Structure of Cardiac and Skeletal Flashcards
What are the 3 different types of muscles?
Skeletal Muscle, Cardiac Muscle and Smooth Muscle
What are the 4 main characteristics of a Skeletal muscle fibre?
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
What is the name for the plasma membrane in Skeletal muscle fibres?
Sarcolemma
What is the name for the cytoplasm in Skeletal muscle fibres?
Sarcoplasm
What is the name for the smooth endoplasmic reticulum in Skeletal muscle fibres?
Sarcoplasmic reticulum
How would you identify a Skeletal muscle cell under a microscope?
It would be striated as it has Myofibrils and Sarcomeres
It would have many nuclei and mitochondria
What are the 5 main properties of the structure of Skeletal muscles?
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
What is the role of a Sarcomere?
It is the contractile part of the muscle fibre
What are Myofibrils?
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
What are the 4 main characteristics of a Cardiac muscle fibre?
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
What is Cardiac muscle structurally like and functionally like ?
Cardiac muscle’s structure is like Skeletal muscles and it functions like smooth muscle
What are the 3 factors which Cardiac muscle activity is dependent upon?
- Intrinsic properties
- Hormones
- Autonomic nervous system (Stimulation of parasympathetic nerve supply will slow down Cardiac muscle)
Why are the gap junctions important in the heart?
They allow the heart to be electrically coupled, so many cells can act together as one
What is the absolute refractory period in the heart?
About 250ms
What does the refractory period in the heart prevent?
Tetanic contractions (a sustained constant muscle contraction when action potentials are very high) which would interfere with the hearts ability to pump
What is the structural and functional role of intercalated discs?
Structural - Mechanical connection between adjacent cardiac muscle cells
Functional - Electrical connection between adjacent cardiac muscle cells
What are gap junctions made from?
A hexagonal array of protein subunits called Connexins
What are the 2 functions of gap junctions?
Sites of low electrical resistance between cells
Act as communicating channels - Connexon
What is the diameter of the central pore of a gap junction and what size of molecules does this allow through?
Central pore of 1.5nm diameter
Small molecules (<500MW) pass through
How does the electrical impulse travel from the brain to skeletal muscles?
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
What is a motor unit ?
A motor unit is defined as the motor neuron and the skeletal muscle it innervates
How many muscle fibres does a single motor neuron innervate?
many muscle fibres
How many motor neurons can innervate one muscle fibre?
Only one motor neuron
What is the number of fibres innervated by a single motor neuron dependent of?
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
What does a motor neuron do to innervate multiple muscle fibres?
Branches
What is the only way that Skeletal muscles can generate action potentials?
By stimulation of nerve fibres from motor neurons/somatic efferent neurons (same thing)
Where are the motor neurons cell bodies located?
In either the brainstem or spinal cord
What is special about the axons of motor neurons?
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
What do the axon terminals of motor neurons contain?
Vesicles containing the neurotransmitter Acetylcholine (ACh)
What is the region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon known as?
The motor end plate
What is the junction of an axon terminal with motor end plate known as?
Neuromuscular junction
What 3 things do neuromuscular junctions consist of?
- Axon terminals
- Motor end plates on muscle membrane
- Schwann cell sheaths
Once ACh/Acetylcholine is released into the synaptic cleft, where does it go?
ACh is rapidly degraded enzymatically by the enzyme acetylcholinesterase
What kind of receptor does ACh open?
Nicotinic Acetylcholine (cholinergic) receptor
What happens when ACh binds to its target receptor?
The nicotinic cholinergic receptor binds two ACh molecules, opening a non-specific Monovalent cation channel allowing Na+ and K+ to pass.
What are the 9 events which occur at the motor end plate and draw this?
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
What is Myasthenia Gravis?
Myasthenia Gravis is an Auto-immune disease where skeletal muscle weakness increases during periods of activity and improves after periods of rest
What functions are susceptible to Myasthenia Gravis ?
Eye and eyelid movement, facial expression, chewing, talking and swallowing
and paralysis of the respiratory muscles
What goes wrong in people with Myasthenia Gravis ?
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
