Topic 4.1: Neuromuscular Junction Flashcards
What is the purpose of the nervous system?
- To regulate the body’s internal environment as well as respond to the external environment
- Controls voluntary movement of skeletal muscle, involuntary regulation of cardiac, smooth muscle and glands
What does the nervous system consists of?
Central Nervous System (CNS)
- Brain and Spinal Cord
Peripheral Nervous System (PSN)
- Nerves extending from the spinal cord to the body
4.1.1
Label a diagram of a motor unit
Limit to dendrite, cell body, nucleus, axon, motor end plate, synapse and muscle.
4.1.4
Explain how slow and fast twitch fibre types differ in structure and function.
Muscle fibers come in two types:
- Fast twitch
- Slow twich
They are mixed together
Types:
- Slow Oxidative Fiber
- Fast Oxidative Fiber (IIa)
- Fast Glycolytic Fiber (IIb)
Motor Units
- A motor unit consists of a group of muscle fibers controlled by one motor neuron.
- The number of fibers per neuron ranges from 10 to 2000, depending on movement type.
- Gross movement = more fibers/neuron
- Fine movement = less fibers/neuron
- Muscle contraction strength depends on the number of activated motor units.
Sliding Filaments Theory
- The sarcoplasmic reticulum (SR) surrounding the T tubules release calcium ions (Ca2+) into the sarcoplasm
- The Ca2+ binds to troponin located on the actin filament, causing tropomyosin to move and expose binding sites for myosin
- The myosin contains ADP and P head now binds to actin and forms a cross-bridge.
- ADP and P are released from myosin, which causes the myosin to move. This movement is called the power stroke.
- ATP binds to myosin causing it to release the actin and reverting ATP into ADP and P.
Sacromere
- Repeating structural units of myofibril
Components of sacromere:
- Thin filaments (actin)
- Thick filaments (myosin)
- Z line at each ends
Thin filaments are made up by 3 proteins:
- Actin
- Troponin
- Tropomyosin
Z Line
- Provides an attachment for actin filaments
- Border for each sacromere, the end of each sacromere
A band
Runs the length of the myosin filament
H band/H zone
The ligher section in the middle of the sarcromere where only myosin is present
Tropomyosin
When the muscle relax, tropomyosin blocks myosin’s attachment site on actin
Muscular contraction process
- The brain and spinal cord sends an impulse to the muscle
- The impluse travels down the motor neuron and reaches a neuromuscular junction where it releases acetylcholine, which triggers the impulse in the muscle
- The impulse travels through the plasma membrane (sarcolemma) and down T tubules surrounding the myofibrils
- As the impulse passes through the T tubules, it causes the sarcoplasmic reticulum (SR) surrounding the T tubule to release calcium ions (Ca2+) into the sarcoplasm, eventually reaching the sarcomere
- The Ca2+ binds to troponin located on the actin filaments, causing tropomyosin to move and expose biding sites for myosin
- Myosin head attach to actin and form a cross bridge
- ADP and P are released from myosin, which causes the myosin to move. This movement is called the power stroke
- ATP binds to the myosin causing it to release the actin and reverting ATP –> ADP and P
- Once the impulse stops, Ca2+ is released from troponin causing tropomyosin to cover the binding sites and prevent contraction. Ca2+ returns to the SR and waits for another impulse. This is relaxion
Actin
- Thin filaments
- 2 proteins found on actin filaments = troponin and tropomyosin
Myosin
Thick filaments
Myofibril
Bundles of protein filaments, contains actin and myosin
Explain the nervous system pathway
Sensory receptors detect changes inside or outside the body and send signals via sensory neurons to the central nervous system (CNS) where this information is processed. If a response is required, the CNS sends an electrical signal via motor neurons to the effector (muscle and glands).
Name the 3 types of neurons
- Sensory (afferent)
- Motor (efferent)
- Interneurons/Relay neurons
What is a neuromuscular junction?
The connection between a motor neuron and a muscle fiber
4.1.2
Explain the role of neurotransmitters in stimulating skeletal muscle contraction.
- The presynaptic terminal (the end of a neuron) are situated over a specialized region called the postsynaptic membrane
- The presynaptic terminal is separated from the end plate/postsynaptic membrane by a space called synaptic cleft
- Synaptic cleft contains high numbers of ligand gated ions channel receptors called acetylcholine receptors
- When an action potential travels down the neuron, it causes the releases of acetylcholine into the synaptic cleft
- Acetylcholine binds to acetylcholine receptors, which causes ion channels to open to allow positive Na2+ ions to flow into the postsynaptic cell utilmately leading to a contraction
- After acetylcholine binds to its receptors, Acetylcholinesterase is released for acetylcholine removal and stops the signal and allow the muscle fiber to relax
Troponin
- Complex of three proteins that binds to actin, tropomyosin and calcium ions
- Bound very tightly to tropomyosin
- Troponin moves tropomyosin away from myosin binding site during muscle contraction
Tropomyosin
Long protein fiber that wraps around actin and covers myosin binding site on actin
Sarcoplasmic Reticulum (SR)
- Network surrounding each myofibril
- Reservoir containing Ca2+
Sarcoplasm
- Contains organelles
- Ca2+ is released from SR into the sarcoplasm which inititates muscle contraction
