16.20
Chemical Synapses
Chemische synapsen zijn gespecialiseerde plaatsen tussen twee neuronen of tussen een neuron en een niet-neuronale cel zoals een spier-, klier- of sens…
Chemical synapses are specialized sites where electrical signals from a presynaptic neuron are relayed by chemical messengers, known as neurotransmitters, to the postsynaptic cell to propagate an electrical impulse.
They consist of an axon terminal of a presynaptic cell, here a neuron, containing numerous neurotransmitter-filled synaptic vesicles, a postsynaptic cell, in this case, another neuron, with neurotransmitter receptors, and the synaptic cleft, a fluid-filled space between the two.
The unidirectional information transfer at a chemical synapse begins with the arrival of an action potential at the axon terminal of the presynaptic neuron. The resulting membrane depolarization triggers voltage-gated calcium channels to open, allowing calcium ions to rush in. This calcium surge causes the synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
After diffusing through the synaptic cleft, the neurotransmitters bind to specific ion channels on the postsynaptic membrane. This binding opens specific ion channels, allowing appropriate ions to enter the postsynaptic cell and initiate excitatory or inhibitory responses.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main structural components of a chemical synapse?
A chemical synapse consists of three key structures: the axon terminal of the presynaptic neuron containing synaptic vesicles filled with neurotransmitters, the postsynaptic cell membrane with neurotransmitter receptors, and the synaptic cleft, a fluid-filled space typically 20-50 nanometers wide separating the two membranes. These components work together to enable communication between neurons or between neurons and other cell types.
Q2: How does calcium trigger neurotransmitter release at the synapse?
When an action potential reaches the axon terminal, membrane depolarization opens voltage-gated calcium channels, allowing calcium ions to rush into the presynaptic cell. This calcium surge initiates a signaling cascade that causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. The strength of the calcium response determines how many vesicles release their contents.
Q3: Why is there a delay between presynaptic stimulation and postsynaptic response?
Chemical synapses exhibit an approximately one millisecond delay because neurotransmitters must be released from synaptic vesicles, diffuse across the synaptic cleft, and bind to postsynaptic receptors before ion channels open. This synaptic delay is inherent to chemical transmission and differs from electrical synapses, which transmit signals directly and instantaneously between cells.
Q4: What determines whether a neurotransmitter produces excitatory or inhibitory effects?
The type of neurotransmitter and the specific ion channels it opens on the postsynaptic membrane determine whether the response is excitatory or inhibitory. When neurotransmitters bind to postsynaptic receptors, they open ligand-gated ion channels that allow specific ions to enter, either depolarizing the membrane to trigger an action potential or hyperpolarizing it to prevent one.
Q5: How does the synaptic cleft width affect neurotransmitter signaling?
The synaptic cleft, typically 20-50 nanometers wide, provides the extracellular space through which neurotransmitters diffuse from the presynaptic to postsynaptic membrane. This narrow gap ensures efficient neurotransmitter delivery while maintaining the separation necessary for unidirectional signal transmission. The cleft width also influences the time required for neurotransmitter diffusion and receptor binding.
Q6: Why must neurotransmitters be removed from the synaptic cleft after transmission?
Neurotransmitter removal from the synaptic cleft allows the postsynaptic membrane to reset and become ready to receive new signals. By regulating neurotransmitter availability, the synapse can fine-tune the strength and timing of neuronal communication. Without clearance, neurotransmitters would continue binding to receptors, preventing the postsynaptic cell from responding to subsequent stimuli.
Q7: How do autoimmune disorders disrupt normal synaptic function?
In Lambert-Eaton myasthenic syndrome, antibodies target voltage-gated calcium channels, reducing acetylcholine release and causing muscle weakness. In myasthenia gravis, autoantibodies block acetylcholine receptors on the postsynaptic membrane, preventing neurotransmitter binding and inhibiting muscle contraction. Both conditions demonstrate how disrupting synaptic components impairs neuromuscular communication.