Chapter 13 Neuronal Communication

Question 1

State different types of stimuli the body can detect.

Answer 1

> Blood glucose concentration.
Internal temperature.
Water availability.
Cell pH.
Humidity.
External temperature.
Light intensity.
A sudden sound.

Question 2

Why is coordination and communication important in the body?

Answer 2

Different components of an organism is dependent on other parts of an organism in order for it to function effectively.

Question 3

What is homeostasis?

Answer 3

Maintaining a relatively constant internal environment in the body in case a change were to occur.

Question 4

State ways cells can signal throughout the body.

Answer 4

Cells can transfer signals locally, like between neurones at synapses- this signal is a neurotransmitter.
Cells can transfer signals over long distances, using hormones.

Question 5

What are neurones?

Answer 5

Neurones transmits electrical signals (impulses) around the body so that the organism can respond to changes in its internal and external environment.

Question 6

State the nervous pathway.

Answer 6

> Receptor
Sensory neurone
Relay Neurone with the CNS
Motor neurone
Effector (glands and muscles)

Question 7

What is the general structure and function of dendrons?

Answer 7

Dendrons are nerve fibres that lead to the cell body. They are responsible for transmitting nerve impulses to the cell body. Dendrons are able to split into smaller branches called dendrites.

Question 8

What is the general structure of a cell body in a neurone?

Answer 8

The cell body receives nerve impulses from the dendron. The cell body contains a nucleus surrounded by a cytoplasm. It contains a lot of endoplasmic reticulum, mitochondria and ribosomes; organelles used to synthesise neurotransmitters. Neurotransmitters are chemicals that pass signals from one neurone to the next.

Question 9

What is the general structure and function of the axon?

Answer 9

The axon is an elongated nerve fibre that transmits nerve impulse away from the cell body. The fibres are very long and cylindrical shape with narrow region of cytoplasm, then a plasma membrane.

Question 10

Describe the structure and function of a sensory neurone.

Answer 10

Sensory neurones transmits nerve impulses from a sensory receptor cell to relay neurones within the CNS.
The cell body lies in the middle of a dendron and axon. Both the dendron and axon is myelinated with a myelin sheath. The dendron also splits into dendrites.

Question 11

Describe the structure and function of a relay neurone.

Answer 11

Relay neurones are found within the CNS and receives nerve impulses from sensory neurones, while carrying nerve impulses to motor neurones.
The cell body lies in the middle of may short axons and short dendrons (dendrons being on one side and axons being on another side).

Question 12

Describe the structure and function of a motor neurone.

Answer 12

A motor neurone carries a nerve impulse from a relay neurone in the CNS to an effector- a gland or muscle.
The cell body is found at the end of the neurone, attached to may short dendrons and dendrites on one side. On the other side is one long axon that is myelinated.

Question 13

What is the advantage if a neurone is myelinated?

Answer 13

If a neurone is myelinated, there is a faster transmission speed of a nerve impulse.

Question 14

What is a myelin sheath made of, and how is it produced?

Answer 14

The myelin sheath is just layers of plasma membrane, hence made of phospholipids. These layers of plasma membranes are produced by Schwann cells that wraps around the nerve fibres. Every time a Schwann cell grows a membrane, a layer of phospholipid bilayer is laid down.

Question 15

Why is the transmission speed faster in a myelinated neurone?

Answer 15

The myelin sheath is made of phospholipids which are non-polar, so they do not have a charge. Hence, they cannot conduct a nerve impulse. As the myelin sheath insulates the nerve fibre, the nerve impulses are made to jump to gaps between the myelin sheath- the nodes of Ranvier. As the nerve impulse travels by saltatory conduction rather than simply moving along the fibre itself, the nerve impulse moves faster this way.

Question 16

State the transmission speed of a myelinated neurone compared with a non-myelinated neurone.

Answer 16

Myelinated= 100-120 m/s
Non-myelinated= 2-20 m/s

Question 17

What does it mean if the plasma membrane of a neurone cell is polarised?

Answer 17

If the membrane is polarised, there is an uneven distribution of charges across the cell surface membrane.

Question 18

What is the value of the resting potential?

Answer 18

-70 mV

Question 19

How is a resting potential in a neurone established?

Answer 19

A sodium potassium pump actively transports 3 sodium ions outside the membrane and 2 potassium ions inside the membrane. This requires energy- ATP breaks down into ADP and a phosphate ion. The phosphate ion binds to the pump, and changes its shape in order to transport the ions.
Now there is a higher concentration of potassium ions on the inside of the membrane. Hence, the potassium ions move to the outside via potassium ion channels, down the concentration gradient. This occurs via facilitated diffusion.
There is also a higher concentration of sodium ions on the outside of the membrane, but there is less movements of these ions to the inside, as less sodium ion channels are open.
Due to this, there is a higher concentration of sodium and potassium ions on the outside of the membrane. On the inside of the membrane, there is still negative ions and negatively charged proteins. Due to this, the membrane is polarised because of the uneven distribution of charges across the membrane.

Question 20

What is an action potential?

Answer 20

An action potential is a nerve impulse that is being transmitted across a membrane.

Question 21

What occurs during an action potential?

Answer 21

During an action potential, as the nerve impulse moves across the membrane, some sodium ions also move along with it (sodium ions from the generator potential).
As the sodium ions move along the membrane, the positive charge increases the potential difference from -70 mV. When the potential difference reaches the potential difference of -55 mV, voltage gated sodium ion channels open. There is now a great influx of sodium ions into the membrane, and this causes the membrane to depolarise. This is also an example of positive feedback.
Eventually, the potential difference reaches +40 mV. All of this is called depolarisation.

When the potential difference reaches +40 mV, the voltage gated sodium ion channels close, and voltage gated potassium ion channels open. Now potassium ions move from the inside of the membrane (axoplasm) to the outside of the membrane. This decreases the potential difference from +40 mV.
The movement of potassium ions outside of the membrane keeps happening, and eventually the potential difference decreases and exceeds the resting potential (more negative than -70 mV). This is called hyperpolarisation.
To go back to the resting potential, sodium potassium ions pumps are activated, so there is some potassium ions into the axoplasm. Now the membrane has reached a potential difference has reached -70 mV.

Question 22

What is a refractory period?

Answer 22

A refractory period is a short period of time where the nerve fibre cannot be excited again. During this time, voltage gated sodium ion channels are closed.

Question 23

Why is a refractory period essential to the normal functioning of nerve impulses?

Answer 23

A refractory period prevents an action potential moving backwards along the axon. It also ensures the action potentials do not overlap and occur as discrete impulses.

Question 24

Why is saltatory conduction more efficient than the normal transmission of a nerve impulse?

Answer 24

Every time channels opens and ions move across the membrane, it takes time. By reducing the number of places it happens, it speeds up the action potential (sodium ions cannot pass though protein channels of the membrane in myelinated area).
Repolarisation requires ATP in sodium potassium pumps, so by reducing the amount of repolarisation needed, saltatory conduction makes conduction of nerve impulses more efficient, as less energy is needed.

Question 25

Apart from myelination, what affects the speed of a nerve impulse?

Answer 25

> Axon diameter- The bigger the diameter, the less resistance to flow of ions in the cytoplasm.
Temperature- The higher the temperature, the faster the nerve impulse (generally up to about 40 degrees Celsius. Higher temperature would cause proteins to denature).

Question 26

A stimulus may not always produce a response. Why is this?

Answer 26

The stimulus needs to reach a threshold value.

Question 27

How does the size of a stimulus correspond with the size of an action potential?

Answer 27

A large stimulus would still produce the same size action potential as a small stimulus. However, for a large stimulus, there will be more frequent action potentials generated.

Question 28

What is a transducer?

Answer 28

A transducer converts a stimulus into a nerve impulse. The nerve impulse is referred to as generator potential.

Question 29

What type of stimulus does a mechanoreceptor detect? In what sensory organ can it be found? Give an example of a mechanoreceptor.

Answer 29

A mechanoreceptor detects a change in pressure/ movement.
It is found in the skin.
An example is the Pacinian corpuscle.

Question 30

What type of stimulus does a chemoreceptor detect? In what sensory organ can it be found? Give an example of a chemoreceptor.

Answer 30

A chemoreceptor detects smell/ taste.
It is found in the taste buds on tongue or nose.
An example is olfactory receptor.

Question 31

What type of stimulus does a thermoreceptor detect? In what sensory organ can it be found? Give an example of a thermoreceptor.

Answer 31

A thermoreceptor detects a change in temperature.
It can be found on the surface of skin or tongue.
An example is end bulbs of Krause.

Question 32

What type of stimulus does a photoreceptor detect? In what sensory organ can it be found? Give an example of a photoreceptor.

Answer 32

A photoreceptor detects light.
It can be found in the retinas in the eye.
An example of a cone cell.

Question 33

What type of channels are found in the Pacinian corpuscle?
How do these channels change when pressure is exerted on the receptor?

Answer 33

Stretch mediated sodium ion channels are found in the Pacinian corpuscle. Initially, with no pressure at all, the stretch mediated sodium ion channels are closed. However, when pressure is exerted, the receptor changes shape, so the stretch mediated sodium ion channels widens and opens.

Question 34

How is a generator potential generated in the Pacinian corpuscle?

Answer 34

Before a stimulus, the Pacinian corpuscle is at resting potential. There is a higher concentration of sodium ions on the outside of the receptor membrane than the inside.
During a stimulus, pressure is applied to the Pacinian corpuscle, so the corpuscle changes shape and causes membrane surrounding the neurone to stretch.
Now stretch mediated sodium ion channels are open.
There is a movement of sodium ions diffusing into the neurone, and this causes the membrane to depolarise. This results in a generator potential.
Generator potentials can lead to action potential that passes along the membrane.

Question 35

Where can the Pacinian corpuscle be found?

Answer 35

The Pacinian corpuscle can be found at the tips of fingers, soles of feet, in reproductive organs and more.

Question 36

What is a synapse?

Answer 36

A synapse is the gap between two neurones, where a nerve impulse crosses with the help of neurotransmitters.

Question 37

What are the two types of neurotransmitters? Give examples of them.

Answer 37

> Excitatory neurotransmitters triggers an action potential in the postsynaptic neurone. An example is acetylcholine.
Inhibitory neurotransmitters do not trigger an action potential in the postsynaptic neurone. An example is GABA.

Question 38

Describe the key components in the presynaptic neurone and postsynaptic neurone.

Answer 38

In/On the presynaptic neurone:
Mitochondria
Voltage gated calcium ion channels
Vesicles containing neurotransmitter acetylcholine

On the postsynaptic neurone:
Postsynaptic sodium ion channels
Receptors binded on to sodium ion channels

Question 39

In the transmission of nerve impulses across a synapse, the arrival of an action potential at a presynaptic neurone, causes what channels to open and why?

Answer 39

The arrival of an action potential at the presynaptic neurone depolarises the membrane. The depolarisation reaches a threshold value and this causes voltage gated calcium ion channels to open. Calcium ions rushes into the presynaptic neurone via facilitated diffusion (a higher concentration on outside of neurone than inside).

Question 40

What does the movement of calcium ions into the presynaptic neurone cause?

Answer 40

The movement of calcium ions into the presynaptic neurone causes vesicles containing acetylcholine to move to the presynaptic membrane and fuse with it.

Question 41

What happens after ACh fuses with the presynaptic neurone?

Answer 41

After ACh is released from the membrane into the synaptic cleft and diffuses across the synapse towards to the postsynaptic neurone.

Question 42

What allows sodium ions to diffuse into the postsynaptic neurone?

Answer 42

ACh released from the presynaptic neurones binds to the receptor sites on postsynaptic sodium ion channels and causes them to open. This allows sodium ion to diffuse into the postsynaptic neurone.

Question 43

What is caused by the movement of sodium ions into the postsynaptic neurone?

Answer 43

The movement of sodium ions into the postsynaptic neurone causes depolarisation and this triggers a new action potential.

Question 44

Why can’t nerve impulses move from a postsynaptic neurone to a presynaptic neurone?

Answer 44

There are no receptor sites on the presynaptic neurone for ACh to bind to and allow sodium ions to flow in the opposite direction.

Question 45

Why is it essential that ACh is removed from the receptor sites on the postsynaptic neurone?

Answer 45

If ACh is constantly binded to the receptor sites on the postsynaptic neurone, there will be a constant flow of sodium ions into the postsynaptic neurone, which can lead to a seizure.

Question 46

What breaks down ACh at the postsynaptic neurone? What does the ACh break down into?

Answer 46

Acetylcholinesterase (AChE) breaks down ACh into acetic acid (ethanoic acid) and choline.

Question 47

What happens after ACh is broken down into their components?

Answer 47

The choline and acetic acid re-enters the presynaptic neurone.
Mitochondria in the presynaptic neurone produces ATP via respiration. The ATP is used to combine the choline and acetic acid to give ACh; it is then stored in vesicles. In this sense, the components of ACh is recycled.

Question 48

Why are synapses important to the functioning of the body?

Answer 48

Synapses ensures impulses are unidirectional and does not flow backwards.
An impulse from one neurone can be transmitted to multiple neurones via multiple synapses. A single stimulus can cause multiple responses.
Different neurones can feed into the same synapse to a single postsynaptic neurone; stimuli from different receptors can interact to give a single result.

Question 49

Why may an impulse not be able to generate an action potential in the next neurone?

Answer 49

In some synapses, there may not be enough neurotransmitter to trigger an action potential in the next neurone. The neurotransmitters will have to build up sufficiently to reach a threshold value to trigger an action potential (this is called summation).

Question 50

What is summation? What are the two types of summation?

Answer 50

Summation is when neurotransmitters builds up at the receptor sites at postsynaptic neurones until a threshold value is reached.

Spatial summation- When multiple presynaptic neurones connects to one postsynaptic neurone. Each releases neurotransmitters which builds up to a high enough level in the synapse to trigger an action potential.

Temporal summation- When a single presynaptic neurone releases neurotransmitters (as a result of an action potential) several times over a short period. Builds up in the synapse until the quantity is sufficient enough to trigger an action potential.

Question 51

How does size of stimulus correspond to the amount of neurotransmitters released at a presynaptic neurone?

Answer 51

Any sized stimulus releases the same amount of neurotransmitters. However, a bigger sized stimulus produces more frequent action potentials (every action potential is the same size for any stimulus). Due to the frequent potential, neurotransmitters will be released more frequently at synapses.

Question 52

How can the mammalian nervous system be split into 2 systems in terms of structure?

Answer 52

Central nervous system- Consists of brain and spinal cord.
Peripheral nervous system- Consists of neurones that connects the CNS to the rest of the body (e.g sensory neurones and motor neurones).

Question 53

How can the mammalian nervous system be split into 2 systems in terms of function?

Answer 53

Somatic nervous system- System is under conscious control; when you voluntarily decide to do something, like moving a limb.

Autonomic nervous system- This system is under subconscious control and constantly working. When the body does something automatically without deciding to do it, like causing the heart to beat, or digesting food.

Question 54

The autonomic nervous system can be spilt into 2 systems. What are they?

Answer 54

Sympathetic nervous system- Generally, outcome increases activity, like an increase in heart rate.
Parasympathetic nervous system- Generally, outcome decreases activity.

Question 55

What is the largest part of the brain?

Answer 55

Cerebrum

Question 56

What is the function of the cerebrum?

Answer 56

Controls voluntary action.
Controls conscious thoughts.
Learning, memory, personality.
The cerebrum is spilt into 2 cerebral hemispheres.

Question 57

What is the function of the cerebellum?

Answer 57

Coordinates muscle movement.
Balance and posture.

Question 58

What is the function of the medulla oblongata?

Answer 58

In charge of autonomic control, like heart rate and ventilation.

Question 59

What is the function of the hypothalamus?

Answer 59

Helps with homeostasis (thermoregulation, water balance and more).
Can release hormones.
Main control of autonomic nervous system.

Question 60

What is the function of the pituitary gland?

Answer 60

Produces hormones (anterior).
Stores and releases hormones made by the hypothalamus (posterior).

Question 61

What is the function of the spinal cord?

Answer 61

Carries nerve impulses from the body to the brain and vice versa.

Question 62

Why are reflexes important?

Answer 62

Reflexes are involuntary actions.
Reflexes are innate (naturally born with) actions and are not learnt.
Reflexes are extremely fast actions.
Some everyday actions are reflexes- blinking and flinching.

Question 63

State the structure of reflex arc.

Answer 63

Receptor.
Sensory neurone.
Relay neurone in spinal cord.
Motor neurone.
Effector.

Question 64

In the kneejerk reflex, tapping under the kneecap causes what to stretch?

Answer 64

Tapping under the kneecap causes the patellar tendon to stretch physically, which also causes the extensor muscle to stretch.

Question 65

In the kneejerk reflex, what causes a generator potential in the sensory neurone?

Answer 65

The stretching of the extensor muscle causes a reflex arc impulse in a sensory neurone.

Question 66

In the kneejerk reflex, the impulse takes two different routes along 2 different motor neurones. Where do each of them lead, and what do they cause?

Answer 66

The impulse travels along one motor neurone to the extensor muscle. It causes the extensor muscle to contract.
The impulse also travels along another motor neurone to a flexor muscle. The impulse signals the flexor muscle to relax.

Question 67

In the kneejerk reflex, the contraction of the extensor muscle and relaxing of the flexor muscle causes what?

Answer 67

The simultaneous contraction and relaxing causes a pull on the patellar tendon; it causes the leg to kick upwards.

Question 68

What is the difference between a spinal and cranial reflex?

Answer 68

Generally, all relay neurones are found in the spinal cord; so either the brain or spine, or both.
In a spinal reflex, the relay neurone is found in the spine.
In a cranial reflex, the relay neurone is found in the brain.

Question 69

How can the blinking reflex be used to check if unconscious patients are brain dead?

Answer 69

If the brain stem is functioning, the blinking reflex process is carried out and hence, they are not brain dead. This is because the relay neurone involved in the blinking reflex is found in the lower brain stem. If you are brain dead, then the neurones in the brain is dead, and hence impulses cannot be carried- so reflexes won’t work.

Question 70

How can you initiate the blinking reflex?

Answer 70

The blinking reflex can be initiated using a corneal reflex action (directly touching the cornea), or by an optical reflex action (using light).

Question 71

State the steps of a blinking reflex.

Answer 71

Cornea is irritated (stimulus).
Triggers impulse along sensory neurone.
Relay neurone in lower brain stem passes impulse along.
Impulse passes along motor neurones on to eyelid muscles.
Both eyes shut as a response.