3.4 The Cells of the Nervous System & Neurotransmitters at Synapses

Question 1

Structure of a Neuron

Answer 1

Includes dendrites, axon and cell body.
Dendrites - Receive nerve impulses & pass them TOWARDS the cell body.
Axon - A single nerve fibre that carries impulses AWAY from the cell body.
Cell Body - Contains the nucleus
- Contains most of the cytoplasm
- Controls the cell’s + METABOLISM+
- Contains clusters of ribosomes – to make proteins e.g. enzymes needed to
make neurotransmitters.

Question 2

Sensory Neurons

Answer 2

Take impulses from sensory receptors to the central nervous system.

Question 3

Motor Neurons

Answer 3

Take impulses from central nervous system to an effector (e.g. muscle).

Question 4

Inter Neurons

Answer 4

Transmit impulses between sensory and motor neurons within the central nervous system.

Question 5

Myelin Sheath

Answer 5

Surrounds the axon, which insulates it and increases the speed of impulse conduction.
Certain diseases destroy the myelin sheath causing a loss of co-ordination.

Question 6

Myelination

Answer 6

The process of myelination is brought about by Glial cells.
Continues from birth to adolescence.

Question 7

Glial Cells

Answer 7

Produce myelin sheath and physically support the neuron/axon.

Question 8

Synapse

Answer 8

A small gap between the axon of one neuron and dendrite of another.
Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.

Question 9

Neurotransmitter

Answer 9

A chemical that relays impulses across the synaptic cleft from the pre-synaptic neuron to the post-synaptic neuron.

Question 10

Neurotransmitter

Answer 10

A chemical that relays impulses across the synaptic cleft from the pre-synaptic neuron to the post-synaptic neuron.

Question 11

Chemical Transmission at a Synapse

Answer 11

1. Nerve impulse reaches the axon ending of the presynaptic neuron.
2. Vesicles containing neutotransmitter molecules move to the presynaptic
   membrane and fuse with it releasing their neurotransmitters into the synaptic
   cleft.
   3.The neurotransmitter diffuses across the gap and combine with receptors on
   the post-synaptic membrane.
3. If sufficient neurotransmitter molecules combine with receptors, a new
   impulse is generated in the postsynaptic neuron.
   Because vesicles are only on one side of the synapse the message can only
   be transmitted in one direction.

Question 12

Removal of Neurotransmitters

Answer 12

Once the impulse has been transmitted, the neurotransmitters need to be
removed from the receptors to prevent continuous stimulation of post-
synaptic neurons.

Question 13

Enzyme Degradation
(methods of removal)

Answer 13

As soon as the impulse is transmitted, certain neurotransmitters are broken
down into non active products and reabsorbed by the presynaptic neuron,
where they are reassembled and stored in vesicles.

Question 14

Re-uptake
(methods of removal)

Answer 14

Norepinephrine (Noradrenaline) on the other hand is reabsorbed by the
presynaptic neurone which secreted it and is stored in vesicles ready to be
reused.

Question 15

Inhibitory or Excitatory

Answer 15

Type of receptor determines whether a signal is inhibitory or excitatory.

Question 16

Threshold

Answer 16

The minimum number of neurotransmitter molecules attached to the receptors in order for the postsynaptic neuron to transmit the impulse is known as the THRESHOLD.

Question 17

Summation

Answer 17

A summation of a series of weak stimuli can release enough neurotransmitter to trigger an impulse.

Question 18

Endorphins

Answer 18

Neurotransmitters that stimulate neurons involved in reducing the intensity of pain.
Endorphins are also connected to feelings of euphoria, appetite control and release of sex hormones.

Question 19

Production of Endorphins

Answer 19

Endorphin production increases in response to severe injury, prolonged exercise, stress and certain foods e.g. chocolate.

Question 20

Dopamine

Answer 20

A neurotransmitter that induces feelings of pleasure and reinforces particular behaviours in the reward pathway.

Question 21

Drugs

Answer 21

Drugs used to treat neurotransmitter-related diseases are similar to neurotransmitters e.g. agonists and antagonists.

Question 22

Agonists

Answer 22

Binds to and stimulates receptors, mimicking the action of the neurotransmitter at a synapse.
Agonist drugs stimulate specific receptors leading to a decrease in sensitivity and number of receptors and as a consequence results in drug tolerance = desensitisation
- individual must take more of the drug to get an effect.

Question 23

Antagonists

Answer 23

Bind to specific receptors blocking the action of the neurotransmitter at a synapse.
Antagonist drugs block specific receptors leading to an increase in sensitivity
and number of receptors and as a consequence results in addiction = sensitisation - individual craves more of the drug.

Question 24

Other Drugs

Answer 24

Inhibit the enzymes which breakdown neurotransmitters or inhibit re-uptake at the synapse.

Question 25

Recreational Drugs

Answer 25

Can also act as agonists or antagonists.
As a consequence of taking recreational drugs changes in neurochemistry alter mood, cognition, perception and behaviour.
Many recreational drugs affect neurotransmission in the reward circuit of the brain.