Somatic nervous system and muscle contractions

Question 1

What are 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 lining of hollow visceral organs (e.g. digestive tract, respiratory system, blood vessels, bladder) except the heart

Question 2

Describe the Overall Muscle Structure.

Answer 2

Muscle Belly Cross-Section
-numerous facsiculi surrounded by perimycium membrane

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

Question 3

What is the Sarcolemma?

Answer 3

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 4

How do Myofibrils look under a microscope?

Answer 4

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 5

Describe the Sliding Filament Theory.

Answer 5

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

ATP facilitates the sliding action

Question 6

Outline the process of Calcium Stimulated Muscle Contraction.

Answer 6

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 7

What is Rogor Mortis?

Answer 7

The pumps which 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 8

What is the Somatic Nervous System (SNS)?

Answer 8

the part of the peripheral nervous system that controls voluntary movement of skeletal muscles

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 9

What is a motor unit?

Answer 9

A motor neurone and all of the muscle fibres it innervates

Question 10

How can you regulate strength of muscle contraction?

Answer 10

by changing the amount of motor units activated

Question 11

What is the Neuromuscular Junction (NMJ)?

Answer 11

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

1) Lower motor neurone synapsed to skeletal muscle and releases acetylcholine (ACh)
2) Action potential moves through plasma membrane into t tubules
3) Stored calcium released from 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 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 12

What are T-tubules (transverse tubules)?

Answer 12

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

Question 13

What are the 2 types of neuronal receptors?

Answer 13

Muscarinic Receptors

• primarily in CNS
• G-protein coupled receptors
• slow

Nicotinic Receptors

• neuronal/NMJs
• fast

Question 14

Describe the Propagating action potential via nicotinic receptor.

Answer 14

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 15

A single action potential will produce a…

Answer 15

single twitch in muscle (lasts 100ms)

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

Question 16

Define summation.

Answer 16

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

Question 17

What is tetanus?

Answer 17

a continuous contraction that shows no evidence of relaxation

Question 18

What is Duchenne Muscular Dystrophy (DMD)?

Answer 18

• 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 19

What is the Role of dystrophin protein?

Answer 19

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

Question 20

What does a lack of dystrophin in DMD cause?

Answer 20

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

Question 21

What is the Indication of DMD?

Answer 21

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

Question 22

What is Motor Neurones Disease?

Answer 22

group of disorders that selectively affect motor neurones, the neurones that control voluntary muscle activity including speaking, walking and swallowing

Question 23

What is Amyotrophic Lateral Sclerosis (ALS)?

Answer 23

• 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 24

What is Myasthenia Gravis?

Answer 24

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 25

What is the Treatment of myasthenia gravis?

Answer 25

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

Immunosuppressants
-prevent ACh receptor degradation

*patients live reasonably normal life and condition is not life limiting