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A Level Biology > 6.2 Nervous Coordination > Flashcards

6.2 Nervous Coordination Flashcards

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1
Q

Describe the general structure of a motor neuron

A

Cell body: contains organelles & high proportion of RER

Dendrons: branch into dendrites which carry impulses towards cell body

Axon: long,unbranched fibre carries nerve impulses away from cell body

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2
Q

Describe the additional features of a myelinated motor neuron

A

Schwann cells: wrap around axon many times

Myelin sheath: made from myelin-rich membranes of Schwann cells

Nodes of ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath

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3
Q

Name 3 processes Schwann cells are involved in

A

Electrical insulation
Phagocytosis
Nerve regeneration

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4
Q

How does an action potential pass along an unmyelinated neuron?

A
  1. Stimulus leads to influx of Na+ ions. First section of membrane depolarises
  2. Local electrical currents cause sodium voltage-gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise.
  3. Sequential wave of depolarisation
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5
Q

Explain why myelinated axons conduct impulses faster than unmyelinated axons

A

Salatatory conduction: impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator.

So impulse does not travel along whole axon length

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6
Q

What is resting potential

A

Potential difference (voltage) across neuron membrane when not stimulated (-50 to -90 mV, usually about -70 mV in humans)

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7
Q

How is resting potential established?

A
  1. Membrane is more permeable to K+ than Na+.
  2. Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell

Establishes electrochemical gradient: cell contents more negative than extracellular environment

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8
Q

Name the stages in generating an action potential

A
  1. Depolarisation
  2. Repolarisation
  3. Hyperpolarisation
  4. Return to resting potential
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9
Q

What happens during depolarisation?

A
  1. Stimulus —> facilitated diffusion of Na+ ions into cell down electrochemical gradient
  2. p.d. across membrane becomes more positive
  3. If membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open
  4. Significant influx of Na+ ions reverses p.d. to +40mV
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10
Q

What happens during repolarisation

A
  1. Voltage-gated Na+ channels close and voltage-gated K+ channels open
  2. Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
  3. p.d. across membrane becomes more negative
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11
Q

What happens during hyperpolarisation

A
  1. ‘Overshoot’ when K+ ions diffuse out = p.d. becomes more negative than resting potential.
  2. Refractory period: no stimulus is large enough to raise membrane potential to threshold
  3. Voltage-gated K+ channels close & sodium-potassium pump re-establishes resting potential
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12
Q

Explain the importance of the refractory period

A

No action potential can be generated in hyperpolarised sections of membrane:

  • ensures unidirectional impulses
  • ensures discrete impulses
  • limits frequency of impulses transmission
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13
Q

What is the ‘all or nothing’ principle?

A

Any stimulus that causes the membrane to reach threshold potential will generate an action potential.

All action potentials have same magnitude

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14
Q

Name the factors that affect the speed of conductance

A

Myelin sheath
Axon diameter
Temperature

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15
Q

How does axon diameter affect the speed of conductance?

A

Greater diameter = faster

Less resistance to flow of ions (depolarisation & repolarisation).
Less ‘leakage’ of ions (easier to maintain membrane potential).

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16
Q

How does temperature affect speed of conductance

A

Higher temperature = faster

Faster rate of diffusion (depolarisation & repolarisation).
Faster rate of respiration (enzyme-controlled) = more ATP for active transport to re-establish resting potential

Temperature too high = membrane proteins denature

17
Q

Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance

A

Student’s t-test (comparing means of continuous data)

18
Q

Suggest appropriate units for the maximum frequency of impulse conduction

A

Hz

19
Q

How can an organism detect the strength of a stimulus

A

Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses

20
Q

What is the function of synapses?

A

Electrical impulse cannot travel over junction between neurons
Neurotransmitters send impulses between neurons/from neurons to effectors
New impulses can be initiated in several different neurons for multiple simultaneous responses

21
Q

Describe the structure of a synapse

A

Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter

Synaptic cleft: 20-30 nm gap between neurons.

Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand gated Na+ channels).

22
Q

Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another

A
  1. Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open.
  2. Vesicles move towards & fuse with presynaptic membrane.
  3. Exocytosis of neurotransmitter into synaptic cleft
23
Q

How do neurotransmitters cross the synaptic cleft?

A

Via simple diffusion

24
Q

Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another

A
  1. Neurotransmitter binds to specific receptors on postsynaptic membrane
  2. Ligand-gated Na+ channels open
  3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated
25
Q

Explain why synaptic transmission is unidirectional

A

Only presynaptic neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.

So impulse always travels presynaptic—> postsynaptic

26
Q

Define summation and name the 2 types

A

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential:

Temporal summation
Spatial summation

NB no summation at neuromuscular junctions

27
Q

What is the difference between temporal and spatial summation?

A

Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession

Spatial: multiple presynaptic neurons release neurotransmitter

28
Q

What are cholinergic synapses

A

Use acetylcholine as primary neurotransmitter. Excitatory or inhibitory. Located at:

-motor end plate (muscle contraction)
- preganglionic neurons (excitation)
- parasympathetic postganglionic neurons (inhibition e.g. of heart or breathing rate).

29
Q

What happens to acetylcholine from the synaptic cleft?

A
  1. Hydrolysis into acetyl and choline by acetylcholinesterase (AChE).
  2. Acetyl & choline diffuse back into presynaptic membrane
  3. ATP is used to reform acetylcholine for storage in vesicles
30
Q

Explain the importance of AChE

A

Prevents overstimulation of skeletal muscle cells
Enables acetyl and choline to be recycled

31
Q

What happens in an inhibitory synapse?

A
  1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane & triggers K+ channels to open
  2. Cl- moves in & K+ moves out via facilitated diffusion
  3. p.d. becomes more negative: hyperpolarisation
32
Q

Describe the structure of a neuromuscular junction

A

Synaptic cleft between a presynaptic neuron and a skeletal muscle cell

33
Q

Contrast a cholinergic synapse and a neuromuscular junction

A

Slide 68

34
Q

How might drugs increase synaptic transmission

A

Inhibit AChE
Mimic shape of neurotransmitter

35
Q

How might drugs decrease synaptic transmission?

A

Inhibit release of neurotransmitter
Decrease permeability of postsynaptic membrane to ions
Hyperpolarise postsynaptic membrane

A Level Biology (33 decks)

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