Human Physiology I · Neurophysiology
Spinal Reflexes
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In 30 seconds
A Reflex Rapid, involuntary, stereotyped response to a stimulus Full entry → is a rapid, involuntary, stereotyped response to a stimulus, mediated by a Reflex arc Five-part pathway: receptor, sensory neuron, integration center, motor neuron, effector Full entry → of five components: receptor, sensory neuron, integration center, motor neuron, and effector. The Muscle spindle Stretch receptor inside skeletal muscle Full entry → detects muscle stretch and drives the monosynaptic Stretch reflex Muscle contraction triggered by its own stretch Full entry → (the patellar or knee-jerk reflex), while the Golgi tendon organ Tension receptor at the muscle-tendon junction Full entry → detects tension and drives the disynaptic Inverse myotatic reflex Relaxation of a muscle when its tension is excessive Full entry → that relaxes the muscle. Painful stimuli trigger the Flexor withdrawal reflex Pulling a limb away from a painful stimulus Full entry → with Reciprocal inhibition Inhibition of antagonist muscles during a reflex Full entry →, paired with the Crossed-extensor reflex Extension of the opposite limb during withdrawal that supports the opposite limb.
Why this matters
Deep-tendon reflexes such as the Patellar reflex Knee-jerk produced by tapping the patellar tendon Full entry → are a core part of the neurological examination. A brisk or diminished knee jerk can reflect the balance between spinal circuitry and descending control, helping localize problems along the reflex arc or its supraspinal modulation — but interpretation belongs to a qualified clinician, because reflex findings vary with technique and context. Reflex testing is safe and non-invasive and illustrates how the same circuit underlying the patellar reflex underlies everyday postural corrections. Any sudden loss of strength, sensation, or balance warrants immediate evaluation by qualified clinicians or local emergency services. Diagnostic criteria and clinical protocols vary by institution and jurisdiction, and these notes support education rather than replacing clinical instruction or supervision.
The college version
1. The reflex arc and the stretch reflex
A reflex is a fast, involuntary, predictable motor response to a stimulus. Its neural pathway is the reflex arc, with five links in order: receptor (detects the stimulus) → sensory neuron (afferent, carries the signal into the CNS) → integration center (one or more synapses in the CNS) → motor neuron (efferent, carries the command out) → effector (muscle or gland that responds). The muscle spindle is a stretch receptor embedded within skeletal muscle that fires when the muscle is lengthened. In the stretch reflex, muscle stretch activates the spindle, whose sensory axon synapses directly on the alpha motor neuron of the same muscle, causing it to contract and resist the stretch. Because the sensory neuron synapses directly on the motor neuron with only one synapse in the integration center, this is a Monosynaptic reflex Reflex with a single synapse between sensory and motor neuron Full entry →. The patellar reflex (knee jerk) is the standard example: tapping the patellar tendon stretches the quadriceps, and the quadriceps reflexively contracts.
2. The Golgi tendon organ and the inverse myotatic reflex
The Golgi tendon organ (GTO) is a tension receptor located at the muscle-tendon junction, in series with the muscle fibers. Unlike the spindle, which signals length, the GTO signals the force (tension) the muscle generates. When tension becomes dangerously high, GTO sensory fibers excite an inhibitory interneuron in the spinal cord, which in turn inhibits the alpha motor neuron of the same muscle, causing it to relax. This is the inverse myotatic reflex (also called the Golgi tendon reflex), and because it passes through one interneuron — two synapses — it is a Disynaptic reflex Reflex with one interneuron (two synapses) Full entry →. It protects the muscle and tendon from tearing under excessive load and helps regulate force during sustained contraction.
3. Flexor withdrawal, crossed-extensor, and reciprocal inhibition
A painful or damaging stimulus, such as touching something sharp, triggers the flexor withdrawal reflex: a polysynaptic pathway that excites flexor muscles and pulls the limb away from the stimulus. At the same time, reciprocal inhibition silences the antagonist extensor muscles (via inhibitory interneurons) so they do not oppose the withdrawal. If the stimulus is applied to one leg, the crossed-extensor reflex simultaneously extends the opposite leg, supporting body weight so you do not fall. Together these reflexes illustrate the protective role of spinal circuitry: they remove the body from harm, brace against it, and prevent injury far faster than voluntary, cortically mediated movement could.
How it works
- A stimulus (stretch, tension, or pain) activates a specific receptor.
- The receptor fires a sensory neuron that carries the signal into the spinal cord.
- In the integration center, the signal is relayed through one or more synapses.
- The motor neuron (or neurons) then carries the command out to the effector muscle.
- The muscle contracts or relaxes, producing the reflexive movement that resists stretch, reduces tension, or withdraws from harm.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Muscle spindle | Golgi tendon organ | Spindle is in the muscle belly and signals length; GTO is at the tendon and signals tension |
| Stretch reflex | Inverse myotatic reflex | Stretch reflex excites the muscle (contraction); inverse myotatic reflex inhibits it (relaxation) |
| Monosynaptic reflex | Disynaptic reflex | Monosynaptic has one synapse (stretch reflex); disynaptic has two, through one interneuron |
| Flexor withdrawal reflex | Crossed-extensor reflex | Withdrawal flexes the stimulated limb; crossed-extensor extends the opposite limb for support |
| Patellar reflex | Flexor withdrawal reflex | Patellar reflex is a monosynaptic stretch reflex; withdrawal is a polysynaptic protective reflex |
Memory aids
"Run Somewhere, Integrate, Move, Execute" — Receptor, Sensory neuron, Integration center, Motor neuron, Effector (the five arc parts). For the two muscle receptors: "Spindle = Stretch, GTO = Tension" — the spindle fires on stretch to contract the muscle, while the Golgi tendon organ fires on tension to relax it.
Quick review
Topic Recap
A reflex is a rapid, stereotyped response carried by a five-part reflex arc. The muscle spindle detects stretch and drives the monosynaptic stretch reflex (the patellar reflex); the Golgi tendon organ detects tension and drives the disynaptic inverse myotatic reflex that relaxes the muscle to prevent tearing. Painful stimuli trigger the flexor withdrawal reflex with reciprocal inhibition, complemented by the crossed-extensor reflex that braces the opposite limb — together fulfilling the protective role of spinal reflexes.
Knowledge Check
- List the five components of a reflex arc in order.
- What receptor initiates the patellar reflex?
- How many synapses does the inverse myotatic reflex have in its integration center?
- What reflex keeps the opposite leg extended when one leg withdraws from pain?
- Which process silences antagonist muscles so an agonist can act unopposed?
Answers and Rationales
- Receptor, sensory neuron, integration center, motor neuron, effector. Why: these five links describe the complete path every reflex signal travels from stimulus to response.
- The muscle spindle. Why: tapping the patellar tendon stretches the quadriceps, and the muscle spindle detects that stretch and triggers the reflex contraction.
- Two. Why: the Golgi tendon organ's sensory fiber synapses on an inhibitory interneuron, which synapses on the motor neuron — two synapses, making it a disynaptic reflex.
- The crossed-extensor reflex. Why: while the stimulated limb flexes to withdraw, the opposite limb extends to support body weight and prevent a fall.
- Reciprocal inhibition. Why: inhibitory interneurons suppress the antagonist muscles, allowing the agonist to produce the reflex movement without opposition.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of a reflex as a knee-jerk "shortcut" that bypasses the brain's slow decision-making office. When a sensor in the muscle (the muscle spindle) feels a sudden stretch, it sends a message along a fast wire (sensory neuron) straight to the spinal cord, where it connects almost directly to a motor neuron that fires back to the same muscle — one quick handshake and the muscle contracts. A Golgi tendon organ is like a strain gauge in the tendon: if tension climbs dangerously high, it shouts "ease off," and the spinal cord tells the muscle to relax. Withdrawal reflexes are like touching a hot stove — the signal pulls your hand away before you even think about it, while the opposite leg stiffens to keep you from falling.
Where the comparison stops being exact: reflexes are not literally "instant" — even the fastest monosynaptic reflex has a measurable delay for conduction and one synaptic crossing — and "bypassing the brain" is only partly true, because higher centers continuously modulate (usually dampen) spinal reflexes.
Simple Example
Sit with your legs dangling and tap the tendon just below your kneecap. The tap stretches the quadriceps muscle, its muscle spindles fire, and within a fraction of a second the quadriceps contracts and your lower leg kicks forward. That is the patellar reflex — the classic stretch reflex, completed entirely through the spinal cord.
Worked example
This topic has no governing equation, so here is the signal-flow walkthrough for the three key reflexes:
- Stretch reflex (monosynaptic). A tap stretches the quadriceps. Muscle spindle intrafusal fibers distort, opening stretch-sensitive channels, and the spindle's Ia sensory axon fires action potentials. The signal travels into the spinal cord through the dorsal root and synapses directly on the alpha motor neuron of the quadriceps in the ventral horn. The alpha motor neuron fires, and the quadriceps contracts, shortening the muscle and counteracting the stretch. The direction of signal flow is receptor → sensory neuron → motor neuron → muscle, with a single synapse in between.
- Inverse myotatic reflex (disynaptic). Excessive muscle tension activates the Golgi tendon organ, whose Ib afferent synapses on an inhibitory interneuron in the spinal cord. That interneuron releases an inhibitory transmitter onto the alpha motor neuron, reducing its firing so the muscle relaxes. The flow is receptor → sensory neuron → inhibitory interneuron → motor neuron → muscle, with two synapses — hence "disynaptic" — and the sign of the output is inverted (inhibition, not excitation).
- Flexor withdrawal with crossed-extensor reflex. A noxious stimulus activates nociceptors whose afferents diverge onto multiple interneurons. On the stimulated side, excitatory interneurons activate flexor motor neurons while inhibitory interneurons suppress extensors (reciprocal inhibition), withdrawing the limb. Collaterals cross the cord to the opposite side, where the pattern is reversed: extensors are excited and flexors inhibited, extending the opposite limb to bear weight. This divergence and crossing is what makes the response coordinated and protective.
Key takeaways
- High yield: The reflex arc has five parts: receptor, sensory neuron, integration center, motor neuron, effector.
- High yield: The stretch reflex is monosynaptic and is the basis of the patellar reflex.
- High yield: The muscle spindle signals muscle length; the Golgi tendon organ signals muscle tension.
- High yield: The inverse myotatic reflex is disynaptic, relaxes the muscle under high tension, and protects against tearing.
- High yield: The flexor withdrawal reflex is polysynaptic and pairs with the crossed-extensor reflex for balance.
- High yield: Reciprocal inhibition silences antagonists so the agonist can act unopposed.
- Spinal reflexes are modulated (usually damped) by descending input from the brain.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- List the five components of a reflex arc and trace the signal flow through it.
- Explain the stretch reflex and its role in the patellar reflex, including the muscle spindle.
- Describe the Golgi tendon organ and the inverse myotatic reflex, contrasting it with the stretch reflex.
- Explain the flexor withdrawal reflex, crossed-extensor reflex, and reciprocal inhibition in terms of their protective role.
Key vocabulary
- Reflex
- Rapid, involuntary, stereotyped response to a stimulus
- Reflex arc
- Five-part pathway: receptor, sensory neuron, integration center, motor neuron, effector
- Muscle spindle
- Stretch receptor inside skeletal muscle
- Stretch reflex
- Muscle contraction triggered by its own stretch
- Monosynaptic reflex
- Reflex with a single synapse between sensory and motor neuron
- Patellar reflex
- Knee-jerk produced by tapping the patellar tendon
- Golgi tendon organ
- Tension receptor at the muscle-tendon junction
- Inverse myotatic reflex
- Relaxation of a muscle when its tension is excessive
- Disynaptic reflex
- Reflex with one interneuron (two synapses)
- Flexor withdrawal reflex
- Pulling a limb away from a painful stimulus
- Crossed-extensor reflex
- Extension of the opposite limb during withdrawal
- Reciprocal inhibition
- Inhibition of antagonist muscles during a reflex
- Protective role
- The injury-avoiding and stabilizing function of spinal reflexes
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