5.1.3 Neuronal communication

Question 1

Why is communication needed?

Answer 1

-so that organisms can respond to changes in their environment to increase chances of survival
-can be in response to internal or external environment

Question 2

Homeostasis

Answer 2

-coordinating the functions of organs in order to maintain a relatively constant internal environment

Question 3

Cell signalling

Answer 3

-occurs through one cell releasing a chemical which has an effect on a target cell
-cells can transfer signals locally i.e neurotransmitters across synapses or across large distances i.e secretion of hormones

Question 4

What is the nervous system?

Answer 4

-made up of the brain and spinal cord(CNS) and the neurones that connect CNS to the rest of the body(PNS)
-responsible for detecting stimuli and coordinating a response–} faster and more targeted than hormonal communication

Question 5

Neurones

Answer 5

-specialised network of nerve cells that can transmit electrical impulses rapidly around the body so the organism can respond to internal/external changes

Question 6

What are the 3 main types of neurones?

Answer 6

-sensory neurones= transmit nerve impulses from receptors to CNS
-relay neurones= transmit nerve impulses from sensory to motor
-motor neurones= transmit nerve impulses from CNS to effectors

Question 7

Describe the general structure of a neuron

Answer 7

-cell body= contains nucleus, has some ER and mitochondria within cytoplasm(involved in production of neurotransmitters)
-dendrites=transmit electrical impulses towards cell body
-axons=transmit impulses away from cell body
-axon terminal=end of an axon where neuro transmitters are released
-myelin sheath=layers of plasma membrane produced by Schwann cells, act as an insulating layer to speed up electrical impulse, can also provide nutritional support(phospholipids)
-Nodes of Ranvier=gaps in the myelin sheath that speeds up transmission because electrical impulses must jump from one node to the next

Question 8

Structure of a sensory neuron

Answer 8

-one long dendron to carry nerve impulses from receptor cells to cell body
-one short axon that carries nerve impulses from cell body to CNS
-cell body in the middle
-dendrites are not connected to cell body

Question 9

Structure of a motor neuron

Answer 9

-many short dendrites that carry nerve impulses from CNS to cell body
-one long axon to carry nerve impulses from cell body to effector cells
-dendrites are connected to cell body
-cell body at the top

Question 10

Structure of a relay neuron

Answer 10

-many short dendrites that carry nerve impulses from sensory neurons to cell body
-many short axons to carry nerve impulses from cell body to motor neurones
-no myelin sheath
-cell body in the middle

Question 11

Describe the pathway of nervous communication

Answer 11

-stimulus is detected by a receptor
-electrical impulse passes from receptor to CNS via sensory neurone
-impulse is passed from sensory to motor neurone via the relay neurone in the CNS
-the motor neurone passes along electrical impulse from CNS to effectors(muscles/glands)
-effectors coordinate a response

Question 12

What are sensory receptors?

Answer 12

-specialised cells that detect changes in the environment
-convert the stimulus they detect into a nerve impulse–} info is passed through the nervous system where a response is coordinated

Question 13

What are the 2 main features of a sensory receptor?

Answer 13

-it is specific to a single type of stimulus
-act as a transducer= convert a stimulus into a nerve impulse called a generator potential

Question 14

4 types of sensory receptors

Answer 14

-thermoreceptors= respond to heat in the end bulbs of Krause in the tongue
-chemoreceptors= respond to chemicals in the olfactory receptor of the nose
-photoreceptors= respond to light the cone cells of the eye
-mechanoreceptors= respond to pressure in the Pacinian corpuscle in the skin

Question 15

What is the resting potential?

Answer 15

-the potential difference* across a neurone’s membrane when it is at rest
-happens because the inside of the cell is more negatively charged than the outside–} results in a voltage/PD across the membrane, therefore the membrane is polarised
*the difference in charge between the inside and outside of around -70mv

Question 16

What is the generator potential?

Answer 16

-the change in potential difference due to a stimulus
-occurs when a stimulus is detected and the cell membrane becomes more permeable–} allows more ions to move in and out of cell, which alters PD

Question 17

What is an action potential?

Answer 17

-if the generator potential is big enough it will trigger formation of an action potential
-energy of the stimulus temporarily reverses the charges on the axon membrane–} PD across membrane increases to +40mv

Question 18

What are Pacinian corpuscles?

Answer 18

-specific sensory receptors that detect mechanical pressure
-located joints and deep within the skin

Question 19

Describe the structure of a Pacinian corpuscle?

Answer 19

-the end of the sensory neurone is found in the centre surrounded by connective tissue(each layer separated by a layer of gel)
-sodium ion channels within the membrane (responsible for transporting sodium ions across the membrane)
-neurone ending has a stretch-mediated sodium channel–} permeability changes when they change shape

Question 20

Explain the steps of how a Pacinian corpuscle converts mechanical pressure into a nervous impulse

Answer 20

-in its resting state, the stretch-mediated sodium channels in the sensory receptor’s membrane are too narrow to allow sodium ions to pass through them(at resting potential)
-when pressure is applied to the Pacinian corpuscle it changes shape. This causes the membrane surrounding its neurone to stretch
-when the membrane stretches, the sodium ion channels present widen–} sodium ions can diffuse into the neurone
-influx of positive sodium ions changes the PD of the membrane= becomes depolarised–} this results in a generator potential
-in turn, the generator potential creates an action potential (a nerve impulse) that passes along the sensory neurones.
-action potential will then be transmitted along neurones to the CNS

Question 21

Why does the resting potential occur?

Answer 21

-occurs as a result of the movement of sodium and potassium ions across the axon membrane
-must be transported via sodium-potassium pumps + ion channels because the phospholipid bilayer prevents these ions from diffusing across the membrane

Question 22

Describe the events that result in the creation of a resting potential

Answer 22

1) sodium ions are actively transported out of the neurone and potassium ions are actively transported into the neurone by the potassium-sodium pumps–} for every 3 sodium ions pumped out, 2 potassium ions are pumped out
2) therefore, there are more sodium ions outside of the membrane than inside the axon cytoplasm in comparison to potassium(has more inside cytoplasm)
3) sodium ions can diffuse back into the axon down its electrochemical gradient, whereas potassium ions move out of the axon via facilitated diffusion through the potassium ion channels
4) however when the cell is at rest, most of the gated sodium ion channels are closed, preventing the sodium ions to move back into axon whereas many potassium ion channels are open, so lots of potassium ions can diffuse out of the axon
5) therefore, outside of the cell is more positively charged than inside the cell, meaning the resting potential inside the cell is -70mv

Question 23

Describe the sequence of events that leads to an action potential

Answer 23

1) the neurone is at resting potential, not transmitting any impulse. Some K+ ion channels are open but sodium voltage-gated ion channels are closed
2) STIMULUS: energy of stimulus triggers some sodium voltage-gated ion channels to open, making membrane more permeable to sodium. Sodium ions can diffuse into the neurone down their electrochemical gradient–} makes the inside of the neurone less negative
3) DEPOLARISATION: if the potential difference reaches the threshold(around -55mv), more voltage-gated sodium ion channels open and sodium ions to diffuse into neurone(positive feedback)
4) REPOLARISATION: at a PD of around +40 mv, the voltage-gated sodium ion channels close and potassium ion channels open. Sodium ions can no longer enter the neurone, but potassium ions can diffuse out of membrane down electrochemical gradient–} reduces charge inside neurone(negative feedback)
5) HYPERPOLARISATION: potassium ion channels are slow to close so there’s a slight overshoot, where too many potassium ions diffuse out of neurone. PD becomes more negative than the resting potential
6) RESTING POTENTIAL: voltage-gated potassium channels now close. Sodium potassium pump causes sodium ions to move out of the cell + potassium ions to move in–} resting potential is maintained

Question 24

What is the refractory period?

Answer 24

a short period of time after an action potential where the neurone can’t be excited again–} ion channels are recovering and they can’t be made to open
-important because it makes sure that the action potentials are unidirectional and prevents them moving backwards along the axon
-also ensures that action potentials don’t overlap but instead pass along as discrete impulses

Question 25

What is the wave of depolarisation?

Answer 25

-when an action potential happens, some of the sodium ions that enter the neurone diffuse sideways
-causes the sodium ion channels in the next region of the neurone to open + sodium ions will diffuse into that part–} leads to a wave of depolarisation across the neurone
-wave moves away from the parts of the membrane in the refractory period as they can’t fire an action potential

Question 26

Factors affecting the speed at which action potentials travel: Saltatory conduction

Answer 26

-myelinated axons transfer electrical impulses faster because depolarisation can only occur at the nodes of Ranvier(where sodium ions can pass into the membrane)
-the neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, so the action potential can ‘jump’ from one node to another
-faster than a wave of depolarisation where the action potential travels along the whole length of the axon membrane + more places that channel must open so ions can move in

Question 27

Factors affecting the speed at which action potentials travel: Axon diameter

Answer 27

-the bigger the axon diameter, the faster the action potential is transmitted–} less resistance to the flow of ions in the cytoplasm compared to cytoplasm in a smaller axon so depolarisation reaches other parts of the neurone cell membrane better

Question 28

Factors affecting the speed at which action potentials travel: Temperature

Answer 28

-the higher the temperature, the faster the ions can diffuse as they have more kinetic energy
-only occurs until around 40 degrees as higher temps cause proteins i.e sodium-potassium pumps to denature

Question 29

All or nothing

Answer 29

-an action potential will only fire if the threshold value is reached
-the action potential will always have the same change in voltage/same size regardless of how big the stimulus is
-a larger stimulus causes action potentials to fire more frequently

Question 30

What is a synapse?

Answer 30

-the junction between 2 neurones/between neurone + effector where neurotransmitters chemically diffuse

Question 31

Structure of a synapse

Answer 31

-synaptic cleft= gap which separates axon of one neurone and dendrite of another
-synaptic knob= end of a presynaptic neurone, contains many mitochondria + large amounts of endoplasmic reticulum so it can synthesise neurotransmitters
-synaptic vesicles= vesicles containing neurotransmitters
-neurotransmitter receptors= receptors which the neurotransmitters bind to on the postsynaptic membrane

Question 32

What are the 2 types of neurotransmitters?

Answer 32

-excitatory= result in the depolarisation of the postsynaptic neurone–} if the threshold is reached then an action potential is triggered i.e Ach
-inhibitory= result in the hyperpolarisation of the postsynaptic membrane–} prevents an action potential being triggered i.e GABA

Question 33

What is a cholinergic synapse?

Answer 33

-a synapse that uses the neurotransmitter acetylcholine which binds to cholinergic receptors

Question 34

Describe the steps of synaptic transmission in a cholinergic synapse

Answer 34

-the action potential arrives at the presynaptic knob and depolarisation of the presynaptic membrane causes voltage-gated calcium ion channels to open–} calcium ions diffuse into the presynaptic knob
-influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to and fuse with the presynaptic membrane
-Ach is released into the synaptic cleft via exocytosis–} Ach diffuses down conc gradient to postsynaptic mebrane
-Ach binds to receptor sites on the sodium ions channel in the membrane of the postsynaptic neurone–} causes the ligand-gated sodium channels open allowing sodium channels to diffuse in rapidly along a conc gradient
-the influx of sodium ions generate a new action potential in the postsynaptic neurone if the threshold is reached
-Ach is broken down into choline and ethanoic acid(acetyl), which diffuse back into the presynaptic neurone. The breakdown of Ach prevents it from continuously generating a new action potential in the postsynaptic neurone

Question 35

Role of synapses

Answer 35

-synapses ensure that impulses are unidirectional as neurotransmitter receptors are only present on the postsynaptic membrane
-they can allow an impulse from one neurone to be transmitted to a no. of neurones at multiple synapses–} results in a single stimulus creating a no. of responses(divergence)
-can allow the impulses of many neurones to feed in to the same synapse–} results in stimuli from different receptors interacting to produce a single result(convergence)

Question 36

What is summation?

Answer 36

-each stimulus from a presynaptic neurone causes the release of the same amount of neurotransmitter
-however the amount of neurotransmitter from a single impulse is not always enough to trigger an action potential
-summation is where the effect of neurotransmitters released from many neurones is added together

Question 37

Spatial summation

Answer 37

-occurs when a no. of presynaptic neurones connect to one postsynaptic neurone. each released neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Question 38

Temporal summation

Answer 38

-occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period
-this builds up in the synapse until the quantity is sufficient to trigger an action potential

Question 39

Chemicals that can disrupt synaptic transmission

Answer 39

-mimicking= nicotine mimics Ach by binding to certain cholinergic receptors
-stimulating= more neurotransmitters released i.e amphetamines
-inhibiting enzymes= prevents neurotransmitters from being broken down by enzymes i.e nerve gases
-blocking= neurotransmitters unable to bind to receptor i.e curare
-inhibiting release of neurotransmitters from presynaptic neurone i.e Opioids