Lecture 6: Muscle and Muscle Contraction Flashcards
Monday 20th January 2025
What do all muscles do?
They transduce chemical and electrical signals to confer movement.
What are the 2 types of muscle contraction?
Isometric and isotonic
Describe isometric contraction
- In both the relaxed and contracted state, the muscle length remains the same
- Only an increase in tension, no increase in length
- For example, forearm muscles while holding
an object.
Describe isotonic contraction
- Tension remains unchanged
- Length of muscle shortens during contraction
Is it true that muscles can switch between isometric and isotonic contraction?
Yes
What are the 3 types of muscle?
caridac, smooth and skeletal (striated)
Describe the structure of cardiac muscle
- Striated
- Muscle fibres are linked via intercalated disks
- Electrically coupled via gap junctions
- Similar to skeletal muscle (sacromeres, T tubules and SR)
What are the properties of caridac muscle cells?
- Differences between atria, conducting system and ventricles
- Striated like skeletal muscle
- Shows myogenic activity
- Cells are electrically coupled
- T system (ventricular muscle)
- Controlled by autonomic nervous system and hormones
Describe the structure of smooth muscle
Spindle-shaped (fusiform) cells with a single central nucleus
No striations (contractile proteins arranged in a crisscross pattern)
Dense bodies act as anchoring points for actin (similar to Z-discs)
Intermediate filaments provide structural support
No T-tubules, but caveolae (small membrane invaginations) help store calcium
Less developed sarcoplasmic reticulum (SR), relies on both intracellular and extracellular calcium
Connected by gap junctions (in single-unit smooth muscle) for coordinated contraction
Describe the properties of smooth muscle
- Line the muscle of internal organs (blood vessels, gut,
glands etc) - Heterogeneous muscle with many different jobs
- Can maintain a steady level of tension (tone)
- Produce slow long lasting contractions
- Innervated by the ANS (varicosities)
- Very plastic properties: can adjust length over
a much wider range than skeletal or cardiac muscle - Controlled by the autonomic nervous system
Does smooth muscle show different patterns of contraction?
Yes
Is it true that smooth muscle can be controlled by both multi units and single units?
Yes
Describe the structure of skeletal muscle
- Functional unit is the sarcomere
- Sarcomeres align to give a striated appearance
- Sarcomeres are mechanically joned together by Z lines
- Muscle fibres slide over each other and get shorter during contraction (sliding filament model)
- There’s an optimal lenght of muscles for contraction
What’s required for muscle contraction?
- Skeletal muscle
- T system (action potential goes down T system for contraction)
- sarcoplasmic reticulum
- Thin actin
- Thick myosin
What is the force produced by muscle contraction dependant on?
Number of active muscle fibres (recruitment)
Frequency of stimulation (temporal summation, tetanus vs twitch)
Rate at which muscle shortens
Cross sectional area of the muscle
Initial resting length of the muscle
Describe the cross bridge cycle (contraction)
- Nerve impulse arrives → triggers calcium (Ca²⁺) release from the sarcoplasmic reticulum.
- Calcium binds to troponin, causing a shape change.
- This moves tropomyosin away from the actin binding sites, exposing them.
- The myosin head, already in a cocked position (after ATP hydrolysis), binds to an exposed binding site on actin, forming a cross-bridge.
- Binding causes the myosin head to release ADP + Pi.
- This triggers the power stroke: the myosin head pivots, pulling the actin filament inward (~11 nm movement).
- A new ATP molecule binds to the myosin head.
- This causes the myosin head to detach from actin.
- The ATP is hydrolyzed to ADP + Pi.
- This energy re-cocks the myosin head into the high-energy, ready state.
- If calcium is still present and binding sites are still exposed, the myosin head rebinds to another actin site → cycle repeats.
- If calcium is removed (pumped back into the SR), tropomyosin covers the binding sites again → contraction stops.
What is the fundamental mechanism behind muscle contraction?
The cross-bridge cycle
How does the cross bridge cycle lead to muscle contraction? (sliding filament)
- Thousands of myosin heads along thick filaments go through this cycle at the same time, pulling the thin (actin) filaments inward.
- As actin filaments slide inward, the sarcomere shortens — this is the basic unit of a muscle fiber.
- When many sarcomeres shorten together, the entire muscle fiber contracts.
- This coordinated shortening of fibers across the muscle causes a visible contraction, like flexing your bicep or your heart muscle pumping.
Give a simple analogy of the cross-bridge cycle
Think of myosin heads like tiny rowing oars pulling actin filaments toward the center of the sarcomere — over and over again. This is called the sliding filament model of contraction.
Without ATP, does myosin stay bound to actin?
Yes. This is what causes rigor mortis.
what is the cross bridge cycle tightly regulated by?
The process is tightly regulated by calcium ions and the proteins troponin and tropomyosin.
What happens after the cross bridge cycle… sliding filament?
- As many sarcomeres shorten at once, the entire muscle fiber shortens, leading to muscle contraction.
- This is what creates force and movement.
- When the nerve signal stops:
Calcium is pumped back into the SR.
Troponin and tropomyosin block actin again, so no more cross-bridges form.
The muscle relaxes and returns to resting length.
Summarise the sliding filament theory
- Myosin pulls actin inward.
- Actin slides over myosin.
- Sarcomeres shorten.
- Muscle contracts.
Muscle contraction requires ATP. Where does this ATP come from?
- Phosphocreatine (lasts for ~10 s about 15-50 mM present in muscle)
- Glycogen (stored in muscle)
- Glycolysis : 2 ATP per glucose molecule
- Oxidative phosphorylation
Describe slow-twitch fibres (type 1)
- Can maintain tension for prolonged periods
- Resistant to fatigue
- Example: muscles that maintain body posture such as
soleus muscle of lower leg
Describe fast twitch fibres
- Contraction rate: high (40-45 mm per second)
What are the 2 types of fast-twitch fibres?
- Type IIa (fast oxidative fibres)
- Type IIb (large diameter, white muscle)
Describe the 2 different fast-twitch fibres
- Type IIa Fibers (Fast Oxidative Fibers)
Energy Source: Uses oxygen to generate energy (oxidative phosphorylation).
Mitochondria: Lots of mitochondria (high energy production).
Glycogen Storage: Stores a lot of glycogen (quick energy for activity).
Fatigue Resistance: Can keep going for longer periods (endurance).
- Type IIb Fibers (Fast Glycolytic Fibers)
Energy Source: Uses stored sugars without oxygen (glycolysis - anaerobic).
Mitochondria: Few mitochondria (limited oxygen use, less efficient).
Glycogen Storage: Also stores a lot of glycogen for fast energy.
Fatigue Resistance: Tires quickly (short bursts of power).
Function: Best for quick, intense movements like sprinting.
Is it true that most muscles are a mixture of different fibre types?
Yes
Which ions are required for muscle contraction?
Calcium ions
Describe what happens when Calcium ions bind to troponin
- Tropomyosin is shifted out of the way and the actin-myosin binding sites are revealed.
- This allows for the formation of the cross bridges
How does muscle intracellular [Ca2+] increase?
- Opening of voltage gated Ca2+ channels
following depolarisation (SK, SM, C) - Opening of intracellular Ca2+ release channels on SR
(SK, SM, C) - Ca2+ entry from SR (action of hormones etc)
(SM)
Do muscle cells have similar roles to nerve cells?
Yes, can both produce action potentials
Why is the action potential of the cardiac tissue very wide?
To prevent the heart from going in to tetanus
What is skeletal muscle depolarised by?
Acetylcholine at the neuromusclular junction. Actin myosiin sites will eventually be revealed, myoisin will bind to actin, and eventually, contraction will occur
What is the receptor on muscle fibres?
nicotinic receptors (ionotropic/have ion channels for calcium ions to travel through)
Do muscle cells fire action potentials, like neurones?
Yes
What is contraction terminated by?
Contraction is terminated by calcium removal
How is calcium removed to terminate muscle contraction?
Small amount of Ca2+
is extruded from the cell
Most taken up into the SR
by a SERCA-type pump
What does enzyme SERCA stand for?
sarcoplasmic and endoplasmic reticulum calcium ATPASE
What is a twitch?
A single contraction of a muscle
What is a tetanus?
Multiple twitches
Describe contraction in smooth muscle
- Doesn’t have a T system
- Slower contractions that are more long-lasting
- Release of calcium from hormones/neurotransmitters only initially
- Most Ca2+ comes from channels
- Usually doesn’t fire action potentials; opening of channels is sufficient for contraction
- Ca2+ binds to calmodulin; myosin phosphorylates; myosin binds to actin; a corss bridge is formed
- The dephosphorylation of myosin allows the smooth muscle to relax- quite slow
- Latched state- can maintain contraction for a long time without any energy use
- Contraction terminated by Ca2+ removal and dephosphorylation
- Contraction is regulated by myosin, not actin
Multi-unit smooth muscle…
Each muscle cell is controlled individually by its own nerve input. The cells don’t communicate with each other, allowing for fine, precise control—found in places like the iris and large blood vessels.
Single unit smooth muscle…
A few cells receive nerve or hormone signals, and the rest are activated through gap junctions, allowing the whole group to contract together. This type is used for coordinated movements like in the uterus, gut, and bladder.
What happens to the sarcomeres during muscle contraction?
The sarcomere shortens (Z lines come closer).
What happens to the H zone during muscle contraction?
The H zone shrinks
What happens to the I band during muscle contraction?
The I band shrinks
What happens to the A band during muscle contraction?
The A band stays the same (myosin length doesn’t change).
✨ Key Point:
Muscle contraction = actin filaments sliding over myosin filaments, shortening the sarcomere and the entire muscle fiber — like pulling ropes from both ends.
What is the sarcolemma?
The muscle cell membrane that carries electrical signals (action potentials) from the surface deep into the muscle fiber.
What are T-tubules (transverse tubules)?
- These are invaginations of the sarcolemma that tunnel into the muscle fiber.
- They allow the action potential to travel deep into the cell and reach the sarcoplasmic reticulum quickly.
What is the Sarcoplasmic Reticulum (SR)?
- A membranous sac that wraps around myofibrils.
- It stores and releases Ca²⁺, which is essential for muscle contraction.
What is the triad junction?
- This is the critical control site where one T-tubule sits between two SR terminal cisternae.
- It’s located where the A band meets the I band of a sarcomere.
- When an action potential travels down the T-tubule, it triggers the SR to release Ca²⁺, starting contraction.
