Topic 6—B: Nervous Coordination-1. Neurones Flashcards
1
Q
What are nervous impulses?
A
- They are electrical charges transmitted along a neurone
2
Q
How are nervous impulses created?
A
- By the movement of sodium and potassium ions
3
Q
The resting membrane potential
A
- In a neurone’s resting state (when it’s not being stimulated) the outside of the membrane is positively charged compared to the inside.
4
Q
How is the outside of the membrane polarised?
A
- There are more positive ions outside the cell than the inside
5
Q
What does it mean when the membrane is polarised?
A
- There is a difference in charge (called a potential difference or voltage) across it.
6
Q
Resting potential
A
- Voltage across the membrane when it’s at rest
7
Q
How is the resting potential created and maintained?
A
- By the sodium-potassium pumps and potassium ion channels in a neurones membrane
8
Q
Sodium potassium pumps
A
- they use active transport to move three sodium ions out of the neurone for every two potassium ions moved in
- ATP is needed
9
Q
Potassium ion channels
A
- They allow facilitated diffusion of potassium ions out of the neurone down their concentration
10
Q
Sodium ion channels
A
- Diffusion of sodium ions into the neurone
11
Q
Movement of sodium and potassium ions across a resting cell membrane
A
- The sodium potassium pumps move sodium ions out of the neurone but the membra isn’t permeable to sodium ions so they can’t diffuse back in - this creates a sodium ion electro chemical gradient. ( A concentration gradient of ions) because there are more positive sodium ions outside the cell than inside
- The sodium potassium pumps also move potassium ions into the membrane
- When the cells at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions, so some diffuse back out through potassium ion channels
12
Q
How is the outside of the cell positively charged compared to the inside?
A
- More positive ions move out of the cell than enter
13
Q
What happens when a neurone is stimulated?
A
- Other ion channels in the cell membrane called sodium ion channels open
14
Q
What happens if the stimulus is big enough?
A
- It will trigger a rapid change in the potential difference which causes the cell membrane to become depolarised (action potential)
15
Q
Stimulus
A
- This excites the neurone cell membrane
- This causes sodium ion channels to open
- The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down their sodium ion electrochemical gradient.
- This makes the inside of the neurone less negative
16
Q
Resting potential
A
- There is a positive charge on the outside of the neurone due to the presence of sodium and potassium ions
- There is a negative charge in the inside of the neurone due to the presence of anions
- Gated sodium ion channels are closed so little diffusion of sodium into the neurone
- Gated potassium ion channels are open so potassium can diffuse out of neurone
- Sodium potassium pump returns the membrane to it’s resting potential by pumping 3 sodium ions out for every 2 potassium ions in
- Resting potential is maintained until the membranes excited by another stimulus
17
Q
Depolarisation
A
- The outside of the neurone gets more negative due to the loss of sodium and potassium ions
- The inside of the neurone is more positive due to the presence of sodium and potassium ions
- If potential a difference reaches threshold more gated sodium ion channels open increasing permeability to sodium ions
- 2 potassium ions are pumped in for every 3 sodium ions pumped out.
-They diffuse down its concentration gradient into the neurone making potential difference more positive - If there is enough sodium ions diffusing across membrane then threshold potential is reached and neurone membrane is depolarised.
18
Q
Generator potential
A
- The same as depolarisation but less sodium ion channels are open compared to action potential
19
Q
Repolarisation
A
- Outside the neurone is more positive due to the presence of sodium and potassium ions
- Inside of the neurone is negative
- Gated sodium ion channels close so less diffusion of sodium ions into the neurone
- Potassium ion channel open allow potassium ions to diffuse out of neurone
- Sodium-potassium pump removes sodium from neurone cytoplasm
- The membrane is more permeable to potassium so potassium ions diffuse out of neurone down the potassium ion concentration gradient
- This starts to get the membrane back to it’s resting potential
20
Q
Hyperpolarisation
A
- Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone
- The potential difference becomes more negative than the resting potential
21
Q
What happens to the membrane when sodium ion channels are open?
A
- The permeability of membrane to sodium ions increase
22
Q
What happens to the membrane when sodium ion channels are closed?
A
- The permeability of membrane to sodium ions decrease
23
Q
Refractory period
A
- The time taken before a new depolarisation (action potential) can occur (during Repolarisation and Hyperpolarisation)
- It acts as a time delay between one action potential and the next
- This makes sure that action potentials don’t overlap but pass along as discrete (separate) impulses.
24
Q
Waves of depolarisation
A
- When an action potential happens some of the sodium ions that enter the neurone diffuse sideways.
- This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
- This causes waves of depolarisation to travel along the neurone.
- The wave moves way from the parts of the membrane in the refractory period because these parts can’t fire an action potential
25
Q
All or nothing principle
A
- Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is
- If the threshold isn’t reached an action potential wont fire
- A bigger stimulus wont cause a bigger action potential, but it will cause them to fire more frequently
26
Q
What three factors affect the speed of conduction of action potentials?
A
- Myelination
- Axon diameter
- Temperature
27
Q
Myelination
A
- Neurones including motor neurones are myelinated (they have a myelin sheath)
- Myelin sheath is an electrical insulator
- In the peripheral nervous system, the sheath is made up of a type of cell called Schwann cell
- Between the Schwann cells are tiny patches of bare membrane called the nodes of ranvier
- Sodium ion channels are concentrated at the nodes of ranvier
28
Q
Axon diameter
A
- action potentials are conducted quicker along axons with bigger diameters
- This is because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon
- With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
29
Q
Temperature
A
- The speed of conduction increases as the temperature increases too
- This is because ions diffuse faster
- The speed only increases up to around 40 degrees and after that proteins begin to denature and the speed decreases.
30
Q
Differences between nervous and endocrine system
A
- Nervous system quickly sends electrical impulses around the body but the endocrine system acts slower
- Electrical impulses pass around the body through nerves in the nervous system but in the endocrine system hormones are released from glands and are proteins which travel around the blood to reach target cells
- Response is localised in the nervous system but continuous and long lasting in the endocrine system
31
Q
Axon
A
Single long fibre that carries nerve impulses away from the cell body
32
Q
Schuann cells
A
- Surround the axon and protect it and provide. Electrical insulation
- Makes up the myelin sheath around neurones
33
Q
Node of ranvier
A
- Tiny area of bare cell membrane on the surface of a myelinated neurone where depolarisation can take place
34
Q
Myelin sheath
A
- A layer of Schwann cells around neurone that act as an electrical insulator and speed up conduction of nervous impulses
35
Q
Dendrons
A
- Extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses towards the body
36
Q
Differences between hormonal and nervous system
A
- Transmission is by the blood system in a hormonal system but transmission is by neurones in the nervous system
- Transmission is slow in the hormonal system but rapid in the nervous system
- Effect may be permanent in hormonal system but effect is temporary in the nervous system
- Response is long-lasting in hormonal system but shortlived in nervous system
37
Q
Saltatory conduction
A
- (Myelinated neurone) depolarisation only happens at the nodes of ranvier (where sodium ions can get through the membrane)
- The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node
- (non-myelinated neurone) impulse travels as a wave along the whole length of the axon membrane so you get depolarisation along the whole length of the membrane (slower than saltatory conduction though it is still quick)
