In one breath
A painful stimulus to a limb makes the limb bend away: the withdrawal (flexor withdrawal) reflex is the classic polysynaptic spinal reflex, set off through nociceptors and the flexor reflex afferents. On the stimulated side the flexors contract and the extensors relax; a stronger stimulus also makes the opposite limb extend (the crossed extensor, or Phillipson’s, reflex), which keeps the body balanced. The reflex is protective and prepotent, shows local sign, and outlasts its stimulus because of afterdischarge.
Builds on: Pain: concepts, receptors and fibers · Stretch reflex · Leads to: Properties of spinal reflexes · Postural reflexes, spinal animal and spinal shock
What it is
- A polysynaptic spinal reflex: chains of interneurons lie between the afferent fibre and the motor neurons.
- Its other name, the flexor withdrawal reflex, says what it does: the limb flexes, and so pulls away.
Receptors and stimuli
- Receptors: nociceptors in the skin, muscles, tendons and joints.
- Stimulus: a noxious (painful) stimulus to the skin, the subcutaneous tissue or a muscle.
Reflex circuit
- The afferents are the flexor reflex afferents (FRA): the type III and type IV somatic afferents.
- In the cord, each afferent splits into two branches, and each branch ends on its own set of interneurons.
- Commissural interneurons cross the midline to the opposite ventral horn and drive a matching set of interneurons there.
The net effect on the motor neurons:
| Side | Flexors | Extensors |
|---|---|---|
| Stimulated side | Excited | Inhibited |
| Opposite side | Inhibited | Excited |
Draw it: withdrawal and crossed extension
Draw a cord cross-section with a leg on each side. On the left, bring a pain fibre (FRA) in from the skin of the foot. In the left grey matter, give it two interneurons: one ”+” to the flexor motor neuron and one ”−” to the extensor motor neuron of the left leg. Draw a commissural interneuron crossing the midline. On the right, have it drive one interneuron ”+” to the extensor motor neuron and one ”−” to the flexor motor neuron of the right leg. Label “left leg flexes (withdraws)” and “right leg extends (supports)”.
Response
The response grows with the strength of the stimulus:
| Stimulus | Response |
|---|---|
| Noxious | Stimulated limb flexes |
| Stronger | Opposite limb also extends: crossed extensor reflex |
| Stronger still | The other limbs join in: widespread response |
- The crossed extensor reflex is also called Phillipson’s reflex. Flexion pulls the limb away from the harm, and extension of the opposite limb holds the body balanced while this happens.
- Widespread response: impulses spread up and down the cord to the motor neurons of the other limbs. The other limb on the same side extends, and the other limb on the opposite side flexes.
- That spread shows three properties at work:
- irradiation of impulses up and down the cord;
- recruitment of more motor neurons and motor units;
- divergence, since each flexor reflex afferent reaches many interneurons and motor neurons.
- The whole pattern keeps the body upright and steady while the injured part is pulled clear.
Special features
- Protective: it removes the body part from a potentially damaging stimulus, while the extended opposite limb supports the body.
- Prepotent: it takes priority. While it is active the cord’s other reflexes are nearly silenced, so the withdrawal runs without delay.
- Local sign: what the limb does depends on what the stimulus is and where it strikes. Stimulate the medial surface of a limb and it abducts; stimulate the lateral surface and it adducts.
- Afterdischarge: a strong stimulus both spreads the response and stretches it out in time. The motor neurons keep firing, again and again, after the stimulus is over.
Afterdischarge: the two mechanisms
- Parallel pathways of different lengths. Many interneuron chains run from the afferent to the motor neuron. Impulses through short chains, with few interneurons, reach the motor neuron early; impulses through long chains arrive later. The motor neuron is therefore fired over and over.
- Reverberating circuits. Interneurons send branches to other interneurons that project back onto the first ones. Impulses circle round these loops and keep re-exciting the motor neurons.
Worth knowing, though not in your pages: the point of afterdischarge is that the limb stays pulled away for a moment after the pain stops, which gives the body time to get clear.
Draw it: afterdischarge
Put one afferent fibre on the left and one motor neuron on the right. Between them, draw three parallel chains that all end on the motor neuron: one with a single interneuron, one with two in series and one with three. Label them “early”, “later” and “latest”. Below, draw a loop of two interneurons that synapse on each other, with a branch to the motor neuron, and label it “reverberating circuit”. Beside the motor neuron, draw a spike train that goes on after a short stimulus bar has ended, labelled “afterdischarge”.
Stretch reflex vs withdrawal reflex
Wiring
| Stretch | Withdrawal | |
|---|---|---|
| Receptor | Muscle spindle | Nociceptors |
| Afferent | Ia | FRA (III, IV) |
| Synapses | One | Many |
| Latency | Short | Long |
| Divergence | Little | Widespread |
| Muscles driven | The stretched muscle | Same-side flexors, opposite-side extensors |
| Reciprocal innervation | Single | Double |
Behaviour
| Stretch | Withdrawal | |
|---|---|---|
| Response | Linear, stereotyped | Nonlinear, may spread |
| When the stimulus stops | Stops too | Goes on (afterdischarge) |
| Specificity | One set of muscles | Many muscle groups |
| Local sign | Absent | Present |
| Prepotent | No | Yes |
What "double" reciprocal innervation means
In the stretch reflex, reciprocal innervation acts within one limb: agonist on, antagonist off. In the withdrawal reflex it acts on both sides: flexors on and extensors off on the stimulated side, and the reverse on the opposite side.
Exam-answer skeleton: "Withdrawal reflex" (short note)
- Definition: a polysynaptic reflex, also called the flexor withdrawal reflex.
- Receptors (nociceptors) and stimulus (noxious).
- Reflex circuit as a diagram: FRA (III, IV) → interneurons → same-side flexors on and extensors off; commissural interneurons → opposite-side extensors on and flexors off.
- Response as the stimulus grows: flexion → crossed extension (Phillipson’s reflex) → widespread response.
- Special features: protective, prepotent, local sign, afterdischarge.
- Afterdischarge: its two mechanisms, with the diagram.
- Differences from the stretch reflex, as a table.
Asked in exams
- Withdrawal reflex: Dec 2016, 4 marks