Neuronal Communication

Question 1

What features are common to all sensory receptors?

Answer 1

Act as energy transducers which establish a generator potential

Respond to specific stimuli

Question 2

Describe the basic structure of a Pacinian Corpuscle

Answer 2

Single nerve fibre surrounded by layers of connective tissue contained by a capsule

Stretch-mediated Na+ channels on membrane

Capillary runs along the base layer of tissues

Question 3

What stimulus does a Pacinian corpuscle respond to? How?

Answer 3

Pressure deforms membrane, causing Na+ channels to open

If influx of Na+ raises membrane to threshold potential, a generator potential is produced

Action potential moves along sensory neurone

Question 4

Describe the features of all neurons

Answer 4

Cell body - contains organelles and high proportion of RER

Dendrons - branch into dendrites which carry impulses towards the cell body

Axon - long, unbranched fibre carries nerve impulses away from cell body

Question 4

Describe the structure and function of a sensory neuron

Answer 4

Usually unipolar

Transmits impulses from receptors to CNS

Question 5

Describe the structure and function of a relay neuron

Answer 5

Usually bipolar

Transmits impulses between neurons

Question 6

Describe the structure and function of a motor neuron

Answer 6

Usually multipolar

Transmits impulses from relay neurons in the CNS to effectors

Question 7

Describe the additional features of a myelinated neuron

Answer 7

Schwann cells - wrap around axon many times

Myelin sheath - made from myelin-rich membranes of Schwann cells

Nodes of Ranvier - very short gaps between neighbouring Schwann cells where there is no myelin

Question 8

Name the 3 processes Schwann cells are involved in

Answer 8

Electrical insulation

Phagocytosis

Nerve regeneration

Question 9

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 9

Saltatory conduction - impulses jump from nodes of Ranvier. Depolarisation cannot occur where myelin sheaths act as electrical insulator

so impulse does not travel along whole axon length

Question 10

Where are myelinated and non-myelinated neurons found in the body?

Answer 10

Myelinated - most neurons in CNS

Non-myelinated - Group C nerve fibres involved in transmitting secondary pain

Question 11

What is resting potential?

Answer 11

Potential difference across neuron membrane when not stimulated

-70mV

Question 12

How is resting potential established?

Answer 12

Na/K pumps - 3:2
Na+:K+
Some voltage gated K+ channels open, so some K+ leaks out

Voltage gated Na+ channels closed

Organic anions inside axon

Question 13

Name the stages in generating an action potential

Answer 13

Depolarisation

Repolarisation

Hyperpolarisation

Refractory period

Question 14

What happens during depolarisation?

Answer 14

Facilitated diffusion of Na+ into cell

Potential difference across membrane becomes more positive

If membrane reaches threshold potential, voltage-gated Na+ channels open

Potential difference reaches +30mV

Question 15

What happens during repolarisation?

Answer 15

Voltage-gated Na+ channels close and voltage-gated K+ channels open

Facilitated diffusion of K+ ions out of cell

Potential difference across membrane becomes more negative

Question 16

What happens during hyperpolarisation?

Answer 16

‘Overshoot’ when K+ ions diffuse out and potential difference becomes more negative than resting potential

Refractory period - no stimulus is large enough to raise membrane potential to threshold

Voltage gated K+ channels close and sodium-potassium pump creates resting potential

Question 17

Explain the importance of the refractory period?

Answer 17

No action potential can be generated in hyperpolarised sections of membrane

Ensures unidirectional impulse

Ensures discrete impulses

Limits frequency of impulses transmission; larger stimuli have higher frequency

Question 18

Why is the frequency of impulse significant?

Answer 18

Enables organism to distinguish size of stimulus although all action potentials have same magnitude

Larger stimuli results in higher frequency of transmission because they overcome hyperpolarisation faster

Question 19

What is the function for the synapse?

Answer 19

Electrical impulses cannot cross the junction

neurotransmitters send impulses between neurons

Summation of sub-threshold impulses

New impulses can be initiated in several different neurons for multiple simultaneous respnses

Question 20

Describe the structure of a synapse

Answer 20

Presynaptic neuron ends in synaptic bulb - contains lots of mitochondria, ER and vesicles of neurotransmitter

Synaptic cleft - gap between neurons

Postsynaptic neuron - has complementary receptors to neurotransmitters

Question 21

What happens in the presynaptic neuron when an action potential is transmitted between neuron

Answer 21

Wave of depolarisation travels down presynaptic neuron causes voltage-gated Ca2+ channels to open

Vesicles move towards and fuse with the membrane of the presynaptic bulb

exocytosis of neurotransmitters into the synaptic cleft

Question 22

How do neurotransmitters cross the synaptic cleft?

Answer 22

simple diffusion

Question 23

What happens in the postsynaptic neuron when an action potential is transmitted between neurons?

Answer 23

Neurotransmitters bind to specific complementary receptors of postsynaptic membrane

Ligand-gated Na+ channels open

If influx of Na+ ions raises membrane to threshold potential, an action potential is generated

Question 24

What happens in inhibitory synapse?

Answer 24

Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open

Cl- moves in and K+ moves out via facilitated diffusion

potential difference becomes more negative - hyperpolarisation so no actionpotential

Question 25

Define summation and name the 2 types

Answer 25

neurotransmitter from several sub-threshold impulses accumulates to generate action potential

Temporal summation

Spatial summation

Question 26

What are cholinergic synapses?

Answer 26

Use acetylcholine as primary neurotransmitter. Exitatory or inhibitory.

Question 27

What is the difference between temporal and spatial summation?

Answer 27

Temporal - one presynaptic neuron releases neurotransmitters serveral times in quick succession

Spatial - multiple presynaptic neurons release neurotransmitters

Question 28

What happens to acetylcholine from the synaptic cleft

Answer 28

Hydrolysis into acetyl and choline by acetylcholinesterase AChE

Acetate and choline diffuse back into presynaptic membrane

ATP used to reform acetylcholine for storage in vesicles