Muscular System

Chapter 8

What are functions of the muscular system?

1. Gross body movement2. Stabilizing body positions –

“Tone”3. Generate heartbeat4. Movement substances within

the body5. Regulating organ volumes6. Producing body heat

Properties of Muscle Tissue

1. Electrical Excitability – able to respond to stimuli and send an impulse

2. Contractility – ability to contract forcefully, muscle body shortens

3. Extensibility – ability to stretch 4. Elasticity – ability to return to original

length or shape after contraction5. Rejuvenation – moderate to poor

What are the 3 types of muscle tissue?

SkeletalSmoothCardiac

Skeletal MuscleCharacteristics:

* Multinucleated

* Striated

* Voluntary control

Appearance:

Smooth Muscle

Uninucleated cells

Involuntary

No striations

•Location: Internal organs, blood vessels

•“Peristalsis” movement of digestive tract

•Usually organized into two sheet-like layers: longitudinal and circular

•Two types: Visceral and Multiunit

Cardiac• Striated and uni-

nucleated • Branching cells• Intercalated discs

separate cells• Rhythmicity• Only found in wall

of heart• Self-exciting

tissue• Large transverse

tubules

Aponeurosis, Fascia and Tendons• Fascia – “gift

wrap” around muscle body or found under skin (spider webs)

• Tendons -connect muscle to bone

• Aponeurosis – flat/sheet-like tendon connects muscle to CT

Skeletal Muscle Anatomy

The Muscle Fiber (cell) Structure

• Sarcolemma• Sarcoplasm• Nucleus(i)• Sarcoplasmic

reticulum• Mitochondria• Myofibrils

1. Actin (thin)

2. Myosin (thick)

Skeletal Muscle Fiber Structure

Anatomy of a Myofibril

Sarcomere – smallest contracting unit

The Sarcomere!Sarcomere: Single contracting unit within a myofibril.Striation pattern is caused by the organization of the

myofilaments• Length = Z line to Z line• I band = only actin region• A band = myosin and actin region• H zone = only myosin region• M line = center of H zone

Contraction: Z-line moves towards M line

Muscle BodyFascicles

Muscle Fibers

MyofibrilsMyofilaments

Largest

to

Smallest

Now that you are familiar with muscle anatomy, what

is the physiology of contraction?

How does muscle contraction happen?

Section 8.3

Structures Involved in Contraction

Motor Unit:One neuron and all the muscle fibers it connects with.

Stronger contractions have more motor units activated.

The Neuromuscular Junction: where the neuron meets with the muscle

fiber.

Now, look at how this process really works…

Video: “Sarcomere shortening”

The Sliding Filament Theory

Muscle Contraction1. Impulse reaches synaptic

bulb.2. Calcium channels open and

calcium enters.3. Ca causes vesicles to

migrate to cell membrane to release neurotransmitter (Acetylcholine = Ach).

4. Vesicles fuse with membrane and release Ach into synaptic cleft (gap).

5. Ach diffuses across the cleft.6. Ach binds with receptor

proteins on motor end plate.7. Motor end plate depolarizes.8. Action potential is generated

in muscle cell.

Video

Animation

9. Action Potential moves:

• along the sarcolemma

•down the transverse tubules

•into the sarcoplasmic reticulum

10. Calcium is released from the sarcoplasmic reticulum.

Binding site before and after calcium reaches myofilaments.

Troponin/tropomyosin complex pulled away by calcium so myosin can bind!

VIDEO

11.

12.

13.

VIDEO

Relaxation Phase…1. Acetylcholine esterase decomposes Ach and

removes it from synaptic gap.

2. ATP binds to myosin causing linkage to release from actin.

3. Actin and myosin filaments slide apart.

4. Calcium ions are actively transported back into the sarcoplasmic reticulum.

5. Muscle fiber relaxes.

So what about Rigor Mortis?

Section 8.4

How does the muscle fiber get ATP for the contraction?

• Cell Storage • Creatine phosphate

• Cellular respiration or fermentation

Creatine Phosphate• Used when current cell

supply of ATP runs out. (within 6 sec.)

• Cannot be used directly as an energy source.

• Purpose: to transfer stored energy to ADP to make ATP.

• Creatine supply is exhausted in 30 sec.

• Activities that benefit: 100 meter dash and weight lifting

Aerobic Respiration

• With oxygen available, glucose is converted into the max amount of ATP!

Myoglobin: Oxygen-binding pigment found only in muscle cells.

Anaerobic Method of Acquiring ATP

Occurs by fermentationFermentation produces Lactic AcidLactic acid is carried to Liver to be converted back to glucoseLow intensity or moderate exercise can recycle the lactic acid immediately. (not a lot is produced)Can be a good energy source for “stop and go” sports. (soccer, tennis, short-term swimming)

***What if you cannot recycle the lactic acid

quickly enough?

Oxygen Debt

• The amount of O2 liver cells require

to convert the lactic acid into glucose + the O2 muscle cells require to restore stored ATP and creatine phosphate.

• Training can speed up the process.

O2 Muscle need + O2 Liver need = Oxygen Debt

For most activities, all systems are used. It’s just the percentages differ based on the intensity of

the activity.

Activity% Creatine Phosphate

% Lactic Acid

% Aerobic Respiration

Sprint 95 3 2

Mile 15 55 30

Marathon 5 5 90

Muscle Fiber Types

Slow Twitch• Not easily fatigued• Myoglobin – “red”• Use in low intensity

activities – Endurance! • Not a lot of growth – Lean• Lots of mitochondria

Fast Twitch• Easily fatigued• Little myoglobin – “white”• Used in high intensity

activities – Anaerobic Power!

• Increase filaments with use – Hypertrophy!

• Can be irreversibly changed into slow twitch!

Genetically, most people are 50% slow, 50% fast twitch

Slow Fast

What if the muscle cannot contract anymore?

Muscle Fatigue

• Loss of the ability to contract• Causes:

1. Lactic acid buildup, pH drops 2. Blood supply interruption3. Lack of acetylcholine4. Lack of ATP

• Cramps: sudden, involuntary contractions or spasms in one or more of your muscles. Possible causes: lack of ATP, dehydration or electrolyte imbalance

Muscular Responses• Threshold stimulus – minimum

amount of stimulus (Ach) needed to cause a contraction.

• All-or-None Response – a muscle fiber will always fully contract no matter the strength of the stimulus. If the minimum amount of stimulus is not met, no contraction will occur.

Muscle contractions can be measured and recorded by:

The Myogram

Twitch: A single contraction of a muscle fiber.

Myogram Recording

(Latent)

a) Shows a series of twitches with complete relaxation before the next stimulus.

b) Summationanother stimulus comes before complete relaxation can occur, thus increasing the contraction force.

c) Tetanus: Stimulus is held constant, where there is no relaxation.

Recruitment• Stimulating more motor units in a muscle body to

contract.• A greater force applied to the muscle or increase

the frequency of stimuli will cause this.

Muscle Tone• Skeletal muscle fibers constantly being

stimulated by brain. (involuntary)• Stretch receptors in muscles monitor muscle

health.• Loss of posture tone = collapse

How Do I Hypertrophy My Muscles?

1. Do high intensity activities.

2. Use the fast-twitch muscles.

3. Eat protein for building material.

4. Maintain activity or else: atrophy

By age 80, there is a 50% decrease in muscle strength!