Nervous coordination Flashcards

Topic 6.2

1
Q

General structure of 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 additional features of 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

How does action potential pass along an unmyelinated neuron?

A
  1. Stimulus lead 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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4
Q

Explain why myelinated axons conduct impulses faster than myelinated axons

A

Saltatory conduction: Impulse ‘jumps’ from one node of Ranvier to another. Deplarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length.

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5
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 & 2K+ into cell

Establishes electrochemical gradient: cell contents more negative than extracellular environment

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6
Q

What happens during depolarisation? (1st stage of action potential)

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 pf Na+ reverses pd.d to +40mV.
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7
Q

What happens during repolarisation? (2nd stage of action potential)

A
  1. Volatge-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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8
Q

What happens during hyperpolarisation? (3rd stage of action potential)

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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9
Q

Importance of refractory period

A

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

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10
Q

How does axon diameter affect speed of conductance?

A

Great diameter= faster

  • Less resistance to flow of ions (depolarisation & repolarisation)
  • Less ‘leakage’ of ions (easier to maintain membrane potential)
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11
Q

How does temperature affect speed of conductance?

A

Higher temperature= faster

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

Temperature too high = membrane proteins denature

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12
Q

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
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13
Q

Describe structure of a synapse

A

Presynaptic neuron end in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
Synaptic cleft: 20-3- nm gap between neurons
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)

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14
Q

What happens in presynaptic neuron when action potential is transmitted from one neuron to another?

A
  1. Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca= channels to open.
  2. Vesicles move towards & fuse with presynaptic membrane
  3. Exocytosis of neurotransmitter into synpatic cleft
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15
Q

What happens in postsynaptic neuron when action potential is transmitted from one neuron to another

A
  1. Neurotransmitter binds to specific receptor on postsynaptic membrane.
  2. Ligand-gated Na+ channels open.
  3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
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16
Q

Define summation & name 2 types

A

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

NB no summation at neuromuscular junctions.

17
Q

Differences between temporal and spatial summation

A

Temporal: one presynaptic neuron releases neurotransmitter several time in quick succession.
Spatial: multiple presynaptic neurons release neurotransmitter

18
Q

What are cholinergic synapses?

A

Use acetylecholine 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)

19
Q

What happens to acetylcholine from synaptic cleft?

A
  1. Hydrolysis into acetyl and choline by acetylcholinerase (AChE)
  2. Acetyle & choline diffuse back into presynaptic membrane
  3. ATP is used to reform acetylcholine for storage in vesicles
20
Q

What happens in inhibitory synapse?

A
  1. Neurontransmitter binds to & 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
21
Q

How might drugs decrease synaptic transmission

A
  • Inhibits release of neurotransmitter
  • Decreases permeability of postsynaptic membrane to ions
  • Hyperpolarise postsynaptic membrane