Skeletal and cardiac muscle Flashcards
What are the characteristics of skeletal muscle?
- Multinucleated
- Striated - sarcomeres
- Voluntary control
- Attached to bone
- Many mitochondria
- T tubules
- Specific terms: Sarcolemma (membrane), Sarcoplasm (cytoplasm), sarcoplasmic reticulum
What makes up a sarcomere?
From Z line to Z line
What is a myofibril?
Lots of sarcomeres
What is the activity of cardiac muscle dependent on?
- Autonomic nervous system
- Hormones
- Intrinsic properties
What are the characteristics of cardiac muscle?
- One to two nuclei per cell
- Striated appearance
- Intercalated discs with gap junctions and desmosomes
- Nodal cells
What is the absolute refractory period of cardiac muscle and what is the significance of this?
250ms
• Prevents tetanic contractions which would interfere with the heart’s ability to pump
What is the structural function of intercalated discs?
To provide a mechanical connection between adjacent cells
What is the functional use of intercalated discs?
To allow electrical connection between adjacent cardiac muscle cells
Describe the structure of a gap junction
- Formed from an array of 6 hexagonal protein subunits called connexins
- Connexon formed which can act as a communicating channel
- Central pore approx 1.5nm in diameter
Describe the characteristics of a gap junction
- Communicating channel
- Allows small molecules of less than 500MW to pass through
- Site of low electrical resistance
What is a motor unit?
The motor neurone and the muscle fibre it innervates
What determines the number of muscle fibres innervated?
- The muscle and its movements/actions
* e.g. fine motor control requires less motor units with relatively few fibres in comparison to the leg muscle
Which type of neurone innervate skeletal muscles? Describe their properties
Somatic efferent neurones
• Cell bodies in the spinal cord or in the brain stem
• Myelinated axons with the largest diameter axons in the body
• Propagate action potentials at high velocities
What is the motor end plate?
The plasma membrane of the muscle fibre directly under the terminal portion of the axon
What neurotransmitter is used in the neuromuscular junction?
Acetylcholine
What is acetylcholine degraded by?
Acetylcholinesterase
Describe and name the receptor on the motor end plate
- Nicotinic acetylcholine receptor
- 2 binding sites for ACh
- Monovalent non specific cation channel
- Allows Na+ and K+ to pass
Give a summary of the events at the motor end plate
- Action potential arrives at the terminal end of the axons, causing voltage gated calcium channels to open
- Influx of Ca2+ into the axon terminal
- ACh containing vesicles move and fuse to the pre-synaptic membrane
- ACh diffuses across the 20nm synaptic cleft
- 2 ACh bind to the nicotinic acetylcholine receptors
- Net entry of Na+ into the end plate causing depolarisation: end plate potential
- Action potential is triggered in the muscle fibre membrane.
What is myasthenia Gravis?
An autoimmune disease causing muscle weakness that increases in times of activity and improves over periods of rest
What are the causes of myasthenia gravis
• Production of autoimmune bodies against the acetylcholine receptor, either:
- impair the ability of the acetylcholine to bind to the receptor
- Leads to the destruction of the receptor
What is a muscle myopathy?
Neuromuscular disorders in which the primary symptom is muscle weakness due to the dysfunction of the muscle fibre
Give 6 muscle myopathies
- Myositis
- Muscular dystrophy
- Myasthaenia
- Myotonia
- Channelopathy
- Metabolic myopathy
Myositis
Inflammation
Muscular dystrophy
Inherited disorders with progressive weakness
Myasthaenia
Fatigueable weakness (worse on exercise)
Myotonia
Sustained contraction/ Slow relaxation
Channelopathy
Ion channel disorder
Metabolic myopathy
Metabolic/ enzyme defects
