Introduction
A synapse is a junction between two neurons, where the axon or some other portion of one cell (presynaptic cell) terminates on the dendrites, soma, or axon of another neuron (postsynaptic cell).
Classification
Synapses are classified using two methods:
-
Anatomical Classification: This classification is based on where the axon of the presynaptic neuron terminates on the postsynaptic neuron. There are three types:
- Axoaxonal: Axon to axon
- Axodendritic: Axon to dendrite
- Axosomatic: Axon to soma
-
Functional Classification: This classification is based on the mode of impulse transmission. There are two types:
- Electrical Synapses
- Chemical Synapses
Electrical Synapses
- Fastest type of synapse.
- No chemical transduction involved.
- Direct exchange of ions between the two neurons through gap junctions, providing physiological continuity.
- Very little synaptic delay due to direct current flow.
- Impulse transmission occurs in both directions.
- Found in:
- Cardiac muscle fibers
- Smooth muscle fibers
- Epithelial cells of the lens in the eye
- Epithelial cells of the gut
- Escape reflex neurons (in invertebrates)
Chemical Synapses
- A chemical substance (neurotransmitter) acts as an intermediary to convert an electrical signal in the presynaptic cell into a response in the postsynaptic cell.
- No continuity between the two neurons due to the presence of a synaptic cleft.
- Action potential reaching the presynaptic terminal causes the release of neurotransmitter from vesicles.
- Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron, causing a potential change.
Action of the Transmitter Substance on the Post-synaptic Neuron
The postsynaptic neuron membrane contains a large number of receptor proteins at the synapse.
Electrical Events in Postsynaptic Neurons
Excitatory Postsynaptic Potential (EPSP)
- Excitatory neurotransmitters open Na+ or Ca++ channels.
- This causes depolarization of the postsynaptic membrane.
- EPSPs:
- Are graded responses.
- Are proportionate to the stimulus strength.
- Can be summated.
- Produce an action potential if large enough to reach the firing level (threshold).
- A postsynaptic potential of +10 to +20mV is needed to produce an action potential.
Inhibitory Post-Synaptic Potentials (IPSP)
- IPSP is the electrical potential in the form of hyperpolarization that develops during postsynaptic inhibition.
- Stimulation of some presynaptic terminals leads to hyperpolarization of the postsynaptic membrane.
- Inhibitory neurotransmitters act on the postsynaptic membrane by binding with receptors.
- The transmitter-receptor complex opens ligand-gated potassium channels, leading to K+ efflux.
- Chloride channels also open, and chloride ions move inside the cell.
- The exit of K+ and influx of Cl- causes hyperpolarization.
- The hyperpolarized state of the synapse inhibits synaptic transmission.
- Causes:
- Increased membrane permeability to Cl–
- Opening of K+ channels
- Closure of Na+ or Ca++ channels
- IPSP = -5mV
Fate of the Neurotransmitter
After a neurotransmitter binds to a postsynaptic neuron:
- It produces a continuous postsynaptic effect.
- It blocks the reception of additional messages.
The neurotransmitter must then be removed from the synaptic cleft by:
- Diffusion into the surrounding fluid.
- Enzymatic destruction (e.g., Acetylcholinesterase for acetylcholine).
- Active transport back into the presynaptic terminal (e.g., norepinephrine).
Properties of Synapses
- One-way conduction (chemical synapses): Synapses generally permit the conduction of impulses in one direction, from the presynaptic to the postsynaptic neuron.
- Synaptic delay (average delay is 0.5ms): This is the minimum time required for transmission across the synapse.
- Synaptic inhibition: There are two types of synaptic inhibition:
- Direct inhibition: Occurs when an inhibitory neuron (releasing an inhibitory substance) acts on a postsynaptic neuron, leading to hyperpolarization due to the opening of Cl- (IPSPs) and/or K+ channels.
- Indirect inhibition: Presynaptic inhibition using e.g., GABA.
Other forms of inhibition include:
- Reciprocal inhibition: One excitation producing excitation in one pathway and inhibition in another (e.g., Golgi tendon reflex).
- Inhibitory interneuron: Negative feedback inhibitory interneurons (e.g., Renshaw cells of the spinal cord, also some cells in the cerebellum and limbic system).
- Lateral inhibition: Lateral pathways are inhibited more strongly to produce a sharp response.
Neurotransmitters
- Acetylcholine
- Biogenic amines:
- Catecholamines:
- Dopamine (DA)
- Norepinephrine (NE)
- Epinephrine
- Serotonin (5-hydroxytryptamine, 5-HT)
- Histamine
- Catecholamines:
- Amino acids:
- Excitatory amino acids: e.g., Glutamate
- Inhibitory amino acids: e.g., Gamma-aminobutyric acid (GABA)
- Neuropeptides: e.g., The endogenous opioids
- Miscellaneous:
- Gases: e.g., Nitric oxide
- Purines: e.g., Adenosine, ATP
Summation
- Spatial summation: When EPSPs occur in more than one synaptic knob at the same time.
- Temporal summation: When EPSPs in the presynaptic neuron are successively repeated without significant delay, so the effects of the previous stimulus are summated to the next.
Clinical Correlates
Modification of synaptic transmission by drugs and diseases. Drug actions at synapses include:
- Increasing the leakage of neurotransmitter from vesicle to cytoplasm, exposing it to enzyme breakdown.
- Increasing transmitter release.
- Blocking transmitter release.
- Inhibiting transmitter synthesis.
- Blocking transmitter reuptake.
- Blocking enzymes that metabolize the transmitter.
- Binding to the receptor to block (antagonist) or mimic (agonist) transmitter actions.
- Inhibiting or facilitating second-messenger activity.
