Topic 6: Nervous Coordination and Muscles

Question 1

What are the two types of coordination in the body?

Answer 1

• Nervous system
• Endocrine system

Question 2

How does the nervous system coordinate a response? Give an example of a nervous response

Answer 2

Uses nerve cells to transmit electrical signals along their length, and neurotransmitters to stimulate target cells

e.g a reflex action

Question 3

Describe how the endocrine system coordinates a response.

Give an example of an endocrine response

Answer 3

Produces chemicals called hormones that are transmitted in blood plasma to target cells with receptors

e.g control of blood glucose concentration

Question 4

Give some differences between the nervous and endocrine systems

Answer 4

• Endocrine communicates by hormones in the blood system, nervous by nerve impulses through neurones
• Endocrine transmission + response is slow, nervous is rapid
• Hormones travel to all parts of the body but only target cells respond, but nerve impulses travel to specific parts of the body
• Endocrine response is widespread, long-lasting, may be permanent/irreversible, but nervous response is localised, short-lived and usually temporary/reversible

Question 5

What are neurones?

Answer 5

Specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another

Question 6

Give the different cell structures in a myelinated neurone

Answer 6

• Cell body
• Dendrons and dendrites
• Axon
• Schwann cells
• Nodes of Ranvier

Question 7

Describe the cell body and dendrons of a neurone

Answer 7

• Cell body: has the usual cell structures, has a nucleus and lots of rough endoplasmic reticulum to produce proteins and neurotransmitters
• Dendrons: extensions of the cell body which branch into dendrites. Carry impulses to the cell body

Question 8

Describe the axon and Schwann cells of neurones

Answer 8

• Axon: a single long fibre that carries nerve impulses away from the cell body
• Schwann cells: wrap around the axon, protecting it and providing electrical insulation. Also carry out phagocytosis to remove cell debris and play a part in nerve regeneration.
  They wrap around the axon many times, and are known as the myelin sheath since their membranes contain myelin.
  Nodes of Ranvier are the gaps between adjacent Schwann cells with no myelin sheath

Question 9

What is the difference between a myelinated and unmyelinated neurone?

Answer 9

Myelinated neurones have Schwann cells wrapped around their axon (a myelin sheath), providing electrical insulation, while unmyelinated neurones do not

Question 10

Describe the differences in structure between motor, intermediate and sensory neurones

Answer 10

• Motor neurones have a cell body with the dendrons, attached to the axon at one end, while sensory neurones have the cell body in the centre of the long fibre (in a dorsal root ganglion)
• Intermediate neurones have the cell body at the centre, with dendrons and axons extending from the body

Question 11

What is a nerve impulse?

Answer 11

A self-propagating wave of electrical activity that travels along the axon membrane.

A temporary reversal of charges (electrical potential difference) across the axon membrane

Question 12

What is a resting potential?

Answer 12

The potential difference across an axon membrane when there is no nerve impulse,

The outside of the axon has a positive potential compared to the inside, so the resting potential is -65mV. Here, the axon is polarised.

Question 13

How is a resting potential maintained on an axon?

Answer 13

• 3 Na+ are actively transported out and 2K+ in by the sodium/potassium pump
• There are more Na+ in the tissue fluid than K+ in the cytoplasm, so there is an electrochemical gradient across the membrane
• Na+ begin to diffuse into the axon, and K+ diffuse out
• Most voltage-gated Na+ channels are closed and most voltage-gated K+ channels are open, so more K+ diffuse out than Na+ in
• Tissue fluid has a more positive potential than the axon cytoplasm, so the membrane is polarised
• Some K+ diffuse back in, but an equilibrium is reached with no net ion movement

Question 14

Describe what an action potential is

Answer 14

If the stimulation of a neurone reaches the threshold, an action potential of +40mV is reached. The membrane is temporarily depolarised (charges reversed).

Action potentials occur in one small section of the axon, and the wave of electrical activity is propagated along the neurone

Question 15

Describe how an action potential is generated

Answer 15

• At resting potential some voltage-gated K+ channels are open, but voltage-gated Na+ channels are closed
• Energy of stimulus causes some Na+ vg channels to open, Na+ diffuse into the axon.
• This causes more Na+ vg channels to open, so more Na+ move in (positive feedback) = depolarisation
• Once an action potential of +40mV is established, the electrical gradient that was preventing further outward diffusion of K+ is reversed, so K+ vg channels open and Na+ vg channels close.
• K+ diffuse out, causing the axon to depolarise. The depolarisation ‘overshoots’ temporarily, = hyperpolarisation
• K+ vg channels close and the Na+/K+ pump restores the resting potential, repolarising the axon

Question 16

What is saltatory conduction?

Answer 16

The ‘jumping’ of action potentials along an axon between nodes of Ranvier

Question 17

What is the refractory period of a neurone?

Answer 17

The minimum interval between action potentials (period of time where there can be no action potential on that portion of the axon membrane)

Question 18

What is depolarisation?

What is hyperpolarisation?

Answer 18

• Depolarisation: the temporary charge reversal across the axon membrane when a nerve impulse is transmitted
• Hyperpolarisation: more K+ flow out the neurone, so the drop in membrane potential difference ‘overshoots’

Question 19

Describe how an action potential is propagated along an unmyelinated neurone

Answer 19

• At resting potential, Na+ concentration outside is higher than inside the axon, and K+ concentration is higher inside than outside. Overall, the concentration of positive ions is higher outside = the membrane is polarised
• Stimulus causes Na+ vg channels to open. Na+ diffuse into axon, creating an action potential and depolarising the membrane
• First action potential causes the opening of Na+ vg channels further along the axon. This is positive feedback. The resulting influx of Na+ causes a new action potential.
• Behind this, Na+ vg channels close and K+ vg channels open, causing K+ to leave along the electrochemical gradient
• Action potential is propagated along the axon. Outward movement of K+ behind continues until those sections are repolarised to the resting potential, ready to receive a new stimulus

Question 20

Describe how an action potential is propagated along a myelinated neurone and why

Answer 20

The lipid-rich fatty myelin sheath acts as an electrical insulator, preventing the passage of ions, and so action potentials forming in those areas, so action potentials can only occur at the nodes of Ranvier.

Action potentials jump from node to node in a process called saltatory conduction

Question 21

What are the factors affecting the speed of a nerve impulse?

Answer 21

• Myelination
• Axon diameter
• Temperature

Question 22

How does myelination affect the speed of a nerve impulse?

Answer 22

Myelin sheath is an electrical insulator, so action potentials have to jump between nodes of Ranvier, which is quicker than propagating the action potential on the entire length of the axon

Question 23

How does axon diameter affect the speed of a nerve impulse?

Answer 23

Greater diameter means a faster speed of conductance, as there is less leakage of ions from large axons, so the membrane potential is easier to maintain.

There is also a greater surface area for K+ and Na+ channel proteins, meaning a greater rate of diffusion of ions across the axon membrane

Question 24

How does temperature affect the speed of a nerve impulse?

Answer 24

Higher temperature means a higher speed of impulse, as ions diffuse quicker (more kinetic energy), and respiration enzymes work quicker (to produce ATP for the active transport of ions).

The impulse stops when enzymes and the membrane proteins denature

Question 25

Describe the all-or-nothing principle

Answer 25

There is a threshold value which triggers an action potential.

Below the threshold, there is no action potential, and any stimulus above the threshold generated the same sized action potential, no matter the strength of the stimulus

Question 26

How do organisms perceive the size of a stimulus?

Answer 26

• The number of impulses in a given time - higher frequency = larger stimulus
• Having different neurones with different threshold values. Brain interprets the size of the stimulus by the number and the type of neurones passing impulses along

Question 27

Describe what the refractory period is

Answer 27

Once an action potential has been created, there is a period afterwards when further movement of Na+ is prevented because Na+ voltage-gated channels are closed and inactivated.

During the refractory period no further action potentials can be generated.

Question 28

What are the purposes of the refractory period?

Answer 28

• Ensures action potentials are propagated in one direction only. Action potentials cannot be propagated backwards into the refractory region
• Ensures action potentials are discrete. New action potentials cannot be formed immediately behind the first one
• Limits the number of action potentials in a given time

Question 29

What are synapses and their purpose?

Answer 29

They are the gap between two neurones. They transmit information between neurones using neurotransmitters in one direction only

Question 30

Describe the structure of a synapse

Answer 30

• Have many mitochondria and lots of endoplasmic reticulum to produce neurotransmitter, which is stored in synaptic vesicles
• At the end of the presynaptic neurone is a swollen portion called the synaptic knob
• The gap between neurones is the synaptic cleft
• The post-synaptic membrane has channel proteins with specific receptors to the neurotransmitter

Question 31

What is the neurotransmitter in a cholinergic synapse?

Answer 31

Acetylcholine (ACh)

Question 32

Describe how an impulse is transmitted across a cholinergic synapse

Answer 32

• The arrival of an action potential at the presynaptic knob causes Ca2+ channels to open, and calcium ions diffuse in
• Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine by exocytosis
• ACh diffuses across the synaptic cleft quickly because the diffusion pathway is short, and binds to receptor sites on Na+ channels in the postsynaptic membrane
• Na+ channels open, and Na+ diffuse into the postsynaptic neurone, generating a new action potential as the membrane is depolarised

Question 33

How is a synapse ‘reset’ after an impulse has been transmitted?

Answer 33

• Ca2+ is removed from the synaptic knob by active transport using ATP, so Ca2+ concentration is always higher outside
• Acetylcholinesterase hydrolyses ACh into acetyl and choline, which diffuse back into the presynaptic neurone (recycling it and preventing it from continuously generating new action potentials in the postsynaptic membrane)
• Acetyl (ethanoic acid) and choline are recombined and stored in vesicles, using ATP
• Na+ protein channels close in the absence of ACh

Question 34

What is summation?

Answer 34

Low frequency action potentials often lead to insufficient concentrations of neurotransmitters being released to trigger a new action potential in the postsynaptic neurone.

Summation causes a rapid build-up of neurotransmitter in the synapse to fix this problem

Question 35

What are the two types of summation?

Answer 35

• Temporal summation
• Spatial summation

Question 36

What is spatial summation?

Answer 36

Many different presynaptic neurones release enough neurotransmitter to exceed the threshold of the postsynaptic neurone

Question 37

What is temporal summation?

Answer 37

A single presynaptic neurone releases neurotransmitter many times over a short period. If concentration exceeds the threshold, an action potential is generated

Question 38

What is the purpose of summation?

Answer 38

• Avoids the nervous system being overloaded (not every generator potential causes an action potential)
• Synapses act as ‘barriers’
• The effect of a stimulus can be magnified

Question 39

What are inhibitory synapses?

Answer 39

Synapses that make it less likely that a new action potential will be created on the postsynaptic neurone releases

Question 40

What are the two ways that inhibitory synapses work?

Answer 40

1:
- Presynaptic neurones releases a neurotransmitter that causes K+ channels to open on the postsynaptic membrane, K+ diffuse our postsynaptic neurone
- Greater potential difference across the membrane (resting potential more negative)
- Na+ diffusing in aren’t enough for postsynaptic membrane to reach threshold

2:
- Presynaptic neurone releases a neurotransmitter causing Cl- channels to open on postsynaptic membrane, so Cl- diffuse in
- Greater potential difference across the membrane, so Na+ diffusing in aren’t enough for postsynaptic membrane to reach threshold
- This is hyperpolarisation

Question 41

What are excitatory synapses?

Answer 41

Synapses that produce new action potentials in the postsynaptic neurone.

They are unidirectional because neurotransmitter receptor proteins are only on the postsynaptic membrane

Question 42

How do synapses act as junctions?

Answer 42

They allow a single impulse on one neurone to initiate impulses in several neurones, so a single stimulus can have many responses.
Also allow a number of impulses to be combined, so different stimuli can have one response

Question 43

What are muscles and their different types?

Answer 43

Muscles are effectors that contract to bring about movement.

Cardiac muscle is in the heart, smooth muscle is in blood vessel and gut walls, but only skeletal muscle is under conscious control. They act in antagonistic pairs against an incompressible skeleton.

Question 44

Describe the general structure of skeletal muscles

Answer 44

Muscles are made from bundles of muscle fibres, which are then made from myofibril bundles.
Separate cells fuse into muscle fibres, which share nuclei and sarcoplasm (cytoplasm), containing a lot of mitochondria and endoplasmic reticulum.
The sarcolemma is a membrane surrounding muscle fibres

Question 45

What types of protein are in a myofibril?

Answer 45

• Actin: thin with 2 strands twisted around each other
• Myosin: thicker with long tails that have bulbous heads projected to the side
• Tropomyosin: forms a fibrous strand around the actin filament

Question 46

Describe the visible structure of sarcomeres

Answer 46

• Z-line is the boundary between 2 sarcomeres, seen as one dark line
• I-bands appear light and are next to the Z-lines. Are lighter because there is no overlapping of actin and myosin
• A-bands are dark next to the I-bands because they are where actin and myosin overlap
• H-zones are in between A bands and appear light, as only myosin is found here

Question 47

What are the two types of muscle fibre?

Answer 47

• Slow-twitch
• Fast-twitch

Question 48

Describe slow-twitch muscle fibres

Answer 48

Contract slowly and less powerfully than fast-twitch but for a longer time = adapted for endurance.

Adapted for aerobic respiration to avoid lactic acid build-up (less effective + prevents long-duration contraction). Have a large myoglobin store of oxygen, a rich blood vessel supply and many mitochondria

e.g calf muscle

Question 49

Describe fast-twitch muscle fibres

Answer 49

Contract rapidly and powerfully for a short time = adapted for intense exercise.

Are thicker with more myosin filaments, high concentration of glycogen, high concentration of enzymes for anaerobic respiration (rapid ATP), phosphocreatine store (can rapidly and anaerobically produce ATP from ADP)

Question 50

What is a neuromuscular junction and how are they distributed?

Answer 50

The point where a motor neurone meets a skeletal muscle fibre.

There are many junctions along the muscle so all the fibres contract simultaneously for a rapid and coordinated response.

Question 51

What is a motor unit?

Answer 51

The group of muscle fibres all supplied by the same neurone. They act as a single functional unit.

Gives control over the force exerted by the muscle, by controlling the number of motor units stimulated

Question 52

Describe how muscles are stimulated at neuromuscular junctions

Answer 52

• Action potential in motor neurone opens Ca2+ channels, and Ca2+ diffuses into the neurone
• Ca2+ causes vesicles containing Acetylcholine to fuse with the presynaptic membrane and release ACh by exocytosis
• ACh diffuses across the neuromuscular junction and binds to sarcolemma receptor proteins
• Na+ channels open and Na+ diffuse into the sarcoplasm, depolarising the sarcolemma
• Action potential passes along the sarcolemma and down the T-tubules
• Ca2+ diffuse out the sarcoplasmic reticulum and into the cytoplasm, which bind to actin filaments

Question 53

What are some similarities between neuromuscular junctions and cholinergic synapses?

Answer 53

• Have neurotransmitters (ACh) transported by diffusion
• Have receptors that bind to the neurotransmitter and cause an influx of Na+
• Are unidirectional
• Use enzymes to hydrolyse the neurotransmitter

Question 54

What are some differences between neuromuscular junctions and cholinergic synapses?

Answer 54

• N.J are only excitatory, but synapses can be excitatory or inhibitory
• N.J only links neurones to muscles, but synapses link neurones to neurones or effectors
• N.J only uses the motor neurone, but synapses can use motor, sensory or intermediate neurones
• N.J action potentials end here, but new action potentials can be produced at synapses
• At N.Js, ACh binds to the receptors on muscle membrane, but at synapses it binds to postsynaptic membrane receptors

Question 55

What is the mechanism by which skeletal muscle contracts?

Answer 55

Sliding filament mechanism - muscle shortening occurs due to movement of actin filaments over the myosin filament and the formation of actinomyosin bridges

Question 56

What is the evidence for the sliding filament mechanism?

Answer 56

• The sarcomere shortens during contraction
• The I-band and H-zone become narrower during contraction
• The A-band remains the same size during contraction

Question 57

Describe how skeletal muscles contract after stimulation

Answer 57

• Action potential travels deep into fibre through T-tubules (extensions of sarcolemma)
• Tubules are in contact with sarcoplasmic reticulum, which has a store of actively transported Ca2+ ions
• Action potential opens Ca2+ channels on reticulum, Ca2+ diffuses into sarcoplasm
• Ca2+ cause tropomyosin to change shape, exposing binding sites on actin filament
• ADP molecules on myosin heads let them bind to actin + form a cross-bridge
• Once attached, myosin heads change angle, pulling actin filament + releasing ADP
• ATP molecule attaches to each myosin head, detaching it from actin filament
• Ca2+ activate ATP hydrolase, ATP => ADP, myosin head returns to original position
• Myosin head reattaches further along filament with an ADP molecule
• Cycle repeats as long as Ca2+ concentration in myofibril remains high
• As myosin molecules are in opposite facing sets, myosin heads move towards each other, contracting muscle

Question 58

Explain how muscles relax

Answer 58

• When nervous stimulation ceases, Ca2+ are actively transported into the sarcoplasmic reticulum using energy from ATP hydrolysis
• Allows tropomyosin to block actin filament again, so myosin heads can’t bind to actin and contraction ceases
• Force from antagonistic muscles can pull actin filaments out from between myosin

Question 59

Why is ATP needed for muscle contraction and how is the demand met?

Answer 59

Energy is needed for the movement of myosin heads and the reabsorption of Ca2+ into the sarcoplasmic reticulum by active transport.

Sometimes, when ATP demand is too high for the oxygen supply, phosphocreatine is stored as a source of Pi to immediately produce ATP from ADP. Phosphocreatine stores are replenished from ATP when muscles are relaxed