A relay neuron, also called an interneuron, is a nerve cell that sits between other nerve cells and carries messages from one to the next inside the brain and spinal cord. Most of the brain’s neurons are of this in-between kind, rather than the ones that sense the world or move muscles. The name undersells them, because relay neurons decide how much of a message gets through, not just whether it arrives.1PubMed. Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system
Where Relay Neurons Fit Among the Three Neuron Types
Sensory neurons carry signals from the skin, eyes and other organs toward the spinal cord and brain. Motor neurons carry commands out to muscles and glands. Relay neurons stay entirely inside the brain and spinal cord and connect the two. Textbooks also call them connector or association neurons.
“Inter” means between, which describes where the cell sits, and “relay” describes passing a message onward. Both names describe the same cell, so an exam may use either.
On an exam diagram, sensory neurons usually show a cell body off to one side of the fibre. Motor neurons have a cell body at one end and a long fibre running to a muscle. The relay neuron sits in the middle, drawn short inside the spinal cord. The arrow runs sensory, then relay, then motor.
The shape matches the job. Relay neurons are usually short and heavily branched, while motor neurons can have fibres a metre long. A relay neuron only needs to reach nearby cells, so it spends its branches on making many connections. Many never send anything out of their local area.2PubMed. Morphology of identified relay cells and interneurons in the dorsal lateral geniculate nucleus of the rat
The textbook picture of one relay neuron joining one sensory neuron to one motor neuron is a teaching model, so draw it as asked. Real circuits are messier: many sensory inputs converge on a relay neuron, and one relay neuron fans out to many targets. That lets a single cell combine information from several sources, which a plain one-to-one link could never do.
How a Signal Passes Through a Relay Neuron
Neurons do not physically fuse. Messages jump between cells at contact points rather than flowing through one continuous web.3PubMed Central. Cajal and the discovery of the Golgi method: a neuroanatomist’s dream These contact points, called synapses, are tiny gaps where a relay neuron receives and passes on messages. At a synapse the incoming cell releases chemical messengers that cross the gap and act on the next cell.
On the receiving side, the messengers fit onto matching docking sites on the surface of the relay neuron, a bit like a key in a lock. Once docked, they open tiny channels that let charged particles flow in or out, nudging the cell’s electrical charge up or down. A ‘go’ messenger pushes the charge toward the firing point, and a ‘stop’ messenger pushes it away. The messengers are then cleared away or taken back up quickly, so the junction is ready for the next signal.
Each relay neuron receives many inputs at once, some saying ‘go’ and some saying ‘stop’. It does not count them one at a time. It sums the pushes and pulls arriving across its branches within a very short window. A single weak ‘go’ usually fails to make the cell fire, but several arriving together can reach the firing point. A ‘stop’ arriving at the same moment can cancel some of the ‘go’, so the same excitation may or may not trigger firing depending on what else is happening.
If the combined push is strong enough, the cell fires an all-or-nothing electrical pulse along its fibre. The decision is a yes or no, and the strength of a message is carried by how often the cell fires. In some relay cells a signal arriving anywhere on the branches spreads almost unchanged to the cell body.4PubMed Central. Dendritic current flow in relay cells and interneurons of the cat’s lateral geniculate nucleus In others, a signal on a distant branch affects only that branch, so parts of one cell can work independently.5PubMed Central. Dendritic current flow in relay cells and interneurons of the cat’s lateral geniculate nucleus
Take a reflex arc. A sensory neuron fires and releases chemicals at its junction in the spinal cord. The relay neuron’s branches pick these up and the charge builds. The relay neuron fires its own pulse along its short fibre and releases chemicals onto the motor neuron, which fires toward the muscle.
Each neuron makes a brand-new pulse rather than passing the old one along. The chemical step at each gap is what lets signals be changed, and it is also why each extra junction adds a tiny delay.
Relay Neurons Change the Message, Not Just Carry It
Many relay neurons are inhibitory, meaning their message makes the next cell less likely to fire, so it acts as a ‘stop’ signal. They release a chemical called GABA. A nervous system with only ‘go’ signals would spiral into runaway activity, so brakes are needed for any circuit to stay controlled. Because the brake acts on the receiving cell’s charge, the same cell can be pushed up by one input and pulled down by another in the same moment.
In a common wiring pattern, the same incoming signal excites both a target cell and a relay neuron that then puts the brake on that target.6PubMed Central. Feed-forward inhibition as a buffer of the neuronal input-output relation The target gets a quick burst followed by a brake, which keeps its response steady even when input is noisy or very strong.7PubMed Central. Feed-forward inhibition as a buffer of the neuronal input-output relation Because the brake arrives slightly late, the start of the signal gets through clearly. Without it, strong input would overwhelm the cell and it would stop responding usefully. The brake also sharpens timing, helping groups of cells fire together.8eLife. Fast and slow feedforward inhibitory circuits for cortical odor processing
Other relay neurons amplify. In the spinal cord, one group links up into a network that can start and keep up a strong excitatory signal.9eLife. Calretinin positive neurons form an excitatory amplifier network in the spinal cord dorsal horn That network passes its signal straight to cells that carry pain toward the brain.10eLife. Calretinin positive neurons form an excitatory amplifier network in the spinal cord dorsal horn In mice, switching these cells on produced pain-like behaviour, and switching them off made the mice less sensitive.11eLife. Calretinin positive neurons form an excitatory amplifier network in the spinal cord dorsal horn
Braking and amplifying come from different relay neurons in different circuits. Which effect a cell has depends on its chemical messenger and on what it connects to. Amplifying pain on purpose can warn of danger, but it is harmful if it stays switched on.
Relay neurons can also learn. Connections onto relay neurons in the memory area of the brain can strengthen with use and stay stronger for at least 30 minutes.12PubMed Central. Long-term potentiation at pyramidal cell to somatostatin interneuron synapses controls hippocampal network plasticity and memory These relay neurons have two separate ways of strengthening, and two matching ways of weakening.13PubMed. Long-term synaptic plasticity in hippocampal interneurons Strengthening means that after a burst of activity the same input produces a bigger response than it did before. The change is physical, at the junction, so the connection itself works differently afterwards. Two routes each way let a connection be turned up or down in more than one manner, giving the circuit more flexibility.
Relay Neurons in the Spinal Cord and Brain
Touch fibres activate a spinal relay neuron that puts a brake on the cells sending pain upward, closing a ‘gate’ on pain.14PubMed Central. Transmitting pain and itch messages: A contemporary view of the spinal cord circuits that generate Gate Control Pain fibres open the gate by exciting those cells and silencing the braking relay neurons.15PubMed Central. Transmitting pain and itch messages: A contemporary view of the spinal cord circuits that generate Gate Control Gentle touch signals compete with pain signals at the same spinal relay cells, which is why you instinctively rub or shake a bumped elbow or stubbed toe. The relief is usually partial and short-lived, and works best for sharp, brief pain rather than serious injury. The gate is a simplification of a more complex system, and pain also depends on the brain’s own processing.
Pain pathways cross over. Fibres carrying pain and temperature cross to the opposite side of the spinal cord close to where they enter it.16Wikipedia. Spinothalamic tract Because of the crossing, damage to one side of the cord can affect pain and temperature on the other side of the body. Touch and position sense follow a different route, so one injury can disturb different sensations on opposite sides.
Relay neurons also handle walking and organ control. Long ones running down the spinal cord coordinate the legs; removing them in mice upset hind-leg coordination, especially at speed.17PubMed Central. Propriospinal Neurons: Essential Elements of Locomotor Control in the Intact and Possibly the Injured Spinal Cord Separate relay neurons control bladder and bowel reflexes, with little overlap between them.18Central Regulation of Autonomic Functions. Spinal Interneurons in the Control of Autonomic Function Others in the spinal cord help the blood-pressure reflex by putting a brake on nerves that speed the heart and tighten vessels.19PubMed. Spinal interneurons in the baroreceptor reflex arc
Reflexes show the difference a relay neuron makes. The knee-jerk links sensory and motor neurons directly, with no relay neuron. Pulling a hand off a hot stove goes through a relay neuron in the spinal cord, while the pain signal also travels up to the brain, so you often feel the pain slightly after you move. It can also brake the opposite muscle so the arm moves smoothly.
In the brain, the thalamus sits deep inside and relays sight, sound and touch signals to the outer brain.20PubMed. Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system Almost every sense except smell goes through it, which is why it is called the brain’s relay station. Some of its relays pass signals from the senses, while others pass signals between areas of the outer brain.21PubMed. Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system
The brain’s relay station does not pass everything on equally. Feedback from the outer brain changes what it lets through according to what you are paying attention to.22PubMed. Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system This feedback lets its cells keep up with faster streams of input.23PubMed Central. Cortical control of adaptation and sensory relay mode in the thalamus The result is that the station can switch between a mode suited to noticing something and one suited to telling details apart.24PubMed Central. Thalamic Synchrony and the Adaptive Gating of Information Flow to Cortex
When Relay Neurons Go Wrong
Faults in inhibitory relay neurons are linked to some epilepsies that begin in early life.25PubMed Central. Interneuronopathies and their role in early life epilepsies and neurodevelopmental disorders Without enough ‘stop’ signals, brain activity can spread out of control as a seizure. The brain relies on a balance of ‘go’ and ‘stop’, and many epilepsy medicines work by strengthening the ‘stop’ chemical GABA. Too little braking can also disturb timing between brain areas, not just cause seizures. Inhibitory relay neurons are a small share of the cells but act on many others, so a fault in a few can affect a large circuit.
Relay-neuron problems are also involved in autism and schizophrenia, which need not include seizures.26PubMed Central. Interneuronopathies and their role in early life epilepsies and neurodevelopmental disorders This means researchers see altered braking or timing in circuits, not that faulty relay neurons are the single cause of either condition. Genetics and development contribute too.
Development matters as well. Relay neurons are born in one place and must travel to the right position in the developing brain, and the route they take shapes the connections they form.27PubMed Central. Optimization of interneuron function by direct coupling of cell migration and axonal targeting A cell that arrives in the wrong spot may wire up wrongly, and such early errors can show up as symptoms in childhood.
Can Relay Neurons Help After Spinal Cord Injury
In mice whose long brain-to-spine fibres were fully cut, chains of short spinal relay neurons bridged the injury and restored stepping controlled by the brain.28PubMed Central. Recovery of supraspinal control of stepping via indirect propriospinal relay connections after spinal cord injury In lampreys, similar relay neurons carried the brain’s command to the spinal walking circuits after injury, though movements were less well coordinated.29PubMed Central. Descending propriospinal neurons mediate restoration of locomotor function following spinal cord injury Recovery may not need the original long fibres to regrow.
The original fibres run from the brain to the lower spine in one piece, while a relay chain passes the message through several short cells, one junction at a time. Each added junction means a new chemical step, which is one reason the detour is slower and less precise. That fits the stepping, but not perfectly coordinated movement, seen in the lamprey and mouse recoveries. Rehabilitation exercise may help strengthen these detours.
Transplants are a different idea. Natural detours use relay neurons already in the spinal cord, while transplants add new cells to build fresh connections. Human spinal relay neurons transplanted into injured rats both received and sent working connections with the rats’ motor circuits.30PubMed. Human spinal interneurons repair the injured rat spinal cord through synaptic integration That is a necessary first step rather than a cure. This is animal research, and no such treatment is yet available for people. Questions remain about safety, how long the cells survive and whether they improve useful movement.