Chapter 12 – Neurophysiology: Synapses, Neurotransmitters, and Nerve Impulse Transmission

Chapter 12 – Neurophysiology: Synapses, Neurotransmitters, and Nerve Impulse Transmission

Types of Synapses

Synapses are specialized junctions that allow communication between neurons or between neurons and effector cells. There are two main types of synapses: electrical and chemical.

• Electrical Synapses: Transmission is rapid and synchronized due to direct ionic current flow through gap junctions. Common in embryonic tissue and some muscle types (e.g., cardiac and smooth muscle).

• Chemical Synapses: Most common in the nervous system. Involve the release and reception of chemical neurotransmitters. The electrical signal is converted to a chemical signal (neurotransmitter release), which then binds to receptors on the postsynaptic cell, possibly generating a new electrical signal.

Example: Electrical synapses are found in cardiac muscle for coordinated contraction, while chemical synapses are predominant in the central nervous system for complex signaling.

Events at the Chemical Synapse

Chemical synapses transmit signals via neurotransmitter release and receptor activation. The process involves several steps:

1. An impulse arrives at the axon terminal.

2. Voltage-gated Ca2+ channels open, allowing Ca2+ influx.

3. Ca2+ promotes exocytosis of neurotransmitter-containing vesicles.

4. Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

5. Ligand-gated ion channels open, causing influx of Na+ ions and initiating a new impulse.

6. Neurotransmitter action is terminated by enzymatic degradation, reuptake, or diffusion away from the synapse.

Example: Acetylcholine is released at neuromuscular junctions, triggering muscle contraction.

Neurotransmitters: Classification and Function

Neurotransmitters are chemicals that allow communication between neurons. They can be classified by chemical structure and function.

• Amino Acids: e.g., Glutamate, GABA (gamma-aminobutyric acid), Glycine.

• Monoamines: e.g., Dopamine, Norepinephrine, Serotonin.

• Soluble Gases: e.g., Nitric oxide, Carbon monoxide.

• Acetylcholine: Major neurotransmitter at neuromuscular junctions.

• Neuropeptides: e.g., Endorphins.

Amino acids	Monoamines	Soluble gases	Acetylcholine	Neuropeptides
Glutamate GABA Glycine	Dopamine Norepinephrine Serotonin	Nitric oxide Carbon monoxide	Acetylcholine	Endorphins

Example: GABA is the main inhibitory neurotransmitter in the brain.

Chemical & Physical Factors Impairing Conduction of Impulses

Several factors can impair the conduction of nerve impulses by affecting membrane permeability or blood supply.

• Reduce Membrane Permeability to Na+: Alcohol, sedatives, anesthetics block nerve impulses.

• Interrupt Blood Circulation: Cold (e.g., holding an ice cube) and continuous pressure (e.g., sitting on your leg) can cause numbness by reducing oxygen and nutrient delivery to neurons.

Example: Local anesthetics block sodium channels, preventing pain signals during dental procedures.

Excitability of Neurons

Neurons are highly excitable cells capable of generating and transmitting electrical impulses (action potentials).

• Resting Membrane Potential: An electrical voltage exists across the plasma membrane due to ion gradients.

• Ion Channels: Specific channels open and close in response to stimuli, allowing ions to move and generate action potentials.

Example: Sensory neurons respond to touch by opening ion channels and generating impulses.

Ion Channels

Ion channels are proteins in the plasma membrane that regulate the movement of ions, crucial for nerve impulse transmission.

• Passive (Leakage) Channels: Always open, allowing ions to move down their concentration gradients.

• Voltage-Gated Channels: Open or close in response to changes in membrane potential.

• Ligand-Gated Channels: Open when a neurotransmitter binds to the channel protein.

Example: Voltage-gated Na+ channels open during the initiation of an action potential.

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the plasma membrane when the neuron is not transmitting an impulse.

• K+ concentration is higher inside the cell; Na+ is higher outside.

• Membranes are more permeable to K+ than Na+.

• The sodium-potassium pump maintains these gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

Equation:

$E_{m} = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Example: Typical resting membrane potential is about -70 mV in neurons.

Nerve Impulse Transmission (Action Potential)

Nerve impulses are all-or-none electrical signals that travel along the axon. The process involves:

1. Depolarization: Na+ influx makes the inside of the cell less negative.

2. Repolarization: K+ efflux restores the negative membrane potential.

3. Propagation: The action potential moves along the axon, triggering the next segment.

Example: Touching a hot surface triggers rapid nerve impulses to the brain.

Nerve Conduction Velocity

The speed at which an impulse travels along a neuron depends on several factors:

• Temperature: Lower temperature slows conduction.

• Axon Diameter: Larger diameter fibers conduct faster.

• Myelination: Myelinated fibers conduct impulses much faster due to saltatory conduction (impulse jumps between nodes of Ranvier).

Factor	Effect on Conduction Velocity
Temperature	Lower temperature = slower conduction
Axon Diameter	Larger diameter = faster conduction
Myelination	Myelinated = much faster conduction

Example: Motor neurons controlling skeletal muscles are heavily myelinated for rapid response.

Neurotoxins

Neurotoxins are substances that disrupt the normal function of the nervous system, often by interfering with synaptic transmission or ion channel function.

• They may inhibit or promote neurotransmitter release, block ion channels, or disrupt communication between neurons and muscles.

• Sources include plants, animals, bacteria, and synthetic chemicals.

Neurotoxin	Source/Notes
Botulinum Toxin	Bacteria (in canned foods); causes paralysis
Teradotoxin	Pufferfish; blocks Na+ channels
Tetanus Toxin	Bacteria; affects neurotransmitter release
Lead	Paint; disrupts nervous system development
Ethanol	Alcohol; impairs synaptic function
Glutamate (high levels)	Excitotoxicity
Chlorotoxin	Scorpion venom
Conotoxin	Cone snail venom
Mercury	Inhaled, ingested, absorbed; neurodevelopmental effects

Example: Botulinum toxin (BOTOX) is used medically in small doses to reduce muscle spasticity.

Additional info: This summary expands on the provided slides and notes with definitions, examples, and academic context for clarity and completeness.