Spinal Reflexes

Reflex

rapid automatic response to specific stimuli

• Predictable with little variability
• Negative feedback mechanism, often protective

Types of reflexes

• Innate - neural connections form during development, modifiable
• Acquired - complex learned motor patterns, improved by repetition
• Spinal - connections & processing occur in spinal cord
• Cranial - occurs in brain
• Somatic - involuntary control of skeletal muscular system
• Visceral - involuntary control of glands, smooth & cardiac muscle

Reflex arc

1. Receptor responds to a stimulus
2. Afferent sensory neuron carries impulse to the CNS
3. Integration of information in CNS (involves several synapses)
   1. Monosynaptic - 1 synapse, rapid
   2. Polysynaptic - 2 or more synapses, slower but more complex
4. Efferent motor neuron carries impulse from the CNS
5. Effector is a muscle or gland

Stretch reflex

monosynaptic & ipsilateral (stimulus & response on same side)

• Regulates skeletal muscle length, ex. patellar reflex (fig. in class)
• Increasing muscle length activates muscle spindle receptor which synapses with a motor neuron causing the muscle to contract

Muscle spindle comprises 3-10 intrafusal muscle fibers in a connective tissue capsule attached to the endomysium or perimysium of skeletal muscle, spindle responds to the stretch of extrafusal skeletal muscle fibers surrounding the spindle, the central part of the spindle is wrapped by sensory fibers that continuously send impulses to the CNS, stretching the spindle increases spindle output and increases muscle tone of extrafusal fibers, compressing the spindle decreases output and decreases muscle tone of extrafusal fibers

Withdrawal reflex

polysynaptic & ipsilateral

• Painful stimulus produces flexor reflex with reciprocal inhibition
• Flexor reflex
  1. Pain receptor is stimulated by a painful stimulus & causes the sensory neuron to fire
  2. Sensory neuron synapses with interneuron(s) in the spinal cord
  3. Excitatory interneuron synapses with motor neuron
  4. Causing motor neuron (in anterior horn) to fire
  5. Producing a contraction in a flexor muscle pulling the appendage from the stimulus
• Reciprocal inhibition
  1. Contraction of the flexor muscle (agonist) will stretch the antagonistic extensor muscle and cause a stretch reflex in the antagonist muscle
  2. When a muscle contracts the antagonist must relax to permit the movement
  3. Inhibitory interneurons prevent the antagonist muscle from firing (fig. in class)

Crossed extensor reflex

polysynaptic & contralateral (stimulus & response on different sides)

• Occurs at same time and complements the flexor reflex
• Involves movements on the opposite side of the body for support or balance (fig. in class)
  1. Sensory neuron synapses with interneurons in the spinal cord
  2. Collateral branch of the excitatory interneuron extends across the gray commissure to the motor neuron of the extensor muscle on the opposite side of the body
     1. Causing the motor neuron (in anterior horn) to fire and the extensor muscle to contract
  3. Collateral branch of the inhibitory interneuron extends across the gray commissure to the motor neuron of the antagonistic flexor muscle on the opposite side
     1. Inhibiting the motor neuron from firing and preventing the flexor muscle from contracting (reciprocal inhibition)

Tendon reflex

generally prevents muscle from producing sufficient tension to break the tendons

• Tendon organs monitor external tension in tendon of contracting muscle
• As external tension increases excessively, inhibitory interneurons increase the inhibitory effect on the motor neuron to the contracting muscle