3.8 - The Nervous System

Question 1

What are the 3 functions of the nervous system?

Answer 1

• Detects changes or stimuli, inside the body and in the environment
• processes and stores information
• initiates responses

Question 2

What are the 3 features of the central nervous system?

Answer 2

• comprises of the brain and the spinal cord
• processes the information provided by a stimulus
  -surrounded by a tough protective membrane called meninges

Question 3

What are the 2 parts of the peripheral nervous system?

Answer 3

Somatic
Autonomic

Question 4

What is the somatic nervous system?

Answer 4

Pairs of nerves that originate in the brain or the spinal cord and their branches
These nerves contain the fibres of sensory neurones, which carry impulses from receptors to the CNS and motor neurones, which carry impulses away from the CNS to the effectors

Question 5

What is the autonomic nervous system?

Answer 5

Provides unconscious control of the functions of internal organs
eg - heartbeat and digestion

Question 6

What is a reflex action?

Answer 6

A rapid, automatic response resulting from the nervous impulses initiated by a stimulus. The decision making areas of the brain are not involved and the action is involuntary
It is protective in function

Question 7

What are the 7 stages in the reflex arc?

Answer 7

Stimulus
Receptor
Sensory neurone
Relay neurone in CNS
Motor neurone
Effector
Response

Question 8

What is a nerve net and the 2 cells which make it up?

Answer 8

SImplest type of nervous system
Diffuse network of cells that group into ganglia but do not form a brain
Ganglion cells provide connections in several directions
Sensory cells detect stimuli

Question 9

What are neurones and their 3 types?

Answer 9

Nerve cells or neurones are specialised cells adapted to rapidly carry nervous impulses from one part of the body to another
The three types are sensory, motor and relay

Question 10

What is a sensory neurone?

Answer 10

Carry impulses from the sensory receptors or organs into the CNS

Question 11

What are motor neurones?

Answer 11

Carry impulses from the CNS to the effector organs ie muscles or glands

Question 12

What are relay neurones?

Answer 12

Connector or association, receive impulses from sensory neurones or other relay neurones and transmit them to motor neurones or other relay neurones

Question 13

What is a cell body/centron?

Answer 13

Contains a nucleus and granular cytoplasm

Question 14

What is a cytoplasm?

Answer 14

Granular - contains many ribosomes

Question 15

What is a nucleus?

Answer 15

Holds DNA

Question 16

What are nissl granules?

Answer 16

Cytoplasmic granules comprising ribosomes grouped on RER

Question 17

What is a dendrite?

Answer 17

Thin fibre carrying impulses towards the cell body, a cell body may have several dendrites

Question 18

What is an axon?

Answer 18

Thin fibre carrying impulses away from the cell body. A cell body only has one axon.

Question 20

What are schwann cells?

Answer 20

Glial cells that surround and support nerve fibres. In vertebrates embryos, they wrap around developing axons many times and withdraw their cytoplasm, leaving a multliayered phospholipid myelin sheath

Question 21

What is a myelin sheath?

Answer 21

Electrical insulator, speeds up transmission of impulses

Question 22

What are nodes of Ranvier?

Answer 22

1um gaps in myelin sheath, where adjacent Schwann cells meet and where the axon membrane is exposed. They allow impulses to be transmitted rapidly

Question 23

What is a synaptic knob?

Answer 23

Swelling at the end of an axon, in which neurotransmitters are synthesised

Question 24

What is an axon ending/terminal?

Answer 24

Secretes neurotransmitters, which transmits impulse to adjacent neurone

Question 25

What is resting potential?

Answer 25

The potential difference across the membrane of a cell when no nervous system impulse is being conducted
A neurone is an excitable cell, which means it can change resting potential

Question 26

What is the potential difference across a cell and why, what does this result in?

Answer 26

• the membrane is more negative in the inside so -70mV
• as there is a potential difference across the cell membrane, it is described as polarised
• the negative resting potential results from the negative ions of large proteins of organic acids such as pyruvate and organic phosphates

Question 27

What is the concentration of sodium and potassium like across a cell?

Answer 27

The inside of a cell has both a higher concentration of potassium ions and a lower concentration of sodium ions than the outside, so the K+ ions tend to diffuse out and the Na+ ions tend to diffuse in

Question 28

What are sodium potassium exchange pumps?

Answer 28

• pump K+ back into the cell and Na+ ions back out
• they are trans-membrane proteins with ATPase activity that transports K+ and Na+ ions across the membrane against a concentration gradient by active transport
• they maintain an uneven distribution of ions across the membrane

Question 29

Why is the inside of a membrane negative?

Answer 29

The na+ ions are pumped out faster than the K+ ions are pumped in so overall result is a more negative inside

Question 30

What are the 4 stages of action potential?

Answer 30

1- the energy of the stimulus causes some of the voltage gated sodium channels in the axon to open. The sudden increase in the permabiliity of the membrane to Na+ ions allows them to rapidly diffuse into the axon, down their concentration gradient. Therefore the negative charge inside the gradient becomes positive at +40mV. This is the action potential, and here the cell is depolarised. Sodium channels begin to close to prevent further influx

2 - the potassium channels open and k+ ions diffuse out down their concentration gradient. The cell becomes less positive inside as more diffuses out and the membrane is repolarised

3 - More k+ ions diffuse out than Na+ ions diffuse in, so the potential difference across the membrane becomes even more negative than the resting potential. This makes the membrane hyperpolarised

4 - the sodium potassium pumps pump K+ ions back in and Na+ ions back out, restoring the ion balance of the resting potential

Question 31

How does the action potential travel along an axon?

Answer 31

1) At the sight of an action potential, Na+ ions diffuse into the axon and the K+ ions diffuse out
2) The resulting reversal of potential sets up local currents, as Na+ ions diffuse along the axon because of their locally high concentration, they depolarise the adjacent section of the membrane
3) This opens more voltage gated sodium channels in those regions and more sodium floods in, depolarising the axon at this point
4) Sodium ions then diffuse further down the axon and in this way, a self perpetuating wave of depolarisation spreads along the axon

Question 32

What is this ‘absolute refractory period’ and what does it ensure?

Answer 32

At the site of action potential, the sodium channels are inactivated and cannot open again until the resting potential has been re-established, so a new action potential cannot be established there.
It ensures that the action potential is not propagated back in the direction from which it came, and the nervous impulse travels in one direction only

Question 33

What is the all or nothing law?

Answer 33

• if the intensity of a stimulus is below a certain threshold value, no action potential is established
• an increase in the intensity of the stimulus does not give greater action potential, instead the frequency of the action potentials increase
• this prevents overloading the nervous system with processing and impulses

Question 34

What are the 3 factors affecting the speed of a nervous impulse?

Answer 34

• temperature
• the diameter of an axon
• myelination

Question 35

How does temperature affect the speed of a nervous impulse?

Answer 35

Ions move faster at higher temperatures as they have more kinetic energy

Question 36

How does the diameter of the axon affect the speed of a nervous impulse?

Answer 36

The greater the diameter of the axon, the greater its volume in relation to its area of the membrane. More sodium ions can flow through the axon, so impulses travel faster

Question 37

How does myelination affect the speed of a nervous impulse?

Answer 37

Speeds up the transmission by insulating the axon. Sodium ions flow through the axon, but a myelinated nerve fibre only depolarises where resistance is low (ie the nodes of ranvier). The voltage gated ion channels only occur at the nodes of ranvier, so these are where sodium ions enter.
The consequence is that the action potential appears to jump from one node to node along the axon (process called saltatory conduction)

Question 38

What is a synapse and what are the 2 different types?

Answer 38

Neurones are separated by synapses, which send the nervous impulse between neurones in one direction only
Electrical and chemical

Question 39

What is an electrical synapse?

Answer 39

3nm across, small enough that an electrical impulse is transmitted directly from one neurone to the next

Question 40

What is a chemical synapse?

Answer 40

20nm gap, too big for the nervous impulse to jump. Most juctions between neurones are chemical synapses. Branches of axons lie close to the dendrites of other neurones but do not touch, the impulse is transmitted by an neurotransmitter

Question 41

What is a neurotransmitter?

Answer 41

A chemical that diffuses across the synaptic cleft, from the pre-synaptic membrane of one neurone to the post synaptic membrane of an adjacent neurone, where anew impulse is initiated

Question 42

What are the 4 stages of synaptic transmission?

Answer 42

• the arrival of the impulse at the synaptic end bulb alters its membrane permeability, opening voltage-dependant calcium channels, so calcium ions diffuse into the end bulb, down their concentration gradient
• the influx of calcium ions causes the synaptic vessels to move towards and fuse with the pre-synaptic membrane. this releases the neurotransmitter acetylcholine, by exocytosis, into the synaptic cleft
• the neurotransmitter diffuses across the synaptic cleft and bounds to a receptor, an intrisic protein spanning the post synaptic mebrane. The protein sub-units have 2 receptor sites and 2 acetylcholine molecules show co-operative binding when they attach
• when acetylcholine molecules bind with both of these sites, the receptor protein changes shape, opening a channel and sodium ion diffuse in, down their concentration gradient. The post synaptic neurone is consequently depolarised and if the membrane is depolarised enough, the threshold potential is reached and an action potential is initiated

Question 43

What would happen if acetylcholine is left in the synaptic cleft and what are the 3 ways it can be prevented?

Answer 43

If acetylcholine is left in the synaptic cleft, it would constantly initiate new impulses in the post synaptic membrane and impulses would not be distinct.
- direct uptake of acetylcholine into the presynaptic neurone
- active transport of calcium ions so no more exocytosis can occur
- hydrolysis of acetylcholine, which is destroyed by the enzyme acetylcholinesterase in the synaptic cleft

Question 44

Why do neurones only transmit nuerones in one direction?

Answer 44

• hyperpolarisation happens behind an action potential and so depolarisation could not happen at that point
• synaptic vesicles may only occur at the end bulb of the pre-synaptic neurone
• neurotransmitter receptors onl occur on the post-synaptic membrane

Question 45

What are the properties of the synapse?

Answer 45

-transmit information between neurones
- pass impulses in one direction, generating precision in the nervous system
- act as junctions
- protect the response system from overstimulation, because the impulse is always the same size no matter the size of the stimulus
- filter out low level stimuli (through threshold value)

Question 46

What is temporal summation?

Answer 46

Depolarisation builds up over time to reach the threshold at which an action potential is initiated (2 waves then peak)

Question 47

What is spatial summation?

Answer 47

Several presynaptic neurones synapse with the same post synaptic neurone and all contribute to the growing depolarisation, which generates an action potential when it is large enough (1 wave then peak)

Question 48

What are the 4 most common neurotransmitters?

Answer 48

• acetylcholine
• GABA
• Monoamines eg dopamine, serotonin and noradrenalin
• neuropeptides eg endorphins

Question 49

What is a sedatitive?

Answer 49

Eg alcohol inhibits the nervous system, creating fewer action potentials in post synaptic neurones

Question 50

What are stimulants?

Answer 50

also called agonists eg amphetamines stimulate the nervous system by allowing more action potentials in post synaptic neurones.