Topic: Nervous System Structure And Function

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus and does not require any thought input

When a person accidentally touches a hot object, they automatically jerk their hand away without thinking. The path taken by the nerve impulses from the stimulus to sensory neuron to motor neuron to reflex muscle movement in a reflex is called a reflex arc.

There are two types of reflex arcs: the autonomic reflex arc, affecting inner organs, and the somatic reflex arc, affecting muscles. When a reflex arc consists of only two neurons, one sensory neuron, and one motor neuron, it is defined as monosynaptic. Monosynaptic refers to the presence of a direct single synapse. No interneuron is present. By contrast, in polysynaptic reflex arcs, one or more interneurons connect afferent (sensory) and efferent (motor) signals. It causes the stimulation of sensory, association, and motor neurons.

The spinal cord provides the synapse for the reflex arc, even though the reflex arc is bypassing the brain, the brain is still aware of the occurrence. 

The feedback loop is a situation when the response impacts or influences the stimulus; it is of two main types namely, positive feedback and negative feedback; the reflex arc is usually negative feedback.

  1. Positive feedback loops are unstable systems, in which a change in a given direction causes an additional change in the same direction. For example, uterine contractions lead to oxytocin release, causing more contractions.
  2. Negative feedback loops are stable systems, in which a change in a given direction causes a change in the opposite direction. For example, a drop in blood pressure causes an increase in the antidiuretic hormone, which increases blood pressure.


Practice Questions


Khan Academy


MCAT Official Prep (AAMC)

Online Flashcards Biology Question 9

Biology Question Pack, Vol. 1 Question 39


Key Points

• Reflexes, or reflex actions, are involuntary, almost instantaneous movements in response to a specific stimulus.

• Reflex arcs that contain only two neurons, a sensory and a motor neuron, are considered monosynaptic. 

• Monosynaptic refers to the presence of a direct single synapse; no interneuron is present.

• Most reflex arcs are polysynaptic, meaning multiple interneurons (also called relay neurons) interface between the sensory and motor neurons in the reflex pathway.

• The spinal cord provides the synapse for the reflex arc, even though the reflex arc is bypassing the brain, the brain is still aware of the occurrence. 

• The supraspinal circuit involves input from the brain or brainstem to process stimuli.


Key Terms

motor neuron: a neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly controls muscles

sensory neuron: a neuron responsible for converting various external stimuli that come from the environment into corresponding internal stimuli

reflex arc: a neural pathway that controls an action reflex

autonomic reflexes: are unconscious motor reflexes relayed from the organs and glands to the CNS

somatic reflexes: one of the two types of reflex arcs, and specifically involves the skeletal muscles. They are unlearned muscle reflexes that are mediated by the brainstem

interneurons: connect neurons within specific regions of the central nervous system

polysynaptic: have many neurones connected



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