Neuronal communication-neurones

Question 1

function of the neurone?

Answer 1

transmit electrical impulses rapidly around the body so that the organism can respond to changes in its internal and external enviroment

Question 2

Describe the structure of the neurone?

Answer 2

-cell body: contains nucleus, ER, mitochondria for the production of neurotransmitters.
-dendrons: short extensions which branch into dendrites, responsible for transmitting electrical impulses towards the cell body
-axons: singular, elongated nerve fibres that transmit impulses away from the cell body

Question 3

what are neurotransmitters?

Answer 3

chemicals which pass signals from one neurone to the next

Question 4

what are the three types of neurones and their function?

Answer 4

-sensory neurone: transmits impulses from sensory receptor cell to a relay neurone, motor neurone or the brain. One dendron and one axon
-relay neurone: transmits impulses between neurones, short axons and dendrons
-motor neurones: transmit impulses from relay or sensory neurone to an effector, long axon and many short dendrites

Question 5

how is the myelin sheath formed

Answer 5

Schwann cells produces layers of the membrane growing around the axon many times, a double phospholipid bilayer is formed. 20 layers of membrane can be formed

Question 6

function of myelin sheath?

Answer 6

acts as an insulating layer and increases speed of transmission

Question 7

between each adjacent Schwann cell what is the small gap (2-3um) called?

Answer 7

node of ranvier

Question 8

why does myelin sheath increase speed of transmission?

Answer 8

myelin sheath is an electrical insulator, due to the node of ranvier the electrical impulse jumps form one node to another. In non-myelinated neurones the impulse doesn’t jump and transmits continuously which is slower

Question 9

what are the two main features of sensory receptors?

Answer 9

-they are specific to a single type of stimulus
-they act as a transducer (converting stimulus into nerve impulse)

Question 10

describe the role of a transducer?

Answer 10

receptor converts stimulus into a nerve impulse called generator potential

Question 11

for a mechanoreceptor, what is the stimulus, example of receptor and example of sense organ?

Answer 11

stimulus- pressure and movement
example of receptor- pacinian corpuscle
example of sense organ- skin

Question 12

describe the pacinian corpsucle?

Answer 12

-sensory receptors detecting mechanical pressure
-located deep in skin (of fingers and feet) and joints
-centre of corpuscle is the end of a sensory neurone surrounded by layers of connective tissue operated by gel

Question 13

Describe the neurone ending in a pacinian corpuscle?

Answer 13

neurone has stretch mediated sodium channels, they can change shape

Question 14

explain how the pacinian corpuscle converts mechanical pressure into a nervous impulse?

Answer 14

1)at resting state, the stretch mediated sodium ion channels are too narrow to allow sodium ions to pass- resting potential
2)When pressure is applied, the pacinian corpuscle changes shape causing neurone membrane to stretch
3)sodium ion channels open and sodium ions diffuse into the neurone
4)influx of positive sodium ions changes the potential of the membrane- it depolarises resulting in a generator potential
5) generator potential creates action potential (nerve impulse) which passes along the sensory neurone.

Question 15

what is potential difference?

Answer 15

difference in charge between the inside and outside of the axon

Question 16

what is resting potential

Answer 16

the potential difference across the membrane when neurone is not transmitting an electrical impulse. At resting, the outside of the membrane is more positively charged than inside the axon therefore the membrane is polarised (-70mV)

Question 17

explain why the resting potential is at -70mV

Answer 17

-Na+ ions are actively transported out of the axon where as K+ ions are actively transported into the axon by a sodium potassium pump. However the movements are not equal. For every 3 Na+ ions pumped out 2 K+ ions are pumped in.
-Na+ ions diffuse back into the axon down the electrochemical gradient where as potassium will diffuse out the axon
-However most gated sodium ion channels are closed, preventing movement of sodium ions whereas many potassium ion channels are open, allowing K+ ions to diffuse out. There fore this creates a more positively charged ions outside the axon than inside.

Question 18

what is depolarisation?

Answer 18

a change in potential difference from negative to positive

Question 19

what is repolarisation?

Answer 19

a change in potential difference from positive to negative

Question 20

Explain the process from depolarisation to repolarisation?

Answer 20

1)At resting potential, some potassium ion channels (not-voltage gated) are open but all sodium voltage gated ion channels are closed
2)stimulus triggers some voltage gated sodium ion channels to open. Sodium ions diffuse into the axon- inside neurone become less negative
3)change causes more sodium ions to diffuse in the axon- positive feedback
4)When potential difference reaches +40mV, voltage gated sodium ions close and voltage gated potassium ion channels open.
5)potassium ions diffuse outside the axon and inside axon becomes more negative
6)hyperpolarisation occurs where the inside of the axon becomes more negative than normal resting state.

Question 21

what occurs after hyper polarisation?

Answer 21

voltage gated potassium ion channels close. The sodium-potassium pump causes sodium ions to move outside the cell and potassium ions in. Axon returns to resting potential- repolarised

Question 22

Describe the propagation of action potentials?

Answer 22

region where depolarisation occurs and action potential is triggered acts as a stimulus for depolarisation of the next region of the membrane. Process continues forming a wave of depolarisation. When sodium ions are inside the axon, they are attracted by the negative charge ahead and diffuse along the axon, triggering depolarisation.

Question 23

what is a refractory period?

Answer 23

period where axon cannot be excited again after action potential. The voltage gated sodium ion channels remain closed preventing movement of sodium ions into the axon

Question 24

why is the refractory period important?

Answer 24

prevents the propagation of an action potential backwards and forwards, ensuring it action potentials are unidirectional and do not overlap causing discrete impulses

Question 25

Why do myelinated axons transfer energy faster than non-myelinated axons (saltatory conduction)?

Answer 25

depolarisation only occurs at the nodes of ranvier. Longer and localised circuits arise between adjacent nodes causing action potential to ‘jump’ from one node to another which is faster than a wave of depolarisation along the axon.

Question 26

Describe saltatory conduction?

Answer 26

-repolarisation uses ATP in the sodium pump so by reducing the amount of repolarisation needed it is more energy efficient

Question 27

what factors affect speed at which action potential travels?

Answer 27

-myelination
-axon diameter : the bigger the diameter, the faster the impulse is transmitted as there is less resistance to the flow of ions in the cytoplasm
-temperature: higher the temp, faster the nerve impulse. However when temp is too high, the proteins (sodium-potassium ion pump) becomes denatured

Question 28

describe the all or nothing principle?

Answer 28

a certain level of stimulus, a threshold value always triggers a response. If this threshold is reached an action potential will be created.

Question 29

Describe a very weak stimulus, weak stimulus, strong stimulus and very strong stimulus?

Answer 29

• very weak stimulus: threshold not exceeded, no depolarisation, no action potential
  -weak stimulus: threshold exceeded with depolarisation and action potential
  -strong stimulus: threshold exceeded with depolarisation and action potential the same size but more frequent
  -very strong stimulus: threshold exceeded with depolarisation and action potential the same size but even more frequent

Question 30

the larger the stimulus…

Answer 30

the more frequent action potentials are generated

Question 31

what is a synapse?

Answer 31

the junction between two neurones (or a neurone and effector)

Question 32

what is a neurotransmitter?

Answer 32

chemical that transmits impulses across the synapse

Question 33

what is a synaptic cleft?

Answer 33

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

Question 34

what is the presynaptic neurone?

Answer 34

neurone along which the impulse has arrived

Question 35

what is the postsynaptic neurone?

Answer 35

neurone that receives the neurotransmitter

Question 36

what is the synaptic knob?

Answer 36

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

Question 37

what are synaptic vesicles?

Answer 37

vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft.

Question 38

what are neurotransmitter receptors?

Answer 38

receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.

Question 39

what is an excitatory neurotransmitter?

Answer 39

These neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is met, the action potential is generated. Acetylcholine is an example of a excitatory neurone.

Question 40

what is an inhibitory neurotransmitter?

Answer 40

these neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being triggered. Gamma-aminobutyric (GABA) is an example of an inhibitory neurotransmitter that is found in some synapses in the brain.

Question 41

How does an impulse travel across a synapse?

Answer 41

-action potential reaches end of presynaptic neurone causing depolarisation of presynaptic membrane.
-depolarisation causes voltage gated calcium ions to open
-the influx of calcium ions within the presynaptic knob causes vesicles containing neurotransmitters to fuse with the membrane and leave by exocytosis
-neurotransmitter diffuses across synaptic cleft and binds with specific receptor on postsynaptic cleft.
-binding causes sodium ion channels in postsynaptic membrane to open
-sodium ions diffuse into postsynaptic neurone, triggering an action potential

Question 42

How are neurotransmitters left in the synaptic cleft removed? Use example of acetylcholine

Answer 42

enzyme acetylcholinesterase breaks acetylcholine into acetate and choline. Acetate and choline diffuse into the presynaptic knob. In the presynaptic knob, acetate and choline are combined used ATP (from mitochondria) to regenerate acetylcholine.

Question 43

What are the three main roles of synapses?

Answer 43

-ensure impulses are unidirectional. As neurotransmitter receptors are only found on the postsynaptic neurone.
-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.
-A number of neurones can feed into the same synapse with a single postsynaptic neurone. This results in stimuli from different receptors interacting to produce a single result.

Question 44

what is summation?

Answer 44

process that determines whether or not an action potential will be generated by the combined effects of neurotransmitters. If there is not enough neurotransmitter to trigger an action potential, the threshold is not met.

Question 45

What is spatial summation?

Answer 45

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

Question 46

what is temporal summation?

Answer 46

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