6.5 Neuron Synapse

Question 1

Nervous System

Answer 1

The nervous system consists of two main divisions:
* Central nervous system (CNS) = brain and spinal cord
* Peripheral nervous system (PNS) = peripheral nerves
The nervous system is composed of specialised cells called
neurons that function to transmit electrical signals
The CNS coordinates sensory & motor signals from the PNS
* Sensory neurons send signals to the CNS (afferent pathway)
* Motor neurons send signals from the CNS (efferent pathway)
* Relay neurons (interneurons) send signals within the CNS

Question 2

Structure of Motor Neuron

Answer 2

Dendrite Soma
(cell body)
Myelin
sheath
Axon
Axon terminal

Question 3

Membrane Potentials

Answer 3

Neurons have a difference in charge across their membranes
due to the distribution of positively-charged ions (Na+ / K+)
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 4

Nerve Impulses

Answer 4

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
An action potential changes the resting membrane potential
* The opening of sodium channels causes a sodium influx
* This creates a positive membrane potential (depolarisation)
* Opening potassium channels causes a potassium efflux
* This restores a negative membrane potential (repolarisation)
The ion distribution must be restored to original conditions
before a neuron can fire again (this is the refractory period)

Question 5

Myelination

Answer 5

Nerve impulses are action potentials propagated via axons
* Action potentials are ‘all or none’ and are only propagated
if a certain threshold potential is reached (~ -55mV)
In certain neurons, the axon is covered by a myelin sheath
* This enables saltatory conduction (⇧ transmission speed)
* The action potential ‘hops’ between gaps in the myelin
sheath (called nodes of Ranvier) for faster transmission

Question 6

Synaptic Transfer

Answer 6

Synapses are the physical junctions between two neurons
* Electrical impulses cannot cross these physical gaps
Neurons release neurotransmitters into the synapse cleft
* 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 7

Neonicotinoid Pesticides

Answer 7

Acetylcholine is a neurotransmitter used in CNS and PNS
* It is broken down in synapses by acetylcholinesterase
* This prevents the overstimulation of the receptors
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