Nervous coordination

Question 1

What is the nervous system?

Answer 1

It uses nerve cells to pass electrical impulses along their length.
They stimulate target cells by secreting neurotransmitters directly on to them.
This results in rapid communication between specific parts of an organism.
Responses are short-lived and restricted to a localised region.

Question 2

What is the hormonal system?

Answer 2

It produces chemicals that are transported in the blood plasma to their target cells.
The target cells have specific receptors on their cell-surface membranes and the change in concentration of hormones stimulates them.
This results in slower, less specific communication.
Responses are long lasting and widespread.

Question 3

What are neurones?

Answer 3

Specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another.
A motor neurone is made of a cell body, dendrons, an axon, Schwann cells, a myelin sheath and nodes of Ranvier.

Question 4

What is a cell body?

Answer 4

This contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum.
The RER is associated with the production of proteins and neurotransmitters.

Question 5

What are dendrons?

Answer 5

Extensions of the cell body which subdivide into smaller branched fibres, dendrites.
The dendrites carry nerve impulse towards the cell body.

Question 6

What is an axon and Schwann cells?

Answer 6

An axon is a single long fibre that carries nerve impulses away from the cell body.
The Schwann cells surround the axon, protecting it and providing electrical insulation.
They also carry out phagocytosis and involved in nerve regeneration.
They wrap themselves around the axon so that layers of membrane build up around it.

Question 7

What is a myelin sheath?

Answer 7

It forms a covering to the axon and is made up of the membranes of Schwann cells.
These membranes are rich in the lipid myelin.
Neurones with a myelin sheath are myelinated neurones.

Question 8

What are nodes of Ranvier?

Answer 8

Constrictions between adjacent Schwann cells where there is no myelin sheath.
The constrictions are 2-3 micro metres long, and occur ever 1-3mm in humans.

Question 9

What are sensory neurones?

Answer 9

They transmit nerve impulses from a receptor to an intermediate or motor neurone.
They have one dendron that is often very long.
It carries the impulse towards the cell body and one axon that carries it away from the cell body.

Question 10

What are motor neurones?

Answer 10

They transmit nerve impulses from an intermediate or relay neurone to an effector.
They have a long axon and many short dendrites.

Question 11

What are relay neurones?

Answer 11

Or intermediate neurones.
They transmit impulses between neurones.
They have numerous short processes.

Question 12

What are nerve impulses?

Answer 12

It is a self-propogating wave of electrical activity that travels along the axon membrane.
It is a temporary reversal of the electrical potential difference across the axon membrane.
This reversal is between resting potential and action potential.

Question 13

What are the characteristics of the hormonal system?

Answer 13

Transmission by blood stream, and relatively slow.
Hormones travel to all parts of the body, but only target cells respond.
Response is widespread, slow and long-lasting.
Effects may be permanent and irreversible.

Question 14

What are the characteristics of the nervous system?

Answer 14

Transmission is by neurones and very rapid.
Nerve impulses travel to specific parts of the body.
Response is localised, rapid and short-lived.
Effect is usually temporary and reversible.

Question 15

How is the movement of ions across the axon membrane controlled?

Answer 15

The phospholipid bilayer prevents Na and K ions diffusing across it.
Channel proteins in the bilayer have ion channels which pass through them. Gates open and close so that these ions can move through them by facilitated diffusion at times.
Some channels remain open all the time though.
Some carrier proteins actively transport K ions in and Na ions out - the sodium-potassium pump.

Question 16

What is resting potential?

Answer 16

This control of ions mean the inside of the axon is negatively charged relative to the outside.
Resting potential ranges from 50 to 90 millivolts, but is 65mV in humans.
The axon is polarised.

Question 17

How is the potential difference of the axon established?

Answer 17

Sodium ions are actively transported out by the sodium-potassium pumps.
Potassium ions are actively transported in by the pumps.
3 sodium ions move in for every 2 potassium ions out.
So there’s more sodium ions in the tissue fluid surrounding the axon that in the cytoplasm, and more potassium in the cytoplasm that the tissue fluid, creating an electrochemical gradient.
The sodium ions diffuse back in naturally while potassium diffused back out.
Most of the channels for potassium are open, but sodium are closed.

Question 18

What is action potential?

Answer 18

When a stimulus is detected by a receptor the energy temporarily reverses the charges.
If the stimulus is great enough, the -65mV charge inside the membrane becomes +40mV.
The axon membrane is depolarised.

Question 19

Why does depolarisation occur?

Answer 19

The channels in the axon membrane change shape, and open or close depending on the voltage - voltage-gated channels.

Question 20

What is the first part of depolarisation - sodium ions?

Answer 20

The energy of the stimulus causes some sodium voltage-gated channels to open so sodium diffuses into the axon. They are positively charged so trigger a reversal in the potential difference across the membrane.
As sodium ions diffuse in, more sodium channels open, causing even more sodium ions to enter by diffusion.
Once the action potential is +40mV, the sodium ion channels close, and the potassium ions channels open.

Question 21

What is the first part of depolarisation - potassium ions and repolarisation?

Answer 21

The electrical gradient that was preventing further outward movement of potassium ions is reversed, causing more potassium ion channels to open, and more K ions diffuse out, causing repolarisation.
This causes the axon to temporarily be more negative than usual (hyperpolarisation).
The gates on the potassium channels close and the sodium-potassium pump begins again.
The resting potential of -65mV is re-established.

Question 22

What is hyperpolarisation?

Answer 22

After repolarisation, the membrane resets. The ion channels close, and the sodium potassium ion pump restores membrane potential by removing sodium ions, and for a few milliseconds, the membrane can’t conduct an impulse.
The potassium ion channels are slow to close so there is a slight overshoot in potential, and for a few milliseconds, membrane potential drops below -65mV.

Question 23

Why is action potential and resting potential misleading?

Answer 23

Movement of ions in action potential is by diffusion and is a passive process.
Resting potential is maintained by active transport, an active process.

Question 24

What is basic passage of an action potential?

Answer 24

As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region.
So the action potential is a travelling wave of depolarisation.
Meanwhile, the previous region of the membrane returns to resting potential, it undergoes repolarisation.

Question 25

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

Answer 25

The axon membrane is polarised - the outside is positive compared to the inside.
A stimulus causes an influx of sodium ions and reverses the charge - depolarised.
The localised electrical currents by sodium ion influx causes the opening of sodium ion channels further along the axon. Behind the new region, the sodium channels close and potassium channels open. Potassium ions leave the axon.
The axon potential continues along the axon. The outward movement of potassium has continued so that the axon membrane behind it has been repolarised.
This allows sodium ions to be actively transported out, returning the axon to resting potential.

Question 26

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

Answer 26

The myelin acts as an electrical insulator, preventing action potentials from forming.
Action potentials can occur at the nodes of Ranvier, the intervals at 1-3mm.
The action potentials jump from node to node by saltatory conduction, instead of all the way along the axon.
So the action potential passes faster than along an unmyelinated neurone.

Question 27

How does the myelin sheath affect the speed at which the action potential travels?

Answer 27

The myelin sheath prevents an action potential forming in the part of the axon covered in myelin.
So it jumps from one node of Ranvier to the next - saltatory conduction.
This increases the speed of conductance from 30ms^-1 in unmyelinated to 90ms^-1.

Question 28

How does the diameter of the axon affect the speed at which the action potential travels?

Answer 28

The greater the diameter of an axon, the faster the speed of conductance.
This is due to less leakage of ions from a large axon, as leakage makes membrane potentials harder to maintain.

Question 29

How does temperature affect the speed at which the action potential travels?

Answer 29

The higher the temperature the faster the nerve impulse due to faster rate of diffusion of ions.
The energy for active transport comes from respiration, controlled by enzymes, which function more rapidly at higher temperatures.
At a certain temperature, the enzymes and proteins are denatured and impulses fail to be conducted at all.
Temperature also affects the speed and strength of muscle contractions.

Question 30

What is the all or nothing principle?

Answer 30

A certain level of stimulus, the threshold value, triggers an action potential.
Below this, there is no action potential and so no impulse is generated.
Any stimulus above the value will generate an action potential and so nerve impulse, but will not increase the size of action potential.

Question 31

How can an organism perceive the size of a stimulus?

Answer 31

By the number of impulses passing in a given time. The larger the stimulus the more impulses that are generated in a given time.
By having different neurones threshold values. The brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size.

Question 32

What is the refractory period?

Answer 32

Once an action has been created in a region of an axon, there is a period afterwards where inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed.
It is impossible for a further action potential to be generated.

Question 33

What are the purposes of the refractory period?

Answer 33

It ensures action potentials are propagated in one direction only, from active to resting region. This is because action potentials cannot be propagated in a refractory region, so only go forwards.
It produces discrete impulses. The refractory period means a new action potential cannot be formed immediately behind the first one, so is separated.
It limits the number of action potentials that can pass along an axon in a given time, because they are separated. This limits the strength of stimulus that can be detected.

Question 34

What is a synapse?

Answer 34

The point where one neurone communicates with another or an effector.
They transmit information, but not impulses, from neurone to neurone by neurotransmitters.

Question 35

What is the structure of a synapse?

Answer 35

Neurones are separated by the synaptic cleft, 20-30nm wide.
The presynaptic neurone releases the neurotransmitter.
The axon of this neurone ends in a swollen portion, the synaptic knob.
This has many mitochondria and endoplasmic reticulum, for the manufacture of the neurotransmitter in the axon.

Question 36

What is the synaptic vesicles?

Answer 36

Neurotransmitter is stored in the synaptic vesicles.
Once it is released it diffuses across to the postsynaptic neurone, which possesses specific receptor proteins on its membrane to receive it.

Question 37

What are the features of synapses?

Answer 37

Unidirectionality - only pass information from the presynaptic neurone to the postsynaptic neurone.
Summation.
Inhibition.

Question 38

What is spatial summation?

Answer 38

Where a number of presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone, and so together trigger a new action potential.

Question 39

What is temporal summation?

Answer 39

A single presynaptic neurone releases neurotransmitter many times over a very short period.
If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, a new action potential is triggered.

Question 40

What are inhibitory synapses?

Answer 40

The presynaptic neurone releases a neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone.
The neurotransmitter causes the chloride ion channels to open.
Chloride ions move in the postsynaptic neurone by facilitated diffusion.
The neurotransmitter causes the opening of potassium protein channels.
Potassium ions move out of the postsynaptic neurone into the synapse.

Question 41

What are inhibitory synapses - hyperpolarisation?

Answer 41

The movement of potassium ions out makes the inside of the postsynaptic membrane more negative and the outside more positive.
The membrane increases to -80mV rather than usual -65mV.
This is hyperpolarisation and makes it less likely that a new action potential will be created because a large influx of sodium ions is needed to produce one.

Question 42

What is the function of synapses?

Answer 42

They act as junctions, allowing:
A single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse. This allows a single stimulus to create a number of simultaneous responses.
A number of impulses to be combined at a synapse. This allows nerve impulses from receptors reacting to different stimuli to contribute to a single response.

Question 43

What is it important to know about synapses?

Answer 43

A neurotransmitter is only made in the presynaptic neurone.
When the action potential reaches the synaptic knob the membranes of the synaptic vesicles fuse with the presynaptic membrane to release the neurotransmitter.
The neurotransmitter then diffuses across the synaptic cleft to bind to specific receptor proteins found only on the postsynaptic neurone.
This leads to a new action potential in the postsynaptic neurone.
These are excitory synapses.

Question 44

What is a cholinergic synapse?

Answer 44

Where the neurotransmitter is acetylcholine.
This is made from acetyl (ethanoic acid) and choline.
They are common in vertebrates, and occur in the central nervous system and at neuromuscular junctions.

Question 45

What is the process of transmission across a cholinergic synapse?

Answer 45

The arrival of the action potential at the presynaptic neurone causes calcium ion protein channels to open and calcium ions enter the synaptic knob by diffusion.
The influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.

Question 46

What is the process of transmission across a cholinergic synapse - acetylcholine?

Answer 46

Acetylcholine diffuses from the narrow synaptic cleft quickly because of the short diffusion pathway.
It then binds to receptor sites on sodium ion protein channels in the postsynaptic membrane.
This causes sodium ion protein channels to open, allowing sodium ions to diffuse in rapidly.
The influx of sodium ions generates a new action potential in the postsynaptic neurone.

Question 47

What is the process of transmission across a cholinergic synapse - acetylcholinesterase?

Answer 47

Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone.
The rapid breakdown of acetylcholine also prevents it from continuously generating a new action potential in the postsynaptic neurone, so leads to discrete transfer of information across synapses.

Question 48

What is the process of transmission across a cholinergic synapse - ATP?

Answer 48

ATP is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use.
Sodium ion protein channels close in the absence of acetylcholine in the receptor sites.

Question 49

What are other examples of neurotransmitters?

Answer 49

Noradrenaline is released by adrenergic neurones in the sympathetic nervous system.
Brain neurotransmitters include glutamic acid and dopamine.