Muscle Tissues

A&P 1

Excitability

The ability to respond to stimuli

Contractility

The ability to shorten when stimulated

Elasticity

The ability to return to original length after being stretched

What are the three types of muscle tissue?

Skeletal, Cardiac, and Smooth

Extensibility

The ability to be stretched between contractions.

What are the key features of skeletal muscle?

It is voluntary (subject to conscious control) , usually attached to bones , and is striated (has alternating light and dark bands)

Name the three connective tissue sheaths of skeletal muscle, from innermost to outermost.

Endomysium, Perimysium, Epimysium

Endomysium

Surrounds an individual muscle fiber (cell).

Perimysium

Surrounds a muscle fascicle (a bundle of fibers).

Epimysium

Surrounds the entire muscle.

Direct muscle attachment

The epimysium fuses directly to the periosteum of a bone.

Indirect muscle attachment

Connective tissue extends beyond the muscle as a rope-like tendon or a sheet-like aponeurosis to attach to the bone.

Origin in muscle attachments

The muscle's attachment to the immovable or less movable bone.

Insertion in muscle attachments

The muscle's attachment to the movable bone.

What is the sarcolemma?

The plasma membrane of a muscle fiber.

What is the sarcoplasm and what does it contain?

The cytoplasm of a muscle fiber. It contains myofibrils (protein cords) , mitochondria , glycogen (stored carbohydrate) , and myoglobin (oxygen-storing protein).

What is the function of the Sarcoplasmic Reticulum (SR)?

It is a network around each myofibril that acts as a calcium reservoir, releasing calcium to activate contraction.

What is a T tubule?

A tubular infolding of the sarcolemma that penetrates through the cell, carrying electrical signals deep into the fiber.

What components make up a triad?

One T tubule and its two associated terminal cisterns (enlarged end-sacs of the SR).

What protein makes up thick filaments?

Myosin. The myosin molecules have a tail and a double globular head.

What three proteins make up thin filaments?

Actin, tropomyosin, and troponin.

Actin

Forms a chain of subunits, each with an active site that can bind to a myosin head.

Tropomyosin

A long, thin protein that blocks the active sites on actin when the muscle is relaxed.

Troponin

A calcium-binding protein attached to tropomyosin.

Contractile proteins

Myosin and actin, which do the work of contraction.

Regulatory proteins

Tropomyosin and troponin, which act as a switch to determine when the fiber can contract.

What is a sarcomere?

The contractile unit of a muscle fiber, which is the segment from one Z disc to the next.

A band of a sarcomere

The dark band where thick filaments are located, including where they overlap with thin filaments.

I band of a sarcomere

The light band containing only thin filaments.

H zone of a sarcomere

The lighter region in the middle of the A band containing only thick filaments.

Z disc of a sarcomere

Provides anchorage for thin filaments and bisects the I band.

What is a motor unit?

A single somatic motor neuron and all the muscle fibers it innervates.

Small motor units

Have as few as 3-6 fibers per neuron and are used for fine control (e.g., eyes, hands).

Large motor units

Can have hundreds or thousands of fibers per neuron and are used for strength (e.g., gastrocnemius).

What is a Neuromuscular Junction (NMJ)?

The synapse where a nerve fiber meets a muscle fiber.

What neurotransmitter is released at the NMJ, and what enzyme breaks it down?

The neurotransmitter is acetylcholine (ACh). It is broken down by acetylcholinesterase (AChE) to allow for muscle relaxation.

Briefly outline the steps of Excitation at the NMJ.

• A nerve signal arrives at the axon terminal.

• ACh is released into the synaptic cleft.

• ACh binds to receptors on the sarcolemma.

• Receptor channels open, allowing Na+ ions to enter the muscle cell, causing depolarization.

What is Excitation-Contraction Coupling?

he events that connect the action potential on the sarcolemma to the contraction of the sarcomere. The action potential travels down T tubules, triggering the Sarcoplasmic Reticulum to release Ca²⁺

What is the role of Calcium (Ca²⁺) in initiating contraction?

Ca²⁺ released from the SR binds to troponin. This causes troponin to change shape and move tropomyosin off the active sites on actin, allowing myosin heads to bind.

Outline the four steps of the Cross-Bridge Cycle.

1. Cross-bridge formation, where myosin heads attach to actin. 2. Power stroke, pivoting the myosin head to pull actin. 3. Cross-bridge detachment, ATP binding frees myosin. 4. Cocking of the myosin head, preparing for the next cycle.

What happens during muscle Relaxation?

Nerve stimulation ends, and Ca²⁺ is pumped back into the SR. Tropomyosin moves back to cover the actin binding sites, preventing myosin from reattaching.

What is Rigor Mortis?

The stiffening of muscles after death. It occurs because ATP is no longer produced, so myosin heads cannot detach from actin.

What are the three phases of a muscle twitch as seen on a myogram?

1. Latent period: Delay between stimulus and contraction.

2. Contraction period: Muscle generates external tension.

3. Relaxation period: Tension declines as Ca²⁺ is reabsorbed.

What is the length-tension relationship?

The amount of tension a muscle can generate depends on how stretched or shortened it was before it was stimulated. Optimal force is generated when sarcomeres are at 80-120% of their normal resting length.

Isometric Contractions

Muscle tension increases, but length remains constant (no movement).

Isotonic Contractions

Muscle length changes, but tension remains constant. This includes concentric (muscle shortens) and eccentric (muscle lengthens) contractions.

What are the three main pathways for synthesizing ATP in muscle?

1. Creatine Phosphate metabolism: Immediate energy (~15 seconds).

2. Anaerobic Fermentation (Glycolysis): Short-term energy (30-40 seconds), produces little ATP and lactate.

3. Aerobic Respiration: Long-term energy, requires oxygen and produces far more ATP.

What is EPOC (Excess Postexercise Oxygen Consumption)?

Also known as oxygen debt, it is the elevated oxygen consumption following exercise needed to replenish ATP and dispose of lactate.

Slow Oxidative (Type I) skeletal muscle fiber

Adapted for endurance; fatigue slowly.

Fast Glycolytic (Type IIb) skeletal muscle fiber

Adapted for quick, powerful responses; fatigue quickly.

Fast Oxidative-Glycolytic (Type IIa) skeletal muscle fiber

Intermediate fibers used for movements like walking.

Describe the characteristics of Slow Oxidative Fibers.

They have many mitochondria and capillaries, high myoglobin ("red meat"), and contract slowly. They are adapted for endurance and resisting fatigue.

Describe the characteristics of Fast Glycolytic Fibers.

They have fewer mitochondria, less myoglobin ("white meat"), abundant glycogen, and contract quickly and powerfully using anaerobic glycolysis.

What are the effects of endurance (aerobic) training on muscles?

It improves fatigue resistance by causing slow-twitch fibers to produce more mitochondria, store more glycogen, and synthesize more myoglobin.

What are the effects of resistance training on muscles?

It stimulates muscle growth (hypertrophy) by increasing the number of myofilaments and myofibrils within muscle fibers.

What are the key features of Cardiac Muscle?

It is striated, involuntary, and autorhythmic. Its cells (cardiomyocytes) are connected by intercalated discs, which contain gap junctions and desmosomes.

What are the key features of Smooth Muscle?

It is non-striated and involuntary. Its cells (myocytes) are spindle-shaped with one nucleus. It is capable of regeneration.

Where is smooth muscle typically found?

In the walls of hollow organs like the digestive tract , circulatory system , and respiratory system.

How does smooth muscle contraction differ from skeletal muscle?

• Ca²⁺ comes mostly from the ECF, not the SR.

• Ca²⁺ binds to calmodulin instead of troponin.

• Contraction is very slow and sustained.

• It exhibits a stress-relaxation response, allowing hollow organs to fill.

Function of Satellite Cells

They are unspecialized myoblasts responsible for the regeneration of damaged skeletal muscle.

Function of the Sarcoplasmic Reticulum (SR) vs. T-Tubules

The SR is a calcium reservoir that releases Ca^{2+} to activate contraction. T-Tubules are infoldings of the sarcolemma that carry the electrical signal into the cell.

The two Contractile proteins vs. the two Regulatory proteins

Contractile: Myosin and Actin. Regulatory: Tropomyosin and Troponin.

Function of Acetylcholinesterase (AChE)

It is an enzyme in the basal lamina that breaks down ACh, allowing the muscle to relax.

The 4 steps of Excitation-Contraction Coupling

• Action potential travels down T-tubules.

• Electrical signal releases Ca^{2+} from SR.

• Ca^{2+} binds to troponin.

• Troponin moves tropomyosin, exposing binding sites on actin.

The 4 steps of the Cross-Bridge Cycle

1. Cross-bridge formation (myosin head attaches).

2. Working (power) stroke (myosin head pivots).

3. Cross-bridge detachment (ATP attaches to myosin).

4. Cocking of myosin head (ATP energy resets head).

Eccentric contraction

Muscle lengthens as it maintains tension (lowering weight).

The 3 main pathways of ATP synthesis

1. Creatine Phosphate metabolism (immediate).

2. Anaerobic fermentation (short-term).

3. Aerobic respiration (long-term).