Modulation of Stretch Reflexes During Behaviour

  • R. B. Stein
  • S. J. Deserres
  • R. E. Kearney


Far from being the simple, stereotyped tendon jerk tested by physicians, stretch reflexes are highly modulated within a given motor task and the type of modulation is task-dependent. There are several sources of this modulation. Static and dynamic γ motoneurones are modulated differently during cat locomotion in a way that could permit the primary and secondary muscle spindle afferents to play distinct roles. Central modulation also occurs in both the cat and the human, since electrical stimulation of the afferent nerve (H-reflex) produces different responses at different times in the step cycle. Moreover, the reflex response is smaller in humans running compared to walking on a treadmill, even when the speed and EMG levels are matched. Mechanical inputs produce reflex responses with marked amplitude and time-dependent nonlinearities, which are being investigated in further cat and human experiments to provide better estimates of the contribution of reflexes to ongoing movements.

Despite being studied for seventy years, the role of stretch reflexes in the control of movement remains something of a mystery (Liddell and Sherrington, 1924; Matthews, 1990). This is even more surprising when one considers that there is a strong monosynaptic connection from primary muscle spindle afferents to α-motoneurones. Inputs and outputs can be easily monitored with no intervening interneurones, a simplification that is found nowhere else in the nervous system.

With this ease of recording, one might anticipate that a wealth of data would be available from which logical, consistent theories could be derived with broad predictive power. Instead, a number of theories, such as the length servo theory of Merton (1953) and the stiffness regulation hypothesis of Houk (1979) have been tested against experimental data and found to be wanting. Others such as the equilibrium point hypothesis (Levin et al, 1992) have been repeatedly updated to take account of the complexities of real systems.

There are other complexities, such as the longer latency components of the stretch reflex, including those that involve pathways from the limbs to the brain and back. Indeed, Melvill Jones and Watt (1971) named these longer latency pathways the “functional stretch reflex” and suggested that the monosynaptic component was relatively weak and stereotyped (see also Dietz, 1992).

However, even the monosynaptic component can be modulated so that it may be important under one condition and insignificant under another. Three classes of modulation can be considered: 1) fusimotor, 2) synaptic and 3) mechanical. A discussion of these types of modulation forms the body of this paper.


Automatic Gain Control Presynaptic Inhibition Stretch Reflex Muscle Spindle Afferents Equilibrium Point Hypothesis 
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Copyright information

© Springer Science+Business Media New York 1995

Authors and Affiliations

  • R. B. Stein
    • 1
  • S. J. Deserres
    • 1
  • R. E. Kearney
    • 2
  1. 1.Division of NeuroscienceUniversity of AlbertaEdmontonCanada
  2. 2.Department of Biomedical EngineeringMcGill UniversityMontrealCanada

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