Neuronal Communication

Question 1

What are neurons

Answer 1

• specialised nerve cells
• transmit electrical impulses rapidly around the body so that an organism can respond to changes in its internal and external environment

Question 2

Outline the structure of a neuron

Answer 2

• cell body; contains a nucleus surrounded by cytoplasm which contains large amounts of mitochondria and endoplamsic reticulum which are involved in the production of neurotransmitters
• dendron; short extensions which divide into dendrites and transmit electrical impulses towards the cell body
• axon; singular, elongated nerve fibres that transmit impulses away from the cell body, it is long and has a cylindrical shape consisting of a narrow region of cytoplasm surrounded by a plasma membrane

Question 3

Name the 3 types of neuron

Answer 3

• sensory - transmits impulses from sensory receptors to the CNS
• relay - transmits impulses between sensory neurons and motor neurons
• motor - transmits impulses from CNS to effectors

Question 4

What are myelinated neurons

Answer 4

• neurones surrounded by a myelin sheath which is made from Schwann cells
• acts as an insulating layer which allows myelinated neurons to conduct the electrical impulse at a much faster speed
• gaps in between the myelin sheath are the nodes of ranvier

Question 5

How do the nodes of ranvier contribute to the speed of an impulse?

Answer 5

• they allow the impulse to jump from one node to the next node as it travels along the neurone which causes it to be transmitted much faster rather than it transmitting continuously along the nerve fibre which would be slower

Question 6

What are sensors receptors?

Answer 6

• transducers which detect stimuli and convert it into a nerve impulse, producing a generator potential
• each specific to a single type of stimulus

Question 7

Give the 4 main examples of sensory receptors in animals

Answer 7

• mechanoreceptors ; detect pressure and movement, found in the skin (e.g Pacinian Corpuscles)
• chemoreceptors ; detect chemicals, found in the nose (e.g olfactory receptors)
• thermoreceptors ; detect heat, found in the tongue (e.g end-bulbs of Krause)
• photoreceptors; detect light, found in the eyes (e.g cone cell)

Question 8

Outline what Pacinian Corpuscles are

Answer 8

• sensory receptors which detect mechanical pressure
• found deep within the skin
• contain a sensory nerve ending

Question 9

How do sensory receptors work?

Answer 9

• during resting potential, when a stimulus is detected the cell
  membrane is excited and becomes more permeable allowing more ions to move in/out of the cell, altering the potential difference
• this change in potential difference caused by a stimulus is known as the generator potential
• the bigger the stimulus, the bigger the generator potential produced
• if the generator potential reaches the threshold then an action potential will be triggered

Question 10

Describe how Pacinian Corpuscles work

Answer 10

• during resting state, the stretch mediated sodium ion channels in the sensory neurone membrane are too narrow for ions to pass through
• when pressure is applied to the Pacinian Corpuscles, it changes shape and causes the membrane surrounding its neurone to stretch
• this widens the sodium ion channels and ions diffuse into the neurone
• the influx of positive sodium ions changes the potential of the membrane; it becomes depolarised and results in a generator potential
• generator potential creates an action potential which is transmitted along neurones to the CNS

Question 11

What is resting potential?

Answer 11

• usually -70mV
• when the outside of the membrane is more positively charged
• the membrane is polarised due to the difference in charge

Question 12

How is a resting potential created?

Answer 12

• Sodium ions are actively transported out of the axon and Potassium ions in using the sodium potassium pump (an intrinsic protein)
• every 3 sodium ions pumped out, 2 potassium ions are pumped in
• there are more sodium ions outside of the membrane than in the axon cytoplasm so sodium ions diffuse back into the axon down the electrochemical gradient
• the gated sodium channels close and potassium channels open so potassium ions can diffuse out of the axon
• there are more positively charged ions outside of the axon creating a resting potential

Question 13

What is an action potential?

Answer 13

• the sequence of events that occur due to the rapid movement of sodium and potassium ions across the axon membrane through voltage gated ion channels

Question 14

Name the 5 stages involved in an action potential

Answer 14

• stimulus
• depolarisation
• repolarisation
• hyperpolarisation
• resting potential

Question 15

Discuss the first stage of an action potential

Answer 15

• stimulus
• excites the cell membrane causing some sodium ion channels to open, means the membrane becomes more permeable to sodium ions so sodium ions diffuse down their electrochemical gradient into the neurone, making the inside of the neurone less negative

Question 16

Describe the second stage of an action potential

Answer 16

• depolarisation
• if the potential difference reaches a threshold then the voltage gated sodium ion channels will open so more sodium ions diffuse into the neurone
• this is positive feedback

Question 17

Describe the third stage of an action potential

Answer 17

• repolarisation
• at around 30mV, all the voltage gated sodium channels close and voltage gated potassium ion channels open membrane becomes more permeable to potassium ions allowing them to diffuse out of the neurone down their conc gradient, starts to get membrane back to its resting potential
• this is negative feedback

Question 18

Describe the 4th stage of an action potential

Answer 18

• hyperpolarisation
• the potassium ion channels are too slow to close causing a slight overshoot of potassium ions where too many diffuse out of the neurone
• potential difference is more negative than the resting potential

Question 19

Describe the 5th stage of an action potential

Answer 19

• resting potential
• once all potassium ion channels are closed, the sodium-potassium pump restores the resting potential

Question 20

what is the refractory period?

Answer 20

• occurs when the sodium ion channels close and when the potassium ion channels are closed (between repolarisation and hyperpolarisation)
• because the ion channels are recovering and can’t be made to open
• ensures that action potentials are separate and that the impulse only travels in one direction

Question 21

Discuss the all or nothing principle

Answer 21

• idea that an impulse will only be transmitted if the initial stimulus is sufficient enough that increase the membrane potential above a threshold

Question 22

what is the speed of conduction

Answer 22

• how quickly an impulse is transmitted along a neurone
• affected by myelination, the diameter of an axon and temperature

Question 23

how does myelination affect the speed of conduction?

Answer 23

• increase the speed that action potentials travel
• depolarisation cannot occur at the myelin sheath
• action potentials only occur at the nodes
• schwann cells allow the impulse to jump from one node to the next (saltatori conduction)

Question 24

How does the diameter of the axon affect the speed of conduction?

Answer 24

• thicker the axon is, the higher speed the impulse travels at
• axon membrane has a greater surface area
• increases the rate of diffusion of ions which increases the rate of action potentials
• greater volume of cytoplasm reduces electrical resistance and action potential moves faster

Question 25

How does temperature affect the speed of conduction?

Answer 25

• impacts ectotherms more
• cold conditions will slow down nerve impulses/warmer conditions will speed them up
• colder means less kinetic energy available for the facilitated diffusion if sodium and potassium ions

Question 26

what is the synaptic cleft?

Answer 26

• the gap between two neurones that depresses the axon of one neuron from the denture of the next

Question 27

what is the presynaptic neurone?

Answer 27

the neurone where the impulse arrives

Question 28

what is the postsynaptic neurone?

Answer 28

• neurone that received the neurotransmitter

Question 29

what are synaptic vesicles

Answer 29

• vesicles containing neurotransmitters that fuse with the presynaptic membrane and release the contents into the synaptic cleft

Question 30

what do excitatory neurotransmitters do? give an example

Answer 30

• result in depolarisation of the postsynaptic neurone
• if threshold is met than an action potential is triggered
• example is acetylcholine

Question 31

what do inhibitory neurotransmitters do?

Answer 31

• result in hyperpolarisation of the postsynaptic membrane
• prevents an action potential being triggered