Neuronal communication

Question 1

What are 4 different types of receptor

Answer 1

1. Mechanoreceptor
2. Chemoreceptors
3. Thermoreceptors
4. Photoreceptors

Question 2

What does the mecahnoreceptor do

Answer 2

1. Stimulus- pressure and movement
2. Example- Pacinian corpuscle (detects pressure
3. Example of organ- skin

Question 3

What does the chemoreceptors

Answer 3

1. Stimulus- chemicals
2. Example receptor- Olfactory receptor (detects smell)
3. Example of organ- nose

Question 4

What does thermoreceptor

Answer 4

1. Stimulus- heat
2. Example receptor- end-bulbs of Krause
3. Example of organ- Tongue

Question 5

What does the photoreceptors

Answer 5

1. Stimulus- light
2. Example- cone cell (detects different light wavelengths)
3. Example of organ- eye

Question 6

Define sensory receptor

Answer 6

Specialised cell that detects a stimulus

Question 7

Define transducer

Answer 7

A device that converts one type of energy or signal into another.
In sensory receptors they convert a stimulus into a nerve impulse

Question 8

Define stimulus

Answer 8

Detectable change in the internal or external environment of an organism which is detected by the nervous system and can cause a response

Question 9

State 3 characteristics of sensory receptors and for each explain why they are important.

Answer 9

1. They are specific to a single type of stimulus- you don’t a stimulus to trigger multiple receptors because the response may not be suitable for the stimulus
2. They act as a transducer- convert a stimulus into a nerve impulse- means the information can be passed through the nervous system to cause a response
3. Sensitive

Question 10

Draw and label a diagram showing the structure of a Pacinian corpuscle.

Answer 10

1. The sensory neurone is found within the centre of the centre of the corpuscle surrounded by layers of connective tissue
2. Each layer is separated by a layer of gel
3. Within the membrane of the neurone there are sodium ion channels- responsible for transporting sodium ions across the membrane
4. The neurone ending in the Pacinian corpuscle has a special type of sodium channel called a stretch mediated sodium channel
5. When these channels change shape their permeability to sodium also changes

Question 11

Explain how a Pacinian corpuscle converts mechanical pressure into a nerve impulse.

Answer 11

1. In its normal state (resting state) the stretch-mediated sodium ion channels in the sensory neurone’s membranes are too narrow to allow sodium ions to pass through them.
2. Pacinian corpuscle has a resting potential
3. When pressure is applied to the Pacinian corpuscle, the corpuscle changes shape, this causes the membrane surrounding its neurone to stretch
4. When the membrane stretches, the sodium ion channels present widen. Sodium can now diffuse into the neurone
5. The influx of positive sodium ions changes the potential of the membrane- becomes depolarised. This results in a generator potential.
6. In turn, the generator potential creates an action potential (a nerve impulse) that passes along the sensory neurone
7. The action potential will then be transmitted along neurones to the CNS

Question 12

Outline the steps in a stimulus-response pathway and identify the role of the sensory, relay, and motor neurones in this pathway.

Answer 12

1. Receptor- detects a stimulus and creates an action potential in the sensory neurone
2. Sensory neurone- carries impulse to spinal cord
3. Relay neurone- connects the sensory neurone to the motor neurone within the spinal cord or brain
4. Motor neurone- carries impulse to the effector to carry out the appropriate response

Question 13

Draw and label diagrams of a motor neurone

Answer 13

1. They have many short dendrites and a long axon
2. Cell body with nucleus in it on the left side
3. Dendrites attached to the cell body
4. Long axon leading to dendrites on the right side
5. Myelin sheath around the axon with the unmyelinated parts called the nodes of Ranvier

Question 14

Draw and label diagrams of a sensory neurone

Answer 14

1. They have one dendron which caries the impulse to the cell body
2. They have one short axon that carries the impulse away from the cell body
3. Dendrites branch of from the dendron and axon
4. The cell body is in the middle of the axon
5. Have a myelinated sheath and Nodes of Ranvier

Question 15

Draw and label diagrams of a relay neurone

Answer 15

1. Many short axons and dendrons
2. Cell body in the middle as a circle surrounded by small dendrons and axons
3. Dendrites branch from the axons and dendrons
4. Myelinated

Question 16

Define dendrite

Answer 16

Projections of a neurone that receives signals from other neurones they conduct electrical neurones to the cell body for the cell to function

Question 17

Define dendron

Answer 17

1. These are short extensions which come from the cell body
   2, These extensions divide into smaller branches called dendrites
2. They are responsible for transmitting electrical impulses towards the cell body

Question 18

Define axon

Answer 18

1. These are singular, elongated nerve fibres that transmit impulses away from the cell body
2. These fibres can be very long, e.g. those that transmit impulses from the tips of toes and fingers to the spinal cord.
3. The fibre is cylindrical in shape consisting of a very narrow region of cytoplasm surrounded by a plasma membrane

Question 19

Define cell body

Answer 19

1. This contains the nucleus surrounded by cytoplasm
2. Within the cytoplasm there are large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters
3. Neurotransmitters are chemicals which are used to pass signals from one neurone to the next

Question 20

Define myelinated sheath

Answer 20

Membrane rich in lipid which surrounds the axon of some neurones, speeding up impulse transmission

Question 21

Define Schwann cell

Answer 21

Produce layers of membrane by growing around the axon many times- producing the myelin sheath

Question 22

Define nodes of Ranvier

Answer 22

Gaps between each adjacent Schwann cell. With the myelin sheath they allow faster speed of transmission

Question 23

Define nodes of Ranvier

Answer 23

Gaps between each adjacent Schwann cell. With the myelin sheath they allow faster speed of transmission

Question 24

Describe and explain the advantage of myelination

Answer 24

1. The myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than un-myelinated neurones
2. The nodes of Ranvier create gaps in the myelin sheath.
3. The myelin sheath is an electrical insulator
4. In myelinated neurones the electrical impulse jumps from one node to the next as it travels along the neurone
5. This allows the impulse to be transmitted much faster
6. In non-myelinated neurones the impulse does not jump- transmits continuously along the nerve fibre so is much slower.

Question 25

Explain why some neurones are myelinated and others are unmyelinated.

Answer 25

1. When the stimulus is non urgent they don't need the very fast transmission e.g temperature change or a dull ache 2. When the stimulus is urgent they need to have fast a fast transmission to protect the body from danger e.g burning candle

Question 26

Describe what the resting potential is

Answer 26

1. When a neurone is not transmitting an impulse the potential different across its membrane is known as a resting potential 2. In this state, the outside of the membrane is more positively charged than the inside of the axon. 3. The membranes is said to be polarised as there is a potential difference across it 4. It is normally about -70mV 5. Some potassium ion channels are open (mainly those that aren't voltage-gated) 6. Sodium voltage-gated ion channels are closed.

Question 27

Explain the role of sodium ions, potassium ions, organic anions, the sodium/potassium ion pump and potassium ion channels in establishing and maintaining the resting potential.

Answer 27

1. Sodium ions are actively transported out of the axon whereas potassium ions are actively transported into the axon by a specific intrinsic protein known as the sodium potassium pump 2, However their movement is not equal. For every 3 sodium ions pumped out, two potassium ions are pumped in. 3. As a result there are more sodium ions outside the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the cytoplasm than outside the axon. 4. Therefore, the sodium ions diffuse back into the axon down its electrochemical gradient whereas potassium ions diffuse out of the axon 5. However, most gated sodium ion channels are closed preventing the movement of sodium ions, whereas many potassium ion channels are open, so the potassium ions can diffuse out of the axon 6. Therefore, there are more positively charged ions outside the axon than inside the cell. 7. This creates the resting potential of -70mV.

Question 28

Explain why a neurone is active when it is said to be resting.

Answer 28

1. Because it is still actively transporting ions using the sodium potassium pump

Question 29

Define resting potential

Answer 29

Potential difference across the membrane of the axon of a neurone at rest

Question 30

Define voltage gated channel

Answer 30

Any ion channel that opens and closes in response to changes in electrical potential across the cell membrane in which the channel is situated

Question 31

Define threshold potential

Answer 31

The membrane voltage that must be reached in an excitable cell during depolarisation in order to generate an action potential

Question 32

Define action potential

Answer 32

The change in the potential difference across the neurone membrane of the axon when stimulated (approx +40mV)

Question 33

Define nerve impulses

Answer 33

The movement of an action potential along a nerve fibre in response to a stimulus

Question 34

Define polarised

Answer 34

When there is a difference in mV inside a cell compared to outside a cell

Question 35

Define depolarisation

Answer 35

A loss of the difference in charge between the inside and outside of the plasma membrane

Question 36

Define repolarisation

Answer 36

Restoration of the difference in charge between the inside and outside of the plasma membrane following depolarisation

Question 37

Define hyperpolarisation

Answer 37

Hyperpolarisation is a change in a cell's membrane potential that makes it more negative.

Question 38

Describe how the depolarisation of the membrane occurs and what is required for it to lead to an action potential.

Answer 38

1. A stimulus is detected by a sensory receptor which sets of these events 2. The energy from the stimulus triggers some sodium voltage gated ion channels to open, making the membrane more permeable to sodium ions. 3. Sodium ions therefore diffuse into the axon- making the inside of the neurone less negative 4. The potential difference reaches threshold potential meaning an action potential will happen. 5. This change in charge causes more voltage-gated sodium ion channels to open allowing more sodium ions to diffuse into the axon- positive feedback 6. When the potential difference reaches approx +40mV the voltage-gated sodium ion channels close and voltage-gates potassium ion channels open 7. Sodium can no longer enter the axon but the membrane is now more permeable to potassium ions

Question 39

Describe the process of repolarisation

Answer 39

1. The voltage-gated sodium ion channels are closed and potassium ion channels are open 2. Potassium ions diffuse out of the axon down their electrochemical gradient. This reduces the charge, resulting in the inside of the axon becoming more negative than the outside 3. Initially lots of potassium ions diffuse out of the axon, resulting in the inside of the axon becoming more negative than its normal resting state- hyperpolarisation 4. The voltage-gated potassium channels now close. 5. The sodium-potassium pump causes sodium ions to move out of the cell, and potassium ions move in. 6. The axon returns to resting potential- repolarised

Question 40

Draw, label and annotate a graph of an action potential occurring over time to show the different stages of an action potential and what happens at each stage.

Answer 40

1. look p350 2. flat line at around -70 3. Goes up to form a curve, the peak of the curve at +40 4. A slight bump on the ascending line to show the threshold level. 5. Descending line goes slightly past -70 and then goes back up to resting potential

Question 41

Define the term “refractory period” and explain its importance for the conduction of the action potential.

Answer 41

1. A short period of time when the axon cannot be excited again, known as the refractory period. 2. During this time the voltage-gated sodium ion channels remain closed preventing the movement of sodium ions into the axon 3. It is important because it prevents the propagation of an action potential backwards along the axon as well as forwards. 4. The refractory period makes sure action potentials are unidirectional and it also ensures that action potentials do not overlap and occur as discrete impulses.

Question 42

Define local circuits

Answer 42

1.

Question 43

Define saltatory conduction

Answer 43

A form of nerve impulse conduction in which the impulse jumps from one Ranvier's node to the next, rather than traveling the entire length of the nerve fibre.

Question 44

Describe and explain how an action potential is transmitted along an unmyelinated axon.

Answer 44

1. The initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised 2. This acts as a stimulus for the depolarisation of the next region of the membrane. 3. The process continues along the length of the axon forming a wave of depolarisation . 4. Once sodium ions are inside the axon, they are attracted by the negative charge ahead and the concentration gradient to diffuse further along inside the axon, triggering the depolarisation of the next section

Question 45

Describe and explain how an action potential is transmitted along a myelinated axon.

Answer 45

1. Myelinated axons transfer electrical impulses much faster than non-myelinated axons 2. This is because depolarisation of the axon membrane can only occur at the node of Ranvier where no myelin is present 3. Here the sodium ions can pass through the protein channels in the membrane. Longer localised circuits therefore arise between adjacent nodes. 4. The action potential then jumps from one node to another in a process known as saltatory conduction. 5. This is much faster than the wave of depolarisation along the whole length of the axon membrane, 6. Every time channels open and ions move it takes time, so reducing the number of places where this happens speeds up the action potential transmission 7. Long-term saltatory conduction is also more energy efficient. Repolarisation uses ATP in the sodium-potassium pump, so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction makes the conduction of impulses.

Question 46

Draw and annotate a graph of an action potential conducting down an unmyelinated axon.

Answer 46

1. Distance along the bottom and action potential on the y axis 2. Reverse of membrane potential to time 3. Hyperpolarisation first, then repolarisation, then depolarisation 4. Opposite because as you go along an axon the part closest will be furthest through its action potential

Question 47

Explain what is meant by the “all-or-nothing” response of neurones

Answer 47

1. A certain level of stimulus- threshold value- always triggers a response 2. If the threshold is reached an action potential will always be created. 3. No matter how large the stimulus is, the same sized action potential will always be triggered. 4. If the threshold is no reached, no action potential will be triggered

Question 48

Explain why the “all-or-nothing” response of neurones means that information must be transmitted by the frequency of impulse transmission.

Answer 48

1. The size of the stimulus doesn't affect the size of the action potential 2. But it does effect the number of action potentials that are generated in a given time. 3. The larger the stimulus the more frequently the action potentials are generated. 4. It is the frequency of action potentials that affect the response initiated not the size

Question 49

Apart from myelination what are the two other factors that affect the speed of transmission

Answer 49

1. Axon diameter- The bigger the axon diameter, the faster the impulse is transmitted. This is because there is less resistance to the flow of ions in the cytoplasm, compared with those in a smaller axon 2. Temperature- The higher the temperature, the faster the nerve impulse. This is because ions diffuse faster at higher temperature. Only occurs to about 40 degrees as higher causes the proteins to become denatured.

Question 50

Define neurotransmitter

Answer 50

Chemical involved in communication across a synapse between adjacent neurones and a muscle cell

Question 51

Define synapse

Answer 51

The junction (small gap) between two neurones, or a neurone and an effector

Question 52

Define cholinergic synapse

Answer 52

A synapse is cholinergic if it uses acetylcholine as its neurotransmitter.

Question 53

Define synaptic knob

Answer 53

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

Question 54

Define presynaptic membrane

Answer 54

The membrane of the neurone along which the impulse has arrived

Question 55

Define synaptic cleft

Answer 55

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

Question 56

Define postsynaptic membrane

Answer 56

The membrane of the neurone that receives the neurotransmitter

Question 57

Define acetylcholine

Answer 57

An example of an excitatory neurotransmitter

Question 58

Define acetylcholinesterase

Answer 58

An enzyme that causes rapid hydrolysis of acetylcholine. Its action serves to stop excitation of a nerve after transmission of an impulse.

Question 59

Define synaptic vesicle

Answer 59

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

Question 60

Define neurotransmitter receptors

Answer 60

Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane

Question 61

Draw, label and annotate a diagram to show the structures present in a cholinergic synapse

Answer 61

p355 Memorise a diagram they all include these features: 1. Synaptic cleft 2. Presynaptic neurone 3. Postsynaptic neurone 4. Synaptic knob 5. Synaptic vesicles 6. Neurotransmitter receptors

Question 62

Describe the structure of the sodium channels on the post synaptic membrane.

Answer 62

1. Made up of 5 proteins sub-units | 2. Has receptor sites where the neurotransmitter binds to

Question 63

Describe the sequences of events that occur at a synapse that can result in an action potential being generated in the post-synaptic neurone

Answer 63

1. The action potential reaches the end of the presynaptic neurone 2. Depolarisation of the presynaptic membrane causes calcium ion channels to open 3. Calcium ions diffuse into the presynaptic knob 4. This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis 5. Neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane 6. This causes sodium ion channels to open 7. Sodium ions diffuse into the postsynaptic neurone 8. This triggers an action potential and the impulse is propagated along the postsynaptic neurone.

Question 64

Describe the role of acetylcholinesterase and explain how acetylcholine is recycled.

Answer 64

1. Once a neurotransmitter has triggered an action potential in the postsynaptic neurone, it is important that it is removed so the stimulus is not maintained, and so another stimulus can arrive and affect the synapse 2. Any neurotransmitter left in the synaptic cleft is removed 3. Acetylcholine is broken down by enzymes which also releases them from the receptors on the postsynaptic membrane 4. The products are taken back into the presynaptic knob where ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. 5. This is stored in vesicles for future use. 6. Sodium ion channels close in the absence of acetylcholine in the receptor site 7. Removing the transmitter from the synaptic cleft prevents the response from happening again and allows the neurotransmitter to be recycled

Question 65

Explain why synapses are unidirectional.

Answer 65

1. As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone

Question 66

Define summation

Answer 66

Build up of neurotransmitter in a synapse to sufficient levels to trigger an action potential

Question 67

Define spatial summation

Answer 67

1. This occurs when a number of presynaptic neurones connect to one postsynaptic neurone 2. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Question 68

Define temporal summation

Answer 68

1. This occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period 2. This builds up in the synapse until the quantity is sufficient to trigger an action potential

Question 69

State 3 roles of synapses and for each describe the importance of this role to the nervous system.

Answer 69

1. They ensure impulses are unidirectional 2. 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 3. Alternatively, a number of neurones may feed in to the same synapse with a single postsynaptic neurone. This results in stimuli for different receptors interacting to produce a single response

Question 70

Describe the difference between excitatory and inhibitory neurotransmitter

Answer 70

1. Excitatory- these neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered. Acetylcholine is an example 2. Inhibitory- These neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being triggered. Gamma-aminobutyric acid is an example