9.5 nervous transmission

Question 1

1. when is an axon at rest
2. whats the resting potential

Answer 1

when it’s not conducting a nerve impulse

Question 2

how is the resting potential created inside the axon

Answer 2

• the extracellular concentration of ions in greater than the concentration inside the cell
• sodium/ potassium pump has an enzyme called Na+/K+ ATPase, uses ATP to move sodium ions out of axons
• pumps sodium ions out of axon, lowering concentration, can’t diffuse back in due to relative permeability
• potassium is actively pumped into cell and diffuses back out along concentration gradient
• membrane is polarised

Question 3

how is an action potential generated

Answer 3

• neurone is stimulated and axon increases in permeability to sodium ions
• sodium gates open, sodium ions diffuse into axon rapidly down conc. gradient
• potential difference reverses, depolarisation occurs, potential diff is +40mV (action potential)
• sodium gates close. sodium ions pumped out by active sodium pump
• permeability to potassium ions increases, voltage dependent potassium gates open, potassium ions move out
• inside of axon becomes more negative than outside, resting potential restored

Question 4

why is the action potential described as an all or nothing response

Answer 4

• the threshold for any nerve fibre is the point where enough sodium gates open for the rush of sodium ions into the axon to be greater than the outflow of potassium ions
• once the threshold has been reached, action potential occurs and the size of the action potential is always the same, not dependent on the strength of stimulus

Question 5

whats the refractory period and what happens during it

Answer 5

• the recovery time of an axon to restore the resting potential
• ionic movements repolarise the membrane
• sodium gates close, sodium is pumped out
• permeability to potassium ions increases, voltage dependent potassium gates open and potassium moves out
• inside of axon becomes more negative than outside

Question 6

what happens after action potential is generated

Answer 6

• nerve impulse moves along the entire length of the nerve fibre

Question 7

what is saltatory conduction

Answer 7

• occurs in myelinated neurones, ions pass in and out of axon at the nodes of ranvier which are 1mm apart so they must ‘jump’
• this speeds up transmission by allowing ionic movements to occur less frequently

Question 8

1. describe and explain structure of a myelinated nerve cell
2. why is the sheath important

Answer 8

1.
- Schwann cell wraps its membrane repeatedly around axon, contains a nucleus
- this forms a fatty myelin sheath around nerve cell
- usually gaps between sheaths which are called nodes of ranvier
2.
sheath is important because it protects nerve cells and speeds up transmission of nerve impulse

Question 9

what does nerve impulse speed depend on

Answer 9

1. diameter of the nerve fibre, the thicker the fibre, the quicker impulses travel along it
2. the presence of a myelin sheath, the presence of a myelin sheath speeds up nerve impulses without the needs for giant axons which take up a lot of space

Question 10

what is saltatory conduction and why is it important

Answer 10

• occurs in myelinated neurones
• ions can only pass in and out of axon at nodes of ranvier which are 1mm apart so they must jump
• this speeds up transmission of electrical impulse instead of slower continuous progression of depolarisation

Question 11

where are synapses

Answer 11

where two different neurones meet
- every cell in CNS is covered with synaptic knobs from other cells

Question 12

how does a synapse function

Answer 12

• impulse arrives at the synaptic knob, increases permeability of presynaptic membrane to calcium ions, calcium gates open
• calcium ions move into presynaptic knob down concentration gradient
• synaptic vesicles move to pre synaptic membrane
• some fuse with membrane and release transmitter substance into synaptic cleft
• transmitter substance molecules diffuse across gap and become attached to specific protein receptor sites on sodium channels on posy synaptic membrane
• sodium gates open, resulting in influx of sodium ions, causes EPSP, action potential is set up in post synaptic neurone

Question 13

what are the two common neurotransmitters

Answer 13

• nerves that use ACh are cholinergic, once its done, it is hydrolysed into acetate and choline by acetylcholinesterase, in vertebrae
• sympathetic nervous system usually use noradrenaline. nerves that use noradrenaline are adrenergic nerves, usually released by post synaptic receptors back into the synaptic cleft then re-taken up by pre synaptic knob

Question 14

how does nicotine produce its effect

Answer 14

• binds to acetylcholine receptors in post synaptic membrane (nicotinic receptors), triggers action potential in PSN, then receptor remains unresponsive for some time
• causes raised heart rate and BP, triggers dopamine release, associated with pleasure
• blocks acetylcholine receptors at high levels and can kill

Question 15

how does lidocaine produce its effect

Answer 15

• local anaesthetic by dentists
• block voltage gated sodium ion channels, preventing the production of an action potential in sensory nerves therefore preventing you from feeling pain

Question 16

how does cobra venom produce its effects

Answer 16

• a-cobratoxin
• often fatal in snake bites, binds to acetylcholine receptors (blocks them) in post synaptic membrane and neuromuscular junctions
• prevents transmission of impulses across synapses
• muscles are not stimulated to contract and person eventually becomes paralysed, toxins reach the muscles involved in breathing and paralyses them

Question 17

what is the role of the retina

Answer 17

• focusing the light onto the retina is the first step, retina must perceive the light and provide information the brain needs to make sense of the image

Question 18

what is the function of:
1. rods
2. cones

Answer 18

they are both photoreceptors
1. rods - provide greyscale vision, used to see in low light / night time, sensitive to light and several synapse 1 sensory neurone
2. tightly packed together in the fovea, used for vision in bright light and provide colour vision each cone has its own sensory neurone

Question 19

what is the role off rhodopsin

Answer 19

• sodium ions move into rod cell through sodium ion channels, Na+/K+ pump moves them back out
• photon of light hits rhodopsin converts cis-retinal into trans-retinal changes shape of retinal
• makes it break up into opsin and retinal (bleaching)
• rhodopsin (visual purple) opsin + retinal
• sodium ion channels close, rod cell membranes become a lot less permeable
• sodium pump is pimping sodium out of cell, interior becomes negative (the generator potential)
• if threshold is reached, generator potential has been created the neurotransmitter substance is released across synapse

Question 20

what is the function of a nephron

Answer 20

• filter the blood ro remove waste and selectively re absorb useful substances back into the blood
• to ensure urine only contains water, dissolved salts, urea and small substances e.g. hormones, vitamins

Question 21

what does urine NOT contain

Answer 21

• proteins
• blood cells
• glucose

Question 22

what occurs in ultrafiltration

Answer 22

• occurs due to high blood pressure in glomerular capillaries, squeezes blood out through pores into bowman’s capsule (only blood cells and large proteins can’t get out through pores)
• podocytes on Bowman’s capsule ensure any platelets or plasma proteins don’t pass through

Question 23

what occurs in selective reabsorption

Answer 23

1. proximal convoluted tubule
2. loop of henle
3. distal convoluted tubule
4. collecting duct

Question 24

what occurs in proximal convoluted tubule

Answer 24

1. sodium ions actively transported into the intracellular spaces, chloride and water ions diffuse down a concentration gradient
2. substances pass by diffusion into capillary network
3. blood is constantly moving in the capillary network to maintain the concentration gradient

Question 25

how is the proximal convoluted tubule adapted for its function

Answer 25

1. contains microvilli to provide a large surface area for glucose reabsorption
2. contains lots of mitochondria to provide energy for active transport

Question 26

what occurs in the loop of henle

Answer 26

• sodium ions are pumped out of ascending limb, chloride follows, making medulla high in salt concentration
• high medulla salt conc. makes water move out of descending limb into medulla, and water moves out of collecting duct, making urine more concentrated
• filtrate in descending limb becomes more concentrated as it goes down due to countercurrent multiplier
• pumping out of sodium ions in descending limb decreases conc of filtrate increases medulla salt conc
• medulla salt conc equilibrates with medulla conc.

Question 27

what occurs in the distal convoluted tube

Answer 27

• if there isn’t enough salt in the body, sodium is pumped out with chloride ions following
• water moves out of tube if walls are permeable
• permeability is affected by ADH

Question 28

what occurs in the collecting duct

Answer 28

• permeability is affected by ADH
• water moves out of collecting duct down a water potential gradient

Question 29

how is urea produced

Answer 29

deamination of excess amino acids in the liver in protein metabolism, converted into ammonia which is very toxic, then converted into urea which is less toxic by ornithine cycle, so it can be excreted by kidneys