Spinal Reflexes

Stretch Reflex

The stretch reflex is a fundamental spinal reflex that helps prevent muscle strain and tear injuries by responding to muscle stretching. It is crucial for maintaining muscle tone and posture.

• Purpose: Prevents muscle strain and tear injuries by responding to muscle being stretched.

• Initiation: Triggered by muscle spindles, which are sensory receptors within the muscle that detect changes in muscle length.

• Synapse Type: The stretch reflex is monosynaptic, meaning it involves a direct synapse between the sensory and motor neuron.

• Effect: Causes the stretched muscle to contract in response.

Reciprocal Inhibition: During the stretch reflex, neurons controlling the antagonistic muscle are inhibited via a polysynaptic pathway, allowing the stretched muscle to contract unopposed.

• Antagonistic muscle is caused to relax.

Example: The knee-jerk reflex (patellar reflex) helps maintain balance if the knees start to buckle by generating a strong muscle contraction in the quadriceps.

Practice Question: Reciprocal inhibition is important because it amplifies the contraction of the stretched muscle.

Key Terms:

• Muscle spindle: Sensory receptor detecting muscle stretch.

• Monosynaptic reflex: Reflex arc with a single synapse between sensory and motor neuron.

• Reciprocal inhibition: Inhibition of antagonistic muscles during a reflex.

Tendon Reflex

The tendon reflex is a protective spinal reflex that prevents tendon injury by responding to excessive muscle tension. It is a polysynaptic reflex involving multiple neurons.

• Purpose: Prevents tendon injury by responding to excessive tension in the muscle.

• Initiation: Triggered by Golgi tendon organs, which are sensory receptors located in tendons that detect changes in muscle tension.

• Synapse Type: The tendon reflex is polysynaptic, involving interneurons.

• Effect: Causes the muscle to relax when tension is too high.

Reciprocal Activation: Neurons controlling antagonistic muscles are activated during the tendon reflex, causing the antagonistic muscle to contract.

Example: During strength training, the tendon reflex prevents excessive muscle tension, protecting the muscle and tendon from injury.

Practice Question: The tendon reflex prevents excessive muscle tension, not by increasing sensory perception or stimulating contraction, but by causing muscle relaxation.

Key Terms:

• Golgi tendon organ: Sensory receptor detecting muscle tension.

• Polysynaptic reflex: Reflex arc involving one or more interneurons.

• Reciprocal activation: Activation of antagonistic muscles during a reflex.

Flexor & Crossed-Extensor Reflexes

The flexor (withdrawal) reflex and crossed-extensor reflex are protective spinal reflexes that help withdraw a body part from harmful stimuli and maintain balance.

• Flexor (withdrawal) reflex: Rapid withdrawal of a body part from a harmful stimulus, causing flexor muscles to contract.

• Ipsilateral reflex: Motor activity occurs on the same side of the body as the stimulus.

• Crossed-extensor reflex: Occurs simultaneously with the flexor reflex, causing extension of the opposite limb to maintain balance.

• Contralateral reflex: Motor activity occurs on the opposite side of the body as the stimulus.

Example: Stepping on a sharp object causes the flexor reflex to withdraw the affected leg, while the crossed-extensor reflex extends the opposite leg to maintain balance.

Practice Question: The crossed-extensor reflex is contralateral because it involves the opposite side of the body.

Key Terms:

• Flexor reflex: Withdrawal of a limb from a painful stimulus.

• Crossed-extensor reflex: Extension of the opposite limb to maintain balance.

• Ipsilateral: Same side as the stimulus.

• Contralateral: Opposite side as the stimulus.

Summary Table: Comparison of Spinal Reflexes

Additional info: The notes include diagrams illustrating the neural pathways of each reflex, showing sensory and motor neurons, interneurons, and the muscles involved. These reflexes are essential for rapid, involuntary responses to stimuli that protect the body from injury and help maintain posture and balance.