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Biology > CH15 Nervous coordination > Flashcards

CH15 Nervous coordination Flashcards

1
Q

Describe general structure of a motor neurone

A

Cell body - contains organelles and 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 additional features of myelinated motor neurone

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

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3
Q

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 neurone

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
  3. Sequential wave of depolarisation
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5
Q

Explain why myelinated axons conduct impulses faster than unmyelinated axons

A

Saltatory conduction - Impulse jumps from 1 node of Ranvier to another
Depolarisation cannot occur where myelin sheath acts as insulator
Impulse does not travel whole length of axon

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6
Q

What is resting potential

A

Potential difference across neurone membrane when not stimulated (-70mV in humans)

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7
Q

How is resting potential established

A
  1. Membrane more permeable to K+ than Na+
  2. Sodium-Potassium pump actively transports 3Na+ out of cell and 2K+ into cell
    Establishes an electrochemical fradient - cell contents more negative than extracellular environment
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8
Q

Stages in generating action potential

A

Depolarisation
Repolarisation
Hyperpolarisation
Returning to resting potential

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9
Q

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

Repolarisation

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

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 and sodium-potassium pump re-establishes resting potential
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12
Q

Importance of refractory period

A

No action potential can be generated in hyperpolarised sections of membrane:
Ensures unidirectional impulse
Ensures discrete impulses
Limits frequency of impulse transmission

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13
Q

What is all or nothing principle

A

Any stimulus that causes the membrane to reach threshold potential will generate an action potential
All action potentials have the same magnitude

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14
Q

Factors that affect speed of conductance

A

Myelin sheath
Axon diameter
Temperature

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15
Q

How does axon diameter affect speed of conductance

A

Greater diameter = faster
Less resistance to flow of ions
Less leakage of ions

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16
Q

How does temperature affect speed of conductance

A

Higher temperature = faster
Faster rate of diffusion
Faster rate of respiration = more ATP for active transport to re-establish resting potential
Temperature too high = denaturing of membrane proteins

17
Q

How can an organism detect strength of stimulus

A

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

18
Q

What is function of synapses

A

Electrical impulse cannot travel over junction between neurones
Neurotransmitters send impulses between neurones and from neurones to effectors

19
Q

New impulses can be initiated in several different neurones for multiple simultaneous responses

A
20
Q

Describe structure of a synapse

A

Presynaptic neurone ends in synaptic knob - contains lots of mitochondria, ER and vesicles of neurotransmitter
Synaptic cleft - gap between neurones
Postsynaptic neurone - has complementary recpetors to neurotransmitter

21
Q

What happens in presynaptic neurone when action potential is transmitted

A
  1. Wave of depolarisation travels down presynaptic neurone causing voltage-gated Ca2+ channels to open
  2. Causes vesicles to move and fuse with presynaptic membrane
  3. Exocytosis of neurotransmitter into synaptic cleft
22
Q

How do neurotransmitters cross the synaptic cleft

A

Simple diffusion

23
Q

What happens in postsynaptic neurone

A
  1. Neurotransmitter binds to specific receptor on postsynaptic membrane
  2. Na+ channels open
  3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated
24
Q

Explain why synaptic transmission is unidirectional

A

Only presynaptic neurone contains vesicles of neurotransmitter
Only postsynaptic membrane has complementary receptors

25
Q

Define summation

A

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

26
Q

2 types of summation

A

Temporal summation

Spatial summation

27
Q

What is temporal summation

A

1 presynaptic neurone releases neurotransmitter several times in quick succession

28
Q

What is spatial summation

A

Multiple presynaptic neurones release neurotransmitter

29
Q

What are cholinergic synapses

A

Uses acetylcholine as primary neurotransmitter

30
Q

What happens to acetylcholine from the synaptic cleft

A
  1. Hydrolysis into acetyl and choline by acetylcholinesterase (AChE)
  2. Acetyl and choline diffuse back into presynaptic membrane
  3. ATP used to reform acetylcholine for storage in vesicles
31
Q

Importance of AChE

A

Prevents overstimulation of skeletal muscle cells

Enables acetyl and choline to be recycled

32
Q

What happens in an inhibitory synapse

A
  1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open
  2. Cl- moves in and K+ moves out via facilitated diffusion
  3. p.d. becomes more negative - hyperpolarisation
33
Q

Describe structure of neuromuscular junction

A

Synaptic cleft between presynaptic neurone and a skeletal muscle cell

34
Q

How might drugs increase synaptic transmission

A

Inhibit AChE

Mimic shape of neurotransmitter

Biology (9 decks)

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