6.5 Neurons and Synapses

Question 1

Nervous system

Answer 1

the body’s control and communication centre
three functions:
- detecting stimuli
- interpreting them
- initiating appropriate responses

Question 2

Central nervous system (CNS)

Answer 2

comprises of the brain and spinal cord
spinal cord transmits messages to and from the brain and controls spinal reflexes

Question 3

Peripheral nervous system

Answer 3

all the nerves and sensory receptors outside the CNS

Question 4

Autonomic nervous system

Answer 4

regulates visceral functions over which there is generally no conscious control

Question 5

Neurons

Answer 5

are specialised cells that function to transmit electrical impulses within the nervous system

Question 6

three main components of neurons

Answer 6

Dendrites
Axon
Soma

Question 7

Myelin sheath

Answer 7

an insulating layer
the myelin sheath improves the conduction speed of electrical impulses along the axon, but require additional space and energy

Question 8

Motor neurons

Answer 8

transmit impulses from the CNS to muscles or glands
cell body at one end (one directional nerve impulses)

Question 9

Sensory neurons

Answer 9

transmits impulses from sensory receptors to CNS
cell body in the middle
receive nerve impulses from both directions

Question 10

Axon

Answer 10

An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors

Question 11

Dendrites

Answer 11

Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals

Question 12

Soma

Answer 12

A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

Question 13

Axon branches

Answer 13

extensively branching with tiny knobs at each end
these release neurotransmitter chemicals which transmit the message between neurons or between a neuron and a muscle cell

Question 14

Nodes of Ranvier

Answer 14

at intervals along the axon, there are gaps between the sheath called nodes of Ranvier

Question 15

Saltatory conduction

Answer 15

Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation
In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier
This results in an increase in the speed of electrical conduction by a factor of up to 100-fold

Question 16

Synapses

Answer 16

junctions between the end of one axon and the dendrite of cell body of a receiving neuron
enable the transmission of impulses rapidly all around the body

Question 17

Resting potential

Answer 17

the difference in charge across the membrane when a neuron is not firing

In a typical resting potential, the inside of the neuron is more negative relative to the outside

Question 18

How do neurons generate electrical signals

Answer 18

by pumping positively charged ions (Na+ and K+) across their membrane

The unequal distribution of ions on different sides of the membrane creates a charge difference called a membrane potential

Question 19

Depolarisation

Answer 19

sudden change in membrane potential – usually from a (relatively) negative to positive internal charge

Question 20

Repolarisation

Answer 20

the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge)

Question 21

Refractory period

Answer 21

the period of time following a nerve impulse before the neuron is able to fire again

Question 22

Threshold

Answer 22

if the threshold stimulation is not reached then the action potential is not generated

Question 23

Process of Action Potential

Answer 23

1. when a neuron is stimulated, the distribution of charged on each side of the membrane changes as Na+ channels open and Na+ begins to flow into the cell
2. for a millisecond, the charges reverse, and the inside of the cell becomes +ve charged (depolarisation). This causes a burst of electrical activity to pass along the axon of the neuron. As the charge reversal reached one region, local currents depolarise the next region. In this way, the nerve impulse spreads along the axon.
3. with no further stimulation repolarisation occurs as K+ and Na+ channels open
4. K+ flows out and the charges inside of the cell become negatively charged again
5. resting potential is achieved when the cell is once again electrically polarised

Question 24

Impulse transmission of a synapse

Answer 24

1. the arrival of a nerve impulse (action potential_ at the end of the presynaptic axon
2. the depolarisation of the presynaptic membrane causes Ca ion channels to open
3. Ca2+ diffuse into the presynaptic neuron
4. the influx of Ca2+ signals the vesicles to move to the membrane
5. vesicles move to the membrane
6. vesicles release their neurotransmitter by exocytosis into the space between the neurons (synaptic cleft)
7. the neurotransmitter diffuses across the synaptic cleft to receptors on the postsynaptic membrane
8. the neurotransmitter binds to receptors on the post synaptic membrane. Ion channels in the membrane open, causing an influx of Na+. This causes depolarisation of the post synaptic membrane
9. the depolarisation passes on down the post synaptic neuron as an action potential
10. the neurotransmitter is deactivated by enzymes located on the membrane. Components of the neurotransmitter are actively reabsorbed back into the synaptic knob, recycled and repackaged. Calcium ions are pumped out of the presynaptic neuron and into the synaptic cleft.

Question 25

Cholinergic synapses

Answer 25

which releases acetylcholine neurotransmitter (e.g. released by all motor neurones, activating skeletal muscles)

Question 26

Adrenergic synapses

Answer 26

which releases noradrenaline or adrenaline neurotransmitters (involved in the sympathetic nervous system)

Question 27

Excitory neurotransmitters

Answer 27

e.g. glutamate, adrenaline
cause depolarisation by opening Na or Ca channels

Question 28

Inhibitory neurotransmitters

Answer 28

cause hyperpolarisation by opening K or Cl chnnels

Question 29

Acetylcholine

Answer 29

is a neurotransmitter found between neurons and muscle cells (as well as other places)
- it travels across the synapse to bind to its receptor
- the enzyme acetylcholinesterase rapidly breaks down acetylcholine in the synapse (into choline and acetate)
- choline is reabsorbed by pre-synaptic neuron and re-used to make more acetylcholine

Question 30

Neonicotinoid pesticides

Answer 30

Neonicotinoid pesticides are able to irreversibly bind to nicotinic acetylcholine receptors and trigger a sustained response hence they block synaptic transmission at cholinergic synapses.

Neonicotinoid pesticides cannot be broken down by acetylcholinesterase, resulting in permanent overstimulation of target cells

While low activation of acetylcholine receptors promotes nerve signalling, overstimulation results in fatal convulsions and paralysis

Insects have a different composition of acetylcholine receptors which bind to neonicotinoids much more strongly
Hence, neonicotinoids are significantly more toxic to insects than mammals, making them a highly effective pesticide

While neonicotinoids have been successfully used to protect crops from pest species, there are disadvantages to their usage

Neonicotinoid use has been linked to a reduction in honey bee populations (bees are important pollinators within ecosystems)
Neonicotinoid use has also been linked to a reduction in bird populations (due to the loss of insects as a food source)
Consequently, certain countries (including the European Union) have restricted the use of neonicotinoid pesticides