Reflex arc and Reflexes

• There are two types of interactions in the human nervous system: involuntary and voluntary
• Involuntary or autonomic responses are not under the conscious control of the brain and are classified as automatic, rapid and instinctive responses that do not need to be learned
• They are known as reflexes or reflex actions and are among the simplest kind of behaviours in humans 
• A reflex arc is the pathway along which nerve impulses travel when a reflex action occurs
• In this action a receptor is stimulated and an impulse travels along a sensory neuron to an interneuron found in the spinal cord via a synapse
• The interneuron stimulates a motor neuron via another synapse and the impulse travels to an effector organ which brings about a response
• In a reflex response, the brain is not directly involved to ensure a fast and automatic response to increase the likeness of an individual being protected and surviving
• Reflex actions have a protective function, for example, when you touch a hot object your hand jerks away rapidly
• Other reflexes include sneezing, secretion of saliva, coughing, blinking and the constriction or dilation of the pupil of the eye
• Reflex responses are automatic, rapid reactions to stimuli that help protect the body from harm and maintain homeostasis
• They involved a specific neural pathways known as the reflex arc
• The reflex arc does not involve the brain
• It is an unconscious action
• Reflex arcs act on an impulse before that impulse can reach the brain
• Sensory neurons do not pass directly into the brain, but synapse directly with motor neurons in the spinal cord
• Allows reflex actions to occur quickly by activating spinal motor neurons without the delay of having to go through the brain
• Pathway of reflex responses

  1. Detection

  2. Transmission to the spinal cord

  3. Processing in the spinal cord

  4. Transmission to the effector

  5. Response
• Detection


  A stimulus activates sensory receptors in the skin
• Transmission to the spinal cord

  The sensory receptors generate an electrical signal that travels along the sensory neuron to the spinal cord
• Processing in the spinal cord


  The signal is processed by an integration centre consisting of interneurons
• Transmission to the effector


  The response signal travels along a motor neuron to the effector organ
• Response

  The effector organ performs the reflex action
• For example, the muscle contracts to pull the hand away from the hot stove
• Types of Reflexes

  • Somatic Reflexes: involves skeletal muscles, example include the knee jerk reaction (patellar reflex) and the withdrawal reflex when touching something hot
  • Autonomic reflexes: involves smooth muscles, cardiac muscles, and glands. Examples include the regulation or heart rate, digestion and pupil dilation