Neurotransmitters can be classified into two main categories based on their effects on the postsynaptic neuron: excitatory neurotransmitters and inhibitory neurotransmitters.

Excitatory neurotransmitters cause depolarization of the postsynaptic neuron, potentially triggering an action potential if the threshold is reached. Acetylcholine is an example of an excitatory neurotransmitter.

Example: Acetylcholine is an excitatory neurotransmitter that plays a crucial role in muscle contraction and cognitive functions.

Inhibitory neurotransmitters result in hyperpolarization of the postsynaptic membrane, making it less likely for an action potential to occur. Gamma-aminobutyric acid $GABA$ is an example of an inhibitory neurotransmitter found in some brain synapses.

Vocabulary: Hyperpolarization is the state in which a neuron's membrane potential becomes more negative, making it less likely to fire an action potential.

The transmission of an impulse across a synapse involves a series of steps:

1. An action potential reaches the presynaptic knob, causing voltage-gated calcium channels to open.
2. Calcium ions diffuse into the presynaptic knob, triggering the mobilization and fusion of synaptic vesicles with the cell membrane.
3. Neurotransmitters are released into the synaptic cleft through exocytosis.
4. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic neuron.
5. This binding causes ligand-gated sodium channels to open, allowing sodium ions to enter the postsynaptic neuron.
6. If the resulting depolarization reaches the threshold, it triggers an action potential in the postsynaptic neuron.

Highlight: The process of neurotransmitter release and reception is a highly regulated and precise mechanism that ensures accurate signal transmission between neurons.

Synapses play crucial roles in the nervous system, far beyond simple signal transmission. In reality, a single neuron may form thousands of synapses using its dendrites, dendrons, and axons.

The key functions of synapses include:

1. Ensuring unidirectional signal transmission: Neurotransmitter receptors are only present on the postsynaptic membrane, allowing impulses to travel exclusively from the presynaptic to the postsynaptic neuron.

Definition: Unidirectional signal transmission refers to the one-way flow of information across a synapse, from the presynaptic to the postsynaptic neuron.

2. Facilitating signal divergence: A single impulse from one neuron can be transmitted to multiple neurons through various synapses, resulting in simultaneous responses to a single stimulus.
3. Enabling signal convergence: Multiple neurons can form synapses with a single postsynaptic neuron, allowing stimuli from different receptors to interact and produce a unified response.

Highlight: The ability of synapses to facilitate both signal divergence and convergence is crucial for complex information processing in the nervous system.

Synapses also play a role in summation and control of neural signals. Each stimulus from a presynaptic neuron releases a consistent amount of neurotransmitter into the synapse. However, in some cases, the neurotransmitter released from a single impulse may not be sufficient to trigger an action potential in the postsynaptic neuron.

Vocabulary: Summation refers to the process by which multiple synaptic inputs are combined to influence the postsynaptic neuron's response.

This mechanism allows for fine-tuning of neural responses and enables complex information processing in the nervous system. The interplay between excitatory and inhibitory inputs, as well as the temporal and spatial summation of signals, contributes to the sophisticated functioning of neural networks.

Example: In the brain, the integration of multiple synaptic inputs at a single neuron allows for complex decision-making processes and the formation of memories.

Synapses serve multiple crucial functions in neural networks, enabling complex information processing and signal integration. The function of synapse extends beyond simple signal transmission.

Highlight: A single neuron can form thousands of synaptic connections through its dendrites and axons.

Definition: Summation occurs when multiple neurotransmitter releases combine to reach the threshold for triggering an action potential.

Example: Multiple presynaptic neurons can converge on a single postsynaptic neuron, enabling integration of different signals.

The structure of the synapse in neurons is a specialized junction where signals are transmitted between nerve cells. The synapse consists of three main components: the presynaptic neuron, the synaptic cleft, and the postsynaptic neuron.

The presynaptic neuron's axon terminal contains synaptic knobs, which are swollen ends filled with mitochondria and endoplasmic reticulum. These organelles are crucial for producing neurotransmitters and storing them in synaptic vesicles.

Vocabulary: Synaptic vesicles are small membrane-bound sacs that contain neurotransmitters.

The synaptic cleft is a narrow gap between the presynaptic and postsynaptic neurons, typically measuring 20-30 nanometers wide. This space allows for the diffusion of neurotransmitters from one neuron to another.

Definition: The synaptic cleft is the space between the presynaptic and postsynaptic neurons where neurotransmitters are released and received.

The postsynaptic neuron contains specialized receptor proteins on its membrane that bind to specific neurotransmitters. These receptors are crucial for receiving and interpreting the chemical signals sent by the presynaptic neuron.

Highlight: The structure of synapses is highly specialized to facilitate efficient signal transmission between neurons.

The process of neurotransmitter release involves several steps, including the arrival of an action potential at the axon terminal, the opening of voltage-gated calcium channels, and the fusion of synaptic vesicles with the presynaptic membrane.

Example: When an action potential reaches the axon terminal, it triggers the influx of calcium ions, which causes synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.