Muscle

Physiology

3 Types of Muscles

1. skeletal- longest muscle cell,

striated, voluntary, elongated,

multi-nucleated

2. cardiac- heart muscle, striated,

involuntary, intercalated discs

3. smooth- non-striated, involuntary,

visceral muscle, slow and

sustained contractions, single

nucleus, less connective tissue

than skeletal muscle

3 Types of Muscles

Skeletal

Muscle

Smooth

Muscle

Cardiac

Muscle

Muscle Functions

• produces movement

• maintains posture

• stabilizes joints

• generates heat

Muscle Characteristics

• excitability- ability to receive and respond to a stimulus

• contractility- ability to shorten- special to muscle tissue

• extensibility- ability to stretch or lengthen beyond resting length

• elasticity- ability to recoil and resume its resting length

Organization of Skeletal Muscle

• muscle fibers are covered with

the endomysium and bundled into

groups or fasicles covered by

perimysium;

• fasicles are bundled and covered

by epimysium; the connective

tissue coverings are continuous

with the tendons connected to the

bones

Organization of Skeletal Muscle

Attachment

• insertion- the movable bone

• origin- immovable or less movable

bone (usually proximal)

• direct attachment- muscle

attaches directly to the bone

• indirect attachment- muscle

attaches through a tendon

Attachment

Muscle Cell Anatomy • sarcolemma- plasma membrane

• sarcoplasm- cytoplasm

• myoglobin- red pigment, stores oxygen in

muscles

• myofibrils- contractile elements of skeletal

muscles

• sarcomere- myofibril region between 2

successive Z discs

• A band- thick myofilaments – myosin

• I band- thin myofilaments – actin

• M line- center of the H zone

• Z disc- anchors the thin filaments and

connects the myofibrils

• H zone- area within the A band where thin and

thick filaments do not overlap

The Sarcomere

Myosin- thick filaments

– 2 interwoven protein chains

– 2 globular heads terminate the

chain AKA: cross bridges, myosin

heads

– bundles of myosin form a central

smooth section and its ends are

studded with myosin heads

– actin and ATP binding sites are

present

Actin- long, thin filaments

– helical structure with active

binding sites for myosin heads

– contains tropomyosin- a protein

to help stiffen the actin core; in

relaxed muscles they block

myosin binding sites

– contains troponin- 3 protein

complex

1. binds to actin

2. binds to tropomyosin and

helps position it

3. binds to Ca+2

Elastic Filaments

–composed of titin

–holds the thick filament in

place

–helps the muscle stretch and

recoil

Muscle Contraction

• Contraction is stimulated by the muscles

single motor nerve innervation which forms a

neuromuscular junction.

• Neuromuscular junction- axonal end of the

motor neuron and muscle fiber which are in

close contact, yet separated by a gel like

space, the synaptic cleft.

• Synaptic vesicles- membrane sacs filled with

acetylcholine (ACh)

• Motor end plate- highly folded area of

sarcolemma with ACh receptors

Neuromuscular Junction

Excitation-Contraction Coupling

• occurs during the latent period, the

time from action potential to

mechanical activity

• action potential continues down the

sarcolemma

• calcium ions are released and binds to

troponin, changing shape and removes

the blocking action of tropomyosin

• myosin cross bridges become

activated

– Sliding Filament Theory Occurs (To Come)

• calcium level falls and tropomyosin

blocks active sites

Muscle Contraction

Sliding Filament Theory of Contraction-

• Activated myosin heads are attracted

to exposed actin binding sites & cross

bridge attachment occurs

• The myosin head changes shape and

pulls on the actin sliding it to the

center of the sarcomere (ATP is used)

• A new ATP molecule binds to the

myosin head. The cross bridge

detaches from the actin.

• ATP causes the myosin head to change

shape and it reattaches to the actin to

repeat the process.

Sliding Filament Theory

View the Sliding Filament

Muscle Contraction

Stopping A Muscle Contraction

1. Nerve communication to contract

ends.

2. Lack of ATP will stop a contraction.

3. Low calcium ion concentration will

slow or stop contraction.

- calcium channel blockers – treat high blood

pressure – calcium is not released so

heart cannot beat as quickly

2 Categories of Contractions

1. isotonic contractions- muscles

change length, decreases the

angle at the joint

– concentric contractions- muscle

shortens

– eccentric contractions- muscle

contracts as it lengthens

2. isometric contractions- muscle

does not shorten or lengthen yet

tension continues to increase

Categories of Contractions

Classification of Muscle Fibers

• Speed of contraction- how fast or slow

they contract based upon how fast ATP

is split

• The major pathway for ATP production

– oxidative fibers- aerobic pathways

– glycolytic fibers- anaerobic path

(glycolysis)

• 3 types of skeletal muscle cells: slow

oxidative fibers, fast oxidative fibers,

fast glycolytic fibers

Myograms Drawings or tracings of the electrical current as it

travels through a muscle.

Twitch

Contraction-

one muscle cell

contracting,

one time

Myograms

4 Phases of a Twitch Contraction

1. Latent Phase

-AP has started

-Ca+2 is being released from sarcoplasmic

reticulum -Ca+2 is binding to troponin

-tropomyosin is exposing myosin binding

sites

2. Contraction Phase

-AP continues

-Na+1 moves into the cell -Muscle cell becomes depolarized

Myograms 4 Phases of a Twitch Contraction

3. Refractory Period

-cell has no membrane potential

-cell cannot respond to any neural stimulus

4. Relaxation Phase

-cell is repolarizing by pumping sodium out of

the cell

-will regain membrane potential

Strength of Contraction

Two ways to control the strength of a muscle contraction

1. By the size and number of motor units we recruit

2. Frequency of stimulation

3. Amount of Calcium ion released

4. Hydration

5. pH of sarcoplasm

6. Temperature of the muscle

7. Amount of stretch before contraction

Muscle Fatigue

• weakness and loss of contractility

Causes

• ATP synthesis declines

• ATP shortage slows Na+/K+ pump

• Motor nerves use their supply of Ach – junctional fatigue

• Lactic acid lowers pH of sarcoplasm

• K+ builds in extracellular fluid

Muscle Strength

Strength of muscle depends upon

• Muscle size

• Fasicle arrangement

• Size of active motor units

• Multiple motor unit summation

• Temporal summation

• Fatigue

• Length-tension relationships

Energy Requirements

Energy is provided by ATP & /CP

Night→Produce Extra ATP→ATP breakdown occurs to form ADP and Pi→Creatine Kinase attaches the Pi to creatine→Creatine Phosphate (CP)

First Energy Source –

Creatine Kinase reverses the reaction to remove Pi and then put the Pi with ADP to form ATP

Second Energy Source – Glycolysis

Third Energy Source – Cellular Respiration

Smooth Muscle Types

• single unit or visceral muscle-

contracts as a unit and

contracts rhythmically

• multi-unit- independent cells,

graded contractions, lots of

nerve endings

Smooth Muscle - Smooth muscle has dense bodies with

intermediate filaments attached

- As the muscle contracts, it forms a spiral or corkscrew that pulls the dense bodies together

- The dense bodies are pulling at different angles

Smooth Muscle

– 2 layers of smooth muscle are present at

right angles

– longitudinal- dilate & shorten the

organ

– circular- constricts the cavity of the

organ and causes the organ to

elongate

• peristalsis- the alternating contraction and

relaxation of the opposing longitudinal and

circular layers - mixes substances and

squeezes them through the organ's internal

pathway

Smooth Muscle