Neurones and synapses

Question 1

What is the nervous system based on?

Answer 1

A system of neurones that transmit electrical nerve impulses throughout the body. Fine control is provided through a system of synapses.

Question 2

What are nerves?

Answer 2

Bundles of neurones

Question 3

How is nervous control better than hormonal control?

Answer 3

Faster and more precise.

Question 4

What does nervous control usually involve?

Answer 4

Receptors and effectors with an interlinking coordinator.

Question 5

What are receptors, effectors and coordinators?

Answer 5

Receptors - Sensitive to a stimulus
Effectors - Produce the response
Coordinator - Involves central nervous system comprising of the brain and spinal cord.

Question 6

What are the three main types of neurone?

Answer 6

1) Motor neurones - Carry impulses from CNS to effectors
2) Sensory neurones - Carry impulse from receptors to CNS
3) Connector (relay/association) neurones - Connect neurones within the CNS

Question 7

Describe how a motor neurone, sensory neurone and connector neurone look.

Answer 7

Motor - Just a cell body and synaptic bulbs connected by an axon that is surrounded by a myelin sheath.
Sensory - Dendron –> Cell body –> Axon–> synaptic bulbs
Connector - Cell body

Question 8

What does the cell body contain?

Answer 8

Nucleus, mitochondria, Nissl’s granules and other organelles.

Question 9

What is an axon?

Answer 9

Transmits nerve impulses away from the cell body - in motor neurones the entire fibre is an axon.

Question 10

What is a dendron and dendrites?

Answer 10

Dendron - Carries impulses towards the cell body

Dendrites - Carry impulses towards the dendron

Question 11

Where do axons terminate?

Answer 11

Synaptic bulbs.

Question 12

What does myelinated mean?

Answer 12

Their dendrons and axons are covered in a myelin sheath. Rich in the lipid myelin. Formed from the greatly extended cell surface membrane of the Schwann cells.

Question 13

What are the nodes of Ranvier?

Answer 13

The Schwann cells are arranged at intervals along the nerve fibre with small gaps between them called the nodes of ranvier.

Question 14

What is the function of myelination

Answer 14

Speed up nervous conduction and protection.

Question 15

What is the resting potential?

Answer 15

The potential difference across the cell surface membrane. This means the neurones are polarised. Excess of Na+ ions on the outside. The inside of the neurone is negative with a potential difference of -70mV. This is maintained because the cell surface membrane is largely impermeable to the flow of Na+ ions when not conducting an impulse.

Question 16

What happens when a neurone is stimulated?

Answer 16

The cell surface membrane becomes permeable to ions. Na+ ions diffuse in along the concentration gradient, making the inside of the neurone less negative.

Question 17

What is the threshold vale and what happens when it is reached?

Answer 17

At -55mV a number of gated channels open and Na+ ions surge in and the neurone quickly becomes depolarised. The inside of the neurone becomes positive in relation to the outside reaching a potential difference of +40mV. This depolarisation and reversal of the potential difference is referred to as an action potential.

Question 18

What is the all or nothing law?

Answer 18

Once the threshold stimulus is reached an action potential will result, i.e. an action potential either occurs or it doesn’t. There are no different intensities, they are all the same.

Question 19

What is the next phase called and what happens during it?

Answer 19

The recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. The membrane repolarises and recovers its resting potential. It is also referred to as the refractory period, a further impulse cannot be fired as the gated channels are closed and the resting potential has not been fully restored. Hyper-polarisation.

Question 20

What are the uses for the refractory period?

Answer 20

1) Ensures action potentials are propagated in one direction only.
2) Limits the number of action potentials that are able to be fired and ensures each is a discrete entity.

Question 21

What are the important features of nerve impulse?

Answer 21

1) The threshold stimulus is the level of stimulus neurones require before an action potential is produced.
2) The all or nothing law as previously discussed.

Question 22

How are nerve impulses propagated?

Answer 22

A wave of depolarisation moves rapidly along neurones. As one part is depolarised it sets up local circuits either side. Positive ions from the depolarised zone pass along the inside of the neurone to the polarised zone immediately in front. And vice versa outside the neurone.

Question 23

What affects the speed of nerve impulse?

Answer 23

1) Myelination - Depolarisation takes place at the Nodes of Ravier, local circuits form between nodes only and the action potential jumps from one node to the next in a process called Saltatory Conduction.
2) Large diameter of the axon - The thicker the axon the faster the impulse
3) Temperature - Affects the rate of diffusion.

Question 24

Describe the process of transmission of impulses at synapses.

Answer 24

1) When an impulse arrives at the synaptic bulb, Calcium ion channels open allowing Ca2+ ions to diffuse in.
2) The Ca2+ ions cause the synaptic vesicles to move towards the pre-synaptic membrane.
3) The vesicles fuse with the pre-synaptic membrane, releasing Acetylcholine by exocytosis into the synaptic cleft.
4) The Acetylcholine diffuses across the synaptic cleft and binds with Acetylcholine receptors in the post-synaptic membrane.
5) This causes the opening of the Na+ ion channels in the membrane of the post synaptic neurone. As positive ions diffuse in, the membrane becomes gradually depolarised and an Excitatory Post-synaptic potential (EPSP)
6) If a threshold level of depolarisation occurs and action potential is produced in the post-synaptic neurone.
7) Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid which are then released into the cleft. If its not broken down it will continually generate an action potential in the post-synaptic membrane.
8) Choline and ethanoic acid diffuse across the cleft and are reabsorbed into the synaptic bulb. Re-synthesised into Acetylcholine and stored in synaptic vesicles to be used again. ATP required is produced by mitochondria.

Question 25

What advantages does the presence of synapses give?

Answer 25

1) Allows nervous control to continue
2) Ensure the impulse travels in one direction only
3) Prevent the overstimulation of effectors, or else they will fatigue.
4) Provide integration - a number of presynaptic neurones forming one junction with a post-synaptic neurone.

Question 26

What is summation?

Answer 26

A series of impulses travelling along the same neurone each releasing neurotransmitter chemicals.

Question 27

What is an Inhibitory post-synaptic potential (IPSP)?

Answer 27

Make it more difficult for synaptic transmission to occur. i.e. the neurotransmitter they release makes it harder for an EPSP to form. They cause an influx of negative ions in the post synaptic membrane - Hyperpolarisation.

Question 28

Whats noradrenaline?

Answer 28

Basically the same as ACh, Acetylcholine. Causes Na+ ion channels to open in the post synaptic membrane.

Question 29

What is GABA?

Answer 29

Causes negative ions to flow into the post-synaptic membrane - Hyperpolarisation. Inhibitory Post-synaptic Potential (IPSP).