Nervous coordination

Question 1

What is a neurone?

Answer 1

A single nerve cell

Question 2

What is a nerve?

Answer 2

A bundle of axons surrounded by connective tissues which binds them together

Question 3

What is the structure of a neruone?

Answer 3

• cell body
• dendrons - extensions of the cell body which subdivide into smaller branches called dendrites, which carry nerve impulses towards the cell body
• axon - a single long fibre that carries nerve impulses away from the cell body
• schwann cells - surround the axon to build up a layer of myelin sheath which provides electrical insulation
• nodes of Ranvier - gaps between adjacent schwann cells where there is no myelin sheath

Question 4

What are the 3 types of neurone and what are their functions?

Answer 4

• sensory - transmit nerve impulses from a receptor to an intermediate neurone in the CNS
• intermediate - transmit nerve impulses from a sensory neurone to a motor neurone across the CNS
• motor - transmit impulses from an intermediate neurone to an effector

Question 5

What is the structure of a sensory neurone?

Answer 5

one dendron that carries an impulse towards the cell body and one axon that carries it away. Synaptic knobs connect to other neurones

Question 6

What is the structure of an intermediate neurone?

Answer 6

• unmyelinated
• numerous short processes

Question 7

What is the structure of a motor neurone?

Answer 7

A long axon and many short dendrites

Question 8

What do cell bodys contain and why?

Answer 8

A nucleus and a large amount of RER used in the production of proteins and neurotransmitters

Question 9

What is a nerve impulse?

Answer 9

A wave of electrical activity that travels along the axon membrane. It is a temporary reversal of the electrical potential differences across the axon membrane

Question 10

What is resting potential and how is it established?

Answer 10

The inside of the axon is negatively charged relative to the outside - the axon is described as polarised

• 3 sodium ions are actively transported out of the axon by sodium-potassium pumps
• 2 potassium ions are actively transported into the axon
• the outward movement of sodium ions is greater than the inward movement of potassium ions which creates an electrochemical gradient (around -70mV)

Question 11

How is movement across the membrane controlled?

Answer 11

• phospholipid bilayer prevents sodium and potassium ions from diffusing across it
• sodium and potassium channel proteins
• sodium-potassium pumps

Question 12

What is an action potential and how is it caused?

Answer 12

When a stimulus is detected by a receptor, it causes a temporary reversal of charges in the axon, causing it to become depolarised

• the energy of the stimulus causes sodium voltage-gated channels in the axon membrane to open, and sodium ions diffuse into the axon through channels along their electrochemical gradient
• as more sodium ions diffuse into the axon, more sodium channels open causing a greater influx of sodium ions by diffusion
• this causes a reversal in charge because the inside of the axon becomes more positive relative to the outside (around +40mV)

Question 13

How do axons become repolarised?

Answer 13

• once an action potential of around +40mV has been established, the voltage gates on sodium ion channels close preventing further sodium ions from entering
• voltage gate potassium ion channels begin to open, so potassium ions diffuse out of the axon membrane, causing it to begin to repolarise as the charge becomes more positive outside the axon relative to the inside

Question 14

What is hyperpolarisation?

Answer 14

• potassium ion channels are slow to close and this causes potassium ions to diffuse out of the axon, making the charge inside the axon more negative
• this causes a temporary overshoot of the electrical gradient
• K+ channels then close and the sodium potassium pump will increase Na+ entering into the axon, meaning the charge inside becomes less negative, and resting potential is restored

Question 15

By what process do sodium ions move in during an action potential?

Answer 15

Diffusion

Question 16

By what process do sodium ions move during resting potential?

Answer 16

active transport

Question 17

How does action potential change as it moves along an axon?

Answer 17

Stays the same

Question 18

What is the passage of action potential along an unmyelinated axon?

Answer 18

• at resting potential, the axon membrane is polarised because the outside of the membrane is more positively charged than the inside of the membrane
• A stimulus causes a sudden influx of sodium ions which causes the charges to be reversed - membrane becomes depolarised
• The localised electrical currents established by the influx of sodium ions cause the opening of voltage-gated sodium ions further along the axon causing a wave of depolarisation
• behind this wave of depolarisation, sodium ion channels close and potassium ion channels open. The outward movement of potassium ions causes this section of membrane to become repolarized
• repolarisation allows sodium ions to be actively transported out, as so resting potential is restored

Question 19

What is the passage of action potential along a myelinated axon?

Answer 19

process known as saltatory conduction

• action potentials can only occur at the nodes of ranvier, so electrical impulses will skip along the axon without having to depolarise the entire length of the axon, making this much faster conduction

Question 20

Which factors affect the speed at which an action potential travels?

Answer 20

• myelin sheath - provide electrical insulation and lead to saltatory conduction, which speeds up conductance
• axon diameter - greater axon diameter = faster conductance - less leakage of ions from a large axon - making it more difficult to maintain membrane potentials in smaller axons
• temperature - higher temperature = faster rate of diffusion of ions so faster nerve impulses. Active transport also requires ATP which requires enzymes, faster production of ATP at higher temperatures

Question 21

What are the absolute and relative refractory periods?

Answer 21

absolute = the interval during which a second action potential absolutely cannot be initiated, no matter how large the stimulus

relative = the interval immediately following during which initiation of a second action potential is inhibited but not impossible

Question 22

Why is there a refractory period?

Answer 22

Inward movement of sodium ions is prevented because voltage-gated sodium ion channels are closed, meaning it is impossible for a further action potential to be generated

Question 23

What is the purpose of the refractory period?

Answer 23

• ensures that action potentials occur in one direction only - they cannot move backwards as the membrane is in the refractory period
• produces discrete impulses - ensures that action potentials are separated from each other due to refractory period
• limits the number of action potentials - limits the number of action potentials that can pass along an axon at a given time

Question 24

Why do neurones contain a large amount of RER?

Answer 24

used to produce proteins and neurotransmitters

Question 25

What is the all-or-nothing principal?

Answer 25

as long as the stimulus allows enough sodium ions to move in to reach threshold, action potential is always the same stronger stimulus = more frequent action potentials

Question 26

What are the stages of synaptic transmission?

Answer 26

- action potential arrives at the end of the presynaptic neurone and the membrane of the presynaptic membrane depolarizes - calcium ion channels open and calcium ions enter into the synaptic knob - calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release acetlycholine (a neurotransmitter) - acetylcholine diffuses across the synaptic cleft and attaches to receptors on the postsynaptic membrane - sodium ion channels open and sodium ions enter the postsynaptic membrane leading to depolarisation - an action potential is produced in the postsynaptic neurone if threshold is reached

Question 27

Why is synaptic transmission unidirectional?

Answer 27

there are only vesicles in the presynaptic membrane and only receptors on the postsynaptic membrane

Question 28

What is temporal summation?

Answer 28

accumulation of neurotransmitters due to several action potentials occuring over a short time period, leading to neurone reaching threshold

Question 29

What is spatial summation?

Answer 29

accumulation of neurotransmitters due to several neurones producing an action potentials over a short time period, releasing more neurotransmitters and leading to neurone reaching threshold