Skeletal and Cardiac Muscle

Question 1

Describe some of the characteristics of skeletal muscle

Answer 1

• Multinuclear
• Many mitochondria
• Special structures called transverse tubules
• Has myofibrils and sarcomeres

Question 2

What is a sarcolemma?

Answer 2

Plasma membrane of a skeletal muscle fibre

Question 3

What is a sarcoplasm?

Answer 3

Cytoplasm of a skeletal muscle fibre

Question 4

What is a sarcoplasmic reticulum?

Answer 4

Smooth ER of skeletal muscle fibre

Question 5

What are myofibrils?

Answer 5

They are the structures that give skeletal and cardiac muscle fibres their striated appearance.

Question 6

How do striations occur?

Answer 6

From the orderly arrangement of thin and thick myofibrils

Question 7

Describe some of the characteristics of cardiac muscle

Answer 7

• Cells have one/two nuclei which are central
• Use sliding filament mechanism to contract
• Branching cells with intercalated discs with desmosomes and gap junctions

Question 8

Give a feature of nodal cells

Answer 8

They have the ability to generate their own action potentials. This is called automaticity or auto-rhythmicity

Question 9

What are intercalated discs?

Answer 9

Mechanical and electrical connections between cardiac cells

Question 10

What protein subunits make up gap junctions?

Answer 10

Connexins

Question 11

What is the central diameter of gap junctions

Answer 11

1.5nm

Question 12

Where do neurons from the primary motor cortex synapse?

Answer 12

On the motor neuron located in the grey matter in the ventral horn of the spinal chord.

Question 13

What is a motor unit composed of?

Answer 13

Axon and muscle fibre.

Question 14

What is an Alpha Motoneuron

Answer 14

It is a motor neurons, the names mean the same thing.

Question 15

What is a motor unit defined as?

Answer 15

The motor neuron (which can innervate many fibres) and the skeletal muscle fibres it innervates.

Question 16

What do fine motor movements like (typing) rely on?

Answer 16

Motor units with relatively few fibres

Question 17

What allows for motor neurons to propagate action potentials at high velocities?

Answer 17

They are myelinated and are the largest-diameter axons in the body.

Question 18

What is the motor end plate?

Answer 18

The region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon.

Question 19

Within Neurovascular junctions what is the neurotransmitter released and how is it degraded

Answer 19

Acetylcholine and it is degraded enzymatically by acetylcholinesterases

Question 20

On the muscle fibre sarcolemma, where are the ACh receptors and voltage activated Na channels located?

Answer 20

ACh receptors are at the top of the junctional folds. Na channels are located at the bottoms of junctional folds

Question 21

How do Nicotinic Acetylcholine/cholinergic receptors work

Answer 21

Two ACh molecules bind to the receptor causing the opening of the nonspecific monovalent cation channel

Question 22

Describe the events that occur at the motor end plate

Answer 22

• Action potentials cause calcium channels to open at the axon terminal allowing for inward diffusion of Ca into the axon.
• There is a fusion of vesicles containing ACh with the presynaptic membrane
• ACh then diffuses across the synaptic cleft where they can bind to nicotinic ACh receptors on the sarcolemma.
• There is then a net entry of Na into the end plate region causing depolarisation (epp - end plate potential)
• Action potential is triggered

Question 23

What is Myasthenia Gravis

Answer 23

It is an auto immune disease characterised by muscle weakness that increases during periods of activity but improves after rest. It is mostly likely to effect eye and eyelid movement, facial expression, chewing, talking and swallowing. However can also cause paralysis of respiratory muscles

Question 24

What causes Myasthenia gravis

Answer 24

The production of auto-antibodies. This most commonly targets nicotinic acetylcholine receptors but some impair the ability of ACh to bind to receptors and some lead to the destruction of receptor.

Question 25

Name some muscle myopathies and what they cause/their explination

Answer 25

Myositis - inflammation
Muscular dystrophy - Inherited disorder that with progressive weakness
Myasthaenia - weakness that gets worse with exercise
Myotonia - Sustained contraction with slow relaxation
Channelopathy - Ion channel disorder
Metabolic Myopathies - Metabolic/enzyme defects