W1 - Proprioceptors & Spinal Reflexes

Proprioception

• Proprioception: "sense of self."

• In limbs:

  • Proprioceptors are sensors providing information on:

    • Joint angle.

    • Muscle length.

    • Muscle tension.

  • Integrated to give information about limb position in space.

• Types of proprioceptors:

  • Muscle spindle: Provides information about changes in muscle length.

  • Golgi tendon organ: Provides information about changes in muscle tension.

Muscle Spindle Function

• Muscle spindles:

  • Small sensory organs enclosed within a capsule.

• Mechanism:

  • Muscle lengthens→muscle spindle stretches ↔ action potentials triggered.

  • Reflexively stimulates muscle contraction to prevent overstretching and muscle fiber damage.

• Stretch or myotatic reflex:

  • Stretching muscle spindle→impulse sent to spinal cord ↔ response to contract the muscle received.

  • Protects from forceful pulling or beyond normal range.

  • Quick impulse: only goes to spinal cord and back.

Example: Knee Jerk Reflex (Patellar Reflex)

• Sudden kicking movement of lower leg in response to a sharp tap on the patellar tendon (below kneecap).

• Mechanism:

  • Tapping tendon of knee extensor muscle group stretches muscle spindle fibers.

  • Activation of extrafusal muscle fibers in the same muscle.

  • Knee jerk occurs as these fibers actively shorten.

  • Shortening of intrafusal fibers causes their discharge to cease.

• Clinical significance:

  • Exaggeration or absence suggests potential CNS damage.

  • Helpful in recognizing thyroid disease.

Golgi Tendon Organs (GTOs)

• Location:

  • In tendons near the myotendinous junction.

  • In series (end to end) with extrafusal muscle fibers.

  • Unlike muscle spindles (parallel with muscle fibers).

• Activation:

  • Activated when tendon attached to an active muscle is stretched.

  • As tension increases, GTO discharge increases.

• Mechanism:

  • Sensory neuron of GTO synapses with inhibitory interneuron in spinal cord.

  • Inhibitory interneuron synapses with and inhibits a motor neuron serving the same muscle.

  • Result: Reduction in tension within the muscle and tendon.

• Function:

  • Spindles facilitate muscle activation.

  • Neural input from GTOs inhibits muscle activation.

Reflex Arcs (or Spinal Reflexes)

• Reflex action: Involuntary and nearly instantaneous movement in response to a stimulus (e.g., jerking hand away from a hot object).

• No thought input required.

• Reflex arc: Path taken by nerve impulses in a reflex.

• Mechanism:

  • Sensory neurons synapse in the spinal cord (usually).

  • Allows quick actions by activating spinal motor neurons without delay of routing signals through the brain.

  • Brain receives sensory input while reflex action occurs.

• Examples: Stretch (myotatic) reflex and Golgi tendon reflex.

Reflex Arc Example

• Most reflex arcs involve three neurons.

• Mechanism:

  • Stimulus (e.g., needle stick) stimulates pain receptors in skin, initiating impulse in a sensory neuron.

  • Travels to spinal cord where it synapses with a relay neuron.

  • Relay neuron synapses with one or more motor neurons.

  • Motor neurons transmit impulse to muscles, causing them to contract and pull away.

• Brain involvement:

  • Reflexes do not require brain involvement.

  • In some cases, the brain can prevent reflex action.

Research Activity

• Stretch reflex and Golgi tendon reflex are two examples of reflex arcs.

• Further Exploration:

  • Identify and describe other examples of reflex arcs in the human body.

  • In what situations do these reflexes occur?

  • What function do they serve?