Muscular System

4 characteristics of muscle fibres

• Excitability

• Contractility

• Extensibility

• Elasticity

Excitability

Ability to receive and respond to a stimulus

Contractility

Ability to shorten forcibly when stimulated

Extensibility

Ability to stretch past its original length when relaxed

Elasticity

Ability to return to its original length when stretched

5 functions of muscles

Producing body movements
Stabilizing body positions
Regulating organ volume
Moving substances within the body
Producing heat

8 classes of skeletal muscles

Fusiform

Parallel

Convergent

Circular

Pennate

Unipennate

Bipennate

Multipennate

Fusiform

Long, parallel fibres
Spindle shaped
Involved with a large range of motion

Parallel

Long fibres that run parallel to each
other
Large movements
Not very strong but good for
endurance

Convergent

Origins are wider than their insertion
Allows maximum force production

Circular

Circular shape, normally sphincter muscles
Surround openings, such as the mouth

Pennate

Short, diagonal fibres

Involved with movements that require great strength

Unipennate

Have their fibres arranged to insert in a diagonal direction onto the tendon

Allows great strength

Bipennate

Have two rows of muscle fibres

Facing in opposite diagonal directions

With a central tendon, like
a feather

Multipennate

Multiple rows of diagonal fibres

With a central tendon which
branches into two or more tendons

Epimysium

Dense, fibrous, tough, irregular connective tissue which surrounds the muscle belly

Perimysium

Dense, fibrous, tough, irregular connective tissue which covers the fascicle

Difference between areolar and dense, irregular connective tissue

Areolar is less resistant, tough and has less collagen

Fascicle

Bundle of muscle fibres

Endomysium

Areolar connective tissue which wraps around the sarcolemma

Sarcolemma

Plasma membrane which surrounds the muscle fibre/cell within the fascicle

Tendon

Rope-like, collagen-rich connective tissue which connects muscle to bone

Aponeurosis

Sheet-like connective tissue which connects muscle to bone

Flow of muscle contraction regarding connective tissue and tendon

Muscle fibre contracts

CT sheaths are pulled on (all 3)

Pull on tendons

Pull or move bone

Creating motion

Insertion

Bone that is moving

Origin

Bone that is not moving

Indirect muscle connection

Common form of muscle connection which are mediated through tendons and aponeuroses

Direct muscle connection

Rarer form of muscle connection where the epimysium fuses to the bone

Location of the sarcomere

In between two Z lines

Main protein of the Z line

Alpha-actinin

Myosin function (thick filament)

Shortens individual sarcomeres

Resulting in overall muscle contraction and tension.

M line function

Accessory proteins that connect to titin and stabilise myosin

3 proteins in the M line

Creatine kinase

Myomesin

C-proteins

Titin

Protein that links myosin to the Z line + M line thus acting as a stabiliser

Anisotropic band (A-band)

Entire length of the thick filament

Isotropic band (I-band)

Connects end of thick filament in sarcomere 1 to start of thick filament in sarcomere 2

Actin (thin filament) function

Interact with thick myosin filaments to generate force and drive muscle contraction

H-zone

Distance from end of thin filament 1 to start of thin filament 2 in the same sarcomere

Nebulin

Protein that connects thin filaments to the Z line

Tropomyosin

Rope-like protein that wraps around the active sites

Not allowing the myosin to bind during rest

Troponin

Protein that binds to actin, tropomyosin and actin

3 structural components of myosin

• head

• hinge

• tail

3 functions of the myosin head

• bind to active sites

• move the myofilaments

• break down ATP into ADP + P

2 structural components of the myosin neck

• regulatory light chain

• essential light chain

Function of regulatory light chain

Bind the phosphate and change the overall shape in order to play a role in contraction

Function of essential light chain

Stabilises the neck and head

Dystrophin

Links the actin to the sarcolemma and then to the ECM

Muscular Dystrophy

Caused by a mutation in the dystrophin gene which results in no dystrophin production

2 facts about the somatic nervous system

• voluntary

• supplies skeletal muscles

Anterior grey horn

Located at the anterior spinal cord

Consists of the cell bodies of somatic motor neurons

Alpha-motor neuron

Motor neuron that supplies the impulse to the skeletal muscles

Action potential

Impulse that travels from the cerebral cortex along the a-motor neuron

Opens up voltage gated sodium channels

Voltage-gated sodium channels

Causes the inside of the cell to change from negative to positive

Due to positive sodium ions flowing into the cell once the electrical threshold is met

Depolarising wave

Result of positive charges sweeping across the axolemma (sodium stage)

And the axoplasm membrane (calcium stage)

Causing the cell to become positive

Voltage-sensitive calcium channels

Once the depolarising wave travels down the cell membrane

The voltage opens the calcium channels (once it reaches the electrical threshold)

Allowing calcium to flow into the cell

Vesicle function

Store and release acetylcholine + other neurotransmitters

Acetylcholine

Neurotransmitter made in the synaptic bulb

Anterograde transport

Process of vesicle movement from creation in the cell body to the synaptic bulb

Acetyl COA

Intermediate for the Kreb’s cycle

Reacts with choline, releasing COA and synthesising ACH.

Which enzyme stimulates the Acetyl COA and choline reaction?

Choline Acetyl Transferase

Why is the breakdown of ATP so important regarding proton concentration?

The proton concentration in the vesicle compared to the cell is higher

Therefore, energy is required to go against the concentration gradient

In order to allow acetylcholine to make it in the vesicle

Importance of calcium regarding proteins

Acts as the link to the snare proteins

Which pull the vesicle towards the cell membrane of the synaptic bulb

What happens after the vesicle fuses with the cell membrane?

ACH is released and diffuses across the synaptic cleft

What is the importance of potassium leaving the cell?

Allows for repolarisation due to positive ions leaving the cell

Meaning it returns to negative during the resting period

Post-junctional folds

Located at the muscle cell membrane

Allows for the muscle cell to receive the stimulus for the motor neuron

Ligan-gated ion channel (nicotinic receptor type 1)

Proteins embedded in the cell membrane that binds neurotransmitters

Allows ions to flow to the across the membrane (closed at rest)

What occurs to open the nicotinic receptors?

ACH binds onto the receptor

Means (more) sodium diffuses into the cell and (less) potassium diffuses out of the cell

End plate potential

Process of sodium and potassium diffusion across the sarcolemma (membrane)

Brings the potential closer to threshold

Why is EPP important regarding sodium?

Allows the cell to reach the peak potential threshold

When the activation gate on the voltage sensitive sodium channel opens

And sodium flows into the cell

What is stimulated when threshold is reached?

The inactivation gate closes preventing sodium from going into the cell

Transverse Tubule

Invagination of the sarcolemma

What is stimulated in the T-tubule when the charges travel down?

Dihydropyridine receptors which are connected to the sarcoplasmic reticulum (rich in calcium)

Ryanodine receptor

Protein that allows calcium to bind onto troponin when pulled out of the sarcoplasmic reticulum by DRs

Due to positive sodium ions and into the sarcoplasm

What occurs when voltage reaches peak potential regarding potassium?

The activation gate of the potassium channels opens

Causing potassium to flow out

Resulting in repolarisation

What does calcium do to the tropomyosin in the sarcoplasm?

Calcium binds on the troponin

Which then pulls on the tropomyosin that was originally covering the active sites at rest

What occurs after tropomyosin is moved?

Hydrolysis of ATP occurs

Results in ADP + P to be bound at the myosin head

Which reattaches to actin and then phosphate is released

What occurs after organic phosphate is released?

ADP remains, and the thin filament is pulled towards the M line

What happens after ADP is displaced from the myosin head?

ATP binds onto the myosin head and detaches them from the actin

What happens to the H-zone in the contractile state?

It disappears

What happens to the Z-line in the contractile state

They get pulled by titin, meaning they get closer together

What happens to the I band in the contractile state

It decreases

When can the muscle then contract?

Once the thin filament slides past the thick filament

Continues until there is not enough ATP and calcium ions

How does the muscle relax?

Tropomyosin moves back in its original position over the active sites

Allowing the myosin heads to detach from the actin

Results in the lengthening and relaxation of the muscle when the filaments slide back over each other

What is the entire process of ATP-driven muscle contraction called?

The cross-bridge cycle