I. Synapse definition – junction between 2 neurons or neuromuscular junction
A. Action potentials in presynaptic neuron may produce action potentials in postsynaptic neuron/muscle cell
B. Action potential (electrical) travels along neuron resulting in neurotransmitter release which can produce a graded potential in the postsynaptic neuron
II. Electrical Synapses
B. Direct ionic coupling via connexin channels of gap junctions
C. Fast current flow
D. Reciprocal ES –gap junctions can pass electrical current in either direction
E. Rectifying ES –gap junctions pass electrical current in only one direction
F. ES are found – Cardiac muscle fibers- Smooth muscle fibers- Sensory receptor cells- Nervous system of crayfish- Giant nerve fibers of the earthworm
III. Chemical Synapses
A. Synaptic cleft– physical gap between pre- and postsynaptic neurons
B. Axon terminal (synaptic knob) contains synaptic vesicles filled with neurotransmitter
C. AP in presynaptic neuron causes voltage regulated Ca2+ gates to open
D. Ca2+ flows into the knob and triggers the release of neurotransmitter
E. Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane (ligand-gated ion channels)
F. Postsynaptic ion channels open for Na+ (or it could be some other ion) to enter and the resting membrane potential of the postsynaptic neuron changes
G. Neurotransmitter is removed from the cleft and ion channels return to the resting condition
H. Acetylcholine receptor
1. Found at neuromuscular junction (motor neuron + muscle cell)
2. Ligand-gated Na-K channel that has two binding sites for Ach
3. AP results in release of Ach, similar to above, and Ach binds the receptor of the motor end plate (muscle cell) resulting in cations (mainly Na+) to enter muscle cell
4. Depolarization in the muscle cell is called an end plate potential and is graded
5. If depolarization of muscle cell reaches threshold then the voltage-gated Na channels open on the muscle cell and the AP propagates resulting in muscle contraction
6. Cleaning up involves removing Ca2+ by Ca-ATPase that removes Ca2+ from the cytoplasm, recycling of neurotransmitter, diffusion of neurotransmitter and destruction of Ach by acetylcholinesterase
I. Other neurotransmitters and neuromodulators (which influence response to neurotransmitters)
Epinephrine, norepinephrine , dopamine, serotonin and histamine, GABA, glutamate and glycine, Nitric oxide, Endorphin and enkephalines, Substance P.
IV. Neural Integration
A. Small changes are seen in resting membrane potentials at neuronal synapses in comparison to the neuromuscular junction
B. EPSP– The excitatory postsynaptic potential is a graded potential in which the postsynaptic neuron depolarizes due to Na+ or Ca2+ entering postsynaptic neuron
C. IPSP –The inhibitory postsynaptic potential is a graded potential in which the postsynaptic neuron hyperpolarizes due to K+ leaving or Cl– entering the postsynaptic neuron
D. EPSPs and IPSPs are graded potentials
Fast excitatory and inhibitory inputs are constantly arriving at the postsynaptic neuron.
Each neuron integrates (sums) all this information and as a result the V m fluctuates
If the V m reaches threshold an AP is produced.
Threshold can be reached as a result of increase EPSPs or a decreased of IPSP.
E. Spatial summation– Convergence, in which several presynaptic neurons synapse with one postsynaptic neuron increasing the likelihood of reaching threshold in the postsynaptic neuron.
F. Temporal summation– summation in time in which postsynaptic potentials come in rapid succession and could result in larger EPSP or IPSP
G. Presynaptic Inhibition –a different inhibitory neuron can decrease the amount of neurotransmitter released by the presynaptic neuron. For example, Substance P is a pain neurotransmitter that contributes to the pain transmission pathway. Enkephalins are opioid peptides, similar to endorphins, that are made by the body to block pain. In this case, enkephalins are considered to be neuromodulators
H. Presynaptic Facilitation –neuromodulators can also increase the amount of neurotransmitter released.