University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2020
Views:

267

Pages:

13
Author:

chin5tgu

Synapses Between Neurons • Neural Synapse: The specialized junction between the membranes of two neurons. – 1st neuron is presynaptic neuron – 2nd neuron is postsynaptic neuron • First neuron affects the second neuron – The first neuron releases a chemical called a neurotransmitter that will bind to a specific receptor on the second neuron. There are over 100 known neurotransmitters • Synaptic delay = 0.5 milliseconds – time for the signal to move from the presynaptic cell to the postsynaptic cell • Synapse may be axodendritic, axosomatic or axoaxonic • Number of synapses on postsynaptic cell is variable: – 8,000 on spinal motor neuron – 100,000 on some brain neurons like the Purkinje cell Neuron Cytoplasm (red) Nucleus (purple) Synapses (green) Synapse Between Neurons Synaptic Vessicles containing Neurotransmitter Presynaptic Neuron Synaptic Cleft Postsynaptic Neuron Types of Neurotransmitters There are over 100 different neurotransmitters, and these are classified in 4 major categories: 1. Acetylcholine (ACh) 2. Amino Acid Neurotransmitters GABA, glycine, aspartic acid 3. Biogenic Amines (Monoamines) – – Catecholamines: epinephrine, norepinephrine, dopamine Indolamines: serotonin, histamine 4. Neuropeptides (see next slide) Neuropeptides Neuropeptides: • are chains of 2-40 amino acids. • are stored in synaptic vesicles in the terminal boutton. • are powerful even at low concentrations. • have long lasting effects. • may also function as hormones if they are released into the blood. Synaptic Transmission Two examples of synapses with different modes of action: • Excitatory Adrenergic Synapse • Inhibitory GABA-ergic Synapse Excitatory Adrenergic Synapse • Action potential opens voltage-gated Ca++ channels on presynaptic neuron. • Ca++ influx triggers release of Norepinephrine (NE) into the synapse. • NE works through a Second Messenger System. – NE binds to receptors on the postsynaptic cell that starts a chemical cascade resulting in production of a second messenger, cAMP that can have multiple effects including activating enzymes, activating genes, and activating ligand-gated ion channels. • NE is taken back up by the presynaptic neuron and recycled. Excitatory Adrenergic Synapse Inhibitory GABA-ergic Synapse • Pre-synaptic neuron releases GABA (-aminobutyric acid) into the synapse. • GABA binds to receptors on the postsynaptic cell and triggers the opening of Cl- channels producing a hyperpolarization of the postsynaptic cell. • Postsynaptic neuron is inhibited because it is now further away from threshold.