Synapses And Muscles

Question 1

How does an action potential move across a synapse?

Answer 1

• the action potential depolarises the presynaptic membrane and causes the calcium ion channels to open and allowing calcium ions to enter the presynaptic membrane
• this causes the fusion of synaptic vesicles containing acetylcholine with the presynaptic membrane
• acetylcholine is released into the synaptic cleft
• acetylcholine diffuses across the synaptic cleft towards the post synaptic neurone and binds to the receptors on the postsynaptic membrane and stimulating the opening of the sodium ion channels, which allow sodium ions to diffuse into the neurone
• acetylcholinesterase hydrolyses acetylcholine into choline and acetyl which diffuses back into the presynaptic neurone where it is reassembled and reused
• the neurotransmitter is recycled and continuous generation of an action potential in the post synaptic neurone is prevented

Question 2

How do synapses prevent action potentials from going in the wrong direction?

Answer 2

• the neurotransmitter is only made in the presynaptic neurone, with receptors only on the postsynaptic neurone

Question 3

What are the effects of summation in synapses?

Answer 3

• amplify the effects of low frequency action potentials

Question 4

What is temporal summation?

Answer 4

• single presynaptic neurone releases neurotransmitter many time, over a short time period, causing threshold potential to be reached in the postsynaptic neurone

Question 5

What is spatial summation?

Answer 5

• multiple presynaptic neurones release neurotransmitter to reach the threshold value

Question 6

Are synapses inhibitory?

Answer 6

• some can be inhibitory and prevent the movement of action potentials
• most are excitatory

Question 7

What are myofibrils?

Answer 7

• protein fibres called myofibrils run through groups of muscle cells increasing their strength

Question 8

What are myofibrils made from?

Answer 8

• thick and thin filaments which overlap to give a banded appearance
• thick filaments are made of myosin and the thin filaments are made of actin
• two actin molecules are twisted together
• myosin has heads which can attach to specific binding sites on the actin when the muscle is contracting

Question 9

How does muscle contraction occur?

Answer 9

• calcium ions diffuse from the sarcoplasmic reticulum
• calcium ions bind to troponin and TROPOMYOSIN and expose myosin binding sites on the actin filament
• myosin heads bind to actin and ADP is released CROSS BRIDGES FORM
• the myosin head changes shape/bends and the filaments slide past each other/pulls actin and the muscle contracts
• ATP binds to myosin heads, causing it to release actin/detach
• calcium ions are returned to the sarcoplasmic reticulum and the muscle relaxes

Question 10

How is ATP provided for muscle contraction?

Answer 10

• produced through aerobic respiration
• myoglobin
• phosphocreatine can supply phosphate for ADP phosphorylation so that ATP can continue to be made

Question 11

What are slow twitch muscle fibres?

Answer 11

• don’t fatigue quickly
• adapted to aerobic exercise by having a large store of myoglobin, a rich supply of blood vessels and numerous mitochondria

Question 12

What are fast twitch muscle fibres?

Answer 12

• adapted for rapid release of energy
• contractions are intense and in short bursts
• thick and numerous myosin filaments, high concentration of glycogen, high concentration of enzymes needed for anaerobic respiration and a store of phosphocreatine so that ATP can be rapidly generated

Question 13

Where are synapses found?

Answer 13

• between the axon of one neurone and the dendrite of another
• between the axon of one neurone and the cell body of another

Question 14

What is a sarcomere?

Answer 14

• a section of one of the myofibrils

Question 16

What are the letters in the sarcomere?

Answer 16

• Z are the ends of the sarcomere
• H is only myosin
• A is the length of the myosin filament
• I is only actin