Introduction to neurotransmitters and receptors

Neurons often communicate at specialised junctions called chemical synapses. When an electrical signal reaches the end of one neuron, chemical messengers can be released into the small gap between cells. These messengers cross the gap and bind to receptors on the receiving cell.

A receptor is a protein or specialised structure that recognises a particular chemical signal or physical stimulus. The result depends on the receptor and the cells involved: a message may increase activity, reduce activity, or change how the cell responds later.

Neurotransmitters

Neurotransmitters are chemical messengers released by neurons. At a chemical synapse, release is usually triggered when an arriving electrical signal opens calcium channels in the nerve terminal. Vesicles then merge with the cell membrane and release their contents into the synaptic cleft.

The messenger binds to receptors on the next cell. Some receptors directly open ion channels, while others activate slower signalling pathways inside the cell. Afterward, the messenger may be broken down, taken back up by a cell, or removed by diffusion.

How neurotransmitters are grouped

A useful introductory distinction is between small, rapidly acting messengers and larger neuropeptides. Small messengers are well suited to quick communication, including many sensory and motor signals. Neuropeptides are produced and transported differently and often alter cell activity for longer periods.

Small, rapidly acting neurotransmitters

Small neurotransmitters are commonly made in or near the nerve terminal and stored in small vesicles. Their release and action can occur very quickly. The effect is not determined by the messenger alone; the receptor, location, and surrounding cells all matter.

Acetylcholine

Acetylcholine is used in both the central and peripheral nervous systems. It helps carry signals to skeletal muscle and is important in several autonomic pathways. Depending on the receptor, it can excite or inhibit a target cell.

Norepinephrine

Norepinephrine participates in attention, arousal, and autonomic regulation. Its effect varies according to the receptor subtype and the tissue receiving the signal.

Dopamine

Dopamine is involved in movement, motivation, learning, and the regulation of some hormones. Different brain pathways use dopamine in different ways, so it is too simple to describe it as only excitatory or only inhibitory.

Glycine

Glycine commonly reduces neuronal activity in parts of the spinal cord and brainstem. It contributes to the coordination of sensory and motor information.

GABA

Gamma-aminobutyric acid, usually called GABA, is a major inhibitory messenger in the brain. By changing ion flow across the cell membrane, GABA can make it less likely that a neuron will generate an electrical signal.

Glutamate

Glutamate is a major excitatory messenger in the central nervous system. It contributes to communication between neurons and to processes involved in learning and memory.

Serotonin

Serotonin helps regulate communication in the brain and digestive system. It is involved in mood, sleep, appetite, and other functions, with effects that depend on its receptor type and location.

Nitric oxide

Nitric oxide differs from many vesicle-stored messengers because it can be made when needed and diffuse across cell membranes. It helps influence signalling and longer-lasting changes in some neural circuits.

Read more about receptors

Neuropeptides

Neuropeptides are chains of amino acids made in the cell body and transported to nerve terminals. They are released in response to particular patterns of activity and often act through receptors that trigger longer-lasting changes. Examples include endorphins, enkephalins, substance P, somatostatin, and several hormone-related peptides.

Receptors

Receptors detect chemical messengers or physical changes in the body. In sensory physiology, receptors are often grouped by the stimulus they detect:

• Mechanoreceptors: respond to touch, pressure, stretch, or vibration.
• Thermoreceptors: respond to changes in temperature.
• Nociceptors: respond to potentially damaging stimuli and contribute to pain sensation.
• Photoreceptors: detect light in the retina.
• Chemoreceptors: detect chemicals, including those involved in taste, smell, and internal regulation.

Receptors are selective rather than perfectly exclusive. Their sensitivity, location, and connections help the nervous system distinguish different kinds of information.

Mechanoreceptors

Mechanoreceptors respond when tissue is deformed. Touch receptors in the skin, stretch receptors in muscles, and sensors involved in balance are examples of mechanical sensing.

Thermoreceptors

Thermoreceptors are free nerve endings that respond to warming or cooling. Their distribution and sensitivity vary across the body.

Nociceptors

Nociceptors detect conditions that may damage tissue. Their signals contribute to protective responses and the experience of pain.

Photoreceptors

Rods and cones in the retina detect different properties of light. Their signals are then processed through visual pathways.

Chemoreceptors

Chemoreceptors respond to chemicals in the environment or body fluids. Taste and smell receptors are familiar examples, while other chemoreceptors help monitor internal conditions.

Summary

Neurotransmitters carry chemical messages across synapses. Their effects depend on the receptors they reach and the cells in which those receptors are found. Some messengers act quickly through ion channels, while others activate slower intracellular pathways.

Neuropeptides generally take a different route of synthesis and can produce longer-lasting changes. Sensory receptors detect mechanical, thermal, painful, light-based, or chemical information and convert it into signals the nervous system can process.

References

For further reading, consult an up-to-date physiology or neuroscience textbook and reliable clinical education resources. Terminology and classifications can vary slightly between sources.