Lecture 11 : Muscle 1 Flashcards
Describe the 3 types of muscle. What? Where? Control?
- Cardiac - heart, rhythmic and coordinated contractions for pumping blood around the body, autonomic nervous system (involuntary)
- Smooth - GI tract, gut motility for digestion, autonomic nervous system (involuntary)
- Skeletal - Core and limb, movement of skeleton, posture, thermoregulation, somatic nervous system (voluntary)
What are the layers of skeletal muscle from largest to smallest?
Muscle -> muscle fassicle -> myofibre -> myofibrils
What is the sarcolemma?
Barrier - surrounds each myofibre, cell membrane keeping it all together
What is a sarcomere?
Functional unit - comprised of myofilaments actin (thin) and myosin (thick)
- Contractile unit, striated muscle
What are transverse-tubules?
Regulatory unit - extensions of the sarcolemma that dive deep into the muscle; sarcoplasmic reticulum
What are mitochondria?
Energy unit - create ATP for muscle contraction
How does skeletal muscle develop?
- Muscle precursor cells (myoblasts) fuse together to form large multinucleated cells
- Each muscle fibre has hundreds to thousands of nuclei
- Fibres are ~ 20—40μm in diameter, but may be many cm long
Describe excitation at the neuromuscular junction:
- Depolarisation of axon terminal → voltage-gated Ca2+ channels open → Ca2+ enters axon terminal
- Ca2+ triggers ACh release from vesicles into the synaptic cleft
- ACh diffuses across the synaptic cleft
- ACh binds to its ACh-receptor (chemically gated Na+ ion channel) on the motor end plate of the myofibre
- Na+ enters the myofibre → membrane potential increases from -70 to -60 mV, → depolarisation of the myofibre
- The action potential propagates along the sarcolemma of the myofibre
Describe the process from excitation to contraction at the neuromuscular junction:
- Action potential propagates along the sarcolemma of the myofibre
- The sarcolemma dives into the myofibre → t-tubule
- When the action potential arrives at the t-tubule, it initiates Ca2+ release from the SR
- Ca2+ in the sarcoplasm diffuses to the myofilaments
- This initiates cross bridge cycling - the sarcomere shortens (muscle contraction)
- Z-lines move closer together, actin-myosin slide into each other sliding filaments - sarcomere shortens
Describe the process of relaxation?
- Ca2+ in the sarcoplasm diffuses away from the myofilaments and the Ca2+ is taken back up into the sarcoplasmic reticulum (SR)
- This consumes ATP (energy) - This terminates cross bridge cycling - sarcomere lenghtens - muscle relaxation
What is botulinum toxin?
- A neurotoxic protein produced by the bacterium Clostridium botulinum.
- Types A and B cause muscular paralysis (flaccid)
- Reduces release of ACh at nerve terminals, blocks neuromuscular transmission - no muscle contraction
Describe the cross-bridge cycle:
- Cross bridge forms
* Ca2+ binds to troponin
* This moves tropomyosin off myosin binding sites
* Myosin binds to actin - Power stroke
* ADP + Pi dissociates from myosin
* Myosin head flexes → power stroke → sarcomere shortens → tension developed - Cross bridge detaches
* ATP binds to myosin head causing it to detach from actin - Myosin head reactivates
* ATP is hydrolysed into ADP + Pi
* The myosin head is “recocked” into an active state
How does a neuron-skeletal muscle interaction occur?
Via a motor unit
- at the neuromuscular junction (NMJ)
- muscle fibres should contract near simultaneously along their entire length
Compare and contrast isometric, isotonic, concentric and eccentric contraction:
- All forms of muscle contraction develops tension in the muscle (cross bridges are formed between actin and myosin)
- Isometric contraction: No change in muscle length while developing tension
- Isotonic contraction: Change in muscle length to develop tension
- Concentric; shortens
- Eccentric; Lengthens
