Reflex Arc & Reflex Action | Biology for JAMB PDF Download

Reflex Arc

A reflex arc is a neural pathway that controls involuntary, rapid, and automatic responses to specific stimuli. It involves the transmission of sensory information from receptors to the central nervous system (CNS) and the subsequent generation of motor responses without the involvement of conscious thought. Reflex arcs are crucial for our survival as they enable us to respond quickly to potentially harmful or dangerous situations.

The reflex arc consists of five essential components:

• Receptor: This is the structure that detects the stimulus. It can be a specialized sensory organ or a sensory neuron ending. For example, in the case of the knee-jerk reflex, the receptor is the sensory neuron located in the muscle tendon just below the knee cap.
• Sensory Neuron: Once the receptor detects the stimulus, it transmits the sensory information to the CNS through a sensory neuron. This neuron carries the information to the spinal cord or brain, depending on the type of reflex. In the knee-jerk reflex, the sensory neuron carries the information to the spinal cord.
• Integration Center: The integration center refers to the region within the CNS where the sensory information is processed. In simple reflexes, such as the knee-jerk reflex, the integration center is located in the spinal cord. Complex reflexes may involve processing in the brain as well.
• Motor Neuron: After the sensory information is processed, the integration center generates a motor response. The motor neuron carries this response from the CNS to the effector organ, which is usually a muscle or gland. In the knee-jerk reflex, the motor neuron carries the signal to the quadriceps muscle in the thigh.
• Effector: The effector is the target organ or muscle that carries out the motor response. In the knee-jerk reflex, the effector is the quadriceps muscle, which contracts in response to the stimulus.

An example of a reflex is the knee-jerk reflex, also known as the patellar reflex. When the knee is tapped just below the kneecap, the quadriceps muscle stretches abruptly. This stretching activates the sensory receptors (sensory neurons) in the muscle tendon, which send a signal to the spinal cord (integration center). The spinal cord processes the information and generates an immediate response. In this case, the response is the contraction of the quadriceps muscle (motor response), causing the leg to kick forward.

Other examples of reflexes include:

• Blink reflex: When an object suddenly comes close to the eye, the eyelids close involuntarily to protect the eye from potential harm.
• Withdrawal reflex: If you accidentally touch a hot surface, such as a stove, your hand immediately pulls away to prevent further injury.
• Gag reflex: When the back of the throat is stimulated, such as by an object touching the uvula, it triggers a reflexive contraction of the muscles, causing a gagging or choking sensation.
• Pupillary reflex: In response to changes in light intensity, the size of the pupil in the eye adjusts automatically to control the amount of light entering the eye.

Understanding the reflex arc and reflexes is vital in studying the nervous system. It allows us to appreciate the body's ability to respond rapidly to various stimuli, ensuring our safety and well-being.