5 - MZH - Animal responses - Muscles Flashcards
What are muscles an example of?
Effectors - They contract in response to nervous impulses in order to bring about a response
Why are muscles called antagonistic?
- Muscles work in pairs as they can exert a force by contraction, not by lengthening
- Pairs are arranged to pull in opposite directions so when one contracts the other relax/ recoils
How is the coordination of muscle movement ensured?
- Areas of the cerebrum which recieve sensory informoration (sensory areas) send impulses to association areas. Impulses then pass to motor areas and from here to effectors
- In the association area concerned with planning actions & movements, the brain integrates these seonsory inputs and motor inputs to ensure muscle movements is coordinated and appropriate.
- This requires the controlled action of skeletal muscles about joints
State the name of the 3 different types of muscle you need to know
- Skeletal (voluntary/ striated)
- Cardiac (specialised striated)
- Involuntary (Smooth/ visceral)
Structure of skeletal muscle and the muscle fibres in them
Skeletal muslce (also known as striped/ striated muscle) - consists of many long muscle fibres
Muscle fibres:
- Cylindrical in shape and enclosed in a CSM of sarcolema
- Sarcolema has many deep infoldings which penetrate deep into the interior of the fibre (called T-tubules).
- These run close to the sarcoplasmic reticulum (ER for muscle cells)
- Have many nuclei - multinucleate
- Contain protein strands (myofibrils) with characteristic cross-striations
- Are arranged in parallel, giving a striped appearance as cross-striations line up
- Are surrounded by collagen & connective tissue which extends to form the tendon connecting the muscle to bond
Ultrastructure of skeletal muscle - What does one myofibril look like?
Banding pattern of skeletal muscle is due to the arrangement of myosin (thick myofilaments) and actin (thin myofilaments) filaments.
Each repeating unit = Sarcomere
_Dark = A band_
_Light = I band - actin only_
-
I band = From end of A band to start of next A band i.e. bit of actin filament that doesn’t overlap with any myosin
- Actin only
-
A band = Length of myosin filament
- H zone is in A band - myosin only (darker than I band but lighter than the edges of A band)
- Myosin and actin filaments overlap at the sides of the A band making it the dark band
-
H Zone = Region between 2 sets of actin
- Myosin only
- M Line = Middle of H Zone
- Z Line/ Z Disc = Marks ends of a sarcomere
A lot of “?” is present next to sarcomeres?
why?
- Many mitochondria is packed next to sarcomeres to provide energy for contraction
- ATP is from respiration
Explain the sliding-filament model of muscle contraction of skeletal muscle
During contraction:
- Thin actin filaments are pulled past and between the thick myosin filaments
- ∴ Causes shortening of the muscle fibres and changes in the banding pattern
- NOTE: the filaments themselves don’t contract or shorten
-
H zone inside the A band narrows (and sometimes it’s referred to as disappears)
- Myosin only
-
Outer darker regions of the A band widen
- Now contain overlapping actin and myosin filaments
-
I band (light band) shortens
- Contains no overlapping portion of the actin filaments
-
A band stays the same
- Length of myosin doesn’t change
- Z lines are pulled towards each other
Describe the role of ATP (3) in muscle contraction and the Rachet Mechanism (5)
Rachet mechanism:
Muscle contraction is brought about by the formation of cross bridges between actin and myosin filaments.
- Each myosin filament have long rod-like regions and a myosin head
- Actin filaments have binding sites that are complementary to the myosin heads
- During contraction actomyosin bridges form as myosin heads attach to actin filaments
- The head then rotates and pulls the 2 filaments into each other
- It finally detatches and returns back to its original position
The cycle of attaching → changing position by rotating → detaching → returning to its original position is repeated many times bringing about muscle contraction.
ATP’s role:
- Provides energy for the release of myosin heads from actin
- ATP is hydrolysed by ATP synthase on the myosin heads
- Numerous mitochondria supply ATP via aerobic respiration
Describe the role of calcium ions in muscle contraction (5)
Muscle contraction is activated by the release of Ca2+ from the sarcoplasmic reticulum when the fibre is stimulated.
- The binding site on actin filaments in relaxed muscle is covered by a protein called Tropomyosin.
- Tropomyosin - switches on/off the contraction mechanism
- Tropomyosin is attached to another protein - Troponin
- When Ca2+ is released they bind to troponin, causing it’s shape to change. This complex displaces the tropomyosin
- Actin filament is now switched on and myosin binds to form cross bridges
- Bridges rapidly break and reform causing shortening of each sarcomere
- When muscle is no longer stimulated Ca2+ are actively moved back into sarcoplasmic reticulum and tropomyosin/troponin complexes block off the binding sites
Name 2 proteins involved with muscle contraction
Tropomyosin
Troponin
When do skeletal muscles contract?
Skeletal muscles contract when it recieves an impulse from a neurone
What is the name of the “thing” that neurones and muscles meet?
Neurones and muscles meet at specialised synapses called Neuromuscular junctions
Describe the sequence of events that occur at a neuromuscular junction
- An action potential arrives at a presynaptic bulb of the terminal motor neurone
- The action potential causes voltage gates Ca2+ channels to open and Ca2+ diffuses into the presynaptic bulb
- Ca2+ causes vesicles containing ACh to fuse with the presynaptic membrane
- ACh is released and diffuses across the synaptic cleft
- ACh molecules bind with specific receptors on the postsynaptic membrane - The Sarcolema, causes Na+ channels to open
- Na+ diffuse into the sarcolema.
- Depolarises the membrane and initiates an action potential which spreads along the membrane
- Depolarisation of the sarcolema spreads down T-tubules
- This stimulates the opening of Ca2+ channels in the sarcoplasmic reticulum and Ca2+ diffuse out into the sarcoplasm (specialised cytoplasm)
- Ca2+ bind to troponin
- Causes tropomyosin to move away from the actin-myosin binding sites on the actin filaments
- Myosin head rotate towards the actin filament and form cross-linkages with them
- Myosin heads then bend, pulling the actin filaments and inwards, so the overlap more with the myosin - Power stroke
- Causes ADP + Pi to be released
- ATP is hydrolysed and the energy released is used to move the myosin heads away from the actin so they can form new cross-linkages further along
This process is repeated, with the myosin heads binding to actin, bending, pulling the actin, releasing and binding further along… UNTIL:
- Ca2+ are actively transported back into sarcoplasmic reticulum
- Actin myosin cross-linkages are broken
- Troponin moves back, causing tropomyosin to move and block the binding sites
- Muscle doesn’t change length until an antagonistic muscle pulls it back to its relaxed length
Compare synapses and neuromuscular junctions:
- Type of postsynaptic membrane
- Type of neurotransmitter
- What is being depolarised and is it stimulatory or inhibitory
Give one similarity
Compare the 3 different types of muscles that you need to know:
- Striated?
- Shape
- Where is it found
- Which branch of the nervous system controls it
- Does it contract fast?
- Does it get tired
