MSS: Somatic Nervous System and Muscle Contraction

Question 1

Describe the different types of muscle

Answer 1

Cardiac Muscle
-striated and involuntary muscle in the heart

Skeletal Muscle

• striated and voluntary muscle
• connected to bone and helps in movement

Smooth Muscle
-non-striated and involuntary muscle found in the lining of hollow visceral organs (e.g. digestive tract, respiratory system, blood vessels, bladder) except the heart

Question 2

Overall Muscle Structure

Answer 2

Muscle Belly Cross-Section
-numerous fasciculi surrounded by perimysium membrane

• each fasciculus contains a bundle of muscle fibres (150 muscle fibres) surrounded by an endomysium membrane
• each muscle fibre in the fasciculi is made of 100-1000s of myofibrils which contain sarcomeres, surrounded by a sarcolemma membrane

Question 3

Fasciculi

Answer 3

a bundle of muscle fibres (150 muscle fibres)

*each muscle fibre is essentially one cell

100-1000s of myofibrils which contain sarcomeres (contractile units)

Question 4

Perimycium

Answer 4

Connective tissue that surrounds a fasciculi

Question 5

Endomysium

Answer 5

The connective tissue surrounding a single muscle fibre in a fasciculi

Question 6

Sarcolemma

Answer 6

the membrane of myofibrils of skeletal muscle, containing a sarcoplasm surrounding the sarcomeres within the myofibrils

the sarcoplasm contains fat, enzymes, mitochondria, glycogen necessary for muscle function

intertwined between these myofibrils is the sarcoplasmic reticulum where calcium is stored

Question 7

Myofibrils under a microscope

Answer 7

Contain sarcomeres (contractile units) with areas of dark signal and light signal:

Anisotropic Band (A-band)
>high density; contain thick myosin filaments

Isotropic Band (I-band)
-light density; contain thin actin filaments

Question 8

Titin

Answer 8

elastic protein (large molecular spring) attaching myosin to Z band that contributes to elastic recoil of muscle and prevents over-stretching

Question 9

Sliding Filament Theory

Answer 9

actin filaments slide in between thick myosin filaments to shorten the sarcomere and provide a contraction

ATP facilitates the sliding action

Question 10

Calcium Stimulated Muscle Contraction

Answer 10

1) Action potential reaches sarcoplasmic reticulum, and calcium released into the sarcoplasm
2) Calcium binds troponin, causing a conformational change in the tropomyosin and exposes the myosin binding sites on the actin
3) Myosin head binds ATP which is hydrolysed by ATPase into ADP and Pi to prime myosin head into a high energy state for binding to myosin binding site on actin filament
4) Using a power stroke, it moves the actin filament towards the centre of the sarcomere for contraction
5) ATP then breaks the bond between actin and myosin head and cycle starts again

Question 11

Rigor mortis

Answer 11

pumps that pump out calcium fail and there is a massive flood of calcium into the cell, causing muscle fibres to contract, and because there is no ATP being formed, the myosin head remains attached to the actin (sustained contraction) which can last for days

-eventually, the muscle relaxes as the cells break down

Question 12

Describe the somatic nervous system.

Answer 12

• part of the peripheral nervous system
• provides voluntary control over skeletal muscle.
• The efferent neurons that innervate muscle are called motor neurones.

upper motor neurones from the brain, brainstem and spinal cord synapse with lower motor neurones using glutamate as a neurotransmitter

upper motor neurones from the spinal cord also synapse with individual muscle cells using ACh as the neurotransmitter

Question 13

Describe the motor unit.

Answer 13

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.

Question 14

How can you regulate the strength of muscle contraction?

Answer 14

by changing the amount of motor units activated

Question 15

Neuromuscular Junction (NMJ)

Answer 15

point of contact between a lower motor neurone and a skeletal muscle cell

Question 16

Describe the process of muscle contraction from the neuromuscular junctions.

Answer 16

1) Lower motor neurone synapsed to skeletal muscle and releases acetylcholine (ACh)
2) Action potential moves through the plasma membrane into t tubules
3) Stored calcium released from the sarcoplasmic reticulum into sarcoplasm
4) Calcium binds troponin, causing tropomyosin conformational change revealing the myosin-binding sites
5) Myosin head combines with ATP which is hydrolysed to ADP and Pi, myosin head is cocked/primed and binds myosin-binding sites on actin filament, and moves the actin filament towards the centre of sarcomere for contraction using power stroke
6) Calcium is then pumped out of the cytosol and back into the sarcoplasmic reticulum
7) ATP then breaks the bond between actin and myosin head and cycle starts again

Question 17

T-tubules (transverse tubules)

Answer 17

invaginations of the sarcolemma which extend excitation deep into the cell where it communicates and interacts with the sarcoplasmic reticulum (calcium store)

Question 18

Describe cholinergic receptors.

Answer 18

There are two types of cholinergic receptors:

1. Muscarinic Receptors (primarily CNS, GPCR, slow)
2. Nicotinic Receptors (neuronal/NMJs, fast)

Question 19

Propagating action potential via nicotinic receptor

Answer 19

2 molecules of Ach bind, resulting in a conformational change in the receptor

ion channel (pore) opens

rapid increase in Na+/Ca2+ and membrane is depolarised leading to muscle contraction

Question 20

A single action potential will produce a…

Answer 20

single twitch in muscle (lasts 100ms)

-not enough to confer tension to the elastic tendons and bone

Question 21

Summation

Answer 21

when a second action potential arrives right after the first and before the muscle relaxes, there is the summation of the two, and there is greater tension applied

Question 22

Tetanus

Answer 22

a continuous contraction that shows no evidence of relaxation

When the rate of action potentials is so high that the muscle doesn’t relax between stimuli, there is a sustained contraction

The fusion of individual twitches generates tetanus

Question 23

Skeletal muscle tone

Answer 23

Most muscles at rest exhibit some low level of contractile activity
Denervation leads to complete relaxation (flaccid)
Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)

Question 24

Describe Duchenne Muscular Dystrophy.

Answer 24

• X-linked degenerative disease
• mutations in gene coding for protein dystrophin
• degradation of structural proteins
• affects both skeletal and cardiac muscle
• unable to walk by 10-12 years; death by 20s due to heart failure

Question 25

Role of dystrophin protein

Answer 25

connects actin filaments to the sarcolemma/extracellular matrix for mechanical stability and fibre integrity

Question 26

What does a lack of dystrophin in DMD cause?

Answer 26

dysfunction of sarcolemma stretch, opening ion pores and resulting in an abnormal increase in intracellular Ca2+

Question 27

Indication of DMD

Answer 27

Due to degradation of structural proteins, creatine kinase (required for ATP) is lost from damaged muscle cells into blood, indicating muscle damage

Question 28

Describe motor neurone disease.

Answer 28

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 29

Give some examples of motor neurone disease.

Answer 29

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 30

Describe Amyotrophic Lateral Sclerosis (ALS).

Answer 30

• Neurodegenerative disease affecting various motor neurones
• Death usually occurs from respiratory failure within 5 years of diagnosis
• Sporadic probably caused by a combination of environmental and genetic factors- largely unknown

Question 31

Describe Myasthenia Gravis.

Answer 31

chronic autoimmune neuromuscular disease caused by antibodies attacking ACh receptors at neuromuscular junctions

antibodies block ACh receptor and increase degradation of receptor, causing impaired signal transduction as motor neurones unable to innervate muscle fibres, leading to muscle weakness

affects younger women (20-30) and older men (50-60)
-affects women more 3:2 (F:M)

Question 32

Treatment of myasthenia gravis

Answer 32

Acetylcholinesterase Inhibitors
-increase ACh at synaptic cleft, more ACh available to innervate muscle

Immunosuppressants
-prevent ACh receptor degradation

*patients live a reasonably normal life and the condition is not life-limiting