MSS: Somatic Nervous System and Muscle Contraction

Question 1

Describe skeletal muscle structure.

Answer 1

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.

Question 2

Describe the somatic nervous system.

Answer 2

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.

Question 3

Describe the motor unit.

Answer 3

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.

Question 4

Describe the process of muscle contraction from the neuromuscular junctions.

Answer 4

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.

Question 5

Describe cholinergic receptors.

Answer 5

There are two types of cholinergic receptors:

1. Muscarinic Receptors (primarily CNS, GPCR, slow)
2. 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.

Question 6

Describe the graded muscle contractions in skeletal muscle.

Answer 6

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).

Question 7

Describe Duchenne Muscular Dystrophy.

Answer 7

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.

Question 8

Describe motor neurone disease.

Answer 8

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.

Question 9

Give some examples of motor neurone disease.

Answer 9

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]

Question 10

Describe Amyotrophic Lateral Sclerosis (ALS).

Answer 10

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.

Question 11

Describe Myasthenia Gravis.

Answer 11

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.

Question 12

What are the different types of muscles and explain the type of control they are under?

Answer 12

Cardiac - lines walls of heart, striated, involuntary

Smooth - lines visceral organs, spindle shaped, involuntary

Skeletal - muscle attached to skeleton, striated, voluntary

Question 13

Explain the process of Ca2+ mediated muscle contraction

Answer 13

Un stimulated muscle: Actin’s Myosin binding sites blocked.

MN stimulated Ca2+ release.

Ca2+ binds to Troponin on tropomyosin, which causes a tropomysin conformation change. Myosin binding sites unblocked.

Hydrolysis of ATP converts myosin to a high-energy form.
This form of myosin binds to actin, forms a cross-bridge, and pulls the thin filament toward the center of the sarcomere.

New ATP causes Myosin bridges to detach. ATP re-cocks the Myosin head ready for the next cycle

Question 14

What do we use to power repetitive contractions?

Answer 14

the muscle relies on two other storage compounds:

creatinine - transfer of a Pi group from creatinine phosphate to ADP in an enzyme catalysed reaction synthesises additional ATP

glycogen

Question 15

What the contraction of skeletal muscle based on?

Answer 15

Myosin cross-bridges binding to actin and transitioning from a high-energy to a low-energy state

Question 16

What happens when a skeletal muscle is deprived of adequate ATP supplies?

Answer 16

It will enter a state where actin and myosin are unable to separate

Question 17

What is rigor mortis?

Answer 17

The muscles of a recently deceased human can remain in a contracted state, termed rigor mortis, for several hours, due to the lack of : ATP needed to break actin-myosin bonds

Question 18

What control muscle contraction and relaxation?

Answer 18

Calcium ions and regulatory proteins control muscle contraction and relaxation.

The regulatory proteins tropomyosin and the troponin complex bind to thin filaments.
In a muscle fiber at rest, tropomyosin covers the myosin binding sites along the thin filament and prevents the interaction of actin and myosin.
When Ca2+ is present in the cytosol, it binds to the troponin complex, causing the proteins bound along the thin filament to shift position, exposing the myosin-binding sites on the thin filament.
The thin and thick filaments slide past each other, and the muscle fiber contracts.
When the Ca2+ concentration falls, the binding sites are covered and contraction stops.
Motor neurons cause muscle contraction by triggering the release of Ca2+ into the cytosol of muscle cells with which they synapse.