Muscular System

Muscle Tissue

4 Functions:

1. Produce skeletal movement and all types of movement

2. Maintain posture and body position

3. Guard the entrances and exits of the body

4. Maintain body temperature

Characteristics of MusclesMuscle cells are elongated

Muscle cell are called muscle fibers

Contraction of muscles is due to the movement of microfilaments in cell

4 major functional characteristics of muscle are:

1. CONTRACTILITY – CAN SHORTEN FORCEFULLY

2. EXCITABILITY – STIMULATED BY HORMONES AND NERVES

Four major functional characteristics of muscle are:1. CONTRACTILITY 2. EXCITABILITY

3. EXTENSIBILITY – CAN BE STRETCHED OUT

4. ELASTICITY – EXTEND BEYOND NORMAL LENGTH AND RECOIL

3 basic types of muscle are found in the body:

1. Cardiac muscle found only in heart Striated (striped in appearance) single nucleus involuntary

3 basic types of muscle are found in the body:

2. Smooth muscle found in organs nonstriated involuntary

3 basic types of muscle are found in the body:

3. Skeletal Striated voluntary multinucleated

Muscles can be also categorized by whether they are:

VOLUNTARY – CONSCIOUSLY CONTROLLED

Or

INVOLUTARY – NOT CONSCIOUSLY CONTROLLED

Muscles can be also categorized by whether they are:

STRIATED – STRIPED IN APPEARANCE

orNONSTRIATED – NOT STRIPED IN APPEARANCE

Skeletal Muscle Characteristicsmultinucleate – Have more than one nucleus per cell

Nucleus is on the outside of the cell

Skeletal muscle characteristics continued:

Muscle are attached to bone by tendons.

tendons are dense regular connective tissue

Muscle itself iscalled the belly.

belly

Skeletal muscle characteristics continued:

The belly of the muscle is made of bundles of muscle fibers (cells).

Bundles of muscle fibers wrapped together are called fascicles.

Connective Tissue Wrappings of Skeletal Muscle

1. Endomysium – wraps a single muscle fiber

2. Perimysium – wraps a fascicle (bundle) of fibers

3. Epimysium– wraps entire muscle

Connective Tissue Wrappings of Skeletal Muscle

These three wrappings extend past the muscle to make the tendon and connect to bone.

Skeletal muscle characteristics continued:

have nerves and blood vessels.controlled by nervous systemrequire lots of energy because of cellular respiration therefore need lotsof oxygen

Special terms for muscle fiber organelles:

Sarcosome = mitochondria

Sarcoplasm = cytoplasm

Microscopic Anatomy of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle

Sarcoplasmic reticulum = smooth endoplasmic reticulum

Sarcolemma = specialized plasma (cell) membrane

Microscopic Anatomy of Skeletal Muscle

Also has:T-tubules= tiny tubes sprouting into the sarcolemma forming pits in the sarcolemma which allow signals to travel and stimulate movement.

Also has:

Myofibril – threadlike structures in muscle fiber

Myofibril made of two types of filaments:

1.Myosin protein is a thick filament

2. Actin protein is aThin filament

Also have:

Sarcomeres

• smaller sections of a myofilament

•around 2 m in length

•functional unit of the skeletal muscle

Also have: Sarcomeres

• Sarcomere is the repeating units of the myofibrils

• Sarcomeres give skeletal muscle its striated appearance

Skeletal muscle

Also have: Sarcomeres

Structures of the Sarcomere:

A bands – dark thick bands made from myosin protein

I bands – lighter bands made from actin protein

H zone – area between the ends of the actin myofilaments which point to one another from opposite ends of the sarcomere

H zone

myosin

actinheads

Z disk – where the actin or thin filaments attach at one end -Z disk are made out of protein

The myosin proteins have head-like structures on them

Sliding-Filament Model

Is the model explaining how a muscle cell or fiber is believed to contract

Sliding Filament Model

Actin is surrounded by two other proteins:

1. tropomyosin which winds around the actin blocking active sites

2. troponin which lies across the tropomyosin and keeps it in place

Sliding filament model

Nerve impulse by neurotransmitter acetylcholine stimulates the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm

Ca2+ bind to the troponin and cause the position of the tropomyosin to change exposing the active sites on the actin filament

Sliding filament model

The heads of the myosin filament then bind to the active site of the actin filament forming a cross bridge

Sliding filament model

The Ca2+ ions are then released

Once cross bridges are formed the myosin filament heads will shift called a power stroke pulling the actin filaments closer together;

Sliding filament model

This happens simultaneously in all the sacromeres and muscle fibers causing the muscle to contract.

During sarcomere contraction:Reduced are the:

H zone (shortens)I band (shortens)Overall length of sarcomere (reduced)

I

During sarcomere contraction:

Stays same: actin filamentmyosin filamentA band (made by myosin)

Energy Requirements

•Muscle contraction requires lots of energy provided in the form of ATP (adenosine triphosphate).

•When the first phosphate bond in ATP is broken large amounts of energy is released.

•ATP becomes ADP and P molecule.

ATP related to sliding filament model:

•ATP bonds to head of myosin filament•ATP ADP + P energy becomes available to the myosin head to move.•P released from myosin, myosin then binds to the actin strongly and power stroke occurs

ATP related to sliding filament model:

•ADP falls off and is recycled

•New ATP can attach to provide more energy

•New ATP attached, crossbridge is broken

What stops the contraction?

Same thing that started it – Ca2+ ions

When nerve impulse stops, sarcoplasmic reticulum stops releasing Ca2+ ions which no longer binds with troponin, and the active sites are no longer available.

Quiz

Motor Units

Motor unit isOne motor neuron (nerve cell) and all the

Muscle cells stimulated by that neuron

Parts of the motor neuron

Axon – process that extends from the cell body and carries signal away Note: may branch

Presynaptic terminal –very end of the axon neurotransmitter containing vesicles are found

Parts of the motor neuron

Neuromuscular junction – where the nerve and muscle fiber meet (don’t touch).

Parts of the neuromuscular junction

Synapse – interface between a nerve cell and another cell (muscle cell)

Synaptic cleft – gap between nerve and muscle cells that is filled with interstitial fluid

Postsynaptic membrane – membrane of the muscle fiber in the region of the synapse

Acetylcholine (ACh)– Most common neurotransmitter (chemical) released by neuron traveling across the cleft interacting with the postsynaptic membrane of muscle cell stimulating it to contract.

Synaptic vesicles – found inside the presynaptic terminal containing neurotransmitters.

Acetylcholinesterase – enzyme that inactivates ACh to stop muscle contraction.

How the motor neuron stimulates muscle contraction:

1.Action potential (electrical signal) travels down axon

2. Vesicles in the presynaptic terminal release ACh.

3. ACh travels across the synaptic cleft.

How the motor neuron stimulates muscle contraction:

4. ACh stimulates a new action potential in the sarcolemma through the T-tubules

5. T-tubules stimulate the sarcoplasmic reticulum to release Ca+2 ions by diffusion.

How the motor neuron stimulates muscle contraction:

6. We know Ca+2 ions attach to troponin changing arrangement of tropomyosin exposing sites on actin- crossbridges-powerstroke- contraction

How motor neuron is responsible for stopping contraction:

1.Action potential stops

2.ACh is no longer released and is broken down by acetylcholinesterase.

3. Calcium is no longer released but is pumped back into sarcoplasmic reticulum.

How motor neuron is responsible for stopping contraction:

5.Troponin releases Ca+2 ions and tropomyosin rearranges covering the actin active sites again.

6.Crossbridges of myosin/actin are broken

7.Muscle is relaxed!!

All-or-none law of skeletal muscle contraction

Individual muscle fibers contract with equal force in response to each action potential

Either a fiber is contracted or relaxed no inbetween.

How do you get degrees of muscle movement?

Slight, fine movement (thread a needle) compared to coarse, extreme movements (pick up a 100 lb weight).

How do you get degrees of muscle movement?

1. By the size of the motor unit or amount of axon branching

A. Large motor unit – axon branches hundreds of times stimulating hundreds of muscle fibers at the same time resulting in coarse movement with less control

or

How do you get degrees of muscle movement?Two ways:1. The size of the motor unit or amount of axon branching

A. Large motor unit – axon branches hundreds of times stimulating hundreds of muscle fibers at the same time resulting in coarse movement with less control

or

B. Small motor unit – axon branches only a few times stimulating only a few muscle fibers or just one at the time resulting in fine movement with more control.

How do you get degrees of muscle movement?

2.Frequency of the action potential can affect muscle contraction

Stronger the stimulus the higher the frequency of action potentials that travel down motor neuron

There are 4 levels of frequency:

A.Subthreshold stimulus – too small to create an action potential

B.Threshold stimulus – strong enough to create an action potential

B. Threshold stimulus -stimulus is strong enough to create an action potential

C. Submaximal stimuli – stimuli of increasing strength that create more action potentials along more neurons

C. Submaximal stimuli – stimuli of increasing strength that create more action potentials along more neurons

D.Maximal stimulus – a stimulus which is strong enough to create action potentials in all the neurons innervating a whole nerve

Innervating = controlling

Nerve stimulates several motor units together.

The more motor units (multiple motor unit summation) activated the more muscle fibers contract the greater the force of the muscle contraction.

If motor unit is stimulated it has been recruited.

•Muscle tone – the state of partial contraction of the muscle, even when not being used.

•Less muscle tone when asleep, than when awake.

•Motor units will take turns contracting and relaxing to have muscle tone.

Energy for muscle fibers comes from one of three ways:

1.Aerobic respiration

glucose synthesized into ATP most efficient; 36 ATP – 1 Glucose slow and requires oxygen

2. Creatine phosphate

• gives up a phosphate to ADP converting it to ATP

• stored in the muscle cells in only small amounts

• 1 ATP for each creatine phosphate stored, less efficient than aerobic

3. Anaerobic respiration

• Glucose broken down to pyruvic acid to ATP

• No oxygen required

• Produces lactic acid toxic to muscles causes burning

• Not very efficient, 1 pyruvic acid yields 2 ATP

If anaerobic is less efficient than aerobic why use it?

Sometimes body cannot keep the muscle supplied with oxygen at a fast enough rate.

Myoglobin molecule stores oxygen in muscle cell but sometimes not enough.