PSIO 201 - Block 3 (Lecture)

What are the three types of muscle?

skeletal, cardiac, smooth

What is the basic function of all muscle?

Generate tension

Functions of skeletal muscle

Move the body: locomotion + facial expression
Posture and body position
Regulation of body temperature

Is skeletal muscle voluntary or involuntary?

voluntary

What does it mean when we say that skeletal muscle is voluntary?

Requires nervous system input

Can some skeletal muscles be automatic?

Yes, but we have the ability to change their activity

What is an example of an automatic skeletal muscle?

Diaphragm

Do muscles pull or push on bones?

Pull

How can you predict muscle action?

Based on location

Origin

The place where the muscle starts on a bone - stays stationary

Insertion

The place where the muscle ends on a bone - moves towards the origin

How does muscle action work in respect to the origin and insertion?

The insertion moves towards the origin

What is the body movement in standard anatomical position?

Everything is extended except the feet

How do you name a movement?

Action + the name of the segment that is moving

Flexion

Reduces the angle between two bones at a joint, typically along a sagittal plane

Extension

Increases the angle between two bones at a joint, typically along a sagittal plane

Abduction

Movement away from the midline of the body, typically along a frontal plane

Adduction

Movement towards the midline of the body, typically along a frontal plane

Reverse Muscle Action (RMA)

When the insertion is anchored, the origin moves towards the insertion

What is an example of RMA?

Pull-ups

Agonist

Muscle primarily responsible for the movement (aka prime mover)

Antagonist

Muscle which opposes the action of the agonist

Synergist

Assists the agonist in making the action more efficient

Fixator

Special synergists which help to prevent movement at the muscle origin

Examples of classification of muscles based on abduction of the arm

Agonist: Deltoid
Antagonist: Latissimus dorsi
Synergist: Supraspinatus
Fixator: Trapezius

Examples of classification of muscles based on flexion of the forearm

Agonist: Brachialis
Antagonist: Triceps brachii
Synergist: Biceps brachii
Fixator: Pectoralis minor

Why is brachialis the agonist in flexion of the forearm rather than biceps brachii?

Better leverage

Lever

Rigid bar - bone

Fulcrum

The fixed point around which a lever moves - joint

How do muscles act to facilitate movement at a fulcrum (joint)?

By exerting effort on the lever (bone) to move a load

What are the three types of lever systems?

First class, second class, third class

First class levers

LFE, not many examples in the human body

Example of a first class lever

Nodding the head

Second class levers

FLE, not many examples in the human body

Example of a second class lever

Calf raise on tip toes

Third class levers

FEL, most common lever system in the human body

Example of a third class lever

Flexion of the forearm

Epimysium

Covers the entire skeletal muscle

Perimysium

Covers the fascicle

Endomysium

Covers individual muscle fibers (different from muscle cell membrane)

Fascicle

Bundle of muscle fibers

What are muscle cells called?

Muscle fibers

How is the size of muscle cells special?

Can be very large
100µm in diameter, many cm long

What is one way in which skeletal muscle fibers are different from other cells in the body?

They are typically multinucleated

How do multiple nuclei arise in muscle fibers?

Fusion of myoblasts

What are the three functional units of organization in muscle?

Conduction of electrical signlas
Control of muscle contraction
Contraction

Sarcolemma

plasma membrane of a muscle fiber that surrounds the cytoplasm

Transverse tubules (T-tubules)

Transmits action potential through cell, arises from the sarcolemma

How does conduction occur in a muscle fiber?

The action potential travels along transversel tubules to allow the signal to reach deep within the muscle fiber

What is a triad comprised of?

Transverse tubules and the terminal cisterns

Sarcoplamic reticulum

Stores calcium, close to transverse tubules, surrounds myofibrils

How is muscle contraction controlled?

Calcium is released from the sarcoplasmic reticulum

Myofibril

Made up of thick and thin filaments and are bound together to form muscle fibers

Deep fascia

Covers groups of muscles

How does contraction occur?

Filaments made of actin and myosin which work to contract muscle

Sarcomere

Contractile unit of muscle from one z-line to another

What surrounds each sarcomere?

Sarcoplasmic reticulum

What does the transverse tubule line up with?

The end of each sarcomere - z-discs

What part of the sarcomere appears dark under a microscope?

A band

A band

Spands the length of the thick filament, overlaps with thin filament

I band

Thin filament only

What happens to the sarcomere during contraction?

Sarcomere shortens and there is more overlap between the thick and thin filaments

Sliding filament mechanism

Myosin heads grab on to the thin filaments and pull them towards the center of the sarcomere, creating more overlap

Z discs

Separate one sarcomere from the next

zone of overlap

Where thick and thin filaments overlap

H zone

The part of the A band where there is no overlap

Result in sarcomere length when contraction occurs

Sarcomere length decreases
Myofibril shortens
Muscle fiber shortens

Result in sarcomere components during contraction

Z discs move toward one another
I band length decreases
A band length remains constant
H zone length decreases
Zone of overlap increases

What are the contractile proteins?

Actin and myosin

What are regulatory proteins?

Troponin and tropomyosin

What are the structural proteins?

Titin, dystrophin, a-actinin, myomesin, nebulin

Actin location

Thin filaments

Actin function

Has myosin binding sites for crossbridge formation with myosin

Myosin location

Thick filaments

Myosin function

Motor protein with mysoin head that binds to myosin binding site on actin and forms crossbridge during muscle contraction

How is actin arranged in the thin filament?

Two helically arranged strands of actin that resemble a strand of twisted pearls

Tropomyosin function

Covers the mysoin binding sites on actin when muscle is relaxed

Troponin function

Holds tropomyosin in place when muscle is relaxed

How does calcium interact with troponin?

During contraction, calcium binds to troponin and causes a conformational change that shifts tropomyosin away from the myosin binding sites on actin.

Exposes binding sites to allow for crossbridge formation.

What activity is found in the myosin head when bound to actin?

ATPase activity

Titin location

Spans half of each sarcomere from Z disc to M line

Titin function

Stabilizes the position of the thick filament, gives muscle elasticity and extensibility, and helps the sarcomere return to resting length after contraction

Dystrophin function

Cytoskeletal protein that links the thin filament to the sarcolemma

Attached to the extracellular proteins in the connective tissues surrounding the muscle fibers

Helps transmit tension from the sarcomeres to tendons

Nebulin loaction

Spans length of the thin filament

Nebulin function

Anchors thin filaments to Z discs

a-Actinin location

Found in the Z discs

a-Actinin function

Binds to actin molecules of the thin filament and to titin

Myomesin location

M line

Myomesin function

Binds to titin and thick flament to connect them together at the M line

What occurs to the myosin head prior to entering the contraction cycle?

ATP binds to myosin head

ATP is hydrolyzed by unbound head and releases energy resulting in a conformational "cocking" of the head group

True or False: When the myosin head is cocked, the ADP and Pi remain bound to the cocked head

True

Where does crossbridge cycling occur?

The zone of overlap

What are the steps of the crossbridge cycle?

1) Exposing the myosin binding sites

2) Crossbridge formation

3) The power stroke

4) Cross bridge detachment

5) Reactivation of the mysoin head

When does the contraction cycle end?

When the myosin binding sites on actin are no longer available

When is the muscle ready to contract?

When the myosin head is in the cocked position and the myosin binding sites on actin are exposed

Exposing the myosin binding sites

Calcium binds to troponin and the myosin binding sites on actin become exposed

Crossbridge formation

Myosin head attaches to exposed binding site on actin, forming crossbridges

The power stroke

Myosin heads pivot towards the center of the sarcomere, this is known as the power stroke

Cross bridge detachment

ATP attaches to myosin head and the cross bridge detaches from actin

Reactivation of the mysoin head

ATP is hydrolyzed and puts myosin back in the "cocked" position