Neuronal Communication

Question 1

What are sensory receptors

Answer 1

these are specialised cells that an detect changes in our surroundings, they respond to a stimulus in an internal or external environment in an organism which can create action potentials

Question 2

What do transducers do

Answer 2

they convert one form of energy to another

• each is adapted to detect change in a particular form of energy, this may be a change in light levels, change in pressure on the skin or one of many other changes
• other receptors detect presence of chemicals

Question 3

What is a nerve impulse

Answer 3

• each change in the environment is called a stimulus, the sensory receptors respond by creating a signal n the form of electrical energy this is called a nerve impulse

Question 4

What is a pacinian corpuscle

Answer 4

• a pressure sensor found in the skin

Question 5

Describe the pacinian corpuscle

Answer 5

it is an oval shaped structure that consists of a series of concentric rings of connective tissue that is wrapped around the end of a nerve cell
when pressure changes in the skin this deforms the concentric rings which push against the nerve ending
only sensitive to changes in pressure that deform rings of connective tissue therefore when pressure is constant they stop responding

Question 6

How do you generate nerve impulses: changing membrane permeability

Answer 6

• if channel proteins are permanently open then ions can diffuse across the membrane and they will until the concentrations are equilibrium
• if the channels can be closed then the action of the active pumps create a concentration gradient across the membrane
• cells associated with nervous systems have specialised channel proteins, some of these are sodium channels where others are potassium channels, these channels possess a gate than can open or close the channel
• sodium channels are sensitive to small movements in the membrane so when the membrane is deformed by changing pressure the sodium channels open allowing sodium ions to diffuse into the cell producing a generator potential
• membranes also contain sodium/potassium pumps that actively pump sodium ions out of the cell and potassium into the cell
• when channels are all closed sodium/potassium pumps work to create a concentration gradient
• the concentration of sodium ions outside the cell increases while the concentration of potassium ions inside the cell increases
• the result of these ionic movements is a potential gradient across the cell membrane, the cell is negatively charged inside compared with outside

Question 7

How do you generate nerve impulses: creating a nerve impulse

Answer 7

• when the cell is inactive the cell membrane is said to be polarised as its negatively charged inside compared with the outside
• the nerve impulse is created by altering the permeability of the nerve cell membrane sodium ions, they do this by opening sodium ion channels this increases the membrane permeability and sodium ions can move across the membrane down their concentration gradient into the cell creating a change in the potential difference across the membrane
• the inside of the cell becomes less negative than usual this is called depolarisation
• if enough gates are open and enough sodium ions enter the cell the potential difference across the membrane changes significantly and will initiate an impulse or action potential

Question 8

Whats the difference in response between a small and large stimulus

Answer 8

if a small stimulus is detected than only a few sodium channels will open whereas if the large stimulus is detected than more gated channels will open
- if enough gates are opened and enough sodium ions enter the cell the potential difference across the cell membrane changes significantly and will initiate an action potential

Question 9

Describe the structure of neurones

Answer 9

• many are long so they can transmit the action potential over a long distance
• cell surface membrane has many gated ion channels that control the entry or exit of sodium, potassium and calcium ions
• sodium/potassium pumps use ATP to actively transport sodium ions out of the cell and potassium ions into the cell
• neurones maintain a potential difference across their cell surface membrane
• cell body contains the nucleus, many mitochondria and ribosomes
• numerous dendrites connect to other neurones they carry impulses towards the cell body
• an axon carries impulses away from the cell body
• neurones are surrounded by a fatty layer that insulates the cell from electrical activity in other nerve cells nearby, the fatty layer is composed of Schwann cells which are closely associated with the neurone

Question 10

what are the different types of neurones

Answer 10

• motor neurones - carry an action potential from the CNS to an effector such as a muscle or gland
• sensory neurones - that carry the action potential from a sensory receptor to the CNS
• relay neurones - that connect sensory and motor neurones

Question 11

what are the differences between the types of neurone

Answer 11

• motor neurones have their cell body in the CNS, they have a long axon which carries the action potential out to the effector
• sensory neurones have a long dendron which carrying the action potential from a sensory receptor to the cell body, this is positioned just outside he CNS, they have a short axon which carries the action potential into the CNS
• relay neurones connect the sensory and motor neurones together, they have short dendrites and a short axon, the number of dendrites and the number of divisions of the axon is variable

Question 12

Describe myelinated neurones

Answer 12

• myelin sheath contains Schwann cells which are wrapped tightly around the neurone so the sheath actually consists of several layers of membrane and thin cytoplasm from the Schwann cell
• along the neurones there are gaps in the myelin sheath which are called the nodes of Ranvier
• prevents the movement of ions across the neurone membranes, therefore it can only occur at the nodes of Ravier which means that the impulse or action potential jumps from one node to the next making the conduction more rapid

Question 13

Describe non-myelinated neurones

Answer 13

• also associated with Schwann cells but several neurones may be wrapped loosely by one Schwann cell
• this means that the action potential moves along the neurone in a wave rather than jumping from node to node
• carry over a shorter distance, and are shorter not needed for rapid transmission and they are used for breathing, and the action of the digestive system

Question 14

What are the advantages of myelination

Answer 14

• myelinated neurones can transmit action potential more quickly than non-myelinated neurones
• myelinated neurones carry action potentials from sensory receptors to the CNS and from the CNS to effectors, carry over long distances
• reaches the end of the neurone more quicker so faster response to a stimulus

Question 15

what is an action potential

Answer 15

A brief reversal of the potential across the membrane of a neurone causing a peak of +40mV compared to the resting potential of -60mV

Question 16

What are the stages of an action potential

Answer 16

1. the membrane starts in the resting state which are polarised with the inside of the cell being -60mV compared to the outside. there is a higher concentration of sodium ions outside than inside and a higher concentration of potassium ions inside than outside
2. sodium ion channels open and some sodium ions diffuse into the cell
3. the membrane depolarises and it becomes less negative with respect to the outside and reaches the threshold value of -50mV
4. positive feedback causes nearby voltage-gated sodium ion channels to open and many sodium ions enter, the cell then becomes positively charged inside compared with outside
5. the potential difference across the plasma membrane reaches +4-mV, the inside of the cell is positive compared with the outside
6. the sodium ion channels close and potassium channels open
7. potassium ions diffuse out of the cell bringing the potential difference back to negative inside compared with the outside this is called repolarisation
8. the potential difference overshoots slightly which makes the cell hyperpolarised
9. the original potential difference is restored so that the cell returns to its resting phase

Question 17

What happens in the refractory period

Answer 17

• after action potential sodium and potassium ions are in the wrong places
• ions concentrations must be restored by the action of pumps
• for a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential,
• allows the cell to recover and ensures that action potentials are transmitter in one direction

Question 18

when is a neurone at rest

Answer 18

a neurone is at rest when it is not transmitting an action potential

Question 19

what is a neurone doing when it is at rest

Answer 19

• its actively pumping
• for every three sodium ions pumped out of the cell two potassium ions are pumped inwards
• gated sodium ions are kept closed but some of the potassium channels are open therefore the plasma membrane is more permeable to potassium ions than sodium ions so potassium ions tend to diffuse out of the cells, the cell cytoplasm also contains large inorganic anions therefore the interior of the cells has a negative potential compared with the outside and the cell membrane is polarised
• potential difference is -60mV, this is called the resting potential

Question 20

Generating an action potential

Answer 20

• while the neurone is at rest it maintains a concentration gradient of sodium ions across plasma membrane, the concentration is higher outside than inside
• potassium ion concentration is higher inside than outside
• membrane is impermeable to sodium ions and there are sodium potassium ion pumps that pump them back out if they come in
• when the sodium ion gates are open then the sodium ions will quickly diffuse down their concentration gradient into the cell from the surrounding tissue fluid this causes a depolarisation of the membrane
• in the generator region of the neurone the gated channels are open by the action of synapse, when a few gated channels are open they allow a few sodium ions in this generates a small depolarisation and is known as the generator potential, but when more gated ion channels are open the generator potentials are combined to produce a larger depolarisation and passes a threshold generating an action potential
• sodium ion channels in a neurone are open by change in potential difference, when there are sufficient generator potentials to reach the threshold point then the channels open - this is an example of positive feedback as a small depolarisation of the membrane causes a change that increases the depolarisation further
• all action potentials are same magnitude, once it reaches +40mV on the inside of the cell the neurone will transmit the action potential

Question 21

what creates local currents

Answer 21

when sodium ions are allowed to flood into the neurone causing depolarisation this creates a local current in the cytoplasm of the neurone
sodium ions begin to move along the neurone towards regions where their concentration is still lower and these cause a slight depolarisation of the membrane and cause sodium ion channels further along the membrane to open this is positive feedback

Question 22

What are the steps in the formation of local currents and the transmission of nerve impulse

Answer 22

1. when an action potential occurs the sodium ion channels open at that point in the same neurone
2. the open sodium ion channels allow sodium ions to diffuse across the membrane from the region of higher concentration outside the neurone into the neurone, the concentration of sodium ions inside the neurones rises at the point where the sodium ion channels are open
3. sodium ions continue to diffuse sideways along the neurone and away from the region of increased concentration, this movement of charged particles is a current called local current
4. local current causes a slight depolarisation further along the neurone which affects the voltage-gated sodium ion channels and causes them to open, open channels allow rapid influx of sodium ions causing a full depolarisation further along the neurone therefore the action potential has moved along the neurone

Question 23

what is saltatory conduction

Answer 23

• in myelinated neurones the local currents are elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next meaning that the action potential jumps from one node to the next this is called saltatory conduction

Question 24

What are the advantages of saltatory conduction

Answer 24

• myelin sheath means that action potentials can only occur in the gaps between the Schwann cells that make up the myelin sheath
• action potential jumps from one node of Ranvier to another which speeds up the transmission of the active potential

Question 25

what does a higher frequency mean

Answer 25

• a higher frequency means that the action potentials are more intense, and more sodium ions are opened in the sensory receptor therefore this produces more generator potentials
• means more action potentials in a certain amount of time
• this increases the intensity of the action potential it is not its size that changes as the size is the same for all

Question 26

Describe the structure of a cholinergic synapse

Answer 26

• junction between two or more neurones where one neurone can communicate to another
• between the two neurones is a gap called the synaptic cleft
• action potential cannot bridge the gap between the two neurones therefore the action potential in the pre-synaptic neurone causes the release of a chemical that diffuses across the synaptic cleft and generates a new action potential in the post-synaptic neurone
• use acetylcholine as the neurotransmitters and are called cholinergic synapses because of this

Question 27

Describe the pre-synaptic bulb

Answer 27

• many mitochondria - active process needing ATP
• a large amount of smooth endoplasmic reticulum which packages the neurotransmitter into vesicles
• large number of vesicles containing molecules of the chemical acetylcholine,
• number of voltage-gated calcium ion channels on the cell surface membrane

Question 28

Describe the post-synaptic membrane

Answer 28

• contains specialised sodium ion channels that respond to the neurotransmitter
• these channels consist of 5 polypeptide molecules, two polypeptide chains have special receptor sites that is specific to acetylcholine
• receptor sites have a shape that is complementary to the shape of the acetylcholine molecule
• when the acetylcholine is present in he synaptic cleft it binds to the two receptor sites and causes the sodium ion channel to open

Question 29

Describe the transmission across the synapse

Answer 29

1. action potential arrives at the synaptic bulb
2. the voltage gated calcium ion channels open
3. calcium ion diffuse into the synaptic bulb
4. the calcium ions cause the synaptic vesicles to move to and fuse with pre-synaptic membrane
5. acetylcholine is released by exocytosis
6. acetylcholine molecules diffuse across the cleft
7. acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post-synaptic membrane
8. the sodium ion channels open
9. sodium ions diffuse across the post-synaptic neurone
10. a generator potential or excitatory post-synaptic potential (EPSP) is created
11. if sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential
12. A new action potential is created in the post-synaptic neurone

Question 30

Describe the role of acteylcholinesterase

Answer 30

• it is an enzyme found in the synaptic cleft
• it hydrolyses the acetylcholine to ethanoic acid and choline which stops the signals and so the synapse does not produce any more action potentials
• ethanoic acid and choline are recycled and they re-enter the synaptic bulb by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria

Question 31

Describe nerve junctions

Answer 31

• they often involve several neurones, could be several neurones coming together from differnet places or one neurone sending out signals to several neurones that diverge to different effectors

Question 32

Describe an excitatory post-synaptic potential (EPSP)

Answer 32

This is when one action potential passes down an axon to the synapse and causes a few vesicles to move and fuse with the pre-synaptic membrane, the small number of acetylcholine molecules diffusing across the cleft produces a small depolarisation

• this is not sufficient enough to cause an action potential
• can take several EPSPs to reach threshold and cause an action potential as they combine together to increase the membrane depolarisation until it reaches the threshold this is called the summation

Question 33

What can summation result form

Answer 33

• it results from several action potentials in the same pre-synaptic neurone or from action potentials arriving from several different pre-synaptic neurones

Question 34

How do nerve junctions and synapses control communication

Answer 34

• several pre-synaptic neurones might converge on one post-synaptic neurone allowing action potentials from different parts of the nervous system to contribute to generating an action potential in one post-synaptic neurone, useful when lots of different stimuli are warning us of danger
• combination of several EPSP could be prevented from producing an action potential by one IPSP
• one pre-synaptic neurone might diverge to several post-synaptic neurones, allowing one action potential to be transmitted to several parts of the nervous system, useful in a reflex arc
• synapse ensure that action potentials are transmitted in the right direction, only the pre-synaptic bulb contains vesicles of acetylcholine therefore if an action potential happens to start half way along a neurone and ends at a post-synaptic membrane it will not cause a response in the next cell
• synapses can filter out unwanted low-level signals as if this creates an action potential in the pre-synaptic neurone it is unlikely to pass across a synapse as little vesicles are released
• low level action potentials can be amplified by summation as they will release several vesicles over a short period of time enabling the post-synaptic EPSPs to combine together producing an action potential
• after repeated stimulation a synapse may run out of vesicles containing the neurotransmitter and the synapse is fatigued so the nervous system no longer responds to this stimulus
• the creation and strengthening of specific pathways within the nervous system is thought to be the basis of conscious thought and memory, synaptic membranes are adaptable, post-synaptic membrane can be made more sensitive to acetylcholine by the addition of more receptors meaning that a particular post-synaptic neurone is more likely to fire an action potential creating a specific pathway in response to a stimulus

Question 35

What is summation

Answer 35

occurs when the effects of several excitatory post-synaptic potentials are added together

Question 36

What is temporal summation

Answer 36

• several action potentials in the same pre-synaptic neurone

Question 37

What is spatial summation

Answer 37

• action potentials arriving from several different pre-synaptic neurones