A&P @ LCC by Dr. Prince

Ch 9 Muscle Tissue
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Objectives
  • Know selected key terms
  • List the characteristics of muscle tissue
  • Be able to compare and contrast the three types of muscle tissue and locations
  • List functions of muscle and muscle tissue
  • Describe the structure of a skeletal muscle its subdivisions and associated connective tissues
  • Describe the microscopic structure of a muscle fiber and the functional roles of myobibrils, sarcoplasmic reticulum, and T tubules of skeletal muscle fibers
  • Be able to label all the parts of a sarcomere
  • Name and describe the components of the thin and thick fillaments (name all the proteins)
  • Explain the role of Ca++ in muscle contraction
  • Explain the sliding filament mechanism of skeletal muscle contraction
  • Define and explain a motor unit
  • Differentiate between isometric and isotonic contractions
  • Define muscle twitch and its three phases

Explain what  the functions of muscle tissue are.

The functions of muscle tissue include:  production of body movements together with bones and joints, stabilization of body positions such as posture, regulation of organ volume via sphincters, moving substances within the body such as blood or food, and heat production.

How does skeletal muscle contract?

This is very general, first ACh stimulates the sarcolemma resulting in an action potential (signal or nurve impulse) that travels along the sarcolema and down the T tubules, triggering the release of calcium ions from the SR.  Calcium ions then bind to troponin changing the shape of tropomyosin resulting in the myosin binding sites on the actin to be exposed.  Myosin heads bind to actin, and power stroke (swivel) pulling the Z lines or Z discs closer together shortening the muscle fiber.

Help! Properties of muscle?????

a.  Electrical exitability - the ability to respond to appropriate stimuli.  b.  Contractility - ability to generate force or to get shorter.  c.  Extensibilty - can be stretched without being damaged to a point. d.  Elasticity - ability to return to original length and shape after contraction or extension.

What is the difference between contractile and regulatory proteins?

Well contractile proteins are myosin (thick filaments) and actin (thin filaments).  The myosin is the motor protein that makes the power stroke and the actin is what myosin pulls on.  Regulatory proteins - troponin and tropomyosin block the myosin binding sites on actin.

 

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Notes

Muscles and Muscle Tissue

Dr. Fernando Prince

AP I

Muscle Tissue

Motion results from contraction of muscles. The skeletal system provides leverage for muscles

Types of Muscle Tissue

Skeletal – primarily attached to bones it is striated and voluntary.

Cardiac – found only in the wall of the heart. It is involuntary.

Smooth – (visceral) is in the walls of viscera and blood vessels. It is not striated and it is involuntary.

Functions of Muscle

Motion

Stabilizing the body

Regulation of organ volume

Generation of heat

Characteristics of Muscle Tissue

Excitability – ability to respond to stimuli (action potentials)

Contractility – shorten (to generate force or to do work)

Extensibility (stretched) without damaging

Elasticity to return to original shape

Skeletal Muscle

Myofibrils

Sarcomeres

Myofilaments: Banding Pattern

Thick Filaments

Thick filaments are composed of:

Myosin

Myosin

And you guessed it

Myosin

Ultrastructure of Myofilaments: Thick Filaments

Thin Filaments

Thin filaments are composed of:

Actin

Tropomyosin

troponin

Ultrastructure of Myofilaments: Thin Filaments

Muscle Proteins

Contractile Proteins

Actin

Myosin

Regulatory Proteins

Troponin

Tropomyosin

Fillaments Fibrils Fiber Fascicle Muscle

Arrangement of the Filaments in a Sarcomere

Longitudinal section within one sarcomere

Sarcoplasmic Reticulum (SR)

Neuromuscular Junction

A synapse is where excitable cells make contact with each other

A neuromuscular junction is a synapse between a neuron and a muscle cell

The end bulb to the motor end plate

ACH = Acetylcholine is a neurotransmitter used in the synaptic vesicles to trigger muscle action potentials and thus muscle contraction

"I don’t get it!"

Neuromuscular Junction

Action Potential: Electrical Conditions of a Polarized Sarcolemma

The outside (extracellular) face is positive, while the inside face is negative

This difference in charge is the resting membrane potential

Action Potential: Electrical Conditions of a Polarized Sarcolemma

The predominant extracellular ion is Na+

The predominant intracellular ion is K+

The sarcolemma is relatively impermeable to both ions

Action Potential: Depolarization and Generation of the Action Potential

An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open)

Action Potential: Depolarization and Generation of the Action Potential

Na+ enters the cell, and the resting potential is decreased (depolarization occurs)

If the stimulus is strong enough, an action potential is initiated

Action Potential: Propagation of the Action Potential

Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch

Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize

Action Potential: Propagation of the Action Potential

Thus, the action potential travels rapidly along the sarcolemma

Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

Action Potential: Repolarization

Immediately after the depolarization wave passes, the sarcolemma permeability changes

Na+ channels close and K+ channels open

K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma

Action Potential: Repolarization

Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period)

The ionic concentration of the resting state is restored by the
Na+-K+ pump

Excitation-Contraction Coupling

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

At low intracellular Ca2+ concentration:

Tropomyosin blocks the binding sites on actin

Myosin cross bridges cannot attach to binding sites on actin

The relaxed state of the muscle is enforced

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

At higher intracellular Ca2+ concentrations:

Additional calcium binds to troponin (inactive troponin binds two Ca2+)

Calcium-activated troponin binds an additional two Ca2+ at a separate regulatory site

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

Calcium-activated troponin undergoes a conformational change

This change moves tropomyosin away from actin’s binding sites

Role of Ionic Calcium (Ca2+) in the Contraction Mechanism

Myosin head can now bind and cycle

This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin

Sequential Events of Contraction

Motor Unit

Motor unit is one neuron and all the muscle fibers that it stimulates.

They range from10 muscle fibers to as many as 2000 fibers. Most average 150.

Motor Unit: The Nerve-Muscle Functional Unit

Adjusting Muscle Tension

According to the All-or-none-principle individual muscle fibers contract to their fullest extent that is they do not partially contract.

A muscle as a whole can have graded contractions to perform different tasks

Muscle Twitch

A muscle twitch is the response of a muscle to a single, brief threshold stimulus

The three phases of a muscle twitch are:

Latent period –
first few milli-
seconds after
stimulation
when excitation-
contraction
coupling is
taking place

Muscle Twitch

Period of contraction – cross bridges actively form and the muscle shortens

Period of relaxation –
Ca2+ is reabsorbed
into the SR, and
muscle tension
goes to zero

Refractory Period

The refractory period is the time when a muscle is unable to respond that is it has temporarily lost its excitability

Muscle Response to Varying Stimuli

A single stimulus results in a single contractile response – a muscle twitch

Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation

Muscle Response to Varying Stimuli

More rapidly delivered stimuli result in incomplete tetanus

If stimuli are given quickly enough, complete tetanus results

Stimulus Intensity and Muscle Tension

Treppe: The Staircase Effect

Isotonic Contractions

Isometric Contractions

Muscle Metabolism: Energy for Contraction

Force of Muscle Contraction

Smooth Muscle

Innervation of Smooth Muscle

Proportion and Organization of Myofilaments in Smooth Muscle