Chapter 6: The Muscular System

Chapter 6.1 - 6.5

What are Muscles?

Responsible for body movements, stabilizing joints, and generating heat

How do Muscles Work?

Generate the force required by contracting, a process in which proteins inside the muscle fibers overlap more than when they are at rest.

Why do we need Muscles?

Needed to move substances in our body like air, food and blood.

What unique characteristic sets muscles apart from other body tissues? (Essential Function)

Contract, or Shorten.

3 Types of Muscle Tissue

Skeletal, Smooth and Cardiac

Skeletal Muscle Location

Attached to bones, or for some facial muscles, to skin.

Cardiac Muscle Location

Walls of the heart

Smooth Muscle Location

Walls of hollow organs (other than the heart)

What Muscle Types are only considered as “Muscle Fibers”?

Skeletal & Smooth

Skeletal Cell Shape

Single, very long, cylindrical, multinucleate cells with very obvious striations

Cardiac Cell Shape

Branching chains of cells; uninucleate, striations; intercalated discs

Smooth Cell Shape

Single, fusiform, uninucleate; no striations

Skeletal Tissue components

Epimysium, perimysium, and endomysium

Endomysium (Responsiveness)

Thin Connective Tissue surrounding each muscle cell

Epimysium (‘Overcoat” of Connective Tissue)

Sheath of fibrous connective tissue surrounding a muscle

Fascicle

Bundle of nerve or muscle fibers

What unique characteristic sets muscles apart from other body tissues? (Essential Function)

Contract, or Shorten.

Cardiac Tissue components

Endomysium attached to the fibrous skeleton of the heart

Smooth Tissue components

Attached to bones, or for some facial muscles, to skin.

Skeletal Regulation of Contraction

Voluntary; via nervous system controls

Cardiac Regulation of Contraction

Involuntary; internal heart pacemaker; nervous system controls; hormones

Smooth Regulation of Contraction

Involuntary; nervous system controls; hormones, chemicals, stretch

Skeletal, Cardiac, and Smooth Speed of Contraction

1. Slow to Fast

2. Slow

3. Very Slow

Skeletal, Cardiac, and Smooth Rhythmic Contraction

1. No

2. Yes

3. Yes, in some

T or F: Skeletal and Smooth Muscle Cells are ELONGATED

T

Skeletal Muscle Fibers

Packaged into organs

Skeletal Cell Shape

Single, Very long, Cylindrical, Multinucleate cells with very obvious striations

Skeletal muscle Location

They are:

1. Large (of the Muscle Fibers)

2. Cigar-Shaped

3. Multinucleate Cells

Cardiac muscle Location

Walls of the heart

Tendon (“cord-like”)

A cord of dense fibrous tissue attaching a muscle to a bone.

Aponeurosis (“sheet-like”)

Indirectly attaches the muscle to bone, cartilage, or another connective tissue covering.

What are tendons made out of?

Collagen Fibers

How are cardiac muscles joined together?

Special Gap Junctions (Intercalated Discs)

3 Important Roles in the Body

1. Maintain posture and body position

2. Stabilizes joints

3. Generates Heat

Visceral Organs

Soft Internal organs of the body

Microscopic Anatomy of Skeletal Muscle

Muscle - Bundle of muscle fiber - Muscle Fiber - Myofibrils - Sacromere

Sarcolemma (“Muscle Hunk”)

Many oval nuclei can be seen just beneath the plasma membrane

Sarcomere | Muscle Cell

• Contractile unit of muscle fiber

• Structural and functional unit of skeletal muscle

Myofibrils

A complex organelle or a Muscle Fiber

Myofilaments

Produce banding (striped pattern)

What do Banding Patterns reveal?

The working structure of MYOFIBRILS

I Band

• Has a Darker Area (Z-Disc)

• Has a Midline Interruption

• Contains only thin filaments

A Band

• Has a lighter area (H zone)

• Entire length of thick filaments

What line is in the middle of the H-Zone?

M Line

• Contains tiny protein rods that hold adjacent thick filaments with together

Thin Filament (Actin)

• Actin

• Actin-containing Thin Filaments

Actin

• Contractile Protein

• Regulatory proteins prevent binding of myosin heads to actin Anchored to the Z discs

Actin-containing Thin Filaments

Slide toward each other during contraction

Sarcoplasmic Reticulum

• Interconnecting tubules and sacs surround every myofibril

What does the Sarcoplasmic Reticulum (SR) do?

Stores CALCIUM and RELEASE it on demand when a muscle fiber is stimulated to contract

4 Special Function Properties of Skeletal Muscle

• Irritability / Responsiveness

• Contractibility

• Extensibility

• Elasticity

Irritability / Responsiveness

Ability to receive and respond to stimulus

Contractibility

Ability to forcibly shorten when an adequate stimulus is received

Extensibility

Ability to be stretched

Elasticity

Ability to recoil and resume resting length after stretching

What do Nerve Impulses do?

Stimulate muscle fibers to contract

Motor Unit

Consist of one neuron and all the skeletal muscle fibers it stimulates

Axon

Long, threadlike extension of a neuron

Axon Terminals

Branches that form junctions with the sarcolemma of a different muscle fiber

Neuromuscular Junctions

Contain synaptic vesicles filled with neurotransmitter

Acetylcholin (ACh)

Specific neurotransmitters that stimulates skeletal muscle fibers

Synaptic Cleft

Nerve endings and muscle fiber never touch; the gap is filled with interstitial fluids

Thick Filament (Myosin)

Bundled molecules of myosin; extend the entire length of the dark A band

Midpart

- Smooth ends are studded with small projections

- Containing ATPase enzyme activity and split ATP to release the “energy” used for muscle contraction

1st event at the neuromuscular junction

Nerve impulse reaches axon terminal of motor neuron

2nd event at the neuromuscular junction

In response to a nerve impulse, calcium (Ca2+) channels open and calcium enters the axon terminal

3rd event at the neuromuscular junction

Calcium entry causes some synaptic vesicles to release their contents (the neurotransmitter acetylcholine) by exocytosis

4th event at the neuromuscular junction

Acetylcholine diffuses across the synaptic cleft and binds to the receptors in the sarcolemma

5th event at the neuromuscular junction

ACh binds and opens channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.

More Na+ ions leave, producing a local change in the electrical conditions of the membrane (depolarization) which leads to an Action Potential

6th event at the neuromuscular junction

The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending stimulation of the muscle fiber

7th event at the neuromuscular junction

The action potential triggers the sarcoplasmic reticulum to release Calcium

2 Events that return the cell to its resting state

1. Diffusion of potassium ions out of the cell

2. Sodium-potassium pump

Formation of Cross Bridges

Myosin heads attach to actin which requires calcium ions (Ca2+) and ATP (to “energize” the myosin heads)

Action Potential

Pass deep into the muscle fiber along membranous tubules that fold inward from the sarcolemma.

Excitation-contraction coupling

• Action potential stimulates the sarcoplasmic reticulum to release calcium ions into the cytoplasm

• Calcium ions trigger the binding of myosin to actin = filament sliding

“All-or-none” Law

• a muscle fiber will contract to its fullest extent when it is stimulated adequately; it never partially contracts

Graded Responses

Whole muscle reacts to stimuli

Different degrees of shortening = different amount of force

Graded muscle contractions production

• Changing the frequency of muscle stimulation

• Changing the number of muscle fibers being stimulated at one time

Muscle Twitches

Result from certain nervous system problems; not the normal way  of operation

Unfused (incomplete) Tetanus

• Nerve impulses are delivered to the muscle at a rapid rate

• The effects of the successive contractions are “summed” (added) together

  The muscle contractions get stronger and smoother.

Fused (complete) Tetanus

Muscle is stimulated so rapidly that no evidence of relaxation is seen, and contractions are completely smooth and sustained

Tetanus

The primary role is to produce smooth and prolonged muscle contractions.

3 Metabolic Pathways that regenerate ATP

1. Direct Phosphorylation of ADP by Creatine Phosphate

2. Anaerobic Glycolysis

3. Aerobic Respiration

Direct Phosphorylation of ADP by Creatine Phosphate

Fastest

Anaerobic Pathway (Glycolysis)

Aerobic Pathway

Slowest

2 Types of Head Muscles

1. Facial

2. Chewing

Facial Muscles

Insert into soft tissues like other muscles or skin

Chewing Muscles

Break down food for the body

What are the 8 Facial Muscles?

1. Frontalis

2. Obicularis Oculi

3. Zygomaticus

4. Buccinator

5. Orbicularis Oris

6. Temporalis

7. Occipitalis

8. Masseter

Frontalis

Raises eyebrows

Orbicularis Oculi

Blinks and closes eye

Zygomaticus “Smiling Muscle”

Raises corner of mouth

Buccinator

Compress cheek, hold food between teeth during chewing

Orbicularis Oris “Kissing Muscle”

Closes and protrudes the lips

Temporalis “Time Muscle”

Closes Jaw

Occipitalis

Pulls the scalp posteriorly

Masseter

Closes Jaw

What are the 2 Neck Muscles?

1. Platysma

2. Sternocleidomastoid

What do Neck Muscles do?

They move the head and shoulder girdle and are small and strap-like

Platysma

Tenses skin of the neck (as in shaving)