Structure of skeletal muscles Flashcards
What are muscles?
Muscles are effector organs that respond to nervous stimulation by contracting and so bring about movement
What are the three types of muscle in the body?
Cardiac, smooth and skeletal
What are individual muscles made up of?
Millions of tiny muscle fibres called myofibrils
What are the two types of protein filament that make up myofibrils?
Myosin and Actin
What is the difference between myosin and actin?
Actin is thin and consists of two strands twisted around one another whereas myosin is thick and consists of long rod-shaped fibres with bulbous heads that project to the sides
Why do myofibrils appear striped?
They have alternating light and dark bands
What are the light bands called and why are they light bands?
The light bands are called isotropic bands and they appear lighter because the actin and myosin filaments do not overlap in this region
What are the dark bands called and why are they dark bands?
The dark bands are called anisotropic bands and they appear darker because the actin and myosin filaments overlap in this region
What is at the centre of each anisotropic band?
A lighter-coloured region called the H-zone
What is at the centre of each isotropic band?
A line called the Z-line
What is the distance between the adjacent Z-lines?
A sarcomere
When the muscle contracts, what happens to the sarcomeres?
They shorten and the pattern of light and dark bands changes
What are two other important proteins that are found in muscle?
Tropomyosin and globular protein (troponin) involved in muscle contractions
What are the two types of muscle fibre?
Slow-twitch and fast twitch
What sort of contractions do slow-twitch muscle fibres produce?
They contract more slowly and provide less powerful contractions over a longer period
What sort of work are slow-twitch muscle fibres adapted to?
Endurance work such as running marathon
What type of respiration do slow-twitch muscle fibres carry out and why?
Aerobic respiration in order to avoid a build up of lactic acid which would cause them to function ineffectively
How are slow-twitch muscle fibres adapted to their role?
They have a large store of myoglobin, a supply of glycogen to provide a source of metabolic activity, a rich supply of blood vessels to deliver oxygen and glucose and they have many mitochondria to produce ATP
What sort of contractions do fast-twitch muscle fibres produce?
They contract more rapidly and produce powerful contractions but only for a short period of time
What sort of work are fast-twitch muscles adapted to?
Intense exercise such as weightlifting and sprinting
How are fast twitch muscles adapted to their role?
They have thicker, more numerous myosin filaments; A high concentration of enzymes involved in anaerobic respiration and a store of phosphocreatine that can rapidly generate ATP from ADP in anaerobic conditions
What is a neuromuscular junction?
The point where a motor neurone meets a skeletal muscle fibre
Why are many neuromuscular junctions present throughout a muscle?
Rapid muscle contractions are often essential for survival
What is a motor neurone?
A neurone that transmits action potentials from the central nervous system to an effector
What is a motor unit?
Where all muscle fibres are supplied by a single motor neurone and act together as a single functional unit
How does a motor unit allow greater control over the force of contraction?
If only a slight force is required then only a few units are stimulated, but if a more powerful force is necessary then a larger number of units can be stimulated
What happens when a nerve impulse is received at a neuromuscular junction?
The synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine which diffuses to the postsynaptic membrane, altering its permeability to sodium ions, which rapidly enter and depolarise the membrane
What prevents the over-stimulation of the muscle?
The acetylcholine is broken down into choline and ethanoic can acid by acetylcholinesterase, which diffuse back into the neurone where they recombine using energy provided by mitochondria
