Cardiac and Skeletal Muscle Flashcards
What are the main three types of muscle ?
Cardiac, smooth, skeletal
Is skeletal muscle voluntary/involuntary ? Striated/non-striated ?
Voluntary
Striated
What are the defining characteristics of skeletal muscle cell ?
- Multinucleated
- Striated
- Many mitochondria
- Transverse Tubules (T Tubules)
- Myofibrils & sarcomeres
- Intracellular structures: Sarcolemma (=plasma membrane), sarcoplasm (= cytoplasm), sarcoplasmic reticulum (= smooth ER)
Is skeletal muscle attached to bone ?
Yes
What structures give skeletal muscle its characteristic striated appearance ?
Myofibrils (due to the orderly arrangements of thick and thin filaments)
Which kind of muscle is cardiac muscle most similar to structurally ? Functionally ?
Skeletal
Smooth
What is the activity of cardiac muscle dependant upon ?
- Intrinsic properties
- Hormones
- Autonomic nervous system
What are the defining characteristics of cardiac muscle cell ?
- One to two nuclei, centrally located
- Striated
- branching cells with intercalated discs with desmosomes and gap junctions
Which kind of mechanism do cardiac muscle cells use to contract ?
Sliding filament mechanism
What are nodal cells ?
Cardiac muscle cells which have the ability to stimulate their own action potentials. Called automaticity or auto-rhythmicity.
What is the absolute refractory period of cardiac muscle cells ? Why is it necessary ?
About 250ms
Because it prevents tetanic contractions which would interfere with the heart’s ability to pump.
What are intercalated discs ?
Mechanical and electrical connection between adjacent cardiac muscle cells
What are the subunits of gap junctions ?
6 connexins acting as a channel called a connexon
What is the electrical significance of gap junctions ?
They are sites of low electrical resistance between cells
Critical to the heart’s ability to be electrically coupled.
Where do the neurons which synapse on the motor neuron, in the grey matter of the ventral horn, originate from ? Where does the motor neuron then project/what does it do ?
From the primary motor cortex in the brain
Projects to the periphery and activates skeletal muscle
What is a motor unit defined as ?
Motor neuron + the skeletal muscle fibers it innervates
Can a motor neuron innervate more than one muscle fibre ? Can a muscle fibre be innervated by more than one motor neuron ?
Yes
No
Within one muscle, do you find one, a few, or many motor units ?
Many
Give an example of an action using muscles relying on motor units with relatively few fibres, and an action using muscles relying on motor units with more fibres.
Typing (fine motor movement in general)
Actions using legs
How many branches of motor neurons innervate a muscle fibre ?
Just one branch of motor neuron per muscle fibre
What is the overall mechanism by which action potential is initiatied in skeletal muscle ?
Stimulation of the nerve fibres to a skeletal muscle
What is the name of the nerve cells whose axons innervate skeletal muscle fibres ? Where are the cells of those nerve cells located ?
Motor neurons or somatic efferent neurons
In either the brainstem or spinal cord
What is the myelination/diameter status of the axons of motor neurons ?
What is the effect of this on velocity of action potential propagation ?
Myelinated, and largest-diameter axons in the body
Propagate action potentials at high velocities
Do the axon terminals of a motor neuron contain vesicles similar, or different, to the vesicles found at synaptic junctions between two neurons ?
Similar
What neurotransmitter do the vesicles in the axon terminals of motor neurons contain ?
Acetylcholine
What is the name of the region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon ?
Motor end plate
What is the name of the junction of an axon terminal with the motor end plate ?
Neuromuscular junction
What does the neuromuscular junction consist of ?
Axon terminals, motor end plates on the muscle membrane
Once ACh is released into the synaptic cleft, where does it go?
ACh is rapidly degraded enzyma,cally by the ac,on of acetylcholinesterase
What kind of receptor binds ACh ?
Nicotinic cholinergic receptor (chemically gated ion channels)
How many molecules of ACh does the nicotinic cholinergic receptor bind at once ?
Two
What is the opening mechanism of nicotinic cholinergic receptor for ACh ?
Binds two ACh molecules, which opens a non-specific monovalent cation channel, which allows Na+ to get into end plate region and K+ to leave end plate region.
What are all the events occurring at the motor end plate ?
- Action potentials arriving at the axon terminal open voltage gated Ca2+ channels
- Inward diffusion of Ca2+
- Fusion of acetylcholine-containing vesicles (ACh) with the pre-synaptic membrane
- ACh diffusion across the 20 nm synaptic cleft
- Nicotinic Ach receptors (nAChR) permit monovalent cations to flow through
- Net entry of Na+ into end plate region causes depolarisation - end plate potential (epp)
- Action potential triggered in muscle fibre membrane
- ACh degradation by acetylcholinesterase
What is Myasthenia gravis ? What are its symptoms ?
An autoimmune disease
Symptoms:
- muscle weakness that increases during periods of ac,vity and improves aher periods of rest
- eye and eyelid movement, facial expression, chewing, talking, and swallowing are especially suscep,ble
- paralysis of the respiratory muscles
What is the cause of myasthenia gravis ?
Production of auto-antibodies
• most commonly directed against the acetylcholine receptor (nicotinic)
• some impair the ability of acetylcholine to bind to receptors whilst some lead to the destruction of receptors
What is myostisis ?
Inflammation in muscle(s)
What is muscular dystrophy ?
Inherited disorders with progressive weakness
What is myasthenia ?
Fatigueable weakness (worse on exercise)
What is myotonia ?
Sustained contraction/slow relaxation
What is channelopathy ?
Ion channel disorders
What is metabolic myopathies ?
Metabolic/enzyme defects causing muscle disorders
