Nervous Tissue: Synapses, Neurotransmitters, and Neuromodulation

Nervous Tissue

Introduction to Nervous Tissue

The nervous tissue is a fundamental component of the human nervous system, responsible for transmitting and processing information throughout the body. It consists of neurons and supporting cells, which work together to coordinate bodily functions and responses to stimuli.

• Neurons: Specialized cells that transmit electrical and chemical signals.

• Neuroglia: Supporting cells that provide structural and metabolic support to neurons.

• Function: Enables rapid communication and integration of information.

Neurotransmission at Synapses

Structure and Function of a Synapse

A synapse is a specialized junction where a neuron communicates with another cell, which may be another neuron, a muscle cell, a gland cell, or an adipocyte. Synapses are essential for the transmission of information between cells in the nervous system.

• Presynaptic neuron: The neuron sending the signal.

• Postsynaptic cell: The cell receiving the signal (can be a neuron, muscle, or gland cell).

• Synaptic cleft: The narrow gap between the presynaptic and postsynaptic membranes.

• Types of synapses:

  • Electrical synapse: Direct cytoplasmic connection via gap junctions; rapid transmission.

  • Chemical synapse: Communication via neurotransmitter release; more variable and modifiable.

Additional info: Most synapses in the human nervous system are chemical synapses.

Events at a Chemical Synapse

Chemical synapses involve a series of coordinated events that allow the transfer of information from one neuron to another.

1. Action potential arrives at the presynaptic axon terminal.

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

3. Exocytosis of synaptic vesicles releases neurotransmitters into the synaptic cleft.

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

5. Postsynaptic response: Binding of neurotransmitters to receptors alters membrane permeability, leading to depolarization (excitatory) or hyperpolarization (inhibitory).

6. Termination: Neurotransmitters are removed by enzymatic degradation, reuptake, or diffusion.

Neurotransmitters and Neuromodulators

Major Types and Their Effects

Neurotransmitters are chemical messengers released by neurons to transmit signals across synapses. Neuromodulators are substances that modify the activity of neurons, often by altering neurotransmitter release or postsynaptic response.

• Excitatory neurotransmitters: Cause depolarization and promote action potential generation (e.g., acetylcholine, glutamate).

• Inhibitory neurotransmitters: Cause hyperpolarization and suppress action potential generation (e.g., GABA, glycine).

• Neuromodulators: Typically neuropeptides; alter the rate of neurotransmitter release or postsynaptic response (e.g., substance P, endorphins).

Examples of neurotransmitters:

• Acetylcholine (ACh): Widely used at neuromuscular junctions and many neuron-to-neuron synapses.

• Catecholamines: Dopamine (DA), Norepinephrine (NE), Epinephrine (EP).

• Monoamines: Serotonin (5-HT), Histamine.

• Lipid-soluble gases: Nitric oxide (NO), Carbon monoxide (CO).

Additional info: Lipid-soluble gases act as neurotransmitters by diffusing across membranes and activating intracellular enzymes, leading to effects such as vasodilation.

Mechanisms of Action (MOA)

Neurotransmitters and neuromodulators affect postsynaptic cells through three main mechanisms:

1. Direct effect via ligand-gated ion channels: Neurotransmitter binds to ionotropic receptor, causing ion channel to open and membrane potential to change.

2. Indirect effect via G protein-coupled receptors (GPCR): Neurotransmitter binds to metabotropic receptor, activating intracellular signaling cascades and second messengers.

3. Indirect effect via lipid-soluble gases: Gases diffuse into the cell and activate cytoplasmic enzymes without the need for membrane receptors.

Equation for membrane potential change:

$ext{Membrane Potential Change} = ext{Ion Flux} imes ext{Membrane Permeability}$

Synaptic Delay and Synaptic Fatigue

Synaptic Delay

Synaptic delay refers to the time required for neurotransmitter release, diffusion, and receptor activation at a chemical synapse. This delay typically ranges from 0.2 to 0.5 milliseconds.

• More synapses: Greater cumulative delay in signal transmission.

• Fewer synapses: Faster response; seen in simple reflex arcs.

Example: The fastest reflexes involve only one synapse between a sensory and a motor neuron.

Synaptic Fatigue

Synaptic fatigue occurs when the demand for neurotransmitter release exceeds the ability of the neuron to synthesize and recycle neurotransmitters, leading to a temporary reduction in synaptic transmission.

• Causes: Intensive stimulation, limited resynthesis and transport of neurotransmitters.

• Effect: Reduced synaptic efficacy and possible failure of signal transmission.

Fate of Neurotransmitters

Removal and Recycling

After neurotransmitters are released and bind to receptors, they must be removed to terminate the signal and prepare the synapse for subsequent transmission.

• Enzymatic degradation: Enzymes such as acetylcholinesterase (AChE) break down neurotransmitters (e.g., ACh into acetate and choline).

• Reuptake: Neurotransmitter or its metabolites are actively transported back into the presynaptic neuron for reuse.

• Diffusion: Neurotransmitters diffuse away from the synaptic cleft and are absorbed by surrounding cells.

Example: Choline is reabsorbed by the presynaptic neuron to synthesize new acetylcholine molecules.

Classification Table: Neurotransmitters and Neuromodulators

Type	Examples	Main Effect	Mechanism
Excitatory Neurotransmitter	Acetylcholine, Glutamate	Depolarization	Ligand-gated ion channels
Inhibitory Neurotransmitter	GABA, Glycine	Hyperpolarization	Ligand-gated ion channels
Neuromodulator	Substance P, Endorphins	Modulation of synaptic activity	GPCRs, second messengers
Lipid-soluble Gas	Nitric oxide, Carbon monoxide	Vasodilation, modulation	Direct activation of enzymes

Summary Questions

• Describe the general structure of a synapse.

• What effect would blocking voltage-gated calcium ion channels at a cholinergic synapse have on synaptic communication?

• What determines the speed of impulse transmission in neural pathways with different numbers of neurons?

• What is the difference between a neurotransmitter and a neuromodulator?

• What are the three mechanisms by which neurotransmitters and neuromodulators affect postsynaptic membrane potentials?