Chapter 13 : Neuronal Communication

Question 1

What does the structure of a sensory neurone look like ?

Answer 1

a sensory neurone consists of dendrites, dendron, cell body, axon and axon terminals (it is myelinated)

Question 2

what are dendrons and axons

Answer 2

dendrons transmit impulse towards the cell body

axons transmit impulses away from the cell body

Question 3

where is direction of impulse

Answer 3

facing towards the axon terminals (to the right)

Question 4

What is the dendrites of a sensory neurone attached to ?

Answer 4

a sensory receptor (which it receives impulses from)

Question 5

Where is the relay neurone found and what does its structure look like ?

Answer 5

the relay neurone is found in the CNS and it consists of short branched axons and dendrites from the cell body

Question 6

What is the role of the sensory neurone ?

Answer 6

to carry impulse from the receptors (which have detected a stimulus) to the CNS

Question 7

What is the role of the motor neurone ?

Answer 7

to carry impulse from the CNS (relay neurone) to the effectors (muscles/glands) so that the response can be carried out

Question 8

What does the structure of the motor neurone look like ?

Answer 8

consists of dendrites, cell body, axon then axon terminals

Question 9

What is a myelin sheath, how is it formed and what neurones have one?

Answer 9

a myelin sheath is a layer of plasma membranes acting as an insulating layer and allowing conduction of impulses at a faster speed

it is produced by Schwann cells

only sensory and motor neurones have one

Question 10

What are the nodes of Ranvier and why are they essential ?

Answer 10

the nodes of Ranvier are the unmyelinated sections where the impulse jumps between.

this is where action potential is generated

faster conduction of impulse

Question 11

What are sensory receptors and what do they act as?

Answer 11

sensory receptors are specialised cells specific to a single type of stimulus and they act as transducers

Question 12

What are transducers

Answer 12

transducers convert one form of energy into another
(converting energy provided from a stimulus to an electrical impulse)

Question 13

How does the Pacinian corpuscle work ?

Answer 13

The Pacinian corpuscle detects changes in pressure and movement (stimulus) converting it into an electrical impulse/nerve impulse

Step 1: high concentration of sodium ions outside of the neurone and stretch mediated sodium ions are closed (membrane is polarised)

Step 2: pressure is applied causing the stretch mediated sodium ions to change shape and open and sodium ions diffuse down a electrochemical gradient into neurone

Step 3: membrane becomes depolarised (more positive inside) + action potential is triggered along sensory neurone —> CNS

Question 14

What does generator potential mean ?

Answer 14

the process of the membrane becoming depolarised and initiating action potential in response to a stimulus (Only if the threshold is passed)

Question 15

What is meant by resting potential and how many mV ?

Answer 15

This is when the cell is at rest

outside of the membrane of the axon is more positively charged than inside the axon (more negative)

it is -70 mV

Question 16

why does the resting potential occur

Answer 16

it happens due to:

• sodium/potassium pump (intrinsic protein) pumps out 3 sodium ions and takes in 2 potassium ions (requires ATP)
• voltage gated sodium channel is closed (preventing re-entry) but non voltage gated potassium channels open allowing K+ ions to diffuse out
• therefore there are more positively charged ions outside the axon

Question 17

how is action potential generated ?

Answer 17

• stimulus is detected which must surpass the threshold of -55mV to produce action potential
• voltage gated sodium channels open due to a stimulus (membrane is more permeable to Na+) inside of the neurone becomes less negative
• this change in charge causes a positive feedback response and more sodium ions diffuse into the axon down their electrochemical gradient
• influx of positively charged sodium ions causes the membrane potential to become less negative (depolarize) and moves towards +40 mV.

Question 18

what happens in repolarisation and why ?

Answer 18

when potential difference reaches +40mV voltage gated sodium channels close and voltage gated potassium channels open

membrane is more permeable to K+ ions which diffuse out of the axon down their electrochemical gradient + this reduces positive charge

occur so that seizures do not happen and a continuous wave of depolarisation (transmit nerve impulses along constantly)

Question 19

what is hyperpolarisation ?

Answer 19

Potassium channels remain open for a short time after the membrane potential has returned to around -70 mV.

leads to an overshoot, membrane potential becomes even more negative than the resting potential, decreases to around -80 mV.

Question 20

how is a return to resting potential created ?

Answer 20

Voltage gated potassium ion channels are closed

The sodium-potassium pump restores the original distribution of sodium and potassium ions, pumping sodium ions out and potassium ions back in.

This process restores the resting potential, readying the neuron for another action potential

Question 21

what is saltatory conduction ?

Answer 21

process where electrical impulses are transmitted in myelinated axons (much faster + energy efficient)

depolarisation only occurs at the nodes of Ranvier (contains voltage gated sodium and potassium ion channels)

large localised circuits between adjacent nodes (action potential jumps from one node to adjacent node)

Question 22

what is the refractory period ?

Answer 22

after an action potential there is a short period of time where axon can not be excited again

voltage gated sodium ion channels remain closed preventing movement of Na+ into axons

Question 23

why is the refractory period important

Answer 23

ensures that action potentials only travel in one direction (unidirectional) and that they do not overlap

Question 24

Define threshold

Answer 24

The minimum stimulus required to trigger an action potential

depolarisation above a certain level occurs initiating action potential

Question 25

Describe the all or nothing principle

Answer 25

the power of the stimulus is not proportional to the power of the action potential.

the action potential will either occur or it won’t depending on if it passes the threshold

Question 26

What does a large stimulus mean and what is the relationship between action potential frequency and size of stimulus

Answer 26

more frequent action potentials generated in neurone

the larger the stimulus the more frequent the action potentials generated in a given time (directly proportionate)

Question 27

What is a synapse ?

Answer 27

the junction between two neurones where impulse is passed from one neurone to another (via neurotransmitters)

Question 28

what is the role of a synapse

Answer 28

to ensure impulses are unidirectional and they can only travel from pre –> post synaptic neurone

Question 29

How does the structure of a cholinergic synapse look like

Answer 29

consists of presynaptic neurone ending (with vesicles of acetylcholine and voltage gated calcium channel)

synapse,

post synaptic neurone (with voltage gated sodium ion channels and ACh receptors)

Question 30

How is impulse transmitted across a cholinergic synapse ?

Answer 30

Step 1: impulse arrives at the presynaptic neurone ending causing voltage gated calcium ion channels to open

step 2: calcium ions rush into the presynaptic neurone down the electrochemical gradient causing depolarisation of synaptic knob (action potential generated)

step 3: presence of calcium ions cause the synaptic vesicles with acetylcholine to move and fuse with the pre synaptic membrane

step 4: ACh is released into synaptic cleft and diffuses across the synapse to post synaptic neurone

step 5: ACh binds to receptor sites on post synaptic sodium ion channels causing them to open

step 6: sodium ions diffuse into post synaptic neurone down electrochemical gradient –> depolarisation of post synaptic membrane triggering new action potential
ACh is removed from the receptor and broken down by acetylcholinesterase

Question 31

what is acetyl choline broken down into by acetylcholinesterase

Answer 31

acetate (ethanoic acid) and choline

Question 32

what happens to the products of enzyme breaking down acetylcholine

Answer 32

these products diffuse back into the presynaptic neurone (acetate and choline)

ATP released from the mitochondria can combine acetate and choline into acetylcholine (inside presynaptic neurone)

Question 33

define summation

Answer 33

when the amount of neurotransmitter builds up to reach the threshold and then an action potential is triggered (in next neurone)

because amount of neurotransmitter from a single impulse may not be enough to trigger action potential in postsynaptic neurone

Question 34

what is spatial summation

Answer 34

when a number of presynaptic neurones connect to one post synaptic neurone

each releases neurotransmitters until eventually they build up to a high enough level to trigger an action potential in the post synaptic neurone

passing the threshold potential

Question 35

what is temporal summation

Answer 35

when a single presynaptic neuron releases neurotransmitters in rapid succession (due several action potentials)

high concentration of neurotransmitters build up and trigger an action potential in post synaptic neurone (passing the threshold)

Question 36

what are the 2 types of neurotransmitters

Answer 36

• excitatory neurotransmitter (acetylcholine) triggering action potential in post synaptic neurone
• inhibitory (GABA)- does not trigger new action potentials in post synaptic