ch 9 lecture

Skeletal muscle tissue

Voluntary, striated muscle tissue attached primarily to bones

• it produces body movements

• maintains posture

• generates heat.

Cardiac muscle tissue

Involuntary, striated muscle tissue found in the heart

• its contractions pump blood.

Smooth muscle tissue

Involuntary, non-striated muscle tissue found in the walls of hollow organs and blood vessels

• it moves substances

• regulates passageways.

Why is skeletal muscle considered voluntary?

Its contractions are generally controlled consciously through the somatic nervous system, although many skeletal muscle reflexes occur involuntarily.

Why is cardiac muscle considered involuntary?

Its contractions are initiated and regulated by the heart's own electrical conduction system and modified by autonomic nerves and hormones.

Why does cardiac muscle appear striated?

Its actin and myosin filaments are organized into repeating sarcomeres.

Why does smooth muscle lack visible striations?

Its actin and myosin are not organized into repeating sarcomeres.

Excitability

The ability of muscle tissue to receive and respond to a stimulus.

Contractility

The ability of muscle tissue to develop tension and shorten forcibly when stimulated.

Extensibility

The ability of muscle tissue to be stretched beyond its resting length.

Elasticity

The ability of muscle tissue to recoil and return toward its resting length after stretching or contraction.

Four characteristics of muscle tissue

• Excitability

• contractility

• extensibility

• elasticity.

Three major functions of skeletal muscle

• Produce movement

• maintain posture and body position

• generate heat.

How does skeletal muscle help maintain body temperature?

• Muscle contractions release heat as a by-product of metabolism

• shivering increases heat production through repeated contractions.

Muscle fiber

A single muscle cell

• skeletal muscle fibers are long cells containing many nuclei and many myofibrils.

Sarcolemma

The plasma membrane surrounding a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber.

Myofibril

A long contractile structure inside a muscle fiber composed of repeating sarcomeres.

Myofilaments

The protein filaments actin and myosin that interact to produce muscle contraction.

Epimysium

The connective tissue covering surrounding the entire skeletal muscle.

Perimysium

The connective tissue covering surrounding each fascicle within a skeletal muscle.

Endomysium

The connective tissue covering surrounding an individual muscle fiber.

Fascicle

A bundle of muscle fibers surrounded by perimysium.

Correct order of skeletal muscle organization

1. Whole muscle

2. fascicle

3. muscle fiber

4. myofibril

5. sarcomere

6. myofilaments.

Which connective tissue surrounds an entire muscle?

Epimysium.

Which connective tissue surrounds a bundle of muscle fibers?

Perimysium.

Which connective tissue surrounds an individual muscle fiber?

Endomysium.

A structure contains many muscle fibers bundled together. What is it?

A fascicle.

What is the function of the connective tissue coverings of skeletal muscle?

• support and organize muscle tissue

• help transmit the force of contraction to tendons and bones.

Tendon

A strong connective tissue structure that attaches muscle to bone or another structure.

What happens to the force produced by individual muscle fibers?

The force is transmitted through connective tissue and tendons to produce movement or tension.

Sarcomere

• The repeating functional contractile unit of a myofibril

• extending from one Z disc to the next.

Z disc

• The boundary of a sarcomere

• it anchors thin filaments

• helps define the repeating contractile unit.

A band

• The region spanning the length of the thick filaments

• its length remains constant during contraction.

I band

• The region containing thin filaments without thick-filament overlap

• it becomes narrower during contraction.

H zone

• The central region of the A band where thick filaments are not overlapped by thin filaments

• it narrows or may disappear during contraction.

M line

• The center of the sarcomere

• where proteins help hold thick filaments in position.

Titin

A large elastic protein that helps stabilize thick filaments and contributes to the muscle's recoil after stretching.

Thick filament

Myosin-containing filament that forms cross-bridges with actin during contraction.

Thin filament

Primarily actin, with the regulatory proteins troponin and tropomyosin in skeletal muscle.

Which sarcomere regions change length during contraction?

• The I band and H zone narrow

• the A band stays the same length.

During contraction, do actin and myosin filaments become shorter?

No. They slide past one another, increasing their overlap while the filaments themselves remain essentially the same length.

A sarcomere contracts. What happens to the Z discs?

They move closer together.

During contraction, what happens to the A band?

Its length remains constant because the thick filaments do not shorten.

During contraction, the H zone disappears. What does this indicate?

Thin filaments have slid farther into the region occupied by thick filaments, increasing overlap.

Why does a whole muscle shorten when its sarcomeres contract?

Many sarcomeres shorten as thin filaments slide past thick filaments, shortening myofibrils and muscle fibers.

Sarcoplasmic reticulum (SR)

A specialized form of smooth endoplasmic reticulum in muscle fibers that stores and releases calcium ions.

T-tubule

An inward extension of the sarcolemma that carries the muscle action potential into the interior of the muscle fiber.

Terminal cisternae

Expanded regions of the sarcoplasmic reticulum that store calcium near T-tubules.

Triad

One T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.

• Cisternae

• T-tubule

• Cisternae

Why does a muscle fiber need T-tubules?

They carry electrical signals deep into the fiber so internal contractile regions can be activated rapidly.

What happens if an action potential travels only along the outer sarcolemma but does not reach the T-tubules properly?

Calcium release from the SR may be impaired, reducing or disrupting contraction throughout the fiber.

Neuromuscular junction (NMJ)

The synapse where a motor neuron communicates with a skeletal muscle fiber.

Motor neuron

A nerve cell that carries signals to muscle fibers.

Synaptic cleft

The small gap between a motor neuron terminal and the muscle fiber's motor end plate.

Motor end plate

The specialized region of the muscle fiber's sarcolemma containing acetylcholine receptors.

Acetylcholine (ACh)

A neurotransmitter released by motor neurons at the skeletal neuromuscular junction.

Acetylcholine receptor

A ligand-gated ion channel on the motor end plate that opens when ACh binds.

Ligand-gated ion channel

A channel that opens in response to a chemical messenger binding to it.

Voltage-gated ion channel

An ion channel that opens or closes in response to a change in membrane potential.

What triggers voltage-gated calcium channels in the motor neuron's axon terminal to open?

The arrival of an action potential at the axon terminal.

What does calcium do inside the motor neuron axon terminal?

It triggers synaptic vesicles to release acetylcholine into the synaptic cleft.

What happens when ACh binds its receptors on the motor end plate?

• Ligand-gated channels open

• allowing Na+ into the cell

• producing a net inward positive current that depolarizes the muscle membrane.

Why does ACh usually depolarize the motor end plate?

More positive charge enters in the form of Na+ through the receptor channels than leaves, producing a net inward current.

What triggers voltage-gated sodium channels in the muscle membrane to open?

If the end-plate depolarization reaches threshold, it triggers an action potential.

What is the difference between the end-plate potential and a muscle action potential?

• The end-plate potential is a local, graded depolarization

• the muscle action potential is an all-or-none electrical signal that propagates along the sarcolemma.

What happens after a muscle action potential begins?

It propagates along the sarcolemma and travels down the T-tubules.

What detects the voltage change in the T-tubule during skeletal muscle excitation?

Voltage-sensitive proteins in the T-tubule membrane, which trigger calcium release from the SR.

What happens when the SR releases calcium into the sarcoplasm?

Calcium binds to troponin, allowing tropomyosin to move and expose myosin-binding sites on actin.

What happens if calcium cannot be released from the SR?

Troponin does not receive the normal calcium signal, actin's binding sites remain blocked, and contraction is greatly reduced or prevented.

What happens if ACh cannot bind to receptors on the motor end plate?

The muscle fiber will not normally depolarize enough to generate the action potential needed to initiate contraction.

What happens if voltage-gated calcium channels in the motor neuron's axon terminal fail to open?

Acetylcholine release is greatly reduced or prevented, impairing muscle activation.

What happens if acetylcholinesterase is inhibited?

ACh remains in the synaptic cleft longer than normal, causing prolonged receptor activation and potentially sustained muscle stimulation.

Acetylcholinesterase (AChE)

The enzyme that breaks down acetylcholine in the synaptic cleft, helping terminate its signal.

What happens to the products of ACh breakdown?

• Acetate and choline are produced

• choline can be taken back up by the nerve terminal and reused to make ACh.

Why must ACh signaling be terminated?

To stop continued stimulation and allow the muscle fiber to relax when neural stimulation ends.

Excitation-contraction coupling

The series of events linking a muscle action potential to calcium release and the initiation of contraction.

Excitation-contraction coupling sequence

1. Muscle action potential

2. T-tubule depolarization

3. SR calcium release

4. calcium binds troponin

5. tropomyosin shifts

6. myosin binds actin and contraction proceeds.

Troponin

• A regulatory protein complex in skeletal muscle thin filaments

• calcium binding causes a change that allows tropomyosin to move.

Tropomyosin

A regulatory protein that blocks myosin-binding sites on actin when the muscle is relaxed.

What is the role of calcium in skeletal muscle contraction?

Calcium binds to troponin, shifting tropomyosin away from actin's myosin-binding sites.

What is the role of ATP in directly allowing contraction to begin?

ATP is hydrolyzed to energize the myosin head, and the exposed actin sites allow myosin to form cross-bridges and cycle.

What prevents myosin from binding strongly to actin in a relaxed skeletal muscle?

Tropomyosin blocks the myosin-binding sites on actin while cytosolic calcium is low.

Sliding filament mechanism

Contraction occurs as thin filaments slide past thick filaments, increasing overlap and shortening sarcomeres.

Cross-bridge

A temporary connection formed when a myosin head binds to an exposed site on actin.

Power stroke

The pivoting movement of a myosin head that pulls the thin filament toward the center of the sarcomere.

What causes myosin to detach from actin?

ATP binding to the myosin head.

What happens if ATP is unavailable while myosin is attached to actin?

Myosin cannot detach normally, so the cross-bridge remains attached.

What does ATP hydrolysis do to the myosin head?

It supplies energy to recock the head into a high-energy position for another cycle.

What triggers the power stroke in the cross-bridge cycle?

Release of inorganic phosphate from the myosin head after it binds actin.

What happens to ADP during the cross-bridge cycle?

ADP is released after the power stroke as myosin transitions toward the strongly bound state.

Cross-bridge cycle sequence

1. Myosin binds actin

2. phosphate release triggers the power stroke

3. ADP is released

4. ATP binds and detaches myosin

5. ATP hydrolysis recocks the head.

Why does myosin need ATP even though the power stroke releases energy?

ATP is needed to detach myosin from actin and, through hydrolysis, re-energize the myosin head for another cycle.

Three major ATP uses in skeletal muscle contraction and relaxation

1. Detaching myosin from actin

2. energizing/recocking myosin heads through ATP hydrolysis

3. pumping calcium back into the SR.

SERCA calcium pump

An ATP-powered pump that transports calcium from the sarcoplasm back into the sarcoplasmic reticulum.

What happens if SERCA pumps stop working?

Calcium is not efficiently returned to the SR, so cytosolic calcium remains elevated and relaxation is impaired.

What directly causes a skeletal muscle fiber to relax?

1. Neural stimulation stops

2. ACh is cleared

3. action potentials cease

4. calcium is pumped back into the SR

5. tropomyosin then blocks actin's binding sites again.

Why is ATP required for relaxation as well as contraction?

ATP is needed for myosin detachment from actin and for active calcium transport back into the SR.

What happens to troponin when calcium concentration in the sarcoplasm falls?

Calcium dissociates from troponin, allowing tropomyosin to return to its blocking position on actin.

Why does a muscle not remain contracted after every action potential?

Calcium is removed from the sarcoplasm and regulatory proteins re-cover actin's binding sites, ending cross-bridge cycling.