Chapter 13 The Nervous System

Question 1

What is homeostasis?

Answer 1

Maintaining a stable equilibrium of the conditions within the body, in spite of external changes

Question 2

Why is coordination needed?

Answer 2

Few body systems work in isolation
Each system is specialised for one function, so coordination ended for cell and system to operate efficiently.

Question 3

What is cell signalling?

Answer 3

Communication at a cellular level
Cells secreting chemicals such as hormones or neurotransmitters, to elicit a response of another cell

Question 4

What is the general pathway of a nervous system response?

Answer 4

Stimulus- Sensory neurone - Spinal Cord - Brain - Spinal Cord - Motor neurone - effector

Question 5

What are neurones?

Answer 5

Specialised nerve cells that transmit electrical impulses around the body

Question 6

What are the structural differences between a motor neurone and sensory neurone?

Answer 6

Motor neurones have the cell body at the start of the neurone, and they do not have a dendron, only an axon and dendrites
Sensory neurones have the cell body in the middle of the neurone, contain 1 axon and 1 dendron, and many dendrites/ axon terminals

Both usually have myelin sheath and nodes of ranvier

Question 7

What is the structure of a relay neurone?

Answer 7

A large cell body with many dendrons and dendrites, and 1 short axon and axon terminals

Question 8

What is the myelin sheath and what is its function?

Answer 8

Schwann cells wrapped along the axon, more than 20 times, producing thick layers of phospholipid bilayer
An insulating layer, meaning ion movement can only occur at the nodes of ranvier, increasing the speed of impulse transmission and reducing the energy demands as less sodium potassium pumps are needed

Question 9

What is the role of a sensory receptor? What is a transducer?

Answer 9

Acts as a transducer, converting a certain type of stimulus into electrical impulses,
Transducer- converts one form of energy into another

Question 10

What are the different types of receptors?

Answer 10

Mechano
Chemo
Thermo
Photo

Question 11

What is the structure of the Pacinian Corpuscle?

Answer 11

Neurone ending covered into layers of connective tissue, with viscous gel in between
Capsule on the outside
Blood capillary inside
Contain stretch mediated sodium channels

Question 12

How does the Pacinian Corpuscle transfer pressure to an electrical impulse?

Answer 12

1) Normally, the stretch-mediated channels are closed, they are too narrow to allow Na+ through at resting potential
2) When pressure applied, the shape of the corpuscle changes. The deformation allows the sodium channels to wides, allowing Na+ to diffuse in.
3) Na+ increases potential difference, causes depolarisation of the membrane. This creates a generator potential, which can be propagated into an action potential along the axon.

Question 13

How is the resting potential of a cell membrane established?

Answer 13

Sodium potassium pump- 3Na+ actively pumped out and 2K+ actively pumped in
Voltage gated Na+ channels closed, membrane less permeable to Na+ movement
Potassium channels shut, but leaky, so so K+ continues to diffuse out
So more positive outside than in, -70mV resting, electrochemical gradient

Question 14

What is an action potential? What is a nerve impulse?

Answer 14

Action potential- Change in potential difference across the neurone membrane of the axon when stimulated

Nerve Impulse- Action potential that starts at one end and is propagated along the axon to the other end

Question 15

What is the process of an action potential?

Answer 15

1) The stimulus (normally Na+) cause some voltage-gated Na+ channels to open, increasing the membranes permeability, and so Na+ diffuse making the membrane less negative
2) When the threshold of -55mV is reached, more Na+ channels open, causing a rapid increase in membrane potential, to +40mV as Na+ keeps diffusing in
3) At 40mV, the voltage gated Na+ channels shut, and K+ channels open. K+ rapidly diffuse out, causing the membrane potential to fall to below the resting
4) In this state of hyperpolarisation, the sodium potassium pump reestablishes the resting potential. The neurone enters its refractory period

Question 16

How is the action potential propagated?

Answer 16

Depolarisation of the axon membrane acts as the stimulus for voltage gated Na+ channels to open slightly further down the axon
This will cause depolarisation along the axon and thus propagate as an impulse

Na+ diffuse down the axon due to the electrochemical gradient, this is more specifically what acts as the stimulus

Question 17

How do refractory periods occur?

Answer 17

Hyperpolarisation occurs in the membrane before the one being stimulated
This means it cannot be excited as the Na+ channels will not open

Question 18

What are the benefits of neurones having refractory periods?

Answer 18

It prevents backward propagation- unidirectional
It enables discrete impulses so they do not overlap

Question 19

What is saltatory conduction?

Answer 19

Action potentials jump between the nodes of Ranvier
The myelin sheath is impermeable to ion movement

Question 20

What are the benefits to saltatory conduction?

Answer 20

There are longer localised circuits
This speeds up transmission as less Na+ channel need to open/ close
And less energy is needed by having fewer sodium potassium pumps
Prevents depolarisation across the whole length of the axon

Question 21

What factors affect the speed of impulse transmission?

Answer 21

Axon diameter- larger, faster, as less resistance to the flow of ions
Temperature- higher, faster diffusion of ions, until too high and sodium/potassium pumps denature
Presence of myelin sheath

Question 22

How does the size of stimulus affect action potentials?

Answer 22

All or nothing response - as long as the threshold value is exceeded, an action potential of the same size is triggered
However, larger stimuli increase the rate/frequency action potentials stimulated

Question 23

What are the components of a synapse? What is the gap size?

Answer 23

Synaptic cleft- gap between
Synaptic knob- swollen end of presynaptic
Neurotransmitter

20-30nm

Question 24

What are the two types of neurotransmitter, give an example?

Answer 24

Excitatory- results in depolarisation of the post synaptic neurone , e.g Acetylcholine
Inhibitory- results in hyper polarisation of the post synaptic neurone, effectively refractory period eg GABA, Gamma Amino Butyric Acid

Question 25

What are the processes of a cholinergic synapse?

Answer 25

1) Depolarisation of the presynaptic neurone causes voltage gated calcium channels to open
2) Ca2+ diffuse in and bind to motor proteins causing a conformational change. The motor proteins enable the movement of secretory vesicles containing neurotransmitters to move to the cell surface membrane
3) Exocytosis of the secretory vesicle. 2 Acetylcholine will bind to Ligand-gated Sodium channels in the post synaptic neurone, opening them . Na+ diffuse in and cause depolarisation of the membrane. The action potential is propagated in the post synaptic neurone
4) Acetylcholinesterase catalyses the hydrolysis of the neurotransmitter, and acetyl and choline diffuse across to the presynaptic neurone
5) ATP is used to catalyse the condensation of acetyl and choline into acetylcholine

Question 26

Why is is important that acetylcholine is removed from the channels and sent back?

Answer 26

Prevents constant impulses so another stimulus can be detected again
Recycling of acetylcholine reduces waste and need for the neurotransmitter

Question 27

What happens in the post-synaptic neurone with GABA?

Answer 27

GABA will bind to chloride channels in the post synaptic neurone
This will cause a conformational change, opening them, and allowing chloride ions to diffuse in
As negative, they will cause hyperpolarisation, and inhibit any action potential

Question 28

How does nicotine work?

Answer 28

There are many nicotinic Na+ voltage gated channels
Acetyl choline has a very similar shape to nicotine, so when nicotine binds to these structures, they mimic the role of the transmitter
The sodium channels remain open for longer, creating longer periods of excitation as they cause depolarisation of the post-synaptic neurone

Question 29

What is spatial summation?

Answer 29

Multiple pre-synaptic neurone must also have synapses with one post-synaptic neurone for an action potential to be triggered

Question 30

What is temporal summation?

Answer 30

There is one presynaptic neurone for every post synaptic neurone
There must be multiple action potentials in the presynaptic triggers in a given time to enable depolarisation of the post-synaptic neurone

Question 31

What is the difference between repolarisation and hyperpolarisaton?

Answer 31

Re- period from when K+ channels open to when the original resting potential reached
Hyperpolarisation- when the neurone enters the refractory period and falls below the membrane potential

Question 32

What are the important voltages for neurones and impulses?

Answer 32

Resting= -70mV
Initiation= -55mV
K+ open= +40mV
Hyperpolarisation= -80mv

Question 33

When is the sodium-potassium pump on in a neurone?

Answer 33

Always activate throughout the whole nervous impulse

Question 34

When explanations are needed for how the nerve impulses differ, what do you need to say?

Answer 34

Talk about how resting potential established, more less Na+/K+ pumps
Electrochemical gradient, how strong, how fast established
Channels less or more leaky, which are open

Then related to function

Question 35

What are the roles of synapses?

Answer 35

Enables neurones to communicate with each other, transmission of impulses between neurones
Convergence and Divergence- spatial summation
Promotes 1 way transmission of nerve impulses

Question 36

How is it possible for a constant stimulus to not be continually sending action potentials if still present?

Answer 36

After initial stimulus and action potential, Na+ voltage gated channels remain open
Resting potential not reestablished so cannot be depolarised again, in continuous refractory period