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 ions are moved in and out of the axon across the membrane
More positive ions are moved out of the axon than in
This 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. polarised membrane + + + +
axon membrane - - - -
inside - - - -
outside + + + +

14. THIS CHARGE CAN BE MEASURED

15. the resting potential

16. sodium-potassium pump

17. 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

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

19. action potential
Stimulation of the axon causes the membrane to become depolarised
This is caused by the ions moving in the opposite direction across the axon membrane
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

20. depolarised membrane + + + - + - - - stimulus + - - - + + + -
depolarisation + + + - + - - -
stimulus + - - - + + + -

21. When an action potential is generated at a specific part of the axon membrane
the areas on either side will have opposite charges as they remain polarised
This difference in charge sets up a local current between the area where there is an action potential and the resting area next to it.
The flow of current in a series of these localised currents results in an action potential moving along an axon

22. passage of an impulse along a neurone
local
electrical
current RP RP AP
passage of an impulse along a neurone

23. In order for further action potentials to pass along the axon the original part of the axon must recover its resting potential
A process called REPOLARISATION
The length of time it takes for the resting potential to be re-established is called the REFRACTORY PERIOD.

24. There are two parts to the refractory period:
The absolute refractory period is the interval during which a second action potential absolutely cannot be initiated, no matter how large a stimulus is applied
It is caused by the closing of carrier proteins which transport ions across the axon membrane
The relative refractory period is the interval immediately following during which initiation of a second action potential is inhibited but not impossible.

25. In this way the action potential can only travel in one direction down the neurone because the area behind the action potential is in a state of recovery.
The transfer of action potentials along the length of an axon represents an impulse
Diagram page 330

26. factors which influence the speed of transmission
1. Diameter of the axon
Thicker axons transmit impulses faster as they have a greater surface area over which exchange of ions can occur.
Giant axons found in earthworms and squid are associated with the need
for rapid escape responses.

27. 2. Myelination of the axon
and saltatory conduction
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

28. Action potentials can occur at these points
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

29. draw diagram page 331 Froggy

30. structure
and
function
of a
synapse

31. The axons of neurones end in swellings called synaptic bulbs.
the synapse
The axons of neurones end in swellings called synaptic bulbs.
Present in the bulbs are many mitochondria and synaptic vesicles which contain a chemical messenger (a neurotransmitter substance)

32. The gap between two neurones is called the synaptic cleft.
The membrane before the cleft is called the pre-synaptic membrane

33. On the other side of the synaptic cleft is the postsynaptic membrane.
This contains many ion channels and has a large number of protein molecules on its surface which act as receptor sites for the neurotransmitter

34. 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

35. 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

36. 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
postsynaptic
cell
ion channel

37. synapse at a neuromuscular junction

38. 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 fuse with vesicles containing neurotransmitter substance (acetylcholine) and cause them to move to the pre-synaptic membrane

39. Vesicles fuse with the pre-synaptic membrane releasing neurotransmitter into the synaptic cleft (exocytosis)
Neurotransmitter diffuses across the synaptic cleft
and binds to receptors on ion channels on the post-synaptic membrane causing them to open
Na+ ions diffuse into the post-synaptic cell

40. This causes the development of an excitatory post-synaptic potential
resulting in depolarisation and an action potential in the post-synaptic membrane
The neurotransmitter must be removed from the synaptic cleft to prevent continued stimulation of the post-synaptic membrane

41. This is carried out by the enzyme acetylcholinesterase
The products (choline & ethanoic acid) are reabsorbed by the pre-synaptic cell and recycled, using ATP to resynthesise the neurotransmitter

43. summation
Sufficient neurotransmitter must diffuse across the synaptic cleft in order for
an excitatory post-synaptic potential (EPSP) to be produced.
In some synapses this is achieved through summation.

44. There are 2 ways in which summation may occur
Temporal summation (time)
A high frequency of APs need to arrive at the pre-synaptic membrane, each resulting in the release of neurotransmitter.
The post-synaptic membrane depolarises only when sufficient neurotransmitter builds up

46. 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.
When sufficient is released the
post-synaptic membrane is depolarised

47. spatial
summation

48. froggy
page 347
Q 1,2,4

54. Na channel
K channel K+
Na+ + + + + - - - -
stimulus - - - - + + + +

55. depolarised membrane + + + - + - - - stimulus + - - - + + + -
depolarisation + + + - + - - -
stimulus + - - - + + + -