Somatic Nervous System and Muscle Contraction Flashcards
What are the 3 muscle types?
1) Cardiac
Striated and involuntary. Found in the heart, in the walls of the ventricles in the atrium.
2) Smooth muscle
Spindle-like cells that line hollow visceral organs, e.g. digestive track, respiratory system, blood vessels. Also involuntary
3) Skeletal muscle
Voluntary muscle. We move by choice. This is muscle that is connected to bone
Describe skeletal muscle structure
It can be broken down into various levels of complexity.
There are up to 150 muscle fibres in bundles called fasciculi. The muscle fibres themselves have a membrane called the sarcolemma, and they can contain up to 1000 microfibrils.
The sarcoplasm contains what’s needed for the muscle to function properly: glycogen, fat, enzymes, mitochondria. The sarcoplasmic reticulum is utilised in calcium release.
The sarcomere is known as the contractile unit. It contains an anisotropic band (or A band) which has a high density and is predominantly made up of thick (myosin) fibres. It also contains an isotropic band (or I band) which contains actin filaments.
Titin joins the thick filaments to the Z line. It acts as a giant molecular spring, so when the sarcomere contracts, it passively brings it back in line.
Describe the somatic nervous system
It is part of the peripheral nervous system. It provides voluntary control over skeletal muscle.
The efferent neurons that innervate muscle are called motor neurones.
The upper motor neurones in the brain connect with the lower motor neurons in the spinal cord, signalling to the muscle.
What does the sliding filament model state?
It says the size of the thick filaments remain the same, and what happens is the actin and myosin interact and draw the actin filaments across the myosin filaments and shorten the sarcomere in order to contract. They also showed that ATP caused the contraction to happen, so sarcomere contractions require ATP.
- A band stays the same, but the actin filaments move and shorten the sarcomere
Also showed that it is the myosin which requires the ATP, so if you take away the myosin and treat it with ATP, the sarcomere fails to contract
Why does rigor mortis occur?
The pumps in the cell which normally pump out calcium fail, so there is a flood of calcium into the cell which causes the muscle fibres to contract. As no ATP is formed, the myosin heads remain attached to the actin, so they contract and stay contracted. Eventually it will relax as the cells and the myosin heads will break down.
Describe the motor unit
When we talk about motor units, one motor unit is a single motor neuron attached to all the muscle fibres it controls.
One motor neuron can innervate multiple muscle fibres. When they stimulate a muscle fibre to contract, it is an all or nothing event - it will contract, or it won’t contract.
By modulating how many muscle fibres are contracting you can change the amount of power of the muscle contracting. You can control the amount of movement in your muscle fibres. You can do this via activating multiple motor units.
Describe the process of muscle contraction from the neuromuscular junctions
An action potential travels down the axon, causing the release of acetylcholine. This binds to Ach receptors on the synapse, which cause a depolarisation that travels down the T tubules and enters the muscles.
There, it causes the depolarisation of the sarcoplasmic reticulum, which leads to calcium release from its stores. This calcium is released into the cytosol, which causes the contraction of muscle fibres.
What are the two types of cholinergic receptors?
- Muscarinic Receptors (primarily CNS, GPCR, slow)
2. Nicotinic Receptors (neuronal/NMJs, fast)
How do nicotinic receptors work?
Two molecules of Ach bind to a nicotinic AchR, which causes a conformational change, opening the ion pore.
Consequently there is a rapid increase in Na2+/Ca2+ and the resulting membrane depolarisation leads to muscle contraction.
How do graded muscle contractions work?
You can have a graded response using the motor neurones, through increasing the frequency of synapse release.
What can happen then is that when there is a single action potential, this will produce a twitch which lasts 100 msec. This is not enough to confer tension to the elastic tendons and to the bone. If there is two action potentials that hit a muscle, before the other one has had a chance to relax, you get summation. This is where even more tension is applied to the tendons. If this is continuous and the stimuli doesn’t allow the muscle to relax at all, you get sustained contraction, which is known as tetanus.
Describe Duchenne Muscular Dystrophy
- Severe form of childhood muscular dystrophy (1:5000 males)
- Mutated genes affect large protein called dystrophin, which is required to connect the cytoskeleton to the extracellular matrix in the muscle
- Lots of these mutations are in regions of repeats, which cause frame mutations, and a non-functional protein, which affects the integrity of the muscle during contraction. The muscle fibres will tear
- Affects both skeletal and cardiac muscle
- The patients are unable to walk by 10-12 years, and will die by mid 20s due to heart and respiratory problems
- Currently no cure, though lots of work going into finding out ways to treat it. The most common treatment is using corticosteroids such as prednisone. These have been shown to improve ambulation, and patients survive a lot longer
Describe motor neurone disease
Motor neurone disease (MND) is a group of disorders that selectively affect motor neurons, the cells that control voluntary muscle activity including speaking, walking and swallowing.
What are some examples of motor neurone disease?
- Amyotrophic Lateral Sclerosis (ALS) (affects all motor neurons)
- Progressive Muscular Atrophy (affects only lower motor neurons)
- Primary Lateral Sclerosis (only affects upper motor neurons)
- Progressive Bulbar Palsy (affects motor neurons in the medulla oblongata, involved in swallowing, chewing, etc.)
- Pseudobulbar Palsy (facial muscle movement is affected)
Describe Amyotrophic Lateral Sclerosis (ALS)
It’s a neuron disease affecting motor neurons, causing a severe disability that leads to death due to ventilatory failure.
It affects 1 in 200000 individuals. There is a genetic component, with 5-10% being familial.
It’s sporadic, and probably caused by a combination of environmental and genetic factors - though, that is largely unknown.
Describe Myasthenia Gravis
It is a chronic automimmune NMD that results in skeletal muscle weakness and fatigue.
Its incidence occurrence is 1-7:10000. It’s more common in young women (20-30 yrs) and older men (50-60 years).
It is when the body makes antibodies against AchRs at NMJs. It blocks AchRs, and increases AchR degradation and causes impaired signal transduction.
The patients can go through bouts of having this, and then going through remission, and there is several treatments that are being used. One is using an acetyl cholinesterase inhibitor, which prevents the breakdown of Ach, so more is available to innervate the muscle
