Reflexes Overview

Reflexes are automatic and rapid responses to changes in the environment, acting as essential protective mechanisms to ensure survival and maintain bodily functions. These responses occur without conscious thought, allowing the body to react swiftly to stimuli. Most reflexes are integrated in the gray matter of the spinal cord, highlighting the significance of the spinal cord in processing sensory inputs and generating motor outputs.

Examples of Reflexes

• Knee-Jerk Reflex: Also known as the patellar reflex, it is triggered by tapping the patellar tendon located just below the knee cap. This quickly causes the leg to extend as a response to the stretching of the quadriceps muscle, demonstrating a basic reflex arc.

• Biceps Reflex: This reflex is initiated by tapping the biceps tendon, leading to a contraction and flexion of the elbow. It is a key assessment tool in neurological examinations.

Autonomic/Visceral Reflexes

• Cardiovascular Response: Heart rate increases when a person is scared, due to the activation of the sympathetic nervous system, which prepares the body for 'fight or flight'.

• Respiratory Response: During stress, there is an increase in respiratory rate while digestion slows down, reflecting the body's prioritization of immediate survival over metabolic processes.

Cranial Nerve Reflexes

• Corneal Reflex: This reflex involves cranial nerves V (trigeminal - sensory) and VII (facial - motor). When an object irritates the eye, such as an eyelash or a fly, it triggers a reflexive blinking response to protect the cornea from damage.

• Oculo-cardiac Reflex: Involves cranial nerves V and X (vagus). When pressure is applied to the eyeball, it can stimulate the vagus nerve, which may slow down the heart rate, illustrating the connection between sensory stimuli and autonomic responses.

• Pupillary Reflex: Involves cranial nerves II (optic - sensory) and III (oculomotor - motor). When light is shone into the eye, pupils constrict to limit the amount of light entering, protecting the retina and adjusting vision.

Reflex Arc Components

Reflex arcs consist of five essential components:

1. Sensory Receptor: Detects the stimulus (e.g., pinch from a needle) and converts it into electrical signals.

2. Sensory Neuron: Transmits these signals to the spinal cord via the dorsal horn, carrying information to the central nervous system.

3. Association Neuron: Acts as a relay point, integrating sensory input with motor output to determine an appropriate response.

4. Motor Neuron: Sends message from the spinal cord to the muscles or glands that will execute the response.

5. Effector: The muscle or gland that physically responds to the impulse from the motor neuron, performing the reflex action.

Types of Somatic Spinal Reflexes

• Stretch Reflex:

  • Example: Patellar Reflex

  • Triggered by tapping the patellar tendon with a reflex hammer, this reflex helps to maintain muscle tone and protects against injury. In this reflex, muscle spindle receptors detect stretch in the quadriceps, leading to its contraction while simultaneously inhibiting the hamstrings through reciprocal innervation, allowing for a smooth and controlled movement.

• Golgi Tendon Organ Reflex:

  • This reflex occurs when a muscle experiences significant tension (e.g., while lifting heavy weights). Golgi tendon organs, located within the tendons, detect this tension and send inhibitory signals to the agonist muscle (e.g., quadriceps), allowing the antagonist muscle (e.g., hamstrings) to contract. This mechanism helps prevent muscle damage by balancing the forces exerted on muscles during contractions.

• Flexor Reflex (Withdrawal Reflex):

  • Example: Pulling away from a hot surface.

  • This protective reflex is crucial for avoiding harmful stimuli. Sensory receptors detect the painful stimulus, activating sensory neurons that transmit the information to interneurons within the spinal cord. This leads to the activation of motor neurons, resulting in the contraction of flexor muscles and withdrawal of the limb from the source of pain, demonstrating the body's automatic protective responses.

• Crossed Extensor Reflex:

  • This reflex is engaged in conjunction with the flexor reflex. When one limb is withdrawn from a painful stimulus, the crossed extensor reflex ensures balance by extending the opposite limb. This involves motor output causing flexion of the withdrawn limb through hamstring contraction, while the opposite quadriceps muscle is activated, providing support and stability in response to the withdrawal of the painful limb.