Reflexes and Introduction to Spasticity

Question 1

What is a reflex arc?

Answer 1

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.

Question 2

What is the role of the afferent neuron in the reflex arc?

Answer 2

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.

Question 3

What is the role of the interneuron in the reflex arc?

Answer 3

The interneuron connects the afferent neuron to the efferent neuron and facilitates the reflex response within the CNS.

Question 4

What is the role of the efferent neuron in the reflex arc?

Answer 4

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.

Question 5

What are some examples of reflexes?

Answer 5

Examples include the withdrawal reflex, Golgi tendon organ (GTO) reflex, and joint protective reflexes.

Question 6

What is the exception to the typical three-neuron reflex arc?

Answer 6

The stretch reflex (myotatic or deep tendon reflex) is monosynaptic, involving only two neurons: a sensory neuron (1a) and an alpha motor neuron (α).

Question 7

What is the purpose of the stretch reflex?

Answer 7

The stretch reflex protects the muscle belly from overstretch injury by counteracting excessive elongation of the muscle.

Question 8

Describe the reflex arc of the stretch reflex.

Answer 8

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.

Question 9

What neurotransmitter is involved in the 1a → α synapse of the stretch reflex?

Answer 9

Acetylcholine is released by the 1a sensory neuron toward the alpha motor neuron, causing excitation.

Question 10

Where should the motor end plate be depicted in the stretch reflex?

Answer 10

The motor end plate should be depicted within the muscle belly.

Question 11

What is the purpose of the Golgi Tendon Organ (GTO) reflex?

Answer 11

To protect the tendon from overstretch injury by reducing stress on the tendon through muscle relaxation.

Question 12

Describe the reflex arc of the GTO reflex.

Answer 12

1b sensory neuron → interneuron → alpha motor neuron (α).

Question 13

How does the GTO reflex reduce tendon stress?

Answer 13

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.

Question 14

What neurotransmitters are involved in the GTO reflex?

Answer 14

• A: - 1b sensory neuron → interneuron: Acetylcholine (excitatory).
• Interneuron → alpha motor neuron: GABA (inhibitory)

Question 15

What is the purpose of reciprocal inhibition in relation to the stretch reflex?

Answer 15

To allow muscle relaxation of the opposing muscle (antagonist) while the primary muscle (agonist) contracts in response to stretch.

Question 16

Describe the reflex arc of reciprocal inhibition during the stretch reflex.

Answer 16

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.

Question 17

How can the HMCs override reciprocal inhibition?

Answer 17

The HMCs can inhibit the interneuron to allow activation of the antagonist muscle’s alpha motor neurons, enabling an eccentric contraction rather than relaxation.

Question 18

In the stretch reflex with reciprocal inhibition, what happens when the quadriceps stretch activates the 1a sensory neuron?

Answer 18

• 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).

Question 19

How do the 1a and 1b sensory neurons interact with the spinal cord in the stretch reflex and GTO reflex?

Answer 19

• 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.

Question 20

What is the homeostatic relationship between the stretch reflex and GTO reflex?

Answer 20

These opposite reflexes balance each other out in day-to-day life, maintaining muscle tone within normal parameters.

Question 21

What is unique about the stretch reflex arc compared to other reflexes?

Answer 21

The stretch reflex arc has no interneuron and involves a direct connection between the 1a sensory neuron and the alpha motor neuron.

Question 22

What is the spindle structure in the stretch reflex?

Answer 22

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.

Question 23

How does the spindle structure function during stretch?

Answer 23

• 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.

Question 24

What is the role of gamma (γ) motor neurons in the stretch reflex?

Answer 24

• 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.

Question 25

Where are the gamma (γ) motor neuron cell bodies located, and how are they influenced?

Answer 25

Their cell bodies are in the anterior horn of the spinal cord, and they are directly influenced by the HMCs.

Question 26

How do gamma motor neurons contribute to muscle tone?

Answer 26

They preset 1a sensitivity to stretch by influencing the intrafusal fibers, allowing the muscle spindle to adapt to changes in muscle belly length.

Question 27

What are the two primary pathways through which HMCs influence alpha motor neuron activation in the spinal cord?

Answer 27

1. Pyramidal Pathway
2. Extrapyramidal Pathway

Question 28

How does the HMC influence alpha motor neurons through the pyramidal pathway?

Answer 28

Direct excitation transmission: “Do this” commands.
Nuanced direct inhibition: Makes alpha motor neurons less responsive to incoming tissue afferentation.

Question 29

How does the HMC influence reflexes via the extrapyramidal pathway?

Answer 29

• Influence on receptor responsiveness: E.g., gamma innervation of the intrafusal fibers in the stretch reflex spindle.
• Influence on interneuron responsiveness: Affects reflex arcs such as the GTO, withdrawal, and reciprocal inhibition reflexes.

Question 30

How do the pyramidal tracts descend in the spinal cord?

Answer 30

They descend from the motor cortex to the opposite side of the spinal cord.

Question 31

How do the extrapyramidal tracts descend in the spinal cord?

Answer 31

They descend from the brainstem to the same side gamma motor neurons.

Question 32

What is spasticity?

Answer 32

Spasticity is muscle hypertonia caused by exaggerated, dysregulated reflex responses due to an imbalance between excitation and inhibition in CNS regulation of reflex sensitivity and motor activities.

Question 33

What causes the imbalance in excitation and inhibition that leads to spasticity?

Answer 33

Damage to upper motor neurons (UMNs) in the brain, brainstem, or spinal cord disrupts the ability of the HMCs to balance excitation and inhibition for smooth motor activity.

Question 34

Where does damage typically occur to cause spasticity?

Answer 34

1. In the brain:
   • Stroke: Infarctions causing axonal injuries and retrograde degeneration.
   • Multiple Sclerosis: Accumulation of plaques in UMNs over time.
   • Traumatic brain injuries.
2. In the brainstem.
3. In the spinal cord.

Question 35

What is the role of reflexes and HMC input in spasticity?

Answer 35

Reflexes are autonomous and do not require CNS involvement to perform basic responses. HMCs modulate, regulate, and coordinate reflex responses, predominantly through inhibition, ensuring smooth and efficient motor activities.

Question 36

What distinguishes spasticity from rigidity?

Answer 36

Spasticity is caused by white matter lesions in pyramidal tracts, while rigidity is typically due to extrapyramidal lesions (e.g., basal ganglia).

Question 37

What are the key features of spasticity?

Answer 37

• Caused by upper motor neuron (UMN) lesions.
• Occurs during muscle stretch (not at rest).
• Accompanied by increased tendon reflexes and Babinski’s response.
• Resistance is greater in one direction and higher at the start of movement (“clasp-knife spasticity”).
• Velocity-dependent: Faster movements trigger stronger involuntary contractions or “catches.”

Question 38

What are the key features of rigidity?

Answer 38

• Caused by extrapyramidal lesions (e.g., Parkinson’s disease).
• Muscle tone is increased at rest and during passive range of motion.
• Resistance is the same in all directions.
• Affects all muscles around a joint equally.
• Not velocity-dependent: Resistance does not vary with movement speed.