Neurones And Synaptic Transmission

Question 1

What are neurones and what is their function?

Answer 1

There are around 100 billion neurones in the human nervous system, 80% of which are located in the brain- neurones are simply specialised nerve cells which communicate by transmitting signals electrically and chemically; these provide the nervous system with its primary means of communication

Question 2

What are the 3 types of neurones?

Answer 2

1- motor neurones
2- sensory neurones
3- relay neurones

Question 3

What are motor neurones?

Answer 3

Nerve cell that carries electrical impulses from how CNS to the effector e.g. muscles or glands

Question 4

What is the structure of a motor neurone regarding its dendrites and axons?

Answer 4

Short dendrites, long axons

Question 5

What are sensory neurones?

Answer 5

Nerve cells that carry electrical impulses from a receptor (PNS) to the CNS

Question 6

What is the structure of a sensory neurone regarding its dendrites and axons?

Answer 6

Long dendrites, short axons

Question 7

What is a relay neurone?

Answer 7

Nerve cell that carries electrical impulses between sensory and motor neurones (acts as a coordinator between the sensory and motor neurones)

Question 8

What is the structure of arelay neurone regarding its dendrites and axons?

Answer 8

Short dendrites, short axons

Question 9

What is the general neurone structure?

Answer 9

• cell body: contains all usual organelles including a nucleus and large amounts of ER (associated with production of proteins and neurotransmitters)
• dendrites: extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses away from the cell body
• Schwann cells: surround axon (protection and provide electrical insulation)
• myelin sheath: forms covering to axon and made up of membranes of Schwann cells
• nodes of ranvier: constrictions between adjacent Schwann cells

Question 10

Explain electrical transmission (‘firing’ of a neurone)

Answer 10

• when a neurone is not transmitting an action potential it’s in its resting state and actively getting ready to do so (resting potential) where due to the sodium-potassium pump actively transporting 3 sodium ions out of the membrane and only 2 potassium ions into the membrane, the membrane is said to be polarised (inside of cell positively charged compared to the outside)
• an action potential is transmitted when the inside of the cell becomes more negatively charged than the outside- depolarised membrane when sodium ions diffuse out of the axon but potassium ions cannot diffuse out as potassium ions channel closed
• action potential travels down the axon towards the end of the neurone

Question 11

Each neurone is separated by

Answer 11

A synapse

Question 12

Describe the process of synaptic transmission

Answer 12

• once an action potential reaches the end of a neurone, the influx of sodium ions and subsequent opening of calcium channels caused the pre-synaptic vesicles containing neurotransmitter e.g. adrenaline to fuse with the pre-synaptic membrane
• the neurotransmitter diffuses across the synaptic cleft and binds to a specific, complementary receptor protein on the post-synaptic membrane where the chemical messenger of a neurotransmitter is ‘reformed’ into an electrical messenger so an action potential can be ‘restarted’
• neurotransmitters can be excitatory (increase the likelihood of an action potential being formed at the post-synaptic neurone) e.g.adrenaline or inhibitory (decrease the likelihood of an action potential being formed at the post-synaptic membrane by making it more negatively charged so the threshold to ‘fire’ an action potential cannot be reached e.g. serotonin
• temporal and spatial summation ensure that the threshold is met on the post-synaptic neurone

Question 13

Explain summation

Answer 13

Low-frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone. They can however, do so in summation: this enables a rapid build-up of neurotransmitter in the synapse by spatial summation or temporal summation

Question 14

Explain spatial summation

Answer 14

A number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. Together they therefore trigger a new action potential

Question 15

Explain temporal summation

Answer 15

In which a single presynaptic neurone releases neurotransmitters many times over a very short period. If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered

Question 16

Nerve impulses are described as __-__-_____ responses

Answer 16

All-or-nothing

Question 17

What is the all-or-nothing principle?

Answer 17

There is a certain level of stimulus called the threshold value, which triggers an action potential. Below the threshold value, no action potential and therefore no impulse is generated

Question 18

What are factors that effect the speed of the transmission of impulses?

Answer 18

• myelination increases the speed of impulse transmission due to saltatory conduction
• axon diameter- the wider the axon, the greater the speed of conduction
• temperature- the higher the temperature, the greater the transmission speed due to faster diffusion of ions- however, in mammals and birds, body is kept constant so temperature should not have an effect