Muscles and Movement

Kate Phelan

Eleni Angelopoulos

Anastasia Matkovski

Fun Facts • “Machines of the

Body”

• We have over 700 muscles in our body

• Muscles account for about 40% of body weight

• Allows the body to move

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Types of Muscles

Smooth --->

Striated --->

Cardiac --->

Skeletal Muscles

• http://trc.ucdavis.edu/biosci10v/bis10v/media/ch21/myofibril.html• Important in maintaining posture, providing support, and allowing for movement• Important in homeostasis

– Constant body temp• Contraction of muscle= ATP breakdown, releasing heat

• Striated– light and dark bands

• Overlapping bands of actin and myosin• Attached to the skeleton by tendons

– Bands of fibrous connective tissue• When muscles contract they shorten• Work in antagonistic pairs

– If one muscle of the pair flexes the joint and bends the limb, then the other one must extend the joint and straighten the limb

• Tone- when muscles appear to be at rest, some of their fibers are still contracting – Important in maintaining posture

Structure • Each muscle fiber is a cell with normal

components (nucleus, etc.) but some components have special features

– T (because they are transverse) tubules from the sarcolemma (plasma membrane) dip into the sarcoplasmic reticulum (expanded version of endoplasmic reticulum) and hold Calcium ions, which, when nerve impulses move down the tubules, are essential for muscle contraction. Happens in the sarcomere.

– Myofibrils- composed of two kinds of myofilaments

– Thin filaments -two strands of actin and a strand of regulatory protein

– Thick filaments -staggered arrays of myosin molecules

• contractile parts of muscle fiber- actin and myosin work together to create this contraction

Myosin-Actin Interactions Underlying Muscle Fiber Contraction

Starting here the myosin head is bound to ATP and is in its low energy configuration

The myosin head hydrolyzes the ATP to ADP and inorganic phosphate and is in its high energy configuration

The myosin head binds to actin, forming a cross-bridge

Releasing ADP and inorganic phosphate, myosin returns to its low energy configuration, sliding the thin filament

Binding of a new molecule, ATP, releases the myosin head from actin, and a new cycle begins

Varying the Rate of Muscle Fiber Stimulation

• If a second action potential arrives before the muscle can relax from the first, then the two twitches will sum resulting in greater tension.

• As the rate of stimulation increases, further summation occurs.

• Tetanus- smooth sustained contraction when twitches fuse because the rate is high enough that the muscle fiber cannot relax at all between stimuli.

Role of Calcium &Regulatory Proteins in Muscle Contraction

• When is a muscle is at rest, the myosin binding sites on the thin filament are blocked by the regulatory protein, tropomyosin.For muscle fibers to contract, these sites must be uncovered.

• Calcium ions bind to the troponin complex, exposing the myosin binding sites on the thin filaments.

• Thin &thick filaments slide past each other and the muscle fiber contracts.

• After breaking down ATP, myosin heads bind to an actin filament to move. When another ATP binds to myosin, the head detaches from actin and the sycle begins again.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

Neural Control of Muscle Tension

• When an action potential in a motor neuron releases acetylcholine on a skeletal muscle fiber, the muscle fiber twitches.

• We can voluntarily alter the extent and strength of the contraction of a whole muscle.– Two basic mechanisms:

• By varying the number of muscle fibers that contract • By varying the rate by which muscle fibers are

stimulated.

Varying the Number of Muscle Fibers

• Each muscle fiber is innervated by one motor neuron, though one motor neuron may synapse with many muscle fibers.

– Hundreds of motor neurons controlling a muscle, each with its own muscle fibers.

• When a motor neuron produces an action potential, all the muscle fibers in its motor unit contract as a group.

• The strength of the contraction depends on ow many muscle fibers the motor neuron controls.

• Thus, the nervous system can regulate the strength of contraction in a while muscle by determining how many motor units are activated and by selecting large or small motor units to activate.

Smooth Muscles• Involuntary movement (not under direct voluntary

control)- controlled by the autonomic nervous system• Found in the internal organs, like the intestines and

the bladder• Lack striations because their actin and myosin

filaments are not regularly arrayed along the cell.• Instead the thick filaments are scattered throughout

cytoplasm and thin filaments are attached to structures called dense bodies.

• Less Myosin than striated muscle fibers and myosin isn’t associated with specific actin strands

• Does not have any troponin complex or T tubules

Some More Smooth Muscle Stuff... Stay awake class...

• Smooth muscles contract slowly but over a greater range of length than striated muscle.

• Some smooth muscle cells contracts only when stimulated by neurons of the nervous system.

• Others can generate action potentials without neural input and are electrically coupled to one another.

Cardiac Muscles

• Only found in heart (obvs)• Characteristics of skeletal and smooth muscle

– striated like skeletal• Cardiac muscles have ion channels in their plasma membrane that

cause rhythmic depolarizations, triggering action potentials without imput from nervous system.

• Action potentials of cardiac muscle cells last up to 20 times longer than those of skeletal muscle fibers– Play important role in control of duration of contraction– Interclated disks- specialized regions where plasma membranes of

adjacent cardiac muscle cells interlock• Where gap junctions provide direct electrical coupling between the cells.

– Thus, an action potential generated by a cell in one part of the heart will spread to all other cardiac muscle cells, and the whole heart will contract.