Nervous coordination

Question 1

Describe the structure of a myelinated motor neurone

Answer 1

AXON-long single fiber which carried impulses away from the cell body (forms short terminal branches at the end each with a SYNAPTIC KNOB-(which secrete neurotransmitter at synapse)
DENDRITES-carry impulses towards the cell body
NODES OF RANVIER-space between the myelin sheath,play a role in SALTATORY CONDUCTION of the nerve impulse
CELL BODY- contains all the organelles of a eukaryotic cells, nucleus and much of RER. site of protein synthesis
SCHWANN CELLS-WRAP AROUND THE AXON to provide PROTECTION & ELECTRICAL INSULATION, also creates the myelin sheath

Question 2

what is a plasma membrane

Answer 2

the same as a cell-surface membrane

Question 3

What are the charges or the neurone inside and outside the cell- what ions do they contain at REST

Answer 3

Inside the neuron is has an overall charge POSITIVE CHARGE than outside as it has a HIGHER CONC OF NA+ where as inside has a LOWER CONC OF K+
(its an overall charge because there are other +ve & -ve ions its just that there are more NA+ outside making the charge positive than inside where there is fewer K+ making the overall charge negative)

Question 4

Explain the neuron at rest/ how does it maintain the overall charges

Answer 4

• SODIUM ION-POTASSIUM IONS ACTIVELY PUMP 3NA+ out and 2K+ in the neuron per ATP hydrolysed
• This means more positive ions flowing out than in
• AS well as the neuron cell is MORE PERMEABLE TO POTASSIUM IONS(K+) THAN SODIUM IONS(NA+) as theres is a higher number of POTASSIUM ION LEAK CHANNELS & LOWER NUMBER OF SODIUM ION LEAK CHANNELS
• This makes it easier for K+ to DIFFUSE back out of the neuron cell but difficult for sodium ions to DIFFUSE back in due to the ELECTROCHEMICAL GRADIENT

Question 5

Explain how a resting potential is maintained in a neurone.

Answer 5

• 3 Na+ diffuse out of the neurone with 2 K+ diffuse by PUMP PROTEIN into the neurone cell by active transport per ATP HYDROLYSED
• plasma membrane IS MORE PERMEABLE TO potassium ions THAN Sodium ions as there are more K+ leak channels than Na+ leak channels
• this maintains a higher concentration of Na+ outside than inside and a lower concentration of K+ inside than outside

Question 6

DEFINE A NERVE IMPULSE

Answer 6

its a change in the membrane potential of a neurone that is sent along the cell-surface of that neurone, this change in membrane potential occurs as a impulse is called the ‘action potential’

Question 7

Explain the stages of ACTION POTENTIAL

Answer 7

• membrane starts at resting potential (-65), initial stimulus causes a DEPOLARISINING POTENTIAL at the receptor
• if generator potential reaches THRESHOLD POTENTIAL(if enough depolarisation has happened/enough Na+ diffusing into the neurone) , VOLTAGE GATED SODIUM ION CHANNELS OPEN , so Na+ rapidly diffuse down their electrochemical gradient into the neurone - depolarising it further
• Neurone eventually becomes overall positive inside & overall negative outside- reaches a PEAK MEMBRANE POTENTIAL AT +40mV inside
• This triggers for the voltage gated Na+ channels to CLOSE and the VOLTAGE GATED K+ TO OPEN- thi allows for more K+ to diffuse down their electrochemical gradient out of the neurone- loss of positive charge inside of the cell making it overall -ve- REPOLARISATION
• So many potassium ions leave that the membrane potential shoots below resting potential- membrane becomes HYPERPOLARISED
• This triggers the voltage gates potassium ions to CLOSE
• the resting potential is restored by the Na+/K+ leak channels and pump protein

Question 8

describe the ‘all or nothing potential’

Answer 8

• if threshold potential is REACHED, and action potential will generate
  -If threshold potential ISN’T reached, an action potential will NOT be generated
  -for a neurone if action potential is generated it will always be the same size.
  *thresholds can be different to other neurones thresholds, If threshold is lower, action potentials will be activated by a weaker
  stimulus.

Question 9

if all action potential is generated are the same size how is the strength of the stimulus encoded

Answer 9

the stronger the stimulus the greater the frequency of action potential

Question 10

explain the passage of action potentials along a NON-MYELINATED axon

Answer 10

• sodium ions diffuse into the axon- depolarisation
  -These ions then diffuse sideways within the axon, down their ELECTROCHEMICAL GRADIENT. This diffusion is called a
  LOCAL CURRENT (also called a local circuit).
  -causes a positive charge accumulating under adjacent patches of the cell-membrane taking it to threshold
• Voltage gated sodium ion channels therefore open allowing for Na+ to diffuse sideways down their electrochemical gradient into the neurone,generating new local currents.
  -Local currents flow in both directions forwards and backwards in the axon, therefore to prevent the acting potential going backwards the REFACTORY PERIOD takes place.
  -The refractory period occurs whilst the membrane potential REPOLARISES from +40 mV back to resting membrane potential.
• during this time the membrane is completely unable to generate a new action potential: voltage-gated sodium ion channel proteins are closed and cannot be opened.
  -Once resting potential is restored, a new action potential can be generated.

Question 11

What is the importance of the refractory period- 3 points

Answer 11

1. ensures that the action potential only goes forward/ in one direction
2. ensures that impulses are discrete-action potentials cannot merge with each other.
3. Ensures an upper limit on the frequency of impulse transmission: action potentials cannot merge, so there comes a point at which the maximum action potential frequency must be reached, therefore puts a limit on the strength of stimulus can be perceived

Question 12

what is the disadvantage of having a non-myelinated axon than an myelinated axon

Answer 12

Non-myelinated axons are slower than myelinated axons this is because the whole length of the cell-surface membrane need to be progressively depolarised to take an impulse from one end of the axon to the other. aLSO MORE Na+ can LEAK OUT using the Na+ leak channels which would weaken the current
whereas myelinated axons have myelin sheaths which protect and electrically insulates the impulse. It also blocks the
leak (through leak channel proteins) of cations from the cytoplasm to the outside of the cell.Which MAINTAINS THE STRENGTH OF THE LOCAL CURRENTS allows them to flow for a longer distance before weakening.The “NODES OF RANVIER”, regenerates an action potential so the nodes need to be unmyelinated so the necessary exchange of ions can occur between the neurone and surrounding fluid.

Question 13

What is SALTATORY CONDUCTION

Answer 13

Its in myelinated axons- depolarisation ONLY OCCURS AT THE NODES OF RANVIER so the nerve impulse JUMPS FROM NODE TO NODE,It is much faster than conduction in non-myelinated axons.

Question 14

How does the axon diameter affect the speed of impulse transmission,give 2 reasons for this

Answer 14

the WIDER THE AXON the FASTER THE IMPULSE TRANSMISSION;

1) Organelles can get in the way of local current flow in the axon. Wider axons provide more alternate pathways for the ions to take. Therefore, wider axons have less resistance to the flow of local currents.
2) Wider axons have lower surface area to volume ratios. This reduces the leakage of local current ions per unit volume of axons and therefore strengthens these currents.

Question 15

how does temperature affect the speed of conduction

Answer 15

higher temperatures increases the speed of impulse transmission as there’s more KINETIC ENERGY, but exe=cessive temperatures will denature the channels & pump proteins therefore stopping the impulse transmission