MUSCLES

What Functions Do Muscles Provide?

Basic Physiological Properties

1. Contractility 2. Excitability 3. Extensibility 4. Elasticity

These four properties are all involved in Movement

Muscle is a specialised tissue of mesodermal origin. About 40-50 percent of the body weight of a human adult is contributed by muscles. Based ontheir location, three types of muscles are identified : (i)Skeletal (ii) Visceral and (iii) Cardiac.

Each organised skeletal muscle in ourbody is made of a number of muscle bundles or fascicles held togetherby a common collagenous connective tissue layer called fascia. Eachmuscle bundle contains a number of muscle fibres

Each muscle fibre is lined by the plasma membrane called sarcolemmaenclosing the sarcoplasm. Muscle fibre is a syncitium as the sarcoplasmcontains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmicreticulum of the muscle fibres is the store house of calcium ions. A characteristic feature of the muscle fibre is the presence of a large numberof parallelly arranged filaments in the sarcoplasm called myofilaments ormyofibrils.

Each myofibril has alternate dark and light bands on it. A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins – Actin and Myosin.

The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.

Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere. In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called the ‘H’ zone.

3. Smooth Muscle:

Involuntary peristaltic contractions of digestive tract

Vasoconstriction and vasodilation of blood vessels

Regulated By A.N.S

Smooth Muscle Characteristics

• Has no striations• Spindle-shaped cells• Single nucleus• Involuntary – no

conscious control• Found mainly in the

walls of hollow organs

Figure 6.2a

Skeletal Muscle Characteristics

• Most are attached by tendons to bones

• Cells are multinucleate

• Striated – have visible banding

• Voluntary – subject to conscious control

• Cells are surrounded and bundled by connective tissue

2. Cardiac Muscle:

Rhythmic "synchronized" beat Involuntary - regulatable by A.N.S.

Cardiac Muscle Characteristics

• Has striations• Usually has a single

nucleus• Joined to another

muscle cell at an intercalated disc

• Involuntary• Found only in the

heartFigure 6.2b

Characteristics of Muscles

• Muscle cells are elongated (muscle cell = muscle fiber)

• Contraction of muscles is due to the movement of microfilaments

• All muscles share some terminology– Prefix myo refers to muscle– Prefix mys refers to muscle– Prefix sarco refers to flesh

Skeletal Muscle Attachments

• Epimysium blends into a connective tissue attachment– Tendon – cord-like structure– Aponeuroses – sheet-like structure

• Sites of muscle attachment– Bones– Cartilages– Connective tissue coverings

Connective Tissue Wrappings of Skeletal Muscle (So as to keep Muscle

fibers together)• Endomysium – around

single muscle fiber

• Perimysium – around a fascicle (bundle) of fibers

• Epimysium- The outermost connective tissue

Figure 6.1

Characteristics of Muscles: AND….MYO, MYS and SARCO• Muscle cells are elongated

(muscle cell = muscle fiber)• Contraction of muscles is due to the

movement of microfilaments• All muscles share some terminology:

– Prefix myo refers to muscle

– Prefix mys refers to muscle– Prefix sarco refers to flesh

Muscle Cell =Muscle Fiber= Myocyte: But BEWARE…ALL Muscles cells are NOT ALIKE!!

• The myocyte is elongated

• Their characteristics depend on which type of muscle they are found: skeletal, muscle or cardiac.

• Which Type of Muscle are we referring to here??

The Cell: Called the Muscle Fiber The Muscle Fiber is VERY SPECIALIZED!!! 2. Sarcolemma: Muscle cell membrane 3. Sarcoplasm: Muscle cell cytoplasm it contains: a. Many small oval nuclei b. Many mitochondria c. Sarcoplasmic reticulum d. Transverse tubules

e. Myofibrils ( many of these)

Microscopic Anatomy of Skeletal Muscle Cell: A First Look

• Cells are multinucleate

• Nuclei are just beneath the sarcolemma

Figure 6.3a

Myofibril components… 1. Myofilaments

Actin: Thin threads Myosin: Thick threads

2. Sarcomere Contractile unit of myofibril

3. Sarcoplasmic reticulum Modified smooth E.R. Surrounds myofibril Contains calcium ions

4. Transverse tubules Cross channels of sarcoplasmic reticulum Open to extracellular fluid Function in muscle activation

We Will See The Role of Calcium

During ……Muscle Contraction

Smooth Endoplasmic Reticulum (SR) in Muscle Cells

• VERY SPECIALIZED

• It STORES and MOBILIZES Calcium ions

• What is the role of Ca++ in muscle contraction??

What Does the SER Look Like?

Function of Muscles

• Produce movement

• Maintain posture

• Stabilize joints

• Generate heat

Properties of Skeletal Muscle Activity

• Irritability – ability to receive and respond to a stimulus

• Contractility – ability to shorten when an adequate stimulus is received

Muscle Contraction Begins When..

• Stored Calcium is released into the sarcoplasm

• These ions are released into the sarcomeres

Relaxed and Contracted Sarcomeres

• Muscle cells shorten because their individual sarcomeres shorten – pulling Z discs closer together– pulls on sarcolemma

• Notice neither thick nor thin filaments change length during shortening

• Their overlap changes as sarcomeres shorten

Microscopic Anatomy of Skeletal Muscle

Sarcoplasmic reticulum – specialized endoplasmic reticulum

Figure 6.3a

Figure 6.3b

Microscopic Anatomy of Skeletal Muscle

• Myofibril– Bundles of myofilaments– Myofibrils are aligned to give distinct bands

• I band =

light band

• A band = dark band

Microscopic Anatomy of Skeletal Muscle

• Sarcomere– Contractile unit of a muscle

Figure 6.3b

Microscopic Anatomy of Skeletal Muscle

• Organization of the sarcomere– Thick filaments = myosin filaments

• Composed of the protein myosin

• Has ATPase enzymes

Figure 6.3c

Microscopic Anatomy of Skeletal Muscle

• Organization of the sarcomere– Thin filaments = actin filaments

• Composed of the protein actin

Figure 6.3c

Microscopic Anatomy of Skeletal Muscle

• Myosin filaments have heads (extensions, or cross bridges)

• Myosin and actin overlap somewhat

Figure 6.3d

Microscopic Anatomy of Skeletal Muscle

• At rest, there is a bare zone that lacks actin filaments

• Sarcoplasmic reticulum (SR) – for storage of calcium

Figure 6.3d

Neural Stimulus to Muscles• Skeletal muscles

must be stimulated by a nerve to contract

• Motor unit– One neuron– Muscle cells

stimulated by that neuron

Figure 6.4a

Those Myofiloments: How They Move: The Basis of Muscle Contraction

Plus: The Neuromuscular Junction: where it all happens: This IS Muscle

Physiology!

What Do You Think?

• Glycogen- Stored form of starch found in muscle

• What is the role of Glycogen in muscle physiology?

The Case of the Shrinking Sarcomere

• SEE THE I BAND DISAPPEAR during a contraction of the.. sarcomere? Yes. Which makes up myofilament.

• Will the I band reappear? When?

• What causes the I band to dissapear?

What is ACTUALLY happening when a Sacromere Shrinks?

• The thin filament ACTIN and the THICK filament MYOCIN form a CROSS-BRIDGE that cause a SLIDING motion.

• This shortens the sacromere or closes that GAP or I band and causes a CONTRACTION.

• Neither the ACTIN nor the Myocin filament actually shorten its the sacromere itself.

OBSERVE THE UNCOVERED ACTIN SITES…MYOCIN CAN NOW BIND…

Before Contraction Can Happen

• We must think about it …even if it is a reflex.

• We must involve the CNS…

Nerve Stimulus to Muscles

• Skeletal muscles must be stimulated by a nerve to contract

• Motor unit– One neuron– Muscle cells

stimulated by that neuron

Figure 6.4a

MOTOR END PLATE

• This is where the neuron and myofiber intercept.

• Muscle contraction is possible because of neural impulse at the motor end plate.

• It is the action potential that causes the release of Ca++ ions from the SR,

• The Ca++ can then bind to Troponin and change the configuration of Tropomyocin.

• ACTiN’s G binding sites are then exposed.

Nerve Stimulus to Muscles

• Synaptic cleft – gap between nerve and muscle– Nerve and

muscle do not make contact

– Area between nerve and muscle is filled with interstitial fluid

Figure 6.5b

Transmission of Nerve Impulse to Muscle

• Neurotransmitter – chemical released by nerve upon arrival of nerve impulse– The neurotransmitter for skeletal muscle is

acetylcholine

• Neurotransmitter attaches to receptors on the sarcolemma

• Sarcolemma becomes permeable to sodium (Na+)

Transmission of Nerve Impulse to Muscle

• Sodium rushes into the cell generates an action potential (AP)

• The action potential travels along the T-tubules to the SR to stimulate release of Calcium ions.

The SR Stores Ca ions and Release them When There is an AP!

Find the T-Tubules

Where do Ca ions go?

• The ions travels to the muscle tissue and bind to the ACTIN regulatory proteins ( TROPONIN) .

• This UNCOVERS Myosin Head BINDING Sites on ACTIN so as to allow CROSS BRIDGING ( once myosin is powered by ATP.

The Sliding Filament Theory of Muscle Contraction

• Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin filament

• Myosin heads then bind to the next site of the thin filament

Figure 6.7

The Sliding Filament Theory of Muscle Contraction

• This continued action causes a sliding of the myosin along the actin

• The result is that the muscle is shortened (contracted)

Figure 6.7

The Sliding Filament Theory

Figure 6.8

NOW the muscle Must RELAX

• Acetylcholine, the neurotransmitter is broken down by the enzyme acetylcholinesterase

• SO the stimulus to muscle ceases!• Calcium ions are actively transported back to

the SR• The actin and myocin cross bridges break• RELAXES------S T R E T C H of the

sarcomere. Get it??

WHERE DOES ALL OF THIS ENERGY for CONTRACTION

COME FROM?

Creatine Phosphate• This a high energy molecule found in muscle cells• 4 to 6 time more abundant in muscle fibers than

ATP!!• It CANNOT directly transfer a phosphate group to

a reaction• INSTEAD, when ATP is sufficient an enzyme in

mitochondrian, creatine phosphokinase promotes the synthesis of creatine phosphate.

• As ATP gets degraded, creatine phosphate can donate its phosphate bonds to ADP creating NEW ATP

Energy for Muscle Contraction• Direct phosphorylation

– Muscle cells contain creatine phosphate (CP)• CP is a high-energy

molecule– After ATP is depleted,

ADP is left– CP transfers energy to

ADP, to regenerate ATP– CP supplies are exhausted

in about 20 seconds

Figure 6.10a

Energy for Muscle Contraction

• Initially, muscles used stored ATP for energy– Bonds of ATP are broken to

release energy– Only 4-6 seconds worth of ATP is

stored by muscles• After this initial time, other

pathways must be utilized to produce ATP

Energy for Muscle Contraction

• Aerobic Respiration– Series of metabolic

pathways that occur in the mitochondria

– Glucose is broken down to carbon dioxide and water, releasing energy

– This is a slower reaction that requires continuous oxygen Figure 6.10b

Energy for Muscle Contraction

• Anaerobic glycolysis– Reaction that breaks down

glucose without oxygen– Glucose is broken down to

pyruvic acid to produce some ATP

– Pyruvic acid is converted to lactic acid

Figure 6.10c

Myoglobin• Myoglobin is oxygen carrier ( It is a

pigment)

• Synthesized in muscle

• Higher affinity for oxygen than hemoglobin

• One globin protein, rather than 4: therefore we say this protein has tertiary level structure as apposed to hemoglobin’s quartenary level structure.

• It can STORE oxygen as well as carry it.

Muscles and Body Movements

• Movement is attained due to a muscle moving an attached bone

Figure 6.12

Energy for Muscle Contraction

• Anaerobic glycolysis (continued)– This reaction is not as

efficient, but is fast• Huge amounts of glucose

are needed

• Lactic acid produces muscle fatigue

Figure 6.10c

Muscle Fatigue and Oxygen Debt

• When a muscle is fatigued, it is unable to contract

• The common reason for muscle fatigue is oxygen debt– Oxygen must be “repaid” to tissue to remove

oxygen debt– Oxygen is required to get rid of accumulated

lactic acid

• Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

•SO YOU SAY one of the functions of muscles is to generate HEAT?

•HOW DO THEY DO THAT??

Muscles and Body Movements

• Muscles are attached to at least two points– Origin – attachment to a

immoveable bone– Insertion – attachment

to a movable bone– Action- What this

muscle and bone (together) accomplish

Figure 6.12

Example of how it works:

Name of Muscle: Biceps Brachii

Origin: Scapula of Shoulder girdle

Insertion: Proximal Radius

Action: Flexes elbow and supinates ( to turn backward = supinate ) forearm

Types of Ordinary Body Movements

• Flexion

• Extension

• Rotation

• Abduction

• Circumduction

Body Movements

Figure 6.13a–c

Body Movements

Figure 6.13d

:Prime mover: • major muscle of movement

Antagonist:

opposes movement;when primeMover is active, antagonist is stretched and relaxes

Synergists:• assist prime mover

• fixator--stabilizers

TYPES OF MUSCLES

All skeletal muscles have fascicles

Fascicle arrangement allows different functions of muscles

• parallel (fusiform)--strap• pennate (feather)

• convergent—fan, triangle

• circular (sphincter)—squeeze

Fascicle Arrangement

Determined by fascicular arrangement

Skeletal muscle shortens to 70% of resting length

• parallel--shorter, not powerful

• pennate--lots of fibers, powerful

Power and Range of Motion

Pennate Muscles

Figure 11–1c, d, e

• Unipennate:

– fibers on 1 side of tendon e.g., extensor digitorum

• Bipennate:

– fibers on both sides of tendon e.g., rectus femoris– tendon branches within muscle e.g., deltoid

• Multipennate:• Form angle with tendon

• Don’t move as far as parallel muscles

• Contain more myofibrils than parallel muscles

• Develop more tension than parallel muscles

Diseases of Muscles

Duchenne Muscular Dystrophy• Inherited muscle-destroying diseases

affects muscle (MALES only)• Muscles atrophy- wheelchairs at a

young age. Death from failure of respiratory muscles by young adulthood.

• Due to lack of protein called dystropin-associated glycoprotein (DAG) that helps maintain sarcolemma. NO CURE YET.

Myathenia Gravis

• The disease involves a SHORTAGE of

Acetylcholine receptors at the neuromuscular junction.

NT can’t bind and stimulate AP to muscle.

Myasthenia Gravis

Myasthenia Gravis

• Patients of Myasthenia Gravis usually have drooping eyelids

• Difficulty in swallowing and talking• Generalized muscle weakness and

fatigability• Antibodies to acetylcholine

receptors found in blood …suggests MG is autoimmune disease

• Respiratory Failure due to muscle failure here--Death

Botulism• Bacterium, Clostridium botulinum,

under anaerobic conditions, produces a toxin called botulinum.

• It prevents release of Acetylcholine from nerves at NM junction.

• Paralyzes muscles. Do you understand why?

1. Skeletal Muscle: Voluntary movement Maintenance of posture Heat production