Muscular system

What are the 3 types of muscle tissue?

Skeletal, cardiac, smooth

Describe skeletal muscle tissue

Striated; long, cylindrical cells (muscle fibers); multinucleated

Describe cardiac musle tissue

Striated; branched cells with intercalated discs; uninucleated

Describe smooth muscle tissue

No striations; spindle-shaped cells; uninucleated

Which (ONE) muscle tissue is multinucleated?

Skeletal muscle tissue

What makes up skeletal muscle that makes it an organ?

Skeletal muscle tissue, connective tissue membrane, blood vessels, and nerve endings

What is another word for a skeletal muscle fiber?

Skeletal muscle cell

What is each skeletal muscle fiber wrapped in (and what is it)?

Endomysium, a delicate CT membrane

What is a fasicle?

A group of endomysium-covered muscle fibers wrapped in perimysium

What is the perimysium?

A coarse CT membrane that covers muscle fibers

What is A skeletal muscle?

A group of fasicles wrapped in epimysium

What is the epimysium?

A tough CT membrane that wraps skeletal muscles

What are skeletal muscles attached to?

Bones

What are the 2 attachment sites that skeletal muscles MUST have?

Origin and insertion

What is the insertion?

The bone that moves when skeletal muscle contracts

What is the origin?

The bone that does not move

When skeletal muscle contracts, what moves towards what (attachment sites)?

The insertion moves towards the origin

What muscle of the forearm flexes the forearm at the elbow?

The brachioradialis

What is indirect attachment?

The outer connective tissue (epimysium) forms a tendon that connects skeletal muscle to the periosteum around the bone

Direct attachment

The epimysium of skeletal muscle is fused directly to the periosteum surrounding bone

What is an advantage of direct attachment of skeletal muscles?

Provides physical protection to vital organs in the body cavities

Most skeletal muscles in the human body are attached …

Indirectly

Most skeletal muscles in the human body are attached (how and why)?

Indirectly because tendons use a smaller bone surface, many skeletal muscles binding to the same bones allows for interrelationships in the functions of the muscles (like synergists), it protects skeletal muscles “flesh” from direct contact with rough bone surfaces (that can tear the muscle), and it allows for long bones to act as levers for movement of joint contraction

What is a synergist muscle?

A muscle that helps the agonist (prime mover), or reduces unnecessary action

What is the antagonist muscle?

The muscle that does the opposite of the agonist. It relaxes when the agonist contracts

What is the agonist muscle?

The muscle that does the movement and contracts

What runs the entire length of the skeletal muscle (organ)?

Skeletal muscle fibers

Myofibrils

Rod-like structures that run the entire length of the muscle fiber. Are the smallest. They contain thick or thin myofilaments

Myoglobin

A red pigment protein that binds, stores, and transports oxygen

Inclusions

Glycosomes that contain glycogen

Mitochondria

Organelles used for aerobic respiration to produce ATP/energy

Sarcoplasm

Cytoplasm of the muscle fiber

Sarcoplasmic reticulum (SR)

Smooth endoplasmic reticulum that stores/releases calcium into the sarcoplasm. Has expanded ends (terminal cisterane)

Terminal cisternae

Expanded ends of the SR (sarcoplasmic reticulum)

Sarcolemma

Plasma membrane of a muscle fiber

Transverse tubules (t-tubules)

Invaginations of the sarcolemma into the sarcoplasm

What makes up a triad?

A t-tubule in between 2 terminal cisternae of 2 SR

What is the function of a triad?

It releases calcium ions into the sarcoplasm when the sarcolemma depolarizes

What are the 2 main types of myofilaments?

Thick and thin filaments

Describe thick filaments

16 nm diameter, made of the protein myosin, has 300 myosin molecules, known as an A band

What does each myosin consist of?

A tail and 2 globular heads

What is an A band?

A thick filament

What does each myosin globular head contain?

The enzyme ATPase, binding sites for actin, and binding sites for ATP

Describe thin filaments

8 nm in diameter, anchored by Z line, have 3 proteins-actin, tropomyosin, and troponin

What are the 3 proteins that thin filaments have?

Actin, tropomyosin, troponin

What makes up the framework of thin filaments

Many actin proteins

What does actin contain?

The binding sites for the myosin globular heads

What is tropomyosin

A rod-shaped protein that spirals around actin and blocks myosin binding sites on actin when skeletal muscles are relaxed

What is troponin

A protein made of 3 polypeptides, TnC, TnT, Tnl

TnC

Polypeptide of troponin that binds calcium ions

TnT

Polypeptide of troponin that binds to tropomyosin

Tnl

Polypeptide of troponin that is an inhibitory subunit that binds to actin

What are sarcomeres

The structural and functional units of skeletal muscles

How are sarcomeres arranged?

End to end throughout a myofibril

What is the distance between 2 successive Z discs in myofibril

A sarcomere

Why does skeletal muscle look striated?

The alternating pattern of thick and thin filaments

What are z-discs (lines)

They anchor thin filaments in myofibrils

H-zone

The middle region of the A band that does not overlap with thin filaments

M line

The line in the middle of the H zone that anchors A bands

I bands

Regions of thin filaments that do not overlap with the A band

What is the sliding filament mechanism of muscle contraction

The sliding of thin filaments into the H zone (towards the M line) results in muscle shortening which is muscle contraction

Describe the shortening of sarcomeres all the way to contraction

Shortening of sarcomeres = shortening of myofibrils= shortening of skeletal muscle fibers= shortening/contraction of the skeletal muscle

According to the sliding filament mechanism, when a muscle contracts, there is more overlap between the…

thin filaments (actin) and the A bands

In a skeletal muscle contraction, what decreases or disappears?

The H zone and I-bands

In a muscle contraction, what shortens?

the sarcomere length

What length will not change or shorten?

A bands and thin filaments

What stimulates skeletal muscles to contract?

When motor neurons (that innervate skeletal muscles) are activated

How are motor neurons activated?

They conduct impulses to skeletal muscles

Axon terminals

What a motor neuron uses to make contact with skeletal muscle fibers

What forms the neuromuscular junction?

One axon terminal innervates one skeletal muscle fiber in the muscle

What is a motor unit?

A motor neuron and all the skeletal muscle fibers it innervates using axon terminals

How many motor units can a muscle fiber have attached to it?

Only one motor neuron

Can a motor unit be attached to multiple muscle fibers?

Yes

What are motor units based on?

The number of skeletal muscle fibers innervated by a motor neuron using axon terminals

What does a motor unit consist of?

A motor neuron and all the muscle fibers it innervates

What is the neuromuscular junction?

The junction between the axon terminal of a motor neuron and a skeletal muscle fiber that is separated by the synaptic cleft

What separates the axon terminal of a motor neuron and a skeletal muscle fiber (or the neuromuscular junction)?

The synaptic (neuromuscular) cleft

How many neuromuscular junctions can each skeletal muscle fiber form?

One

What is the motor end plate and what does it express?

The folded region of the sarcolemma at the neuromuscular junction that expresses acetylcholine receptors

Where are motor neurons activated?

in the CNS

What releases acetylcholine from vesicles in the axon terminal to the neuromuscular cleft?

When action potentials are transmitted by the axon of motor neurons

What causes depolarization?

When acetylcholine binds to acetylcholine receptors on the motor end plate

What does depolarization lead to?

Action potentials (electrical currents) at the motor end plate

What causes calcium channels on terminal cisternae to open?

When action potentials spread across the whole sarcolemma into t-tubules of triads

What happens after calcium channels (on terminal cisternae) are opened?

Calcium ions are released into the sarcoplasm and bind to TnC

What does Ca ions binding to TnC result in?

A conformational change and tropomyosin is removed from blocking myosin-binding sites on actin

What happens after tropomyosin is removed from blocking?

Activated myosin heads (cross bridges) bind to the myosin binding sites on actin

When are myosin globular heads activated and why?

When the myosin heads are attached by ADP+P to bind to the myosin binding sites on actin to form cross bridges

What is the Powerstroke?

When the myosin globular heads change orientation from a right angle to a bent position that pull thin filaments into the H zone towards the M line

What does the Powerstroke explain?

How the sliding of thin filaments makes a skeletal muscle contraction

How do skeletal muscles relax after contraction?

Cross bridge detachment where new ATP molecules bond to the attached myosin heads that cause them to detach

What is muscle fatigue?

The inability of a stimulated skeletal muscle to contract due to ATP deficit

What is rigor mortis?

Lack of ATP that results in skeletal muscle contracture, that only occurs when a person dies and ATP synthesis stops where actin and myosin are linked and skeletal muscles stay contracted

What ATP is used first for skeletal muscle contraction and how long does it last?

Stored ATP, lasts 5 seconds

What is creatine phosphate (CP)?

ATP produced from direct phosphorylation of ADP by CP using the enzyme creatine kinase

CP + ADP —>

ATP + creatine

How long does ATP produced by CP last?

the next 10 seconds of skeletal muscle activites

How long does anaerobic catabolism of glucose energy produce?

30-60 seconds of skeletal muscle activities

What happens if anaerobic respiration is extended?

There’s an increase in lactic acid production which reduces blood pH and contributes to muscle fatigue

What produces ATP that supports skeletal muscle activities for hours?

Aerobic catabolism of glucose