LECTURE 8 (Muscle contraction) Flashcards
Describe the chemo-mechanical system
During the muscle contraction, chemical energy is directly transformed into mechanical energy (mechanical work)
What makes chemical synapses different from electrical synapses?
- Unidirectional
- Receptors on Post-synaptic nerve instead of gap junctions
- Neurotransmitters
What are Electrical synapses?
Allow current to flow from one excitable cell to the next via low resistance pathways between the cells called gap junctions
What is the Miniature End Plate Potential (MEPP)?
The content of a single synaptic vesicle produces the smallest possible change in membrane potential of the motor end plate -> this summates to produce the EPP (End plate potential)
What is Curare?
Competes with ACh for nicotinic receptors on the motor end plate -> decreases size of EPP
EFFECT:
- small amounts -> therapeutically causes relaxation of skeletal muscle during anaesthesia
- maximal doses -> paralysis of respiratory muscles + death
What is Botulinus toxin?
Blocks EACh release from presynaptic terminals
EFFECT:
- total blockade
- paralysis of respiratory muscles + death
What is Neostigmine?
Prevent degradation of Ash in synaptic cleft -> prolong and enhance action of EACh at motor end plate
EFFECT:
- can treat myasthenia graves (skeletal weakness + fatigue)
What is Hemicholinium?
Blocks reuptake of choline into presynaptic terminal
EFFECT:
- depletes ACh stores from presynaptic terminal
Describe Skeletal muscles
- Composed of tubular muscle cells myocytes (muscle fibres)
- Striated
- Voluntary
- Repeating sections of sarcomeres -> slide past each other when a muscle contracts or relaxes
Where are Actin molecules bound to?
The Z line
[which forms the borders of the sarcomere]
What happens upon muscle contraction?
The A bands do not change their length but the I-bands and the H-zone shorten which causes the Z lines to come closer together
Describe the process of Muscle contraction
1) Myosin head attaches to the actin myofilament forming a cross bridge
2) Inorganic phosphate generated in previous contraction cycle is released -> initiates power stroke -> Myosin head pivots and bends as it pulls on the actin filament sliding it toward the M line -> ADP released
3) As new ATP attaches to the myosin head, link between myosin and actin weakens -> cross bridge detaches
4) ATP split into ADP and Pi -> Myosin head energised
What is the difference between Skeletal muscle and Smooth muscle?
Skeletal muscles = contract + relax rapidly
Smooth muscles = prolonged tonic contraction, lasting hours or even days
Describe the “Latch” mechanism
Used for prolonged holding of contractions of smooth muscle -> Once smooth muscle has developed full contraction, the amount of continuing excitation usually can be reduced to far less than the initial level -> Muscle still maintains full contraction
What is the importance of the latch mechanism?
It can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy -> Little continued excitatory signal required from nerve fibers or hormonal sources
