Monosynaptic Reflex vs. Polysynaptic Reflex

Definition

Monosynaptic Reflex: The Stretch Reflex

Muscle spindle (receptor) → Ia afferent fiber → dorsal root ganglion → direct synapse on alpha motor neuron in ventral horn → same (homonymous) muscle contracts

• Stimulus is muscle stretch; receptor is the muscle spindle
• Afferents are large, fast, myelinated Group Ia fibers
• Neurotransmitter at the central synapse is glutamate
• The knee jerk (patellar tendon - L4), ankle jerk (Achilles - S1), biceps reflex (C5/C6), and triceps reflex (C7) are all examples

• 0 = absent, 1+ = hypoactive, 2+ = normal, 3+ = hyperactive without clonus, 4+ = hyperactive with mild clonus, 5+ = sustained clonus
• Absent DTR suggests a lesion anywhere in the reflex arc (muscle spindle, Ia afferent, or motor neuron) - e.g., peripheral neuropathy
• Hyperactive DTR suggests an upper motor neuron lesion (loss of descending inhibitory control)

Monosynaptic stretch reflex: blue = afferent Ia fiber from spindle, red = efferent alpha motor neuron. One direct synapse at C5/C6 level.

Polysynaptic Reflex: The Withdrawal (Flexion) Reflex

Nociceptor in skin (receptor) → Aδ afferent fiber → excitatory interneurons → ipsilateral flexor motor neurons (flexion/withdrawal) AND → inhibitory interneurons → ipsilateral extensor motor neurons (relaxed) AND crossed excitatory interneurons → contralateral extensor motor neurons (contract to support weight)

• Multiple interneurons allow convergence of many sensory inputs
• The response is bilateral and multi-segmental
• Interneurons allow the reflex to be modified by descending cortical and brainstem inputs
• Inhibitory interneurons (using GABA and glycine) ensure antagonist muscles relax during the response
• The tonic vibration reflex also has a long latency implying a polysynaptic pathway

Polysynaptic withdrawal reflex: A-delta fiber from skin nociceptor activates multiple interneurons (red). Ipsilateral leg withdraws (flexion); contralateral leg extends for support. Inhibitory interneurons (dark blue stars) ensure antagonist suppression.

Key Differentiating Features - Summary

• One synapse only
• Receptor = muscle spindle (proprioceptor), Ia fiber
• Same-muscle contraction (agonist activation + reciprocal inhibition of antagonist via interneuron)
• Short latency, ipsilateral, simple
• Only example in humans: the deep tendon reflexes

• Multiple synapses (interneurons)
• Receptor = cutaneous exteroceptors (nociceptors), Aδ/C fibers
• Complex response: flexion ipsilateral + extension contralateral
• Longer latency, bilateral, multi-segmental
• Highly modifiable by higher centers; most reflexes in the body are polysynaptic
• Subject to temporal and spatial summation, facilitation, and occlusion

• Ganong's Review of Medical Physiology, 26th Ed., pp. 235-240
• Kandel's Principles of Neural Science, 6th Ed., pp. 808-811 (Chapter 32)
• THIEME Atlas of General Anatomy and Musculoskeletal System, p. 109
• Miller's Review of Orthopaedics, 9th Ed., p. 76

Classification and Properties of Sensory Receptors

What is a Sensory Receptor?

Classification of Receptors

1. By Type of Stimulus (Modality) - Most important classification

2. By Location

3. By Structure (Morphology)

4. By Adaptation Speed

5. By Adequate Stimulus (Principle of Specific Nerve Energies)

• Photoreceptors respond to light, not to touch
• Nociceptors respond to noxious stimuli, not to light

The Four Cutaneous Mechanoreceptors (most tested)

(Ganong's Review of Medical Physiology, Fig. 8-1)

Properties of Sensory Receptors

1. Receptor (Generator) Potential

• When a stimulus activates a receptor, ion channels open/close → current flow → change in membrane potential called the receptor potential (also called generator potential)
• The receptor potential is a graded potential - its amplitude is proportional to stimulus intensity
• It is NOT an action potential; it increases or decreases the probability that an action potential will occur
• If the receptor potential reaches threshold → action potentials fire in the afferent nerve
• The frequency of action potentials encodes stimulus intensity (Costanzo, pp. 82-83)

2. Adequate Stimulus

• Each receptor has a specific form of energy (its "adequate stimulus") to which it has the lowest threshold
• Stimuli of other modalities can activate the receptor, but only at much higher intensities

3. Threshold

• The minimum stimulus intensity required to activate a receptor
• Small subthreshold stimuli produce receptor potentials but no action potentials
• Summation of subthreshold stimuli can reach threshold (spatial and temporal summation)

4. Adaptation

• If a constant-strength stimulus is maintained, firing frequency declines over time - this is adaptation
• Phasic/rapidly adapting receptors: detect the onset and offset of stimuli (changes); useful for detecting motion, vibration (e.g., Pacinian corpuscles - you stop noticing clothing on your skin)
• Tonic/slowly adapting receptors: continue signaling during sustained stimuli; useful for maintaining postural tone, sustained pain warning (e.g., muscle spindles, nociceptors)

5. Receptive Field

• The area of the body surface (or space) that, when stimulated, changes the firing rate of a sensory neuron
• Small receptive fields = high spatial acuity (fingertips - Merkel, Meissner)
• Large receptive fields = low spatial acuity (back - Pacinian, Ruffini)
• Two-point discrimination is directly related to receptive field size

6. Sensory Coding

• Modality - type of stimulus (labeled-line principle: each modality has dedicated pathways)
• Location - encoded by receptive fields and enhanced by lateral inhibition
• Intensity - encoded by firing frequency, number of receptors activated, and type of receptor activated
• Duration - encoded by duration of firing; shaped by adaptation

7. Law of Projection

8. Convergence and Divergence

• Convergence: multiple afferent fibers synapse on a single second-order neuron → amplification
• Divergence: a single afferent fiber contacts multiple second-order neurons → wider broadcast of information

Nociceptors - Special Properties

• Aδ fibers (thin myelinated, 2-5 μm, 12-35 m/s) → First (fast) pain - sharp, well-localized
• C fibers (unmyelinated, 0.4-1.2 μm, 0.5-2 m/s) → Second (slow) pain - dull, burning, diffuse

• TRPV1 - activated by heat (>45°C), acids, and capsaicin
• TRPM8 - activated by cold (<24°C) and menthol
• TRPV3/TRPV4 - warm temperatures (33-39°C / 25-34°C)

• Ganong's Review of Medical Physiology, 26th Ed., pp. 169-177
• Costanzo Physiology, 7th Ed., pp. 82-86
• Medical Physiology (Boron & Boulpaep), pp. 340-343