Neurotransmitters at Synapses

Neurotransmitters at Synapses

Introduction

Neurons communicate with each other and with target cells, such as muscle fibres, through specialised junctions called synapses. The transmission of signals at synapses is a fundamental process in the nervous system and relies on the actions of molecules known as neurotransmitters. In this section, we will delve into the concept of neurotransmitters at synapses, the chemical transmission process involving vesicles, the synaptic cleft, and receptors, as well as mechanisms that prevent continuous stimulation.

Neurotransmitters at Synapses

1. Synaptic Cleft

• The synaptic cleft is a small gap that separates the axon terminal (end) of the presynaptic neuron from the membrane of the postsynaptic neuron or target cell.
• Neurotransmitters must traverse this gap to transmit signals.

2. Vesicles

• Neurotransmitters are stored in vesicles within the axon terminals of the presynaptic neuron.
• These vesicles contain specific neurotransmitters required for signal transmission.

3. Receptors

• Receptors are proteins located on the postsynaptic membrane.
• They bind to neurotransmitter molecules and initiate a response in the postsynaptic neuron or target cell.
• Receptors determine whether the signal received is excitatory (promotes an impulse) or inhibitory (prevents an impulse).

Chemical Transmission at the Synapse

1. Release of Neurotransmitters

• When an action potential (electrical signal) reaches the axon terminal of the presynaptic neuron, it triggers the release of neurotransmitters from the vesicles.
• The neurotransmitters are released into the synaptic cleft.

---

Neurotransmitters at Synapses

2. Diffusion and Binding

• Neurotransmitters diffuse across the synaptic cleft.
• They bind to specific receptors on the postsynaptic membrane.
• The binding of neurotransmitters to receptors initiates a response in the postsynaptic neuron.

3. Threshold

• To transmit an impulse, a minimum number of neurotransmitter molecules must attach to receptors on the postsynaptic membrane.
• This binding must reach a threshold level to generate an impulse in the postsynaptic neuron.

Preventing Continuous Stimulation

1. Removal of Neurotransmitters

• To prevent continuous stimulation of postsynaptic neurons, neurotransmitters need to be removed from the synaptic cleft.
• This removal occurs through two main mechanisms:
  • Enzymatic Degradation: Some neurotransmitters are broken down by specific enzymes present in the synaptic cleft.
  • Reuptake: In reuptake, neurotransmitters are reabsorbed by the presynaptic neuron after binding to receptors. This recycling process prevents their accumulation in the synaptic cleft.

2. Filtering Weak Stimuli

• Synapses have the capability to filter out weak stimuli that may arise from insufficient secretion of neurotransmitters.
• If the quantity of neurotransmitters released is below the threshold required to initiate an impulse, the signal is not transmitted.

3. Summation

• Summation is the process by which a series of weak stimuli, occurring in rapid succession, can collectively release enough neurotransmitter molecules to reach the threshold.
• This cumulative effect enables the generation of an impulse in the postsynaptic neuron.

---

Neurotransmitters at Synapses

Summary

Neurotransmitters are essential for communication between neurons and target cells at synapses. They are stored in vesicles, released into the synaptic cleft, and bind to receptors on the postsynaptic membrane. The binding of neurotransmitters to receptors can initiate or inhibit the generation of an impulse in the postsynaptic neuron. To prevent continuous stimulation, neurotransmitters are removed from the synaptic cleft through enzymatic degradation or reuptake. Additionally, synapses can filter out weak stimuli, and summation allows a series of weak stimuli to collectively reach the threshold for impulse transmission. Understanding the role of neurotransmitters and the mechanisms governing synaptic communication is vital for comprehending the functioning of the nervous system.