ATP (net gain) per glucose molecule.

4.Glycolysis is used for moderate to intense activity.

5.During strenuous exercise, pyruvic acid produced during glycolysis is converted into lactic acid.

  • As lactic acid accumulates further cell respiration becomes more difficult to carry out.
  • When oxygen supplies are available again, lactic acid is converted back into glucose in the liver.
  • Oxygen Debt – amount of oxygen required to convert lactic acid back into glucose after strenuous activity.

C.Aerobic Respiration

1.Requires oxygen

2.Occurs within mitochondria (citric acid cycle and electron transport chain)

  • Electron Transport Chain produces 32-34 ATP per glucose.

3.Aerobic Respiration requires a good blood supply (to meet high oxygen demands) and plenty of mitochondria.

  • Hemoglobin – transports oxygen in red blood cells
  • Myoglobin – stores oxygen in muscle cells
  • Increases amount of oxygen available to muscle cells
  • Imparts the reddish-brown color of muscles.

4.Cells use aerobic respiration while at rest or during light activity. With more intense activity, oxygen demands increase and most ATP is produce via glycolysis.

D.Impacts of exercise on Cell Respiration

1. High intensity exercise - promotes an increased production of glycolytic enzymes, increasing glycolytic capacity.

2.Aerobic Respiration – increases the muscle’s capacity for aerobic respiration.

  • Increased blood flow
  • Increased mitochondria

XII.Muscle Fatigue

A.Muscle fatigue is the inability for a muscle to contract during prolonged exercise

B.Psychological factors – loss of desire to complete the exercise

C.Physiological Factors – might be due to the accumulation of lactic acid. Research is ongoing.

XIII.Muscle Cramp

A.A cramp is an involuntary, sustained, painful muscle contraction.

B.Cause of a cramp is unclear, but it might be due to an electrolyte imbalance in the extracellular fluid.

XIV.Muscle Responses

A.Threshold Stimulus

1.Minimum stimulus required to initiate a muscle contraction.

2.Subthreshold stimulus – any stimulus less than threshold will not initiate a response.

B.Recording a Muscle Contraction

1.Twitch – single contractile response to a stimulus

a.Latent Period – slight delay after stimulus is applied before muscle contracts.

  • About 2 milliseconds in humans

b.Period of Contraction

c.Period of Relaxation

2.Summation –higher frequency stimuli increases force of contraction.

3.Tetanic Contraction – a higher frequency of stimulations can lead to a forceful, sustained contraction with no sign of relaxation.

  • Complete tetany does not occur in the body, but can be observed in the lab.

XV.Recruitment

A.All-or-None Response - Muscle fibers exhibit an all-or-none response. They either contract fully and completely or not at all.

B.Muscles generate more force by recruiting motor units.

C.During a forceful contraction, smaller motor units are stimulated first, then additional motor units are recruited to increase the force of contraction.

D.When all motor units have been recruited a muscle is fully contracted.

XVI.Muscle Tone – continuous state of partial contraction

XVII.Types of muscle contractions

A.Isotonic contraction – muscle changes length, but tension remains the same.

1.Concentric – muscle shortens

2.Eccentric – muscles lengthen

B.Isometric contraction – muscle changes tension, but the length remains the same.

  • Especially important for keeping skeleton upright, maintaining posture.

XVIII.Fast and Slow Twitch Fibers

A.Slow Twitch Fibers (Type I)

1.Oxidative

2.Red Fibers – due to abundance of myoglobin

3.Well supplied with blood vessels to transport oxygen.

4.Fatigue resistant – muscles function a long time without fatiguing.

B.Fast Twitch Fibers (Type IIb)

1.Glycolytic

2.White fibers – due to less myoglobin

3.Relatively poor blood supply.

4.Contacts faster than slow twitch fibers

5.susceptible to fatigue.

C.Fast Twitch Fatigue Resistant (Type IIa)

1.Intermediate

2.Fast twitch speed with intermediate oxidative capacity

STUDY QUESTIONS

1.A muscle fiber is composed of bundles of…

a. fasciclesb. myofibrils

c. myofilamentsd. muscles

2.The functional unit of skeletal muscle is called…

a. fascicleb. muscle fiber

c. sarcomered. sarcolemma

3.The portion of a muscle fiber that makes up the neuromuscular junction is called the…

a. motor end plateb. myofibril

c. synaptic knobd. sarcolemma

4.A muscle ______refers to a single contractile response to a stimulus.

a. impulseb. twitch

c. tetanyd. summation

5.All of the following are sources of ATP for muscle contractions, except…

a. creatine phosphateb. glycolysis

c. aerobic respirationd. sarcoplasmic reticulum

6.Muscle cramps are likely due to…

a. accumulation of lactic acidb. electrolyte imbalance

c. overstretched muscled. accumulation of acetylcholine in the synapse

7.Which molecule is required to release myosin heads from actin filaments?

a. ATPb. ADP

c. Phosphated. Creatine Phosphate

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