Skeletal and Cardiac Muscle

Question 1

What are the 3 different types of muscles?

Answer 1

Skeletal Muscle, Cardiac Muscle and Smooth Muscle

Question 2

What are the 4 main characteristics of a Skeletal muscle fibre?

Answer 2

1) . It has multiple nuclei (Multinucleated)
2) . It contains many mitochondria as it requires energy for contraction and relaxation
3) . It has Transverse tubules (T tubules)
4) . It has myofibrils and sarcomeres

Question 3

What is the name for the plasma membrane in Skeletal muscle fibres?

Answer 3

Sarcolemma

Question 4

What is the name for the cytoplasm in Skeletal muscle fibres?

Answer 4

Sarcoplasm

Question 5

What is the name for the smooth endoplasmic reticulum in Skeletal muscle fibres?

Answer 5

Sarcoplasmic reticulum

Question 6

How would you identify a Skeletal muscle cell under a microscope?

Answer 6

It would be striated as it has Myofibrils and Sarcomeres

It would have many nuclei and mitochondria

Question 7

What are the 5 main properties of the structure of Skeletal muscles?

Answer 7

1) . They are striated
2) . Attached to bone by tendons
3) . They are under voluntary control - We can make a conscious decision to contract or relax
4) . They interact with the external environment
5) . Can shorten or lengthen depending on the position of the joints

Question 8

What is the role of a Sarcomere?

Answer 8

It is the contractile part of the muscle fibre

Question 9

What are Myofibrils?

Answer 9

Myofibrils are the structures that give skeletal and Cardiac muscles their striated appearance

Striations are result from the orderly arrangements of thick and thin filaments

Question 10

What are the 4 main characteristics of a Cardiac muscle fibre?

Answer 10

1) . It has 1 or 2 nuclei, centrally located
2) . Has branching fibres with intercalated discs which contain desmosomes and gap junctions to join muscle cells together
3) . Has Nodal cells which have the ability to stimulate their own action potentials (automaticity or auto-rhythmicity; can generate their own rhythm of contraction)
4) . It has striations and uses the sliding filament mechanism to contract

Question 11

What is Cardiac muscle structurally like and functionally like ?

Answer 11

Cardiac muscle’s structure is like Skeletal muscles and it functions like smooth muscle

Question 12

What are the 3 factors which Cardiac muscle activity is dependent upon?

Answer 12

• Intrinsic properties
• Hormones
• Autonomic nervous system (Stimulation of parasympathetic nerve supply will slow down Cardiac muscle)

Question 13

Why are the gap junctions important in the heart?

Answer 13

They allow the heart to be electrically coupled, so many cells can act together as one

Question 14

What is the absolute refractory period in the heart?

Answer 14

About 250ms

Question 15

What does the refractory period in the heart prevent?

Answer 15

Tetanic contractions (a sustained constant muscle contraction when action potentials are very high) which would interfere with the hearts ability to pump

Question 16

What is the structural and functional role of intercalated discs?

Answer 16

Structural - Mechanical connection between adjacent cardiac muscle cells

Functional - Electrical connection between adjacent cardiac muscle cells

Question 17

What are gap junctions made from?

Answer 17

A hexagonal array of protein subunits called Connexins

Question 18

What are the 2 functions of gap junctions?

Answer 18

Sites of low electrical resistance between cells

Act as communicating channels - Connexon

Question 19

What is the diameter of the central pore of a gap junction and what size of molecules does this allow through?

Answer 19

Central pore of 1.5nm diameter

Small molecules (<500MW) pass through

Question 20

How does the electrical impulse travel from the brain to skeletal muscles?

Answer 20

Neurons from the Primary motor cortex in the brain synapse on the motor neuron (located in the grey matter of the ventral horn) and projects to the periphery and activates skeletal muscle

Question 21

What is a motor unit ?

Answer 21

A motor unit is defined as the motor neuron and the skeletal muscle it innervates

Question 22

How many muscle fibres does a single motor neuron innervate?

Answer 22

many muscle fibres

Question 23

How many motor neurons can innervate one muscle fibre?

Answer 23

Only one motor neuron

Question 24

What is the number of fibres innervated by a single motor neuron dependent of?

Answer 24

If fine motor movements are required like in the fingers when typing, more motor units with relatively less muscle fibres for more precision where as the leg muscles can have more muscle fibres to a single motor neurons

Question 25

What does a motor neuron do to innervate multiple muscle fibres?

Answer 25

Branches

Question 26

What is the only way that Skeletal muscles can generate action potentials?

Answer 26

By stimulation of nerve fibres from motor neurons/somatic efferent neurons (same thing)

Question 27

Where are the motor neurons cell bodies located?

Answer 27

In either the brainstem or spinal cord

Question 28

What is special about the axons of motor neurons?

Answer 28

They are myelinated and are the largest diameter axons in the body so they can transmit signals at high velocities from the CNS to Skeletal muscles fibres with minimal delay

Question 29

What do the axon terminals of motor neurons contain?

Answer 29

Vesicles containing the neurotransmitter Acetylcholine (ACh)

Question 30

What is the region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon known as?

Answer 30

The motor end plate

Question 31

What is the junction of an axon terminal with motor end plate known as?

Answer 31

Neuromuscular junction

Question 32

What 3 things do neuromuscular junctions consist of?

Answer 32

• Axon terminals
• Motor end plates on muscle membrane
• Schwann cell sheaths

Question 33

Once ACh/Acetylcholine is released into the synaptic cleft, where does it go?

Answer 33

ACh is rapidly degraded enzymatically by the enzyme acetylcholinesterase

Question 34

What kind of receptor does ACh open?

Answer 34

Nicotinic Acetylcholine (cholinergic) receptor

Question 35

What happens when ACh binds to its target receptor?

Answer 35

The nicotinic cholinergic receptor binds two ACh molecules, opening a non-specific Monovalent cation channel allowing Na+ and K+ to pass.

Question 36

What are the 9 events which occur at the motor end plate and draw this?

Answer 36

1) . Action potentials arrive at the axon terminal open the voltage gated Ca2+ channels
2) . Inwards diffusion of Ca2+
3) . Fusion of acetylcholine-containing vesicles (ACh) with the pre-synaptic membrane
4) . ACh diffusion across the 20nm synaptic cleft
5) . Nicotinic ACh receptors (nAChR) are chemically gated ion channels which permit monovalent cations to flow through
6) . Net entry of Na+ into end plate region causes depolarisation to end plate potential (epp)
7) . Action potential triggered in muscle fibre membrane
8) . Propagated action potential in muscle plasma membrane
9) . Acetylcholine degradation

Question 37

What is Myasthenia Gravis?

Answer 37

Myasthenia Gravis is an Auto-immune disease where skeletal muscle weakness increases during periods of activity and improves after periods of rest

Question 38

What functions are susceptible to Myasthenia Gravis ?

Answer 38

Eye and eyelid movement, facial expression, chewing, talking and swallowing

and paralysis of the respiratory muscles

Question 39

What goes wrong in people with Myasthenia Gravis ?

Answer 39

There is a production of auto-antibodies which;

• Most commonly are directed against the acetylcholine receptor (nicotinic)
• some impair the ability of acetylcholine to bind to receptors
• some lead to the destruction of receptors