Reflexes and Introduction to Spasticity Flashcards
What is a reflex arc?
A reflex arc is the set of neurons involved in enacting a reflex response. It typically includes three neurons: an afferent neuron, an interneuron, and an efferent neuron.
What is the role of the afferent neuron in the reflex arc?
The afferent neuron is a primary (1⁰) neuron in the sensory system. It communicates tissue information (nociception, temperature, stretch, etc.) to the CNS, entering at the dorsal horn. It is always excitatory.
What is the role of the interneuron in the reflex arc?
The interneuron connects the afferent neuron to the efferent neuron and facilitates the reflex response within the CNS.
What is the role of the efferent neuron in the reflex arc?
The efferent neuron is an alpha motor neuron. Its cell body is in the anterior horn, and its axon is in the peripheral nervous system, where it innervates its motor unit to enact movement.
What are some examples of reflexes?
Examples include the withdrawal reflex, Golgi tendon organ (GTO) reflex, and joint protective reflexes.
What is the exception to the typical three-neuron reflex arc?
The stretch reflex (myotatic or deep tendon reflex) is monosynaptic, involving only two neurons: a sensory neuron (1a) and an alpha motor neuron (α).
What is the purpose of the stretch reflex?
The stretch reflex protects the muscle belly from overstretch injury by counteracting excessive elongation of the muscle.
Describe the reflex arc of the stretch reflex.
The 1a sensory neuron detects muscle elongation, especially during rapid or end-range stretching. It synapses directly with the alpha motor neuron, increasing excitation and causing the muscle to tighten.
What neurotransmitter is involved in the 1a → α synapse of the stretch reflex?
Acetylcholine is released by the 1a sensory neuron toward the alpha motor neuron, causing excitation.
Where should the motor end plate be depicted in the stretch reflex?
The motor end plate should be depicted within the muscle belly.
What is the purpose of the Golgi Tendon Organ (GTO) reflex?
To protect the tendon from overstretch injury by reducing stress on the tendon through muscle relaxation.
Describe the reflex arc of the GTO reflex.
1b sensory neuron → interneuron → alpha motor neuron (α).
How does the GTO reflex reduce tendon stress?
The 1b sensory neuron increases firing as the tendon elongates, synapsing with an interneuron. The interneuron releases an inhibitory neurotransmitter (GABA) to the alpha motor neuron, causing muscle relaxation and reduced pull on the tendon.
What neurotransmitters are involved in the GTO reflex?
• A: - 1b sensory neuron → interneuron: Acetylcholine (excitatory).
• Interneuron → alpha motor neuron: GABA (inhibitory)
What is the purpose of reciprocal inhibition in relation to the stretch reflex?
To allow muscle relaxation of the opposing muscle (antagonist) while the primary muscle (agonist) contracts in response to stretch.
Describe the reflex arc of reciprocal inhibition during the stretch reflex.
A: 1a sensory neuron synapses with:
• Direct Pathway: Alpha motor neuron for the agonist muscle (e.g., quadriceps), causing contraction.
• Indirect Pathway: Interneuron → alpha motor neuron for the antagonist muscle (e.g., hamstrings), causing relaxation.
How can the HMCs override reciprocal inhibition?
The HMCs can inhibit the interneuron to allow activation of the antagonist muscle’s alpha motor neurons, enabling an eccentric contraction rather than relaxation.
In the stretch reflex with reciprocal inhibition, what happens when the quadriceps stretch activates the 1a sensory neuron?
• A: - The 1a neuron directly activates the quadriceps alpha motor neuron for contraction (stretch reflex).
• The 1a neuron also activates an inhibitory interneuron to relax the hamstrings (reciprocal inhibition).
How do the 1a and 1b sensory neurons interact with the spinal cord in the stretch reflex and GTO reflex?
• A: - 1a (stretch reflex): Direct excitation synapse with alpha motor neuron (α).
• 1b (GTO reflex): Synapses with an inhibitory interneuron, which then synapses with the same alpha motor neuron.
What is the homeostatic relationship between the stretch reflex and GTO reflex?
These opposite reflexes balance each other out in day-to-day life, maintaining muscle tone within normal parameters.
What is unique about the stretch reflex arc compared to other reflexes?
The stretch reflex arc has no interneuron and involves a direct connection between the 1a sensory neuron and the alpha motor neuron.
What is the spindle structure in the stretch reflex?
A: A specialized receptor structure in the muscle that includes:
• Central, non-contractile portion: Wrapped by the 1a sensory neuron.
• Contractile portions (intrafusal fibers): Located at either end of the spindle.
How does the spindle structure function during stretch?
• The 1a sensory neuron increases firing as the central, non-contractile portion elongates and reduces firing as it shortens.
• The intrafusal fibers can add to or reduce the stretch detected by 1a by contracting or relaxing.
What is the role of gamma (γ) motor neurons in the stretch reflex?
• They innervate the intrafusal fibers, adjusting their tension.
• They allow the HMCs to influence the sensitivity of the spindle to stretch, thereby helping maintain muscle tone.
