Reflexes

A reflex is a rapid, involuntary response to a stimulus. A reflex arc is the pathway traveled by the nerve impulses during a reflex. Most reflexes are spinal reflexes with pathways that traverse only the spinal cord. During a spinal reflex, information may be transmitted to the brain, but it is the spinal cord, and not the brain, that is responsible for the integration of sensory information and a response transmitted to motor neurons. Some reflexes are cranial reflexes with pathways through cranial nerves and the brain stem.

A reflex arc involves the following components, shown in Figure 1 :

• The receptor is the part of the neuron (usually a dendrite) that detects a stimulus.

• The sensory neuron transmits the impulse to the spinal cord.

• The integration center involves one synapse (monosynaptic reflex arc) or two or more synapses (polysynaptic reflex arc) in the gray matter of the spinal cord. In polysynaptic reflex arcs, one or more interneurons in the gray matter constitute the integration center.

• A motor neuron transmits a nerve impulse from the spinal cord to a peripheral region.

• An effector is a muscle or gland that receives the impulse form the motor neuron. In somatic reflexes, the effector is skeletal muscle. In autonomic (visceral) reflexes, the effector is smooth or cardiac muscle, or a gland.
  
  

  Figure 1 A reflex arc.

• A stretch reflex is a monosynaptic reflex that is a response to a muscle that has been stretched (the knee jerk reflex is an example). When receptors in muscles, called muscle spindles, detect changes in muscle length, they stimulate, through a reflex arc, the contraction of a muscle. Stretch reflexes help maintain posture by stimulating muscles to regain normal body position.

• A flexor (withdrawal) reflex is a polysynaptic reflex that causes a limb to be withdrawn when it encounters pain (refer to Figure 1 ).

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