L67. Physiology - Spinal Reflexes

• Spinal reflexes remain present even when the spinal cord is severed from the brain.
• Spinal reflexes typically have short latencies.
• Spinal reflexes often have known circuitry.
• Spinal reflexes respond to muscle length and tension and touches to specific organs.
• Flexor reflexes often occur in coordination with crossed extensor reflexes.
• Babinski sign (flaring of toes and upward movement of the big toe in response to pressure on the bottom of the foot) is found in babies up to 18 months old; it can also be found in adults if the pyramidal tract from the brain is severed.
• Reflex function depends on stimulus intensity, position (local sign), and duration.
• Most reflexes are polysynaptic, except for the stretch reflex which is monosynaptic.
• Reciprocal innervation involves opposite output to antagonists and similar output to synergists.

Stretch Reflex (Myotatic Reflex)

• Functions as a length monitoring system for muscles.
• Present in all skeletal muscles, but best developed in extensors.
• Knee jerk reflex is an example.
• Components of stretch reflex include:
  • Muscle spindle (intrafusal muscle fiber and sensory endings of stretch receptor neurons)
  • Stretch receptor neuron (sensory ending in muscle spindle, cell body in dorsal root ganglion, synapses in ventral horn)
  • Alpha motorneurons (cell body in ventral horn, axon in ventral root, synapses on extrafusal muscle fibers)
• Functional pathway involves:
  • Stretch of intrafusal fiber.
  • Activation of stretch receptors.
  • Transmission to spinal cord via dorsal root.
  • Synapse directly onto alpha motorneurons.
  • Transmission via ventral roots of motor activity to extrafusal muscle fibers.
  • Contraction of extrafusal muscle fibers.
• Properties include:
  • Monosynaptic spinal reflex with short latency.
  • An adequate stimulus is stretch (e.g., knee jerk reflex).
• Reciprocal innervation involves inhibition of antagonist motorneurons (via inhibitory interneurons).
• The stretch reflex also causes excitation of synergists through weak, parallel monosynaptic pathways.

Functions of the Stretch Reflex

• Load compensation.
• Monitors length during voluntary muscle contraction.
• Sensitivity control via gamma innervation.
• Alpha-gamma co-activation occurs during movement.

Clinical Implications of Stretch Reflex

• Muscle jerks are used to assess spinal cord function to identify motor level lesions.
• Overactive stretch reflexes are present in Parkinson's disease.
• Clonus (oscillating movement of the foot stimulated by upward pressure on the bottom of the foot) may occur spontaneously or in multiple sclerosis patients.

Golgi Tendon Reflex

• Components include the Golgi tendon organ, interneurons, and alpha motorneurons.
• The functional pathway:
  • Tension on the muscle
  • activate Golgi tendon organ receptor
  • stimulate inhibitory interneurons
  • Inhibit alpha motorneurons
• Properties:
  • Shows local sign
  • Disynaptic
  • Activated by tension
  • Inhibitory effect
• Protective function of the tendon reflex.
• Detection and adjustment for changes from desired length or tension, regulation of muscle "stiffness" during normal movement function synergistically.
• Exhibits reciprocal innervation of antagonists.

Flexor Reflex and Crossed-Extensor Reflex

• Stimulus is noxious cutaneous input (touch, pressure, heat, cold, and tissue damage).
• Leads to withdrawal of the stimulated limb and extension of the opposite limb.
• Flexor reflex (ipsilateral) involves:
  • Exciting flexors
  • Inhibiting extensors
  • Short delay (few msec), indicating a few interneurons and synapses.
• Output: alpha-motorneurons, the same "final common path" as used in other reflexes.
• Crossed-extensor reflex excites extensors and inhibits flexors on the opposite side.
• Latency: 0.2 – 0.5 seconds, indicating many interneurons and synapses.
• Long-lasting, indicating spinal mechanisms for maintained "after-discharge" or perseveration.
• A stronger stimulus causes a stronger response.
• Functions include avoidance or limitation of tissue damage, used by higher centers in locomotion.

Complex Coordination/Reflexes

• Scratch reflex is specific to the place of stimulus and involves complex motor outputs.
• Spinal shock is a state of transient reflex depression below the level of injury of the spinal cord.
• Recovery varies among reflexes and may result in hyperreflexia weeks of months after the injury.
• Different species recover at different timescales:
  • Humans: days to weeks or months
  • Dogs: hours to days
  • Rats: minutes
  • Frogs: seconds
• Spinal reflexes disappear as brain circuits exert more positive and negative control on spinal circuits in newborns. The presence and disappearance of spinal mediated reflexes occurs in a stereotyped sequence in normal infants.
• Examples of reflexes in newborns:
  • Extensor thrust reflex (aka positive supportive reaction reflex)
  • Walking reflex
  • Grasp reflex (elicited in fetuses as early as 16 weeks of fetal life, disappears by 3 months postnatal age)
  • Babinski sign (normally disappears at age 12-18 months; appearance in adults indicates pyramidal tract damage).