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6.3 Skeletal muscles - Coggle Diagram
6.3 Skeletal muscles
lesson 1: muscle types and structure
smooth muscle
also called involuntary or unstriated as don't contain coloured banding
walls of blood vessels, gut etc
skeletal muscle
also known as voluntary or striated muscle as it contains light and dark bands
cardiac muscle
found exclusively in the heart
striated - possesses contractile units (sacromeres)
rhythmic contractions not under voluntary control
skeletal muscles
Muscles act in antagonistic pairs against an incompressible skeleton
muscles are attached to the skeleton via tendons
bring about movement
examples of antagonistic muscles
biceps and triceps
quadriceps and hamstrings
the gross structure of skeletal muscles
individual muscles consist of millions of tiny fibre-like structures called myofibrils
myofibrils are grouped into individual muscle fibres which in turn form bundles
these bundles group to form muscles
ultrastructure of a skeletal muscle
A muscle fibre is a highly specialised cell-like unit.
Muscle fibres are not referred to as cells because they contain many nuclei.
Each fibre contains an organised arrangement of contractile proteins in the cytoplasm which is called the sarcoplasm
cell surface membrane is called the sarcolemma
the sarcolemma has many deep tube like projections that fold to form its outer surface known as t-tubules which run close to the sarcoplasmic reticulum
structure of myofibrils
made of protein and transfer chemical energy into movement
made of two types of protein filament: thick myosin filament and thin actin filament
actin
the thin filament
it contains globular actin proteins arranged in two long chains twisted around each other.
The actin filament is associated with tropomyosin.
myosin
The myosin filament contains two types of protein: A fibrous protein arranged into a filament of up to several hundred molecules forming the tail; A globular protein formed into bulbous structures at one end forming the head.
z-disc
Attachment for actin filaments.
A-band
areas where only myosin filaments are present and
areas where myosin and actin filaments overlap
H-band
Only thick myosin filaments are present
m-line
Attachment for myosin filaments.
I-band
Only thin actin filaments presents
sarcomere
The section of myofibril between two Z lines
tropomyosin
Proteins associated with actin that are important for muscle
contraction.
lesson 2: siding filament theory - muscle contraction
sliding filament model
1) action potential arrives as neuromuscular junction. acetylcholine diffuses across neuromuscular junction which causes the sarcolemma to depolarise, action potential travels down t-tubules causing voltage gates calcium ion channels to open
2) calcium ions released from the sarcoplasmic reticulum
3) calcium ions bind to tropomyosin stimulating them to change shape
4) this causes tropomyosin to change position on actin filament
5) myosin binding sites are exposed
6) globular myosin heads bind to myosin binding sites forming a cross-bridge between the two filaments
7) myosin head spontaneously bends - power stroke (releasing ADP and Pi)
8) actin filament pulled closer to centre of sarcomere - muscle contracts a short distance
9) ATP binds to myosin head - changes shape and releases from actin
evidence for sliding filament theory
when a muscle is contracting:
H bands narrow
I bands narrow
z-lines get close to the sarcomere/the sarcomere shortens
A-bands remain the sane - myosin does not shorten
muscle relaxation
calcium ions actively transported back into the sarcoplasmic reticulum
tropomyosin moves back to block myosin sites on actin
why is ATP needed for muscle contraction
energy needed for the active transport of calcium ions back into the sarcoplasmic reticulum
binds to myosin head causing it to detach from actin filament - ATP is hydrolysed and the energy released repositions the myosin head
where does the ATP/energy come from
resting muscles have a small amount of ATP stored - only lasts for a few seconds
muscle fibres produce more ATP through aerobic respiration however this is slow - may use anaerobic respiration as this is faster but still not fast enough - phosphocreatine system used
phosphocreatine system
phosphocreatine is a molecule stored by muscles and can be used for rapid production of ATP
phosphate ion from phosphocreatine is transferred to ADP
ADP + phosphocreatine --> ATP + creatine
allows muscles to contract until enough ATP is produced via respiration
phosphocreatine stores deplete quickly
described as anaerobic (doesn't require oxygen) and analactic (doesnt produce lactic acid)
occurs in the cytosol
lesson 3: types of muscle fibre
different muscle fibre types contain different limited amount of phosphocreatine
two types of muscle fibres found in muscles
1) fast twitch muscle fibres
found in muscles such as the biceps which carry out short periods of intense work
source ATP mainly through phosphocreatine system and anaerobic respiration
structure
less extensive blood supply
little/no myoglobin
large stores of phosphocreatine
Large stores of glycogen for rapid glucose production via glycogenolysis
Fewer mitochondria
Thicker and more numerous myosin filaments
Higher concentration of enzymes for anaerobic respiration
action
contractions are quick and powerful
Contractions are not sustained due to build-up of lactic acid leading to fatigue
2) slow twitch muscles fibres
found in muscles that maintain posture
main source of ATP is phosphocreatine system initially but then aerobic respiration
structure
Many mitochondria for aerobic respiration
Large stores of myoglobin
Rich supply of blood vessels to supply oxygen and glucose
action
Contractions are slower and less powerful
Sustained contractions
