Spinal Reflexes
Reflexes represent some of the most basic neural circuits. Relatively simple reflexes are responsible for simple behaviors. However, they are also involved in more complex circuits, whereby complex behaviours can be built up from sequences of simple reflexive responses.
Stretch Reflex
When a muscle is stretched, the stretch reflex activates a rapid contraction of the same muscle to increase muscle tension and oppose the stretch. It is particularly strong in muscles that oppose gravity.
Increasing a muscle length stimulates spindle afferents, especially Ia fibres. These terminate in the spinal cord on alpha motor neurons that innervate the same muscle.
Monosynapic conenctions account for the rapid component of the stretch reflex, but at the same time parallel circuits, involving interneurons, inhibit the alpha motor neurons of antagonist muscles. This reciprocal innervation increases the effectiveness of the stretch reflex.
Golgi tendon reflexes
Golgi tendon organs are aligned in series in a muscle and are very sensitive to the tension within a tendon. Thus, instead of responding to muscle length, they are activated by the force generated within a muscle and are stimulated most during active contraction.
Ib sensory neurons act through interneurons to inhibit the muscle experiencing stretch while activating the antagonistic muscle, resulting in the opposite effect of the stretch reflex.
Flexor Reflex
The flexor reflex responds to painful stimuli, such as stepping on a sharp stick. The injured limb withdraws while the oppositve leg is extended. It is primarily mediated by Aδ axons.
This bilateral reflex is coordinated within the spinal gray matter.
The flexor reflex is remarkable in that it induces withdrawal away from the stimulus and is realted in its amplitude to the stimulus intensity.
Central Control of Reflexes
Axons descending from multiple sites within the brainstem and cortex synapse on motor interneurons and on some lower motor neurons directly. These descending pathways mediate conscious and unconscious movement but can also alter the strength of both stretch and flexor reflexes. During walking, for example, reflexes of the leg vary dramatically.
Tend to be multisynaptic:
visceral reflexes, which control autonomic outflow
Grading Reflex Strength
0 - no response
1+ diminshed/low normal
2+ average/normal
3+ brisk/high normal
4+ hyperactive with clonus
Babinski
The Babinski reflex is used to diagnose problems with upper motor neurons.
A normal test is a downwards curl of the toes, while a positive test is an upper curling of the toes.