Reflexes

Reflexes Overview

  • A reflex is a simple, involuntary reaction to a stimulus.

  • Key roles in animal behavior and protective mechanisms.

Learning Objectives

  1. Describe how a reflex arc works and its components.

  2. Differentiate between monosynaptic and polysynaptic reflexes.

  3. Differentiate between response and reflex.

  4. Describe examples of reflexes and the involved nerves.

Reflex Arc Components

  • Components:

    • Sensory receptors

    • Sensory neurons

    • Segment of spinal cord

    • Motor neurons

    • Neuromuscular junction

    • Muscle or other end organ

  • Monosynaptic Reflex:

    • Involves a small segment of the nervous system.

    • Tests a specific area (e.g., patella reflex).

  • Polysynaptic Reflex:

    • Involves broader areas with multiple segments.

    • Examples include withdrawal reflex.

Mechanism of Reflexes

Types of Neurons

  • Receptor: End of a sensory neuron responding to a stimulus.

  • Sensory (afferent) neuron: Sends sensory impulses to the CNS.

  • Interneuron: Connects sensory and motor neurons in the spinal cord.

  • Motor (efferent) neuron: Carries impulses to an organ/muscle.

Monosynaptic Reflex

  • Characteristics:

    • Most simple, rapid, and less common.

    • Known as the stretch reflex.

  • Process:

    1. Stretch on the tendon stimulates muscle spindles.

    2. Information sent via afferent neurons to the spinal cord.

    3. Impulse transmitted to the efferent neuron.

    4. Innervates the quadriceps muscle.

    • No interneuron is involved.

Patellar Reflex

  • A stretch reflex testing the femoral nerve.

  • Response: Contraction of the quadricep muscle when patellar ligament is struck.

Polysynaptic Reflex

  • Involves:

    • Motor neurons to multiple muscles.

    • Inhibition of muscle contraction.

    • Includes an interneuron.

  • Process:

    1. Skin receptor activation.

    2. Sensory neuron sends signal to interneuron.

    3. Synapse at motor neuron.

    4. Muscle contraction occurs.

    5. Feedback through spinal cord.

Withdrawal Reflex

  • Triggered by pinching skin between toes.

  • Response: Flexion of joints in the pinched limb.

    • Flexion of all joints across multiple muscles coordinated.

    • Message sent to the opposite leg for extensor muscles activation.

Reflexes vs Responses

  • Reflex: Automatic reaction to stimuli, involves spinal cord only.

  • Response: Involves conscious thought and carries sensory information to the brain for interpretation.

Lower Motor Neurons (LMN) and Upper Motor Neurons (UMN)

  • LMN:

    • Motor neurons in the PNS connecting directly to muscles.

    • Damage leads to permanent flaccid muscle tone, weak to absent reflexes.

  • UMN:

    • Located entirely in the CNS; communicate with LMN for motor movement.

    • Damage may recover with new neuron connections. Symptoms include exaggerated muscle tone.

Spinal Functional Segments

  • Higher Cervical (C1-C5): UMN.

  • Lower Cervical (C6-T2): LMN to front, UMN to rear.

  • Thoracolumbar (T3-L3): UMN to rear.

  • Lumbosacral (L4-S3): LMN to rear legs.

  • Sacrocaudal (S1): LMN to bladder, anus, and tail.

Specific Reflex Examples

  • Menace Response:

    • Blinking in response to a threatening object.

    • Positive response indicates blinking; not present in young animals.

  • Pupillary Light Reflex:

    • Involves CN II (optic nerve) and CN III (oculomotor nerve).

    • Pupil constricts with bright light; does not assess vision.

  • Panniculus Reflex:

    • Light touch along the back causes cutaneous trunci muscle to twitch.

    • Involves lateral thoracic nerve.

  • Palpebral Reflex:

    • Touching the eyelid sends sensory info via trigeminal nerve to facial nerve, causing eyelid contraction.

    • Used to evaluate anesthesia depth.

  • Perineal Reflex:

    • Positive test involves contraction of the anal sphincter upon touching the perineal area.

    • Mediated by the pudendal nerve.

Practice Questions

  1. Example of a monosynaptic reflex: A) Patella reflex.

  2. Identify neuron in the reflex arc: B) Motor somatic.

  3. Injury area affecting LMN to front limb: A) C6 – T2.

Conclusion

  • Understanding reflexes is crucial for veterinary assessments, diagnostics, and treatments.