Topic 6.5: Neurons and Synapses

Question 1

The nervous system consists of two main divisions:

Answer 1

• Central nervous system (CNS) = brain and spinal cord

* Peripheral nervous system (PNS) = peripheral nerves

Question 2

Composition of the Nervous System

Answer 2

Specialized cells called neurons that function to transmit electrical signals

Question 3

The CNS coordinates sensory & motor signals from the PNS

Answer 3

1) Sensory neurons send signals to the CNS (afferent pathway)
2) Motor neurons send signals from the CNS (efferent pathway)
3) Relay neurons (interneurons) send signals within the CNS

Question 4

Structure of a Motor Neuron

Answer 4

1) Dendrite
2) Cell body (Soma)
3) Axon (Myelinated sheath)
4) Axon terminal

Question 5

Membrane Potentials

Answer 5

Neurons have a difference in charge across their membranes due to the distribution of positively-charged ions (Na+ / K+)

Question 6

Action and Resting Potential

Answer 6

Electrical signals are created by changing membrane polarity
• Polarity of a neuron at rest is the resting potential (-70mV)
• Polarity of a firing neuron is the action potential (+30mV)

Question 7

Resting Potential

Answer 7

The resting potential is maintained by a Na+/K+ pump

• It exchange sodium ions (3 out) and potassium ions (2 in) so that the membrane potential becomes slightly negative

Question 8

Action Potential

Answer 8

An action potential changes the resting membrane potential

1) The opening of sodium channels causes a sodium influx
2) This creates a positive membrane potential (depolarisation)
3) Opening potassium channels causes a potassium efflux
4) This restores a negative membrane potential (repolarisation)

Question 9

Refractory Period

Answer 9

The ion distribution must be restored to original conditions before a neuron can fire again

Question 10

Nerve impulses

Answer 10

Action potentials propagated via axons

* Action potentials are ‘all or none’ and are only propagated if a certain threshold potential is reached (~ -55mV)

Question 11

Myelin sheath

Answer 11

• This enables saltatory conduction (⇧ transmission speed)
• The action potential ‘hops’ between gaps in the myelin
sheath (called nodes of Ranvier) for faster transmission

Question 12

Synapses

Answer 12

Physical junctions between two neurons

-Electrical impulses cannot cross these physical gaps

Question 13

Neurons release neurotransmitters into the synapse cleft

Answer 13

Depolarisation in axon terminals opens Ca2+ channels
• Ca2+ influx causes vesicles containing neurotransmitters to release their contents into the synapse (via exocytosis)
• Neurotransmitters bind receptors on post-synaptic cells and generate graded potentials (excitatory or inhibitory)
• The summation of these graded potentials determines if the post-synaptic neuron (or effector cell) is activated

Question 14

Acetylcholine

Answer 14

A neurotransmitter used in CNS and PNS

1) It is broken down in synapses by acetylcholinesterase
2) This prevents the overstimulation of the receptors

Question 15

Neonicotinoid pesticides

Answer 15

Neonicotinoid pesticides irreversibly bind to acetylcholine receptors and cannot be digested by acetylcholinesterase
• Insects have higher levels of these types of receptors
• This makes neonicotinoids highly effective pesticides