The CNS controls consciousness, movement, thoughts and emotions.
PNS – all of the nerves of the body
Adaptations of neurones:
Cell body where nucleus and cell organelles are found
Axon – main long fibre of neurone
Axons can be over a metre long, far more efficient than multiple neurones to convey
information from CNS to effectors, less time is wasted in transferring electrical impulses from
one cells to another
Axon insulated by fatty myelin sheath will small uninsulated sections (nodes) so electrical
impulses do not have to travel along whole axon, but jump from one axon to the next
Dendrites extend out of the cell body – so neurones can connect to many other neurones
and receive impulses from them, forming a network for easy communication
Sensory – long, cell body branching off middle of axon
Relay – short, small cell body with many dendrites
Motor – long, large cell body at one end, long dendrites branching off it
The pathway through the nervous system can be summarised as follows:
stimulus → sensory neurone → relay neurone → motor neurone → effector → response
For maximum marks, you will need to understand the structure and functioning of a synapse and
explain what happens at each step.Exam questions about neurotransmitters are a good opportunity
for examiners to introduce unfamiliar examples and contexts, so remember the following:
Neurotransmitters move by diffusion – remember, this requires a concentration gradient and is a
passive process
Receptors that are complementary in shape to neurotransmitters are located on the postsynaptic
neurone
Drugs (such as heroin, ecstasy and cocaine) can bind to neurotransmitter receptors, triggering
impulses in different regions of the brain
How an impulse is passed across a synapse
The electrical impulse travels along the first axon (of the first neurone, known
as the presynaptic neurone)
This triggers the end of the presynaptic neurone to release chemical
messengers called neurotransmitters from vesicles
o These vesicles fuse with the presynaptic membrane, releasing their
contents into the synaptic cleft
The neurotransmitters diffuse across the synaptic cleft and bind with
receptor molecules on the membrane of the second neurone (known as
the postsynaptic membrane)
This stimulates the second neurone to generate an electrical impulse (which
then travels down the second axon)
, The neurotransmitters are then destroyed to prevent continued
stimulation of the second neurone (otherwise the neurotransmitters would
cause repeated impulses to be sent)
Synapses ensure that impulses only travel in one direction, avoiding the
confusion that would be caused within the nervous system if impulses were
able to travel in both directions
Neurones do not actually come into direct contact with each other
Where the dendrites of two neurones meet (to make a connection between
the neurones) a junction known as a synapse is formed
At a synapse, there is a very small gap between neurones
o This very small gap is known as the synaptic cleft or synaptic gap
Electrical impulses cannot travel directly from one neurone to the next due to
the synaptic cleft (electricity cannot 'jump' the gap)
Instead, the electrical signal is briefly converted to a chemical
signal that can cross the synaptic cleft
o The chemical signalling molecules used to transfer the signal between
neurones at a synapse are known as neurotransmitters
Once these neurotransmitters cross the synaptic cleft and meet the neurone
on the opposite side, the signal is converted back into an electrical impulse,
which can then pass along the neurone
, A reflex response (also known as an involuntary response) does not
involve the conscious part of the brain as the coordinator of the reaction
Awareness of a response having happened occurs after the response has
been carried out
Responses are therefore automatic and rapid – this helps to minimise
damage to the body and aids survival
PNS – all of the nerves of the body
Adaptations of neurones:
Cell body where nucleus and cell organelles are found
Axon – main long fibre of neurone
Axons can be over a metre long, far more efficient than multiple neurones to convey
information from CNS to effectors, less time is wasted in transferring electrical impulses from
one cells to another
Axon insulated by fatty myelin sheath will small uninsulated sections (nodes) so electrical
impulses do not have to travel along whole axon, but jump from one axon to the next
Dendrites extend out of the cell body – so neurones can connect to many other neurones
and receive impulses from them, forming a network for easy communication
Sensory – long, cell body branching off middle of axon
Relay – short, small cell body with many dendrites
Motor – long, large cell body at one end, long dendrites branching off it
The pathway through the nervous system can be summarised as follows:
stimulus → sensory neurone → relay neurone → motor neurone → effector → response
For maximum marks, you will need to understand the structure and functioning of a synapse and
explain what happens at each step.Exam questions about neurotransmitters are a good opportunity
for examiners to introduce unfamiliar examples and contexts, so remember the following:
Neurotransmitters move by diffusion – remember, this requires a concentration gradient and is a
passive process
Receptors that are complementary in shape to neurotransmitters are located on the postsynaptic
neurone
Drugs (such as heroin, ecstasy and cocaine) can bind to neurotransmitter receptors, triggering
impulses in different regions of the brain
How an impulse is passed across a synapse
The electrical impulse travels along the first axon (of the first neurone, known
as the presynaptic neurone)
This triggers the end of the presynaptic neurone to release chemical
messengers called neurotransmitters from vesicles
o These vesicles fuse with the presynaptic membrane, releasing their
contents into the synaptic cleft
The neurotransmitters diffuse across the synaptic cleft and bind with
receptor molecules on the membrane of the second neurone (known as
the postsynaptic membrane)
This stimulates the second neurone to generate an electrical impulse (which
then travels down the second axon)
, The neurotransmitters are then destroyed to prevent continued
stimulation of the second neurone (otherwise the neurotransmitters would
cause repeated impulses to be sent)
Synapses ensure that impulses only travel in one direction, avoiding the
confusion that would be caused within the nervous system if impulses were
able to travel in both directions
Neurones do not actually come into direct contact with each other
Where the dendrites of two neurones meet (to make a connection between
the neurones) a junction known as a synapse is formed
At a synapse, there is a very small gap between neurones
o This very small gap is known as the synaptic cleft or synaptic gap
Electrical impulses cannot travel directly from one neurone to the next due to
the synaptic cleft (electricity cannot 'jump' the gap)
Instead, the electrical signal is briefly converted to a chemical
signal that can cross the synaptic cleft
o The chemical signalling molecules used to transfer the signal between
neurones at a synapse are known as neurotransmitters
Once these neurotransmitters cross the synaptic cleft and meet the neurone
on the opposite side, the signal is converted back into an electrical impulse,
which can then pass along the neurone
, A reflex response (also known as an involuntary response) does not
involve the conscious part of the brain as the coordinator of the reaction
Awareness of a response having happened occurs after the response has
been carried out
Responses are therefore automatic and rapid – this helps to minimise
damage to the body and aids survival
