Muscle Spindle and Stretch Reflex
Stretch Reflex Overview
The stretch reflex is an intrinsic reflex built into our anatomy, separate from other skeletal muscle activities.
Unlike voluntary muscle movements, the stretch reflex does not originate from the primary motor cortex.
Skeletal Muscle Innervation
General Innervation Pathway
Signals for muscle innervation come from:
Cerebellum
Skeletal brainstem
Spinal cord
Primary motor cortex sends signals down the spinal cord to synapse on:
Anterior horn
Alpha motor neuron (second order neuron)
These signals can desiccate at:
The pyramids
The lateral corticospinal tract
Spinal level
Example:
The signal travels down the lateral corticospinal tract, synapsing with the alpha motor neuron, which, through the ventral root, connects with the dorsal root and its ganglia to form the spinal nerve, ultimately innervating skeletal muscle (e.g., quadriceps).
Muscle Fiber Anatomy
Types of Muscle Fibers
Extrafusal muscle fibers:
Striated skeletal muscle fibers innervated by alpha motor neurons.
Intrafusal fibers:
Found within muscle spindles.
Exhibit different properties compared to extrafusal fibers; not fully striated.
Striations are present only at the ends (actin and myosin at dark and light bands) capable of contraction.
Muscle Spindle Structure
Muscle spindles contain intrafusal fibers and act as sensory receptors.
Annulospiral fibers:
Type of sensory receptor coiled around intrafusal fibers.
Function: send feedback about muscle stretch directly to the alpha motor neuron, bypassing the primary sensory cortex.
Activates an excitatory neurotransmitter to indicate stretch and prevent muscle tearing.
Stretch Reflex Mechanism
The unique feature of the stretch reflex is its intrinsic nature; it operates independently of higher brain input.
When a muscle (like the quadriceps) is stretched (e.g., by a reflex hammer hitting the patellar tendon), the muscle spindle detects this stretch.
Coactivation process:
When the alpha motor neuron is excited (either from voluntary or reflex activity), gamma motor neurons are also activated, allowing the intrafusal fibers to adjust and maintain sensitivity to stretch even when the muscle is contracted.
Sensory Feedback and Length Change
Muscle spindles detect:
Changes in muscle length through:
Annulospiral endings: detect both stretch and degree of stretch.
Flower spray endings: detect changes in length but not the degree of change.
Example of Response:
When stepping off a curb, the quadriceps contract reflexively to prevent a fall, all due to the stretch reflex pathway activated by length changes.
Antagonist Muscle Consideration
Reciprocal inhibition will be discussed in further detail in the following video.
Effective activation of the stretch reflex is hindered if the antagonist (e.g., hamstrings) is simultaneously active.
Reflex Arc Components
Stretch reflexes are monosynaptic and ipsilateral:
Monosynaptic: only one synapse between sensory receptor (annulospiral ending) and the alpha motor neuron.
Ipsilateral: the effect occurs on the same side where the reflex originates.
Components of the reflex arc include:
Sensory receptor (annulospiral endings)
Afferent signal (to the spinal cord)
Integration center (alpha motor neuron)
Efferent signal (to muscle)
Effector (the muscle contracting)
The proper stretch reflex contains only one synapse, although additional synapses occur in the dorsal horn influencing higher processing centers like the cerebellum.
Summary
The stretch reflex is a vital sensory mechanism that helps maintain posture and prevent injury through rapid, involuntary muscle contractions in response to muscle stretching.
The integrity of this reflex is critical for activities that require balance and coordination, and it underscores the importance of understanding how muscles and the nervous system interact to produce coordinated movement.