BIOSCI 107- MODULE 7 muscles

Skeletal muscle is

Voluntary, striated, single long cylindrical cells, multiple peripheral nuclei

How long can muscle fibres get?

Up to 35cm

Muscle cell / fibre is made up of

myofibrils

Myofibrils are made up of

contractile units called sarcomeres

z disc

edge of sarcomere where thin filaments join together

A band

Length of thick filaments

I band

Area with only thin filaments

H zone

Area with only thick filaments

M line

Middle of H zone

Where are T tubules

around every end of A band (2 per sarcomere)

What do T Tubules do?

Allow Action potentials to be carried deep into the muscle

Sarcoplasmic reticulum (SR)

calcium storage, close to T tubules to allow fast reaction, specialised ER

Myosin has

A head and tail, tail holds and head binds to actin, polarised fashion because heads at end tails in middle

A myosin head move via

the hydrolysis of ATP into ADP and inorganic phosphate

What does titin do?

Anchors thick filament to the Z line

Actin has

myson binding sites attached, with tropomyosin looped around and troponin attached

First step in cross bridge cycle

Cross bridge formation

Second step in cross bridge cycle

Power stroke

Third step of cross bridge cycle

detachment

fourth step of cross bridge cycle

energisation of myosin head

Formation of cross bridge

Myosin head in activated state binds to actin (only when Ca has binded to troponin causing tropomyosin to move)

Powerstroke

ADP is released from myosin head, changing it from high energy state to low energy state and pulling actin over (shortening of sarcomere)

Detachment

New ATP molecule binds to myosin head, the actin to myosin bond is weakened and myosin detaches

Energisation of myosin head

ATP molecule is hydrolysed into ADP and P

How is calcium released from the SR?

An action potential, calcium leaves through calcium channels

How does calcium return to the SR?

Ca2+/ATPase uses active transport for Ca to return, constantly working

What happens to Ca when there is no muscle contraction from an action potential?

The concentration of Ca in cytoplasm decreases as none is being released, but it is all being reabsorbed by SR

Isotonic movement

Shortening, constant tension, velocity variable

Isometric movement

No shortening, length constant, tension variable

A stretched muscle has less

tension

Maximum active force is dependant on the degree of actin and. myosin overlap. At the optimum overlap of myosin, there is:

Maximum tension

A longer stretched muscle has less

cross bridging because myosin heads cant reach

An understretched muscle has

interferring and overlapping

Total tension=

Active + passive force

Passive force increases as

muscle stretches

Active force developed via

cross bridge cycling, dependant on actin-myosin overlap

At a muscles optiumum length, what kind of tension is present?

Only active force

What is the name of this step (in Excitation-contraction coupling):

AP travels down a motor neuron to the axon terminal, where voltage gated Ca channels open, allowing Ca to enter. The vesicles holding ACh are triggered and fuse with the terminal membrane, releasing ACh into the neuromuscular junction (synaptic cleft)

1. ACh released into neuromuscular junction

What is the name of this step (in Excitation-contraction coupling):

The binding of ACh to the receptors on the muscle end plate causes the opening of the ligand gated ion channels, which allow movement of ions into the muscle (mostly Na) making it less negative (End Plate Potential)

2. Activation of ACh receptors

What is the name of this step (in Excitation-contraction coupling):

If sufficient number of ligand gated ion channels are opened, the end plate potential reaches threshold and the voltage gated Na channels open, an AP is triggered then propagated along the sarcolemma into the T-tubule system

3. Muscle Action potential is triggered

What is the name of this step (in Excitation-contraction coupling):

AP is conducted down the T-Tubules coming in close contact with SR, results in voltage gated Ca channels in the SR opening up, Ca is released into the cytosol.

4. calcium is released from SR

What is the fifth step of Excitation-contraction coupling after calcium is released form the SR?

Ca binds to troponin, then cross bridge cycle happens

When does a contraction end

When Ca levels fall as it is actively pumped back into SR by Ca/ATPase pumps, tropomyosin moves back over myosin binding sites

Three sources of ATP for muscle metabolism

Creatine phosphate, anaerobic glycolysis, Aerobic metabolism

Creatine phosphate

Breif, no O needed, creatine phosphate + ADP = 1 ATP, acts as an ATP store

Anaerobic glycolysis

no O, fast but inefficient (2ATP), energy from glucose, build up of H+ limits to 120s of use, about 10-30s maximum effort

Aerobic metabolism

Endurance, efficient but slow (32ATP), requires Oxygen and therefore needs good blood flow, energy source from glucose, pyruvic acid, fatty acids, amino acids

Regulation of force is dependant on:

Rate of stimulation, number of motor units stimulated

Motor unit

a motor neuron + all the muscle fibers it innervates

Rate of stimulation

A single stimulus = twitch, high stimulation the twitches move together = fused tetanus, temporal summation = unfused tetnaus

Recruitment of motor units

Everyday activities require only type 1 motor units, any further more motor units recruited and tension increases, more efficient to use smaller less fatigued motor units first

Blood starts in the ——-, then moves to the ———-

right atrium, right ventricle

After the right ventricle, blood flows outwards to the lungs via the —— and then back in by the ——

Pulmonary artery, pulmonary veins

After the blood is brough back from the lungs by the pulmonary veins, it travels to the ——- then the ———- then out through aorta

left atrium, left ventricle

Majority of cardiac muscle is found in the ——- because it has to ovecome resistance to pump blood around body

left ventricle

Heartbeat initiates at

right atrium

Ventricular muscle cells are branched, with many —— junctions to form sheets

gap

Ventricular muscle has 1 t tubule placed on ————, and has a more developed —— than skeletal muscle

the z line, SR

Ventricular muscle has ———— which act as the glue between two cells

intercalated discs

Intercalated discs contain ——- which prevent cells from separating during contraction, and —— junctions which allow the action potential to be carried from one cell to the next quickly.

desmosomes, gap

Intercalated discs in muscle cells allow ——— contraction, unlike skeletal muscle where fibres are recruited via the motor nerves

coordinated

Cardiac muscle is myogenic, meaning

contraction starts within the muscle

RMP of cardiac muscle

-90mV, more negative than skeletal muscle

Ventricular muscle Action potentials have a fast —— caused by voltage gated Na channels opening

depolarisation

Plateau phase

during a ventricular muscle AP, voltage gated Calcium channels (extra) slowly open and slowly close, allowing longer AP

Action potentials in ventricular muscle are less likely to —- than skeletal muscle?

join together (tetanus) due to the long refractory period inhibiting release of new APs

Where is LTCC = L-type voltage gated calcium channel (I CaL) found?

T-tubule of cardiac muscle, allows calcium into cytoplasm between T tubule and SR

Where is RyR = ryanodine receptor (Calcium ligand gated channel) found?

In SR of cardiac muscle, allows Ca to bind and release Ca into the cytoplasm

What is the importance of Na/Ca exchanger in the cardiac muscle?

Uses the gradient of Na to remove Ca from cytoplasm into T tubule

Cardiac Muscle: excitation-contraction coupling

AP opens voltage gated type-L calcium channel in t tubule, releasing Ca into cytoplasm towards SR, triggering opening of (RyRa) ligand gated calcium induced calcium release into cytoplasm

How is calcium removed in cardiac muscle?

SR Ca2+-ATPase, or sarcolemmal Na+/Ca2+ exchange

Trigger for Ca2+ release from SR in skeletal muscle?

Voltage gated Na+ channel

Trigger for Ca2+ release from SR in cardiac muscle

ligand gated calcium induced calcium release

How to create a graded contraction in skeletal vs cardiac muscle?

recruitment of more fibres in skeletal, increase Ca in cardiac

Regulation of Cardiac Output (CO) =

SV x HR

Heart Rate (HR) is set by

the pacemaker cells in the sinoatrial node, can be modified by neurotransmitters

pacemaker cells have an unstable

RMP, spontaneously firing

Sympathetic nervous system

increases heart rate (force of contraction) by noradrenaline

Parasympathetic nerves

decrease heart rate by releasing ACh

SV can be increased by

noradrenaline, increased rate of firing, increased stretch of ventricles

Single unit smooth muscle

primarily found in the walls of hollow organs and viscera, sheets of cells that are electrically coupled and act in unison

Multiunit smooth muscle

Tissues made of discrete bundles of independant cells, innervated, contract in response to nerve, eye

What do smooth muscle cells have instead of T tubules?

Caveolae, wrinkles to increase surface area

Dense bodies in smooth muscle anchor ?

actin to sarcolemma

Contractile proteins in smooth muscle

actin and myosin filament

Initiation of contraction in smooth muscle

primarily due to voltage gated Ca2+ channels allowing Ca into cell, less reliant on SR stores

Calcium regulation in smooth muscle

Comes from outside cell, regulation via hormones, neurotransmitters and ions

Calmodulin

floats in cell until its activated by Ca, then binds to MLCK which activates and turns on myosin heads

Contraction in smooth muscle ends when

MLCP dephosphorylates the myosin light chain

Innervation of smooth muscle fibers

autonomic nerve fibers branch and form diffuse junctions with underlying smooth muscle fibers

Smooth muscle response to stretch

counteracts / contracts due to stretch activated calcium channels, overtime slowly adapts to change in length via calcium dependant K+ channels hyperpolarising membrane potential

Calcium source in smooth muscle

Outside cell and SR via IP3