Reflex Arc Definition Anatomy

A reflex is an involuntary reaction in response to a stimulus applied to the periphery (e.g. arms, legs, etc) and transmitted to the Central Nervous System (CNS). The purpose of the reflex is to react to stimuli more quickly than is possible by conscious thought. As such, reflexes do not involve the brain which is only aware of the response after it has taken place. Most reflexes occur in the spinal cord or brainstem. Many reflexes have a high survival value, enabling rapid action to avoid potential danger. For example, the withdrawal reflex; which includes the situation of the sudden withdrawal of the hand in response to a pain stimulus.

There are also learned/conditioned reflexes such as the sight and smell of food which triggers a salivation response. For example, in Pavlov’s Dog experiment: On numerous occasions a bell is rung just before a dog is fed. The dog salivates as usual on receiving its food. Then the bell is rung without any food being presented. The dog still salivates, as it is in response to the bell ringing.

A reflex arc is the neuronal pathway that the reflex follows. It is the basic functional unit of the nervous system because it is the smallest, simplest pathway capable of receiving a stimulus and yielding a response. The reflex arc consists of these 5 components:

• A Sensory Receptor – which senses the stimulus
• A Sensory Neuron – where the electrical signal generated from the sensory receptor travels along
• Interneurons – located between and communicating with two other neurons;
• A Motor Neuron – where the responding signal travels, towards the muscle to activate a response
• An Effector Organ – E.g. The muscle, responding to the stimulus.

The result of a reflex can be seen when a person’s finger touches a hot stove. Sensory receptors called pain receptors in the skin are stimulated by the hot stove, and action potentials (APs, electrical signals) are produced. Sensory neurons conduct the APs to the spinal cord where the signal transmits to interneurons (a ‘synapse’), then to the motor neurons in the spinal cord. Finally, the signal is transmitted back to the body’s periphery to the flexor muscles in the upper limb which contract and pull the finger away from the stove.

No conscious thought is required for this reflex, and withdrawal of the finger from the hot stove begins before the person is consciously aware of any pain.

This is a stretch reflex, meaning that muscles contract in response to a stretching force applied to them. In this particular reflex, stretching occurs when a hammer is tapped just below the knee cap where the patella tendon lies. The patella reflex is important in maintaining posture and in coordinating muscular activity. It is used by doctors to determine if the associated CNS centres are functioning correctly. An absence of or an exaggerated patella reflex is evidence of spinal cord damage.