Why are proprioceptive pathways important?
These pathways are important for:
• knowing the position of the body & limbs
• for maintaining appropriate muscle tone
• for perceiving movement in relation to gravity
Muscle spindles
- located within muscle
- formed from modified muscle fibers - sensory receptors (neurons) but also are contractile
- sensitive to quick stretch of muscle; transient change
- primes system for further movements (general activation)
- innervated by gamma motor neurons; gamma m.n. activity = spindle tone
- provide higher center control of spindle sensitivity
Golgi tendon organ
- located within tendon
- fires whenever muscle is under tension
- either due to passive stretch or to contraction (so it is always firing)
- signals when muscle is actively moved and placed under strain
- highest activity when a stretched muscle is contracted
- the action of a few GTO’s is amplified to influence many muscle fibers
Extrafusal muscle fibers
- are muscle fibers only; no sensory receptors
- primary contractile portion of the muscle
- innervated by alpha motor neurons (= muscle tone)
The Tendon reflex
A rapid stretch of the patellar tendon produces a brief stretch of the quadriceps muscle, and briefly activates muscle spindles. This sensory information is sent to the gray matter of the spinal cord. The alpha-motoneuron for the quads is excited, causing contraction. At the same time, the alpha-motoneuron for the antagonistic muscle (hamstring) is inhibited, causing that muscle to relax proportionally. However, because there is no muscle tension (no load) the GTOs are inactive. The inactivity of the GTOs and the spindles after the original stimulus causes the quads to relax. This process produces the ‘knee-jerk’ reflex. This reflex operates through a spinal arc, with little contribution from higher control centers
The Stretch reflex
This reflex occurs when a weight is added to a limb, and serves to both stabilize the limb and to maintain it in a desired position. The weight shifts the position of the limb and stretches the muscle. Spindles are activated by the stretch, and GTOs indicate the change in muscle tension (passive stretch). Extrafusal muscle fibers are instructed to contract (agonist muscle) or relax (antagonist muscle) proportionally, but muscle tone is maintained in each muscle group in order to stabilize the joint. GTO activity continues as active contraction of the muscle occurs. The level of GTO activity and the lack of spindle activity once the stretch is eliminated are used to determine position of the limb. Note that the final position depends partly on higher control centers determining the ‘correctness’ of the limb’s position (voluntary motor control), and partly upon past experience
