Spinal Reflexes


A. Define: motor actions mediated via synapses entirely within the spinal cord and analogous brainstem motor nuclei

1. stereotyped, primitive, involuntary

2. generally characterized by reciprocal innervation (inhibition of antagonists and facilitation of synergists)

Note: Synergists = muscles that contribute cooperatively to a given motion

3. upon spinal cord section, these are the only motor actions remaining below the transection level (but note cerebral dominance in humans or encephalization (increased dependence on higher centers) and spinal shock (temporary depression of spinal reflexes following cord section)

B. Segmental Reflexes: reflexes involving one or a few spinal segments

1. myotatic (stretch)
2. inverse myotatic (clasp-knife)
3. flexion (withdrawal)
4. crossed extension

Sensory Endings Muscle Spindle Golgi Tendon Organ Nociceptor (pain) endings
Stimulus Muscle length (stretch) Muscle/tendon tension Noxious
Response Muscle contraction Muscle relaxation Ipsilateral withdrawal & contralateral extension
Role Muscle tone & posture Protect muscle/tendon from excess stres Protect stimulated structure from damage while maintaining posture

Note: Muscle spindle sensory endings and Golgi tendon organs also have the role of conveying information to the brain on muscle length and force

Note: Role of the gamma (small) motoneurons in the ventral horn

a. control of the sensitivity of the muscle spindle sensory receptors through activation of the muscle spindle contractile poles

b. keep muscle spindle sensory endings within their sensitive range (prevent “unloading” upon muscle contraction) by activation when muscle contracts

c. set general sensitivity of reflex response; influenced by descending motor pathways

C. Other segmental input to lower motoneurons

1. facilitation arising from sensory endings associated with muscle synergists

2. inhibition arising from sensory endings associated with muscle antagonists

D. Long Spinal Reflexes: reflexes whose pathways extend over many spinal segments (e.g. “scratch reflex”, “righting reflex”)