Neurotransmitter Systems

Communication between Neurons

• The nervous system uses various neurotransmitters for communication between neurons.

• Different groups of neurotransmitters are categorized into systems, each with unique neurotransmitters.

1. Cholinergic System

• Key Neurotransmitter: Acetylcholine

• Functionality: Operates at the neuromuscular junction, autonomic nervous system, and throughout the brain.

• Types of Receptors:

  • Nicotinic Receptors: Found at the NMJ and various synapses; activated by nicotine and acetylcholine.

  • Muscarinic Receptors: Interact with muscarine from certain mushrooms; do not bind to nicotinic receptors and have distinct roles.

2. Amino Acid Neurotransmitters

• Key Neurotransmitters: Glutamate, GABA (gamma-aminobutyric acid), and glycine.

• Systems:

  • Glutamatergic: Corresponds to glutamate; excitatory.

  • GABAergic: Corresponds to GABA; inhibitory.

  • Glycinergic: Corresponds to glycine; inhibitory.

• Reuptake Mechanism: Neurotransmitters removed from synapses via reuptake by presynaptic cells or glial cells.

3. Biogenic Amine Neurotransmitters

• Composition: Enzymatically made from amino acids; lack carboxyl groups.

• Key Neurotransmitters:

  • Serotonin: Made from tryptophan; belongs to the serotonergic system.

  • Dopamine: From tyrosine; involved in the dopaminergic system.

  • Norepinephrine and Epinephrine: Similar and part of the adrenergic system; produced in adrenal glands.

• Reuptake Process: Each neurotransmitter is transported back into presynaptic cells for recycling.

4. Neuropeptides

• Definition: Chains of amino acids linked by peptide bonds, serving as neurotransmitters.

• Examples: Met-enkephalin (5 amino acids) and beta-endorphin (31 amino acids).

• Functionality: Released with other neurotransmitters; can act as hormones (e.g., vasoactive intestinal peptide, substance P).

• Effect on Postsynaptic Cell: Varies with receptor - can cause depolarization or hyperpolarization.

  • Acetylcholine Example: Binds to nicotinic receptor (depolarization) vs. muscarinic receptor (varied effects).

Table of Neurotransmitter Systems

• Overview of characteristics including neurotransmitters, receptors, degradation methods, and postsynaptic effects.

  • Cholinergic System: Acetylcholine, nicotinic/muscarinic, eliminated by acetylcholinesterase.

  • Amino Acids: Glutamate and GABA, reuptake by neurons/glia, predominantly excitatory/inhibitory effects.

  • Biogenic Amines: Serotonin, dopamine, norepinephrine/epinephrine, reuptake, varied postsynaptic effects.

  • Neuropeptides: Variable length peptides, degradation by peptidases, diverse receptor effects.

Types of Receptors

1. Ionotropic Receptors

• Act as ion gates (e.g., nicotinic receptors for acetylcholine).

2. Metabotropic Receptors

• More complex; involve G proteins and effector proteins.

• Mechanism: Neurotransmitter binds, activating G protein, results in production of second messengers (e.g., cAMP, IP3).

• Effects in Neurons: Changes in enzyme activity and potential gene transcription impacts, influencing strength in neuronal connections (learning/memory).

Disorders of the Nervous System

• Proteopathy: Accumulation of misfolded proteins causes neurodegenerative diseases like Alzheimer’s and Parkinson’s.

  • Alzheimer's Disease: Linked to beta-amyloid plaques.

  • Parkinson's Disease: Associated with toxic alpha-synuclein.

• Mechanism: Proper folding of proteins is critical; disruption leads to toxicity and accumulation.

Terms to Know

• Proteopathy: Diseases characterized by abnormal protein accumulation.

• Reuptake: Process of taking neurotransmitters back into neurons post-release.

• Neurodegenerative Diseases: Conditions where nerve cells progressively deteriorate (e.g., Alzheimer’s, Parkinson’s).

Real-Life Application

• Brain Utilization Myth: The idea that humans only use 10% of their brains is false.

• Measurement through functional magnetic resonance imaging (fMRI) shows high brain activity across various regions during tasks (approximately 80% utilized).

• Example Task: Subject performs Visual Recognition Task; fMRI measures activity across sensory and motor areas.