cells of the nervous system and neurotransmitters at synapse

• Myelin sheath insulates the axon and increases the speed of impulse conduction.
• Glial cells produce the myelin sheath and support neurons.

  Myelin enables nerve cells to transmit information faster and allows for more complex brain processes. The myelination process is vitally important to healthy central nervous system functioning.
• Myelination continues from birth to adolescence. Responses to stimuli in the first two years of life are not as rapid or co-ordinated as those of an older child or adult.

  Loss of the Myelin sheath: Certain diseases destroy the myelin sheath causing the loss of co-ordination.Example: Multiple Sclerosis
• Neurons connect with other neurons or muscle fibres at a synaptic cleft.
• Neurotransmitters are stored in vesicles in the axon endings of the pre-synaptic 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.

  The electrical impulse changes to a chemical impulse as it is carried across the synaptic cleft.
• Receptors will determine whether a signal is excitatory or inhibitory.
• There may be a need for the quick removal of neurotransmitters by enzymes or the reuptake of neurotransmitters to prevent continuous stimulation of postsynaptic neurons.
• 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. This mechanism prevents the generation of impulses from weak/harmless stimuli.
• Summation of a series of weak stimuli can release enough neurotransmitter to trigger an impulse. Convergent neural pathways can release enough neurotransmitter molecules to reach threshold and trigger an impulse.
• Endorphins are neurotransmitters that stimulate neurons involved in reducing the intensity of pain.
• Increased levels of endorphins are also linked to the feelings of pleasure obtained from activities such as eating, sex and prolonged exercise.
• Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.
• Dopamine is a neurotransmitter that induces feelings of pleasure and reinforces particular behaviour by activating the reward pathway of 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, for example when eating when hungry.
• Agonists are chemicals that bind to and stimulate specific receptors mimicking the action of a neurotransmitter at a synapse.

  Example: morphine action leading to pain relief.
• Antagonists are chemicals that bind to specific receptors blocking the action of a neurotransmitters at a synapse.
• As there may be a need for the quick removal of neurotransmitters by enzymes or the reuptake of neurotransmitters to prevent continuous stimulation of postsynaptic neurons, other drugs act by inhibiting the enzymes that degrade neurotransmitters or by inhibiting reuptake of the neurotransmitter at the synapse causing an enhanced effect.
• 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.