Flashcards in Synapses and Muscle Contraction (Chapter 15) Deck (61):
What is a synaptic cleft?
The very small gap between two neurones where they meet (20nm wide)
What is a synapse?
The parts of the two neurones near to the synaptic cleft and the synaptic cleft
What is synaptic transmission?
When molecules of neurotransmitter (transmitter substance) are released to stimulate the next neurone
What can impulses not do?
Jump across synapses
Describe the general way in which synaptic transmission occurs
1) an action potential occurs at the CSM of the presynaptic neurone
2) the action potential causes the release of neurotransmitter molecules into the synaptic cleft
3) the molecules of neurotransmitter diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone
4) the postsynaptic neurone responses to all the impulses arriving at any one time by depolarising - if the overall depolarisation is above its threshold, then it will send impulses
What does the cytoplasm of the presynaptic neurone contain?
Vesicles of neurotransmitter e.g. noradrenaline and acetylcholine (ACh)
What are cholinergic synapses?
Synapses that use ACh as the neurotransmitter molecule
Describe how synaptic transmission occurs at a cholinergic synapse (presynaptic neurone)
1) as an action potential occurs at one place on an axon, local circuits depolarise the next piece of membrane, stimulating the opening of Na+ voltage-gated channels ∴ propagating the action potential
2) in the part of the membrane of the presynaptic neurone that is next to the synaptic cleft, the arrival of the action potential causes Ca2+ voltage-gated channels to open
3) Ca2+ diffuses into the cytoplasm of the presynaptic neurone down a steep electrochemical gradient
4) the influx of Ca2+ stimulates vesicles containing ACh to move to the presynaptic membrane (exocytosis) and fuse with it, emptying their contents into the synaptic cleft
5) the ACh diffuses across the membrane
Describe how synaptic transmission occurs at a cholinergic synapse (postsynaptic neurone)
1) molecules of ACh temporarily bind with receptor proteins on the CSM of the postsynaptic neurone - a part of the receptor protein molecule has a complementary shape to part of the ACh molecule
2) this changes the shape of the protein, opening channels through which Na+ can pass
3) Na+ diffuse into the cytoplasm of the postsynaptic neurone and depolarise the membrane - the action potential is transmitted if the threshold potential is reached
Why is there a very steep Ca2+ electrochemical gradient in the presynaptic neurone?
Bc there are virtually no Ca2+ in the cytoplasm but many in the tissue fluid surrounding the synapse
Why are the receptor proteins on the postsynaptic CSM chemically-gated ion channels?
Bc they are stimulated by chemicals (neurotransmitters) and not by voltage change
What would happen if ACh remained bound to the postsynaptic receptors?
The Na+ channels would remain open and the postsynaptic neurone would ∴ be permanently depolarised
What happens to prevent permanent depolarisation of the postsynaptic neurone and waste of ACh?
ACh is recycled
1) the synaptic cleft contains acetylcholinesterase, which catalyses the hydrolysis of each ACh into acetate and choline
2) the choline is taken back into the presynaptic neurone where it is combined with acetyl CoA to reform ACh
3) the ACh is then transported into the presynaptic vesicles, ready for the next action potential
When and only when does the depolarisation of the postsynaptic neurone lead to the generation of an action potential?
If the p.d. is above the threshold for that neurone - if not,, then there is no action potential
How is the chance that an action potential is generated and an impulse is sent in a postsynaptic neurone increased?
If more than one presynaptic neurone releases ACh at the same time or over a short period of time
What is a neuromuscular junction and what happens here?
- Where a motor neurone forms a motor end plate with each muscle fibre + the synapse
- Here, an action potential is produced in the muscle fibre, which may cause it to contract
What are the 4 roles of synapses?
1) to ensure one-way transmission
2) to allow integration of impulses
3) to allow the interconnection of nerve pathways
4) to be involved in memory and learning
How do synapses ensure one-way transmission?
- Impulses can only pass in one direction at synapses because a neurotransmitter is released on one side and its receptors are on the other side
- ∴ no way that a chemical transmission can occur in the opposite direction
How do synapses allow integration of impulses?
1) each sensory neurone has many branches at the end of its axon that form synapses with many relay neurones
2) the cell body of each motor neurone is covered with the terminations of many relay neurones
3) motor neurones only transmit impulses if the net effect of the relay neurones is above the threshold at which it initiates action potentials
4) so, if the depolarisation of the postsynaptic membrane does not reach the threshold, no impulse is sent in that neurone
5) ∴ impulses with low frequencies do not travel from sensory neurones to reach the brain ∴ the brain is not overloaded with sensory information
What does the interconnection of many nerve pathways mean for the nervous system?
That synapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly 'wired up' to each other
What are the two ways in which synapses allow the interconnection of many nerve pathways?
1) individual sensory and relay neurones have axons that branch to form synapses with many different neurones
- ∴ information from one neurone can spread out throughout the body to reach many relay neurones and many effectors, especially in danger
2) there are many neurones that terminate on each relay and motor neurone as they have many dendrites to give a large surface area for many synapses
- ∴ one neurone can integrate the information coming from many different parts of the body (essential for decision-making in the brain)
How are synapses involved in memory and learning?
1) if your brain frequently receives information about two things at the same time (e.g. sound of voice and sight of face), then new synapses form in your brain that link the neurones involved in the passing of information along the particular pathways from your ears and eyes
2) ∴ in the future, when you hear the voice, information flowing from your ears along this pathways automatically flows into the other pathway too, s that your brain pictures the face which goes with the voice
What is the disadvantage of synapses?
- Synapses slow down the rate of transmission of a nerve impulse that has to travel along to or more neurones
- Responses would be much quicker if action potentials generated in a receptor travelled along an unbroken neuronal pathway from receptor to effector, rather than having to cross synapses on the way
What is striated muscle?
- Type of muscle tissue that makes up the many muscles in the body attached to the skeleton
- Only contracts when stimulated to do so by impulses that arrive via motor neurones ∴ it is neurogenic
What is cardiac muscle?
- The muscle in the heart
- It is myogenic ∴ it contracts and relaxes automatically with no need for impulses arriving from neurones
What is smooth muscle?
- Muscle found in organs e.g. gas exchange system, alimentary canal, in the walls of arteries, veins and arterioles
- Most smooth muscle is neurogenic, however smooth muscle in arteries also contracts when stretched by pressure of blood
What does the nervous system ensure about muscles?
That the behaviour of each muscle is coordinated with all the other muscles so that they can bring about desired movement without causing any damage
What is a muscle made up of?
1000s of muscle fibres
What is a muscle fibre?
- A very specialised cell with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by the CSM
- It is a syncytium bc it is multinucleate
Describe the organelles of a muscle fibre
- It has more than one nucleus
- Sarcolemma = CSM
- Sarcoplasm = cytoplasm
- Sarcoplasmic reticulum (SR) = endoplasmic reticulum
What are transverse system tubules (T-tubules)?
- Deep infoldings in the CSM into the interior of the muscle that run close to SR
- They have a large surface area
What do the membranes of the SR have?
Huge numbers of protein pumps that transport Ca2+ into the cisternae of the SR
What does the sarcoplasm contain?
Many mitochondria, which carry out aerobic respiration, generating ATP required for muscle contraction, often packed tightly between the myofibril
What are the striations on muscle fibre produce by?
- A very regular arrangement of many myofibrils in the sarcoplasm
- Each myofibril is striped in exactly the same way and is lined up precisely against the next one
What is each myofibril made up of?
Thick and thin filaments - parallel groups of thick filaments lie between groups of thin filaments
What are thick and thin filaments made up of?
Protein - thick mostly made of myosin, thin mostly made of actin
What is the A band?
The area of thick filaments (darker) including the overlap (the darkest part) - the length of the thick filament
What is the I band?
Only thin filaments (lighter) - distance between adjacent thick filaments
What is the H band?
Only thick filaments, not including overlap - distance between thin filaments at rest
What does the Z line do?
Provides attachment for actin filaments - it is an anchoring point
What does the M line do?
Provides attachment for myosin filaments
What is the sarcomere?
- The part of the myofibril between the two Z lines
- The contractile unit of muscles
Why is the Z line also called a Z disc?
Bc myofibrils are cylindrical ∴ the Z line is a disc separating one sarcomere from another
Describe the structure of the thick filament
- Composed of many molecules of myosin (a fibrous protein with a globular head)
- The fibrous portion helps to anchor the molecule into the thick filament
- Each myosin head is an ATPase
- Within the thick filament, many myosin molecules all lie together in a bundle with their globular heads all pointing away from the M line
Describe the structure of the thin filament
- The main component is actin (a globular protein)
- Many actin molecules are linked together to form a chain
- Two of these chains are twisted together to form a thin filament
- A fibrous protein called tropomyosin is also twisted around the actin chains
- A protein called troponin is attached to the actin chain at regular intervals
How do muscles cause movement?
- The sarcomeres in each myofibril get shorter as the Z discs are pulled closer together (the sliding filament model of muscle contraction)
- The energy for the movement comes from ATP molecules that are attached to the myosin heads
Describe how a muscle contracts
1) Ca2+ are released from stores in the SR and bind to troponin molecules, stimulating them to change shape
2) the troponin and tropomyosin proteins move to a different position ∴ exposing parts of the actin molecules which act as binding sites for myosin
3) the myosin heads bind with these sites, forming cross-bridges between the thick and thin filaments
4) the myosin heads tilt puling the actin filaments along towards the centres of the sarcomere (power stroke) - here, ADP and Pi are released
5) the heads hydrolyse ATP molecules, which provide enough energy to force the heads to let go of actin
6) ATP hydrolyses to ADP + Pi, and the heads tip back to their previous positions and bind again to the exposed sites on the actin
7) the thin filaments have moved as a result of the power stroke
What happens after the muscle has made its first movement?
1) as the thin filaments have now moved, myosin heads now bind to actin further along the thin filaments closer to the Z disc
2) they tilt again, pulling the actin filaments even further along, then hydrolyse more ATP molecules so that they can let go again
3) this goes on and on, so long as the troponin and tropomyosin are not blocking binding sites and so long as the muscle has a supply of ATP
When happens at the synapse of a neuromuscular junction?
1) an impulse moves along the axon of a motor neurone and arrives at the presynaptic membrane
2) the arrival of Ca2+ through voltage-gated channels stimulates the exocytosis of vesicles containing ACh to travel and fuse with the presynaptic membrane
2) ACh diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (postsynaptic membrane)
3) the binding of ACh stimulates the ion channels to open, so that Na+ enter to depolarise the membrane and generate an action potential in the sarcolemma
What happens at the muscle fibre before the contraction occurs?
1) impulses pass along the sarcolemma and along the T tubules towards the centre of the muscle fibre (the membranes of the SR are very close to the T tubules)
2) the arrival of the impulses causes Ca2+ channels in the membrane to open
3) Ca2+ diffuse out, down a very steep conc gradient, into the sarcoplasm surrounding the myofibrils
4) Ca2+ binds with the troponin molecules
5) this changes the shape of the troponin molecules, causing the troponin and tropomyosin to move away and expose the binding sites for the myosin heads
6) the myosin heads attach to the binding sites on the thin filaments and form cross-bridges
What happens when there is no longer any stimulation from the motor neurone?
1) there are no impulses conducted along the T tubules
2) released from stimulation, the Ca2+ channels in the SR close and the calcium pumps move Ca2+ back into stores in the SR
3) as Ca2+ leaves their binding sites on troponin, tropomyosin moves back to cover the myosin-binding sites
What happens when the muscle is in a relaxed state?
1) there are no cross-bridges between thick and thin filaments
2) troponin and tropomyosin are sitting in a position in the actin filament that prevents myosin from binding
3) there is nothing to hold the filaments together ∴ any pulling force applied to the muscle will lengthen the sarcomeres so that they are ready to contract (and shorten) again
What does each skeletal muscle in the body have?
An antagonist - a muscle that restores sarcomeres to their original lengths when it contracts
How is ATP used in muscle contraction?
- A contracting muscle uses a lot of ATP
- The very small quantity of ATP in the muscle fibres in a resting muscle is used up rapidly once the muscle starts to contract
- More ATP is produced by respiration - both aerobic respiration inside the mitochondria and when that cannot supply ATP fast enough, also by lactic fermentation in the sarcoplasm
What is the other source of ATP for muscles?
- ATP produced by creatine phosphate (kept in stores in the sarcoplasm)
- Creatine phosphate is their immediate source of energy once they have used up the small quantity of ATP in the sarcoplasm
How is creatine phosphate a source of ATP?
- A phosphate group can quickly and easily be removed from each creatine phosphate molecule and combined with ADP to produce more ATP
- creatine phosphate + ADP = creatine + ATP
What happens once the demand for energy in the muscles has slowed down/stopped?
- ATP molecules produced by respiration can be used to 'recharge' the creatine
- creatine + ATP = creatine phosphate + ADP
What happens if energy is still being demanded by the muscles and there is no ATP spare to regenerate the creatine phosphate?
The creatine is converted to creatinine and excreted in the urine
What is the structure of muscles in decreasing size?
What happens to the length of the A band during contraction?