5.1.3 Neuronal Communication

Question 1

role of sensory receptors

Answer 1

• respond to specific types of stimuli (eg. pressure Pacinian corpuscle)
• as transducers (convert diff stimulus types into nerve impulse- generator potential)

Question 2

Pacinian corpuscle

what is it

Answer 2

• mechanoreceptor
• converts mechanical energy to electrical energy
• located deep in skin
• sensory neurone surrounded by layers of connective tissue, covered by gel
• neurone ending has stretch mediated Na⁺ channel. Permeability to Na⁺ ions changes when channels change shape

Question 3

Pacinian corpuscle

how does it work

Answer 3

1. in resting state, stretch mediated Na⁺ channels are too narrow to allow Na⁺ in. the neurone has a resting potential.
2. when pressure is applied the corpuscle changes shape, causing the membrane surrounding the neurone to stretch. (Temporary gaps/spaces appear between the phospholipids in the bilayer.)
3. Na⁺ channels widen and Na⁺ diffuses in
4. membrane is depolarised by influx of Na⁺, resulting in generator potential
5. this creates an action potential that passes along sensory neurone to CNS

Question 4

structure and function of sensory neurones

Answer 4

one long dendron
cell body
one short axon

outside of CNS
transmits impulses from sensory receptor cell to relay neurone, motor neurone or brain

Question 5

structure and function of relay neurones

Answer 5

many short dendrons
cell body
many short axons

transmit impulses between sensory and motor neurones

Question 6

structure and function of motor neurones

Answer 6

many short dendrons
cell body
one long axon

Question 7

axons

Answer 7

transmit impulses Away from the cell body
Axon=Away

can be myelinated

Question 8

what is a myelin sheath?

Answer 8

schwann cells grow and wrap around axon many times, laying down double phospholipid layer each time

act as an insulating layer, allowing nerve impulses to be transmitted x100 faster

Question 9

adaptations of cell body

Answer 9

• many endoplasmic reticulum
• many mitochondria

both involved in producing neurotransmitters

Question 10

How is information about the strength & intensity of a stimulus communicated to the brain?

Answer 10

by the frequency of the action potentials sent. high freq = high intensity stimulus

Question 11

why is there a difference in speed of conduction of an AP along myelinated vs non-myelinated neurone?

Answer 11

• depolarisation can only occur where voltage gated Na⁺ channels are present ie. at nodes of ranvier
• myelinated neurones have longer sections with no voltage gated Na⁺ channels
• longer circuits
• saltatory conduction: action potential jumps from node to node

Question 12

how is a resting potential established?

Answer 12

Sodium potassium pump

• active transport: 3Na+ out for every 2K+ in
• as a result there is more Na+ outside the axon and more K+ inside. Therefore Na+ diffuse back into the axon down its electrochemical gradient and K+ diffuse out. This is by diffusion through channel proteins, NOT diff. across phospholipid bilayer
• However, most gated Na+ channels are closed and most K+ channels are open so more K+ diffuses out than Na+ in, so there is a resting potential across the membrane of -70mV (inside more -ve than outside)

Question 13

describe saltatory conduction

Answer 13

depolarisation of the axon can only occur at the nodes of ranvier where no myelin is present as Na⁺ ions can pass through protein channels in membrane.

this creates longer localised circuits between adjacent nodes. the action potential ‘jumps’ from node to node ‘saltatory conduction’. much faster than wave of depolarisation along whole length of axon as it takes time for channels to open and ions to move.

repolarisation also uses ATP in the Na pump so reducing repolarisation means saltatory conduction is much more efficient

Question 14

how is a resting potential established?

Answer 14

Sodium potassium pump

• active transport: 3Na⁺ out for every 2K⁺ in
• as a result there is more Na⁺ outside the axon and more K⁺ inside. Therefore Na⁺ diffuse back into the axon down its electrochemical gradient and K⁺ diffuse out. This is by diffusion through channel proteins, NOT diff. across phospholipid bilayer
• However, most gated Na⁺ channels are closed and most K⁺ channels are open so more K⁺ diffuses out than Na⁺ in, so there is a resting potential across the membrane of -70mV (inside more -ve than outside)

Question 15

at resting potential, the inside of the axon has loads of …….. ions

Answer 15

K⁺

Question 16

at resting potential, outside the axon has loads of …….. ions

Answer 16

Na⁺

Question 17

how is an action potential generated?

Answer 17

1. energy of stimulus triggers opening of Na⁺ channels. Na⁺ diffuses into axon and down electrochemical grad, depolarising the membrane
2. this change causes voltage gated Na⁺ channels to open (positive feedback) so more Na⁺ floods in
3. When potential difference reaches +40mV, Na⁺ voltage gated channels close and K⁺ voltage gated channels open. K⁺ diffuses out of the axon, reducing charge and repolarising axon membrane.
4. There is a delay in the closing of K⁺ channels so membrane hyperpolarises
5. Na⁺/K⁺ pump restores resting potential
6. After an AP there is a short refractory period where the axon can’t be excited again. Voltage gated Na⁺ channels remain closed to stop Na⁺ depolarising membrane. –> prevents propagation of AP backwards along axon. Also ensures APs don’t overlap.
   UNIDIRECTIONAL & DISCRETE

Question 18

what are the 3 factors that affect the speed of an action potential?

Answer 18

• presence of myelin sheath
• axon diameter
• -> the bigger the diameter the faster the impulse is transmitted as there is less resistance to flow of ions in the cytoplasm
• temperature
• -> higher temp = faster impulse as diffusion of ions is faster at higher temperatures. However above 40C proteins start to denature

Question 19

what is the refractory period and its purpose?

Answer 19

period where axon can’t be excited
voltage gated Na⁺ channels remain closed to stop Na⁺ depolarising membrane

ensures APs are UNIDIRECTIONAL and DISCRETE

Question 20

all or nothing principle

Answer 20

if stimulus surpasses threshold value an AP is generated, no matter how large the stimulus. Stronger stimuli create more frequent APs but of the same size.

Question 21

role of synapse

Answer 21

• Allows neurones to communicate
• Ensures transmission between neurones is UNIDIRECTIONAL -> neurotransmitter receptors are on postsynaptic membrane only
• Allows convergence/ impulses from more than one neurone to be passed to a single neurone
• Allows divergence/ impulses from a single neurone to be passed to more than one neurone
• Filters out background/low level stimuli or ensures only stimulation that is strong enough will be passed on
• Prevents fatigue/over-stimulation
• Allows many low level stimuli to be amplified
• Presence of inhibitory and stimulatory synapses allow impulses to follow specific path
• Permits memory/learning/ decision making

Question 22

adaptations of synapse for function

Answer 22

• many endoplasmic reticulum
• many mitochondria

both involved in producing neurotransmitters which are stored in vesicles

Question 23

what are the two types of neurotransmitter

Answer 23

Excitatory

• results in the DEPOLARISATION of the postsynaptic membrane
• if the threshold is reached in the postsynaptic membrane, an AP is generated.
• increase the likelihood of a response
  eg. Acetylcholine

Inhibitory

• results in HYPERPOLARISATION of the postsynaptic membrane
• prevents an AP being triggered
  eg. GABA

Question 24

how is a nerve impulse transmitted across a synapse?

Answer 24

1. AP arrives at presynaptic neurone and depolarises membrane, causing Ca²⁺ channels to open and Ca²⁺ to diffuse into the presynaptic knob
2. Infux of Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane, releasing ACh into synaptic cleft by exocytosis
3. ACh diffuses across synaptic cleft
4. ACh binds to receptors on Na⁺ voltage gated channels on postsynaptic neurone, causing them to open and Na⁺ to diffuse in rapidly
5. Influx of Na⁺ generates an AP in postsynaptic neurone
6. Acetylcholinesterase hydrolyses ACh –> choline and ethanoic acid, which diffuse back across synaptic cleft into presynaptic neurone. Breakdown of ACh prevents it from continuously generating AP.
7. ATP released from mitochondria is used to recombine choline and ethanoic acid to form ACh, which is stored in synaptic vesicles. Na⁺ channels close.

Question 25

cholinergic synapse

Answer 25

uses acetylcholine

common in CNS of vertebrates and at neuromuscular junctions

Question 26

spacial summation

Answer 26

kind of like convergence
multiple neurones from diff places –> one neurone.

each neurone releases neurotransmitter which builds up in high enough levels in the synapse to trigger an AP in the single post synaptic neurone

Question 27

temporal summation

Answer 27

single presynaptic neurone releases neurotransmitter as a result of an AP several times over a short period. neurotransmitter builds up in the synapse until the quantity is sufficient to trigger an AP