Action Potential Generation

This page delves into the process of action potential generation in neurons. It explains how a neuron transitions from its resting state to an excited state capable of transmitting signals.

When a neuron is stimulated, additional ion channels in the membrane $particularly sodium channels$ open. If the stimulus is strong enough, it triggers a rapid change in the potential difference across the membrane, causing depolarization. This sequence of events is known as an action potential.

The document presents a graph illustrating the stages of an action potential, including:

1. Resting state
2. Stimulus and initial depolarization
3. Rapid depolarization
4. Repolarization
5. Hyperpolarization
6. Return to resting state

Highlight: For an action potential to be generated, the stimulus must be greater than the threshold value. This is known as the all-or-nothing principle.

Key points about action potential generation:

• A stimulus below the threshold value is insufficient to open enough sodium channels for full depolarization.
• Once the threshold is reached, the action potential generated will be the same size regardless of stimulus strength.
• This is referred to as an "all-or-nothing" response.

Example: If a weak stimulus only opens a few sodium channels, it may cause a small depolarization but fail to trigger a full action potential. However, if enough channels open to reach the threshold, a full-sized action potential will occur regardless of how much stronger the stimulus might be.

This page provides crucial information for understanding the 7 steps of action potential generation and the physiological mechanisms underlying neuronal signaling.

Synapses and Synaptic Transmission

This page focuses on synapses and the mechanism of synaptic transmission in the nervous system. It provides detailed information about the structure of synapses and how they facilitate communication between neurons.

A synapse is defined as the junction between two neurons or between a neuron and an effector cell. Key features of synapses include:

• The synaptic cleft: a tiny gap between the cells at the synapse
• The synaptic knob: a swelling on the pre-synaptic neuron containing synaptic vesicles filled with neurotransmitters

Vocabulary: Synaptic vesicles are small membrane-bound organelles that store neurotransmitters in the pre-synaptic neuron.

The page explains the role of action potentials in synaptic transmission:

• When an action potential reaches the end of a neuron, it causes neurotransmitters to be released into the synaptic cleft.
• These neurotransmitters diffuse across to the post-synaptic membrane and bind to specific receptors.
• Binding of neurotransmitters may trigger an action potential, cause muscle contraction, or stimulate hormone secretion.

Highlight: Receptors for neurotransmitters are only found on the post-synaptic membrane, ensuring that synaptic transmission is unidirectional.

The document also describes the process of neurotransmitter removal from the synaptic cleft, which is crucial for preventing continuous stimulation:

• Neurotransmitters are taken back into the pre-synaptic membrane
• They are then broken down by enzymes

This page provides essential information for understanding the mechanism of synaptic transmission in the nervous system, including the steps of synaptic transmission and the role of neurotransmitters in neuronal communication.

Cholinergic Synapses and Neurotransmitters

This page focuses on cholinergic synapses, which use acetylcholine $ACh$ as their neurotransmitter, and provides an overview of different types of neurotransmitters.

The process of synaptic transmission at a cholinergic synapse is described in detail:

1. Arrival of an action potential: The action potential reaches the synaptic knob of the pre-synaptic neuron. This stimulates voltage-gated calcium ion channels to open. Calcium ions diffuse into the synaptic knob.
2. Fusion of vesicles: The influx of calcium ions causes synaptic vesicles to fuse with the pre-synaptic membrane. Acetylcholine is released into the synaptic cleft via exocytosis.
3. Diffusion of ACh: ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the post-synaptic membrane. This causes sodium ion channels to open in the post-synaptic neuron, leading to depolarization. If the threshold is reached, an action potential is generated in the post-synaptic neuron.
4. Removal of ACh: ACh is removed from the synaptic cleft to stop the response. It is broken down by acetylcholinesterase. The breakdown products are absorbed by the pre-synaptic neuron and used to make more ACh.

Example: In a neuromuscular junction, ACh released from a motor neuron binds to receptors on a muscle fiber, triggering muscle contraction.

The page also introduces the concept of different types of neurotransmitters:

• Excitatory neurotransmitters: These depolarize the post-synaptic membrane, making it more likely to fire an action potential if the threshold is reached. Acetylcholine is given as an example.

Highlight: The type of neurotransmitter and receptor determines whether a synapse is excitatory or inhibitory, influencing the overall signaling in neural circuits.

This information is crucial for understanding the mechanism of synaptic transmission in the nervous system and the specific steps of synaptic transmission at cholinergic synapses.

Action Potential and Resting Potential

This page introduces the concept of action potentials and explains the resting potential of neurons.

An action potential is defined as a self-propagating wave of electrical activity that travels along the neuron membrane. It involves a temporary reversal of the electrical potential difference across the membrane, changing from a resting state to an excited state.

The neuron membrane contains several important components:

• Sodium-potassium pumps for active transport
• Voltage-gated sodium and potassium ion channels
• Some ion channels that are always open
• Ligand-gated ion channels that bind to neurotransmitters

The resting potential is explained as the state when a neuron is not stimulated. In this state, the outside of the membrane is positively charged compared to the inside, creating a polarized membrane with a potential difference of about -70 mV.

Definition: The resting potential is the electrical potential difference across a neuron's membrane when it is not being stimulated or conducting impulses.

The document details how the resting potential is maintained through the movement of sodium and potassium ions:

• Sodium ions are actively pumped out of the axon
• Potassium ions are actively pumped into the axon
• Sodium ions accumulate outside the axon
• Potassium ions accumulate inside the axon
• Some ion diffusion occurs through always-open channels
• Most voltage-gated channels remain closed

Highlight: The inside of the axon is negatively charged compared to the outside, with a difference of about -70 mV in the resting state.

This information provides a crucial foundation for understanding how neurons maintain their electrical properties and prepare for signal transmission.