(a) Structure and function of neurons

• Dendrites are nerve fibres that receive nerve impulses and pass them towards a cell body.
• The cell body of a neuron contains the nucleus and most of the cytoplasm.
• An axon is a single nerve fibre that carries nerve impulses away from a cell body and, in the case of sensory and inter neurons, on to the next neuron in the sequence.

Myelin sheath

• Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction from node to node.
• Myelination continues from birth to adolescence.
• Responses to stimuli in the first two years of life are not as rapid or coordinated as those of an older child or adult.
• Certain diseases destroy the myelin sheath causing a loss of coordination.
• Glial cells physically support neurons and produce the myelin sheath.

(b) Neurotransmitters at synapses

• Chemical transmission occurs at the synapse by neurotransmitters.
• Neurotransmitters relay messages from nerve to nerve within and out with the brain.
  • Neurons connect with other neurons or muscle fibres at a synaptic cleft.
  • Neurotransmitters relay impulses across the synaptic cleft.
  • Neurotransmitters are stored in vesicles in the axon endings of the presynaptic neuron.
  • They are released into the cleft on arrival of an impulse.
  • They diffuse across the cleft and bind to receptors on the membrane of the postsynaptic neuron.
• There is a need for removal of neurotransmitters by enzymes or reuptake to prevent continuous stimulation of post-synaptic neurons.
• Receptors determine whether the signal is excitatory or inhibitory.
• Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.
• A minimum number of neurotransmitter molecules must attach to receptors in order to reach the threshold on the postsynaptic membrane to transmit the impulse.
• Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse.
• Convergent neural pathways can release enough neurotransmitter molecules to reach threshold and trigger an impulse.

(c) Neurotransmitters, mood and behaviour

The functions of endorphins and dopamine

• Endorphins are neurotransmitters that stimulate neurons involved in reducing the intensity of pain.
• Endorphin production increases in response to:
  • severe injury
  • prolonged and continuous exercise
  • stress
  • certain foods
• Increased levels of endorphins are also linked to the feelings of pleasure obtained from activities such as eating, sex and prolonged exercise.
• Dopamine is a neurotransmitter that induces feelings of pleasure and reinforces particular behaviour by activating the reward pathway in the brain.
• The reward pathway involves neurons which secrete or respond to dopamine.
• The reward pathway is activated when an individual engages in a behaviour that is beneficial to them, eg eating when hungry.

(d) Neurotransmitter related disorders and their treatment

• Many drugs used to treat neurotransmitter-related disorders are agonists or antagonists.
• Agonists are chemicals that bind to and stimulate specific receptors mimicking the action of a neurotransmitter at a synapse.
• Antagonists are chemicals that bind to specific receptors blocking the action of a neurotransmitter at a synapse.
• Other drugs act by inhibiting the enzymes that degrade neurotransmitters or by inhibiting reuptake of the neurotransmitter at the synapse causing an enhanced effect.

(e) Mode of action of recreational drugs

Recreational drugs

• Recreational drugs can also act as agonists or antagonists.
• Recreational drugs affect neurotransmission at synapses in the brain altering an individual’s mood, cognition, perception and behaviour.
• Many recreational drugs affect neurotransmission in the reward pathway of the brain.

Drug addiction/tolerance

• Drug addiction is caused by repeated use of drugs that act as antagonists.
• Antagonists block specific receptors causing the nervous system to increase both the number and sensitivity of these receptors.
• This sensitisation leads to addiction where the individual craves more of the drug.
• Drug tolerance is caused by repeated use of drugs that act as agonists.
• Agonists stimulate specific receptors causing the nervous system to decrease both the number and sensitivity of these receptors.
• This desensitisation leads to drug tolerance where the individual must take more of the drug to get an effect.