1. nervous system

2. The nervous system allows the body to respond to changes in the internal and external environment
Receptors detect changes/stimuli which are rapidly transmitted along neurones to effectors that bring about a corrective response

3. STIMULI RECEPTORS PNS CNS EFFECTOR RESPONSE changes in the environment
sensory cells
PNS
peripheral nervous system
Cranial and spinal nerves (sensory + motor neurones)
CNS
central nervous system
brain + spinal cord processes info
EFFECTOR
muscle or gland
RESPONSE
action

4. neurones Type of neurone Stimulated by Transmits impulses to Sensory
Adapted to transmit information throughout the nervous system.
3 types:
Type of neurone
Stimulated by
Transmits impulses to
Sensory
Stimulus
Association
Motor
Effector

5. association neurone motor sensory neurone neurone effector receptor
CNS
association
neurone
motor
neurone
sensory
neurone
effector
receptor
response
stimulus

6. structure of a myelinated motor neurone
Large cell body: containing a nucleus, nucleolus, mitochondria and ribosomes
Dendrons: long thin strands of cytoplasm that carry impulses towards the cell body and connect to many neurones in the CNS
Axon: a long extension that carries impulses away from the cell body and terminates in motor end plates that connect to muscles or glands

7. Schwann cells: wrap around the axon of myelinated neurones forming the fatty myelin sheath that insulates the impulse.
Nodes of Ranvier: small gaps between adjacent Schwann cells that aid transmission of impulses
Schwann cell
axon

10. the generation
and transmission
of nerve impulses

11. RP is approximately -70mV
resting potential
When a nerve cell is at rest positive ions are moved in and out of the axon across the membrane by facilitated diffusion using gated channel proteins and by active transport, using carrier proteins.
More positive ions are moved out of the axon than in which causes the inside of the axon to become negative in relation to the outside; the membrane is polarised
The charge across the membrane, the (potential difference) is called the
RESTING POTENTIAL
RP is approximately -70mV

12. Once the RP is achieved the gated channel proteins that allow facilitated diffusion to occur close, making the membrane impermeable to ions.
This prevents ions moving back down their concentration gradient.

14. polarised membrane + + + +
axon membrane - - - -
inside - - - -
outside + + + +

17. THIS CHARGE CAN BE MEASURED

18. the resting potential

19. sodium-potassium pump

20. all-or-nothing law
A stimulus must reach a specific level in order for an impulse to be generated
A more intense impulse will not produce a bigger impulse; all impulses are the same size
Strong stimuli produce a greater frequency of action potentials (impulses)
NB: below threshold, no AP
above threshold AP all the same size

21. Below threshold intensity: no action potentials
Action potentials generated
Increasing intensity of stimulus
Threshold intensity
Successive stimuli

22. action potential
Stimulation of the axon causes the membrane to become depolarised
This causes some of the gated channel proteins to open and the membrane becomes permeable to ions.
The positive ions diffuse down their concentration gradient through channel proteins.
The pd across the membrane changes, causing more gated channel proteins to open in a positive feedback mechanism.

23. This reverses the potential difference across the membrane, to +40mV
Making the inside positive in relation to the outside
This change in charge evokes (starts) an ACTION POTENTIAL

24. depolarised membrane + + + - + - - - stimulus + - - - + + + -
depolarisation + + + - + - - -
stimulus + - - - + + + -

25. When the AP reaches 40mV the recovery phase starts.
The positive ions are moved by facilitated diffusion through channel proteins and by active transport by carrier proteins to restore the RP.
When the membrane is REPOLARISEDthe membrane becomes impermeable to ions again.
The length of time it takes for the resting potential to be re-established is called the REFRACTORY PERIOD.
This is necessary in order for further action potentials to pass along the axon.

26. The refractory period:
Ensures the APs are propagated in one direction only, because the area behind the action potential is in a state of recovery.. Without it APs could be generated in both directions along the axon.
Limits the number of APs that can be fired, ensuring each remains separate from subsequent Aps.

27. DIAGRAM & POINTS PAGE 53/54

29. propagation of a nerve impulse
An action potential is generated at a specific part of the axon membrane and moves rapidly as a wave of depolarisation along the neurone.
The area behind the depolarised part of the axon (AP) quickly repolarises.
The membrane adjacent to the depolarised section will have opposite charges as they are polarised.

30. This results in the creation of local currents between the area where there is an action potential and the resting area next to it.
Positive ions from the depolarised section pass along the inside of the membrane towards the polarised zone immediately in front of it.
On the outside of the membrane positive ions move from the polarised zone to the depolarised zone.
The flow of current in a series of these localised currents creates a wave of depolarisation that moves rapidly along the neurone.
Similar circuits enable the RP to be restored directly behind the AP

31. + + + + - + + + + - - - - - - - - + + - - - - - - - - + + + + - + + +
Area behind the AP
undergoes repolarisation
during the refractory period
so is unable to be depolarised,
preventing the impulse moving backwards
IMPULSE RP AP RP + + + + - + + + + - - - - - - - - +
Stimulus
generated
this end + - - - - - - - - + + + + - + + + +
local
electrical circuit

32. factors which influence the speed of transmission
1. Diameter of the axon
Thicker axons transmit impulses faster as there is proportionally less leakage of ions. (SA:VOL)
Giant axons found in earthworms and squid are associated with the need
for rapid escape responses.

33. 2. Myelination of the axon
and saltatory conduction
The myelin sheath acts as an insulator in myelinated neurones.
So areas of the axon that are myelinated cannot be polarised or depolarised.
This is because myelin is a fatty substance that does not allow movement of ions across it.
Myelin is absent at the nodes of Ranvier where the Schwann cells meet, allowing depolarisation of the axon membrane, and AP are generated.

34. Local circuits form between the nodes only, allowing sections of the neurone to be by-passed.
APs ‘jump’ from one node to the next, speeding up transmission by about 100 times.
This is called SALTATORY CONDUCTION
and is found only in the myelinated axons of vertebrates
Saltatory conduction also saves energy as less is required for the active transport of ions across the axon membrane

35. There are relatively few ion channels under the myelin sheath, being concentrated at the nodes of Ranvier.
This allows myelinated neurones to have relatively diameters.
3. Temperature also affects speed of the nerve impulse, as it affects the rate of diffusion of ions involved.

36. impulse ALWAYS moves from cell body away from the axon
NB:
impulse ALWAYS moves from cell body away from the axon
draw diagram page 55 CCEA Text

37. structure
and
function
of a
synapse

38. the synapse
Synapses are junctions between the axon of one neurone and the dendrite of the next.
They also occur between neurones and muscle and are called neuromuscular junctions.
The gap between 2 neurones is called the synaptic cleft.
Chemicals called neurotransmitters diffuse across this very small gap.
The neurone that releases the neurotransmitter is called the pre-synaptic neurone, whilst the neurone that it diffuses to is called the post-synaptic neurone.

39. PRE & POST SYNAPTIC NEURONES.
ADAPTATIONS OF THE
PRE & POST SYNAPTIC NEURONES.
The end of the pre-synaptic neurone is thickened to form a synaptic bulb.
Pre-synaptic bulbs contain many mitochondria (to release energy for neurotransmitter production) and synaptic vesicles, which store the neurotransmitter

40. The postsynaptic membrane contains receptors that are complementary to the specific neurotransmitter used in a particular synapse.
These are linked to many ion channels will cause depolarisation of the post-synaptic neurone, allowing the impulse to continue to travel along the next neurone.

41. structure of the synapse
Synaptic cleft: gap between the pre and post synaptic membranes
Presynaptic membrane: neurone membrane before the synapse
Postsynaptic membrane: neurone membrane after the synapse
Neuromuscular junction: synapse between a motor neurone and a muscle
Neurotransmitter: chemical that carries the impulse across the synaptic cleft, found in synaptic vesicles

42. structure of the synapse
You will be expected to identify and label the following structures from LEM and TEM photographs and diagrams
Synapse: gap between 2 neurones
Synaptic bulb: swelling at the end of an axon
Synaptic vesicles: vesicles containing neurotransmitter found in the synaptic bulb

43. complementary receptor & ion channel postsynaptic cell
axon
myelin sheath
action potential
end of axon
mitochondrion
synaptic vesicle
containing
neurotransmitter
substance
synaptic bulb
Draw diagram from page 332 Froggy
synaptic cleft
presynaptic membrane
dendrite
protein receptor
postsynaptic
membrane
complementary receptor
& ion channel
postsynaptic
cell

44. synapse at a neuromuscular junction

45. transmission of an impulse across a synapse
An impulse arrives at the synaptic bulb
Depolarisation of the membrane causes (voltage activated) calcium channels to open
Ca2+ ions enter the synaptic bulb by diffusion
The Ca2+ ions cause the vesicles containing neurotransmitter substance (usually acetylcholine) to move to the pre-synaptic membrane

46. Vesicles fuse with the pre-synaptic membrane releasing acetylcholine into the synaptic cleft (exocytosis)
Acetylcholine diffuses across the synaptic cleft
and binds to receptors on the post-synaptic membrane
This causes the opening of ion channels and
Na+ ions diffuse into the post-synaptic neurone.

47. The membrane gradually depolarises and an excitatory post-synaptic potential (EPSP) is generated.
If sufficient depolarisation occurs (depending on the number of neurotransmitter molecules filling the receptors) The EPSP will reach the threshold intensity required to produce an AP in the post-synaptic membrane

48. The acetylcholine must be removed from the post-synaptic membrane, to prevent it continuously generating a new AP in the post-synaptic neurone.
The enzyme acetylcholinesterase, which is attached to the post-synaptic membrane, breaks down acetylcholine into choline and ethanoic acid (acetyl) which are released into the synaptic cleft.
They diffuse across the cleft and are reabsorbed by the pre-synaptic bulb, where they are resynthesised into acetylcholine and stored in the synaptic vesicles to be used again. The process requires ATP.

50. function of the synapse
1. Unidirectionality
As the neurotransmitter is only made in the pre-synaptic neurone and the neurotransmitter receptors are present only on the post-synaptic neurone membrane, impulses can only cross from the pre-synaptic neurone to the post-synaptic neurone.

51. 2. Prevent overstimulation of effectors
Too many impulses over a short period will exhaust the supply of neurotransmitter more quickly than it can be resynthesised and the synapse becomes fatigued.
3. Integration
A number of pre-synaptic neurones may synapse with a post-synaptic neurone, providing flexibility. Without synapses the impulse will follow the same route, producing automatic, never changing responses.

52. summation
This process is important in providing integration.
Sufficient neurotransmitter must diffuse across the synaptic cleft in order for an excitatory post-synaptic potential (EPSP) to be produced.
An infrequent AP reaching the synapse may not be sufficient to generate an AP in the post-synaptic neurone.

53. The threshold level for the neurotransmitter can be reached in one of two ways:
1. Spatial summation
2 or more pre-synaptic neurones synapse with a single post-synaptic neurone.
Each releases small quantities of neurotransmitter into the synaptic cleft, which together reaches the threshold level to produce an AP in the
post-synaptic neurone

54. 2. Temporal summation (time)
A single pre-synaptic neurone fires a series of APs each releasing neurotransmitter over a short period of time.
Although any one AP is not sufficient to result in an AP in the post-synaptic neurone, together there is sufficient neurotransmitter to reach the threshold potential, resulting in an EPSP.

56. spatial
summation
diagram p58
CCEA text

58. types of synapse Excitatory
Neurotransmitter released causes an EPSP and AP in post-synaptic neurone.
Inhibitory
Make it difficult for synaptic transmission to occur e.g. by causing negative ions to move into the post-synaptic membrane. The inside becomes more negative than normal RP, hyperpolarisation, so that it is more difficult to reach the threshold level to develop an EPSP.
Their function is to reduce background stimuli that could interfere with brain functioning or may prevent some reflex actions.

59. types of neurotransmitter
Each synaptic bulb produces only one type of neurotransmitter.
Acetylcholine
Main neurotransmitter in CNS of vertebrates.
Noradrenaline
Used in involuntary nervous control, e.g. regulation of gut movement.

60. Drugs
Many drugs stimulate the development of APs as they have a similar shape to the normal neurotransmitters, whilst others may cause the production/release of more neurotransmitter at the synapse.
Some restrict the number of action potentials produced, by preventing the release of neurotransmitter or blocking receptor sites on the post-synaptic membrane.
Nicotine
Curare
Opoids
See p59 CCEA text

61. froggy
page 347
Q 1,2,4

63. Cross section through an axon Schwann cell nucleus Layers of membrane
containing fatty myelin
mitochondrion
axon

64. Cross section
through a synapse

65. Cross section through a synapse
Label:
Pre-synaptic neurone
Synaptic cleft
Post-synaptic neurone
Vesicles containing neurotransmitter
Where EPSP generated
Ca2+ ions diffusing across the membrane
Location of receptors

67. Neuromuscular junction
Cross section
through a
Neuromuscular junction