3.4

• Convergent neural pathway


  Impulses from several neurons travel to one neuron
• Diffuse


  When particles spread out from a region of higher concentration to a region of lower concentration
• Dopamine


  Neurotransmitter that induces feelings of pleasure and reinforces particular behaviour by activating the reward pathway in the brain
• Endorphins


  Neurotransmitters that stimulate neurons involved in reducing the intensity of pain
• Excitatory


  Increases contraction of muscle fibres
• Inhibitory


  Decreases contraction of muscle fibres
• Myelin


  A substance made from protein and lipids (fat) that surrounds the axon of a nerve cell
• Myelination


  The process of myelin developing around the axon fibres
• Neurone


  Nerve cells. They carry an electrical message or impulse when stimulated from one place to another
• Neurotransmitters


  Chemical involved in passing nerve impulses from one nerve cell to the next across a synapse
• Stimuli


  Any change in the environment that can be detected by receptors in an organism
• Synapse


  A tiny gap at the junction between two nerve cells, which nerve signals must cross
• Synaptic cleft


  Neurons connect with other neurons or muscle fibres at a synaptic cleft. Neurotransmitters relay impulses across the synaptic cleft
• Neurons


  • An axon - a single nerve fibre that carries nerve impulses away from a cell body which is insulated by a fatty sheath
  • Dendrites - Branched nerve fibres which receive nerve impulses and pass them towards a cell body
  • Myelin sheath - insulates the axon and increases the speed of impulse conduction
• 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
• Certain diseases destroy the myelin sheath causing a loss of co-ordination
• Glial cells produce the myelin sheath and support neurons
• Neurotransmitters at synapses


  1. An electrical nerve impulse travels along the axon of the presynaptic neuron

  2. Neurotransmitters are stored in vesicles in the axon endings of the presynaptic neuron. They are released into the synaptic cleft on arrival of the impulse

  3. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the membrane of the postsynaptic neuron

  4. Receptors determine whether the signal is excitatory or inhibitory
• Removal of neurotransmitters


  1. Re-uptake - the neurotransmitter is reabsorbed back into the presynaptic neuron and restored inside a vesicle ready to be used again

  2. Enzyme degradation - neurotransmitter is broken down by an enzyme into smaller inactive product which are then reabsorbed by the presynaptic neuron and resynthesised into active neurotransmitter
• Stimuli threshold


  A minimum number of neurotransmitter molecules must attach to receptors on the postsynaptic membrane in order to reach the threshold to transmit the impulse
• Synapses filter out weak stimuli arising from insufficient secretion of neurotransmitters
• Summation of a series of weak stimuli can release enough neurotransmitter to trigger an impulse
• Convergent neural pathways can release enough neurotransmitter to reach threshold and trigger an impulse
• Endorphins


  Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress and 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


  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 e.g. eating when hungry
• Alzheimer's disease is linked to low levels of the neurotransmitter acetylcholine which results in a decline in signals between brain cells
• Parkinson's disease is caused by low levels of dopamine affecting the part of the brain controlling movement causing it not to work properly
• Agonists


  Chemicals that bind to and stimulate specific receptors, mimicking the action of a neurotransmitter at a synapse
• Antagonists


  Chemicals that bind to specific receptors blocking the action of a neurotransmitter at a synapse
• Drugs can act by inhibiting the enzymes that degrade neurotransmitters or by inhibiting reuptake of the neurotransmitter at the synapse causing an enhanced effect
• 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


  Caused by repeated use of drugs that act as antagonists, blocking specific receptors causing the nervous system to increase both the number and sensitivity of these receptors, leading to addiction where the individual craves more of the drug
• Drug tolerance


  Caused by repeated use of drugs that act as agonists, stimulating specific receptors causing the nervous system to decrease both the number and sensitivity of these receptors, leading to the individual needing to take more of the drug to get an effect