Nerves

Question 1

What are sensory receptors?

Answer 1

• Transducers.
• They convert one form of energy into another.
• They convert a change in energy of the stimulus into electrical impulses sent down neurones (called nerve impulses).

Question 2

What types of stimulus are there?

Answer 2

• Light intensity and wavelength- Detected by photosensitive cells on the retina.
• Volatile chemicals- Detected by olfactory cells in the nasal cavity.
• Soluble chemicals- Taste buds on the tongue.
• Pressure on skin- Pressure receptors called Pacinian corpuscules.
• Vibrations in air: Sound receptors in ear canal.
• Length of muscles: Muscle spindles.

Question 3

What are the 3 main types of neurones in the body?

Answer 3

• Sensory neurone: Carries action potential from a sensory receptor to the CNS.
• Motor neurone: Carries action potential from the CNS to an effector.
• Relay neurone: Connects sensory neurones to motor neurones.

Question 4

How are neurones specialised for their purpose?

Answer 4

• Long so are able to transmit action potentials across long distances.
• Gated ion channels (Na+, Ca2+ and K+) on plasma membrane to generate/transmit action potential.
• Na+/K+ pump using ATP to maintain resting potential by pumping Na+ out and K+ in.
• Motor and sensory neurone surrounded by fatty sheath called myelin sheath, which are made from Schwann cells for electrical insulation and increased speed of transmission. Gaps between Schwann cells called nodes of Ranvier.
• Dendrites connect neurones to other neurones.

Question 5

What is the structure of a motor neurone?

Answer 5

• Cell body at one end of the neurone, with dendrites and ‘mini’ dendrons leading into it from all direction.
• Long myelinated axon leading to stimuli.
• Dendrites at end of axon with synaptic knobs to pass on stimulus via synapse.

Question 6

What is the structure of a sensory neurone?

Answer 6

• Dendrites receive signal from the sensory receptor and action potential is generated.
• Myelinated dendrons carry impulse from sensory receptor to cell body.
• Myelinated axon carries impulse from cell body to synaptic knobs on dendrites that transmit signal to CNS.

Question 7

What is the structure of a relay neurone?

Answer 7

• Dendrites receive impulse from sensory neurones which is transmitted to the cell body via short unmyelinated dendron.
• Axon carries impulse from cell body to dendrites that transmit impulse to motor neurones.

Question 8

What are the differences between the 3 types of neurones?

Answer 8

• Location: Cell body of sensory neurone outside CNS whereas they are inside for motor and relay neurones.
• Length of axon: Shorter than dendron for sensory neurone, much longer than axon for motor neurones and short for relay neurones.
• Length of dendrons: Longer than axon for sensory neurones; very short, if not, non-existent for motor neurone and short for relay neurone.
• Myelination: Sensory neurone and motor neurone are always myelinated whereas relay neurones are usually un-myelinated.
• Direction of impulse: Sensory neurone- from receptors to CNS. Motor neurones- from CNS to effectors. Relay neurones- from sensory neurones to motor neurones.
• Type of neurone: Sensory neurone- Afferent neuron. Motor neurone- Efferent neurone.

Question 9

What is the resting potential of a neurone?

Answer 9

-60mV.

Question 10

How is the resting potential of a neurone created and maintained?

Answer 10

• Large concentration of organic anions present on inside of neurone as plasma membrane is impermeable to large organic molecules. This creates negative charge.
• High concentration of Na+ ions on outside and negative charge on inside creates large concentration and electrical gradient for Na+ ions to diffuse into neurone and destroy resting potential.
• Na+/K+ pumps in plasma membrane uses ATP to pump out Na+ ions and in exchange for entrance of K+ ions. For every 3 Na+ ions pumped out, only 2 K+ ions pumped in. This decreases net charge inside neurone overall and maintains negative resting potential inside neurone.
• The neurone is said to be polarised.

Question 11

How is a generator potential created?

Answer 11

• When a stimulus smaller than that of the the threshold is applied to a neurone, some Na+ channels open to let some Na+ ions into neurone.
• This creates a very small impulse called a generator potential to travel across plasma membrane of the neurone.

Question 12

How is an action potential created?

Answer 12

1. The inside of the neurone is polarised at -60mV to the surrounding.
2. When stimulus is applied to neurone, depolarisation occurs as some Na+ channels are opened and allows influx of Na+ ions into neurone. This causes charge inside neurone to increase.
3. As charge increases beyond threshold of -50mV, all voltage-gated Na+ channels and there is a large influx of Na+ ions due to electrical and concentration gradient.
4. Influx of Na+ ions flips relative charge across plasma membrane to +40mV.
5. At this point, Na+ channels close and K+ channels open.
6. There is a large efflux of K+ ions down the electrical and concentration gradient out of the neurone.
7. Relative charge inside neurone decreases and flips again to negative (repolarisation).
8. Efflux of K+ ions causes charge inside neurone to go below resting, causing hyperpolarisation, which is restored to resting potential after very short time.
9. Na+/K+ pumps work to replace high concentration of Na+ inside neurone with K+ from outside neurone, ready for another action potential.

Question 13

What is the all-or-nothing law?

Answer 13

The law states that if a stimulus causes a depolarisation above the threshold, all voltage-gated Na+ channels open to generate an action potential. The size of the action potential is the same no matter the size of the stimulus. This difference can thus only be determined by the frequency of the action potentials.

Question 14

What is the refractory period?

Answer 14

The period of time when part of a neurone is recovering from a previous action potential is called a refractory period as it is impossible to stimulate an action potential during this period no matter the size of the stimulus.

Question 15

How is an action potential transmitted across a neurone?

Answer 15

1. When action potential is generated at one point along a neurone, Na+ channels open at that point and there is large influx of Na+ ions, increasing Na+ concentration at point of entry.
2. Na+ ions diffuse from point of entry along neurone, down concentration and electrical gradient.
3. Movement of Na+ ions creates local currents along neurone.
4. This causes an increase in potential difference further along neurone above the -50mV threshold.
5. Voltage-gated Na+ channels further along from point of original action potential open, allowing further influx of Na+ ions, so action potential moves along neurone.
6. Process repeats from one end of neurone to the other end, transmitting action potential along neurone.

Question 16

What is the purpose of myelination along neurones?

Answer 16

• To speed up rate of transmission of action potential along neurones by process called saltatory conduction.

Question 17

How does myelination speed up rate of action potential transmission?

Answer 17

• Na+/K+ ions are unable to diffuse through lengths of neurone lined by fatty myelin sheath.
• Ionic exchange can only occur in nodes of Ranvier.
• Na+ ions from stimulus point diffuse across length of Schwann cells to reach next node in order to transmit action potential.
• Local currents are elongated and action potential ‘jumps’ between nodes of Ranvier, across Schwann cells, speeding up impulse.
• This is called saltatory conduction.

Question 18

What is the structure of an excitatory (cholinergic) synapse?

Answer 18

1. Synapse begins as synaptic knobs on ends of dendrites at axon ends of neurones.
2. Synaptic knob contains many Ca2+ ions channels.
3. Lots of mitochondria to produce ATP needed to pump Ca2+ ions out of synaptic knob.
4. Neurotransmitter vesicles containing the neurotransmitters.
5. Pre-synaptic membrane.
6. Synaptic cleft, around 20nm in length.
7. Post-synaptic membrane, containing lots of neurotransmitter receptors associated with Na+ channels.
8. Dendrite at end of dendron.

Question 19

What is the difference between an excitatory synapse and an inhibitory synapse?

Answer 19

For excitatory synapse, post-synaptic membrane contains Na+ channels whereas for inhibitory synapse, post-synaptic membrane contains K+/Cl- channels.

Question 20

What is the sequence of events in an excitatory synapse?

Answer 20

1. Nerve impulse travels along axon of pre-synaptic neurone.
2. Nerve impulses travel along the dendrite to reach synaptic knob.
3. Voltage-gated Ca2+ channels open and Ca2+ ions diffuse into synaptic knob down concentration gradient.
4. Ca2+ ions cause neurotransmitter vesicles to move to pre-synaptic membrane.
5. Vesicles fuse with pre-synaptic membrane to release neurotransmitter into synaptic cleft via exocytosis.
6. Neurotransmitter diffuses across synaptic cleft and bind to complementary receptor sites on Na+ channels on post-synaptic membrane.
7. Na+ channels open in post-synaptic membrane.
8. Some Na+ diffuse into post-synaptic cell and depolarisation occurs across post-synaptic membrane- Excitatory Post Synaptic Potential (EPSP), creating generator potential.
9. If generator potential exceeds threshold, action potential generated in post-synaptic neurone.

Question 21

What is the sequence of events in an inhibitory synapse?

Answer 21

1. Nerve impulse travels along axon of pre-synaptic neurone.
2. Nerve impulses travel along the dendrite to reach synaptic knob.
3. Voltage-gated Ca2+ channels open and Ca2+ ions diffuse into synaptic knob down concentration gradient.
4. Ca2+ ions cause neurotransmitter vesicles to move to pre-synaptic membrane.
5. Vesicles fuse with pre-synaptic membrane to release neurotransmitter into synaptic cleft via exocytosis.
6. Neurotransmitter diffuses across synaptic cleft and bind to complementary receptor sites on K+/Cl- channels on post-synaptic membrane.
7. K+/Cl- channels open in post synaptic membrane.
8. Some K+ diffuses out of/Cl- diffuses into post-synaptic cell and hyperpolarisation occurs across post-synaptic membrane- Inhibitory Post Synaptic Potential, IPSP.
9. IPSP decreases likelihood of action potential being generated in post-synaptic neurone.

Question 22

How does a synapse reset after transmission? E.g. acetylcholine.

Answer 22

1. Acetylcholine is broken down by the enzyme acetylcholinesterase which breaks ACh down into ethanoic acid and chlorine, closing Na+ channels and stopping EPSP.
2. Ethanoic acid and chlorine diffuse from post-synaptic membrane back to pre-synaptic membrane and diffuse into synaptic knob.
3. They are recombined into ACh using ATP and are repackaged into vesicles for future use.
4. Ca+ ions are pumped out of synaptic knob by active transport using ATP.

Question 23

What is the general nature of connectivity between neurones?

Answer 23

• One pre-synaptic neurone will be connected to many other neurones via synapses. This allows one stimulus to trigger a series of responses.
• One post-synaptic neurone may be connected to several pre-synaptic neurones, allowing a range of different stimuli to cause the same response.
• Both inhibitory and excitatory synapses may be connected to a post-synaptic neurone.

Question 24

What does the process of summation incur?

Answer 24

• Inhibitory and excitatory synapses connected to a post-synaptic neurone may be active at the same time.
• If the sum of the IPSPs and the EPSPs exceed the threshold potential, an action potential is generated. This is called summation in space.
• Frequent, repeated action potentials may be sent from pre-synaptic neurones to the post-synaptic neurone. This results in the continual release of neurotransmitters and production of small EPSPs.
• Small, frequent EPSPs add up to exceed threshold potential, resulting in action potential. This is called summation in time.

Question 25

What is the process if acclimatisation?

Answer 25

• If an action potential is continuously transmitted across a synapse, neurotransmitters in the synaptic knobs may run out so that no more cross-synapse transmission can occur. The synapse is said to be fatigued.
• This can explain why we get used to certain sensations after a while.