Spinal voice pt.2

Overview of Chapter 13

  • Chapter 13 discusses reflexes within the nervous system, defining their characteristics and detailing specific reflex types.
  • The chapter concludes with planning for upcoming exams.

Reflexes: General Characteristics

  • Definition and Components of Reflexes

    • Reflexes are automatic responses to stimuli.
    • They require a stimulation, meaning there must be a triggering event such as pain, temperature change, or muscle stretch leading to a reflex action.

  • Key Characteristics of Reflexes

    • Reflexes are involuntary: They do not involve conscious thought or decision-making and bypass the brain.
    • They are rapid: Reflex pathways are short as they connect directly to the spinal cord rather than going to the brain first.
    • Reflexes are stereotyped: The same reflex occurs in everyone, regardless of age or sex.

Reflex Pathway Diagram

  • The general pathway of reflexes includes:
    • Stimulus (detected by sensory receptors in the peripheral nervous system)
    • Sensory Afferent Neuron (transmits impulse to the spinal cord)
    • Central Nervous System (spinal cord)
    • Motor Efferent Neuron (sends impulse from spinal cord to effector)
    • Effector (typically muscles that respond by contracting or relaxing)

Specific Reflexes

1. Stretch Reflex

  • Purpose: Helps maintain balance and prevent muscle overstretching.

  • Mechanism:

    • When a muscle is stretched, a reflex kicks in to contract the muscle, effectively shortening it back to its original length.
    • Muscle Spindles: These are sensory receptors in muscles that register stretch. They play a crucial role in proprioception.

  • Example: The knee-jerk response when a doctor taps the patellar tendon; this causes quadricep contraction and an extension of the knee.

  • Pathway of Stretch Reflex:

    • Triggered by a stretching stimulus.
    • Sensory afferent neuron signals the spinal cord.
    • Excitation of quadriceps and inhibition of hamstring muscles to prevent opposition.

2. Golgi Tendon Reflex

  • Purpose: Prevents muscle damage by regulating tension on tendons.

  • Golgi Tendon Organs: Receptors located within tendons that are triggered by excessive muscle tension.

  • Mechanism:

    • When tension becomes too high, Golgi tendon organs activate to induce muscle relaxation, preventing tendon rupture.

3. Flexor Withdrawal Reflex

  • Purpose: Protects the body from harmful stimuli such as pain.

  • Example: Stepping on a piece of glass.

  • Mechanism:

    • The painful stimulus activates sensory afferent neurons sending impulses to the spinal cord.
    • The reflex causes activation of knee flexor muscles (hamstrings) while inhibiting knee extensor muscles (quadriceps) to lift the foot away from danger.

4. Crossed Extension Reflex

  • Purpose: Maintains balance and posture when one side of the body is withdrawn from danger.

  • Coupled with flexor withdrawal reflex to stabilize the body.

  • Mechanism:

    • When the painful stimulus activates the withdrawal reflex on one side (e.g., lifting the leg), the crossed extension reflex helps to keep the opposite leg extended to maintain balance.
    • Contralateral Reflex Arc: Activation occurs on opposite sides of the body (stimulus enters on one side and response isOutput out on the other).

Spinal Cord Anatomy Review

  • Motor functions are associated with the anterior part of the spinal cord (anterior gray matter and ventral roots).

  • Sensory functions are associated with the posterior part (posterior gray matter and dorsal roots).

  • Ganglia: Collections of neuronal cell bodies located within dorsal root ganglia.

  • Neuron Arrangement:

    • Unipolar sensory neurons reside in ganglia with peripheral processes leading to receptors and central processes entering the spinal cord.
    • Multipolar motor neurons reside in the anterior gray matter, leaving via the ventral root to innervate muscles.

Concluding Notes

  • Reflexes serve essential roles in protecting the body and maintaining homeostasis by rapidly responding to stimuli.
  • The arrangements and pathways of neurons constitute a vital aspect of understanding how reflexes operate.