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A-Level Biology > 13.1-13.5 Neural Communication > Flashcards

13.1-13.5 Neural Communication Flashcards

1
Q

What are the main three types of neurones?

A

1) Sensory neurones
2) Motor neurones
3) Relay neurones

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2
Q

What do sensory neurones do?

A

Sensory neurones transmit nerve impulses from the receptors to the CENTRAL NERVOUS SYSTEM- the brain and spinal cord.

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3
Q

What do motor neurones do?

A

Motor neurones transmit nerve impulses from the CNS to effectors.

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4
Q

What do relay neurones do?

A

Relay neurones transmit nerve impulses between sensory neurones and motor neurones.

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5
Q

What key features do neurones have?

A

1) Cell body
2) Dendrons
3) Axons

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6
Q

What are axons?

A

Axons are singular, elongated nerve fibres that transmit impulses away from the cell body.

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7
Q

What are dendrons?

A
  • Dendrons are short extensions which come from the cell body.
  • These dendrons divide into smaller and smaller branches known as dendrites.
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8
Q

What is the cell body in a neurone?

A

The cell body contains the nucleus surrounded by the cytoplasm.

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9
Q

What is a myelin sheath and what does it do?

A
  • Myelin sheath is a membrane rich in lipid which surrounds the axon of some neurones.
  • It acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulses at a much faster speed than unmyelinated neurones.
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10
Q

What cells make up the myelin sheath?

A

Schwann cells

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11
Q

What are the gaps between the myelin sheath called?

A

Nodes of Ranvier

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12
Q

What are the two main features of sensory receptors?

A

1) They are specific to a single type of stimulus.

2) They act as a transducer- they convert a stimulus into a nerve impulse.

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13
Q

What is the stimulus for mechanoreceptors?

A

Pressure and movement

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14
Q

What type of receptors are pacinian corpsucles?

A

Mechanoreceptors

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15
Q

What do sensory receptors convert the stimulus into?

A

A nerve impulse, known as generator potential.

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16
Q

What are pacinian corpuscles?

A

Pacinian corpsucles are specific sensory receptors that detect mechanical pressure.

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17
Q

Where are pacinian corpuscles located?

A
  • Deep within skin
  • Fingers
  • Soles of feet
  • Within joints
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18
Q

Describe the structure of a pacinian corpsule.

A
  • End of sensory neurone found within centre of corpscule.
  • Surrounded by layers of connective tissue
  • Each layer of tissue is separated by layer of gel.
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19
Q

How does the Pacinian Corpsucle convert mechanical pressure into a nerve impulse?

A

1) In a normal state (resting state), the stretch-mediated sodium ion channels in the sensory neurone’s membrane are too narrow to allow sodium ions to pass through them.
2) When pressure is applied to the Pacinian corpsucle, the corpsucle changes shape. This causes the membrane surrounding the neurone to stretch.
3) When the membrane stretches, the sodium ion channels present widen. Sodium ions can now diffuse into the neurone.
4) The influx of positive sodium ions changes the potential of the membrane- it becomes depolarised. This results in a generator potential.
5) In turn, the generator potential creates an action potential that passes along sensory neurone to CNS

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20
Q

What is resting potential?

A

Resting potential is the potential difference across the membrane when a neurone is not transmitting an impulse.

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21
Q

What is the potential difference across the membrane at resting potential?

A

-70mV

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22
Q

How is resting potential created?

A

1) Sodium ions are actively transported out of the axon whereas potassium ions are actively transported into the axon by a sodium-potassium pump. However their movement is not equal because for every 3 sodium ions pumped out, two potassium ions are pumped in.
2) As a result there are more sodium ions outside the membrane than inside the axon cytoplasm.
3) Therefore sodium ions diffuse back into the axon down its electrochemical gradient, and potassium ions diffuse out of the axon.
4) Most of the gated sodium ion channels are closed, preventing movement of sodium ions whereas many potassium ion channels are open, allowing potassium ions to diffuse out of the axon.
5) As there are more positively charged ions outside the axon than inside the axon, a resting potential of -70mV is created across the membrane.

23
Q

When does an action potential occur?

A

An action potential occurs when protein channels in the axon membrane change shape as a result of the change in voltage across its membrane.

24
Q

Describe the events that occur in an action potential.

A

1) The neurone has a resting potential. Some potassium ion channels are open but sodium voltage-gated ion channels are closed.
2) STIMULUS: The energy of the stimulus triggers some sodium voltage-gated ion channels to open, making the membrane more permeable to sodium ions. Sodium ions therefore diffuse into the axon down their electrochemical gradient. This makes the inside of the axon less negative.
3) DEPOLARISATION: If the potential difference reaches the threshold value of around -55mV, voltage-gated sodium ion channels open and more sodium ions diffuse into the axon. This is POSITIVE feedback.
4) REPOLARISATION: When the potential difference reaches approx +40mV, the voltage-gated sodium ion channels close and voltage-gated potassium ion channels open. The membrane is now permeable to potassium so potassium ions diffuse out of the neurone down the electrochemical gradient. This starts to get the membrane back to its resting potential. This is NEGATIVE feedback.
5) HYPERPOLARISATION: Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mV)
6) RESTING POTENTIAL: The ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting potential until the membrane’s excited by another stimulus.

25
Q

What is a nerve impulse?

A

A nerve impulse is an action potential that starts at one end of the neurone and is propagated along the axon to the end of the neurone.

26
Q

What is a synapse?

A

A synapse is the junction between two neurones.

27
Q

What is the synaptic cleft?

A

The gap which separates the axon of one neurone from the dendrite of the next neurone

28
Q

What is the presynaptic neurone?

A

Neurone along which the impulse has arrived.

29
Q

What is the post synaptic neurone?

A

The neurone that receives the neurotransmitters

30
Q

What is the synaptic knob?

A

The swollen end of the presynaptic neurone. It contains many mitochondria and large amounts of endoplasmic reticulum to enable it to manufacture neurotransmitters.

31
Q

What are neurotransmitter receptors?

A

Receptor molecules which the neurotransmitter binds to in the post synaptic membrane.

32
Q

Which two categories can neurotransmitters be grouped into?

A

1) Excitatory

2) Inhibitory

33
Q

What are excitatory neurotransmitters?

A
  • These neurotransmitters result in the depolarisation of the postsynaptic neurone.
  • If the threshold is reached in the post synaptic membrane, an action potential is triggered.
  • An example of an excitatory neurotransmitter is acetylcholine.
34
Q

What are inhibitory neurotransmitters?

A
  • These neurotransmitters result in the hyperpolarisation of the postsynaptic membrane
  • This prevents an action potential being triggered.
  • An example of an inhibitory neurotransmitter is gamma-aminobutryic acid (GABA)
35
Q

Describe how synaptic transmission occurs.

A

1) The action potential reaches the end of the presynaptic neurone.
2) Depolarisation of the presynaptic membrane causes calcium ion channels to open.
3) Calcium ions diffuse into the presynaptic knob.
4) This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
5) Neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane.
6) This causes sodium ion channels to open.
7) Sodium ions diffuse into the postsynaptic neurone.

36
Q

How are neurotransmitters left in the synaptic cleft removed?

A
  • Acetylcholine is broken down by enzyme ACh esterase into choline and acetate
  • The products are taken back into the presynaptic knob.
37
Q

Why is removing the neurotransmitter from the synaptic cleft important?

A

Removing the neurotransmitter from the synaptic cleft prevents the response from happening again and allows the neurotransmitter to be recycled.

38
Q

Which synapses use the neurotransmitter acetylcholine?

A

Cholinergic synapses

39
Q

Where are cholinergic synapses used?

A
  • CNS of vertebrates

- At neuromuscular junctions (where motor neurone and muscle cell/effector meet)

40
Q

Describe how transmission occurs across cholinergic synapses.

A

1) The arrival of an action potential at the end of the pre-synaptic neurone causes calcium ion channels to open and calcium ions (Ca2+) enter the synaptic knob.
2) The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into the synaptic cleft.
3) Acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone. This causes the sodium ion channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient.
4) The influx of sodium ions generate a new action potential in the postsynaptic neurone.
5) Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone. In addition to recycling the choline and ethanoic acid, the breakdown of acetylcholine also prevents it from continously generating a new action potential in the post synaptic neurone.
6) ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites.

41
Q

What role do synapses play in the nervous system?

A

1) They ensure impulses are unidirectional. As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone.
2) They can allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses. This results in a single stimulus creating a number of simultaneous responses. This is SYNAPTIC DIVERGENCE and means that information can be dispersed to different parts of the body.
3) A number of neurones may feed into the same synapse with a single postsynaptic neurone. This results in stimuli from different receptors interacting to produce a single result. This is SYNAPTIC CONVERGENCE and means that information can be amplified.

42
Q

What does summation mean?

A

When the buildup of neurotransmitter in an synapse is sufficient to trigger an action potential

43
Q

In which two ways can summation occur?

A

1) Spatial summation

2) Temporal summation

44
Q

When does spatial summation occur?

A

Spatial summation occurs when a number of presynaptic neurones connect to one postsynaptic neurone. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone.

45
Q

When does temporal summation occur?

A

Temporal summation occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient to trigger an action potential.

46
Q

What effects can drugs have on synapses?

A
  • Mimiciking the shape of the neurotransmitter e.g nicotine
  • Stimulating the release of more neurotransmitter e.g amphetamines
  • Inhibiting the enzyme responsible for breaking down the neurotransmitter in the synapse. E.g nerve gases stop acetylcholine being broken down
  • Blocking receptors so that the neurotransmitter can no longer bind and activate the receptor.
  • Binding to specific receptors on postsynaptic membrane of some neurones and changing the shape of the receptor.
47
Q

What is the refractory period?

A

A short period of time when the axon cannot be excited again. During this time the voltage gated sodium ions remain closed, preventing the movement of sodium ions into the axon.

48
Q

Why are refractory periods important?

A
  • Prevents propagation of an action potential forwards as well as backwards
  • Makes sure action potentials are unidirectional
  • Makes sure action potentials do not overlap and occur as discrete impulses.
49
Q

Why do myelinated axons transfer electrical impulses much faster than unmyelinated?

A
  • Because depolarisation of the axon can only occur at the nodes of Ranvier where no myelin is present.
  • Here sodium ions can pass through the protein channels in the membrane.
  • The action potential then ‘jumps’ from one node to another in a process called saltatory conduction.
  • This is much faster than a wave of depolarisation along the whole length of the axon membrane.
50
Q

What factors affect the speed at which an action potential travels?

A
  • Myelination
  • Axon diameter
  • Temperature
51
Q

How does axon diameter affect the speed at which an action potential travels?

A
  • The bigger the axon diameter, the faster the impulse is transmitted.
  • This is because there is less resistance to the flow of ions in the cytoplasm, compared with those in a smaller axon.
52
Q

How does temperature affect the speed at which an action potential travels?

A
  • The higher the temperature, the faster the nerve impulses.
  • This is because ions diffuse faster at higher temperatures.
  • However this generally only occurs up to about 40 degrees as higher temperatures cause the proteins to become denatured.
53
Q

What does the all-or-nothing principle suggest?

A
  • If the threshold value is reached, an action potential will always be created.
  • No matter how large the stimulus is, the same sized action potential will always be triggered.
  • If the threshold is not reached, no action potential will be triggered.

A-Level Biology (41 decks)

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