nerve impulses

myelinated neurone

1. cell body - large rough ER
2. dendron - carries impulses to cell body
3. axon - carries impulses away from cell body
4. Schwann cells - protective insulator (also carry out phagocytosis, remove cell debris and have a role in nerve regeneration)
5. myelin sheath - membranes of schwann cells, rich in myelin lipid
6. Node of Ranvier - 2-3micrometers constriction (occur every 1-3mm in humans)

potential difference

the difference in electrical charge across the plasma membrane

action potential

4 parts: resting potential, depolarisation, repolarisation, refactory period

occurs when a stimulus is detected by a receptor

resting potential

the movement of Na+ and K+ is restricted across the phospholipid bilayer. proteins span the phospholipid bilayer: sodium-potassium pumps (active transport) and voltage gated ion channels (facilitated diffusion).

sodium-potassium pump: 3 Na+ OUT, 2 K+ IN to axon → higher conc of Na+ outside axon and higher conc of K+ inside axon cytoplasm → both Na+ and K+ voltage-gated ion channels are CLOSED → membrane is less permeable to Na+ (cuz voltage gated Na+ channels are closed), some K+ diffuses back out of axon.

inside axon contains anions that maintain a negative charge, therefore, inside axon negative relative to outside → the axon is polarised - there’s an electrochemical gradient. this is the resting potential and is about -70mV in humans.

sodium-potassium pump maintains the resting potential, until the next stimulus.

what causes Na+ channels to open?

stimulus exciting the neurone cell membrane

depolarisation

- voltage gated Na+ channels open, making the membrane more permeable to Na+. Na+ diffuse into axon, along electrochemical gradient.
- K+ channels closed.
- if the potential difference reaches the threshold, more Na+ channels open. more Na+ diffuse into axon.
- potential difference about +35mV.

repolarisation

- at about +35mV
- Na+ channels closed
- K+ channels open
- membrane is more permeable to K+
- K+ diffuse out of axon, along electrochemical gradient

hyperpolarisation (refractory period)

- voltage gated K+ remain open (slow to close)
- some K+ ions diffuse out of axon
- membrane potential becomes more negative than at resting potential
- always the last stage of an AP

“all or nothing” principle

for an AP to be generated, the stimulus must be greater than the threshold value.

a stimulus will be below the threshold value if insufficient numbers of sodium channels open, preventing full depolarisation of the axon.

once the threshold value is reached, the action potential generated is always the same size regardless of the strength of the stimulus → it’s an “all or nothing” response.

refactory period

the period of time, just after an AP, when it’s impossible to generate another AP. to generate an AP Na+ must be on the outside of the axon. but most of the Na+ are still on the inside of the axon at the end of the current AP and as the membrane is being depolarised by the K+ moving out, all the Na+ voltage gated channels are closed so the membrane can’t be depolarised again yet.

two stages:
(1) absolute refractory period: lasts about 1ms. no new impulses can be propagated. all Na+ are inside - they need to be on the outside to generate an AP.

(2) relative refractory period: lasts about 5ms. new impulses can only be propagated if the stimulus is more intense than the normal threshold level. the Na+ are being pumped outside - depends on how many have been pumped outside.

what are the 3 purposes of the refractory period?

1. ensures that APs are propagated in one direction. the AP can only go from an active to a resting region, it can’t be propagated in a refractory region, so it only moves forward (this means the AP can only travel in 1 direction)
2. produces discrete impulses (a new AP can’t be formed immediately after the first one, therefore, impulses are separated from one another)
3. limits the number of APs cuz the APs are separated from each other (thus limits the frequency of APs and strength of the stimulus that can be detected)

saltatory conduction

myelin sheath is an insulator, it prevents APs from forming.

APs can only occur at the nodes of Ranvier, where the K+ and Na+ channels are exposed.

the APs jump from node to node - saltatory conduction - thus, faster impulse transmission.

the speed of conduction is 90ms-1 in a myelinated neurone.

factors affecting impulse speed

1. axon diameter
2. temperature
3. myelination
4. saltatory conduction

how does axon diameter affect impulse speed?

the greater the diameter of the axon the faster the impulse travels (axons with a small diameter have a larger SA:V, so more ions leak out of the axon, making it more difficult for an action potential to propagate)

how does temperature affect impulse speed?

- higher temp, more kinetic energy, faster rate of ion diffusion, faster speed of nerve impulse
- energy for active transport is from respiration which is controlled by enzymes
- at high temps enzyme and plasma membrane proteins are denatured - so no impulses are conducted (temp is important is cold blooded ectothermic animals)

how does myelination affect impulse speed?

myelin sheath acts as an electrical insulator so APs jump from one node of Ranvier to another (saltatory conduction): increases speed

cell body

contains nucleus and large amounts of RER to aid production of proteins and neurotransmitters

dendrons

small extensions of the cell body which subdivided into smaller branched fibres, called dendrites that carry nerve impulses toward the cell body

axon

single long fibre that carries nerve impulses away from the cell body

Schwann cell

- surrounds the axon providing electrical insulation and protection
- carries out phagocytosis and plays a part in nerve regeneration

myelin sheath

• a lipid covering the axon, made of Schwann cell membranes which are rich in a lipid called myelin

• neurones with myelin sheath are called myelinated and transmit nerve impulses faster

• neurones without myelin sheath are called unmyelinated neurones

Nodes of Ranvier

- gaps between adjacent Schwann cells where there is no myeline sheath
- saltatory conduction

parts of the myeline sheaths surrounding neurones are destroyed. explain how this results in slower responses to stimuli. (2)

1. less saltatory conduction
2. more depolarisation over area of membranes

explain how three features of a plasma membrane adapt it for its function (5)

• phospholipid bilayer as a barrier, forms a barrier to charged substances

• bilayer is fluid, can bend to take up different shapes for phagocytosis

• channel proteins through the bilayer let charged substances through

polarisation

- Na+ voltage gated ion channel closed
- K+ voltage gated ion channel closed
- Na+/K+ pump: 3Na+ out, 2K+ in
- membrane more permeable to K+ less permeable to Na+
- overall: more +ve outside, more -ve inside the neurone. Na+ outside and K+ inside.
- voltage -70mV

nerve impulse

the transmission of the action potential along the axon of a neurone

how do we know the size of a stimulus

- the number of impulses that pass in a given time: the larger the stimulus, the more impulses are generated in a given time
- by having different neurones with different threshold values the brain can interpret the number and type of neurones that pass impulses along - and from this determine the size of the impulse

non-myelinated axons

- slower
- the impulse is transmitted when each part of the membrane depolarised and causes the next section to depolarise
- every part of the membrane must be depolarised

myelinated axon

- myeline sheath around the axon - acts as an electrical insulator and stops the formation of APs
- however, at 1-3mm intervals along the myeline sheath there are breaks called nodes of Ranvier where APs can occur
- so the AP can ‘jump’ from node to node in a process called ‘saltatory conduction’
- NI passes along faster in myelinated than in non-myelinated neurone because less of the membrane needs to depolarise

damage to the myelin sheaths of neurones can lead to problems controlling the contraction of muscles. why?

action potentials travel more slowly. so muscles contract slower.

why are hydrophobic molecules easily able to pass into neurones?

pass through phospholipid bilayer