Reflexes

Chapter 19

KINE 3301Biomechanics of Human Movement

Monosynaptic & Polysynapic Connections

The Ia afferent neuron from the muscle spindle is the only sensor fiber to make a monosynaptic connection.

• The efferent neural response can be modified by monosynaptic or interneuron input.

Presynaptic Inhibition & Facilitation

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.

Nerve Conduction velocity:

Ia 40 – 90 m/sII 30 – 75 m/s

Spinal Connections

The Ia neuron makes monosynaptic connections with alpha and gamma.

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

Stretch Reflex

The number of impulses propagated up the Ia neuron is related to the magnitude and rate of stretch.

Inverse Stretch Reflex

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.

Golgi Tendon Organ (GTO)

A motor unit is one alpha motor neuron and all of the muscle fibers it innervates.

GTO & Muscle Spindle Control Muscle Stiffness

𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠= ∆𝐹𝑜𝑟𝑐𝑒∆ h𝐿𝑒𝑛𝑔𝑡

𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠=𝐺𝑜𝑙𝑔𝑖𝑇𝑒𝑛𝑑𝑜𝑛𝑂𝑟𝑔𝑎𝑛𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠∆𝐹𝑜𝑟𝑐𝑒𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑝𝑖𝑛𝑑𝑙𝑒𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠∆ h𝐿𝑒𝑛𝑔𝑡

The level of muscular preactivation prior to some known event is modulated by the golgi tendon organ and the muscle spindle via cortical commands.

Reflex latencies from cutaneous receptors vary from 50 – 85 ms.

Cutaneous Receptors

Convergence of signals to the motor neuron can modulate the input to the motor neuron and it’s output response to a stimulus.

Convergence & Modulation of Sensory Signals

• 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 and Feedforward Control

• 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?

Feedback & Feedforward