Neuronal Communication 5.3

Question 1

What is a transducer?

Answer 1

-a cell that converts one form of energy into another

Question 2

What is a sensory receptor?

Answer 2

A cell/sensory nerve ending that responds to a stimulus is called a receptor cell
Receptor cells are transducers – they convert energy from one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
They can create action potentials
Each receptor will only respond to a specific stimulus

Question 3

What are the 5 types of receptors found in the body?

Answer 3

Question 4

Outline how different receptors work as transducers

Answer 4

Question 5

What is the Pacinian corpuscles?

Answer 5

Pacinian corpuscles are a type of mechanoreceptor found deep in the skin
They are present in the skin of fingers, soles of the feet as well as in joints, tendons and ligaments
They respond to changes in pressure
When these receptors are stimulated by pressure on the skin it leads to the establishment of a generator potential

Question 6

Describe how the Pacinian corpuscle works

Answer 6

Question 7

Answer 7

Question 9

What are neurones?

Answer 9

Neurones are specialised cells of the nervous system which carry electrical impulses around the body
A bundle of neurones is known as a nerve

Question 10

What features are found in all neurones?

Answer 10

Neurones have a long fibre known as an axon
They have a cell body that contains the nucleus and other cellular structures
The end of the axon, known as the axon terminal, contains many nerve endings
The nerve endings at the axon terminal allow neurones to connect to many other neurones which receive impulses from the axon terminals; this forms a network for easy communication

Question 11

What does it mean if a neurone is myelinated?

Answer 11

-their axon is insulated by a myelin sheath with small uninsulated sections along its length (called nodes of Ranvier)
The myelin sheath is formed by specialised cells known as Schwann cells which wrap themselves around the axon.
This means that electrical impulses do not travel down the whole axon, but jump from one node to the next so that less time is wasted transferring the impulse from one cell to another- this is SALTATORY CONDUCTION and allows the impulse to travel much faster

Question 12

What are non-myelinated neurones?

Answer 12

non-myelinated neurones the axon is uninsulated- several neurones may be enshrouded in one loosely wrapped Schwann cell
The impulse travels more slowly as it moves through the entire length of the axon

Question 13

What are the differences between myelinated and non-myelinated neurones?

Answer 13

-myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones can
-myelinated neurones carry action potentials from the CNS and from the CNS to effectors, they carry action potentials over long distances. They enable a much quicker response to a stimulus
-non-myelinated neurones tend to be shorter and carry action potentials only over a short distance- they are often used to coordinate functions such as breathing and the action f the digestive system

Question 14

What are the three types of neurones and what does each one do?

Answer 14

-Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)

-Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones

-Motor neurones carry impulses from the CNS to effectors (muscles or glands)

Question 15

What is the structure of motor neurones?

Answer 15

A large cell body at one end that lies within the spinal cord or brain
A nucleus that is always in its cell body
Many highly-branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones

Question 16

What is the structure of relay neurones?

Answer 16

Short, but highly branched, axons and dendrites

Question 17

What is the structure of sensory neurones?

Answer 17

A cell body that branches off in the middle of the cell
A single long dendron that carries impulses to the cell body and a single long axon that carries impulses away from the cell body

Question 18

In what direction does the axon always carry impulses?

Answer 18

Away from the cell body

Question 19

Answer 19

Question 20

What is the potential difference at resting potential?

Answer 20

potential difference (when there are no impulses) is usually about -70mV
->there are more positive ions, Na+ and K+ outside compared to inside, therefore the inside of the neurones is comparatively more negative at -70mV

Question 21

What are the two factors that contribute to establishing and maintaining the resting potential?

Answer 21

The active transport of sodium ions and potassium ions
Differential membrane permeability

Question 22

How is a resting potential established and maintained?

Answer 22

-Carrier proteins called sodium-potassium pumps are present in the membranes of neurones
-These pumps use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions that they actively transport in
-This means that there is a larger concentration of positive ions outside the axon than there are inside the axon
-The movement of ions via the sodium-potassium pumps establishes an electrochemical gradient
-The cell-surface membrane of neurones has selective protein channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
The protein channels are less permeable to sodium ions than potassium ions
This means that potassium ions can diffuse back down their concentration gradient, out of the axon, at a faster rate than sodium ions
-this results in inside the axon having a lower charge than outside the axon- resulting in -70mV

Question 23

What are the five steps that occur during an action potential?

Answer 23

stimulus, depolarisation, repolarisation, hyperpolarisation and the return to resting potential

Question 24

How is an action potential generated?

Answer 24

1-A stimulus ( an electrical impulse from another neurone or a chemical change to the membrane of the neurone) triggers sodium ion channels in the membrane to open allowing sodium ions to diffuse into the neurone down an electrochemical gradient
2-When a large enough stimulus is detected by a neurone, the resting potential can be converted into an action potential
The potential difference across the membrane must reach a threshold of around -55mV to trigger depolarisation
3-When the threshold (around -55mV) is reached, an action potential is stimulated and the following steps occur:
Voltage-gated sodium ion channels in the axon membrane open
Sodium ions pass into the axon down the electrochemical gradient (there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
The movement of sodium ions reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – a process known as depolarisation
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
This is an example of positive feedback
The action potential that is generated will reach a potential of around +30mV

Question 25

Describe repolarisation

Answer 25

Very shortly after the potential difference has reached +30mV, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions diffusing into the axon Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient This returns the potential difference to normal (about -70mV) – a process known as repolarisation This is an example of negative feedback.

Question 26

Describe hyperpolarisation

Answer 26

Potassium ion channels are slow to close and as a result, too many potassium ions diffuse out of the neurone causing a short period of hyperpolarisation This means that the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential

Question 27

Describe returning to the resting potential

Answer 27

Once the potassium ion voltage-gated channel proteins are closed the sodium-potassium pump restores the resting potential The sodium ion channel proteins in this section of membrane become responsive to depolarisation again

Question 28

How do action potentials travel?

Answer 28

-action potentials travel as a wave of depolarisation across the length of the neurone

Question 29

How are action potentials transmitted along the axon of a neurone and what does this ensure?

Answer 29

An action potential triggered in the neurone causes depolarisation of that section of the axon The current causes the opening of sodium ion channels a little further up the axon This causes an influx of sodium ions in this section of the axon generating an action potential in this direction The previous section of the axon is in the repolarisation stage (the sodium channels are closed and potassium channels are open) and is unresponsive This makes the action potentials discrete events and means the impulse can only travel in one direction

Question 30

What is the all or nothing principle?

Answer 30

An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential If stimulus is very weak (below threshold), receptor cells wont be sufficiently depolarised -> sensory neurone not activated -> no impulse If stimulus is strong enough to increase the receptor potential above threshold -> stimulates the sensory neurone -> sends impulse Rather than staying constant threshold levels in receptors often increase with continued stimulation, therefore a greater stimulus is requires before impulses are sent along sensory neurones

Question 31

Where does depolarisation occur in myelinated neurones?

Answer 31

depolarisation only occurs at the nodes but still occurs in one direction as the previous node is still in the repolarisation stage.

Question 32

What is the refractory period?

Answer 32

After the action potential, there is a period of time when both sodium ion channels are closed (during repolarisation) and potassium ion channels are closed (during hyperpolarisation). When this occurs, this section of the axon membrane is in a period of recovery and is unresponsive This is known as the refractory period

Question 33

What is the importance of the refractory period?

Answer 33

It ensures that action potentials are discrete events, stopping them from merging into one another It ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential, as the region behind is ‘recovering’ from the action potential that has just occurred As a result, an impulse can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurones The existence of the refractory period also means that there is a minimum time between action potentials occurring at any one place along a neurone The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones

Question 34

A _________ frequency of action potentials means a more intense stimulus When a stimulus is at higher intensity _______ sodium channels are opened in the sensory receptor. This produces ______ generator potentials. As a result there are more frequent _______ _____ in the sensory neurone. Therefore there are more frequent action potentials entering the central nervous system

Answer 34

Higher More More Action potentials

Question 35

The junction where two neurones meet is known as a ______

Answer 35

Synapse

Question 36

What are cholinergic synapses?

Answer 36

Synapses that use acetylcholine (ACh) as a neurotransmitter are described as cholinergic synapses

Question 37

What is acetylcholine (ACh)?

Answer 37

-a neurotransmitter (chemical used as a signalling molecule between two neurones in a synapse)

Question 38

Describe the process of synaptic transmission using ACh

Answer 38

-The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane -This stimulates voltage-gated calcium ion channel proteins to open -Calcium ions (Ca2+) diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the synaptic knob (low concentration of calcium ions) -This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft by exocytosis -The ACh molecules diffuse across the synaptic cleft and temporarily bind to cholinergic receptors in the postsynaptic membrane -This causes sodium ion channels to open Sodium ions diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone -The sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse once the threshold is reached -The ACh molecules are broken down and recycled.The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline -The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives

Question 39

What is the significance of ACh being broken down and recycled at the postsynaptic membrane?

Answer 39

This prevents the sodium ion channels staying permanently open and stops permanent depolarisation of the postsynaptic membrane, The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline

Question 40

How do synapses ensure the one-way transmission of impulses?

Answer 40

Impulses can only pass in one direction at synapses because neurotransmitter is released on one side and its receptors are on the other – chemical transmission cannot occur in the opposite direction This prevents impulses from travelling the wrong way, back to where they were initiated

Question 41

What is summation and what are the benefits of summation?

Answer 41

the effect of multiple impulses can be added together in a process known as summation There are several benefits of summation It allows for the effect of a stimulus to be magnified A combination of different stimuli can trigger a response It avoids the nervous system being overwhelmed by impulses Synapses act as a barrier and slow down the rate of transmission of a nerve impulse that has to travel along two or more neurones They only allow the impulses to pass on if there has been input from other neurones and receptors

Question 42

Sometimes, a single impulse that arrives at a synaptic knob is __________ to generate an action potential in the post-synaptic neurone because: Only a small amount of _________ is released into the synaptic cleft This means only a small number of the gated ion channels are opened in the axon membrane Therefore an insufficient number of ________ ____ pass through the membrane So the threshold potential is not reached The small amount of acetylcholine attached to receptors is broken down rapidly by ___________

Answer 42

Insufficient Acetylcholine Sodium ions Acetylcholinesterase

Question 43

What is temporal summation?

Answer 43

If multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential A large amount of acetylcholine is released into the synaptic cleft Therefore a large number of the gated ion channels open So a sufficient number of sodium ions pass through the membrane

Question 44

What is spatial summation?

Answer 44

Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential through spatial summation The multiple impulses result in a large amount of acetylcholine being released into the synaptic cleft which results in the generation of an action potential

Question 45

What do excitatory neurotransmitters do?

Answer 45

Excitatory neurotransmitters can stimulate the generation of an action potential in a postsynaptic neurone This is done by opening sodium ion channels in the postsynaptic membrane which causes depolarisation if a threshold is reached

Question 46

What do inhibitory neurotransmitters do?

Answer 46

inhibitory neurotransmitters can prevent the generation of an action potential in a postsynaptic neurone They do this by opening potassium ion channels in the postsynaptic membrane which causes hyperpolarisation of the membrane

Question 47

What happens if a neurone is subject to both excitatory and inhibitory synapses?

Answer 47

Sodium ions enter the cell body following stimulation by the excitatory synapse The stimulation of the inhibitory synapse causes potassium ions to diffuse out of the cell body This cancels out the effect of the sodium ions entering The threshold potential is not reached so no action potential is generated

Question 48

Label this cholinergic synapse

Answer 48