muscles

There are thee types of muscle, voluntary(Skeletal), Involuntary(smooth and cardiac) muscle. Skeletal muscle makes up most of our muscle. Indeed, its about 40% of our body mass. Its attached to bones via tendons. The contraction of the muscle brings about movement and is under conscious control. Cardiac muscle ia only found in the heart. Smooth muscle is found in the walls of the gut and blood vessels. Neither carciac or smooth muscle is under conscious or voluntary control. 

The contraction of muscles is under control of the nervous system via the motor end plate, which together, with the muscle cell makes the neuromuscular junction. There are many neuromuscular junctions along a muscle. This allows the muscle to contract simultaneously and forcefully. 

Key features in operation of neuromuscular junction 

1. Motor end plate with vesicles containing neurotransmitter

2. Neurotransmitter diffuses across gap 

3. sarcolemma contains receptors for neurotransmitter 

4. depolarisation spreads to T-System

5. Ca2+ ions are releases from sarcoplasmic reticulum

6. Ca2+ ions bind to troponin and exposes muyosin binding sites on the actin allowing contraction. 

There is a minimum depolarisation required to bring about an action beyond the nerve, However, at the neuromuscular junction, stimuli above the threshold bring about an increased or graded response in a whole muscle

• Stimulus Intensity; If the stimulus is above threshold, there is no contraction of the muscle, If the threshold is exceeded, contraction increases with increased stimulus intensity, i.e. there is a larger response. Individual fibres have an " all or nothing response", but the more fibres that are stimuated, the greater the contraction.
• Summation; If an impulse arrives before the previous contraction has faded, a new contraction is started whch is added to the effect of the previous one. So the overall contraction is greater than a single contraction would jave been if the stimuli were further apart
• Tetanus; Repeated Large stimuli result in the summation of the individual stimuli and produces a sustained and powerful contraction
• Fatigue; If stimulation is continued for a period of time, the contraction may fade as the muscle becomes fatigued. 

Transmitter substance

• neuromuscular junction-yes, Acetycholine
• Synapse; Yes-Acetycholine, Dopamine and Serotonin

Cleft or Gap

• Neuromusuclar Junction-Yes 
• Synapse-Yes, Synaptic Cleft

Effect of Action Potential Arriving

• Neuromuscular Junction-Migration of Vesicles to and fusion with membrane so release Nt
• Synapse-Same as NJ 

Receptors in Post Synaptic Membrane

• Neuromuscular Junction-Yes, they accept Ach(Acetycholine)
• Synpase-Yes, they accept appropriate NT

Enzyme to remove Trasmitter

• Neuromuscular Junction-Yes, Cholinesterase
• Synpase-Yes, as appropriate

Effect of Depolarisation

• Neuromuscular Junction- Spread via T-System to sarcoplasm(sarcoplasmic reticulum) causing release of Ca2+ and muscle contraction
• Synpase- Initiates an action potential in post synaptic membrane

• Skeletal of voluntay muscle is made up of many contractile fibres called myofibrils
• also known as the striated or striped muscle because of its physcial appearance
• The contraction of the muscle is under conscious control, and moves the bones of the skeleton across joints
• Its cylinderated and multinucleated
• Skeletal muscle consists of fibres surrounded by a membrane or sarcolemma, within which there are several nucelo, many mitochondria and an extensive sarcoplasmic reticulum

Muscle is made up of 5 layers in total

• Whole muscle (organ)
• Muscle Fasicle (portion of muscle)
• Muscle fiber (Muscle cell) 
• Myofibril (Specialised Cellular Structure)
• Thick and Thin Filament (Cytoskeletal element)

Sarcolemma

• surrounds striated muscle fiber cells 
• Acts as a barrier between extracellular and intracellular compartments defining the individual muscle fiber from its surroundings. 

Sarcoplasm 

• Cytoplasm of muscle fiber 
• Contains ATP and Phosphagens as well as enzymes involved in many metabolic reactions 

Sarcomere

• Fundamental unit of a muscle and is responsible for contraction
• Composed pf thick and thin filaments bordered by 2 Z discs

Myofibrils

• Thin and Thick Filaments
• Made up of Sarcomeres
• Function is to perform muscle contraction via the sliding-filament model

Nucleus

• Contain DNA combined with histones and other proteins to from chromatin

T-Tubules

• conduct impulses from surface of cell (Sarcolemma) down into cell and to another structure called Sarcoplasmic Reticulum

Sarcoplasmic Reticiulum(SR)

• membrane bound structure found within muscle cells 
• stores Ca2+ ions 

Triad Muscle 

• Translates action potential from plasma membrane to sarcoplasmc reticulum effecting calcium flow into cytoplasm

Myofilaments 

• made up of proteins, Actin (thin filaments)and mysosin (Thick Filaments)

Thick Myosin Filaments; Myosin molecules consist of a long filament with a head projecting from it. Myosin molecules are bundled together to produce thick bundles with equal numbers of myosin heads in each half 

Thin Actin Filaments; Actin consists of many globular protein units linked together into long chains. Two of these actin chains are twisted together with a tropomyosin molecule lying in the groove between them. At intervals, troponin molecules bind to the actin, covering the binding sites of the myosin heads. 

The regular Arrangement of these molecules produces a banding pattern typical of voluntary muscle that can be seen with the light microscope and electron microscope. 

Muscle relaxed

• The Sarcomere is 2.5 micrometeres compared the 2.0 micrometres when contracted

Muscle Contraction

• The actin filaments are pulled across the myosin filaments towards the middle of it. consquently the I Bands  H-Bands get smaller while the A-Band remains the same. 

A-Atp binds to mysoin head and has detached from Actin

B- ATP is hydrolysed (broken down) into ADP/Pi by myosin. Myosin is ready to bind Actin. It binds Actin

C- Pi is released from myosin and this will cause the power stroke(Part D) which moves the Actin Filament 

D-Mysoin Head has moved, The actin filament has moved towards the M-Line. ADP is relased from mysoin head. ATP can now binf the myosin head and we start again

Muscles are effector organs that respond to simulation by contracting. Skeleteal miscle contracts to bring about tbe movement of bones across joints. For example, when the bicep muscle contracts, the arm is flexed. In contrast, when the tricep muscle contracts, thr arm is extended. Contraction of the Smooth Muscle in blood vessels causes the constriction of the vessel (Lumen size is reduced)

ATP is needed for muscle Contraction. It is needed to break the cross bridges or links between myosin heads and Actin, and to reset the myosin head in its pre-power stroke form so that it is able to bring about further muscle contraction. Very little ATP is present in the muscle fibers, only enough for a few seconds contraction. ATP must therefore be resupplied if contraction is to continue. This is achieved by a combination of Aerobic Respiration, Anaerobic Respiration and its regeration from Phosphocreatine found in the muscle cells 

Myglobin in muscle; Serves as a local oxygen reservoir that can temporarily provide oxygen, when blood 02 delivery is insufficent during periods of intense muscular activity. 

High levels of CO2 in active muscle; Makes haemoglobin release oxygen more easily. Haemoglobin gives up mote oxygen. Makes 02 more available at times of high demand. 

What is Glycogen. How does it help in ATP Production?

Its a polymer made up of glucose molecules, its branches allow enzymes access for respiration. 

Aerobic Respiration; The sarcoplasm contains many mitochondria (only take part in aerobic respiration) that contain many enzymes responsible for completion of aerobic respiration- the release of energy by the complete breakdown of glucose into oxygen and water. Respiration rates are high in active muscle and the supply of 02 must be maintained if aerobic respiration is to continue 

Anaerobic Respiration; ensures the resupply of NAD so that glycoloysis and limited ATP production can continue. This cannot continue indefinitely and relatively small amounts of ATP are produced

ATP = ADP + Pi 

Fast twitch fibres; contract quickyly and forcibly for short periods because they fatigue easily. They contribute most to muscle strength but can only sustain relatively short bursts of anaerobic activity. Being adapted for intense or explosive excercise, they are found in the biceps. Sprinters have about 80% of their muscle as Fast twitch fibres

Slow twich Fibres: contract slowly and are less powerful. There cytoplasm is rich in mitochondria and myoglobin and the muscle is supplied by a dense capillary network. They are adapted for endurance and aerobic conditions. The calf muscle and muscles of the lower back are predominantly made up of slow-twitch fibres, There constantly contracting to maintain posture. Endurance athletes, such as distance and marathon runners, have about 80% of their muscle as slow twitch fibres

• Contain a store of Phosphocreatine; energy is reserved to regenerate ATP when muscle becomes anaerobic
• Contain lots of Enzymes for anaerobic respiration; Use a lot of ATP energy in short burst conditions, become anaerobic more quickly
• They have more myosin Filaments; Permits more rapid and forceful contraction
• They appear white or Paler(as in chicken or turkey breast meat)- Fewer cappilaries, they have  a lower blood supply
• They have Fewer Mitochondria; Do not require as many mitochondria per cell as they are less reliant on aerobic respiration
• They have a large store of Glycogen; Glycogen can be broken down to quickly generate glucose for glycolysis

• They contain Large amounts of Myoglobin; to attract 02 for aerobic respiration. Myoglobin has a high affinity (attraction) for oxygen 
• They contain numerous mitochondria; ATP production through aerobic respiration
• They have a smaller store of Glycogen; less needs to be stored as good blood supply delivers all glucose required
• They have a good blood supply; To deliver O2 and glucsoe for aerobic respiration
• They appear darker (red as in chicken of turkey leg meat) =; because of the good blood supply.