Neurobiology 303 -- Chapter 17 Outline

Sensory influence on motor output

sensory input is crucial for generation and proper execution of movement
    for example, sensory input can --
        stabilize performance
        compensate for disturbances
        provide timing cues
        modulate behavior

regulation of muscle contraction by the muscle spindle is an excellent
        example of stabilization of performance by sensory input
    tension in the muscle spindle receptor itself is maintained by
        intrafusal muscle fibers
    contraction of intrafusal muscle fibers is controlled by g motoneurons
   so the g motoneurons set the desired tension of the muscle spindle
    since the muscle spindles are part of the monosynaptic stretch reflex,
        the g motoneurons set the desired length of the whole muscle
        (i.e. the extrafusal muscle fibers innervated by a motoneurons)

the stretch reflex controls muscle length as follows:
   first, the a and g motoneurons fire to achieve some desired length in
      the extrafusal and intrafusal muscles; the Ia and II afferents from
       the spindle fire at the desired rate, and the Ia and II afferents in
       turn drive the a motoneurons to the whole muscle at the rate
       appropriate to the desired length
   if the muscle is stretched beyond the desired length, the spindle
       stretches, the Ia and II afferents fire at a higher rate and increase
       their drive of the a motoneurons; that in turn causes the whole
       muscle to contract thus shortening it back to the desired length
   if the muscle is shortened, the spindle relaxes, the Ia and II afferents
       fire at a lower rate and decrease their drive of the a
       motoneurons; that in turn causes the whole muscle to relax thus
       lengthening it back to the desired length
   thus, the stretch reflex regulates muscle length because it always acts
       to bring the whole muscle back to the desired (or pre-set) length
   the stretch reflex can also regulate rate of change of muscle length

other behaviors are also stabilized by sensory input -- notably posture, which
    is under the control of sensory input from the vestibular system

sensory input also allows the motor system to compensate for disturbances
    larger than those that are corrected by stabilizing mechanisms
   for example, if you trip while walking, sensory input can trigger
       reflexive responses like arm-flinging and double-stepping that
       may help you regain your equilibrium
   such large-scale responses involve muscle groups throughout the body
   all animals that locomote have this sort of ability to use sensory input
       to help them compensate for disturbances
   for example, a flying locust must maintain its attitude in 3-d space,
       just as a pilot must maintain that of an aircraft
   both the locust and the pilot have controls for adjustment of
       orientation about three axis: roll, pitch, and yaw
   the locust, like the pilot, wants to fly straight and level; a disturbance
       (e.g. a wind gust) can cause deviation from straight-and-level
       flight, and this deviation can be sensed by sensory hairs on the
       locust that signal the direction of the wind flowing over its head
   these sensory responses trigger reflexive movements of wings, legs,
       and abdomen that restore the locust to straight-and-level flight
   for example, sticking the legs or abdomen out to one side increases
       drag on that side and causes the locust to turn toward that side
       disturbances in yaw and pitch can be controlled in this way
   other corrective movements involve twisting the wings to increase lift
       for the wings on one side or the other or both; this can correct
       for disturbances in roll, pitch, and yaw
   all of these sensory-motor responses are coordinated by interneurons

sensory input can provide timing cues to CPGs
    CPGs can produce an oscillatory pattern without sensory input
    however, sensory input can influence the CPG pattern in various ways
    swimming in dogfish (a type of small shark) provides an example
    CPGs in the spinal cord of dogfish, which are coupled along the
        length by coordinating interneurons, can produce a stable
        swimming pattern even after all sensory input is eliminated by
        cutting all of the dorsal roots

Sten Grillner studied the effect of sensory input on this swimming pattern by
    recording fictive swimming motoneuron discharges from the spinal
    cord of paralyzed dogfish
   he first isolated the spinal cord by severing it at the brainstem and tail
   then he clamped the tail and oscillated it from side-to-side at cycle periods
       different from the natural swimming period of 3 - 5 sec/cycle
   he found that the period of the fictive swimming could be
       sped up to 1.5 sec/cycle or slowed down to 9 sec/cycle
   what caused the change in period? input from sensory neurons in the body
       of the dogfish that are sensitive to bending of the body
   the experiments demonstrate how sensory input can influence CPG
       oscillation period (and inversely, CPG oscillation frequency)
   in an intact shark, the swimming pattern is thus determined both by the
       intrinsic CPG pattern and modulation of that pattern by sensory input

the wing beat CPG of the locust can also be entrained to an imposed
    movement, but only if it is within 10 to 15% of the natural frequency
   the effect of sensory input upon the locust wing beat CPG is dependent upon
       the phase of the wing beat cycle at which the sensory input is applied
   if its timed right, repetitive stimulation of the stretch receptors at the base of
       the wings can cause the wing beat CPG to begin its cycle a bit earlier
   continued stimulation like this can actually increase the wing beat CPG
       oscillation frequency by 50%
   in intact locusts, input from the stretch receptors causes the wing beat CPG
       to oscillate at a higher frequency
   for this reason, the wing beat CPG oscillation frequency is about 50% slower
       in locusts from which the wings (and wing stretch receptors) have been removed

the effect on CPG timing of sensory input is well illustrated by insect gait
    six-legged insects locomote using the alternating tripod gait
    each leg has its own CPG, and the six CPGs are coordinated to
       produce the appropriate pattern using coordinating interneurons
   sensory input can affect the timing of the whole locomotive pattern, or
       the timing of one CPG relative to the others
   for example, if the insect is forced to pull a weight, sensory input
       from stress receptors in the legs causes the whole locomotive
       pattern to slow down
   if the load on only one side is increased (e.g. by increasing the friction
       for legs on one side) the entire pattern is again slowed down

for an even more dramatic example, if the middle legs of a six-legged
    insect are removed, the gain changes instantly from alternating
    tripod to one in which the front and read legs step alternately!

the behavioral state can influence the way in which a reflex responds
    to sensory input -- this process is called reflex gating
   for example, whether a locust will response to wind hair bending with an
       abdominal movement depends upon whether the locust is flying
   a standing locust will not produce an abdominal movement in response to
       wind hair bending
   low-level input from the wing beat CPG provides sub-threshold excitation of
       the abdominal motoneurons during flight but not otherwise
   this low-level input allows the abdominal motoneurons to respond to the
       wind hair input, which is also sub-threshold by itself
   thus, the behavioral state of flying acts as a gate that allows the wind hair
       sensory input to trigger the abdominal reflex
   the wind hair input has no effect when the locust is not flying

the behavioral state can even reverse the response of a reflex to the same
    sensory input -- this process is called reflex reversal
   for example, resistance reflexes exhibit reflex reversal
   resistance reflex -- causes activation of muscles that oppose a movement
       being imposed on the body
   resistance reflexes have been well described in invertebrates
   in a locust or crab, for example, resistance reflexes will tend to hold the legs
       in some rest position; externally applied flex will be opposed by
       activation of extensor muscles; externally applied extension will be
       opposed by activation of flexor muscles
   however, resistance reflexes are suppressed or even reversed during
       locomotion; in that case extension of the leg leads to even more
       extension, rather than the flexion that would occur with the resistance
       reflex in an animal that is not locomoting
   thus, the reflex effects of various stimuli can be completely different
       depending upon the current behavioral state of the animal

sensory stimuli can also effect ongoing repetitive movements
        this process is called reflex modulation
    reflex modulation of walking well illustrates this phenomenon
    during one cycle of stepping, limb movement can be divided into a
        swing phase and a stance phase
    the effect of a sensory input on the stepping cycle depends upon when
        in the cycle it is applied
    consider the effect on stepping of a tap on the dorsum of the foot:
        stance phase -- increased activity in extensors of tapped leg and
            quickening of step cycle
        swing phase -- increased activity in flexors of tapped leg and
            increase in activity of extensors in leg on opposite side
    in both cases, the effect of the reflex response is to move the foot
        away from the tap, and the best way to do this depends upon
        the phase of the step cycle at the time of stimulus application
    in addition, the amount of reflex activity is continuously modulated by
        exactly where the leg is in each of the stance or swing phases:
            it is strongest mid-phase and weakest at either end
    thus, the effect of the sensory stimulus is modulated according to the
        phase of the step cycle
    this type of reflex modulation of walking has been observed in various
        animals from insects to mammals like cats and humans

reflex modulation may be mediated by pre-synaptic inhibition of the axon
        terminals of the sensory neurons
    it has been shown in cats that the synaptic terminals of the sensory
        neurons are inhibited during locomotion according to the phase
        of the step cycle
    by modulating the pre-synaptic terminals in this way, the amount of
        sensory input to the reflex can be adjusted continuously
        according to the phase of the stepping cycle