Central pattern generators

Why it matters

Walking does not need the brain to time every step. Across the vertebrates studied, from lampreys to humans, a spinal network called the central pattern generator (CPG) can under some conditions produce basic, coordinated, rhythmic locomotor movements even without descending or peripheral input ^[1].

What it is

Rhythmic, stereotyped behaviours such as walking, flying and swimming are controlled largely by a neuronal network generally called the CPG for locomotion ^[1]. Early in the twentieth century Thomas Graham Brown showed that it is located centrally and made of sets of spinal interneurons, rather than being of peripheral origin as Sherrington had concluded ^[1].

How it works

Graham Brown proposed a half-centre model: two groups of spinal neurons, reciprocally organised and mutually inhibiting each other, able to produce the basic rhythm and pattern of stepping ^[1].

1. While the extensor half-centre is active it excites extensor motor neurons and inhibits the flexor half-centre ^[1].
2. After a period of depression of the extensor half-centre, for example fatigue or adaptation, the flexor half-centre takes over for the next phase ^[1].

Half-centre model of the locomotor pattern generatorTwo groups of spinal interneurons inhibit each other. While the extensor half-centre is active it drives extensor motor neurons and silences the flexor half-centre; when it tires, the flexor half-centre takes over.

Step through

• Excitatory synapse (filled arrowhead)
• Inhibitory synapse (bar)
• Modulatory (open circle)
• Signal or data flow, not a synapse (dashed)
• Midline crossing (decussation)

• Graham Brown proposed the model in the early twentieth century; intracellular recordings in the 1960s gave the first cellular evidence.^[1]

Simplified: Real locomotor networks contain many interneuron types on both sides of the cord; the two half-centres stand for them.

Text description of this diagram

1. Two groups of spinal interneurons inhibit each other. While the extensor half-centre is active it drives extensor motor neurons and silences the flexor half-centre; when it tires, the flexor half-centre takes over.
2. Extensor half-centre (left side, Half-centres) to Flexor half-centre (left side, Half-centres); inhibitory synapse; mutual inhibition.
3. Extensor half-centre (left side, Half-centres) to Extensor motor neurons (left side, Motor neurons); excitatory synapse.
4. Extensor motor neurons (left side, Motor neurons) to Extensors (left side, Muscles); excitatory synapse.
5. Flexor half-centre (left side, Half-centres) to Extensor half-centre (left side, Half-centres); inhibitory synapse; after the extensor side tires.
6. Flexor half-centre (left side, Half-centres) to Flexor motor neurons (left side, Motor neurons); excitatory synapse.
7. Flexor motor neurons (left side, Motor neurons) to Flexors (left side, Muscles); excitatory synapse.

When it does not behave

Key rhythm-generating elements of the CPG were found in segments L1 and L2 in mice, and in people with spinal cord injury epidural stimulation near L1 to L2 triggered locomotor-like movements of the legs ^[1]. Changes in parts of the CPG may also contribute to conditions with spontaneous leg movements ^[1].

Frontier

Stimulating the lumbar cord in time with intended movement is now being tested to help people with spinal cord injury walk. Each entry shows its evidence tier ^{[3, 4]}.

• T1T1 peer-reviewed human studyStimulation timed to the intended step restores walking2018-10 · as of 2026-10-02 · trial

  Trains of spatially selective stimulation were delivered to the lumbosacral cord at the moment each leg movement was intended. In people injured more than four years earlier, adaptive control of paralysed muscles returned within a week; after months of rehabilitation some regained voluntary control without stimulation.

  What it does not show: A small group with incomplete or chronic injuries; walking needed the implant and assistance, and results do not predict any one person's recovery.

  Sources and details
• T1T1 peer-reviewed human studyActivity-specific stimulation after complete paralysis2022-02 · as of 2026-10-02 · trial

  A new paddle lead was arranged to target the dorsal roots for leg and trunk movements, with software for activity-specific stimulation programs. Within a single day, three people with complete sensorimotor paralysis could stand, walk, cycle, swim and control trunk movements with stimulation.

  What it does not show: Three participants; the movements need the stimulation programs to run, and the cord injury itself is not healed.

  Sources and details

Common misconceptions

Check yourself

What is a half-centre?

One of two groups of spinal neurons that inhibit each other; while one is active it drives its muscles and suppresses the other ^[1].

Which segments did epidural stimulation target to trigger stepping-like movements?

The region near L1 to L2 ^[1].

Read next

• Walking rhythm lab.
• Brain–spine interfaces.