Unit 4 Muscle Physiology

3 Types of muscle

• Cardiac

• Skeletal

• Smooth

Properties of cardiac muscle

Involuntary and striated

Properties of skeletal muscle

Striated and voluntary

Properties of smooth muscle

Involuntary, not bundled like other muscle types

Contractility

Ability of muscle to generate force through contraction

Excitability

Ability of muscle to respond to a stimulus

Extensibility

Ability of a muscle to stretch beyond resting without causing damage

Elasticity

Ability of a muscle to return to its original length at rest after stretching

Primary function of each type of muscle

• To produce force

  • Skeletal - locomotion

  • Smooth - to move substances such as blood or urine within the body or out of the body

  • Cardiac - to produce blood flow

Secondary function of skeletal muscle

• maintain posture

• stabilize joints

• generate heat through shivering

How are skeletal musces attached to bone?

Tendons

Muscle contraction generates force on ________ which then causes the joint to move

tendons

Muscle origin vs insertion

Origin - the part of a tendon that remains immobile during muscle movement and is usually closer to the midline of the body

Insertion - the part of a bone that moves during muscle action and is usually more distant from the midline of the body

Agonist vs antagonist muscle, + example

• Agonist - prime mover of an action, which initiates contraction

• Antagonist - act on the same joint to produce opposite action

Ex. when the bicep contracts, the tricep relaxes. As a result, the bicep is the agonist, and the tricep is the antagonist in this case

Muscle cells are referred to as muscle _______

fibers or myofibers

Smallest component of a muscle

Muscle fiber

Bundles of muscle fibers are called ______

fasicles

Fasicles are surrounded by what protective layer?

Perimysium

Label all parts of the muscle structure

What are the two ways that fasicles can be arranged? What is the function of each form?

Parallel/fusiform - to generate a range of motion

Pennate (angled) - to generate power

True or false: Myofibers have a single nuclei

False

Sarcolemma

Membrane of a muscle cell

T-tubule

Tubule that transverses the interior of the myofiber

Sarcoplasmic reticulum

Stores calcium, located in close proximity to the t-tubule

Sarcomere

Functional unit of muscle contraction

Thin filament

Actin, which composes part of myofilaments

Thick filament

Myosin, which comprises part of myofilaments

Sarcomeres are defined from end to end by __________ to _________

Z-disc to Z-disc

What causes the striated appearance of muscles?

Sarcomeres

Do smooth muscles have sarcomeres? Why?

No, because sarcomeres are the basis for striation, and smooth muscles are not striated

The distance between z discs _________ during contraction

Shorten

The I band of a muscle is comprised of _______

actin only

The H band of a muscle sarcomere is comprised of _________

Myosin only

The A band of a sarcomere is comprised of __________

the overlap of actin and myosin (I and H bands)

True or False: all sarcomere bands except the A band shorten

True

Main component(s) of the thin filament

Actin, bound to tropomyosin, which holds troponin sites

Primary component(s) of the thick myofilament

Made of thick bundles of myosin with myosin head regions that bind to actin and ATP

ATP binds to which filament of the sarcomere? At what specific part?

Thick filament at the myosin head

4 steps of cross-bridge cycling

1. ATP binds to myosin head, triggering the detachment of the two filaments from eachother

2. ATP is hydrolyzed into ADP and P, resulting in the myosin head being energized to the high energy state (cocked)

3. Myosin head binds to actin filament, forming cross bridge

4. The P generated earlier is released, causing the power stroke to occur

How is cross-bridge cycling regulated?

The calcium released from muscle sarcoplasmic reticulum binds to troponin on the thin filament, which causes the connected tropomyosin to change shape and reveal the myosin binding site of actin

What 2 things are absolutely necessary for muscle contraction?

ATP and Ca2+

Motor unit

A motor neuron and all of the muscle fibers it innervates

True or false: a single muscle fiber receives multiple axon terminals from its motor neuron

false, it only recieves a single

All or none principle

When a motor neuron is activated, ALL of the muscle fibers it innervates (motor unit) will be depolarized and contract

Innervation ratio

Ratio of motor neurons to muscle fibers

The innervation ratio for an eye muscle would be ________ than the innervation ratio for a calf muscle

much lower, such as 1:20 in the eye compared to 1:2000 in the calf

True or False: the muscle fibers within a motor unit are spread throughout a muscle instead of being concentrated in one spot

True

True or False: All muscle fibers in a particular motor unit are the same type of fiber

true

Where is calcium in a muscle cell stored?

Sarcoplasmic reticulum

Before cross-bridge formation, calcium binds to _______ which moves _____ so that the myosin head of the thick filament can bind

troponin, moves tropomyosin

What type of channel is the acetylcholine receptor of a muscle cell?

Ligand-gated channels

How does a somatic motor neuron action potential lead to sarcomere contraction eventually?

1. Action potential in motor neuron leads to release of acetylcholine

2. Acetylcholine binds to nicotinic receptors of sarcolemma, causing sodium to diffuse in and an action potential release

3. Action potential travels through TRANSVERSE TUBULES

4. Action potential eventually reaches the sarcoplasmic reticulum, where calcium is then released

5. Calcium from sarcoplasmic reticulum binds to troponin, moving tropomyosin, and allowing myosin head to bind to actin

How does a muscle relax?

• Action potential has to cease

• Acetylcholin is degraded by acetylcholinesterase

• Calcium release channels of sarcoplasmic reticulum closes

• Calcium is pumped back into the sarcoplasmic reticulum to facilitate muscle relaxation via the Ca2+-ATPase pumps

Myasthenia gravis

An autoimmune disease that attacks acetylcholine receptors, resulting in muscle weakness

True or False: Myasthenia gravis affects muscles with a higher innervation ratio more than those with a smaller one, such as the eye muscles.

False, it tends to affect the eyes first because there are only a handful of muscle fibers per neuron

What ion is primarily responsible for membrane depolarization during muscle contraction?

Calcium

What happens when the voltage gated Ca2+ channel undergoes a change in structure during depolarization?

Ca2+ is released from the sarcoplasmic reticulum

Label each color as force, calcium, or voltage

Tetanus

When relaxation is incomplete between APs as a muscle is repetitively stimulated, with the force increasing with each action potential until maximum force is reached

Length-tension relationship of a muscle

The length of a sarcomere determines myofilament overlap, which determines the amount of relative tension. The optimal length will maximize overlap between thin and thick filaments, which maximizes the amount of cross bridges formed

Why does a very low sarcomere length weaken the relative tension of a muscle, even though there is more overlap between the thin and thick filaments?

Too much overlap disrupts the spacing of the thin and thick filaments, which leads to disruption of cross bridge formation and force drops off

3 Types of muscle contraction

• Isometric - muscle length stays constant, load is greater than the force of contraction, NO MOVEMENT

• Concentric- muscle shortens with contraction, force of contraction exceeds the load

• Eccentric - muscle lengthens with contraction, load may exceed force of muscle contraction

4 determinants of isometric force

• How often muscle is activated

• Number of motor units recruited

• Muscle size/cross section size

• Sarcomere length

Force-velocity curve of muscle

Heavy loads result in less muscle shortening over a smaller amount of time - LESS VELOCITY WITH MORE FORCE

When the velocity of a muscle is at zero, the load is at its _________

greatest

3 Types of energy systems for muscle contraction (+time spans

• Phosphagen (seconds)

• Glycolytic (minutes)

• Aerobic system (unlimited, using fat)

Sprinting would PRIMARILY use the ______ energy system

Phosphagen

a 1500 meter or 1 mile run would PRIMARILY use what energy stem?

Glycolytic system

A marathon run would primarily use what energy sysem?

Aerobic respiration

True or False: All three energy systems operate regardless of the activity causing muscle contraction (ex. sprinting, marathon, etc.)

True, they all operate but at different rates depending on the activity

Phosphogenic system

Uses creatine phosphate to rapidly convert ADP to ATP

Byproducts of aerobic metabolism

• CO2

• H2O

  • Heat

VO2 max

a measure of cardiovascular fitness - maximum rate of energy that your body can consume and use during intense exercise

VO2 max depends on what factors?

• Age

• Training

• Body weight

Slow twitch muscle fiber characteristics

• Slow to fatigue

• Red appearance

• Lots of capillaries

• Many mitochondria

• Aerobic respiration is much more common

• Common in endurance muscles

Fast twitch fibers (2a)

• Fast to contract

• Highly aerobic, with many mitochondria

• Fatigue resistant

Fast twitch fibers (2x)

• Have a white appearance

• Anaerobic

• Fewer capillaries

• Large glycogen stores

• Sprinting muscles

The thickest of the 3 muscle fiber types are _________

Fast twitch glycolytic

The smallest diameter of the 3 fiber types are ________

Type 1 slow oxidative

The widest Z line thickness of the 3 muscle fiber types is _________

Slow oxidative (type 1)

The narrowest Z line thickness of the 3 muscle fiber types is ________

Fast glycolytic (Type 2x)

How many different fiber types do humans have?

3

Stem cells two characteristics that are unique

• They can self renew

  • They can differentiate into mature cell

Satellite cells (muscle)

Skeletal muscle stem cells that play a role in supporting/repairing muscle

True or false: Satellite cells are typically not quiescent cells

False, they are usually quiescent until they are repairing muscle

Duchenne muscular dystrophy

The most common form of muscular dystrophy, caused by dystrophin gene mutation on the x chromosome so it is more common in boys

Dystrophin function

To provide structural stability to muscle cell membrane

Why is dmd more common in boys than girls?

Boys only get one x, and the disease is recessive so inheriting an x chromosome with the mutation leads to having the disease in boys. Girls, however, can be carriers of the disease without symptoms if they inherit an X with the mutation

DMD symptoms

Muscle weakness and muscle mass loss, muscle is replaced by fat fibrotic tissue, vastly reduced life expectancy, often leads to cardiovascular issues eventually

How are cardiac muscles different than skeletal muscles?

• Cardiac muscle fibers are shorter in length

• Cardiac muscle cells have gap junctions to connect the cells as one unit during action potential

Syncitium meaning

When many cells behave as one unit, such as the heart!

How are calcium channels different in the heart than skeletal muscle?

The voltage gated calcium channels of the T tubule trigger calcium to be released from the sarcoplasmic reticulum

Smooth muscle vs other muscle cells

• Have no sarcomeres

• Much higher actin to myosin ratio (16:1)

  • Actin filaments are attached to dense bodies

Which muscle cell involves actin filaments attached to dense bodies?

Smooth muscle

Which type of muscle cell does not have sarcomeres?

Smooth muscle cells

Calmodulin function in smooth muscle

Calmodulin binds to calcium to activate myosin kinase which allows the myosin head to be phosphorylated and thus active

How is smooth muscle contraction different than skeletal and cardiac muscle contraction?

• Smooth muscle contraction is regulated via calmodulin binding to calcium and the subsequent phosphorylation of myosin via a myosin kinase that activates the myosin head to be able to bind to the thin filament

• Cross bridges are released when a phosphatase takes the phosphate off of the myosin head instead