Neurotransmitters

Neurotransmitters

Chemical messengers that neurons use to send signals across synapses to other cells, or function as hormones to regulate actions like heart rate, movement, mood, and sleep

Neurosecretory Cells

Neurons in the brain that secrete hormones.

Neurotransmitters: Classification

Neurotransmitters are divided into two classes based on their size: small-molecule neurotransmitters and neuropeptides.

Small-Molecule Neurotransmitters

Small-molecule neurotransmitters are neurotransmitters that are relatively small in size (e.g., acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide)

Small-Molecule Neurotransmitters: Acetylcholine (ACh)

A neurotransmitter released by many PNS neurons and some CNS neurons that functions as an excitatory or inhibitory neurotransmitters depending on where they bind to

Small-Molecule Neurotransmitters: Acetylcholine (ACh): Excitatory Effect

ACh acts as an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, where it binds to ionotropic receptors that open cation channels, producing depolarization.

Small-Molecule Neurotransmitters: Acetylcholine (ACh): Inhibitory Effect

ACh acts as an inhibitory neurotransmitter at other synapses by binding to metabotropic receptors that are coupled to G proteins that open K⁺ channels, causing hyperpolarization and inhibition.

Small-Molecule Neurotransmitters: Acetylcholine (ACh): Acetylcholinesterase (AChE)

An enzyme that inactivates acetylcholine by breaking it down into acetate and choline fragments, ending its effects.

Small-Molecule Neurotransmitters: Amino Acid

Several amino acids function as neurotransmitters in the CNS.

Small-Molecule Neurotransmitters: Amino Acid: Example

Glutamate and aspartate are powerful excitatory neurotransmitters in the CNS that have powerful excitatory effects

Small-Molecule Neurotransmitters: Amino Acid: Glutamate: CNS Role

Most excitatory neurons in the CNS and perhaps half of the synapses in the brain communicate using glutamate.

Small-Molecule Neurotransmitters: Amino Acid: Glutamate: Ionotropic Receptors

At some glutamate synapses, glutamate binds to ionotropic receptors that open cation channels, allowing mainly Na⁺ to enter and causing an EPSP.

Small-Molecule Neurotransmitters: Amino Acid: Glutamate: Inactivation

Glutamate is then removed from the synaptic cleft by reuptake, where glutamate transporters actively transport it back into the synaptic end bulbs and neighboring neuroglia.

Small-Molecule Neurotransmitters: Amino Acid: Inhibitory Neurotransmitters.

Gamma- aminobutyric acid (GABA) and glycine are important inhibitory neurotransmitters found only in the CNS.

GABA: Ionotropic Receptors

GABA binds to ionotropic receptors that open Cl⁻ channels at many synapses, allowing Cl⁻ to enter the postsynaptic cell, causing hyperpolarization and inhibition.

GABA: Brain Synapses

GABA is the most common inhibitory neurotransmitter in the CNS, as many as one-third of all brain synapses use GABA.

Glycine: Ionotropic Receptors

Glycine binds to ionotropic receptors that open Cl⁻ channels, allowing Cl⁻ to enter the postsynaptic cell, causing hyperpolarization and inhibition.

Glycine: Spinal Cord

About half of the inhibitory synapses in the spinal cord use glycine.

Small-Molecule Neurotransmitters: Biogenic Amines

Neurotransmitters are produced when certain amino acids are modified, their carboxyl group removed (decarboxylated).

Small-Molecule Neurotransmitters: Biogenic Amines: Examples

The major biogenic amines in the nervous system include norepinephrine, epinephrine, dopamine, and serotonin.

Small-Molecule Neurotransmitters: Biogenic Amines: Receptors

Most biogenic amines bind to metabotropic receptors that produce either excitation or inhibition depending on the type of metabotropic receptor at the synapse.

Small-Molecule Neurotransmitters: Biogenic Amines: Norepinephrine

Norepinephrine plays roles in arousal, dreaming, and regulation of mood.

Small-Molecule Neurotransmitters: Biogenic Amines: Epinephrine

A smaller number of neurons in the brain use epinephrine as a neurotransmitter.

Small-Molecule Neurotransmitters: Biogenic Amines: Epinephrine and Norepinephrine: Hormones

Both epinephrine and norepinephrine also function as hormones when released into the bloodstream from the cells of the suprarenal medulla, the inner portion of the suprarenal gland

Small-Molecule Neurotransmitters: Biogenic Amines: Dopamine

Dopamine is involved in emotional responses, addictive behaviors, pleasurable experiences, skeletal muscle tone, and some aspects of movement.

Small-Molecule Neurotransmitters: Biogenic Amines: Catecholamines

Catecholamines are a chemical group that includes norepinephrine, dopamine, and epinephrine because they all contain an amino group (—NH₂) and a catechol ring of six carbons with two hydroxyl (—OH) groups

Small-Molecule Neurotransmitters: Biogenic Amines: Catecholamines: Synthesis

Catecholamines are synthesized from the amino acid tyrosine.

Small-Molecule Neurotransmitters: Biogenic Amines: Catecholamines: Inactivation

Catecholamines are inactivated mainly by reuptake into synaptic end bulbs, where they are either recycled into synaptic vesicles or broken down by enzymes.

Small-Molecule Neurotransmitters: Biogenic Amines: Catecholamines: Breakdown Enzymes

The two enzymes that break down catecholamines are catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO).

Small-Molecule Neurotransmitters: Biogenic Amines: Serotonin

Serotonin, also known as 5-hydroxytryptamine (5-HT), is involved in sensory perception, temperature regulation, mood control, appetite, and sleep induction— concentrated in neurons in a part of the brain called the raphe nucleus,

Small-Molecule Neurotransmitters: ATP and Other Purines: Adenosine

A natural organic compound found in every cell of the human body that functions as an excitatory neurotransmitter in both the CNS and the PNS within its triphosphate, diphosphate, and monophosphate derivatives (ATP, ADP, and AMP)

Small-Molecule Neurotransmitters: Nitric Oxide

A simple gas that acts as an important excitatory neurotransmitter secreted in the brain, spinal cord, suprarenal glands, and the penis, having widespread effects throughout the body.

Small-Molecule Neurotransmitters: Nitric Oxide: Nitric Oxide Synthase

The enzyme nitric oxide synthase (NOS) produces NO from the amino acid arginine.

Small-Molecule Neurotransmitters: Nitric Oxide (NO): Synthesis and Storage

Unlike most neurotransmitters, NO is produced on demand and exists for less than 10 seconds before it combines with oxygen and water to form inactive nitrates and nitrites

Small-Molecule Neurotransmitters: Nitric Oxide (NO): Memory and Learning

Some research suggests that NO plays a role in memory and learning because researchers discovered that endothelium-derived relaxing factor (EDRF) was actually nitric oxide in 1987

Small-Molecule Neurotransmitters: Nitric Oxide (NO): EDRF

Endothelial cells release NO, which diffuses into nearby smooth muscle fibers and causes them to relax, thereby increasing blood vessel diameter to lower blood pressure

Small-Molecule Neurotransmitters: Nitric Oxide (NO): Immune Function

Phagocytic cells, including macrophages and certain white blood cells, produce NO to kill microbes and tumor cells.

Small-Molecule Neurotransmitters: Carbon Monoxide (CO)

A colorless, odorless, and tasteless gas that functions as an excitatory neurotransmitter, acting similarly to nitric oxide (NO), which is produced in the brain and in response to some neuromuscular and neuroglandular functions.

Small-Molecule Neurotransmitters: Carbon Monoxide (CO): Function

CO may help protect neurons against excessive neuronal activity, relating to blood vessel dilation, memory, sense of smell, vision, thermoregulation, insulin release, and anti-inflammatory activity.

Neuropeptides

Neuropeptides are neurotransmitters made of 3–40 amino acids linked together by peptide bonds, often found throughout the CNS and PNS.

Neuropeptides: Effects

Neuropeptides can have either excitatory or inhibitory effects depending on the type of metabotropic receptor they activate.

Neuropeptides: Formation

Neuropeptides are produced in the neuron's cell body, packaged into vesicles, and transported to axon terminals.

Neuropeptides: Hormones

Many neuropeptides also function as hormones that regulate physiological responses elsewhere in the body.

Neuropeptides: Opioid Peptides

Neuropeptides that act on opioid receptors and can produce pain-relieving effects.

Neuropeptides: Opioid Peptides: Enkephalins

Enkephalins are first known naturally occurring opioid peptides made of chains of five amino acids that bind to the same receptors as morphine and heroin.

Neuropeptides: Opioid Peptides: Enkephalins: Pain Relief

Enkephalins have powerful analgesic (pain-relieving) effects, reported as about 200 times stronger than morphine.

Neuropeptides: Opioid Peptides: Examples

Enkephalins, endorphins, and dynorphins are examples of opioid peptides.

Neuropeptides: Opioid Peptides: Other Functions

Opioid peptides have been linked to memory and learning, pleasure or euphoria, body temperature control, and regulation of hormones involved in puberty, sexual drive, and reproduction.

Neuropeptides: Opioid Peptides: Mental Health

Opioid peptides have been linked to mental illnesses such as depression and schizophrenia.

Neuropeptides: Substance P: Definition

A neuropeptide released by neurons that transmit pain-related signals from peripheral pain receptors into the CNS, enhancing the perception of pain

Enkephalin and Endorphin: Effect on Substance P

Enkephalin and endorphin suppress the release of substance P, reducing the number of nerve impulses relayed to the brain for pain sensations.

Substance P: Nerve Protection

Substance P may counter the effects of certain nerve-damaging chemicals, suggesting a possible role in treating nerve degeneration.