KINE 3301
Biomechanics of Human Movement
Reflexes
Chapter 19
Monosynaptic & Polysynapic Connections
The Ia afferent neuron from the muscle spindle is the only
sensor fiber to make a monosynaptic connection.
Presynaptic Inhibition &
Facilitation
• The efferent neural
response can be modified
by monosynaptic or
interneuron input.
Muscle Spindle Structure
Intrafusual Fibers: the term intrafusal fibers refers to the muscle spindle to differentiate
it from the extrafusal (normal muscle).
Fusimotor System: the fusimotor system refers to the muscle spindle, it’s afferent,
efferent and specialized muscle tissue.
Spinal
Connections
Nerve
Conduction
velocity:
Ia 40 – 90 m/s
II 30 – 75 m/s
The Ia neuron
makes
monosynaptic
connections
with alpha and
gamma.
Stretch Reflex
When a muscle spindle is
stretched above it’s
current sensitivity it
elicits a stretch reflex:
1. Ia stimulates alpha –
agonist & synergist.
2. Ia inhibits antagonist.
3. Ia stimulates gamma
The number of impulses
propagated up the Ia
neuron is related to the
magnitude and rate of
stretch.
Inverse Stretch Reflex
Golgi Tendon
Organ (GTO)
The Ib neuron
weaves in and out
of the tendons
collagen fibers.
The GTO is very
sensitive to
changes in muscle
force. It can
respond to a
twitch from a
single motor unit.
Note it uses
interneurons to
communicate with
alpha.
GTO & Muscle Spindle Control Muscle Stiffness
A motor unit is one alpha motor neuron and all of the
muscle fibers it innervates.
∆𝐹𝑜𝑟𝑐𝑒
𝑀𝑢𝑠𝑐𝑙𝑒 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠 =
∆𝐿𝑒𝑛𝑔𝑡ℎ
𝐺𝑜𝑙𝑔𝑖 𝑇𝑒𝑛𝑑𝑜𝑛 𝑂𝑟𝑔𝑎𝑛 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠 ∆𝐹𝑜𝑟𝑐𝑒
𝑀𝑢𝑠𝑐𝑙𝑒 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠 =
𝑀𝑢𝑠𝑐𝑙𝑒 𝑆𝑝𝑖𝑛𝑑𝑙𝑒 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠 ∆𝐿𝑒𝑛𝑔𝑡ℎ
The level of muscular preactivation prior to some known
event is modulated by the golgi tendon organ and the
muscle spindle via cortical commands.
Cutaneous Receptors
Reflex latencies from
cutaneous receptors
vary from 50 – 85 ms.
Convergence & Modulation
of Sensory Signals
Convergence of signals
to the motor neuron
can modulate the input
to the motor neuron
and it’s output
response to a stimulus.
Feedback and Feedforward Control
• Reflexes were once thought to be invariant. It was
assumed that they were completely involuntary.
• We now view the terms reflex and voluntary as being
on a continuum with pure reflexes such as an eye
blink or a stretch reflex on one end and complete and
full conscious control on the other end.
• The speed with which a movement or force is
generated determines the extent to which feedback
is able to correct errors.
• In feedback control the movement is slow enough (>
50 ms) that reflex signals can modify movement.
• In feedforward control the movement is execute so
fast (< 50 ms) so that feedback can not modify
movement.
Feedback & Feedforward
• The minimum time for the neuromuscular system to
respond to a stimulus is 80 – 200 ms.
• Reflex time 30 – 120 ms
• Muscle force time 50 – 300 ms
Consider the following scenarios, which are feedback
and feedforward:
• During the upward phase of a vertical jump can you
change from jumping up to jumping up & forward?
• When landing on a rug, if the rug is unexpectedly
pulled backward at the instant your feet touch the
rug, what happens?
• 10 ms prior to a bat hitting a ball can you switch from
hitting a fly to hitting a grounder?