Nerves & Action Potential

Question 0

Where is the thermoreceptor found and what is it sensitive to?

Answer 0

Skin and changes in temperature.

Question 1

Where is the Pacinian Corpuscle found and what is it sensitive to?

Answer 1

Skin and Pressure

Question 2

Where are the chemoreceptors (taste buds) found and what are they sensitive to?

Answer 2

Tongue and soluble chemicals

Question 3

Where are the rods and cones cells found and what are they sensitive to?

Answer 3

Eyes (retina) and sensitive to light intensity and range of different wavelengths

Question 4

Where is the sound receptor found and what is it sensitive to?

Answer 4

Ears and vibrations/sound waves

Question 5

Where are the olfactory cells found and what are they sensitive to?

Answer 5

Nose and volatile chemicals

Question 6

What do muscles spindles do?

Answer 6

They detect the length of muscle fibres

Question 7

What is the reflex arc?

Answer 7

Change in environment
⬇️
Stimulus
⬇️
Detected by Sensory Receptor
⬇️
Action Potential sent along sensory Neurone⤵️ (To Brain) ⬇️
⬇️ CNS➡️Brain +Spinal Cord◀️⬇️
Action Potential sent along Relay Neurone ⬇
⬇️ (Synapse) ⬇️
Action Potential arrives at Motor Neurone ↩️
⬇️
Signal passed to Effector (Muscle/Gland)
⬇️
Effector Responds

Question 8

What do Sensory Neurones do?

Answer 8

• they are transducers and convert the stimulus into a nerve impulse
• they carry the action potential from the Sensory Receptor to the CNS

Question 9

What do Motor Neurones do?

Answer 9

They carry the Action Potential from the CNS to the Effector

Question 10

How are Neurones adapted?

Answer 10

The neurones are:

• Very Long
To transmit the Action Potential over a long distance

• Myelin Sheath
Insulates the neurone from the electrical activity in nearby cells

• Dendrites
Increase the surface area

Question 11

Describe the structure of the motor neurone

Answer 11

• the motor neurone has a cell body at the end with a large nucleus and lots of RER and Golgi bodies
• it has a many short dendrites that carry imposes to the cell body
• a long axon which caries an impulse from the cell body to the effector

Question 12

Describe the structure of the sensory neurone

Answer 12

• the sensory neurone has long processes on either side of the cell body
• a dendron carrying nerve impulses from a receptor to the cell body
• an axon carrying an impulse from the cell body to the effector

Question 13

What is the difference between Sensory Neurones and Motor Neurones?

Answer 13

In the Sensory Neurone, the DENDRON is called the AXON after the Cell Body.

(It’s only called the AXON AFTER the Cell Body)

Cell Bodies of Motor Neurones are located in the CNS or Brain - (At the end of the Neurone)

Cell Bodies of Sensory Neurones are located around the body - (In the MIDDLE of the Sensory Neurone)

Question 14

Describe and explain how the resting potential is established

Answer 14

• when not conducting an impulse the potential difference across the membrane is about -70 mv
• the outside of the membrane is positively charged compared to the inside so the membrane is polarised
• sodium-potassium pumps actively transport (need ATP) 3 Na+ ions out of the neurone for every 2K+ ions moved in
• the membrane is impermeable to sodium ions so they can’t diffuse back in which creates a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside
• the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels using facilitated diffusion down their concentration gradient
• this makes the outside of the cell positively charged compared to the inside

Question 15

Describe how an action potential is generated

Answer 15

• the membrane is at resting state, -70 mv inside compared to the outside so it is polarised
• the sodium ion channels open and the membrane becomes more permeable to sodium so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient
• this makes the inside of the neurone less negative and reaches the threshold potential of around -50 mv (it is depolarised)
• this causes voltage-gated sodium ion channels to open and many Na+ ions diffuse into the neurone
• the potential difference across the membrane reaches +40 mv so the inside is now positive compared to the outside
• this causes sodium ion channels to close and voltage gated potassium ion channel to open
• the membrane is more permeable to potassium ions so they diffuse out of the neurone down the potassium ion concentration gradient, this starts to bring the potential difference back towards the negative resting potential (repolarisation)
• potassium ion channels are slow to close so there’s a slight overshoot where too many potassium ions diffuse out of the neurone, this causes the potential difference to become more negative than the resting potential (hyperpolarisation)
• the ion channels are then reset and the sodium-potassium pump returns the membrane back to its resting potential

Question 16

Revise potential difference across membrane graph

Answer 16

• resting potential
• generator potential
• threshold potential
• depolarisation
• repolarisation
• hyperpolarisation
• resting potential

Question 17

What are local currents?

Answer 17

• Na+ ions enter the axon through open channels down the concentration gradient. The inside of the axon near the open channel now has a higher concentration of Na+ ions than the area of the axon near the close channel –> this sets up a concentration gradient inside the axon
• Na+ ions diffuse down their gradient inside the axon, making the inside of the axon more positive/ less negative bear the closed channels (depolarising the membrane)
• This causes voltage gated Na+ channels to open so more Na+ ions can enter

Question 18

Describe and explain how an action potential is transmitted in a myelinated neurone

Answer 18

• No Na+ ions can diffuse into the axon where Schwann cells are
• There are open voltage gated Na+ ion channel at each node of ranvier (gap between Schwann cells)
• So the action potential ‘jumps’ from mode to node
• This is known as saltatory conduction

Question 19

What factors can increase the speed of the impulse?

Answer 19

• if the neurone is myelinated
• temperature
• axon diameter

Question 20

Compare the differences in structure and function of myelinated and non-myelinated neurones

Answer 20

• myelinated neurones

  - speed of transmission is around 100-120 Ms-1
  - transmit signals over distances up to 1m
  - faster response time 
  - used in movement 
  - neurones are surrounded by a Schwann cell and have nodes of randier in between each Schwann 

• Non - myelinated

   - speed of transmission 2-20 Ms-1
   - transmission distance is mm or cm 
   - slow response time 
   - used in breathing an digestion

Question 21

What is the refractory period?

Answer 21

It is the process where ion balance is restored using the Na+/K+ pump
• an action potential cannot be generated
• helps to ensure that action potentials only travel in one direction

Question 22

What are the roles of synapses in the nervous system?

Answer 22

• ensures that impulses are transmitted in the correction direction as only the presynaptic knob contains vesicles of acetylcholine
• one presynaptic neurone can diverge to several postsynaptic neurones –> one signal can be transmitted to several parts of the nervous system
• several presynaptic neurones may converge together –> allows signals from different parts of the nervous system to create the same response
• can filter out unwanted low level signals - several vesicles of acetylcholine must be released for an action potential to be created on post-synaptic membrane
• low level signals can be amplified by summation to produce an action potential - if a low level stimulus is persistent it can generate several successive action potentials in the pre synaptic membrane, the release of many vesicles of acetylcholine is a short space of time will enable the postsynaptic generator potentials to combine together to produce an action potential
• acclimatisation - after repeated stimulation a synapse may run out of vesicles containing acetylcholine, the synapse is said to be fatigued which helps to avoid overstimulation of an effector which could damage it

Question 23

What is the significance of the frequency of the transmission?

Answer 23

• when a stimulus is at a higher intensity the sensory receptor will produce more generator potentials –> which will cause more frequent action potentials in the sensory neurone
• when these arrive at a synapse these will cause more vesicles to be released –> creates a higher frequency of action potentials in the postsynaptic neurone
• so there will be a higher frequency of signals sent to the brain
• so the stimulus will be more intense

Question 24

What is the structure of a cholinergic synapse?

Answer 24

• between two neurones there is a gap called the synaptic cleft which an action potential cannot bridge across
• so the presynaptic action potential causes the release of a neurotransmitter that diffuses across the gap and generates an action potential in the postsynaptic membrane
• postsynaptic membrane has specialised sodium ion channel that will only open when acteylcholine binds to them
• synapses that use acetylcholine as the neurotransmitter are called cholinergic synapses

Question 25

Describe the structure of the synaptic knob

Answer 25

• the presynaptic neurone ends in a swelling called the synaptic knob which contains a number of specialised features
- many mitochondria (active process, needs ATP)
- large amount of SER
- vesicles containing acetylcholine
- voltage - gated calcium ion channels in the membrane
DIAGRAM

Question 26

Explain the transmission or action potentials across the synapse

Answer 26

• an action potential arrives at the synaptic knob of the presynaptic neurone
• this triggers the voltage-gated calcium ion (Ca2+) channels to open on the presynaptic membrane so that Ca2+ ions can diffuse into the cell
• the influx of Ca2+ ions causes vesicles containing acetylcholine within the neurone to move to the cell surface membrane and fuse with the presynaptic membrane
• via exocytosis, the vesicles release the acetylcholine into the synaptic cleft which diffuses across the gap down the concentration gradient to reach the postsynaptic membrane
• the acetylcholine binds to receptors on sodium ion (Na+) channels on the membrane which causes the channels to open, so Na+ ions diffuse into the cell
• the influx of sodium ions triggers an action potential, via the depolarisation of the membrane, on the postsynaptic membrane which travels along the membrane of this new neurone