INTS 10: Muscle Contraction, Bone Development, Disease and Injury Flashcards
What are the four major function of the musculoskeletal system?
- producing movement
- maintaining posture
- stabilising joints
- producing heat
What are the three types of muscle in the body?
What is muscle tissue made up of?
- muscle fibres: multinucleate structures made up of fused muscles cells or myocytes
- associated blood vessels, nerves and connective tissues
Describe skeletal muscle structure at a macroscopic level
- at a macroscopic level, skeletal muscle can be seen to be made up of bundles, or fascicles of muscle fibres
- the individual muscle fibres and fascicles are surrounded by layers of connective tissue
Describe the structure of the skeletal muscle at the microscopic level
- each muscle fibre is made up of many, many myofibrils, which are the contractile units
- the major constituents of myofibrils are actin and myosin filaments
- myofibrils are arranged into sarcomeres
- the variable appearance of different parts of the sarcomere is the reason that skeletal muscle is striated
- a network of tubules, called the sarcoplasmic reticulum, surrounds myofibrils
- transverse tubules, or T tubules also cross myofibrils and communicate with the sarcoplasmic reticulum
- neuromuscular junctions are the point of communication between the nervous system and the muscular system
What are skeletal muscles comprised of?
- skeletal muscles are made up of muscle fibres comprised of fused myocytes
- within the muscle fibres are a large number of myofibrils
- predominantly actin and myosin filaments
Why does skeletal (and cardiac) muscle have a striated appearance?
- each muscle fibre is made up of an array of myofibrils
- predominantly actin and myosin filaments
- the arrangement of these fibres into parallel array and the precise overlapping distribution of these myofilaments gives them their striated appearance
Describe sarcomere structure and what it is
- a sarcomere is the region between two Z discs
- actin filaments are attached to the Z disc
- myosin filaments are located in the central part of the sarcomere
- actin filaments are also referred to as thin filaments
- myosin filaments are also referred to as thick filaments
What parts of the muscle causes contraction?
- muscle contraction occurs through the sliding of actin filaments relative to myosin filaments
What is the chain of command for muscle contraction to occur?
Describe the spinal motor nerves involved in muscle contraction and their pathway
- motor signals are generated within the sensorimotor cortex in the brain
- signals are relayed to motor neurons within the ventral horn of the spinal cord
- axons from motor neurons exit the spinal cord via the ventral root to join mixed spinal nerves
- these contain both sensory and motor axons
- motor axons travel to muscles all around the body
- each axons will have many axon terminals which make contact with a single myocyte at the neuromuscular junction
What is the neuromuscular junction?
Describe its features and their functions
- it is the direct point of contact between the nervous system and the musculoskeletal system
- the axon terminal:
- formed when the myelinated motor neuron axon becomes unmyelinated
- contains vesicles containing acetylcholine (neurotransmitter)
- contain voltage-gated calcium channels
- a synaptic cleft is between the presynaptic membrane of the axon terminal and the postsynaptic membrane (the plasma membrane of the muscle fibre)
- acetylcholine binds cation receptors on the postsynaptic membrane
- the enzyme acetylcholinesterase is located on the postsynaptic membrane
Describe the key stages of synaptic transmission at the neuromuscular junction
- An axon potential is propagated along a motor neuron axon to the axon terminal
- At the axon terminal, the action potential opens voltage-gated calcium channels and this leads to calcium ion influx into the axon terminal
- This causes exocytosis of vesicles containing acetylcholine
- Acetylcholine is released into the synaptic cleft
- They bind to receptor cation channels on the postsynaptic membrane
- this leads to the opening of sodium ion channels which allows the influx of sodium ions exceeding the efflux of potassium ions
- this causes depolarization of the postsynaptic membrane - Local current flows between depolarised endplate region and adjacent membrane
- An axon potential is triggered and is spread along the muscle fibre membrane
- Finally, acetylcholine is broken down by acetylcholinesterase
Describe how muscle is activated once the action potential has been generated
- the action potential is propagated through the transverse tubule (T tubule) network
1. The action potential propagates along the muscle fibre into the muscle cell
2. This lead to the release of calcium ions from the sarcoplasmic reticulum (SR)
3. The calcium ions bind to troponin on actin filaments
4. This leads to a conformational change in the position of tropomyosin which moves to unblock myosin-binding sites
5. This allows cross-bridges to form - which enables the generation of force through the sliding of actin filaments
6. While an action potential is being generated, this process occurs continuously with Ca2+ being actively pumped back into the sarcoplasmic reticulum
What is the sliding filament theory?
- this is that muscle contraction results from a shortening of the sarcomere
- occurs when actin filaments move towards the centre of the sarcomere, drawing the associated Z discs toward one another
- this event occurs in a coordinated fashion across the whole muscle to result in force generation