In one breath
Spinal reflexes share a set of properties that come from the way their neurons and synapses are wired: adequate stimulus, convergence and divergence, facilitation, inhibition, summation, occlusion and habituation. Many afferents converging on one motor neuron make facilitation and spatial summation possible, while one afferent diverging to many lets a reflex spread. Reflexes can be inhibited (reciprocal and Renshaw inhibition), can occlude one another, and change with experience; all of them reach muscle through the α motor neuron, the final common path.
Builds on: Properties of synapses · Synapses and synaptic transmission · Leads to: Postural reflexes, spinal animal and spinal shock · Upper vs lower motor neuron lesions
The features peculiar to the withdrawal reflex (prepotency, local sign, irradiation, recruitment and afterdischarge) are on Withdrawal reflex.
Adequate stimulus
- Each reflex is set off by one particular, precisely defined kind of stimulus: its adequate stimulus.
- Example: for the stretch reflex, the adequate stimulus is stretch of the muscle.
Convergence
- Many to one: several neurons end on a single target neuron.
- Example: many Ia fibres from the spindles of one muscle end, monosynaptically, on the same α motor neuron.
Divergence
- One to many: a single neuron ends on many target neurons.
- Example: one flexor reflex afferent reaches many motor neurons through different sets of interneurons, which is how the withdrawal reflex spreads.
Facilitation
- It rests on convergence.
- One afferent fibre firing alone may fail to fire a motor neuron. When several afferents that converge on it fire together, the motor neuron fires: that is facilitation.
Inhibition
- Inhibition is a fall in the activity of the target neurons.
- Two spinal examples:
- Reciprocal inhibition: in the stretch reflex, an inhibitory interneuron switches off the antagonist (see Stretch reflex).
- Renshaw cell inhibition: a motor neuron sends a collateral to an interneuron, the Renshaw cell, which projects back and inhibits that same motor neuron. The neuron damps its own output.
- Two ways of classifying inhibition:
| Basis | Types |
|---|---|
| Where it acts | Direct (postsynaptic) or indirect (presynaptic) |
| Circuit | Feedback (e.g. Renshaw) or feedforward |
- Worth knowing, though not in your pages: the motor neuron collateral excites the Renshaw cell with acetylcholine, and the Renshaw cell inhibits with glycine. Strychnine blocks glycine receptors and tetanus toxin blocks glycine release, so both strip away glycine inhibition in the cord and cause violent muscle spasms.
Draw it: Renshaw cell inhibition
Draw an α motor neuron in the ventral horn with its axon heading out to the muscle. Just after the axon leaves the cell body, draw a collateral that curves back to a small black Renshaw cell. From the Renshaw cell, draw a ”−” synapse back on the same motor neuron. Label “recurrent (feedback) inhibition”.
Summation
| Type | What adds up |
|---|---|
| Spatial | Several afferents firing at the same time on one neuron |
| Temporal | One afferent firing repeatedly, in quick succession |
- Spatial summation: one afferent too weak to fire the motor neuron still brings it into the subliminal fringe, partly excited but below threshold. If other afferents to the same neuron fire at the same moment, their effects add up and push it past threshold, and it fires.
- Temporal summation: a single impulse in one afferent may not be enough, but a rapid train of impulses in that same afferent adds up and fires the target.
Occlusion
- Among the motor neurons an afferent reaches, there are two zones:
- the discharge zone (central), whose neurons it fires;
- the subliminal fringe (peripheral), whose neurons it only partly excites.
- When the discharge zones of two neighbouring afferents overlap, the shared neurons are fired by either afferent alone. Firing both together therefore fires fewer neurons than the two would fire separately: this is occlusion.
- When the subliminal fringes overlap, the shared neurons get part of their excitation from each afferent and fire only when both act together. Firing both together fires more neurons than the two would fire separately: this is facilitation.
A slip in your book
- Your book says that overlapping discharge zones give occlusion, and in the next line that overlapping discharge zones give facilitation. The second should be the subliminal fringes.
- Write in the exam: discharge zones overlap → occlusion (response less than the sum); subliminal fringes overlap → facilitation (response more than the sum).
Draw it: occlusion and facilitation
Draw two afferents, A and B, each ending on a cluster of motor neurons. Around each afferent’s targets draw a solid inner circle (discharge zone) and a dotted outer ring (subliminal fringe). For occlusion, let the two solid circles overlap, shade the shared neurons and label them “fired by A or B alone”. For facilitation, let only the dotted rings overlap, shade the shared neurons and label them “fired only when A and B act together”.
Habituation and sensitization
- Reflexes are mostly fixed, stereotyped responses, yet experience can reshape them.
- Habituation: the response grows weaker when the same stimulus is given again and again.
- Sensitization: the response grows stronger, typically after a strong or noxious stimulus (see the warning below).
- These two simple forms of change underlie learning and memory in the CNS.
A slip in your book
- Your book defines sensitization as a decreased response to a stimulus that is given repeatedly. That is the definition of habituation.
- Sensitization is the opposite: an increased response, typically after a strong or noxious stimulus.
- Write in the exam: habituation, response ↓ with repetition; sensitization, response ↑ after a strong stimulus.
Final common path
- All motor commands, whatever their source in the CNS, reach skeletal muscle through its motor neurons, chiefly the α motor neurons.
- That is why these motor neurons are called the final common path of motor output.
Applied: the final common path in disease
Worth knowing, though not in your pages: when the final common path itself is damaged, as in a lower motor neuron lesion, the muscle is flaccid, its reflexes are lost and it wastes, whatever the brain above is doing (see Upper vs lower motor neuron lesions).
Exam-answer skeleton: "Properties of spinal reflexes" (short note)
- Adequate stimulus, with an example.
- Convergence and divergence, with a spinal example of each.
- Facilitation, built on convergence.
- Inhibition: reciprocal and Renshaw cell; postsynaptic vs presynaptic, feedback vs feedforward.
- Summation: spatial and temporal.
- Occlusion vs facilitation, with the discharge zone and subliminal fringe diagram.
- Habituation and sensitization; the final common path.
- For the withdrawal reflex’s own features (afterdischarge, local sign, prepotency), see Withdrawal reflex.