Spinal Reflexes

  • simplest neurological pathways

    • basic reflexes: we are born with these inherent reflexes, usually protective, also called spinal reflexes

    • acquired reflexes: become reflexive after a lot of practice and learning

  • reflex arc: components involved in making a reflex happen, consist of:

    • receptor: detects stimulus

    • afferent pathway: triggers action potential in afferent neuron

    • integrating centre: processes and decodes afferent information (usually the spinal cord in basic reflexes)

    • efferent pathway: transmits integrating centre’s ‘decision’/motor action

    • effector: receives signal from efferent pathway, typically a muscle

Myotatic (Stretch) Reflex

  • triggered by passively stretching a muscle, which then causes it to contract

    • the passive stretch of the whole muscle is detected by the muscle spindle

      • muscle spindle is the receptor in a stretch reflex, found deep within muscle belly

    • muscle spindle’s stretch triggers action potentials in the afferent pathway

  • Monosynaptic reflex: when two neurons, and therefore one synapse, are involved

    • ex. patellar tendon reflex

Reciprocal Innervation

  • most stretch reflexes have reciprocal innervation to prevent antagonist muscle from contracting (keeps it relaxed)

  • sensory afferent neuron synapses with the inhibitory interneuron —> inhibitory neuron synapses with antagonistic muscle

Withdrawal Reflex

  • painful stimulus will activate a receptor (ex. touching hot surface)

  • sensory afferent neuron synapses with:

    • excitatory interneuron —> motor neuron (ex. to biceps)

    • inhibitory interneuron —> motor neuron (ex. to triceps)

    • interneuron —> brain

  • Polysynaptic reflex: involve more than 1 synapse between afferent and efferent neurons

Crossed Extensor Reflex + Withdrawal Reflex

  • painful stimulus activates receptor

  • sensory afferent neuron synapses with:

    • Excitatory interneuron —> motor neuron to flexor

    • Inhibitory interneuron —> motor neuron to extensor

    • Interneuron crosses spinal cord then up to brain

  • sensory afferent neuron cross spinal cord and then synapses with:

    • Excitatory interneuron —> motor neuron to extensor

    • Inhibitory interneuron —> motor neuron to flexor