MSS: Somatic Nervous System and Muscle Contraction Flashcards
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.
Describe the motor unit.
The motor unit is a single motor neurone and all the muscle fibres it controls.
The single fibre contracts completely or not at all.
Motor neurones can innervate a few to a hundred fibres.
They can regulate the strength of muscle contraction by varying how many motor units are activated.
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.
Describe cholinergic receptors.
There are two types of cholinergic receptors:
- Muscarinic Receptors (primarily CNS, GPCR, slow)
- Nicotinic Receptors (neuronal/NMJs, fast)
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.
Describe the graded muscle contractions in skeletal muscle.
A single action potential will produce a twitching that will last for 100 milliseconds; this is known as a single twitch.
If the second action potential arrives before the muscle has relaxed, you get a summation of the two, which leads to greater tension.
When the rate of action potentials is so high that the muscle doesn’t relax between stimuli, there is a sustained contraction known as tetanus (continuous muscle contraction).
Describe Duchenne Muscular Dystrophy.
DMD is the most common severe form of childhood muscular dystrophy (1:5000 males from birth). It affects skeletal and cardiac muscle. The patient will be unable to walk by 10-12 years, and death follows by early to mid 20s (usually due to heart failure or respiratory problems).
It’s caused by a mutation in the dystrophin gene. Dystrophin connects actin filaments to the sarcolemma, which is required for mechanical stability.
The lack of dystrophin causes dysfunction of the sarcolemma, causing it to stretch and open the ion pores, increasing the intracellular Ca2+. There is also degradation of structural proteins, and creatine kinase is lost from the cell into the blood (used as a marker indicative of muscle damage). Creatine kinase is required for ATP recycling.
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.
Give some examples of motor neurone disease.
Examples of MNDs are:
- 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.
