cells of the nervous system and neurotransmitters at synapse

• The nervous system is composed of two main types of cell: neurons and glial cells. Nerve cells receive and transmit signals. Glial cells support neurons
• Neurons (nerve cells) are composed of dendrites, a cell body, and an axon.

  
• Myelin sheath


  A fatty substance produced by glial cells which insulates the axon and increases the speed of impulse conduction
• Unmyelinated axons


  • Conduct impulses at 0.5-10 m/s
• Myelinated axons


  • Conduct impulses up to 150 m/s
• Myelination


  1. Begins during the fourth month of pregnancy

  2. 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 due to myelination being incomplete in our early years
• Certain diseases

  Destroy the myelin sheath causing a loss of co-ordination
• Diseases that destroy the myelin sheath


  • Multiple sclerosis (MS)
• Where two neurons meet is a small gap called a synapse. The plasma membranes of each neuron are in close contact and are separated by a synaptic cleft. An electrical impulse can't directly cross the gap; the signals are transmitted across the synapse by chemicals called neurotransmitters.
• Neurotransmitters are stored in vesicles in the axon endings of the presynaptic neuron.

  On the arrival of an impulse, they are released into the cleft. They diffuse across the cleft and bind to receptors on the membrane of the post-synaptic neuron. In doing so the neurotransmitter relays impulses across the synaptic cleft.

  Neurotransmitters need to removed to prevent continuous stimulation of postsynaptic neurons either by enzyme degradation or reuptake to the presynaptic neuron
• A minimum number of neurotransmitter molecules must attach to receptors in order to reach the threshold on the post synaptic membrane to transmit the impulse.

  Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.

  A series of weak stimuli can release enough neurotransmitters to reach the threshold to trigger an impulse - summation (converging pathway)
• Two neurotransmitters are endorphins and dopamine

  Endorphins stimulate neurons involved in reducing the feeling of pain. They act on receptors to inhibit neurons involved in pain transmission.

  Endorphin production increases in response to severe injury, exercise, stress, certain foods. Increased levels of endorphins are also linked to feelings of pleasure as dopamine is released due to eating, sex, exercise.
• Dopamine is a neurotransmitter involved in the feeling of pleasure. It also reinforces particular behaviours by activating the reward pathway in the brain.

  The reward pathway is activated when an individual engages in a behaviour that is beneficial to them - eating when hungry, drinking when thirsty. It tells the individual to repeat what it just did to get that reward.
• Common diseases are related to neurotransmitters.

  Alzheimer's - low levels of acetylcholine which results in a decline in signals between brain cells.

  Parkinson's - low levels of dopamine affecting the part of the brain affecting movement.
• Drugs are either agonists/antagonists of neurotransmitters.

  Agonists - chemicals that mimic the action of neurotransmitters at a synapse by binding to and stimulating specific receptors and triggering the same response as the neurotransmitter. Example - morphine is an agonist of endorphines, acts on the same opioid receptors and leads to pain relief
• Antagonists - chemicals that bind to specific receptors and block the action of the neurotransmitter at the synapse. Example - antihistamines treat allergies like hayfever. They block histamine receptors that the allergy has triggered. Also, the poison strychnine is an antagonist.
• An enhanced effect of the neurotransmitter can be achieved by using drugs that inhibit the enzymes that degrade neurotransmitters or by blocking the reuptake of neurotransmitters at the synapse. This is used to treat neurotransmitter disorders.

  Examples - cholinesterase inhibitors to treat Alzheimer's and serotonin reuptake inhibitors to treat depression
• Recreational drugs are chemical substances taken for enjoyment rather than medical reasons eg heroin and meth.

  They can act as agonists, antagonists, stimulate release of neurotransmitters, inhibit neurotransmitter reuptake.

  They can affect the neurotransmission at synapses and so cause changes in mood, cognition, perception, behaviour.
• Drug addiction is caused by repeated use of drugs that act as antagonists.

  Antagonists block specific receptors causing the nervous system to increase the number and sensitivity of these receptors - sensitisation.

  This sensitisation leads to addiction where the person 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 - desensitisation.

  This desensitisation leads to drug tolerance where the individual must take more of the drug to get the same effect.