Effectors MUSCLES

3 Types of Muscles

• Smooth Muscle- Contracts without conscious control. Its found in walls of internal organs (apart from the heart) e.g. Stomach, intestine and blood vessels.

• Cardiac Muscle- contracts without conscious control, like smooth muscle but its only found in the heart.

• Skeletal Muscle- is the type of muscle you use to move, e.g. Biceps and triceps move the lower arm.

How do muscles work?• Skeletal muscles cause the skeleton to move at joints. • They are attached to the skeleton by tendons, which transmit the

muscle force to the bone and can also change the direction of the force.

• Tendons are made of collagen fibres and are very strong and stiff (i.e. not elastic).

• The skeleton provides leverage, magnifying either the movement or the force.

• Muscles are either relaxed or contracted. • In the relaxed state muscle is compliant (can be stretched)• In the contracted state muscle exerts a pulling force, causing it to

shorten or generate force. • Since muscles can only pull (not push), they work in pairs called

antagonistic muscles. The muscle that bends (flexes) the joint is called the flexor muscle, and the muscle that straightens (extends) the joint is called the extensor muscle.

Skeletal Muscle

• Made up of large bundles of cells called muscle fibres.

• Sacrolemma folds inwards across the muscle fibre, these folds are called transverse (T) tubules, they help spread electrical impulses throughout the sarcoplasm.

The structure of skeletal muscle

• A single muscle contains around 1000 muscle fibres running down the length of the muscle and joined together.

• Each fibre is actually a single muscle cell. They contain many nuclei, their cytoplasm is packed full of microfibrils, bundles of protein filaments that cause contraction, they also contain ATP for contraction.

Myofibirls Structure

Sliding Filament Theory

• Muscle contraction is explained by the sliding filament theory. This is where myosin and actin filaments slide over one another to make sarcomeres contract. The simultaneous contraction of lots of sacromeres means the myofibrils and muscle fibres contract. Sacromeres return to their original length as the muscle relaxes.

The sarcomere – structure to functionHansen and Huxley realized that the interlocking structure of the thick and thin filaments allows them to slide past one another. This reduces the length of the sarcomere.

At the same time the banding pattern of the sarcomere changes; light bands, formed by actin, shrink as the filaments become more interlocked.

contraction

The sliding filament theory

Practice Questions

• At rest, the actin-myosin binding site is blocked by tropomyosin, held in place by troponin

• Myosin heads cannot bind to actin filaments

actin filament

troponin

tropomyosin

myosin complex

myosin filament

ADPPi

• Ca2+ binds to troponin, changing its shape• Tropomyosin is pulled out of the binding site• Myosin head can bind – bond is an actin-

myosin cross bridge

Ca2+

• Ca2+ activates ATPase, breaking down ATP to ADP + Pi

• Energy provided moves myosin head, pulling acting filament along in a ratchet motion

• Free ATP binds to head, changing shape• Actin-myosin cross bridge breaks• ATP is hydrolysed, and head returns to original

shape

• With continued stimulation the cycle is repeated

• If stimulation ceases, Ca2+ is pumped back into sarcoplasmic reticulum

• Troponin and tropomyosin return to original positions

• Muscle fibre is relaxed

Energy for Muscle Contraction

So much energy is needed when muscles contract that ATP gets used up very quickly. ATP has to be continuously generated so exercise can continue-this happens in 3 main ways

Aerobic Respiration

• Most ATP is generated via oxidative phosphorylation in the cell’s mitochondria. Aerobic respiration only works when there’s oxygen so its good from long periods of low-intensity exercise, e.g. A long walk.

Anaerobic Respiration• ATP is rapidly made by glycolysis. The end

product of glycolysis is pyruvate which is converted to lactate by lactate fermentation. Lactate can quickly build up in the muscles and cause muscle fatigue. Anaerobic respiration is good for short periods of hard exercise, e.g a 400m sprint.

ATP- Phosphocreatine (PCr) System

• ATP is made by phosphorylating ADP. The equation for this is

ADP + PCr ATP + Cr (Creatine)

• PCr is stored inside cells and the ATP-PCr system generates ATP very quickly. PCr runs out after a few seconds so its used during short bursts of vigorous exercise, e.g. A tennis serve. The ATP-PCr system is anerobic and its alactic.

Slow and Fast Twitch Muscle Fibres

• Skeletal muscles are made up of two types of muscle fibres- slow twitch and fast twitch. Different muscles have different proportions of slow and fast twitch fibres.

Slow Twitch

Fast Twitch

SLOW TWITCH FIBRES FAST TWITCH FIBRES

Appearance __________ __________

Characteristics •Rich in _________ (a red-coloured protein that stores oxygen).•A rich supply of blood vessels to deliver _________ and glucose.•Numerous _____________to produce ATP.•A supply of glycogen to provide a source of metabolic energy but can also use fat stores.•High concentration of enzymes that regulate ________ cycle.

•Low ____________ content (can’t store much oxygen).•Thicker and more numerous ____________ filaments.•A high concentration of enzymes that control _____________.•Few ________________ and blood vessels.•Has a store of _______________ to rapidly generate ATP.•Has a higher concentration of _____________ than slow twitch to hydrolyse ATP quickly.

How they work •Muscle fibres contract __________•Produce less powerful contractions that can be sustained over a long period of time – high resistance to ____________•Energy released through __________ respiration.

•Muscle fibres contract _________•Produce powerful contractions ONLY over a short period of time – _______ resistance to fatigue.•Energy released through ____________ respiration using glycogen (stored glucose).

Uses •Good for _____________ activities E.g. Maintaining posture, _________________________

•Good for short bursts of _________and power E.g. Eye movement, ____________.

SLOW TWITCH FIBRES FAST TWITCH FIBRESAppearance Red White

Characteristics •Rich in myoglobin (a red-coloured protein that stores oxygen).•A rich supply of blood vessels to deliver oxygen and glucose.•Numerous mitochondria to produce ATP.•A supply of glycogen to provide a source of metabolic energy but can also use fat stores.•High concentration of enzymes that regulate Krebs cycle.

•Low myoglobin content (can’t store much oxygen).•Thicker and more numerous myosin filaments.•A high concentration of enzymes that control glycolysis.•Few mitochondria and blood vessels.•Has a store of phosphocreatine to rapidly generate ATP.•Has a higher concentration of ATPase than slow twitch to hydrolyse ATP quickly.

How they work •Muscle fibres contract slowly.•Produce less powerful contractions that can be sustained over a long period of time – high resistance to fatigue•Energy released through aerobic respiration.

•Muscle fibres contract rapidly.•Produce powerful contractions ONLY over a short period of time – low resistance to fatigue.•Energy released through anaerobic respiration using glycogen (stored glucose).

Uses •Good for endurance activities E.g. Maintaining posture, long-distance running.

•Good for short bursts of speed and power E.g. Eye movement, sprinting.