A2/A-level Biology - Muscle Movement (1)

• Muscles can work in antagonistic pairs.
• In order for smooth movement at a muscular junction, such as the elbow, two muscles must be involved.
• A muscle can only produce a force when it contracts, so if there are two muscles, the bone at the joint can only move smoothly if one muscle contracts, and the other relaxes.

• The elbow joint is an example of a synovial joint where a lot of movement occurs.
• These muscles do operate as an antagonistic pair, and although they cannot actively contract, they re-extend when pulled by the opposite antagonistic muscle. 

• Synovial joints produce synovial fluid (from the synovial membrane) which acts as a lubricant for the joint.
• This is there to ease the movement of the bones at the joint.
• At the terminus of each bone, there is cartilage which has the purpose of reducing friction due to movement of the bones.
• The protective ligament casing keeps the bones together when they move.

• Muscle action is controlled by the nervous system.
• There are motor neurons connected to muscles cells over a junction called a neuromuscular junction.  

• A nerve impulse which arrives at the neuromuscular junction is transmitted across the gap via a method very similar to that of the cholinergic synapse and a small chain of events eventually stimulates a contraction of the muscle:

1. - An electrical impulse arrives at the terminus of the motor neurone, where there are vesicles containing the neurotransmitter acetylcholine.
   - The action potential triggers the vesicles to move the neurone membrane and fuse, releasing the chemical across the neuromuscular cleft.
    
2. - The acetylcholine binds to receptors on the muscle fibre membrane, which triggers a depolarisation in the membrane.
    
3. - The wave of depolarisation travels along the muscular membrane, until it reaches a tubule of the T-system, called a T-tubule, a deep cleft in the sarcolemma (muscle fibre membrane).
    
4. - The depolarisation in the T-system sends out a message causing sarcoplasmic reticulum (a specialised form of endoplasmic reticulum found only in muscle fibres) to release calcium ions (Ca2+) from its vast stores.
    
5. - The calcium moves towards proteins embedded in the muscle, and binds to those proteins, causing a contraction.
    
6. - Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid, so that the neuromuscular junction is not constantly activated, but is only active when a new impulse arrives at the junction.

When it comes to A2/A-level Biology, can you recognize the differences and similarities between the neuromuscular junction and the synapse?

Similarities

• Both use acetylcholine as the neurotransmitter (provided the synapse in question is the cholinergic synapse).
• The enzyme acetylcholinesterase is involved in both for breaking down acetylcholine to maintain a concentration gradient and prevent constant impulses being transmitted.
• Both are triggered by the arrival of an action potential on the pre-synaptic/motor neurone membrane.

Differences

• T-tubules carry the electrical signal quickly into the inside of the muscle cell, whereas at a synaptic junction the message is passed on by the movement of sodium ions.
• The neuromuscular junction is only ever excitatory,whereas synapses can be either excitatory or inhibitory.
• The synapse sends a message from neurone-to-neurone, whereas the neuromuscular junction transmits a signal from neurone-to-muscle.

In part 2, you will know more about muscles.

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