Muscle spindles and the myotatic reflex
Muscle spindles are stretch receptors. They are intrafusal muscle fibers located strictly parallel to the regular (extrafusal) skeletal muscle fibers.
The structure of an intrafusal fiber is heterogeneous: the poles contain contractile elements, while the center features a connective tissue capsule where the sensory nerve ending lies. When the muscle is stretched, the central part of the spindle is excited. The myotatic reflex—a reflexive contraction in response to stretch—is triggered. Its arc is monosynaptic (two-neuron): the signal from the receptor goes directly to the alpha motor neurons of the spinal cord, causing the same muscle to contract. This is necessary to maintain a constant muscle length and preserve posture.
Gamma-efferent innervation plays a special role. Gamma motor neurons send impulses to the peripheral parts of the spindle, causing them to contract. This stretches the central capsule of the receptor, and it begins to continuously generate signals to alpha motor neurons. This is how constant muscle tone is maintained. If posture needs to be changed, brainstem centers activate gamma motor neurons (or both gamma and alpha motor neurons together).
Golgi tendon organs
Unlike spindles, Golgi tendon receptors are arranged in series within the muscle-tendon-bone system. They respond not to stretch, but to active muscle contraction.
Their task is to maintain constant tension and protect tissues from tearing. During strong muscle contraction, Golgi tendon receptors are excited and trigger a polysynaptic (three-neuron) reflex. The impulse is transmitted to inhibitory spinal interneurons—Renshaw cells. They mediate direct postsynaptic inhibition of alpha motor neurons, resulting in muscle relaxation.
If this inhibition is blocked (for example, by strychnine or tetanus toxin), the protective reflex is switched off. Contractions become so strong and prolonged that they can lead to fractures of long tubular bones of the limbs and asphyxia due to tetanic spasm of the respiratory muscles.
The role of neck proprioceptors in tonic reflexes
Proprioceptors are involved not only in local reflexes of a single muscle, but also in complex motor acts encompassing the entire body. A striking example is Magnus neck tonic reflexes.
They provide reflexive redistribution of muscle tone in the limbs during any tilts or turns of the head. When the head position changes, the proprioceptors of the neck muscles are stimulated. Signals from them reach the cervical segments of the spinal cord. These segments have a special status: they exert a commanding influence on the motor centers of the thoracic (for forelimbs) and lumbar (for hindlimbs) regions. These are the most complex manifestations of motor activity at the spinal cord level.
How proprioceptive information reaches the cortex
Signals from muscle and joint spindles, as well as joint receptors, are transmitted to the brain via deep sensibility pathways (the dorsal column system). This pathway is structured as a three-neuron chain:
- First neuron is located in the dorsal root ganglion. Its central process enters the spinal cord via the dorsal root, but does not form synapses in the gray matter. The fibers ascend immediately on their own side as part of the dorsal columns. Medially runs the fasciculus gracilis (fasciculus gracilis), carrying information from the legs and trunk below Th4. Laterally lies the fasciculus cuneatus (fasciculus cuneatus), transmitting signals from the arms and trunk above Th4.
- Second neuron lies in the medulla oblongata—in the gracile and cuneate nuclei (nucleus gracilis and nucleus cuneatus). Here, axons cross to the opposite side, forming the sensory decussation (decussatio lemniscorum medialium), and ascend within the medial lemniscus to the thalamus.
- Third neuron is located in the ventrolateral nucleus of the thalamus. From there, via the internal capsule, impulses reach the cortex of the postcentral gyrus, where fine discrimination of muscle-joint sense occurs.
The topography of deep sensibility pathways in the spinal cord is easy to remember through body anatomy: the cuneate fasciculus lies laterally (like arms at the sides of the torso, from which it carries impulses), and the gracile fasciculus lies medially (like legs, located closer to the central axis).
Class and exam questions
How does the arrangement of muscle spindles differ from Golgi receptors?
Spindles lie parallel to the main (extrafusal) muscle fibers, whereas Golgi receptors lie in series, at the border of the muscle and tendon.
What is the purpose of gamma-efferent innervation?
Gamma motor neurons contract the peripheral parts of the intrafusal fiber. This stretches its center, causing the receptor to continuously send signals to alpha motor neurons to maintain muscle tone.
Where do the pathways of muscle-joint sense cross?
In the medulla oblongata. The axons of the second neurons form the medial lemniscal decussation (decussatio lemniscorum medialium), after which they head towards the thalamus.
Why do severe spasms occur in tetanus?
Tetanus toxin blocks Renshaw interneurons. Because of this, the inhibitory reflex from Golgi receptors is turned off, and muscle contraction becomes excessive, even leading to bone fractures.
What else to study on this topic
- Flexor and extensor phasic reflexes
- Stepping automatism and rhythmic reflexes of the spinal cord
- Spinothalamic pathways of pain and temperature sensibility
- Somatotopic organization of pathways
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