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Stretch Reflex, Muscle Spindle Receptor, Reciprocal Innervation & Renshaw Cell Inhibition

1. Stretch Reflex (Myotatic Reflex) [2 marks]

The stretch reflex (also called the myotatic reflex) is the simplest spinal reflex, defined as: a reflex contraction of a muscle in response to its sudden passive stretching. Whenever a muscle is stretched suddenly, excitation of the spindle receptors causes reflex contraction of the large skeletal (extrafusal) muscle fibers of that same stretched muscle, and of closely allied synergistic muscles. Its purpose is to resist sudden changes in muscle length, to damp oscillatory movements, and to maintain muscle tone.

Key features:

Monosynaptic - the afferent (Ia fiber) synapses directly on the anterior horn motor neuron (shortest possible reflex arc latency)
Ipsilateral - acts on the same muscle that was stretched
Homonymous - acts on the same muscle from which the signal originated

Classic clinical example: Knee jerk reflex - striking the patellar tendon stretches the quadriceps, triggering a dynamic stretch reflex that causes the lower leg to kick forward.

2. Receptor Involved: The Muscle Spindle [6 marks]

The receptor for the stretch reflex is the muscle spindle (a proprioceptor).

Structure

Each muscle spindle is 3-10 mm long and is built around 3-12 intrafusal muscle fibers attached to the surrounding large extrafusal skeletal muscle fibers. There are two types of intrafusal fibers:

Feature	Nuclear Bag Fiber	Nuclear Chain Fiber
Number per spindle	1-3	3-9
Nuclei arrangement	Clustered in a bag	In a chain
Response	Dynamic (detects rate of stretch)	Static (detects sustained length)

The central (equatorial) region has few actin/myosin filaments - it acts as a sensory receptor. The polar ends contract when stimulated by gamma (γ) motor neurons.

Muscle spindle structure showing nuclear bag and nuclear chain fibers with their innervation

Fig: Muscle spindle - nuclear bag fiber (top) and nuclear chain fiber (bottom), with efferent gamma fibers and afferent Ia/II fibers.

Sensory (Afferent) Innervation

Primary afferent ending (annulospiral ending):

Formed by a Type Ia fiber (17 µm diameter)
Wraps around the equatorial region of both nuclear bag and nuclear chain fibers
Conduction velocity: 70-120 m/sec (fastest in the body)
Responds to both dynamic (rate of stretch) and static length changes

Secondary afferent ending (flower-spray ending):

Formed by Type II fiber (~8 µm diameter)
Located on one or both sides of the primary ending, mainly on nuclear chain fibers
Responds primarily to sustained (static) stretch
Terminates on interneurons in the cord, providing delayed polysynaptic signals

Motor (Efferent) Innervation

Alpha (α) motor neurons - innervate the extrafusal muscle fibers (the main muscle body)
Gamma (γ) motor neurons - innervate the polar contractile ends of the intrafusal fibers
- Dynamic γ fibers - innervate nuclear bag fibers (increase spindle sensitivity to rate of stretch)
- Static γ fibers - innervate nuclear chain fibers (increase spindle sensitivity to length)
Alpha-gamma coactivation: During voluntary movement, both α and γ neurons are activated together, preventing the spindle from going "silent" during muscle shortening, thus maintaining spindle sensitivity throughout contraction.

3. Reflex Arc of the Stretch Reflex (Diagram)

Neuronal circuit of the stretch reflex showing Type Ia sensory nerve from muscle spindle entering dorsal root, synapsing directly on anterior horn motor neuron, which sends motor nerve back to the same muscle

Fig 55.5: Neuronal circuit of the stretch reflex (Guyton & Hall). Type Ia sensory fiber from muscle spindle → dorsal root → directly synapses on anterior horn motor neuron → motor nerve → muscle contraction.

The reflex arc consists of:

Receptor - Muscle spindle (in stretched muscle)
Afferent limb - Type Ia fiber → enters via dorsal root
Integration center - Anterior horn of spinal cord (monosynaptic connection)
Efferent limb - Alpha motor neuron → ventral root
Effector - Extrafusal skeletal muscle fibers (contraction of stretched muscle)

The main arc is monosynaptic (one synapse, shortest latency). Type II fibers additionally activate interneurons for polysynaptic pathways.

4. Reciprocal Innervation [2 marks]

Definition: When the stretch reflex excites (contracts) one muscle (the agonist), it simultaneously inhibits the motor neurons supplying the antagonist muscles. This phenomenon is called reciprocal inhibition, and the neuronal circuit that mediates this is called reciprocal innervation.

Mechanism (Ganong's Physiology):

Ia afferent fibers from the stretched agonist muscle spindle enter the spinal cord
A collateral branch of each Ia fiber synapses on an inhibitory interneuron (the Ia inhibitory interneuron) in the cord gray matter
This inhibitory interneuron then synapses on the alpha motor neurons supplying the antagonist muscles
This is an example of postsynaptic inhibition (IPSP in the antagonist motor neuron)

Functional significance: When a flexor muscle contracts (e.g., biceps during bicep curl), the extensor (e.g., triceps) must simultaneously relax - this reciprocal inhibition allows smooth, coordinated movement without the two muscle groups opposing each other. Descending corticospinal pathways can also modulate the Ia inhibitory interneuron to allow co-contraction at joints when needed.

5. Renshaw Cell Inhibition [2 marks]

Definition: Renshaw cell inhibition (also called recurrent collateral inhibition or negative feedback inhibition) is a form of postsynaptic inhibition in which a motor neuron's own axon collateral feeds back to inhibit that same motor neuron via a specialized inhibitory interneuron called the Renshaw cell.

Circuit (Ganong's & Kandel's Neuroscience):

The alpha motor neuron axon, as it exits the anterior horn, gives off a recurrent collateral branch
This collateral synapses on a Renshaw cell (an inhibitory interneuron in the anterior horn)
The Renshaw cell releases glycine (inhibitory neurotransmitter)
Glycine causes IPSPs on the same motor neuron (and on neighboring motor neurons of synergist muscles, and on Ia inhibitory interneurons)

Negative feedback inhibition via Renshaw cell - axon of motor neuron gives recurrent collateral to inhibitory interneuron (Renshaw cell), which feeds back to inhibit the same and nearby motor neurons

Fig 6-10 (Ganong): Recurrent collateral from motor neuron axon synapses on inhibitory interneuron (Renshaw cell), which inhibits the same and other motor neurons. Neurotransmitter = glycine.

Functional significance:

Acts as a negative feedback system to regulate motor neuron excitability
Stabilizes firing rates - prevents motor neurons from firing at excessively high frequencies
When motor neuron activity increases, Renshaw cell inhibition increases proportionally, capping output
Also inhibits synergist motor neurons and Ia inhibitory interneurons, helping to coordinate overall joint motor output
Clinical relevance: Tetanus toxin preferentially blocks glycine release from Renshaw cells → loss of recurrent inhibition → sustained tonic spasm of muscles

Sources: Guyton & Hall Textbook of Medical Physiology, Ch. 55; Ganong's Review of Medical Physiology, 26th Ed., Ch. 12; Kandel's Principles of Neural Science, 6th Ed., Ch. 32.

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