Reflexes and Reaction Times

Reflex

An involuntary, rapid movement that occurs almost immediately after a stimulus

Peripheral reflex

A fast reflex that occurs in the spinal cord without needing to travel to the brain

Somatic reflex

Reflex involving the somatic nervous system that we are aware of (e.g., stepping on a tack)

Autonomic reflex

Reflex involving the autonomic nervous system that we are unaware of (e.g., regulating organ systems)

Reflex arc

The neural pathway through which reflexes occur

Monosynaptic reflex arc

Reflex arc with one sensory neuron and one motor neuron; example = knee-jerk reflex

Polysynaptic reflex arc

Reflex arc with a sensory neuron, one or more interneurons, and a motor neuron; example = withdrawal reflex

Stretch reflex

A reflex in response to muscle stretch; the fastest spinal reflex

Knee-jerk reflex

A stretch reflex involving the patellar ligament, sensory neuron, and quadriceps contraction

Muscle spindles

Sensory receptors that detect changes in muscle length and regulate reflex sensitivity

Flexion withdrawal reflex

Protective reflex that withdraws a limb from a painful stimulus (e.g., pinprick, heat)

Pathway of withdrawal reflex

Pain sensory neurons → interneurons → motor neurons → muscle contraction to withdraw limb

Crossed extensor reflex

A withdrawal reflex where the opposite limb strengthens to support body weight when the stimulated limb withdraws

Example of crossed extensor reflex

Stepping on sharp glass → one foot withdraws, opposite leg supports balance

Pupillary light reflex

Reflex controlling pupil size in response to light intensity

Pupil constriction (miosis)

Parasympathetic nerves contract iris sphincter muscles, reducing pupil aperture in bright light

Pupil dilation (mydriasis)

Sympathetic nerves contract iris dilator muscles, enlarging pupil aperture in dim light

Direct pupillary response

Constriction of the pupil in the eye directly exposed to light

Consensual pupillary response

Constriction of the opposite eye’s pupil when light is directed into one eye