Acetylcholine in the synaptic cleft binds toMultiple ChoiceCarrier proteinsCalcium ionsLigand-gated sodium channelsSynaptic vesicles

Step 1: Break Down the Problem

In this multiple-choice question, we need to identify which option acetylcholine binds to in the synaptic cleft. The options are:

1. Carrier proteins
2. Calcium ions
3. Ligand-gated sodium channels
4. Synaptic vesicles

Step 2: Relevant Concepts

Acetylcholine (ACh) is a neurotransmitter that plays a crucial role in transmitting signals across the synapse. It primarily binds to specific receptor proteins located on the postsynaptic membrane. The relevant protein types for neurotransmitter action include:

• Receptor proteins (for binding)
• Ligand-gated ion channels (which open upon binding)

Step 3: Analysis and Detail

Out of the provided options:

1. Carrier proteins: These proteins transport substances across the cell membrane but are not directly involved in receptor action for acetylcholine.
2. Calcium ions: These are crucial for neurotransmitter release but are not the target for acetylcholine's binding–they enter the presynaptic neuron upon action potentials.
3. Ligand-gated sodium channels: These channels open when acetylcholine binds to them, allowing Na+ ions to enter the neuron and create an action potential.
4. Synaptic vesicles: These store neurotransmitters but do not represent a binding target for acetylcholine.

Step 4: Verify and Summarize

The option that directly involves binding is the ligand-gated sodium channels. Acetylcholine binds to these channels, causing them to open, which leads to depolarization of the postsynaptic membrane.

Final Answer

Ligand-gated sodium channels are the correct option, as acetylcholine binds to these channels in the synaptic cleft.