Nervous System and Neurotransmission Notes

Components of the Nervous System

  • Two major types of cells in the nervous system: Neurons or nerve cells; Glia or glial cells.

Neuron Structure

  • Every neuron has four regions: Cell Body, Dendrites, Axon, and Axon Terminals. Figure 4.1: Every Neuron Has Four Regions: Cell Body, Dendrites, Axon, and Axon Terminals.

Glia

  • Major functions:
    • Providing firmness and structure to the brain
    • Getting nutrients into the system
    • Eliminating waste
    • Forming myelin
    • Communicating with other glia and neurons
    • Creating the blood-brain barrier
    • A drug molecule must be able to pass the barrier to be psychoactive
    • Protecting the brain from toxic chemicals

Neurotransmission

  • Neurotransmission: Transferring information from one neuron to another at a synapse.

Somatic and Autonomic Nervous Systems

  • Somatic Nervous System: Part of the Peripheral Nervous System; Controls voluntary actions.
  • Autonomic Nervous System: (no additional details provided in the transcript)

Central Nervous System

  • Central Nervous System consists of the brain and spinal cord.
  • Functions:
    • Integration of information
    • Learning and memory
    • Coordination of activity

Chemical Pathways in the Brain

  • Dopamine
    • Mesolimbic dopamine pathway: From the ventral tegmental area in the midbrain to the nucleus accumbens; Proposed to mediate some types of psychotic behavior; Possible component of the “rewarding” properties of drugs.
    • Nigrostriatal dopamine pathway: From the substantia nigra to the striatum, past the hypothalamus; Substantial loss of cells leads to Parkinson’s Disease.
    • Figure 4.3: Mesolimbic and Nigrostriatal Dopamine Pathways.
  • Acetylcholine
    • Pathways arise from cell bodies in the nucleus basalis in the lower part of the brain and project widely throughout the cerebral cortex
    • Involved in Alzheimer’s disease
    • Acetylcholine blockers impair memory
  • Norepinephrine pathways
    • Arise from the locus ceruleus in the brain-stem and project both up and down in the brain
    • Influence the level of arousal and attentiveness
    • Play a role in the initiation of food intake
  • Serotonin pathways
    • Arise from the brain-stem raphe nuclei and project both upward to brain and downward to spinal cord
    • May have a role in impulsivity and aggression, depression, food intake and weight control, as well as alcohol use
    • Serotonin receptors play a role in the actions of some hallucinogenic drugs
  • GABA or Gamma-Amino Butyric Acid
    • Not neatly organized into discrete pathways
    • Found in most regions of the CNS
    • Inhibitory neurotransmitter, which exerts generalized inhibitory functions
  • Glutamate
    • Found in most regions of the brain
    • Excitatory neurotransmitter
    • Evidence indicates that specific glutamate pathways may be important for the expression of some psychoactive drug effects
  • Endorphins
    • Found throughout the brain
    • Naturally occurring morphinelike chemicals
    • Play a role in pain relief and other functions

Common Neurotransmitters

  • Dopamine: Inhibitory or excitatory; CNS changes include wanting, euphoria, agitation, paranoia; Drugs of abuse include amphetamines and cocaine.
  • GABA: Inhibitory; CNS changes include sedation, relaxation, drowsiness, depression; Drugs include alcohol and barbiturates.
  • Serotonin: Excitatory or inhibitory; CNS changes include sleep, relaxation, sedation; Drugs include LSD.
  • Acetylcholine: Excitatory or inhibitory; CNS changes include mild euphoria, excitation, insomnia; Drugs include nicotine.
  • Endorphins: Inhibitory; CNS changes include mild euphoria, block pain, slow respiration; Drugs include opioids.

Life Cycle of a Neurotransmitter

  • Step 1: Neurotransmitter precursors are found circulating in the blood supply.
  • Step 2: Uptake — Selected precursors are taken up by cells.

Figure 4.6

  • Figure 4.6: Neurons Use Enzymes to Synthesize the Neurotransmitters Dopamine and Norepinephrine.

Life Cycle of a Neurotransmitter (continued)

  • Step 3: Synthesis — Precursors are synthesized into neurotransmitters through the actions of enzymes.
  • Step 4: Storage — Neurotransmitters are stored in synaptic vesicles near the terminal from which they will be released.

Life Cycle of a Neurotransmitter (continued)

  • Step 5: Release — When the action potential arrives, neurotransmitters are released into the synapse.
  • Step 6: Binding — Released neurotransmitters bind with receptors on the membrane of the postsynaptic cell; neurotransmitters may have excitatory or inhibitory effects depending on the type of receptor.

Life Cycle of a Neurotransmitter (continued)

  • Step 7: Metabolism — Once a signal has been sent, neurotransmitters are removed from the synapse.

Examples of Drug Actions

  • Altering neurotransmitter availability in the synapse through actions on synthesis, storage, release, uptake, or metabolism.
  • Example: Many antidepressants block the reuptake of dopamine, serotonin, and norepinephrine.
  • Direct action on the receptor:
    • Drug as an agonist mimics the action of neurotransmitters by activating the receptor.
    • Drug as an antagonist occupies the receptor and prevents the neurotransmitter from activating it.